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Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges Soumyabrata Roy, Arjun Cherevotan, and Sebastian C. Peter ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00740 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Energy Letters

Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges Soumyabrata Roy#, Arjun Cherevotan# and Sebastian C. Peter* New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India #

The authors contributed equally.

Abstract Catalytic conversion of CO2 into chemicals and fuels is a “two birds one stone” approach towards solving climate change problem and energy demand-supply deficit in the modern world. The recent advances in the mechanistic insights and design of suitable catalysts for direct thermocatalytic hydrogenation of CO2 to C1 products have been thoroughly discussed in this perspective. The role of catalyst composition and process conditions in determining the selective pathways to various products like carbon monoxide, methanol, methane and dimethyl ether, has been overviewed in the light of thermodynamic and kinetic considerations. After extensive elaboration of the main motivation of reaction pathways, catalytic roles, and reaction thermodynamics, we summarized the most important macroscopic aspects of CO2 hydrogenation technology development which includes reactor innovations, industrial status of the technology and life cycle assessment and technoeconomic analysis. Finally, a critical perspective of the future challenges and opportunities in both the fronts of core and overall technology development has been provided.

*Corresponding author. Phone: 080-22082998, Fax: 080-22082627 [email protected] (S. C. Peter)

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The energy production and utilization in the modern society exploit the enormous energy that has been stored in Earth’s crust by nature, over the ages, in the form of fossilized sunlight namely, carbonaceous fuels like coal, petroleum and natural gas. While this treasure has led to an unprecedented exponential growth of human civilization, it has also resulted in enormous pressure built on the natural carbon cycle which has been altered due to a tremendous amount of CO2 emission from anthropogenic activities, mainly fossil fuel burning. The annual global CO2 emission in 2014 was recorded to be 39,250 Mt of which, fossil fuel emissions (including cement production) accounted for about 91% of the emissions from human sources. The distribution of the total emission comes from coal (42%), oil (33%), gas (19%), cement (6%) and gas flaring (1%). Consequently, the atmospheric CO2 concentration increased from ~270 ppm in the pre-industrialisation era to ~410 ppm in 2017, leading to serious climate change issues and detrimental ocean acidification. As a result, various global initiatives like Intergovernmental Panel on Climate Change (IPCC) and the United Nations Climate Change Conference (COP21, Paris, 2015) have emphasized the urgency to mitigate CO2 emissions by at least one half of the current value by 2050, to deter the average global temperature increase to a maximum of 2 °C.1 On the other hand, with increasing energy demand, the global consumption of energy per hour is predicted to reach 1.1 x 1021 J by 2050, which is 80% more than what can be derived from fossil fuel resources.2 However, with the recent discovery of shale gas & oil resources, and the idea of utilizing methane hydrates and large existing coal deposits, it is evident that running out of fossil fuels is a definite but distant possibility, thus unsignifying the so-called global Hubbert’s peak. However, with no practical and economical solution for the CO2 crisis being found, the usage of fossil fuels will indirectly be constrained by the levels of CO2 increase and the accompanying environmental hazards. The ultimate solution thus will be to capture carbon in the form of CO2 from any source, and eventually from the atmosphere, and recycle it to new chemicals and fuels using alternative sources of energy.3 The mitigation of CO2 emission is envisaged to occur through three possible strategies: (a) Extensive shift of global energy base from fossil to renewable energy and greener fuels like hydrogen, (b) storage of CO2, and (c) CO2 utilization.4 Strategy (a), though very important, have serious disadvantages necessitating major changes in energy infrastructure of the transportation sector, and also political issues of implementing these changes in large geographical areas with abundant fossil fuel deposits. Storage of CO2 (strategy b) have limitations of high cost, intensive energy requirements for separation & pumping, permanency of stored CO2 in sites and an enlarged use of fossil-C (from 20% to 60%) in the process.5 Thus the strategy (c) of carbon utilization through CO2 conversion, which is 20–40 times more efficient than sequestration over a span of 20 years,6 is the most practical and viable solution to arrest climate change along with curbing of anthropogenic emissions. Many analytical studies have been conducted to estimate the amount of CO2 that has to be captured and converted, to make a substantial impact on the climate change.7 These searches particularly focussed on solutions to quantitatively reduce CO2 emissions of the fossil-based energy system. However, the combustion of the CO2-converted fuels produces an equivalent amount of CO2 and thus will not lead to a direct net “consumption” of CO2. In spite of that, CO2 conversion in a man-maneuvered carbon cycle is very crucial for a sustainable future.7 CO2 is a highly oxidized, thermodynamically stable molecule (∆G° = -400 kJ/mol) with two linear double bonds,2 having ultra-low reactivity, which requires overcoming a thermodynamic barrier for its activation. Therefore, its chemical conversion and economical utilization is a formidable scientific and technical challenge. Currently, there are very few existing industrial syntheses using CO2, e.g., urea synthesis (for nitrogen fertilizers and plastics), salicylic acid synthesis (pharmaceutical ingredient), and polycarbonates

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ACS Energy Letters

manufacturing (for plastics). Song et al. elaborated the thermodynamic considerations of CO2 conversion and emphasized on the requirements of high input energy, efficient catalysts and optimal reaction conditions for CO2 conversion.8 However, given the widely divergent scales of the material value chain and energy/fuel sector, it is quite apparent that CO2-derived chemicals cannot provide a quantitative “sink” for CO2 to balance the enormous anthropogenic emissions, particularly from fossil fuel sector. Thus, CO2 embodied into liquid fuels, fuel substituents and fuel precursors for transportation sectors and energy production, is the right direction for CO2 utilization owing to its commendable market size and “sinking capacity”.7 To talk of numbers, assuming all fuels and chemicals being produced from CO2, demand for organic chemicals accounts for only 4% of CO2 emissions, while fuels account for 30% of total CO2 emissions and 100% of emissions from power plants.9 In this regard, existing chemical and petrochemical industrial infrastructure, with a large production capacity of chemicals and fuels using heterogeneous catalysis, has the potential to perceive the needed energy transition, by utilization of CO2 at globally significant scales. The growth of refineries and fuel industries based on CO2 is one of the major approaches envisioned to facilitate greenhouse gas reduction and ameliorate climate change.10 Hydrogenation reaction has been identified as the most important among various chemical conversions of CO2 as it offers a good opportunity for sustainable development in the energy and environmental sectors.4 Additionally, the products of CO2 conversion are value-added and can be used as fuels or precursors to produce more complex chemicals and fuels.6 The “Sabatier reaction” discovered in the 1910s,11 was the first industrially developed CO2 hydrogenation reaction to form methane, which has been a pivotal discovery in understanding the basic underlying principles of modern-day catalysis. However, with the discovery of the Fischer−Tropsch process for synthesizing hydrocarbons from syngas (CO, H2, CO2 mix), Sabatier reaction became less industrially relevant.11 CH4, the easiest C1 hydrocarbon of CO2 conversion, however is much more stable and less reactive than methanol in forming further derivatives and chemicals.11 Methanol, one of the most important products of CO2 reduction, is industrially synthesized from syngas (with CO and H2 in the ratio 1:2) which is also called the metgas, recently.11, 12 A lot of industrial or industrially relevant downstream processes from methanol are well studied, e.g. conversion to HCHO (formaldehyde) and HCOOH (formic acid) and their further conversions to formamide, sugars, nucleic acid bases, amino acids and even peptides.11 Methanol also has extensive applications in producing higher hydrocarbons & derivatives or gasoline through the MTO (methanol-to-olefins) and MTG (methanol-to-gasoline) reactions.13 CO on the other hand has tremendous applications in the chemical and fuel industries through FT process and as a major syngas component. Moreover, it has wide applications in steel industry, metal fabrication industry and in the field of pharmaceuticals. It is important to note that process of CO2 hydrogenation to various products like MeOH, CO, DME (dimethyl ether) etc. is now being widely practiced industrially in many countries, with the first industrial plant being put up in Iceland, by Carbon Recycling International to produce methanol from CO2 using geothermal energy source. Other notable endeavours include the methanol plant in Osaka, Japan by Mitsui Chemicals, DME production by KOGAS Corp. in Korea, methanol production at the Lünen power plant, Germany by Mitsubishi Hitachi Power Systems Europe (MHPSE). In spite of all these efforts, it is only rather apparent that CO2 hydrogenation technology is still steps further from extensive commercialization, which is necessary to bring about the intended environmental change. This is primarily owing to insufficient conversion, product selectivity, which are effects of competitive and unfavourable kinetic and thermodynamic factors. The only way to address these issues is to develop more efficient & selective catalysts and integrated reactor systems which can form targeted products at high

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conversion rates with minimum energy expense over industrial time scales. Designing better catalysts demands thorough understanding of the holy grail of reaction mechanisms. However, determining reaction mechanisms in case of thermochemical hydrogenation of CO2 is complicated, and rather challenging, primarily because of the lack of powerful in-situ probing techniques, complexities of extreme reaction conditions and spectroscopically opaque reactor designs. Thus, current endeavours into CO2 hydrogenation research focus on the development of highly active, selective and stable catalysts for thermochemical CO2 reduction based on the mechanistic understanding of the catalytic processes.6 The importance of catalytic activation of CO2 and its embodiment into value-added chemicals using cheap and earth-abundant catalytic materials have been highlighted in several recent reviews and perspective articles.3, 5, 14-19 There were also substantial focus and improvements in the field of homogeneous CO2 hydrogenation mainly to methanol and formic acid, mostly were pincer ligands which easily activates the first row metals like Mn, Co and Fe.20-23 Daza et al.24 presented an important summary of catalytic insights for CO2 conversion to fuels via RWGS (Reverse water gas shift). A comprehensive review of catalysts for CO2 conversion to a wide range of products like CO, CH3OH and light alkenes and olefins was presented by Porosoff et al.6 Dorner et al.18 provided an overview of CO2 conversion to value-added hydrocarbons by the modified FT catalysts. Rodriguez et al. discussed the promotional effect of metal/oxide or carbide interfaces on CO2 to MeOH activity.19 Finally, a critical theoretical review by Li et al.25 outlined the reaction mechanisms for CO2 conversion to C1 fuels. Selectivity, an even more important factor of industrial relevance, is also closely related to the catalytic composition and governing mechanism of a particular conversion. Complexities of reaction mechanisms for CO2 hydrogenation leads to multiple product formation, the separation of which becomes a serious technological and economic issue in practical applications. The current perspective aims to identify the critical mechanisms for CO2 thermocatalytic conversion to C1 products by providing deeper insights into the synergistic interactions at the various catalytic interfaces which tunes the reaction mechanisms and in turn the selectivity of CO2 hydrogenation. The role of bonding strength26-29 and electronic effects of different catalytic sites have been elucidated by several in-situ operando studies and theoretical investigations on multi-functional composite catalysts generally used for thermochemical CO2 reduction.14, 30, 31 The multifunctional interface in many cases provides different adsorption sites to stabilize the key intermediates of the reduction pathway, which plays a crucial role in determining the catalytic mechanism.26, 28, 32, 33 The strong electronic interactions between different parts of a multi-functional composite catalyst creates unique hybrid properties that facilitate CO2 activation and its subsequent transformation. However, it is very critical to resolve the exact roles and effects of different catalytic sites/interfaces in a composite structure.34, 35 Here, we have particularly tried to elaborate on catalytic reaction pathways and the role of catalysts in determining the product selectivity of CO2 hydrogenation to C1 and C1 derived carbon compounds: carbon monoxide (CO), methanol (CH3OH), dimethyl ether (DME) and methane (CH4). The success of the overall technology also depends on other macroscopic factors like reactor designing, emission assessment and techno-economic potential of the CO2 hydrogenation process. We have first summarized the activity and reaction mechanisms of the different catalysts that are selective to produce CO, CH3OH, DME, and CH4. That has been followed by brief discussions on reactor innovations, instruments for mechanistic studies, industrial status of the technology, life cycle assessment and future challenges and opportunities. 1. Products and mechanistic outlook 1.1. CO2 to CO: Carbon monoxide, one of the most common and industrially important products of CO2 reduction, is primarily formed through reverse water gas shift reaction (RWGS). CO has extensive usage in industries, particularly in the field of Fischer Tropsch

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ACS Energy Letters

process, metal and steel fabrication industry, and pharma & biotechnology. RWGS is an endothermic reaction (Eqn. 1) requiring higher temperature (550°C to 750°C) but lower pressure (0-5 Bar) conditions. Catalysts used for RWGS can be classified mainly as (i) Cubased, (ii) Noble metals supported on metal oxides, (iii) Reducible oxide supported and (iv) Transition metal carbide based ones. RWGS reaction on all these classes of catalysts goes through either the redox mechanism or through the formate dissociation pathway. The redox mechanism occurs primarily in Cu-based catalysts where CO2 oxidizes Cu0 to Cu+1 and generates CO, while H2 reduces Cu+1 back and forms water.36-38 In the formate dissociation mechanism, CO2 gets hydrogenated into formate followed by subsequent cleaving of C=O, which requires bi-functionality in the catalyst.6 In metal nanoparticles supported on metaloxide systems (M-NPs@M-O) the metals dissociatively adsorbs H2 and spills it over to the M-O sites where CO2 is adsorbed.39 In view of all the catalysts reported to date, it could be concluded that both the mechanisms are prevalent in any reaction, with relative dominance of one pathway over the other, depending on the catalysts. Examples of such cases have been discussed in the following section (Table 1). Organized studies on RWGS mechanism which led to better understanding of the catalysts’ role were initially done using IR studies, be it in-situ FTIR, ATR or DRIFTS. One of the highly studied systems for mechanistic investigations of RWGS is Pd/Al2O3. Arunajatesan et. al. in his work on Pd/Al2O3, using in-situ FTIR, proved that bare alumina exposed to CO2 and H2 (P = 138 bar: T = 343 K; molar CO2/H2 = 19 in high-pressure transmission cell) gave mainly carbonates and traces of formates.40 Table 1: Examples of the exemplary RWGS catalysts which were used to elucidate the most dominant catalytic mechanisms of CO formation on various catalytic surfaces. Catalysts compositions, results of conversion and the detected/predicted mechanisms (wherever available) of these selected catalysts for CO2 to CO conversion listed in this table. Catalyst & Composition Fe doped-Cu/SiO2 (Cu/Fe = 10:0.3)36

Synthesis Route

Reaction Condition

Impregnation of silica by nitrate salts of metal

CuO/ZnO/Al2O3 (35% of Cu)37

Co-precipitation by Na2CO3 as precipitant -

Fixed bed reactor; 40 ml/min (1:1 of CO2:H2); 600°C & atm pressure Fixed bed reactor; 493K-523K & atm pressure 1:1 of CO2:H2; 342K & 138 bar Fixed bed reactor; 1:1 of CO2:H2 600°C & atm pressure

Pd/Al2O3 (1% Pd loading)41 Cu/K/SiO2 (9% Cu +1.9% K)42 Rh/SiO243 NiO/SBA-15 (10% NiO)44, 45 CuO(5wt%)NiO(1wt%)/SBA & CuO(10wt%)CeO2(1wt%)/SBA1544 2%Pt/CeO246

Impregnation of Cab-O-Sil M-5 SiO2 by nitrate metal salts Incipient wetness impregnation by acetate precursor In-situ metal oxide dispersion during SBA-15 synthesis In-situ metal oxide dispersion during SBA-15 synthesis

Commercial

Conversion & Selectivity 15% (conversion)

Mechanism assigned _

-

Redox Mechanism

-

Formate Decomposition Formate decomposition

12.8%

Fixed bed reactor; 100 0.52% & 88% ml/min at H2/CO2 = 3; selectivity 403K 5 MPa Fixed bed reactor; 1:1 20% and 100% selectivity of CO2:H2; 600°C & atm. Pressure >50% Fixed bed reactor; 1:1 of CO2:H2;400-900°C conversion at 900°C and & atm. Pressure 100% selectivity 21000 H-1 GHSV (1% -

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-

-

Hydrogenation


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CO2 + 4% H2 in Ar) 498 K atm pressure

1%wtPt/TiO247-49

Wetness impregnation by PtCl4 salt

Fixed bed reactor;100 ml/min; H2/CO2 = 3; 12,000h-1; 873K & atm pressure

55% conversion

Cu-Ni/γ−Al2O350

Co-impregnation using metal nitrates as precursor

28.7% conversion and 79.7% selectivity

PtCo/CeO2 (1.7%Pt; 1.5% Co)51

Co-impregnation of by using Pt(NH3)4(NO3)2 & Co(NO3)2.6H2O

Fixed bed reactor; 2000h-1 GHSV with CO2/H2 ratio = 1; 873K and atm. pressure Batch reactor; 1:3 partial pressure ratio of CO2:H2; 573K & 30Torr

10% conversion and 259.4 of CO/CH4 selectivity

of surface carbonates on CeO2 support by H-spill over from Pt. Through easy carbonate formation on TiO2 which gets reduced by Ti3+ Redox mechanism by Cu

Formate decomposition

When Pd/Al2O3 was exposed with the same, there were presence of formates, CO, and carbonates. Bare alumina exposed with only CO2 gave only carbonates. These observations confirmed the above-mentioned formate mechanism for Pd/Al2O3. Authors also pointed the emergence of a new peak at 1900 cm-1 after 20 mins of reaction, corresponding to bridged CO on Pd metal, which became significant close to 5h, thus hampering the catalytic activity. Unlike other cases, no hydrogenation of bridged CO to CH4 was observed in this case. These studies led to the conclusion that short-residence time continuous reactors are preferred over batch reactor for RWGS.40 One of the main challenges of using M-NPs@M-O for RWGS is the role of metal doping percentages in determining the balance between conversion and selectivity. For example, in case of Pd/Al2O3, CO2 conversion hiked with active metal loading, but a higher Pd loading on Al2O3 (ie 5% Pd) led to 2-3 fold increase in methane selectivity compared to 0.5% Pd/Al2O3.41 The authors here confirmed the formate mechanism where CO formed spills back to Pd and gets adsorbed. The subsequent desorption of CO depends upon the strength of adsorbed CO. Higher Pd loading led to more active terrace sites with multiple CO-Pd bonds which on subsequent hydrogenation yielded CH4 (Figure 1). The catalysts with 0.5% Pd loading, although less active, showed greater stability as there was no deactivation due to CO-adsorption on M-sites.41

Figure 1. Schematic representation of CH4 and CO production on Pd/Al2O3 with different loading. Reprinted from ref.41

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ACS Energy Letters

Further detailed ATR studies, reported earlier on Pt/Al2O3 by Ferri et al showed that the CO frequency and intensity of COL:COB (linear:bidentate) was unusually low on Pt/Al2O3.52 This revealed that CO was binding in the active unsaturated interfacial site between Pt and Al2O3 (Figure 2). Thus the CO formed during reaction acted as the probe for the active site. This was confirmed by observing normal CO frequency and COL:COB on CO adsorbed catalyst from solution.52

Figure 2. Pictorial presentation of RWGS mechanism on Pt/Al2O3. Reproduced with permission from ref.52 Copyright 2002 Royal Society of Chemistry. Since RWGS is a reverse equilibrium reaction for WGS, catalysts exhibiting WGS can also exhibit RWGS.53 Cu-based catalysts are thus most studied for RWGS. The alumina supported Cu-ZnO with Cu:Zn ratio>3 was more active when compared to that of unsupported Cu-ZnO. The alumina was proved to affect the CuO and ZnO dispersion which led to a linear increase in activity with an increase in Cu0 surface area. A better activity for RWGS was seen in Cu-SiO2 prepared by impregnation when promoted by potassium (K). This was attributed to the formation of interfacial sites between Cu and K where the formates were produced. The H gets spilled over from Cu sites to the interfacial sites between Cu and K, where CO2 is actively adsorbed to form formate which decomposes to CO.53 Interestingly, in the un-promoted Cu catalyst, the redox mechanism (described earlier) of RWGS dominates over the formate mechanism.54 The rate of reduction of Cu2O is far less than that of CO formation from CO2 dissociation. Thus there is a resultant oxidation of Cu sites which ultimately leads to catalyst deactivation. The Cu based catalyst is also prone to sintering of Cu at temperatures higher than 600°C36, 55 with endothermic RWGS often demanding elevated temperatures.36, 55 However, promotion of impregnated Cu/SiO2 with Fe gave good activities even at temperatures above 600°C. Fe species acted as a textural promoter which prevented Cu sintering. Also, the O species formed on Cu, spilled over and oxidized Fe which acted as a sacrificial promoter to prevent Cu oxidation and increased the activity.55 In another method to improve the dispersion and stability of Cu-NPs, Cu/SiO2 was synthesized by atomic layer epitaxy.56 But this is a tedious method and has its own limitations for application in industrial scale catalyst synthesis for CO production, unlike impregnation. NiO dispersion on silica (SBA-15) was improved by direct synthesis of metal on SiO2 instead of post-synthetic impregnation to SBA-15.45 Interestingly it was observed that while the conversion was independent of NiO loading and the temperature, higher loading caused a decrease in CO selectivity at temperatures less than 600 0C. Authors concluded that isolated Ni atoms gave CO at all temperature while paired Ni led to CH4 at a lower temperature.45 High conversion and selectivity towards RWGS are only possible if NiO loading is high without pairing of NiO to cause monodispersed NiO/SBA-15. This breakthrough was achieved by changing the precursor of direct NiO/SBA-15 synthesis. Two precursors mainly

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compared were Ni(NO3)2-HCl and NiSO4-HCl. The former gave nitrates of metal which are smaller sized and thus moved easily and re-dispersed during heat treatment, getting agglomerated above 2% loading. The latter gave bigger sized sulphates of metal which are difficult to re-disperse during heat treatment and so got well dispersed even at 25% loading. Direct hydrothermal synthesis was employed for the first time in dispersing bimetallic oxide on SBA-15 for RWGS. In CuO-NiO/SBA-15 and CuO-CeO2/SBA-15, the first M-O is well distributed on SBA-15 and the other was distributed over the primary metal oxide.44 This occurred according to the hydrophilicity of M-nitrates and TEOS with ethylene oxide of Pluronic P123. Though generally, these mixed metal oxide systems gave better activity than single M-O dispersed by direct synthesis, in NiO-CeO2/SBA-15 case, the mixtures of NiO and CeO2 distributed over SBA-15 showed lesser CO selectivity at 700 °C due to the proximity between Ni sites.44 The reports using reducible supports like CeO2 started appearing in the early 2000s, which easily outperformed the inert non-reducible supports like alumina and silica.46 Su Kim et al.48 used TiO2 as the support and proved the presence of strong metal support interaction due to reducibility of TiO2 to form Pt-Ov-Ti+3 (Ov: O vacancy) sites which have high potential to donate electrons. The presence of CO adsorbed on Pt-Ov-Ti+3, confirmed by FTIR, supports the redox mechanism as contrary to the Pt/Al2O3 Case. The XPS studies also revealed the presence of two oxidation states of Pt in Pt/TiO2.48 Better mechanistic insights into the role CeO2 (reducible supports) were presented by a series of works that followed. The broad mechanism starts with the exchange of O of CO2 with CeO2 to give CO (direct dissociation of CO2 to CO),57-61 which necessitated the ceria surface to be reduced prior to the reaction. This was followed by subsequent reaction of spilled over hydrogen from the metal with O of CeO2 to regenerate O vacancy (RWGS via redox reaction). In another study, Porosoff et. al. loaded different bimetallic systems on CeO2.51 The activity hiked enormously, with better activity being attributed to the ability of the ceria surface to take away O leading to greater bimetallic bonds. PtCo/CeO2 was found to be the best catalyst towards RWGS. This was due to the low interaction energy between the metallic system and CO, which was corroborated by the d-band picture (Figure 3).51

Figure 3. d-band centre effect on the ratio of CO to CH4 selectivity at 10% CO2 conversion. For ease of comparison, open and solid symbols represent catalysts with and without Ni, respectively. Reproduced with permission from ref.51 Copyright 2013 ELSEVIER. Another class of frequently studied RWGS catalysts is the transition metal carbides (TMCs) which have the unique property in which CO2 adsorption, hydrogenation, and C=O scission takes place in a single phase. Their cost effectiveness and similarity in catalytic/ 8


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ACS Energy Letters

electronic behaviour to precious metals62 have made them attractive towards commercial usage.63 The most potent TMC is Mo2C, due to its high activity and low cost.64 The unique features of Mo2C, like dissociative adsorption of H2, cleavage of C=O and most importantly ability to remove O from Mo2C-O state were studied by temperature programmed surface reaction (TPSR) and ambient pressure XPS (AP-XPS). TPSR measurements were conducted at a ratio of 100 mTorr to 700 mTorr of CO2 to H2. This led to the identification of only CO at the Mo2C surface with the absence of methane (m/e= 15) and methanol (m/e= 31). The interesting observation was that, an exposure of 150 mTorr of CO2 gave a peak at 283.6 eV which corresponds to oxy-carbide (O-Mo-C) in AP-XPS experiment. Also during the reaction at 523K on Mo2C, oxy-carbide was identified as the major species by AP-XPS instead of carbonate, carboxyl or formate, as in the case of metal oxide. Thus the pathway followed was concluded as the direct dissociation of CO2 on Mo2C to give O-Mo-C and CO where O-Mo-C will hydrogenate back to Mo2C and H2O, similar to the redox mechanism occurring on Cu. The formation of O-Mo-C is again similar to reducible metal oxide support. Here the authors further loaded Co on Mo2C and observed improvement in CO to CH4 selectivity. This was attributed to the ability of Co on Mo2C to decompose methane to C and H which was already proved by Izhar et. al..64, 65 In-operando XANES measurement on Co-Mo2C showed the carburization of O-Mo-C to Mo2C, which is the active phase. This re-carburization is sustained due to decomposition of CH4 over Co sites to yield carbon and hydrogen.64 The high activity of TMCs are well validated and explained by Porosoff et al by oxygen binding energy (OBE) on TMC as a descriptor.63 The CO2 gets chemisorbed on TMC and dissociate to give CO and O, the latter of which gets incorporated into TMC forming oxy-carbide. The rate of desorption of O from oxy-carbide is crucial as it gets reduced to water by H2 and reactivates the TMC. The TMC with least OBE was found to have the highest activity. Interestingly, no other methane producing C-H intermediates like formates etc., form on TMCs, thus making them ideal catalysts for selective CO formation. Thus, it has out-performed even Pt/CeO2 in CO selectivity and activity.64 2.2 CO2 to CH3OH CO2 to methanol conversion is one of the primary processes of industrial relevance in the current scenario, where global warming is threatening the existence of human civilization. With a annual global market demand of 70-80 million metric tons, which is expected to increase at a very fast rate, methanol is primarily used for important downstream processes of formaldehyde, methyl acetic acid, dimethyl ether and methyl tertbuthyl ether (MTBE) syntheses. Along with its utilization in the MTO and MTG processes of petrochemical value chain, methanol is also projected as a transportation fuel for the future, or atleast a major fuel substituent.7 This thermochemical process is a combined outcome of three reactions, as shown below, where the preferred direct methanol formation (Eqn. 2) from CO2 may often be accompanied by a combination of RWGS reaction (Eqn. 1) followed by subsequent hydrogenation of CO (Eqn 3) to methanol.66, 67 The relative occurrences of these parallel or consecutive reactions are difficult to be quantified under operating conditions and are generally functions of thermodynamic process conditions and chemical nature of the catalyst. Commercially methanol is produced by a catalytic process reacting syngas which is an optimum mixture of CO, H2, and CO2. CO2 + H2 CO + H2O ------ ∆H298K = 41.2kJ/mol ----- Eqn. 1 CO2 + 3H2 CH3OH + H2O ------∆H298K = -49.5kJ/mol ----- Eqn. 2 CO + 2H2 CH3OH ------ ∆H298K = -90.6kJ/mol ----- Eqn.3 From a thermodynamic point of view of the above reactions (Eqns. 1-3) in terms of enthalpy and entropy considerations, it can be concluded that low temperature and high pressure is favourable for direct CO2 to methanol (CTM) conversion (Table 2). Equation 1 is endothermic but its subsequent hydrogenation to methanol (Eqn. 3) is highly exothermic with

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decreasing entropy in contrast to RWGS where the number of moles remains constant. Thus, the overall process becomes an equilibrium limited one, where thermodynamics and equilibrium shifts interplay to control the conversion and selectivity of the products. One common way adapted for shifting the equilibrium is to remove the products, in-situ, either in form of water or methanol by dehydrating it to dimethyl ether (DME) through an exothermic (∆H= -23.4kJ/mol) process.67 This has been discussed in details in a later section. Till date, the majority of the catalysts reported for CTM are also used to reduce CO to methanol. More than 75% of the catalysts reported over the last 10-15 years are primarily Cubased systems.68 Among those, the Cu-ZnO (CZ) motif is at the core of the industrial catalysts used for CO2 conversion to MeOH. Cu-ZnO-Al2O3 (CZA), the most extensively used industrial catalyst is most conveniently prepared by co-precipitation technique using sodium carbonate as the precipitant and metal nitrates as the precursors. Each step of the coprecipitation procedure (precipitation-aging-washing-drying-calcination and reduction) are represented in Figure 4.69-71 With the classic CTM catalysts, CZ or CZA, also being active for RWGS, the main factors governing the reaction pathway to CO or methanol mostly happens to be the CO2:H2 ratio and the reaction temperatures and pressures, owing to opposing enthalpy and entropy requirements.

Figure 4. Major steps in coprecipitation synthesis of CZA catalyst. Reproduced with permission from ref.71 Copyright 2015 ELSEVIER. One of the primary factors for choosing CZA for industrial applications is its stability which is attributed to the extensive distribution of Cu-NPs by ZnO matrix and inertness of alumina at higher temperature, pressure and chemical conditions.72 The exact mechanism of CTM is an elusive one. There are no conclusive inferences about the nature of the activity of Cu (Cu0 or Cu+1) present at interface.73, 74 The basic step starts with CO2 adsorption on the oxides surface followed by H2 spilling over from Cu sites.75 The critical catalytic process is said to take place at the interface of Cu and M-O. However, the identity of the exact active site and the role of ZnO is still a subject of debate in the catalysis community.76, 77 The sites consisting of Cu-steps decorated by Zn species have been identified as the most probable active site. According to the report by Behrens et. al.,77 Cu-steps act as the active site with a nearby Zn serving as the adsorption site for O-bound intermediates. The 2014 work by Kuld et al. tried to prove that the interaction between ZnO and Cu were wrongly interpreted as 10


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ZnOx on Cu.76, 78-81 However, it was proved that the formation of the CuZn alloy, which was accounted by surface sensitive techniques such as XPS, H2-TPD (Temperature Programmed Desorption), N2O-RFC (Nitrous Oxide - Reactive Frontal Chromatography) and H2-TA and the activity was found to increase upon Zn atom presence near Cu.76 Shyam Kateel et. al.32 while providing a different view to CZA catalytic mechanism, reported that during the reaction, CuZn alloy undergoes oxidation to give ZnO by O* due to dissociation of CO2. This is attributed to the good dispersion of Zn on top of Cu leading to easy oxidation of Zn. The synergy between Cu and ZnO enhances the methanol productivity through formate mechanism which is discussed later.32 However, in a recent work it was shown that regardless of the presence of Zn-species as a promoter, the activity towards methanol formation suffered a significant decrease for particles below 8 nm size, as the smaller particles below a particular limit cannot accommodate the unique configurations like step edge sites.81 There are two main pathways defined for methanol formation from CO2: 1) formate pathway82, 83 and 2) CO pathway through RWGS and subsequent hydrogenation. Some of the representative catalysts and their pathways are included in Table 2.

Figure 5. Potential energy diagrams for the hydrogenation of CO2(g) to CH3OH(g) on (a) Ti3O6H6/Cu(111) and (b) Zr3O6H6/Cu(111) via the RWGS + CO-Hydro and formate pathways. “TS” corresponds to the transition state. Reprinted from ref 28. Formate pathway (Figure 5): In this pathway, CO2 + H2 in the initial steps gives * HCOO, which hydrogenates to produce *H2COO, then *H2CO via HCOOH. Subsequent hydrogenation leads to the formation of *H3CO and H3COH.84 In case of Cu-particles, both supported on SiO2 and unsupported, it was proved that formate behaves as a spectator for methanol synthesis.84 Yang et. al.84 introduced bi-dentate formates on the Cu-catalyst surface by (1) steady-state catalytic condition of CO2 and H2 and by (2) exposure to the formic acid condition. Then they attempted the hydrogenation of bi-dentate formate adlayers on Cucatalyst. The studies on these were monitored by simultaneous Mass and IR spectroscopic

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techniques. It was seen that there was an insignificant amount of methanol formation with bidentate formate and spilled over-H. But when the bi-dentate formates were converted to mono by exposing to O2 or N2O it was observed that subsequent hydrogenation increased methanol yield along with the high production of water as observed from MS data. The methanol formation from CO2 was still attributed to mono-dentate formate hydrogenation to methoxy, with dry H and some water-derived co-adsorbates.84 Powerful theoretical techniques of density functional theory (DFT) were used to further study the formate mechanism on Cu-particles. It was identified that the RDS step for CH3OH formation through formate pathway is conversion HCOO to H2COO.85 But the rate of H2COO dehydrogenation to HCOO was of the order of 2x107-8 higher than the forward step. Also, the decomposition rates of H2COO to CO were very high compared to that of HCOO to H2COO.85 Zhao et. al.86 further proved that the dehydrogenation of H2COO to HCOO takes place before further hydrogenation of HCOO owing to its (H2COO) lower stability as calculated using Periodic DFT. Their calculation further revealed a barrier of 1.17 eV for the transition of H2COOH to H2CO which is thus, kinetically more hindered than that of HCOO to H2COO conversion.86 All the above works confirm that in spite of formate formation on catalyst surface, they mostly act as mere spectators during further reaction pathways of methanol formation. In case of Cu/ZnO (CZ) catalyst, the formate pathway dominates over CO (RWGS).32 For both ZnO/Cu(111) and CuZn(211) systems, the Zn species helps in stabilizing HCOOH via O-Zn interaction which leads to the activation of the HCOO* through hydrogenation (Figure 6a and 6b).32 They also showed that compared to ZnO/Cu, CuZn exhibits lower reaction rate and faster decay of methanol activity. Theoretical calculations showed that the higher energy barrier of *HCOO to HCOOH* transformation (1.19 eV), makes the reverse dehydrogenation more facile owing to a lower energy pathway (0.5 eV). This, in turn, leads to high production of *HCOO which passivates the active sites.32

Figure 6a. Side and top views of DFT optimized geometries of (a) CuZn(211) surface and (b) *CO2, (c) *HOCO, (d) *CO, (e) *HCO, (f) *H2CO, (g) *H3CO, (h) *HCOO, (i)*HCOOH, (j) *H2COOH, (k) *CH3OH, (l) *OH, (m) *H2O, (n) *O adsorbed on the CuZn(211) surface. * = adsorbed species. Cu: reddish-orange, Zn: dark blue, O: red, C: grey and H: white. Reproduced with permission from ref. 32 Copyright 2017 Science. Employing supports like TiO2, ZrO2 or both TiO2-ZrO2 increased the dispersion of Cu and adsorption capacity of CO2 and H2 due to increased basicity, which showed a direct linear relationship with the methanol yield.87 The increase in surface area of Cu was in the 12


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order of ZnO95% conversion & 100% selectivity 90% conversion & 100% selectivity

Mechanism assigned Typical mechanism of adsorption of CO2 by MSN as carbonates & reduction of it by Ni by spillover -

-

91.1% conversion & 100% selectivity

Via CO route

80% conversion & 100% selectivity Almost 76% conversion & >98% selectivity 85% conversion and 85% selectivity 17% conversion and 100% selectivity

Via carbonates and formates

-

Via carbonates and formates

Via CO route

Reactor innovations Many of the recent works in the CO2 reduction field has focused on innovations in the reactor design, particularly in the CTM/syngas to methanol technology, which has been briefly discussed in this section. Considering the important aspects of conversion and selectivity, like the efficient removal of reaction heat, high conversion at low-temperature equilibrium, and recycling of the unreacted feed, many groups have reported different reactor

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designs with varying catalyst bed arrangements.57 Three commonly used industrial CTM reactors are (a) Tubular boiling water reactor applied by Lurgi, (b) series quench reactor designed by ICI’s Synetix (presently Johnson Matthey) and (c) Spherical type series adiabatic reactor utilized by the Kellogg and Haldor-Topsøe.68 The Lurgi reactor, which is costintensive owing to design complexities, is of isothermal nature, where the catalyst bed temperature is regulated by circulating boiling water, yielding high conversion and catalyst lifetime (5 years). The series quench adiabatic reactor, developed by Synetix, having multiple catalysts beds, though relatively cheap, has disadvantages of feed gas dilution (due to cooling gas), higher by-product formation and inefficient catalyst utilization.147 The spherical adiabatic series reactor of Kellogg and Haldor-Topsoe presents the advantages of lower cost, low pressure drops, and high production rate of methanol over conventional cylindrical reactors.68 Reactors for liquid phase methanol synthesis, though not commercially used, have been developed by Air Products and Chemicals. In this type of reactor, reaction heat is efficiently removed by an inert oil containing the powdered catalysts. The high conversion rates in this system suffice for a single pass process. Fluidized beds have been utilized efficiently for multiphase exothermic reactions such as the catalytic hydrogenation of CO2, owing to its higher mass and heat transfer capacity in comparison to other contacting modes. For example, Fe/Cu/K/Al catalyst, used for CO2 conversion through RWGS showed substantially higher activity (46.8%) in a fluidized bed reactor than that in the fixed bed reactor (32.3%). In an attempt to combine catalytic conversion with separation properties, Gallucci et al. (2004)148 showed using a zeolite membrane reactor, that CO2 conversion was enhanced in terms of both (XCO2 = 11.6%), methanol selectivity (SMeOH = 75%) and yield (YMeOH = 8.7%) at T = 206 °C than in a conventional reactor (XCO2 = 5%, SMeOH = 48% and YMeOH = 2.4%) at T = 210 ◦C, P = 2 MPa, H2/CO2 ratio = 3 and space velocity of 6000 1/h. This happened due to the shift in the equilibrium because of selective removal of liquid products (methanol and water) from the reaction system by the zeolitic separation membranes. By reactor simulations, Barbieri et. al. (2002)149 showed that at similar reaction conditions (T = 210 °C and P = 1 MPa), reactors with an organophilic (X = 22.7%, SMeOH = 60.2%, YMeOH = 13.7%) and hydrophilic membranes (X = 23.9%, SMeOH = 54.2%, YMeOH = 13%) performed better than that of a conventional tubular reactor (X = 14.2%, SMeOH = 40.5%, YMeOH = 5.8%). It was shown that a membrane reactor leads to increased methanol yield, thus decreasing the consumption of reactants and facilitating operation at lower pressures and higher temperatures. This, in turn, enhances the kinetics of the reaction by reducing residence time and the reactor volume. Rahimpour (2008)150 investigated a two-stage catalyst bed reactor for methanol production from CO2, where in the first catalyst bed, the synthesis gas was partly converted to methanol in a conventional water-cooled reactor, operating at higher temperature and high yield mode. In the 2nd bed of lower reaction rate, the feed gases to the first bed were pre-heated using the heat of the reaction. The constant reduction in temperature of this bed pushed the thermodynamic equilibrium. The lower heat of reaction due to slow reaction rates resulted in milder temperature profiles in the 2nd bed. Thus catalyst lifetime increased due to milder reaction temperatures, which avoided sintering of particles. Thus, the two-stage reactor proved to be more efficient for methanol formation, than the single-bed one, due to higher conversion rates and enhanced catalyst lifetime.8 Recycled reactor for CO2 conversion to MeOH surpasses the traditional fixed-bed tubular reactors by high margins by mitigating the effects of thermal conflicts along the reactor. Using the common catalyst (Cu/Zn/ZrO2), the effects of operating conditions were compared in a fixed-bed and recycle reactor where the latter showed higher CO2 conversion and product selectivity at milder temperature and higher pressure conditions.4 As mentioned previously, Rahimpour group attempted at overcoming the thermodynamic limitation,

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enhancing the reaction kinetics, and controlled the stoichiometric feeds for CO2 hydrogenation in a membrane dual-type reactor. Owing to the possibility of equilibrium shift by continuous removal of products, many scientists have studied the use of membrane reactor (MR) in methanol production. A combination of experimental and modeling study showed that MR gives better conversion and selectivity than the conventional fixed bed reactor under the same experimental conditions.151 However, a big disadvantage with MRs is their limitation of working temperature, which is ≤ 200 °C. Thus the development of hightemperature membranes has the potential to revolutionize the CO2 hydrogenation process through more efficient MRs. Chen et. al. coupled experimental data with the simulation of CTM in a silicone rubber/ceramic composite MR to show an enhancement in the conversion by 22% compared to the fixed bed one.152 Moreover, many groups have reported enhancements in methanol selectivity and yield in MR with the various types of membranes (e.g., ceramic zeolite).148, 149, 153 Many groups have also resorted to the use of the liquid medium to overcome the thermodynamic limitations of MeOH synthesis.154 MeOH synthesis in liquid medium has the leverage of high heat transfer efficiency, enhanced CO2 adaptability, high single run conversion efficiency, and low operational cost.155-158 Liu et al. developed a novel low temperature (170 °C) but high pressure (5 MPa) method for high conversion of CO2 (25.9%) with good and methanol selectivity (72.9%) in a semi-bath autoclave reactor over copper catalyst.159 The promotional effect of Cr, Zr, and Th doping in ultrafine copper boride catalysts (Me–CuB (Me: Cr, Zr, Th)) in liquid phase CO2 hydrogenation for enhanced methanol production was demonstrated by Liaw et al.160, 161 All these studies demonstrate that there is still enormous scope in development of reactors and processes for CO2 hydrogenation, which can make the technology more efficient and commercially viable. 3. Industrial Status of CO2 Hydrogenation Technology With the aggravating environmental impact of global industrialization and the growing promise of the CO2 reduction technology for mitigation of climate change, there have been tremendous global endeavors to commercialize this technology, particularly that to methanol and methane. Pilot scale production of MeOH using CO2 and H2 as raw materials was demonstrated by Lurgi in 1994.7 Around 35-45% methanol conversion with high selectivity was achieved using a particular Cu/Zn/Al catalyst obtained from Süd-Chemie (presently Clariant), at 260 °C and 60 bar of pressure. Two Japanese institutes, NIRE and RITE, in 1996 demonstrated an upper bench scale (50 kg/day) CO2 reduction process with high MeOH selectivity, using multicomponent Cu/ZnO/ZrO2/Al2O3/SiO2 catalyst at high temperature (200-270 °C) and pressure (30-50 Bar).7 Mitsui Chemicals and Carbon Recycling International (CRI) are two very well-recognized and initial companies that demonstrated the conversion of sustainable CO2 to MeOH at industrial scales using H2 as secondary raw material. Mitsui Chemicals is claimed to have invented catalysts operating at the scales of 100 tons of methanol per year from CO2 released in ethylene production and H2 generated by photochemical water splitting.162 CRI in Iceland considered as one of the global leaders in commercializing CO2-to-methanol processes, have built a 4000 tons per year methanol plant, where the CO2 captured from the flue gas of an adjacent geothermal power plant is reduced by H2 produced by water electrolysis powered by green geothermal energy. The estimated capital expenditure for a CO2-hydrogenation methanol plant is similar to that of a conventional syngas plant. However, extensive commercialization of CO2 to methanol technology is limited by weak economic viability, owing to high price and quantitative requirement of H2 in the process and the current low price of methanol owing to cheap natural gas. For the technology to be commercially successful, the production price of methanol has to be less than its current market price, which is about 480 $/ton (US market price). Of course, secondary factors like cheap accessibility and advancement of renewable energy technologies, additional social regulations on fossil fuels to mitigate climate change

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and general introduction of a strict carbon tax, will foster the growth and maturation of carbon neutral/negative CO2-to-methanol and CO2-to-DME technologies. Blue Fuel Energy in Canada is another company trying to develop CTM technology using hydroelectricity and concentrated CO2 emissions from natural gas. The OTTO-R platform (Technical University of Freiberg) proposed to form MeOH using CO2, H2O and renewable electricity and use it for downstream MTG.7 Thus, the last decade witnessed a widespread industrial interest in the development of economical CTM technologies in China, Australia, the EU and other countries, in conjunction with a huge global research interest.3 The Korean Institute of Science and Technologies developed the CAMERE process, which produces methanol in two steps using CO2 and H2. Mitsubishi Hitachi Power Systems Europe (MHPSE) is setting a methanol plant at the Lünen power plant, Germany, owned by STEAG GmbH. KOGAS Corp in Korea is developing the technology for CO2 to DME conversion. Haldor Topsoe, a Danish company is developing the technology for electrochemical conversion of CO2 to CO. The presence of these plants and endeavours allows us to conclude the Technology Readiness Level (TRL) for MeOH production from CO2 is in the range of 6-7. The industrial relevance of the CO2 methantion process is evident from the large emerging number of pilot scale endeavours mainly across Europe and centering around Germany. This is believed to be instigated by the increasing importance and awareness of renewable energy utilization and storage in these countries. A very interesting example of success along this line of thought is the Audi E-Gas project achieving a scale of 6.3 MW.163 This is a very innovative and comprehensive project where, a plant producing methane form renewable hydrogen (alkanine water electrolysis) and biomass-derived CO2 feeds exactly the same amount of natural gas into the German national grid, as that consumed by cars (recorded through the Audi “e-gas fuel card” system) from various gas stations, thus generating a parallel and almost CO2 neutral transportation infrastructure. 4. Life Cycle Assessment of CO2 hydrogenation Technology Life Cycle Assessment (LCA) studies on CO2 conversion technologies are essential in determining important targets in the course of its development. However, till date, there have been very few comprehensive LCA studies on CO2 reduction technology, which present a complete analysis of the environmental benefits and challenges focusing on energy balances and global warming impact (GWI) reduction. GWI, though a very important metric, is not the only performance measure that should be considered for a complete LCA study. Interestingly, the existing integrated LCA studies for CO2-conversion technology demonstrates that current state-of-the-art conversion processes can provide benefits beyond GWIs and simultaneously reduce a broad spectrum of environmental impacts. However, it has also been shown that use of electrolyzers in syngas technologies may lead to trade-offs between environmental impacts. CO2-based fuel productions in its present form can also lead to increased environmental impacts. This scenario can, however, be reversed by integrating renewable energy technologies with the CO2 to fuels supply chain. This, in future, necessitates comprehensive and integrated LCA studies on such technologies, going beyond the conventional GWI assessment as the singular determining metric. Remarkably, as a counterintuitive fact of CO2 being a highly thermodynamically stable compound, the available results show that there can be methods where CO2 conversion processes may potentially reduce global warming impacts over fossil fuel based energy systems. In many of these technologies, using CO2 as the direct carbon source can significantly reduce the number of conversion steps of the conventional process using naturally occurring methane as the starting point. This can result in an advantageous balance of the environmental impacts. In simple words, though CO2 conversion to chemicals cannot form a carbon sink over the life cycle of the products, which eventually produces equivalent

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amount CO2 at the cycle’s end, it can lead to more environmentally benign production processes.7 The use of CO2, even if not completely embodied, as a C1 building block for the catalytic construction of chemicals presents the potential to create entirely new synthetic pathways substituting the conventional methodologies. Classical green chemistry principles such as substitution of hazardous reagents and atom economy signify a very potent effect of these direct CO2 routes. Conventional synthesis of carboxylic acids from chlorinated substrates through Grignard reaction or methylation by methyl iodides are golden examples of such cases. The current global environmental crisis deserves experimental validation and accurate quantitative LCA studies of such class of technologies, which have been theoretically predicted to have positive environmental impacts. However, CO2 to chemicals technology is severely limited by their global market size, in alleviating the huge scale of global warming from extensive anthropogenic CO2 emission. Mitigation of an impactful amount of GHG emissions could only be achieved by reducing CO2 from highly concentrated sources with carbon-free hydrogen or electrons from renewable power. Modern world’s diverse energy infrastructure necessitates a regional differentiation and a specific outlook for individual scenarios that set the boundary conditions of the assessment. For example, CMT technology by CRI in Iceland exhibits a low and acceptable carbon footprint owing to accessibility to low-carbon electricity in the region. Thus, while power-to-methanol provides promise in geographical areas of accessible and economically viable renewable electricity, it would turn counterproductive in a strictly fossil-based energy infrastructure. This effect aggravates when MeOH is consumed into the fuel industry, as compared to the chemical industry sector, owing to the differential life cycle of the embodied CO2. Similar to the case of CO2 to chemicals, CO2 to fuels, also, cannot provide a carbon “sink” over their complete life cycle. But it can provide very substantial environmental benefits by reducing the GWIs as compared to the current technologies in a well-to-wheel analysis. Thus, instead of being a CO2 mitigation strategy, CO2 to fuels eventually will pose itself as a process of harvesting renewable electricity to the transportation sector. Thus one day in future, CO2 to fuels/chemicals technology has to compete with potential parallel technologies exploiting renewable electricity, such as green vehicles, fuel cells, or thermoelectrics, which can offer higher reductions of CO2 emissions per unit of renewable energy used. However, as discussed earlier, CO2 reduction efficiency will not be the only factor determining a technology’s social acceptance, sustainability and overall environmental impact. It will also depend on its adaptability to existing energy infrastructures, economic viability, convenience and efficiency of usage, product market size, energy security & sustainability, scalability and indirect emission & side hazards. Such additional aspects will become un-foreseeingly critical, in addition to its direct environmental impact, for societies to decide about the implementation and integration of CO2 conversion technologies into the industrial supply chain grid.7 LCAs performed by Aresta et al.68 to identify the most sustainable and green process of MeOH synthesis comparing syngas technology and CO2 capture-hydrogenation showed that the latter process is way greener and less energy expensive than steam reforming of natural gases. The most environmentally benign process involved reduction of recovered CO2 from flue gas with H2 obtained from photovoltaic empowered water electrolysis. In another extensive LCA analyses by von der Assen et al.164 of potential methanol production processes, wind energy supported electrolyzers for H2 production proved to be more beneficial than the photovoltaic process or steam reformation. The capture of CO2 from concentrated emission sources has a more negative GW value than normal air capture, owing to less energy requirements. Broadly speaking, the carbon footprint of the overall process primarily depends on the choice of various energy sources in its different subsection.

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Interestingly, though the CTM technology has the potential to be greener than the convention methanol technology, with a predicted GW value of half of the latter, the use of conventional H2 from steam reforming makes it 3 times more CO2 emitting. From various comparative LCA studies done on CTM technology, it is evident that approximately 60−70% of the total energy consumed was used for H2 production, inferring that H2 generation process will eventually determine the environmental impact, energy consumption, cost and commercial viability of the overall process.68 There are several LCA analyses available165-168 for direct thermochemical hydrogenation of CO2 to methanol. The GWIs for CO2-based methanol technology varies widely from −1.7 to +9.7 kg of CO2 equiv/kg of methanol. Solar driven thermochemical process (solar heat) has the lowest GWI. Production of methanol from syngas coelectrolysis technologies shows GWIs between −1.4 to +4.3 kg of CO2 equiv/kg of methanol. GWIs of direct hydrogenation varies between −1.6 to +9.7 kg of CO2 equiv/kg of methanol. In the latter case, lowest GWIs are exhibited by wind-electricity powered hydrogen production by water electrolysis and highest GWI for water electrolysis powered by coal electricity. The present available data indicates suggest that the methanol production from hydrogenation of CO2 show a negative GWI within the range of −1.2 to −1.3 kg of CO2 equiv/kg of CH3OH if renewable electricity is used for electrolysis/coelectrolysis as compared to the positive range of GWI +0.7 to +1.1 kg of CO2 equiv/kg of CH3OH for the conventional fossil-fuel based technology.7 On the contrary, use of non-renewable fossilenergy for electrolysis or H2 from steam reforming renders CTM a higher GWI compared to the conventional process. With an abundant availability of renewable electicity, the carbon footprint of the RWGS mediated syngas production followed by conventional process and the direct CO2 hydrogenation will be almost equally impacted as both technologies are suitable for adapting the the new energy infrastructure. However, other factors like operational conditions, scale of operations, and complexity of the process flow will manipulate the economic and ecological outcomes. Though there are no direct LCA analyses available for CO production, there are studies on syngas production (H2/CO = 3) considering CO and H2 as individual products, both from direct CO2 hydrogentation (RWGS) and from the dry methane reforming process (DRM). Considering similar gate boundaries and operations scales, a comparative LCA analyses of the RWGS, DRM an fossil-based steam methane reforming pathways were provided in the comprehensive review by Artz et al.7 A electrolyser based H2 production unit was considered for both RWGS (no H2 production) and DRM (insufficient H2 prodcution, H2/CO Rh > Ni > Co > Os> Pt > Fe > Mo > Pd > Ag.194 when unsupported metal is considered , while the order slightly changes (Ru > Fe > Ni > Co > Rh > Pd > Pt > Ir)195 when specific active metal surface area is considered on supports. The selectivity trend was found to follow the order Ni > Co > Fe > Ru.196 Thus Ru, though the most active metal loses to the natural industrial choice, Ni in terms of selectivity and cost. In terms of stability, Mo among all these metals shows highes sulfur tolerance.196 though its low conversion efficiency and higher selectivity towards C2+ hydrocarbons (like iron) affects its potential as a suitable methanation catalyst. Co having an almost comparable activity for methanation to Ni, again is more expensive than the latter. The most used and suitable supports for CO2 hydrogenation to methane are γAl2O3, SiO2, and TiO2 owing to their thermal stability at elevated temperatures and CO2 adsorption properties.197 A class of mesoporous supports like mesostructured silica nanoparticles (MSNs), MCM-41, HY-Zeolites, SBA-15, have also been extensively explored recently with MSNs emerging as the champion of activity and stability, owing to the high concentration of accessible basic sites in their porous, high surface area and stable architectures.198 Typical promoters shown to enhance the methanation acitivity till now are Na, K, V, or La197, 199, 200 where in one case K showed to divert the selectivity towards higher HCs.201 MgO doping was found to increase thermal stability and resist coke formation202 while ceria doping enhanced reducibility and stability,203 both promoting the overall Ni activity. The primary challenge for future studies in catalyst design for the CO2 methanation process lie in the aspect of stability which is ocassionally affected by coke formation, sulfur poisoning (from reactant feed), attrition and thermal stress. Methanol Cu/ZnO are the motif used for methanol similar to CO, but in contrast with CO, methanol production is favoured by 3:1 ratio of H2 to CO2, temperature around 200-300°C with very high pressure of 30-50 bar. The easiest way of catalyst synthesis for industrial application is co-precipitation.67 The synergy between Cu-ZnO was identified as enhancing factor for methanol production in formate mechanism (CZA catalyst proceed majorly through formate route), though there were proof of CuZn alloy formation.28 This CuZn alloy gets oxidized and gives Cu-ZnO the active component.28 Better methanol selectivity was observed in Rh-system on MgO, ZrO2, and TiO2 operated between 100 and 300 °C.204 In case of commercial Cu/ZnO catalyst, use of solid oxide supports like Ga2O3, ZrO2, and Cr2O3 produced greater catalytic stability.205, 206 due to stabilization of an intermediate surface Cu oxidation state (between 0 & +2) induced by particle size.503 Hydrophobic silica supported Cu/ZnO/Ga2O3 achieved >99% methanol selectivity at temperatures ranges of 250−270 ° C.207, 208 Importance of stepped Cu surfaces and bulk defects inducing better intermediate binding and lower transition energy barriers for better methanol activity was demonstrated using Cu/ZnO/Al2O3 by Behrens et al.77, 209 Zn substitution at these step sites of Cu had additional promotion effect on the activity. In studies using various substituents (B, In, Mg, Ga, M, Y, Mg) in Cu/ZnO/ZrO2 system, Ga once again proved to be the best candidate for promoting selective methanol activity with the highest yield.210, 211 Exploration of different supporting materials (TiO2, SiO2, Al2O3)3, 161 elucidated that reducibility of the oxide matrix plays a crucial role in the overall activity and greatly enhaces the catalyst stability of heterogeneous systems. Though late transition metal based systems like those of Pd,212 Pt,213 33



and Ni91 and transition-metal carbides19 have ocassionally proved to be good catalysts, replacing Cu/Zn at the industrial scale demands further innovations in catalyst design and engineering. However, uniques reports of catalysts like those of Ni5Ga3 by Studt et al.91 or Pd2Ga214 show that there is still a positive scope for catalysts research in this century long field of CTM technology for achieving better conversions and higher selectivity at less energy consuming milder operating conditions. DME All the Cu/ZnO systems physically mixed with acidic materials like γ-alumina and HZSM5 act as excellent catalysts for direct CO2 conversion to DME. In γ-alumina, the Lewis acidic sites, which are the primary condensation sites are often prone to poisoning by water produced in the methanol process. This poisoning effect due to higher binding affinity of these sites to H2O, highly degrades the DME production. This issue was solved using HZSM5, having a higher density of Bronsted acidic sites which are highly selective to methanol over water. The acidic sites in these systems are, thus not poisoned giving stable DME yields.98 The loading arrangement of the bifunctional catlysts on fixed bed reactors dictates the conversion highly where the homogeneous mixing of pre-pelletized bifunctional motifs are best due to the physical proximity of methanol synthesizing and dehydrating units. 105, 108-110

5.2. (a)

Overall Technology Development More efficient Carbon Capture Technology The success of CO2 reduction technology is largely dependent on the commercialization of efficient and economic carbon capture technologies. As discussed in the LCA section, the energy expense of CO2 capture from concentrated sources, e.g. flue gas from natural gas or thermal power stations, or steel industry is much less than that of atmospheric capture. The carbon capture technologies must assure high percentages of CO2 capture from flue streams having high dry weight purity at a minimal expense of energy and finance. At the same time, these technologies should have the potential to operate at the industrial scale of time and production quantity. Though atmospheric carbon capture is the ultimate solution and is indispensable in the future, localized CO2 capture from emission sources poses to be more feasible at current timescales. There has been a significant global advancement in carbon capture technologies starting from membrane to resin/solvent-based solutions. The cost of CO2 coming out of the capture plant is crucial to determine the nature of the CO2 utilization technology and its commercial viability. For example, it is believed that a capture technology producing sufficiently pure CO2 at a rate of approximately 20 $/ton operating at commercial scale may boost the growth of a parallel petrochemical industry, based on CO2 as the primary raw material.68 The current rates of CO2 from different capture technologies varies as a function of the flue stream composition, purity of the output CO2 and scale and geographical location of the capture plant. However, capture technologies producing CO2 at competitive prices of 40-100 $/ton do exist in the current markets, which already reflects the technological readiness level of such technologies. It is, however, equally important to continue research and development of such technologies particularly in the frontiers of better membranes, solvent designs, and heat integration processes. Parallel research on atmospheric CO2 capture should also proceed using adsorption and absorption techniques empowered by renewable electricity, to turn this potential solution into a commercial process in the years to come. (b) Carbon-free hydrogen production Cost and indirect emission of hydrogen production for reducing CO2, are probably the most important factors determining the commercial viability and environmental impact of thermochemical CO2 reduction technology. In the current scenario, H2 is majorly produced

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from hydrocarbon-based feedstocks (steam reforming of CH4, coal gasification and partial oxidation of light oil residues), which have substantial indirect or direct CO2 emission. For CO2 reduction to become carbon-neutral or carbon-positive technology, large-scale utilization of CO2-free hydrogen coming from renewable energy sources is of utmost importance. Renewable or green hydrogen can be produced from biomass gasification, electrolysis of water using photovoltaic or wind energy sources. However, the extensive use of such H2production technologies is hindered by commercial availability, scalability of onsite production and most importantly the high price of the produced H2 and capital cost of the technology. The current market price of H2 from cheap natural gas and coal plants varies between 1-3 $/kg, while that of H2 from renewable energy sources varies between 4-10 $/kg, depending on the energy source. Though H2 from wind power or biomass is comparatively cheaper, these suffer from the disadvantages of sustainability, intermittency and geographical limitations. Thus, H2 from water electrolysis using solar power appears as the most practically viable solution among the various renewable H2 production technologies. It is estimated that a production cost of $2.75/kg or below for renewable H2, will make CO2 reduced fuels/fuel substituents cost-competitive with gasoline and a CO2 based chemical production technology economically viable.6 Thus research and innovation in the field of green hydrogen production through sustainable, cost-effective and scalable pathways will directly determine the fate of the CO2 to fuel/chemical technology. (c) Carbon-footprint of the integrated technology As discussed in the LCA analyses section, comprehensive studies are essential to determine the targets at various developmental stages of the CO2 reduction technology. While the overall technology has the potential to have a positive environmental impact, it is severely dependent on the integration of secondary technologies like green hydrogen production, renewable electricity and solar thermochemistry for a carbon neutral/positive output. Detailed LCA incorporating GWI calculations, lifetime of embodied CO2, sustainability, indirect emissions and competence with parallel renewable technologies, will determine the overall environmental effect that this process can bring in. (d) Techno-economic viability Techno-economic viability is perhaps the most critical concern, over the CO2 hydrogenation technology. While the capital investment of this technology is similar to that of conventional syngas technology, the operational cost in the current scenario is unfavourable primarily due to high hydrogen requirements and involvement of substantial amount of electricity. Also, the cost of CO2 capture per unit mass of the raw material is crucial in determining the economic viability of this technology. The current price of MeOH being very competitively placed owing to cheap natural gas availability, innovations and economic analyses on several fronts are required to make this technology commercially viable. The current technological readiness level of CTM technology is estimated to be 6-7. The CTM technology, if used extensively instead of the conventional plant has the potential to impact a CO2 change (reduction) of 77 %, due to decrease in direct CO2 emissions.215 However for the entire technological process to be environmentally benign, introduction of renewable energy is an indespensible necessity. The primary power consumer in the overall technology are the electrolysers and the compressors.215 Thus innovative research developments to decrease electrolyser power consumption and capital cost in necessary. The opex for the CTM technology exceeds the benefits by a large margin, primary controlled by the electicity cost. In a very comprehensive technoeconomic analyses by Mar Pérez-Fortes and Evangelos Tzimas,215 it has been shown that under different sensitivity analyses, electricity and methanol prices are the key factors behind the economic competitiveness (zero Net Present Value-NPV) of the technology in the current market. They showed and we quote that “Prices of electricity lower than EUR 9/MWh, prices of MeOH higher than EUR 1378/t (reference

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market price, EUR 350/t), or an income from feedstock CO2 higher than EUR 665/t, would allow a positive NPV for the MeOH CDU plant”.215 In a bivariate analysis it was shown that with a reduced electricity price of EUR 14/MWh, the plant operations is sufficient tp payr hor the utilized CO2, while with "free" electricity, the methanol selling price can be lower than the market price (240€/ton).215 Thus for the overall economic success of the technology, substantial decrease in prices of electricity & steam, better process integration and higher revenues for converted CO2 and products are necessary factors. More efficient catalyst discovery, innovative reactor designs, less power-intensive H2 generation technologies are also crucial factors governing the profitability of the process. In comparison to competing Power-to-X technologies, operating cost decrease though extensive research and development should also be achieved for the overall growth of the technology readiness level. An annual global increase in methanol demand, and its utilization in different transportation sectors, fuel cells and residential energy consuming appliances will prove beneficial for the immediate market penetration of this technology. Along with these, governmental initiatives for curbing CO2 emissions and development of methanol-based economy will also foster the market demand growth of such non-conventional technologies based on CO2 utilization.

Soumyabrata Roy received his bachelors degree in Chemistrty from Presidency College, Kolkata, India and pursued his Masters Studies in Chemistry major, from Indian Institute of Technology, Kharagpur. He is currently a senior research fellow in the group of Prof. Sebastian C. Peter, in the New Chemistry Unit, JNCASR, Bangalore, India.

Arjun Cherevotan received his BSc Chemistry Degree from Kannur University, Kerala in the year 2013 and completed his MSc Degree from National Institute of Technology, Karnataka in 2015. He joined Professor Sebastian C. Peter group of JNCASR at 2016 July after qualifying National Level exam (UGC-JRF).

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Sebastian C. Peter received his M.Sc degree from Calicut University (India), and M.Tech degree from CUSAT (India). He received his PhD degree from University of Münster, Germany in 2006 and was a postdoctoral researcher at Max Plank Institute for Chemical Physics of Solids, Dresden, Germany and Northwestern University, USA. He is presently an Associate Professor at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. His research interests include fuel cells and CO2 reduction. http://www.jncasr.ac.in/sebastiancp/ Acknowledgments We thank Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) and the Department of Science and Technology, India (DST), DST nanomission (Grant Number: SR/NM/NS-1125/2015(C)) and Technical Reeseach (TRC) Centre in JNCASR for financial support. SR and ACH thank University Grant Commission for the research fellowship. References: (1) Catizzone, E.; Bonura, G.; Migliori, M.; Frusteri, F.; Giordano, G. CO2 Recycling to Dimethyl Ether: State-of-the-Art and Perspectives. Molecules 2018, 23, 1-28. (2) Zhao, G. X.; Huang, X. B.; Wang, X. X.; Wang, X. K. Progress in Catalyst Exploration for Heterogeneous CO2 Reduction and Utilization: A Critical Review. J. Mater. Chem. A 2017, 5, 2162521649. (3) Goeppert, A.; Czaun, M.; Jones, J. P.; Prakash, G. K. S.; Olah, G. A. Recycling of Carbon Dioxide to Methanol and Derived Products - Closing the Loop. Chem. Soc. Rev. 2014, 43, 7995-8048. (4) Saeidi, S.; Amin, N. A. S.; Rahimpour, M. R. Hydrogenation of CO2 to Value-added Products-A Review and Potential Future Developments. J. CO2. Util. 2014, 5, 66-81. (5) Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114, 17091742. (6) Porosoff, M. D.; Yan, B. H.; Chen, J. G. G. Catalytic Reduction of CO2 by H2 for Synthesis of CO, Methanol and Hydrocarbons: Challenges and Opportunities. Energ. Environ. Sci. 2016, 9, 62-73. (7) Artz, J.; Muller, T. E.; Thenert, K.; Kleinekorte, J.; Meys, R.; Sternberg, A.; Bardow, A.; Leitner, W. Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment. Chem. Rev. 2018, 118, 434-504. (8) Jadhav, S. G.; Vaidya, P. D.; Bhanage, B. M.; Joshi, J. B. Catalytic Carbon Dioxide Hydrogenation to Methanol: A Review of Recent Studies. Chem. Eng. Res. Des. 2014, 92, 2557-2567. (9) Kaiser, P.; Unde, R. B.; Kern, C.; Jess, A. Production of Liquid Hydrocarbons with CO2 as Carbon Source based on Reverse Water-Gas Shift and Fischer-Tropsch Synthesis. Chem-Ing-Tech 2013, 85, 489-499.

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(29) Kattel, S.; Yu, W. T.; Yang, X. F.; Yan, B. H.; Huang, Y. Q.; Wan, W. M.; Liu, P.; Chen, J. G. G. CO2 Hydrogenation over Oxide-Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity. Angew. Chem. Int. Edit. 2016, 55, 7968-7973. (30) Zhou, X. W.; Qu, J.; Xu, F.; Hu, J. P.; Foord, J. S.; Zeng, Z. Y.; Hong, X. L.; Tsang, S. C. E. Shape Selective Plate-Form Ga2O3 with Strong Metal-Support Interaction to Overlying Pd For Hydrogenation of CO2 to CH3OH. Chem. Commun. 2013, 49, 1747-1749. (31) Wang, J.; Funk, S.; Burghaus, U. Indications for Metal-Support Interactions: The Case of CO2 Adsorption on Cu/ZnO(0001). Catal. Lett. 2005, 103, 219-223. (32) Kattel, S.; Ramirez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active sites for CO2 Hydrogenation to Methanol on Cu/ZnO Catalysts. Science 2017, 355, 1296-1299. (33) Larmier, K.; Liao, W. C.; Tada, S.; Lam, E.; Verel, R.; Bansode, A.; Urakawa, A.; Comas-Vives, A.; Coperet, C. CO2-to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface. Angew. Chem. Int. Edit. 2017, 56, 2318-2323. (34) Kattel, S.; Yan, B. H.; Chen, J. G. G.; Liu, P. CO2 Hydrogenation on Pt, Pt/SiO2 and Pt/TiO2: Importance of Synergy between Pt and Oxide Support. J. Catal. 2016, 343, 115-126. (35) Mehta, P.; Greeley, J.; Delgass, W. N.; Schneider, W. F. Adsorption Energy Correlations at the Metal-Support Boundary. Acs Catal. 2017, 7, 4707-4715. (36) Chen, C. S.; Cheng, W. H.; Lin, S. S. Study of Iron-promoted Cu/Sio2 Catalyst on High Temperature Reverse Water Gas Shift Reaction. Appl. Catal. A-Gen. 2004, 257, 97-106. (37) Gines, M. J. L.; Marchi, A. J.; Apesteguia, C. R. Kinetic Study of the Reverse Water-Gas Shift Reaction over CuO/ZnO/Al2O3 Catalysts. Appl. Catal. A-Gen. 1997, 154, 155-171. (38) Shen, G. C.; Fujita, S. I.; Takezawa, N. Preparation of Precursors for the Cu/ZnO Methanol Synthesis Catalysts by Coprecipitation Methods - Effects of the Preparation Conditions Upon the Structures of the Precursors. J. Catal. 1992, 138, 754-758. (39) Conner, W. C.; Falconer, J. L. Spillover in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 759788. (40) Arunajatesan, V.; Subramaniam, B.; Hutchenson, K. W.; Herkes, F. E. In situ FTIR Investigations of Reverse Water Gas Shift Reaction Activity at Supercritical Conditions. Chem. Eng. Sci. 2007, 62, 5062-5069. (41) Wang, X.; Shi, H.; Kwak, J. H.; Szanyi, J. Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies. Acs Catal. 2015, 5, 6337-6349. (42) Chen, C. S.; Cheng, W. H.; Lin, S. S. Study of Reverse Water Gas Shift Reaction by TPD, TPR and CO2 Hydrogenation over Potassium-Promoted Cu/Sio2 Catalyst. Appl. Catal. A-Gen. 2003, 238, 55-67. (43) Kusama, H.; Bando, K. K.; Okabe, K.; Arakawa, H. CO2 Hydrogenation Reactivity and Structure of Rh/SiO2 Catalysts Prepared from Acetate, Chloride and Nitrate Precursors. Appl. Catal. A-Gen. 2001, 205, 285-294. (44) Lu, B. W.; Ju, Y. W.; Abe, T.; Kawamoto, K. Dispersion and Distribution of Bimetallic Oxides in SBA-15, and Their Enhanced Activity for Reverse Water Gas Shift Reaction. Inorg. Chem. Front. 2015, 2, 741-748. (45) Lu, B. W.; Kawamoto, K. Direct Synthesis of Highly Loaded and Well-dispersed NiO/SBA-15 for Producer Gas Conversion. Rsc Adv. 2012, 2, 6800-6805. (46) Goguet, A.; Meunier, F. C.; Tibiletti, D.; Breen, J. P.; Burch, R. Spectrokinetic Investigation of Reverse Water-Gas-Shift Reaction Intermediates over a Pt/CeO2 Catalyst. J. Phys. Chem. B 2004, 108, 20240-20246. (47) Kim, S. S.; Park, K. H.; Hong, S. C. A Study of the Selectivity of the Reverse Water Gas-Shift Reaction over Pt/Tio2 Catalysts. Fuel Process. Technol. 2013, 108, 47-54. (48) Kim, S. S.; Lee, H. H.; Hong, S. C. A Study on the Effect of Support's Reducibility on the Reverse Water-Gas Shift Reaction over Pt Catalysts. Appl. Catal. A-Gen. 2012, 423, 100-107.

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TOC

Graphical Abstract Catalytic conversion of CO2 into chemicals and fuels is a “two birds one stone” approach towards solving climate change problem and energy demand-supply deficit in the modern world.

Quotes to highlight in paper 1. Catalytic conversion of CO2 into chemicals and fuels is a “two birds one stone” approach towards solving climate change problem and energy demand-supply deficit in the modern world. 2. One day in future, CO2 to fuels/chemicals technology has to compete with potential parallel technologies exploiting renewable electricity, such as green vehicles, fuel cells, or thermoelectrics, which can offer higher reductions of CO2 emissions per unit of renewable energy used. 3. Achieving commercial maturity level for CO2 hydrogenation technology calls for innovation in catalytic designs, reactor technology, carbon-free hydrogen production, carbon capture technology and renewable electricity development.

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