# Catalytic CO2 Conversion to Added-Value Energy Rich C1 Products

• Jangam Ashok
• Leonardo Falbo
• Sonali Das
• Nikita Dewangan
• Carlo Giorgio Visconti
• Sibudjing Kawi
Chapter

## Abstract

Carbon-dioxide emission from various sources is the primary cause of rapid climate change. Its utilization and storage are becoming a pivotal issue to reduce the risk of future devastating effect. The conversion of carbon-dioxide as an abundant and inexpensive feedstock to valuable chemicals is a challenging contemporary issue having multi-facets. There is a need to elucidate the process of utilizing CO2 to gain a fundamental understanding to overcome the challenges. This chapter focuses on converting CO2 to C1 valuable chemicals via hydrogenation (methane, methanol, syngas, formic acid) and reforming reactions (syngas). The first four parts of this chapter cover the production of methane, methanol and formic acid via hydrogenation reaction and syngas via reverse water gas shift reaction. Moreover, the last part of the chapters consists of reforming whereby CO2 acts as a mild oxidant for the production of syngas (CO + H2). The chapter covers different aspects, including the current challenges in the process, the state of the art and design of catalysts, and mechanistic consideration, all of which are critically evaluated to give the insight into each reaction.

## 5.1 Introduction

Anthropogenic carbon dioxide production is widely accepted as a major reason for accelerated climate change and global warming. In recent years, there has been a wide amount of global interest in finding sustainable ways to reverse the increasing CO2 levels in the atmosphere. Globally, treaties such as the Kyoto Protocol and the Paris Agreement identify reduction in carbon emissions as vital in preventing the potentially disastrous effects of further global warming. Carbon Capture and Utilization (CCU) is one of the key areas that can achieve CO2 emission targets at the same time contributing to the production of energy, fuels and chemicals to support the increasing demand. In CCU, carbon dioxide is captured and separated from emission gases and then converted into valuable products. CO2 may be used as a raw material for conversion into syngas and energy products  such as methane, methanol, dimethyl ether etc., or as chemical feedstock for production of inorganic or organic carbonates, carboxylates, urea etc. [1, 2, 3]. Many of the technologies to convert CO2 into value-added products are still immature, and the focus of active research. In this chapter, we will cover the technologies for the reduction of CO2 into C1 molecules that are the fundamental building blocks of the fuel and chemical industry. Technologies covered are CO2 hydrogenation to methane, syngas, methanol, and formic acid, and methane reforming to syngas production.

## 5.2 CO2 Hydrogenation to Methane

A noteworthy product of CO2 hydrogenation is Synthetic (or Substitute) Natural Gas (SNG). The SNG, which is mainly constituted of methane, has attracted much attention in the last few years because of its wide potential utilization market and because it is the only CO2 hydrogenation product with an existing distribution network available in many industrialized countries. In principle, SNG can be injected in the existing low, medium or high-pressure natural gas grid, so as to be promptly distributed or stored.

The production of SNG from CO2 and renewable H2 is commonly referred to as “Power to Gas” (PtG) process [4, 5, 6]. Low-cost hydrogen can be produced by water electrolysis, exploiting the excess (renewable or nuclear) electricity when available in the power grid. In this regard, CO2 can be considered as a low-cost carbon vector, that is converted into methane to allow more effective storage of H2 and, at the same time, assist the stabilization of the electric grid. For this reason, CO2 methanation is considered as an enabling technology for the long term (chemical) storage of electricity [5].

### 5.2.1 Thermodynamic Consideration

CO2 methanation [Sabatier reaction, (5.1)] is strongly exothermic and brings about the molar contraction of the reacting mixture. Furthermore, the equimolar production of CO through the mildly endothermic reverse water gas shift [RWGS, (5.2)] reaction occurs during the synthesis.
$${\text{CO}}_{2} + 4{\text{H}}_{2} \to {\text{CH}}_{4} + 2{\text{H}}_{2} {\text{O}}\quad \Delta {\text{H}}^{\circ}_{{298\,{\text{K}}}} = - 164\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.1)
$${\text{CO}}_{2} + {\text{H}}_{2} \to {\text{CO}} + {\text{H}}_{2} {\text{O}}\quad \Delta \text{H}^{\circ}_{{298{\text{K}}}} = 41.2\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.2)
Due to the exothermic nature and the decrease in number of moles, thermodynamics indicates that low temperature and high pressure boost the CH4 yield (Fig. 5.1).

As shown in Fig. 5.1, CO is the only important by-product of the CO2 methanation. C2+ hydrocarbons can be also found in the products pool, but their content is usually negligible, especially when operating at low to moderate pressures.

The development of a CO2 methanation technology is historically associated to the production of H2 from syngas. Indeed, when pure hydrogen has to be produced from fossil fuels, usually natural gas, syngas is first produced by one or two reforming steps (usually steam reforming + autothermal reforming), then CO is converted into CO2 in high and low-temperature water gas shift units, most of CO2 is captured through adsorption processes and finally the remaining traces of COx are hydrogenated to methane. In this latter process, methanation reaction is used as a purification step, and the process conditions are very different from those of interest for the methanation of concentrated CO2 streams: the inlet concentration of COx is very low during H2 purification steps, while H2 is in large excess. Under these highly diluted conditions, thermodynamics allows COx conversion close to 100%, and regardless of the high exothermicity of the COx hydrogenation reactions, low duties have to be managed in the reactor.

Massive production of SNG through COx methanation is more demanding: very active catalysts are needed, that are able to work at lower temperature to prevent strong thermodynamic limitations. Also, reactors able to effectively remove high reaction duties must be designed. Nevertheless, COx hydrogenation processes aimed at the production of SNG gained importance during the oil crisis in the late 1970s, when methane was produced by using syngas obtained from coal gasification as feedstock [7]. Several plants were built worldwide in that period and some of them are still in operation [8, 9]. Furthermore, during the last years, the increase in biomass utilization led to the construction of new plants for the conversion of biomass to methane [7].

Massive production of SNG through CO2 hydrogenation is much more recent, and is related, as previously discussed, both to the need of effectively storing excess renewable power for long periods, and to the increased sensibility on the need of limiting CO2 emissions in the atmosphere. The typical feedstock of these methanation plants is the carbon dioxide separated from biogas or from the flue gases of power plants [10, 11, 12]. However, more interest in this process has been also demonstrated by energivorous industries, such as steel manufactory, which are important CO2 emitters. The first pilot plant for CO2 methanation was built in the 1990s, and nowadays the first commercial applications are available [10].

### 5.2.2 Catalysts

#### 5.2.2.1 Nature of Active Center

At the process conditions where CO2 methanation is thermodynamically favored, significant kinetic limitations exist. Thus, catalyst design plays a pivotal-role to achieve acceptable process performance.

At the beginning of the 20th century, Sabatier and Senderens [13] discovered that metallic nickel is able to catalyze the hydrogenation of carbon oxides producing methane and water. In the following decades, CO2 methanation was investigated over several suitable catalytic systems based on VIII B group metals [14]. Cobalt, iron and almost all the noble metals were also found to be able to activate CO2. Different selectivity to methane were measured [7]. Nickel [15, 16], cobalt [17, 18] and noble metals [19, 20] produce mainly CH4 and CO, while Fe also catalyzes the formation of significant amounts of C2+ hydrocarbons [21, 22].

Although several suitable catalytic systems have been considered, the Ni-based catalysts are those more studied and the only ones currently used at industrial scale because of their low cost and wide availability of this metal [23]. Usually, commercially used catalysts consist of high concentrations of nickel dispersed on a high surface area support. Unsupported Ni nanoparticles [18, 24], Ni-Raney [14, 23, 25] and hydrotalcite-like NiAl(O)x samples [26] have been also proposed as effective catalysts, but their effective performances (activity/selectivity/stability/cost) cannot outperform that of conventional catalyst when used in real conditions. Typically, Ni-based catalysts are active at temperatures where CO2 conversion is extensively limited by the thermodynamics (green areas in Fig. 5.1). Moreover, due to the rather high process temperature, significant amount of CO is produced during CO2 methanation, in addition to traces of C2+. Also, at the typical process conditions, Ni-based catalysts are rather prone to deactivation [27]. Sintering phenomena are observed due to the high process temperature [28]. Furthermore, these catalysts suffer from carbonyls and carbides formation, resulting in deactivation by active phase volatilization [7, 27, 28, 29, 30]. Fouling phenomena have been observed, and carbon deposition leading to whisker carbon formation has been reported [29, 31]. In the case of contaminated feed stream, severe sulfur poisoning has been also shown [29].

Efforts are currently ongoing in many laboratories worldwide to design more active nickel catalysts, that can be operated at process conditions where thermodynamics is more favorable, and temperature driven deactivation mechanism are slowed-down. Important progress has been reported in the very last years, but optimization work is still needed before introducing a new generation of Ni-based methanation catalysts.

Noble metals are much more active than nickel [14, 23], therefore low-temperature operations are possible, even using metal loading lower than 1 wt%. Solymosi and Erdöhelyi [19], working noble metal alumina-supported catalysts prepared using metal chlorides, have shown that the specific $${{\text{CO}}_{2}}$$ methanation rate decreases in the order Ru > Rh ≫ Pt ∼ Ir ∼ Pd, with turnover numbers for Ru and Rh two orders of magnitude higher than for the other noble metals. Also, Ru- and Rh-based catalysts have been found to be more selective to methane than Pt- and Pd-based ones, which lead instead to high CO selectivity [7, 14]. Regarding the resistance to deactivation, noble metal-based catalysts are more tolerant than nickel to sulfur poisoning, carbon deposition or carbides formation [14, 32, 33, 34, 35, 36, 37]. Also, high temperature treatments in hydrogen have been shown to be able to restore the initial activity of a Ru-based catalyst when carbonaceous species are accumulated on the surface [38, 39, 40].

The longer catalytic life and the higher activity and CH4 selectivity are the essential characteristics that allow some catalysts based on precious metals to favorably compete with the cheaper Ni-based materials. On the basis of these premises, to date, Ru-based catalysts seem to be the most promising alternative to Ni-based catalysts for the intensification of SNG production from CO2. Indeed, Ru-catalysts are able to operate with CO2 yield to CH4 over 99% already at atmospheric pressure, exploiting temperature range where the thermodynamics is particularly favorable (red areas in Fig. 5.1). This paves the way for once-through reactor configurations and for the production of a SNG that does not need complex and expensive purification steps to comply with the feed-in regulations of several countries’ gas grid [6].

#### 5.2.2.2 Role of Support

The stability, activity and selectivity of CO2 methanation catalysts have been shown to depend on the crystal size [41, 42], on the shape of the metal particles dispersed on the support [43, 44], as well as on the interaction between the active metals and the oxide supports [45, 46]. In the case of Ni-based catalysts, the support has been shown to have a significant influence on the morphology of the active phase, as well as on the reactants adsorption and on the catalytic properties [47]. Several preparation methods have been developed to control the dispersion of the active phase [44, 48, 49]. On the contrary, the production of highly dispersed noble metals supported catalyst is more straightforward. Due to the low-metal loading and the strong interaction with the support, atomic dispersion is often approached [50]. Here, the problem may be that of the non-homogeneous distribution of the active phase in the support pellet, which may result in egg-shell configurations. If this represents an issue, solutions can be easily found based on the use of competitors during the impregnation phase, or by changing the pH of the impregnating solution.

In addition to those more “conventional” characteristics, that are common for many supports used in heterogeneous catalysis, it has been shown that during CO2 methanation the support has a crucial role in granting CO2 adsorption and activation. Szanyi and coworkers [50] reported that no CO2 is hydrogenated by using a catalyst made of 1 wt% of Pd supported on carbon nanotubes, while methanation activity was observed on 1 wt% Pd supported on alumina. Similar results were reported with different active phases and supports, such as Ni on ceria-zirconia [51] or Ru on titania [52].

Although it has been shown that, at least on some metals, CO2 methanation can occur also on unsupported catalysts, intensified catalyst performances can be achieved by exploiting the additional active centers that are created at the metal-support interface. These centers “at the interface” offer additional catalytic sites for CO2 methanation [53, 54], where CO2 molecules adsorbed on the support can react with hydrogen dissociatively adsorbed on the metal sites. As a result, the catalyst performances are boosted.

High surface area supports, usually oxides, are extensively used for the preparation of methanation catalysts. Among them, γ-Al2O3 is that most studied [55, 56] mostly due to its high surface area, well known properties, effective interaction with active metals, stability and low price. Other inorganic oxides, like TiO2 [43, 52, 57, 58], SiO2 [15, 59] and CeO2 [49, 60, 61, 62], have been investigated, both as single or mixed oxide structures [63, 64, 65]. Furthermore, TiO2, which is a good semiconductor support material, is widely studied especially for the CO2 photocatalytic methanation [66]. Eventually, zeolites [67], metal-organic frameworks [68] have been investigated [44]. Detailed information on the performances of these supports can be found in recent reviews [41, 42, 44].

#### 5.2.2.3 Role of Promoters

As previously discussed, conventional Ni-based catalysts suffer from remarkable catalyst deactivation during the CO2 methanation reaction. In order to overcome such an issue, promoters may be added in the catalyst formulation which provide auxiliary functions, such as sulfur-, sintering- and carbon-resistance properties [29].

It has been also proposed that the addition of a second metal can enhance the stability and activity of Ni-based catalysts [44]. The addition of Fe increases the amount of adsorbed CO2 and reduces the CO dissociation energy, thus favoring the methanation reaction [69]. The doping with La2O3 modifies the catalyst electronic properties, thus boosting the CO2 activation [70]. Eventually, the addition of MgO increases the fouling resistance, enhances the thermal stability and minimize the sintering of Ni-based catalysts [7]. Both La2O3 [50] and MgO [71, 72] can be also effectively added to some noble-metal catalysts supported on materials that are not able to activate carbon dioxide. In this case, a bi-functional catalyst for CO2 methanation is produced, where carbon dioxide is activated on the promoter, while hydrogen is dissociated on the noble metal [23]. Ru-based catalysts are usually unpromoted: their intrinsic activity and stability is already appropriate to catalyze CO2 methanation for a long period of time.

### 5.2.3 Reaction Pathway and Kinetics

For what concerns the reaction pathway, CO2 methanation mechanism is still debated and there is evidence that the nature of the metal, the typology of the support and the process conditions can strongly affect the reaction mechanism [14, 23, 29, 41]. Through in situ infrared spectroscopy it has been shown that, during CO2 methanation, adsorbed CO is the key reaction intermediate [52, 54]. The CO formation mechanism, however, is debated as well. Most of the authors believe that it is produced via RWGS, where the CO2 is adsorbed as bicarbonate on the support surface and then transformed into formate at the metal-support interface [53, 54]. Other authors suggest that CO2 is adsorbed dissociatively, with the consequent formation of CO and some formates, that act as spectators [73, 74].

Regarding the mechanism of hydrogenation of adsorbed CO, both H-assisted [54] and unassisted [31, 75] dissociation pathways have been reported.

Efforts have been devoted to the development of empirical and mechanistic kinetic models, especially in the case of Ni-based catalysts [7, 76, 77]. One of the most comprehensive kinetic studies on the Sabatier process was proposed by Weatherbee and Bartholomew [78], who studied a 3% Ni/SiO2 catalyst in a single-pass differential fixed-bed reactor working at low pressure. A second kinetic study worthy of note is the one reported by Xu and Froment [79], who modeled the process through 3 reactions happening at the same time: the methanation of carbon dioxide, the methane steam reforming (i.e. the reverse reaction with respect to the CO-methanation), as well as the water gas shift reaction. Although the model was developed and validated specifically for the steam reforming process on Ni-catalysts, it has been used also to describe experiments related to the CO2 methanation over similar catalysts [80]. Mechanistic kinetic expression, based on the Langmuir-Hinshelwood-Hougen-Watson approach, have been also proposed [77]. Some of these expressions, which are frequently used in the literature for process or reactor modeling studies, are listed in Table 5.1.
Table 5.1

Kinetic equations reported in the literature for CO2 methanation

Kinetic equation

Catalyst

References

$$r_{{\text{CO}_{2} }} = \frac{{kP_{\text{CO}}^{0.5} P_{\text{H}_{2}}^{0.5} }}{{\left( {1 + K_{1} \frac{{P_{{\text{CO}_{2} }}^{0.5} }}{{P_{\text{H}_{2}}^{0.5} }} + K_{2} P_{{\text{CO}_{2} }}^{0.5} P_{\text{H}_{2}}^{0.5} + K_{3} P_{\text{CO}} } \right)^{2} }}$$

3 wt% Ni/SiO2

[78]

$$r_{1} = \frac{{k_{1} }}{{P_{\text{H}_{2}}^{2.5} }}\frac{{\left( {P_{{\text{CH}_{4} }} P_{{\text{H}_{2} \text{O}}} - \frac{{P_{{\text{H}_{2} }}^{3} P_{\text{CO}} }}{{K_{{\text{eq},1}} }}} \right)}}{{\left( {\text{DEN}} \right)^{2} }}$$

$$r_{2} = \frac{{k_{2} }}{{P_{{{\text{H}}_{2} }} }}\frac{{\left( {P_{\text{CO}} P_{{{\text{H}}_{2} {\text{O}}}} - \frac{{P_{{{\text{H}}_{2} }} P_{{{\text{CO}}_{2} }} }}{{K_{{{\text{eq}},2}} }}} \right)}}{{\left( {\text{DEN}} \right)^{2} }}$$

$$r_{3} = \frac{{k_{3} }}{{P_{\text{H}_{2}}^{3.5} }}\frac{{\left( {P_{{\text{CH}_{4} }} P_{{\text{H}_{2} \text{O}}}^{2} - \frac{{P_{{\text{H}_{2} }}^{4} P_{\text{CO}} }}{{K_{{\text{eq},3}} }}} \right)}}{{\left( {\text{DEN}} \right)^{2} }}$$

$${\text{DEN}} = 1 + K_{\text{CO}} P_{\text{CO}} + K_{{{\text{H}}_{2} }} P_{{{\text{H}}_{2} }} + K_{{{\text{CH}}_{4} }} P_{{{\text{CH}}_{4} }} + K_{{{\text{H}}_{2} {\text{O}}}} \frac{{P_{{{\text{H}}_{2} {\text{O}}}} }}{{P_{{{\text{H}}_{2} }} }}$$

15.2 wt% Ni/MgAl2O4

[79]

$$r_{{\text{CO}_{2} }} = \frac{{kP_{{\text{CO}_{2}}}^{0.5} P_{\text{H}_{2}}^{0.5} \left( {1 - \frac{{P_{{\text{CH}_{4} }} P_{{\text{H}_{2} \text{O}}}^{2} }}{{P_{{\text{CO}_{2} }} P_{{\text{H}_{2} }}^{4} K_{\text{eq}} }}} \right)}}{{\left( {1 + K_{\text{OH}} \frac{{P_{{\text{H}_{2} \text{O}}} }}{{P_{\text{H}_{2}}^{0.5} }} + K_{{\text{H}_{2} }} P_{\text{H}_{2}}^{0.5} + K_{\text{mix}} P_{{\text{CO}_{2}}}^{0.5} } \right)^{2} }}$$

NiAl(O)x

[26]

$$r_{{{\text{CO}}_{2} }} = k\left\{ {\left[ {P_{{{\text{CO}}_{2} }} } \right]^{n} \left[ {P_{{{\text{H}}_{2} }} } \right]^{4n} - \frac{{\left[ {P_{{{\text{CH}}_{4} }} } \right]^{n} \left[ {P_{{{\text{H}}_{2} {\text{O}}}} } \right]^{2n} }}{{[K_{\text{eq}} ]^{n} }}} \right\}$$

0.5 wt. Ru/Al2O3

[32, 81]

Looking to CO2 methanation on Ru-based catalysts, literature reports mainly power-law empirical equations [32, 81], even though some mechanistic equations are available [54]. Most of the empirical models, developed by exploiting experimental data collected in laboratory reactors operating under differential conditions [81], show that the reaction rate has a dependence on H2 partial pressure (reaction orders in the range 0.3–2.5) stronger than on CO2 (reaction order 0–1) at low CO2 conversion values. Unfortunately, no valuable information can be derived from these models at the conditions of industrial interest for PtG applications, i.e. at high conversion and concentrated reactant streams.

On the contrary, the kinetic model proposed by Lunde and Kester [32], which also accounts for the approach to thermodynamic equilibrium, is widely used for the description of CO2 methanation over Ru-based catalysts at the process conditions of industrial interest. Recently, Falbo et al. [81] extended the validity of this kinetic model by proposing a new set of kinetic parameter able to describe the catalyst performances also under pressure (Table 5.1). This is particularly relevant for the design of modern optimized PtG processes aiming at producing pressurized SNG to be injected in the natural gas grid.

### 5.2.4 Engineering Challenges

A key-challenge in the engineering of the highly exothermic CO2 methanation process is the temperature control in the reactor: assuming a space velocity of 5000 h−1 and a complete CO2 conversion, around $$2\,{{{\text{ MW}}/{\text{m}}}^{3}}_{\text{cat}}$$ of heat need to be removed (methanol synthesis requires $$0.6\,{{\text{MW}/{\text{m}}}^{3}}_{\text{cat}}$$) [4]. Several reactor configurations have been proposed to grant the strong reaction  exothermicity: adiabatic packed-bed with interstage cooling, multitubular packed-bed with external cooling, multitubular structured reactors with external cooling, fluidized-bed reactors.

For Ni-based catalysts, which are limited by thermodynamic equilibrium, adiabatic multistage fixed-bed reactors (from 2 to 5 catalyst stages) with interstage heat exchanger and unconverted reactants recirculation is the most viable and less expensive option [7].

For Ru-based catalysts, non-adiabatic reactors are most appropriate. Among those, fluidized-bed reactors with internal heat exchanger would grant the best isothermicity [82, 83]. Nevertheless, due to high mechanical stress resulting from fluidization, attrition processes may strongly affect the catalyst and the reactor performances. Multitubular packed-bed represent a viable option to this configuration, even though these reactors are less effective in preventing the formation of hot-spots, risk of pore diffusion limitation exists if big catalyst pellets are selected and inacceptable pressure drops are encountered when small pellets are employed. In order to further enhance the heat transfer and to prevent hot spots, microchannel reactors [84], or structured reactors based on highly conductive metallic honeycomb monoliths or open-cell foams (sponges) [85], coated with the catalyst, have been proposed. Due to their internal metallic structure, monolith reactors grant enhanced heat transport due to heat conduction within the continuous substrate. Also, the thin catalyst layer prevents pore diffusion limitations and the high void fraction in the reactor limits the pressure drops. Drawbacks of structured reactors are the demanding catalyst deposition on the structured substrate, as well as the difficulty of catalyst loading, unloading and replacement [4, 7].

## 5.3 CO2 Hydrogenation to CO (Reverse Water Gas Shift Reaction)

Reverse water gas shift reaction is a crucial process for the production of CO which is a building block for the production of various useful chemicals such as methanol or other long-chain hydrocarbons [86]. CO that is considered to be the initial step of CO2 hydrogenation on metal catalysts is the primary product from RWGS reaction. This reaction exists in conjunction with FT synthesis for the production of hydrocarbons from syngas [87]. Indeed, this process is more technically feasible as compared to the alternative technologies converting CO2 to CO and gives an added versatility in the products obtained from CO transformation. The RWGS is also of great interest to be used in space exploration due to high (95%) atmospheric CO2 concentration on Mars and availability of H2 as a by-product of oxygen generation [88].

### 5.3.1 Thermodynamic Considerations

RWGS reaction is thermodynamically favourable at high temperature since it is an endothermic process. However, at low temperature region the WGS reaction is prominent as the reaction is exothermic. This reaction is also accompanied by other side reaction such as methanation that reduces the selectivity towards CO formation [89, 90, 91, 92, 93].

The RWGS reaction occurs according to (5.2).
$${\text{CO}}_{2} + {\text{H}}_{2} \leftrightarrow {\text{CO}} + {\text{ H}}_{2} {\text{O}}\quad {{\Delta \text{H}}^{\circ}}_{{298\,{\text{K}}}} = 41.2\,{\text{ kJ}}\,{\text{mol}}^{ - 1}$$
(5.2)
However, several undesired parallel and side reactions (5.1), (5.3) and (5.4) tend to occur as well
$$2{\text{CO }} \leftrightarrow {\text{C}} + {\text{CO}}_{2} \quad {\Delta \text{H}^{\circ}}_{{298\,{\text{K}}}} = - 172.6\,{\text{ kJ}}\,{\text{mol}}^{ - 1}$$
(5.3)
$${\text{CO }} + 3{\text{H}}_{2} \leftrightarrow {\text{CH}}_{4} + {\text{ H}}_{2} {\text{O}}\quad {\Delta \text{H}^{\circ}}_{{298\,{\text{K}}}} = - 206\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.4)
All reactions above occur simultaneously producing H2O, CO, CO2, H2, and C in the reaction medium. For any other reaction generating CO as a main product, the reaction generally requires H2:CO2 ratio of about 2 [94]. This additional amount of H2 imposes around 50% cost increase, thus not substantiating the overall process economy and feasibility. There are several other studies which compared the RWGS reaction with other processes producing CO, and among the investigated methods, RWGS reaction showed greater potential and higher efficiency when flue gas is the source of CO2. Therefore, an additional reason to improve catalytic activity for the RWGS reaction at lower range temperatures is to reduce heat requirements necessary for the FT process which follows RWGS. In the lower temperature region of 600 °C, methanation reaction becomes prominent and only at a temperature higher than 700 °C RWGS can produce CO as a significant product. Therefore, a novel catalyst design is needed to overcome this problem and achieve higher activity at an even lower temperature.

### 5.3.2 Catalyst Types

Design of catalyst is crucial to obtain a high activity and selectivity. Generally, the catalyst is designed to promote the dual functionality of active metal/metal oxide sites and sites on the surface of the support. The correct choice for metal/metal oxide and supports promotes the better adsorption of reactants, followed by reaction and finally the desorption of  products. There are mainly three categories of catalysts  for RWGS, namely: supported metal catalyst, mixed metal oxide catalyst, and transition metal carbide catalyst.

#### 5.3.2.1 Supported Metal Catalyst

There are several combinations of active metal and support which correspond to the production of a wide range of chemicals from hydrogenation of CO2. For examples, Wambach et al. reported metals (Ni, Cu, Ag, Rh, Ru, Pt, Pd and Au) supported on ZrO2 samples for CO2 hydrogenation reaction [95]. In this study, it was found that Ag and Cu are favorable for methanol, whereas Ni and Ru lead to methane as the major product, and the rest produced mainly CO, methanol and methane. Dai et al. studied mesoporous M (Ni, Cu, Co, Fe, Mn)–CeO2. Although the conversion for Ni–CeO2 was higher among all other metal-CeO2, however the selectivity was lower for Ni and Co. Several modifications such as alloying with Cu [91, 96, 97, 98], doping with alkali metals [93, 99], and enhanced metal-support interaction [100, 101] for Ni based catalysts have been reported to improve the selectivity towards desired product by methane suppression [102, 103]. Metals such as Cu, Fe, Mn showed almost 100% selectivity for CO during RWGS reaction [104].

From theoretical insights and experimental results, one of the pivotal criteria to consider while designing a catalyst for RWGS reaction is based on the electron properties of d-orbital holes of metals and the difference between desorption energy and dissociation barrier of metal carbonyls determined by the adsorption configuration [105]. The presence of incompletely filled d-orbitals leads to ease in the adsorption of reactants to form an intermediate, thereby improving the catalytic activity. Thus, noble metal based catalysts are considered as a highly important class of catalyst for RWGS reaction. Chen et al. studied the Pt/TiO2 supported catalyst and reported that the presence of oxygen defects in the reducible TiO2 support increase the number of interfacial sites and showed varying reactant adsorption at low and high temperature region as shown in Fig. 5.2 [106]. Additionally, the presence of other metals such as Ni, Co enhances the electronic property of Pt and forming bimetallic catalysts enhances the CO selectivity as compared to Pt alone which favors the formation of methane [107]. In contrast to reducible oxide supports, non-reducible supports such as Al2O3, SiO2, and zeolites show lower activity [108, 109]. Besides, when reducible phase is added to the parent metal oxide like MoOx, it decreases the activation energy barrier, thus enhancing the RWGS activity. Reducible oxide support for noble metals such as Pt, improves the activity due to high oxygen mobility in the presence of oxidant CO2 [110]. Addition of alkali metals such as potassium also enhances the stability of Pt thereby preventing metal sintering [111], provides active site for formate decomposition, reduces the adsorption strength of CO and shows higher TOF than un-doped Pt. Highly dispersed supported Au has been studied in depth for RWGS reaction and showed promising performance in terms of stability of metal sites, and decreased the required reaction temperature to below 400 °C.

Apart from noble metal supported catalysts, transition metals such as Cu and Fe are considered favorable for RWGS reaction. One of the comparative studies between Cu–ZnO and Cu–ZnO supported on alumina [112], showed the presence of alumina increases the dispersion of CuO and ZnO species. Nickel catalysts are rarely considered as effective RWGS catalysts because of their excellent hydrogenation to methane behavior [48, 49, 102, 113]. Catalysts with highly dispersed Ni nanoparticles on supports with large oxygen exchanging capacity are still found to be effective for RWGS reaction.

Single atom based catalyst showed a recent boom in terms of research interest to understand its property and wide application. Metal particle size plays a unique role in maintaining the stability of catalyst during the CO2 hydrogenation reaction. Matsubu et al., studied Rh/TiO2 supported catalysts. During the reaction, it was observed that Rh nanoparticles disintegrated to form isolated Rh sites. This change in size, controls the changing reactivity with time on stream. A strong correlation was observed between the reaction mechanism, TOF and number of Rh-isolated sites and methanation reaction [114]. Another investigation revealed the importance of Ru loading on Al2O3. With the loading percentage of less than 0.5%, the active metal was mostly atomically dispersed and high selectivity for CO was obtained [115]. Therefore, from all the studies done so far on supported metal catalyst for RWGS reaction, it can be concluded that the metal particle size, type of support (reducible and non-reducible), morphology of support and bimetallic catalyst showed distinctive behavior to enhance the activity and stability of catalyst under different reaction conditions.

#### 5.3.2.2 Mixed Metal Oxide Catalyst

Transition metal oxides such as ZnO, Fe2O3, Cr2O3 and mixed oxide solid solutions have been reported to be the promising catalyst for the RWGS reaction. Due to high temperature reaction condition, ZnO based catalyst loses activity with time; however this metal oxide, when mixed with a proper ratio of Al2O3, forms a spinel phase, ZnAl2O4 at higher temperature and it was found to be stable at 600 °C for 100 h of operation [116]. Indeed, ZnO/Cr2O3 showed excellent performance and stability with no coke formation. On the other hand, Fe2O3/Cr2O3 showed slight deactivation. Catalyst deactivation due to coke deposition is one the main issues associated with RWGS reaction. Thus, utilizing high oxygen storage elements such as CeO2 is another possible way to improve the performance [117]. Ce doped ZnO showed enhanced performance and stability even at the high temperature of 800 °C [118]. Co-CeO2 mixed oxides prepared by co-precipitation method showed excellent performance and low coke deposition. The presence of well dispersed Co on CeO2 support suppressed methane formation whereas high loading of cobalt led to an increase in the possibility of methanation reaction. Another group of mixed metal oxides extensively applied for RWGS are ZnxZr1 − xO2 −x [119], NixCe0.75 Zr0.25− xO2 [120]. These catalysts offer promising properties such as high oxygen storage and reducibility during high temperature reaction condition. Under severe reaction conditions, the catalyst undergoes deactivation after several cycles. Zn replaces the Zr ions in the lattice to form a surface solid solution. This solid solution increases the reducibility by improving oxygen vacancy which improves the oxygen mobility, thereby suppressing carbon formation.

Another type of mixed oxides are in the form of perovskite. Perovskite type oxides are widely used for various catalytic applications including reverse water gas shift reaction. Mixed oxide perovskites consist of A- and B-site ions. This, combined with their structural stability, allows for the variation of composition, oxygen vacancies and oxidation state of metal ions. For these reasons, mixed oxide perovskites are suitable models to study the relation between the solid state chemistry and the catalytic activity of the mixed metal oxides. In order to break thermodynamic equilibrium limitation and suppress methane formation, another technique reported is coupling RWGS reaction with chemical looping (CL). Perovskite based catalyst possess several possibilities to be modified to achieve desired oxygen vacancy in the structure and thus show huge potential for the process of RWGS coupled with chemical looping applications. In this process, carbon dioxide is first captured from its emissions source or separated from air and purified. The RWGS-CL operation converts CO2 and H2 to produce separate streams of CO and water. The produced CO can then be combined with additional H2 for liquid fuel production via FTS or methanol synthesis [121]. These separate product streams eliminate the possibility of methanation as a side reaction, because there is no direct interaction between CO2 and H2, and aid in avoiding thermodynamic limitations as shown in Fig. 5.3. Daza et al. studied La0.75Sr0.25CoO3-δ and LaFeO3 for RWGS coupled with CL and found that, with the oxidation-reduction process, a higher CO production was achieved, with good recyclability of catalysts [122]. Ba–Zr-based perovskite catalysts with Zn, Y and Ce doped in the structure were also studied. Among the three doping elements, Zn- and Y-doped BZYZ catalyst showed an outstanding activity for the RWGS reaction at 600 °C, whereas Ce doped perovskite showed no positive effect on RWGS reaction [123, 124].

#### 5.3.2.3 Transition Metal Carbide (TMC) Catalyst: An inexpensive and Emerging Class

Transition metal carbides are considered as an alternative to the noble metal based catalysts, with a low cost of materials. Pioneering work by Levy and Boudart showed addition of carbon to metal like tungsten modified the electronic properties, making it similar to the noble metal Pt [125]. Presence of different transition metal changes the surface properties. From previous studies, TMCs are considered as a promising substrate for metal dispersion, thereby enhancing the hydrogen dissociation and C=O scissoring [126]. There are several studies done on TMC which showed CO2 activation over β-Mo2C, where CO2 binds to Mo2C in bent configuration breaking the C=O bond [125, 127]. The dissociated CO desorbs while O interacts with Mo2C to form Mo2C–O that is then removed by H2 to complete the cycle [128]. This oxy-carbide formation is crucial as it is a descriptor for determining the activity of WGS reaction [129]. TMC can also be used as a catalyst support for the dispersion of active metal sites to form small sized and stable nanoclusters. Zhang et al. studied Cu/Mo2C prepared using Cu–MoO3 as a precursor [130]. This catalyst showed excellent performance at higher temperature range of 600 °C and maintained 85% activity for 40 h of operation. The stability was attributed to the strong metal support interaction, enhancing Cu dispersion and preventing Cu agglomeration as shown in Fig. 5.4.

### 5.3.3 Mechanistic Considerations

Based on current development in understanding the kinetics and mechanism of RWGS reaction, there are several advanced techniques used, such as isotopic tracer method, operando DRIFTS and DFT calculations. For RWGS reaction, the reaction mechanism can be classified into two main reaction mechanisms as shown below in Table 5.2.
Table 5.2

Reaction mechanism for RWGS reaction

Redox mechanism

Dissociative mechanism

$${\text{H}}_{2} + 2{}^{*} \to {\text{H}} {}^{*} + {\text{H}} {}^{*}$$

$${\text{CO}}_{2} + {}^{*} \to {\text{CO}}_{2} {}^{*}$$

$${\text{CO}}_{2} {}^{*} + {}^{*} \to {\text{CO}} {}^{*} + {\text{O}} {}^{*}$$

$${\text{H}} {}^{*} + {\text{O}} {}^{*} \to {\text{OH}} {}^{*} + {}^{*}$$

$${\text{H}} {}^{*} + {\text{OH}} {}^{*} \to {\text{H}}_{2} {\text{O}} {}^{*}$$

$${\text{OH}} {}^{*} + {\text{OH}} {}^{*} \to {\text{ H}}_{2} {\text{O}} {}^{*} + {\text{O}} {}^{*}$$

$${\text{H}}_{2} {\text{O}} {}^{*} \to {\text{ H}}_{2} {\text{O }} + {}^{*}$$

$${\text{CO}} {}^{*} \to {\text{CO }} + {}^{*}$$

$${\text{H}}_{2} + 2 {}^{*} \to {\text{H}} {}^{*} + {\text{H}} {}^{*}$$

$${\text{CO}}_{2} + {}^{*} \to {\text{ CO}}_{2} {}^{*}$$

$${\text{CO}}_{2} {}^{*} + {\text{H}} {}^{*} \to {\text{HCOO}} {}^{*} \left( {{\text{COOH}} {}^{*} } \right) + {}^{*}$$

$${\text{HCOO}} {}^{*} \left( {{\text{COOH}} {}^{*} } \right) + {}^{*} \to {\text{HCO}} {}^{*} \left( {{\text{COH}} {}^{*} } \right) \, + {\text{ O}} {}^{*} {\text{HCO}} {}^{*} \left( {{\text{COH}} {}^{*} } \right) + {}^{*} \to {\text{CO}} {}^{*} + {\text{H}} {}^{*}$$

$${\text{COOH}} {}^{*} + {}^{*} \to {\text{CO}} {}^{*} + {\text{OH}} {}^{*}$$

$${\text{H}} {}^{*} + {\text{OH}} {}^{*} \to {\text{ H}}_{2} {\text{O}} {}^{*}$$

$${\text{H}}_{2} {\text{O}} {}^{*} \to {\text{ H}}_{2} {\text{O}} + {}^{*}$$

$${\text{CO}} {}^{*} \to {\text{CO}} + {}^{*}$$

#### 5.3.3.1 Surface Redox Mechanism

During redox mechanism, a rapid change on the active sites was observed in the presence of CO2 and H2 due to oxidation and reduction steps involved. The redox mechanism envisions absorbed CO over reduced metals that reacts with an oxygen atom contributed from the support to form CO2. Reduced support re-oxidizes by H2O, releasing hydrogen. Wang et al. investigated the redox mechanism using gold metal supported on CeO2 as a catalyst for RWGS. The authors used TAP analysis to elucidate the mechanism and to show the interaction of CO2 with the catalyst [110].

In this study it was found that the surface oxygen can be removed by reaction with H2 shown in (5.5).
$${\text{H}}_{2} + {\text{ O}}_{{{\text{CeO}}_{2}}} \to {\text{H}}_{2} {\text{O}} + {\square}_{{{\text{CeO}}_{2}}}$$
(5.5)
where OCeO2 is the oxygen atom and $${\square}_{{{\text{CeO}}_{2}}}$$ oxygen vacancies at the surface of the CeO2 support.
In the second step, CO2 acts as an oxidant for the partly reduced Au/CeO2 catalyst surface, as shown in (5.6).
$${\text{CO}}_{2} + {\square}_{{{\text{CeO}}_{2}}} \to {\text{CO}} + {\text{O}}_{{{\text{CeO}}_{2}}}$$
(5.6)
A few studies reported the role of CO2 as an oxidant even at low or mild temperature condition. Sharma et al. found that Pd/CeO2, Pd/ZrO2 and Pt/CeO2 can partially be reduced by CO2 at 350 and 200 °C, respectively (Fig. 5.5) [131]. A vast range of studies were also focused on investigating Cu-based catalysts. In this system, CO2 was dissociated on metallic Cu atoms as active sites and the reduction of the oxidized Cu catalyst was shown to be faster than the oxidation process [132], as shown in (5.7) and (5.8).
$${\text{CO}}_{{2({\text{g}})}} + 2{{\text{Cu}}^{0}}_{{({\text{s}})}} \to {\text{CO}}_{{({\text{g}})}} + {\text{Cu}}_{2} {\text{O}}_{{({\text{s}})}}$$
(5.7)
$${\text{H}}_{{2({\text{g}})}} + {\text{Cu}}_{2} {\text{O}}_{{({\text{s}})}} \to {\text{H}}_{2} {\text{O}}_{{({\text{g}})}} + 2{{\text{Cu}}^{0}}_{{({\text{s}})}}$$
(5.8)
Gines et al. studied the role of H2/CO2 ratio on CuO/ZnO/Al2O3 based catalysts. The change in ratio shifts the reaction from first order in H2 to first order in CO2 by varying the pressure of the two gases [133]. Fujita et al. concluded that surface redox mechanism pathway is caused by the presence of subsurface hydrogen trapped on the reconstructed copper surface caused by oxygen overlayer. This serves in making the surface more reactive to CO2 adsorption [134].

#### 5.3.3.2 Associative Mechanism

In this mechanism, the formation of intermediates such as formate, carbonates, and carbonyl, is crucial for RWGS reaction. An in-situ ATR-IRS study was used over Pt/Al2O3 based catalyst to show that RWGS reaction occurs at the sites of oxygen defects present on the thin surface of Al2O3 and a carbonate like species was formed on Pt sites, followed by CO2 adsorption on the oxygen vacancy of Al2O3 near the interface of Pt which further reacts with H2 to form CO [135]. The formate mechanism postulates a bidentate formate reaction intermediate produced through the reaction of CO with terminal hydroxyl groups over the oxide support. This intermediate decomposes to form H2 and a mono-dentate carbonate. With Cu as the catalyst active site, the formate species derived from the association of H2 and CO2 is mainly proposed to be the key intermediate for CO production. Another technique involves mass spectroscopy and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) during steady-state isotopic transient kinetic analysis (SSITKA) experiments to dynamically detect the surface species over Pt/CeO2 catalyst [136]. By performing pulse-response TAP experiments, 1% CO2 + 4% H2 reaction mixtures containing isotopic CO2 were introduced alternatively in a single reactor. Similarly, some other catalysts, Pd, Ni and Ru on Al2O3 also showed the formation of carbonates on alumina support [137].

In another study, the RWGS mechanism on Pd/Al2O3 was studied and it was shown from DRIFTS analysis that surface species and their evolution patterns are comparable during transient and steady-state experiments, during the switch of feed gases among CO2, H2, and CO2 + H2. Indeed, there was no direct dissociation of CO2; instead, the CO2 first reacted with surface hydroxyls on the oxide support. The formed bicarbonates react with adsorbed hydrogen dissociated on Pd particles to produce adsorbed formate species. Formates near the Pd particles rapidly reacts with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. In this analysis it was found that there are two types of Pd sites available-one with a weak interaction with CO and the other having stronger interaction with CO. The latter sites are reactive toward adsorbed H atoms on Pd, leading eventually to CH4 formation, as shown in Fig. 5.6.

## 5.4 CO2 Hydrogenation to Methanol

Global methanol production is in the region of 80 Mt/y. Methanol, a convenient and safe liquid (b.p.: 64.78 °C) at ambient conditions, can be used as:
• fuel, although it has half the volumetric energy density relative to gasoline or diesel,

• blending component for gasoline,

• feed for fuel cells, where it is oxidized with air to carbon dioxide and water to produce electricity,

• reactant for the MTO (methanol-to-olefins) process to produce ethylene or propylene,

• reactant for the MTG (methanol-to-gasoline),

• building block for the chemical industry,

• solvent,

• energy storage material.

Nowadays, most of the quantity of methanol is produced from a mixture of carbon monoxide, carbon dioxide, and hydrogen (syngas) at elevated pressures and moderate temperatures. In this process, CO2 is the carbon source for methanol at the molecular scale, while CO reacts with the water produced in the process to form CO2 and H2 via the water-gas shift (WGS) reaction. These reactions are represented by (5.9) and (5.10):
$${\text{CO}}_{2} + 3{\text{H}}_{2} \to {\text{CH}}_{3} {\text{OH}} + {\text{H}}_{2} {\text{O}}\quad \Delta {\text{H}}_{{298{\text{K}}}} = - 49\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.9)
$${\text{CO}} + {\text{H}}_{2} {\text{O}} \to {\text{CO}}_{2} + {\text{H}}_{2}\quad \Delta {\text{H}}_{{298{\text{K}}}} = - 41{\text{ kJ}}\,{\text{mol}}^{ - 1}$$
(5.10)
The hydrogenation of CO2 to produce methanol for use as both a fuel and a chemical building block is one possible route towards the development of an economy where CO2 (from industrial effluents or the atmosphere) is regarded as an abundant carbon source (i.e. as alternative energy source), instead of being considered as a waste molecule. This “methanol economy” concept, proposed at the end of the ‘90s, is generally attributed to the Noble Laureate Olah [138], and is often referred to as the anthropogenic carbon cycle.

The hydrogenation of carbon dioxide can be carried out either by catalytic conversion with H2 or by electrochemical reduction. This chapter will only focus on the first route, which appears to be closer to commercial application. For more details on the electrochemical reduction, the reader is referred to a recent review by Albo et al. and references therein [139].

Even though scientists discovered during the early 1960s that small amounts of CO2 added to the feed enhance the yield of methanol [140], the first experiments on pure catalytic CO2 hydrogenation to methanol were performed only in 1975 [141]. The interest at that time was the comprehension of the role of CO and CO2 in the syngas hydrogenation to methanol. The utilisation of pure CO2 as a carbon source for methanol synthesis is a more recent concept, that has been studied extensively, though it is yet to be commercialised on a large scale. In the following, an overview will be given on this process, with a special attention to gap analysis and needs assessment.

Key barriers to CO2 hydrogenation to methanol include [142]:

• Restriction by thermodynamic equilibria

CO2 hydrogenation to methanol is an exergonic process, which is favoured at low reaction temperatures or high reaction pressure. Even at 240–260 °C, which is typical of the low-T and low-P methanol synthesis process from syngas, the equilibrium constant of reaction (5.1) lies between 10−5 and 10−6, allowing for a single-pass methanol yield of 15–25% and thus necessitating the implementation of costly recycling loops to achieve similar yields as in the syngas conversion in the presence of CO [143].
• The need of highly active and robust catalysts against product inhibition

In the light of the equilibrium limitations, the need for operation at lower temperature becomes apparent, and thus CO2 hydrogenation to methanol requires highly active catalysts. Also, the absence of CO in the feed (i) prevents the role of CO as a scavenger of surface H2O molecules (through the WGS reaction), which strongly stick on the catalyst surface, and thus resulting in a kinetic inhibition of the process [144]; (ii) activates the RWGS reaction, instead of the WGS. Thus, while the WGS equilibrium helps in the syngas conversion to keep the surface of the catalyst clean of adsorbed water, the RWGS suppresses the methanol formation rate because RWGS produces more water and consumes valuable hydrogen that cannot be used for methanol formation.
• The need of highly selective catalysts

Methanol production from CO2 is indeed in competition with CO formation via the reverse water gas shift reaction, and with C–O bond dissociation and hydrogenation reactions. Side reactions yield CO, CH4 and C2–C4 hydrocarbons, which incur significant downstream separation costs. Also, as pointed out in the previous point, RWGS is an undesired competitive reaction that consumes H2 and forms H2O, both of which kinetically and thermodynamically inhibit methanol synthesis from CO2. H2O also poses a criticism for the catalyst stability (see next point). Accordingly, the RWGS activity of catalysts for CO2 hydrogenation to methanol should be suppressed.
• The need of stable catalysts

Research to date has been focused on finding selective catalysts that can activate CO2 under mild reaction conditions with high selectivity towards methanol.

### 5.4.1 Catalyst Considerations

Since catalyst design is vital, a good catalyst must show [142]:

• strong adsorption and transportation of CO2

• high concentration of hydrogenation sites

• ability to stabilise intermediates

• resistance towards water induced deactivation.

Wang et al. [46] recently reviewed the various catalytic systems that have been studied for the synthesis of methanol by CO2 hydrogenation. The reader is referred to this paper for more details. In the following, the main aspects of most studied catalysts will be considered.

#### 5.4.1.1 Cu-Based Catalysts

The catalysts used for industrial methanol synthesis (from syngas) are composed of copper, zinc, and alumina, and are based on the catalysts originally designed by ICI during the 1960s [145]. Even though the reaction mechanism during CO2 hydrogenation to methanol may be rather different than those during syngas hydrogenation, due to its pivotal role in the methanol synthesis from syngas, Cu has been extensively studied also for CO2 hydrogenation, often in the presence of secondary oxides such as Zn, Mn, Mg or Ce. Al2O3 is usually employed as the catalyst support, but TiO2 and SiO2 have been also considered. Cu/ZnO/Al2O3 catalyst proved to offer satisfactory performances at a pressure of 60 bar, space velocity of 22,000 h−1, at temperatures between 260 and 270 °C. The CO2 per-pass conversions were in the range of 35–45% and showed a slight decrease over time-on-stream.

It has been reported that three major strategies for optimization of Cu/ZnO-based for CO2 hydrogenation to methanol are [143]:

• Making smaller Cu particles

There is a consensus on the fact that the methanol productivity is strongly correlated with the specific copper surface area of a catalyst [146]. This calls for new synthesis strategies, which utilize new precursor phases, advanced impregnation and thermal treatment methods or new synthesis approaches.
• Improving the beneficial interaction between ZnOx and Cu

Zn is the most studied secondary oxide for Cu-based catalysts. It has been shown that ZnO acts as structural support that helps to keep Cu in a highly dispersed state. Also, ZnO is an electronic promoter for Cu, due to its partial reducibility and to the resulting strong metal support interaction [147]. Accordingly, both the oxidation state of the copper and the copper–zinc interaction have an effect on the catalyst activity [145]. Doping of ZnO with small amounts of trivalent ions like Al3+, Ga3+ or Cr3+ has been shown to lead to promising results likely due to the enhanced reducibility of doped catalyst [148]. Also zirconia has been successfully used as a promoter for CO2 hydrogenation over Cu/ZnO catalysts [149].
• Increasing the defect concentration in metallic nanoparticles and amount of surface steps and edges of the Cu phase

It has been proposed that this is possible by playing with the solid state chemistry of CuO reduction to Cu metal [150]. One of the major limitations of Cu/ZnO-based catalysts for CO2 hydrogenation to methanol is their deactivation on-stream. This arises as catalytic activity is highly dependent on copper surface area whilst water, which is formed through the RWGS, induces sintering of copper nanoparticles [151], as a consequence of the modification of ZnO crystallization [152]. So, although the initial copper surface area is important, the ability to retain this surface area under reaction conditions appear to be the key consideration [153]. In conclusion, a lot of work remains to be done to develop a successful CO2 hydrogenation catalyst based on Cu/ZnO [143]. The major challenges are related to synthetic inorganic chemistry, defect generation in metallic nanoparticles and surface/interface design between the two major catalyst components Cu and ZnO [143].

#### 5.4.1.2 Pd-Based Catalysts

Pd has been also found to be an active metal for CO2 hydrogenation [154], and its higher stability than Cu towards water-induced sintering has been also shown during methanol steam reforming [155]. Notably, Pd has attracted interest due to the property malleability derived from the presence of a nearly full d-band [156]. As for Cu, it has been shown that both the preparation method and Pd precursor may affect the selectivity of the products [154]. The catalytic activity of Pd is also strongly associated with the type of metal oxide support including ZnO, CeO2, Ga2O3, TiO2 and Al2O3 [156]. In fact, the proximity between metal and substrate stabilizes the intermediate formate species, which is often reported as the reaction intermediate. Notably, Pd on ZnO, TiO2 and Al2O3 has low methanol productivity. On the contrary, catalysts containing alloyed PdZn nanoparticles with mean diameters in the region of 3–6 nm are characterized by an increased productivity. Those active centres are formed by pre-reduction of the catalyst at high temperature [156]. Also the presence of a ZnTiO3 phase has been reported to boost the methanol yield. This phase is obtained when ZnO is reduced at a temperature of 650 °C, in the presence of TiO2 as support [142]. As in many heterogeneous catalytic systems, the dispersion of Pd nanoparticles is important to grant activity to the catalyst, but this increases the risk of active area loss by sintering during the process.

#### 5.4.1.3 In-Based Catalysts

Indium oxide (In2O3), as such or supported on ZrO2, has been recently proposed as a very active, selective and stable catalyst for methanol synthesis by CO2 hydrogenation [157]. The measured performances have been shown to be much better than a benchmark Cu–ZnO–Al2O3 catalyst, which has been found to be unselective rapidly deactivating at the same conditions where 100% methanol selectivity and stable performances were measured for the indium based catalyst. This behaviour has been attributed to rapid creation and annihilation of oxygen vacancies as active sites during CO2 hydrogenation, which has been also simulated by DFT on pure In2O3 catalyst [158]. Notably, steady-state experiments carried out at 50 bar, at variable temperature around 300 °C, indicated a lower apparent activation energy for CO2 hydrogenation than for the RWGS, which was used to explain the high methanol selectivity observed [159]. Water was found to strongly inhibit the reaction rate, and to deactivate the catalyst by sintering for partial pressures exceeding 1.25 bar [159]. Pd has been shown to be able to enhance the activity of indium oxide by facilitating hydrogen splitting [160].

#### 5.4.1.4 Ag-Based Catalysts

Ag/ZrO2 catalysts have been also proposed, showing interesting performances when Ag+ cations are present in the vicinity of oxygen vacancies. In this case, the introduction of ZnO in the catalyst formulation was shown not to have significant effect on the methanol yield which differed from the behavior of Cu-based catalyst [161]. More complex catalyst formulation containing both Ag/ZrO2 and Cu have been also proposed: it has been shown that the presence of Ag in the Ag/CuO–ZrO2 catalyst changes the surface condition of metallic Cu via the formation of a Ag-Cu alloy, which imparts high activity and selectivity on CO2-to-methanol hydrogenation [162].

### 5.4.2 Engineering Challenges

One of the main challenge associated with CO2-to-methanol is that CO2 is less reactive than CO, so that the yield of methanol is much lower than that obtained from syngas conversion under the same temperature and pressure [163]. This leads to larger and more expensive reactors. This is why, as discussed, many attentions are currently devoted to the development of more active catalysts.

Also, as already mentioned, due to the RWGS, more water is also produced when pure CO2 is used instead of syngas to manufacture methanol. Crude methanol from the CO2-based process contains approximately 30–40% water. Apart from the discussed issues related with catalyst deactivation, high water concentrations inhibit the process kinetics, further requiring for large reactors. Notably, the synthesis of methanol from CO2 is less exothermic than the synthesis of methanol from CO. This simplifies the reactor design, which in the case of methanol from syngas is so critical to require multi-tubular reactors with external cooling (usually boiling water reactors, BWRs) or stage conversion reactors with intermediate quenching or cold-shot gas injection. In the case of methanol from CO2, tube-cooled reactors are appropriate [164]. In those reactors the feed gas to the reactor controls the temperatures in the catalyst bed. Fresh feed gas enters typically at the bottom of the reactor and is preheated as it flows upwards through tubes in the catalyst bed. The heated feed gas leaves the top of the tubes and flows down through the catalyst bed where the reaction takes place. The use of a tube-cooled reactor is advantageous over externally cooled reactor (like BWRs) in terms of the lower cost, higher efficiency, and relative simplicity of operation [164]. Additionally, tube-cooled reactors are preferred as being more efficient than adiabatic or cold-shot reactors which may require multiple reactors in series to achieve desired conversion rates. The flexibility with respect to load changes may be however a critical issue related to tube-cooled reactor.

The use of CO2 instead of CO/CO2 mixtures as C-source also limits the formation of by-products such as higher alcohols (mostly ethanol), esters, ethers (mostly dimethyl ether), and ketones (such as acetone and methyl ethyl ketone) [164]. It has been reported that CO2-based process yields methanol in higher purity with five times lower by-product contents [152]. This can be partly explained by the high temperature sensitivity of the by-product formation reactions and by the better temperature control in the reactor during the less exothermic catalytic hydrogenation of CO2. This simplifies the purification steps needed, limiting the number of distillation operations and consequently the energy consumption of the process. It has been reported that, when starting from pure CO2, effective purification of methanol may be achieved by a single column separation (lean methanol leaves the column as bottom liquid) followed by a stripping operation on the condensed distillates to separate unconverted CO2 from pure methanol. When dealing with separation, it is worth mentioning that there is typically more CO2 in the crude methanol when pure CO2 is hydrogenated [164].

### 5.4.3 Status of Industrial Development

Worldwide first demonstration of converting the greenhouse gas CO2 to methanol as a useful chemical was reported by Lurgi in 1994 [165]. Another pilot plant was built in 1996 in Japan [166]. At that time, this new technology was attractive for producers with access to pure CO2 and excess H2, such as methyl tertiary butyl ether makers with dehydrogenation units, thus making the process as cost-effective as conventional methods [140]. Other technologies have been proposed for “CO2 hydrogenation to methanol”, but many of them were in reality technologies for the hydrogenation of CO/CO2 mixtures with high CO2 contents. A review of these technologies can be found in [140].

Since 2012, Carbon Recycling International (CRI) is operating the “George Olah” plant (Iceland, formerly known as Svartsengi plant), that has a capacity of 4000 tons per annum of methanol produced from carbon dioxide and renewable hydrogen. The CO2 is extracted and purified from the flue gases of the nearby geothermal power plant, while the hydrogen required for the production is generated by alkaline water electrolysis using Iceland’s entirely renewable grid electricity [164].

## 5.5 CO2 Hydrogenation to Formic Acid

Another approach for utilizing CO2 as C1 source for fuels/chemicals is to convert CO2 into formic acid (FA) via CO2 hydrogenation technique [1, 167]. FA is a valuable chemical commonly used as preservative and antibacterial agent [168, 169]. Additionally, it is an established hydrogen storage component. Upon decomposition, it generates CO2 and H2 with a possible reversible transformation back to regenerate formic acid. FA contains 53 gL−1 H2 at room temperature and atmospheric pressure, and by weight, it contains 4.3 wt% of H2 in pure formic acid. Being liquid at ambient conditions, its transportation and storage is more straightforward than that of molecular hydrogen. Additionally, formic acid can also be used in a formic acid based fuel cell, and/or can be further reduced to a carbon-based fuel. Generally, FA can be produced using photocatalytic reduction, electrochemical processes, enzymatic conversion, and hydrogenation methods [46, 170, 171]. Moreover, current industrial methods for producing formic acid include carbonylation of methanol to methyl formate followed by hydrolysis of methyl formate to generate formic acid [1, 172]. In comparison with conventional synthesis methods, the direct hydrogenation of carbon dioxide into formic acid serves two important purposes-CO2 utilization as C1 source and hydrogen storage in a liquid form.

### 5.5.1 Thermodynamic Considerations

Direct conversion of gaseous carbon dioxide and hydrogen into liquid formic acid is mildly exothermic, but it is an entropically disfavored reaction (5.11) due to the involvement of phase change from gaseous reactants into a liquid product [1].
$$\begin{array}{*{20}l} {{\text{H}}_{{2({\text{g}})}} + {\text{CO}}_{{2({\text{g}})}} \leftrightarrow {\text{HCO}}_{2} {\text{H}}_{{({\text{l}})}}} & {{\Delta G^{\circ }}_{{298\,{\text{K}}}} = 32.9\,{\text{kJ}}\,{\text{mol}}^{ - 1} ;} \\ & { \Delta {\text{H}}_{{298\,{\text{K}}}} = - 31.2\,{\text{kJ}}\,{\text{mol}}^{ - 1}} \\ \end{array}$$
(5.11)
$${\text{H}}_{{2({\text{aq}})}} + {\text{CO}}_{{2({\text{aq}})}} \leftrightarrow {\text{HCO}}_{ 2} {\text{H}}_{{({\text{aq}})}} \quad{\Delta G^{\circ}}_{{298\,{\text{K}}}} = - 4\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.12)
$$\begin{array}{*{20}l} {{\text{H}}_{{2({\text{aq}})}} + {\text{CO}}_{{2({\text{aq}})}} + {\text{NH}}_{4} \leftrightarrow {{\text{HCO}}_{2}}^{ - }{}_{{({\text{aq}})}} + {{\text{NH}}_{4}}^{ + }{}_{{({\text{aq}})}}} & {{ \Delta G^{\circ}}_{{298\,{\text{K}}}} = - 9.5\,{\text{kJ}}\,{\text{mol}}^{ - 1} ;} \\ & {\Delta {\text{H}}_{{298\,{\text{K}}}} = - 84.3\,{\text{kJ}}\,{\text{mol}}^{ - 1}} \\ \end{array}$$
(5.13)
Alternatively, this reaction becomes slightly exothermic and favorable when operated in an aqueous phase as shown in (5.12). Moreover, if this reaction is carried out in a suitable solvent, it can be more favorable towards formation of formate/formic acid (5.13). The presence of suitable solvent alters the thermodynamics of the reaction. Thus, the most commonly adopted practice for transforming CO2 into formic acid is to carry out the reaction with strong base additives such as amino or alkali/alkaline earth bicarbonates in water and/or alcohols (methanol, ethanol) solvent. In liquid phase reaction, though the CO2 hydrogenation to FA is thermodynamically favored, significant kinetic limitations exist. Thus, catalyst design and suitable solvent plays a pivotal-role to achieve acceptable process performance and selectivity towards desired product.

### 5.5.2 Catalytic Systems

The transformation of CO2 into formic acid was first reported by Inoue et al. using homogenous Ru-based catalyst [173]. Since then, CO2 hydrogenation into FA using homogeneous catalysts has been extensively studied [174]. For this reaction, Ru and Ir-based homogenous catalytic systems were much explored and reviewed by many researchers. The homogenous route offers the advantage of operating at milder reaction conditions. However, it also shows some disadvantages such as inefficient capture and recycling of the precious-metal catalysts, and the limited liquid-phase solubility of hydrogen. Comparatively, the research on heterogeneous catalysts is less explored than homogenous catalysts. However, in recent times, interest towards development of heterogeneous catalysts for CO2 hydrogenation to formic acid is drawing interest due to the advantages of  heterogeneous catalysts such as easy recovery after the reaction and application in a continuous reaction system in a large industrial process. Recently, Alvarez et al. [1] categorically reviewed the heterogeneous catalysts for CO2 hydrogenation to formic acid/formate and methanol/DME synthesis. In this section, we will be covering the recent developments of heterogeneous catalysts for CO2 hydrogenation to FA reaction. The unsupported/bulk metal particles and supported mono and bi-metallic catalysts for CO2 hydrogenation reactions with respect to the catalytic performance and the acceptable reaction pathways over supported and bulk metal catalysts will be highlighted.

#### 5.5.2.1 Unsupported/Bulk Metal Particles

In order to perform direct conversion of CO2 and H2 into formic acid in aqueous media, activation of both CO2 and H2 is required. As discussed before, CO2 can be activated by additives such as amino or alkali/alkaline earth bicarbonates, and hydrogen molecule can be activated using metal species. Thus, during initial periods, pure metals such as Pd, Raney Ni, Au, and Ru were employed as heterogeneous catalysts for this reaction. Bredig and Carter et al. [175] synthesized formic acid using Pd black as a catalyst material using alkali/alkaline earth (bi)carbonates as the CO2 source in the presence of H2 (reaction condition: 70−95 °C, 30−60 bar of H2, 0−30 bar of CO2). Similarly, Raney Ni was employed as a catalyst for synthesizing formaldehyde at 400 bars pressure in the presence of amines and alcohol as solvent [176]. Likewise, Takahashi et al. [177] reported selective formation of formic acid in a hydrothermal reactor using Ni powder as hydrogenating catalyst and K2CO3 as CO2 source at 300 °C. Upon mixing with Fe powder, the selectivity towards methanol formation was observed. Jin et al. [178] reported a novel strategy to generate formic acid via CO2 reduction using zero valent metal/metal oxide redox cycle under hydrothermal conditions. They have showed that metals such as Mg, Zn, Al in reduced form catalyze the formation of formic acid by CO2 reduction in the presence of water at 300 °C. The yield of FA formation can be further improved in the presence of Ni and Cu metal particles. The oxidized metals can be regenerated back to reduced form using biochemical reductants such as glycerol. Unsupported Au nanoparticles were employed for CO2 hydrogenation at 20 bars pressure in the presence of ternary amine and ethanol [179]. In most of the above reported catalytic systems, the turnover number (TON) was reported to be quite low, which is possible due to the lower availability of active metal centers to perform hydrogenation reaction. Recently, Srivastava et al. [180] reported the prominent catalytic activity of Ru nanoparticles, wherein the catalytically active Ru nanoparticles were synthesized in an ionic liquid. In-situ generated Ru NPs in [DAMI][NTf2] ionic liquid was found to be highly active in terms of formic acid formation during the CO2 hydrogenation reaction to other ionic liquid immobilized standing Ru NPs. They reported the TOF of 245 h−1 at 100 °C. Likewise, in another report, Ru nanoparticles prepared in a methyl alcohol solution under solvothermal condition were employed to perform the reaction with supercritical CO2 in the presence of trimethylamine and water as promoter. They have achieved a high TON of 6351 after 3 h at 80 °C [181]. In both cases, the presence of water showed a positive effect in catalytic performance of Ru nanoparticles.

#### 5.5.2.2 Supported Mono/Bimetallic Metal Catalysts

In supported metal catalysts, the role of support is mainly to disperse active metal species, which in turn helps to improve the number of available surface active metal species and prevents the agglomeration of metals during hydrogenation reaction. Several supports such as Al2O3, TiO2, ZnO, CeO2, Mg–Al2O3, activated carbon, CaCO3 and BaSO4 were employed for this reaction to disperse metals such as Pd, Au, Ru and Ni. For instance, Stalder et al. [182] reported Al2O3 supported Ru, Rh, Pd and Pt catalysts for conversion of aqueous sodium bicarbonate to sodium formate. Among them, Ru/Al2O3 showed better catalytic performance. In another work, Pd supported over activated carbon gave better catalytic performance than Pd supported over Al2O3, CaCO3 and BaSO4 [183]. Generally, positive effect on the catalytic performance for supported catalysts was reported as compared to bulk metal catalysts. Furthermore, Bi et al. [184] studied the effect of Pd loading for Pd/r-GO (reduced graphene oxide) catalysts by preparing several loadings of Pd (1–5 wt%) supported catalysts. Among all, the lowest Pd (1 wt%) loaded r-GO showed better catalytic performance; according to the authors, it is caused by the large lattice strains. Similarly, Song et al. [185] observed that 0.25%Pd/chitin gave impressive catalytic performance with a TOF of 257 h−1. They have reported that the dispersion of Pd particles was promoted by the acetamide group present in chitin support. In another report, Hao et al. [186] reported Ru supported on activated carbon, MgO and Al2O3 for this reaction. According to them, Ru/Al2O3 showed better catalytic performance due to the presence of higher surface hydroxyl group. The availability of surface hydroxyl groups on the support showed positive effect on catalytic performance. Other than Pd and Ru as the active metal component, Au supported catalysts were also explored for this reaction. Filonenko et al. [179] observed that the supported Au particles showed superior catalytic performance than unsupported Au particles. They have also reported that among all the supported (TiO2, Al2O3, ZnO, CeO2, MgAl−hydrotalcite, MgCr−hydrotalcite, and CuCr2O4) catalysts, Au/Al2O3 gave highest catalytic performance. According to them, the basic sites of the Al2O3 support play an important role, acting cooperatively with Au0 nanoparticles in improving catalytic performance.

Besides monometallic supported catalysts, bimetallic supported catalysts were also utilized in this reaction. Takahashi et al. [177] examined the mixture of Ni and Fe oxide powder for CO2 hydrogenation to formic acid using K2CO3 as CO2 source at 300 °C. They have reported that the product selectivity can be controlled by varying the metal oxide compositions. In such a type of metal oxide mixture, the interaction between the active metal species is minimal, thus they can act as independent catalytic centers. In another work, Nguyen et al. [187] reported the synthesis of formic acid via CO2 hydrogenation using PdNi alloys supported on carbon nanotube-graphene (PdNi/CNT-GR) catalyst in the absence of a base and water as solvent. Alloying Pd with Ni brought a significant enhancement in catalytic activity compared to the monometallic Pd catalyst. The composite support improved the dispersion and intimate contact between both metal species.

According to literature, among noble metals, Pd and Au are the most verified active metals for the synthesis of formic acid/formates and their catalytic activity can be enhanced by a proper choice of support material. Hydrophobic carbon-based materials are preferred choices as support for Pd catalysts, whereas more hydrophilic support materials such as Al2O3 and TiO2 are preferably employed for Au catalysts (the same is also indicated for Ru catalysts). Among non-noble metals, Ni and Fe are the mostly explored active metals. Comparatively, noble metals showed more promising catalytic performance than non-noble metal based catalysts with respect to TON and longevity. Still, the number of studies on bare metal particles and supported metal particles as catalysts for CO2 reduction reaction are limited and there are discrepancies on the fundamental aspects. Further investigations on this catalytic system for formic acid and formate synthesis are absolutely required to establish clearer catalyst structure-activity relationships.

### 5.5.3 Reaction Mechanism

As discussed before, formation of formic acid via direct CO2 hydrogenation reaction is entropically disfavored when both reactants are in gaseous state. The reaction is feasible if both reactants are in aqueous state. Therefore, the role of suitable solvent medium and active metal species are critical. The presence of additives in reaction media mainly fixes the gaseous CO2 in the form of an aqueous carbonates/bicarbonates and helps the dissolution of H2 to reach metal species to carryout hydrogenation reaction. Actually, in formic acid synthesis, CO2 is unlikely to be the reactant, but in most cases, carbonates/bicarbonates act as source for CO2. The carbonates/bicarbonates can be generated by dissolving CO2 in the base solvents. Dissolved gaseous H2 can be the source for hydrogen, in order to dissolve in the solvent, and the reaction should be operated at high pressure conditions. In general, the reaction mechanism of a typical heterogeneous catalysis involves four main steps. Firstly, the reactants diffuse and contact with the catalyst. Secondly, the reactants are adsorbed at an active site. Thirdly, the surface reaction occurs and finally, the formed product is desorbed from the active site. In literature, the reaction pathways for transformation of CO2 into formic acid in the presence of unsupported, supported mono and bi-metallic catalysts are reported. For monometallic supported catalysts, Song et al. [185] proposed a plausible CO2 hydrogenation to formic acid reaction pathway using Pd supported chitin catalysts (Fig. 5.7). According to them, the diffused hydrogen molecule is homolytically cleaved and attached to the vacant sites on Pd. The cleavage of hydrogen molecule can be promoted by the amino groups present on the catalyst surface. Similarly, CO2 in bicarbonate form is then adsorbed to vacant sites on palladium. A hydrogen molecule is then inserted into the adsorbed carbonate anion, followed by a breakage of the C–O bond, the desorption of formate, and formation of formic acid.
Furthermore, the advantage of having bimetallic surfaces during CO2 hydrogenation to formic acid is clearly highlighted by Nguyen et al. (Fig. 5.8) [187]. The reaction mechanism of CO2 hydrogenation to formic acid over bimetallic PdNi supported over carbon nanotube-graphene was investigated and the proposed mechanism is illustrated in Fig. 5.8. Unlike the mono metal supported catalysts, in bi-metallic catalysts both metals can be involved in activating CO2 and H2 molecules. According to them, in the first step, an electron transfer from Ni to Pd atoms occurs. Therefore, Pd and Ni are in the electron-rich and—deficient state, respectively. It is followed by H2 dissociative adsorption on Pd surface and CO2 adsorption through its O-atoms on the Ni surface. Reaction between H on Pd and adsorbed CO2 leads to the formation of adsorbed HCOOH.

Thongnuam et al. [188] computationally investigated CO2 hydrogenation to formic acid reaction on modified zeolites (ZSM-5, BEA, FAU) using density function theory (DFT) with the M06-L functional. According to them, the reaction proceeds in two steps on the surface of zeolite material. At first, abstraction of hydrogen atom by CO2 forms a formate intermediate. After that, the intermediate takes another hydrogen atom to form formic acid. The formation of formate intermediate is observed to be the rate-determining step of the reaction for both the perfect and defect Sn-ZSM-5 zeolites. Similar result was also reported by Eseafili and Dinparast [189] over Ti-doped graphene nanoflakes (Ti-GNF) for CO2 hydrogenation to formic acid reaction. According to them, the presence of large positive charge on the Ti atom can greatly regulate the surface reactivity of GNF. The hydrogenation of CO2 over Ti-GNF also occurred as two-step reaction, i.e. (a) H2 + CO2 → HCOO + H, and (b) HCOO + H → HCOOH. Step (a) for the formation of formate intermediate is found to be the rate determining step. The activation energy for the first step was calculated to be 0.85 eV, while the second step could occur quickly due to the small reaction barrier (0.08 eV).

### 5.5.4 Technological Challenges

When comparing the technology readiness level (TRL) for heterogeneous CO2 hydrogenation to formic acid with CO2 hydrogenation to methane, methanol and CO, the TRL for this process is far lower (TRL 1–2). With the available state of art on catalytic systems, which have been proven to generate formic acid with decent reaction rates, the economic feasibility for this reaction needs to be demonstrated. The technology of homogenously catalyzed synthesis of formic acid was first introduced by BO Chemicals in the 1980s and has been developed by BASF [190]. In this technology, a soluble Ru-complex is employed to catalyze CO2 to formic acid using a mixture of ternary amine and alcohol at 50–70 °C and 10–12 MPa. The process must keep the expensive transition metal complex catalyst active yet avoid even traces of it being present in active form in formic acid distillation, because it can catalyze decomposition of the acid. Possible catalyst residues can be reversibly deactivated with CO. Such issues can be addressed comfortably by adopting heterogeneously catalyzed technology. For this, a better catalyst plays a crucial role; in many cases the nature of the active metal species and the structure of the catalyst material have been explored. However, there is a need to understand the catalyst structural changes occurring under the reaction conditions and their consequences to catalyst longevity and reusability. Moreover, recent advances in developing catalysts for dehydrogenation of formic acid to generate CO2 and H2 make FA an efficient chemical H2 carrier [191]. The main advantages of formic acid over other H2 carriers include easy handling, refueling, and transportation. For this, research for development of efficient catalyst system is required to catalyze both hydrogenating CO2 to FA and decomposition to generate H2 and CO2 with minimal operational changes.

## 5.6 CO2-Methane Reforming to Syngas

Dry reforming of methane (DRM) using CO2 as an oxidant is one of the primary areas being researched for the transformation of CO2 into higher chemicals through syngas (CO + H2) as an intermediate. By principle, it is similar to the steam reforming of methane, which is an established and widely used technology for H2 or syngas production, but DRM uses CO2 as an oxidizing agent instead of steam, which makes it more attractive for CO2 mitigation and a carbon-neutral economy. DRM consumes two major greenhouse gases, CO2 and CH4, and converts them to syngas which can then be converted into higher chemicals through Fischer-Tropsch synthesis or used as a source of H2. The H2/CO ratio in the product syngas is 1, which is lower than that achieved by steam reforming, and is suitable for subsequent usage in Fischer-Tropsch synthesis of long-chain hydrocarbons.

The major technological challenges for the industrial application of DRM are its high endothermicity, requiring high temperatures for appreciable conversion and rapid deactivation of catalysts under reaction conditions, caused by coking or sintering. The high energy input required by the process causes a serious penalty on the efficiency of CO2 utilization, since the heat input is mainly provided through the combustion of fossil fuels. The option of using solar heating to supplement the energy requirement of DRM is a highly attractive green alternative and is one area of active research. For industrial application of DRM, there is a need to develop cost-effective catalysts that can maintain stable performance for extended durations in DRM. In the past decade, there has been a substantial increase in research focus on catalyst development for DRM and tremendous progress has been made in increasing the activity and stability of catalysts.

### 5.6.1 Reaction Thermodynamics

Dry Reforming of methane (5.14) is a highly endothermic reaction, since both CH4 and CO2 are very stable molecules with high bond dissociation energy (435 kJ/mol for CH3–H and 526 kJ mol−1 for CO–O). High temperatures (>800 °C) are hence required to achieve good conversion of methane and carbon dioxide to syngas. DRM is more endothermic than steam reforming of methane or partial oxidation of methane or autothermal reforming of methane reactions.
$${\text{CH}}_{4} + {\text{CO}}_{2} \to 2{\text{CO}} + 2{\text{H}}_{2} \quad{\Delta \text{H}}_{298} = 248\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.14)
The ideal H2/CO ratio in the DRM product is 1 but this is influenced by the simultaneous occurrence of Reverse Water Gas Shift reaction (5.2) which lowers the H2/CO ratio to <1 by producing water. Based on the relative endothermicity of the DRM and RWGS reaction, the effect of RWGS on product selectivity is more significant in the temperature range of 400–800 °C [192]. Apart from RWGS, other significant side-reactions in DRM are methane decomposition (5.15) and the CO disproportionation (Boudouard reaction) (5.3). Both of these reactions result in the formation of solid carbon that can cover the catalytically active sites and cause rapid deactivation.
$${\text{CH}}_{4} \to {\text{C}} + 2\,{\text{H}}_{2} \quad\Delta {\text{H}}_{298} = 75\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.15)
Wang et al. reported that CH4 decomposition occurs above 557 °C, while the Boudouard reaction occurs below 700 °C [193]. In a temperature range of 557–700 °C, carbon deposition in DRM happens from both methane decomposition and CO disproportionation, which ultimately leads to a suppression of catalytic activity. At higher temperatures (>800 °C), the carbon deposition is derived mostly from methane decomposition, where the formed carbon species is relatively more reactive and can be easily oxidized. Wang et al. suggested that the optimum temperature at the feed ratio of CO2/CH4=1:1 is between 870 and 1040 °C, considering the conversion and carbon formation. Several researchers have conducted thermodynamic simulations for DRM under various process conditions and it has commonly been concluded that the operation of DRM at high temperatures above 850 °C and low pressures is required to attain high conversion [192].

### 5.6.2 Catalysts for DRM

A variety of catalysts such as supported metal catalysts including both noble metals like Pt, Pd, Ru, Rh and transition metals like Ni, Cu, Co etc. are active for dry reforming of methane. The overall activity of a catalyst in DRM depends on a combination of factors such as the active metal, nature of support, metal-support interaction, particle size, support surface area etc. Along with high activity, a crucial parameter for DRM catalyst is that it should be resistant to deactivation due to sintering at the high reaction temperature conditions or due to deposition of coke from the reforming reaction.

#### 5.6.2.1 Type of Active Metal

Noble metals such at Pt, Pd etc. show higher resistance to sintering and coke formation in DRM compared to transition metals, but their usage is limited due to high cost. Due to easier availability and low cost, transition metals like Ni, Cu are gaining more interest as DRM catalysts, and tremendous efforts are being made to stabilize these catalysts under DRM conditions.

In terms of catalytic activity, it has been shown that the metals follow an order of Ru > Rh > Ni > Pt > Pd on SiO2 support, Ru > Rh > Ni > Pd > Pt on a MgO support and Rh > Ni > Pt > Ir > Ru > Co on an Al2O3 support [194]. The higher activity of Ru and Rh than Ni, Pt, Pd of the same dispersion has also been proven by first principle calculations [195]. Alloying or promoting transition metals like Ni with noble metals to form bimetallic systems have been shown to significantly improve the activity and resistance to deactivation by coke formation or sintering [196]. For instance, doping trace amounts of noble metals like Pt, Pd, Rh can increase the reducibility of Ni by a hydrogen spill-over effect, wherein H2 molecules preferentially dissociate on noble metal atoms to hydrogen atoms, which diffuse to the non-noble metals and enhance their reduction to create more active sites [196, 197, 198]. It has also been shown that based on the synthesis conditions and the reduction potential/kinetics of the different metals in a bimetallic catalyst, the two metals may exist in a uniform alloy phase or segregate to form surface enrichment of one metal species, that can affect the catalytic activity [199, 200]. Bimetallic systems involving only transition metals such as Ni–Co, Ni–Fe, Ni–Co have also shown considerable benefits over monometallic catalysts due to a synergistic effect on activity and coke resistance [196].

#### 5.6.2.2 Role of Support

Active metal components are usually dispersed on metal oxide supports that provide high surface area for metal dispersion, exposing higher amounts of active sites and prevent metal agglomeration. A number of supports for these active metals have also been investigated, including SiO2, La2O3, ZrO2, TiO2, CeO2, Al2O3, and MgO [201]. Depending on the chemical nature of the support, it may also participate in the reaction mechanism of DRM and affect the product selectivity and tendency for coke formation. Inert supports like SiO2 serve mainly as a medium to disperse the active metal and mostly do not contribute to the reaction pathway. On the other hand, basic or redox supports like MgO, CeO2 etc. can actively reduce the formation of coke species in DRM by easily activating the CO2 and providing more oxygen species for the removal of coke.

The specific surface area and porosity of the support plays a key role in the effective dispersion of the active metal phase. High surface area supports with ordered mesopores can stabilize very small metal nanoparticles within the pores while providing good accessibility to the reactant gas molecules. Silica based ordered mesoporous supports like SBA-15, MCM-41, KIT-6 etc. have been widely used to support metal nanoparticles, and can provide high metal dispersion, stability and sintering resistance by confining the nanoparticles within the mesopores [202, 203].

The interaction between the metal and support is another very important factor in determining the activity and stability of the catalyst in DRM. Stronger metal-support interaction reduces the mobility of metal particles on the support under high reaction temperatures and can thus yield sinter-resistant catalysts with higher metal dispersion [201]. On the other hand, too strong metal support interaction can make it extremely hard to reduce the metals to active metallic state, which causes a decline in activity. In case of reducible oxides, it has been shown that for high metal-support interaction, the support may partially cover the metal particles, thus blocking active sites and reducing the overall activity [204]. The metal support interaction is dependent on the catalyst preparation method and can be tweaked by changing synthesis conditions or catalyst pre-treatment conditions.

Some supports may also form different inorganic phases with the active metal under reaction conditions, that may affect the catalyst performance. For example, under high temperatures, Ni/Al2O3 catalysts may form a perovskite phase NiAlO3 that is inactive for DRM and causes rapid deactivation [205].

#### 5.6.2.3 Effect of Promoters

Promoters are non-active materials that can help in improving the catalytic activity or selectivity by inducing structural or electronic changes in the catalyst. Promoters may affect the metal-support interaction and hence, the dispersion of active metals on the catalyst. For instance, Sigl et al. showed that using V2O5 as a promoter forms an overlayer of VOx on Rh/SiO2 which breaks down the larger ensembles into smaller Rh particles, thereby increasing number of sites for activation of CH4 [206]. Pan et al. observed that introducing Ga2O3 onto SiO2 helped in activating CO2 in the form of surface carbonate and bicarbonate species, and consequently promoted the coke resistance of the catalyst [207]. Addition of alkali metal and alkaline earth metal oxides such as Na2O, K2O, MgO etc. can neutralize the surface acidity of the catalyst, reduce methane dehydrogenation activity, increase CO2 adsorption, and improve the coke elimination in DRM [208, 209]. Promoters may also affect the nature of coke formed during DRM [210]; for instance, Ag-promoted catalysts alter the type of coke formed on the catalyst surface from whisker to amorphous species [211].

### CH 4 & CO 2 Activation

Methane activation is usually considered to be the most kinetically significant and rate determining step in the DRM process. CH4 activation requires the presence of metals like Ni, Pt, Ru etc. which can adsorb and dissociate methane either directly or through intermediates like CHx or formates. Methane activation is believed to happen through intermediate formation at lower temperatures (<550 °C) while direct dissociation is favoured at higher temperatures. It is usually agreed in literature that on catalysts using inert supports like SiO2, DRM follows a mono-functional pathway where both CH4 and CO2 get activated on the metal surface. On acidic/basic supports like Al2O3, MgO, CeO2 etc., the mechanism is usually bi-functional wherein CH4 is activated on the metal and CO2 is activated on the acidic/basic support. CO2 activation occurs through the formation of formates on acidic supports with the surface hydroxyls and through oxy-carbonates/carbonates on basic supports. In such catalysts, the catalytic activity becomes a function of the interfacial area between the metal and support instead of the metal surface area alone [212]. The mechanism and the rate determining step strongly depend on the catalyst system, and a wide variety of mechanisms and rate determining steps have been reported for different systems [213, 214]. Some of the commonly reported mechanisms are described in Table 5.3.
Table 5.3

Reaction mechanism for dry reforming of methane

CH4 activation

CO2 activation

Direct decomposition

Direct decomposition on metal

1. $${\text{CH}}_{4} \left( {\text{g}} \right) + {}^{*} \leftrightarrow {\text{CH}}_{4} {}^{*}$$

14. $${\text{CO}}_{2} \left( {\text{g}} \right) + {}^{*} \leftrightarrow {\text{CO}}_{2} {}^{*}$$

2. $${\text{CH}}_{4} {}^{*} + {}^{*} \leftrightarrow {\text{CH}}_{3} {}^{*} + {\text{H}}{}^{*}$$

15. $${\text{CO}}_{2} {}^{*} + {}^{*} \leftrightarrow {\text{CO}}{}^{*} + {\text{O}}{}^{*}$$

3. $${{\text{CH}}_{3}}{}^{*} + {}^{*}\leftrightarrow {{\text{CH}}_{2}}{}^{*} + {\text{H}}{}^{*}$$

H assisted activation

4. $${\text{CH}}_{2} {}^{*} + {}^{*} \leftrightarrow {\text{CH}}{}^{*} + {\text{H}}{}^{*}$$

16. $${\text{CO}}_{2} {}^{*} + {\text{H}}{}^{*} \leftrightarrow {\text{COOH}}{}^{*}$$

5. $${\text{CH}}{}^{*} + {}^{*} \leftrightarrow {\text{C}}{}^{*} + {\text{H}}{}^{*}$$

Redox mechanism

Activation by Surface Hydroxyl species

17. $${\text{CO}}_{2} {}^{*} + {\text{O}}_{{{x} - 1}} \leftrightarrow {\text{CO}}{}^{*} + {\text{O}}_{x}$$ (Ox, Ox−1 lattice oxygen and oxygen vacancy in support)

6. $${\text{CH}}_{3} {}^{*} + {\text{OH}}{}^{*} \leftrightarrow {\text{CH}}_{3} {\text{OH}}{}^{*}$$

CO and H2 formation

7. $${\text{CH}}_{3} {\text{OH}}{}^{*} + {}^{*} \leftrightarrow {\text{CH}}_{2} {\text{OH}}{}^{*} + {\text{H}}{}^{*}$$

18. $${\text{CHO}}{}^{*} + {}^{*} \leftrightarrow {\text{CO}}{}^{*} + {\text{H}}{}^{*}$$

8. $${\text{CH}}_{2} {\text{OH}}{}^{*} + {}^{*} \leftrightarrow {\text{CHOH}}{}^{*} + {\text{H}}{}^{*}$$

19. $${\text{C}}{}^{*} + {\text{O}}{}^{*} \leftrightarrow {\text{CO}}{}^{*}$$

9. $${\text{CHOH}}{}^{*} + {}^{*} \leftrightarrow {\text{COH}}{}^{*} + {\text{H}}{}^{*}$$

20. $${\text{COOH}}{}^{*} + {}^{*} \leftrightarrow {\text{CO}}{}^{*} + {\text{OH}}{}^{*}$$

10. $${\text{C}}{}^{*} + {\text{OH}}{}^{*} \leftrightarrow {\text{COH}}{}^{*}$$

21. $${\text{C}}{}^{*} + {\text{O}}_{x} \leftrightarrow {\text{CO}}{}^{*} + {\text{O}}_{{{x} - 1}}$$

Activation by surface O species

22. $${\text{CO}}{}^{*} \leftrightarrow {\text{CO}}\left( {\text{g}} \right) + {}^{*}$$

11. $${\text{CH}}_{3} {}^{*} + {\text{O}}{}^{*} \leftrightarrow {\text{CH}}_{3} {\text{O}}{}^{*}$$

23. $${\text{H}}{}^{*} + {\text{H}}{}^{*} \leftrightarrow {\text{H}}_{2} {}^{*} + {}^{*}$$

12. $${\text{CH}}_{3} {\text{O}}{}^{*} + {}^{*} \leftrightarrow {\text{CH}}_{2} {\text{O}}{}^{*} + {\text{H}}{}^{*}$$

24. $${\text{H}}_{2} {}^{*} \leftrightarrow {\text{H}}_{2} \left( {\text{g}} \right) + {}^{*}$$

13. $${\text{CH}}_{2} {\text{O}}{}^{*} + {}^{*} \leftrightarrow {\text{CHO}}{}^{*} + {\text{H}}{}^{*}$$

Kinetically, DRM has been mostly observed to be first order in CH4, with the CO2 reaction order varying from 0 on supports like Al2O3 to low positive values (<0.4) on supports like La2O3, MgO etc. [192, 213, 214]. Isotopic studies performed on Rh/Al2O3 by Wang et al. showed that the conversion of CH4 was greater than the conversion of CD4, suggesting that the dissociation of the C–H bond is the rate determining step in the reforming reaction [215]. Similar conclusions have been reported by a number of other researchers such as Wei and Iglesia [216]. The dissociation of CH4 has also been observed to be very sensitive to the catalyst surface structure, with the rate of CH4 decomposition reducing in the order of Ni(110) > Ni(100) > Ni(111) on different faces of Ni crystallites. However, on various systems, other elementary reactions such as surface reaction between CHx and O species, carbon oxidation etc. have been observed to be the slower and rate-controlling step [213].

### 5.6.4 Reactor Systems for DRM

Continuous fixed bed reactors (FBR) are the most widely studied reaction systems for DRM. Isothermal operation is necessary due to the high endothermicity of the reaction. Several other experimental setups such as Fluidized Bed reactors (FIBR), membrane reactors (MR) etc. have also been studied by several groups [217]. In a single and multi-mode switching study on a FBR and FIBR with Ni/Al2O3 catalyst, Chen et al. showed that a FIBR showed higher methane and CO2 conversion while also more efficiently suppressing coke formation [218]. However, for lab-scale studies, it is expensive and labour intensive to operate fluidized bed systems.

Membrane reactors with H2 permeable membranes provide the benefit of simultaneously separating produced H2 from the syngas, driving the reaction in the forward direction beyond thermodynamic equilibrium constraints. Pd-based hollow fiber membranes provide extremely high selectivity in separating pure H2 and high surface to volume ratio for separation and coating of catalysts. A hollow fiber membrane reactor with Pd/Al2O3 composite membrane has been shown to have higher methane conversion (34% higher than thermodynamic equilibrium) than a fixed bed reactor in DRM [219].

Recently, unconventional systems such as plasma reactors or reactors with electric field have also been applied in DRM. Plasma reactors provide the benefit of activating the reactant molecules at very low temperature through electron impact excitation, dissociation and ionization, yielding high conversion but suffer from low energy utilization efficiency and low selectivity to syngas due to the formation of substantial amount of higher hydrocarbon side-products [220]. Since the first use of plasma reactors for DRM around 40 years back, different types of plasma like microwave plasma, gliding arc discharge, thermal plasma, dielectric barrier discharge and corona discharge plasma has been applied for DRM. It has been reported that combining catalysts with plasma can provide a synergistic effect in increasing both activity and selectivity of the reaction through a complex interplay of highly energetic reaction intermediates created by the plasma, their interaction with the catalyst surface and the in-situ modification of catalyst properties by the plasma.

### 5.6.5 Minimizing Catalyst Deactivation in DRM

Catalyst deactivation over time is a critical challenge for DRM, stemming mainly from sintering of active metal phase at high temperatures and the blocking of active sites by deposition of carbonaceous species formed in the DRM reaction. Catalyst deactivation can also happen due to other reasons such as surface oxidation of active metal sites, encapsulation of active metal by reducible supports like CeO2 MgO etc., poisoning by sulfur compounds etc. However, coking is the most predominant reason for loss of catalytic activity, and hence has garnered significant research interest in the recent years. Some of the strategies that can be adopted to minimize the issue of catalyst deactivation are described below.

#### 5.6.5.1 Reaction Conditions

As discussed before, the coke forming reactions in DRM show a strong dependence on the reaction conditions. Figure 5.9 shows the equilibrium product composition from DRM as a function of temperature, wherein solid coke is considered as one of the reaction products. Equilibrium coke formation is negligible above reaction temperatures of 900 °C [221]. The highly exothermic Boudouard reaction (5.3) is inhibited above 800 °C and coke formation occurs mainly as a result of methane decomposition. The carbon species from cracking of methane is more reactive than that generated from Boudouard reaction and can be oxidized by CO2 at such high temperatures. Thus, at high temperature (>800–900 °C), the rate of carbon gasification by CO2 is equivalent to or faster than the rate of formation of coke precursors, leading to minimal coking [196].

Another major factor that affects the coke formation rate is the CO2/CH4 ratio. Higher CO2/CH4 ratio naturally favours low coke formation, but affects product composition, further lowering the H2/CO ratio. Increasing pressure increases the propensity of coke deposition; for instance, increasing pressure from 1 to 10 bars leads to an increase in the carbon deposition limit (temperature beyond which coke deposition is negligible) from 870 to 1030 °C [217]. Thus, deactivation by coking may be limited by operating in suitable regimes.

#### 5.6.5.2 Coupling DRM with Other Reactions

Coupling DRM with Partial Oxidation of Methane (POM) [222, 223, 224] (5.16) or Steam Reforming of Methane (SRM) [225] (5.17) can reduce the coke formation tendency due to the presence of O2 and H2O, which are stronger oxidants than CO2. At the same time, coupling these reactions also provide a handle in varying the H2/CO ratio of the final product syngas. Oxidative DRM (combining DRM and POM) also results in a more favourable process thermodynamics due to the exothermic nature of POM and the strong endothermic nature of DRM [226].
$$2{\text{CH}}_{4} + {\text{ O}}_{2} \to 2{\text{CO}} + 4{\text{H}}_{2} \quad{\Delta \text{H}}_{298} = - 36\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.16)
$${\text{CH}}_{4} + {\text{H}}_{2}{\text{O}} \to {\text{CO}} + 3{\text{H}}_{2}\quad\Delta {\text{H}}_{298} = 228\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$
(5.17)
While coupling such reactions lead almost invariably to higher methane conversion, it can affect the hydrogen selectivity due to over oxidation of methane to CO2. Membrane reactors have been developed to address such issues, where O2 permeable membranes are used to provide a precisely controlled flow of additional O2 to the DRM process, leading to high selectivity. Kawi et al. reported several perovskite membrane reactor systems to integrate POM with DRM that showed high conversion (c.a. 94% methane conversion at 725 °C), product selectivity and stable operation up to 160 h with negligible coke deposition [227, 228].

#### 5.6.5.3 Catalyst Design

The rate of coke formation and deposition in DRM is a strong function of the catalyst structure and composition. A plethora of recent studies have focused on designing catalysts that can inherently eliminate coke formation and can be operated stably under a wide range of operating conditions.

##### Support Modification
A very effective way of resisting carbon formation is the incorporation of the active metal in to the support structure with high oxygen mobility. Redox oxides like CeO2, ZrO2 and perovskite oxides possess highly mobile oxygen species (either as lattice oxygen or surface oxygen species), that can adsorb CO2 and activate it to accelerate the carbon oxidation process. CeO2 can store and release lattice oxygen through a stable transition between Ce+4 to Ce+3 states. Doping redox oxides like CeO2 in the support has shown high effectiveness in controlling coke formation by accelerating the oxygen-transport properties of the system [229, 230]. The oxygen mobility of oxide supports can be further enhanced by the introduction of trace amounts of heteroatoms in the oxide structure, that creates structural defects and increases the oxygen mobility [209, 231]. For example, Sutthiumporn et al. showed that partially substituted La0.8Sr0.2Ni0.8M0.2O3 perovskite (M=Bi, Cu, Cr, Co, Fe) catalysts possessed higher lattice oxygen mobility, that helped in C–H activation and also in reducing carbon formation by activating CO2 through Lanthanum Oxycarbonate (La2O2CO3) formation (Fig. 5.10) [232]. In an other study [233], the Lewis acid species (Al-F) present in modified Ni/Al2O3 catalyst stabilized Ni species from high temperature sintering through metal-support interactions and reduce the electron density to alleviate the fast CH4 decomposition.
##### Reducing Metal Particle Size

The rate of coke formation has been shown to be a strong function of the metal particle size, with coke formation being favoured on bigger ensembles. Kim et al. investigated the effect of size of nickel particles supported on alumina aerogel on coke formation in DRM and found that a minimum diameter of about 7 nm is required for the Ni particles to generate filamentous carbon [234, 235]. Hence, synthesizing catalysts with high metal dispersion and small particle size is considered an effective strategy to reduce coke formation in DRM. Several synthesis methods such as sol-gel, deposition-precipitation, microemulsion, colloidal method etc, have been explored to synthesize more dispersed metal nanoparticles. Impregnation in the presence of complex organic chelating agents like oleic acid/amine [101], PVP [236], etc. has proven to be effective in reducing metal particle size and consequently reduce coke formation in DRM. Kawi et al. reported self-assembly of transition metals into small nanoparticles coated with organic agents like oleic acid/amine, which resulted in high metal dispersion and sintering resistance [101, 113, 237, 238, 239, 240, 241]. Another method of controlling metal dispersion and preventing sintering is to use structured catalyst precursors with mineral type structures such as phyllosilicates [225, 242, 243], perovskites [232], hydrotalcites [244] etc. that can generate supported metal catalysts with very strong metal support interaction with lower metal mobility and sintering. Suitable promoters may be used to enhance the metal-support interaction. Confining metal nanoparticles within the pore structures of ordered mesoporous supports like SBA-15, MCM-41 etc. also prevents metal agglomeration by physically separating the particles [245].

Another class of catalysts that have gained high interest in recent years for their high thermal stability and anti-coking properties are core@shell catalysts [246, 247]. By coating the active metal cores with thermally stable porous metal oxide shells, the particles can be segregated, preventing their agglomeration and growth under high temperatures. Several types of core@shell structured catalysts have been reported to show high stability under DRM, even under low temperature conditions with negligible coke formation (Fig. 5.11) [184, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257]. While SiO2 remains the most widely used shell material for the ease of controlling the coating process by modifier Stöber process, other materials such as CeO2 [250], ZrO2 [258] can also provide more coke resistance by aiding in coke removal through CO2 activation.
##### Bimetallic Systems

Using bimetallic catalysts can provide benefits in increasing the coke resistance of the catalyst through multiple ways, by modifying both the electronic and geometric properties of the active component [196]. Geometrically, the presence of a second metal acts to break up the metal ensembles by a ‘dilution effect’, which can reduce the coke formation tendency. For bimetallic catalysts involving transition metals and noble metals such as Ni–Pt, Ni–Ru etc., doping minute amounts of noble metals has shown a significant increase in activity and stability due to the inherent properties of noble metals, synergistic effect caused by alloying and an increase in metal dispersion. Depending on the method of catalyst preparation, noble metals may preferentially be enriched on the surface in the bimetallic particle, which leads to more significant improvement in performance [200].

Alloying Ni with transition metals with redox property such as Co and Fe has also shown significant improvement in coke resistance due to the higher oxygen affinity of these metals. For instance, Fan et al. studied the role of doping Co in CoxNiyMg100-xyO catalysts and showed that Co is enriched on Ni–Co alloy surface and enhances the chemisorption of oxygen species, thereby accelerating the gasification of coke intermediates [259]. However, it is crucial to optimize the Co/Ni ratio in the bimetallic catalyst as too high Co content may lead to oxidation of Ni and a consequent loss in activity. Similarly, adding Fe to Ni has been shown to reduce coke formation in DRM, although at a slight trade-off in activity, due to the redox nature of Fe. Kim et al. showed that under DRM conditions, there is some de-alloying of Ni and Fe and a FeO phase is formed that migrates towards the NiFe particle surface and provides oxygen species for coke removal through a redox mechanism [260].

## 5.7 Concluding Remarks

In this chapter, we have highlighted the technological developments of CO2 to various C1 valuables, with respect to heterogeneous catalysts, kinetic considerations and engineering challenges. The technologies discussed include CO2 to methane, methanol, formic acid and syngas formation via hydrogenation reaction and CO2 to syngas production using methane reforming process. On the common note, the role of efficient catalyst is pivotal for these technologies to be economically feasible. The nature and properties of the catalysts, such as redox or acid-base, play an important role in determining the catalytic activity as well as selectivity towards desired product. For CO2 methanation reaction, being an exothermic process, the current development in the catalyst mainly focuses on low-temperature active catalysts to overcome the thermodynamic barrier and also requires appropriate reactor design to remove additional heat. Nanosized nickel particles appear to be the most suitable non-precious metal to achieve high conversion at low temperatures. Additionally, development of the catalyst is based on improving the selectivity of methane by avoiding the formation route of carbonyls and carbides which causes deactivation. Apart from active catalytic sites, designing support and addition of promoters play a crucial role in improving the performance by H2 and CO2 activation, metal dispersion and enhancement of metal support interaction. The fixed-bed reactors are the most widely used systems for CO2 methanation. Fluidized-bed reactors have proven highly reliable for CO methanation, and other types of reactor are still under development.

Similarly, reverse water gas shift (RWGS) reaction which converts CO2 to CO by combining with hydrogen is one the most straightforward processes used in industries as an intermediate process for the production of more valuable chemicals such as methanol. Additionally, RWGS is the most straightforward way for large-scale production of value-added chemical when combined with CO2-FT synthesis units. To design a catalyst for RWGS reaction, it is important to consider the problems associated with the reaction such as catalyst deactivation. Some of the possible reasons for catalyst deactivation are the large amount of water formation, metal sintering and coke deposition which decreases the selectivity towards CO formation. Controlling the selectivity of RWGS requires a deep understanding of thermodynamics, reaction kinetics, and mechanism which becomes easier by the utilization of advanced techniques such as transient quantitative temporal analysis, in-situ DRIFTS, and few more. In parallel with the experimental study and simulation method involving DFT calculations, the structures of well -performing catalysts can be designed to make the RWGS process more efficient to be used industrially.

Another product from CO2 hydrogenation reaction is methanol, which is a widely used feedstock for fuel, olefins and other chemicals. Direct conversion of CO2 to methanol is one of the most economical and feasible ways after oil and gas. CO2 to methanol reaction is highly exothermic and thus it can be facilitated only at high pressure and low temperature. However, this reaction suffers from low selectivity towards methanol and catalyst deactivation due to crystallization by water formed during the reaction. There are possibly two reaction routes to methanol as described in the literature: firstly via a reverse WGS for CO2 decomposition to CO and secondly via an intermediate formate route. Cu, Pd, In, Ag are some of the active metals suitable for methanol production. Therefore, synthesis of highly-dispersed metal catalyst, with enhanced metal support interaction and increasing defect concentration in metallic nanoparticles, amount of surface steps and edges of the Cu phase helps to achieve the target yield for this process to be commercialized.

Besides methanol, synthesis of formic acid is another approach for utilizing CO2 as C1 source for fuels/chemicals. Heterogeneous direct synthesis of formic acid from CO2 is also an exothermic process, so lower reaction temperature and high pressure favor the reaction towards product formation. Unlike the methanol technology, direct formic acid synthesis technology is still at conceptual research and development stage. Improvements in the yield of formic acid production remains a key factor which requires much on-going research. The key challenges that need to be addressed include the development of suitable heterogeneous solvent medium. As per the current development, Ru and Pd based catalytic systems are the most active metals for this reaction; however, the performance of active metals is greatly influenced by the choice of support. A suitable catalyst support can increase the number of available catalytic centers and minimize their agglomeration under reaction condition, thus resulting in longevity and reusability. In addition to the nature of catalyst, suitable additives and solvents have a great role in fixing gaseous reactants to make them available for the reaction to occur on the catalyst surface. DFT studies and experimental observation revealed that reaction proceeds in two steps on the metal surface i.e., abstraction of hydrogen atom by CO2 to form a formate intermediate and this formate intermediate then takes another hydrogen atom to form formic acid. The formation of formate intermediate is observed to be the rate-determining step of the reaction.

CO2 reforming of methane is another avenue for the fixation of CO2 into useful fuels, and significant progress has been made in the development of catalysts and novel reactor systems that allows long-term stable operation, circumventing the inherent issues of deactivation. However, further work is still required for the translation of lab-scale research to industrial-scale implementation. In 2015, the Linde Group officially opened a dry-reforming based pilot facility at Pullach near Munich. With more research both on key catalyst development and on addressing the practical challenges of scale-up, DRM may be at the cusp of commercialization shortly.

The highlighted CO2 hydrogenation and methane reforming reactions make a complex network which are inter-related, not only in terms of schematic reaction pathways (Fig. 5.12), but also in terms of the catalysts used.

For instance, for CO2 methanation reaction (Route 1 in Fig. 5.12), supported Ni catalysts are one of the extensively explored catalyst and proved to be promising at slightly higher temperature (above 400 °C). Similar catalytic system can also be adopted for CO2-CH4 reforming reaction (route 2). However, due to the endothermic nature, the latter needs to be operated at significantly higher temperature than the former. Moreover, further research is going on to develop DRM catalyst active at lower reaction temperature. Furthermore, methane generated from scheme 1 can be used to carry out the second reaction. Vapour phase CO2 hydrogenation at low temperature and high pressure and the presence of suitable catalyst generates methanol (route 3). And liquid phase CO2 hydrogenation at low temperature and high pressure with suitable catalyst and additive forms formic acid (route 4). The formation of methanol can be either via formation for CO (route 5) or formate species (route 6) as an intermediate products. Thus, the reaction systems, which can catalyse CO2 hydrogenation to CO via RWGS reaction, can also show promising methanol synthesis activity at its reaction conditions. Similarly, if the catalyst catalysing CO2 hydrogenation to formate can be able to generate methanol from formate species with some modifications, it can open the doors for the synthesis of methanol at milder reaction conditions i.e., low temperature and high pressure. There are studies showing methanol synthesis from formic acid using H2 with Cu–Zn and Cu–Al catalysts [261]. The maximum methanol yield of 36% could be achieved at 300 °C for 5 h. Further research on this direction is required to boost CO2 conversion to C1 valuable chemicals.

In a nutshell, the conversion of CO2 to synthesize valuable chemicals has a tremendous potential to be commercialized on a large scale [262]. These processes require practical design of the catalyst system and an energy efficient reactor design to improve the performance regarding activity, selectivity, and stability. Finally, except CO2 to formic acid reaction, other processes are technologically matured and can become economically viable, if the costs associated with CO2 capture and green hydrogen production is mitigated.

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© Springer Nature Switzerland AG 2019

## Authors and Affiliations

• Jangam Ashok
• 1
• Leonardo Falbo
• 2
• Sonali Das
• 1
• Nikita Dewangan
• 1
• Carlo Giorgio Visconti
• 2
Email author
• Sibudjing Kawi
• 1
Email author
1. 1.Department of Chemical and Biomolecular EngineeringNational University of SingaporeSingaporeRepublic of Singapore
2. 2.Laboratory of Catalysis and Catalytic Processes, Department of EnergyPolitecnico di MilanoMilanItaly