Chapter - 10

Perspectives - CO2 Conversion to Fuels and Chemicals

Chemists have always been interested in converting the molecules into value added substances.

Among the various molecules, carbon dioxide conversion has attracted the attention recently

from various points of view especially in the context of global warming and other environmental

concerns. It has been always a debate whether carbon dioxide is a waste or wealth and this

debate is still going on [1, 2]. This debate has been intensified in view of the possibility that the

extent of greenhouse gas in the atmosphere may exceed the limits normally specified as tolerable

limit. Carbon dioxide is a linear molecule and as such activation and utilization of CO2 as a

chemical feed stock is difficult even though numerous attempts have been made. Even though

the physical and chemical properties of CO2 have been discussed previously, a more

comprehensive listing is given in Table 10.1.

Table 10.1. Physical and Chemical properties of carbon dioxide. [Reproduced and modified

from Ref. [3].

Property Value and unit

Molecular weight 44.01 g/mol

Sublimation Point at 1 atm (101.3 kPa) -78.5 oC

Triple point at 5.1 atm (518kPa) -56.5oC

Triple point pressure 5.1185 bar

Critical Temperature (Tc) 31.04oC

Critical Pressure (Pc) 7383 kPa

Critical density (ρc) 0.468 g/L

Gas density at 0oC and 1 atm 1.976 g/L

Liquid density at 0oC and 1 atm 928 g/L

Solid density 1560 g/L

Specific Volume at at 21oC and 1 atm 0.546 m3/kg

Latent heat of vaporization at the triple point (78.5oC) at 0o-C 353.4 J/g

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231.3 J/g

Viscosity at 25oC and1 atm of CO2 0.015 cP

Solubility in water at 0oC and 1 atm 0.3346 g/100g-H2O

Solubility in water at 25oC and 1 atm 0.1449 g/100g-H2O

Heat of formation at 25oC -393.5 kJ/mol

Entropy of formation at 25oC 213.6 J/mol

Gibbs free energy of formation at 25oC -394.3 kJ/mol

Heat capacity under constant pressure at 25oC 37.1 J/(mol K)

Heat capacity under constant volume at 25oC 28.1 J/(mol K)

Thermal conductivity 14.65 mW/(m K)

IR anti-symmetric stretching mode 2349 cm-1

IR Bending mode 666 cm-1

Symmetric mode ( Raman spectrum) 1388 cm-1

[Further extensive data are available in Sara Anwar and John J. Carroll, Carbon Dioxide,

Thermodynamic Properties Handbook Second Edition, Wiley 2016.]

The bonding in CO2 has been rationalized in an earlier chapter. Briefly, sp hybridized orbital

on carbon overlaps with Pz orbital of oxygen atoms to give rise to two σ-orbital while π-bonding

is formed between oxygen 2px or 2py orbitals with empty orbitals of the same kind on the central

carbon atom. In this sense the central carbon atom behaves as electrophilic centre and therefore

can interact by chemisorption with an electron rich surface (typically metals). Being

centrosymmetric, the molecule has no net electric dipole. Being a linear molecule, the

activation of carbon dioxide and its conversion into useful fuels and chemicals is a difficult

proposition. A pictorial representation of the process one visualizes is given in Fig.10.1 [4].

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Fig 10.1. General pictorial representation of the conversion of carbon dioxide to useful value

added chemicals and fuels [Reproduced from Ref [4]].

It is seen from the data given in Table 10.1, that the entropy contribution by means of the term

(-T∆S) is negligible to the thermodynamic driving force for the reaction at room temperature, so

that the value of the enthalpy change (∆H) is a fairly good guide to predict the thermodynamic

feasibility. Therefore using CO2 as single reactant and carryout the process is high energy

demanding and as such the dissociation of CO2 to CO requires high positive ∆H°.

Before considering the reaction of carbon dioxide with other substrates, the self-activation of

carbon dioxide on catalytic surfaces is of great relevance. The inherent difficulty in reduction of

carbon dioxide stems from the fact that the standard redox potential of the CO2-./CO2 couple is

highly negative -1.97 V vs SHE in dimethylformamide. The one electron reduction of CO 2 to

generate CO2-. is not easily possible. This is because that the carbon atom in CO -

2. is sp2

hybridized while in the free CO2 it is sp hybridized. This change in hybridization involved

large reorganization energy. One such model has been proposed in literature as shown in

Fig.10.2.

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Fig.10.2. The constrained geometry of adsorbed carbon dioxide molecule on metallic surfaces.

Reproduced with permission from Ref [5].

In spite of this difficulty, a variety of alternate modes of activation have been examined and

some of the important ones are considered in the preceding chapters. The activation of carbon

dioxide on solid surfaces depends on the mode of adsorption of CO2 and this requires special

geometry of the active site on solid surfaces [2, 5].

Marc Robert claims that “for more than two centuries, we have been running the CO2

counterclockwise that is burning fossil fuels releasing huge amounts of CO2 and it is time to run

the clock clockwise to recycle and reuse the CO2 in a responsible and useful way that would

benefit and be accessible to everyone” [6]. He has visualized a CO2 clock illustrating the

successive reduction steps which is shown in Fig.10.3.

Fig.10.3. The beneficial way to run the CO2 clock [Reproduced from [6].

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According to him, Rhenium is the best metal converting CO2 to CO but the world production of

Re is small and hence one has to develop the most abundant metal like iron as the catalyst for the

conversion of CO2 to CO.

Thermochemical splitting of carbon dioxide on oxides like CeO2 has been reported to occur at

temperatures around 1773 K in literature but the results are not satisfactory in terms of yield of

fuel source.La1-xSrxMnO3 (LSM) and its modifications with rare earth ions have been employed

as catalyst for STCS (Solar Thermochemical Splitting of CO2) at temperatures 1573 (on

substituted LSM) or 1673 K (on LSM) with yields 2 to 3 times that is obtained with CeO 2 at

1773 K [7].

Among the chemical conversion routes, the thermal conversion of CO2 is the only possible route

and industrially adopted process. This is shown in Fig.10.4. The production of syngas [CO

+H2] the natural feedstock for chemical industry is produced by dry reforming or reverse water

gas shift reaction namely, CO2 + H2→ CO + H2O(g).

Fig.10.4. Chemical processes of converting CO2 into useful and value added chemicals and fuels

[Reproduced with permission from Ref. 3].

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The catalytic hydrogenation of CO2 to methanol, methane and other hydrogenation processes is

now to some extent at a commercial scale. These technologies are, however, viable because of

the presence of (i) CO2 streams from local biomass or power plants and (ii) Hydrogen produced

through water electrolysis using inexpensive renewable power sources. Alternate methods for

the production of hydrogen must be economically developed. Their economy will be

significantly improved when novel cost- and energy- efficient methods for providing large

amounts of pure CO2 are developed. In addition, more active catalysts are required since CO2

is a highly stable molecule that is reflected in low catalyst productivity when comparing with

classical processes operating with CO and syngas. This is also valid for CO2-based Fischer–

Tropsch synthesis, and other hydrogenation processes which have high potential for recovering

large amounts of CO2 to synthetic fuels. Their industrial application is, however, limited by

the high price of renewable hydrogen and the availability of large-volume sources of pure CO2.

Future developments in this direction will benefit the catalytic hydrogenation of CO2. There is

an urgent need to improve the yield and selectivity of the direct conversion processes and also

devise new methodologies for alternate products from the CO2.

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Fig.10.5. Periodic Table showing possible metal electrodes that are promoting either water

decomposition or carbon dioxide reduction. It is seen that the metals fall into distinct four

groups namely hydrogen evolving systems, CO, HCOOH or HC forming systems respectively.

[Reproduced with permission from Ref 5].

Significant technical and catalytic advances are still required for the large-scale use of electro-

catalytic routes for CO2 conversion into fuels and chemicals due to their low energy efficiency

and productivity. Novel electrodes enabling operation at current densities close to commercially

viable situation have to be developed. From a practical point of view, solid oxide electrodes

appear to be suitable candidates. Electro-catalytic processes have seen developments in

employing metal electrodes and different metals promote different products. With regard to the

direct electrocatalytic reduction of CO2 to value-added chemicals, substantial advances in

electrodes, electrolyte, and reactor design are still required to permit the development of

commercial processes. The role of metal electrodes and product selectivity are summarized in

the form of the periodic Table in Fig.10.5.

In this monograph, fundamental aspects of the chemistry of activation of carbon dioxide and the

available strategies for the rational design of materials for effective transformations of CO2 to

value-added chemicals with the help of substrates like H2, electricity and/or light have been

considered in detail with a view that future research directions in this important area can be

pursued. This is pictorially shown in Fig.10.6.

Fig.10.6. Pictorial representation of conversion of CO2 to value added products by fuel,

substrates including hydrogen and light.

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Photocatalytic or photo-electrochemical conversion of carbon dioxide is a well investigated

reaction [8]. Photocatalytic conversion of carbon dioxide to value added chemicals is a

promising route to the sustainable solar energy conversion. The existing level of knowledge

does not permit the design of economically valuabe process for the conversion of carbon dioxide

into chemicals and value added fuels. It could happen only by the cooperative and collaborative

research efforts from different groups irrespective of the implicit bias among the scientific

community. The material development and the lack of a standard procedure have been the

bottleneck. The successful material should give at least 10% quantum efficiency in the visible

region of the solar spectrum. Currently all such carbon dioxide to fuel research efforts have

been scattered around the various parts of the globe. A global initiative is needed for the

resurrection of CO2 to fuel technology in a real life process. The global initiative should be like

a compound parabolic concentrator in which all the scattered research efforts are concentrated

into a central point. The current pace of research shows that the plants will face aa tight

competition from CO2 to fuel business in the near future.

Fig.10.7. Possible projection of CO2 conversion in short, medium and long terms in comparison

to what is possible today [reproduced with permission from Ref. 4]

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A possible projection of CO2 in the short, medium and long term has been made in the literature

[4]. This projection is shown in the form of bar chart in Fig.10.7.

Table 10. 2. The main products of CO2 and water reduction and the corresponding reduction

potentials with reference to NHE at pH 7 in aqueous solution, at 298 K and 1 atm gas pressure.

Product Reaction E0 (V/NHE) Equation

Hydrogen 2H2O + 2e- → 2OH- + H2 0.41 10.1

Methane CO2 + 8H+ + 8e- → CH4 + H2O 0.24 10.2

Carbon monoxide CO2 + 2H+ + 2e- → CO + H2O 0.51 10.3

Methanol CO2 + 6H+ + 6e- → CH3OH + 3H2O 0.39 10.4

Formic acid CO2 + 2H+ + 2e- → HCOOH 0.58 10.5

Ethane 2CO2 + 14H+ + 14e- → C2H6 + 4H2O 0.27 10.6

Ethanol 2CO2 + 12H+ + 12e- → C2H5OH + 3H2O 0.33 10.7

Oxalate 2CO2 + 2H+ + 2e- → H2C2O4 0.87 10.8

Recently Perathoner and Centi [10] in their analysis of CO2 recycling as a key strategy in the

chemical production, advocated the use of renewable energy (solar, wind or hydro) in

combination with CO2will be a new way for chemical production with many technical

challenges. In Fig 10.8 (reproduced from their publication) a road map as visualized by them for

successful implementation of the CO2 recycling in the chain of chemicals production processes is

given.

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Fig.10.8 Indicative roadmap for CO2 recycling to introduce green energy in the chemical

production [Reprinted from ref.10.]

Till today achievements in the photocatalytic reduction of CO2 with H2O based on heterogeneous

catalysis have been limited. The insights into photocatalysis and the mechanism of CO2

photoreduction have been gathered and the design and fabrication of efficient photocatalytic

systems have become possible. However, the photo-conversion efficiency and selectivity for

desired products of CO2 photoreduction are lower than that in natural photosynthesis or in

photocatalytic hydrogen evolution reaction. Besides light harvesting and charge generation,

charge separation and transportation, which are common to both reactions namely H2O splitting

and CO2 reduction, but the surface reaction and activation of CO2 in the non-linear form require

clarity and definite solutions. These steps are important in suppressing the competitive HER to

increase the selectivity towards the carbonaeous products. One approach to achieve these

objectives is to design catalyst systems with definite morphology and textural characteristics

with appropriate surface functionalities. In addition, the introduction of surface defects (e. g.,

oxygen and surface vacancies), surface electrophilic centers or basic sites and simultaneous

presence of noble metal co-catalysts are widely employed strategies to maximize the adsorption

and activation of the acidic CO2 molecules. Furthermore, the selectivity of products from CO2

photoreduction is another important parameter that should be considered in order to produce

more valuable hydrocarbons and to reduce the cost of product separation. The mechanistic steps

of CO2 reduction needs to be investigated in detail in the further studies. The finding of the

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reaction barriers and rate determining steps in CO2 photoreduction is important for the further

improvement of the conversion efficiency. It is generally realized in overall solar water splitting

that the O2 evaluation half reaction is the challenging and rate-determining step because it

involves four-proton coupled four electron transfer process. The organic adsorbates on the

surface of photocatalysts formed during the formation process can affect the photocatalytic

products.

EPILOGUE

As an epilogue to this exercise the following questions on which more detailed information are

required are listed below: It must be remarked that the list is not comprehensive but indicative of

what areas of research lack full information on the issues relating to CO2 to fuels and chemicals.

1. The basic molecule CO2 can be activated through the electrophilic center (namely carbon

atom) or nucleophilic center through the oxygen atoms. Which one is the preferred route and

why?

2. It is generally believed that the bond angle should be reduced from 180ᵒ in free CO2 molecule

to nearly tetrahedral angle, if so to what value and why? Why this condition has to be

satisfied? Other triatomic systems like O3 are reactive enough while CO2 is not?

3. What is the geometry of the active sites on metallic surfaces and on other surfaces like oxides

and sulphides?

4. What are the nature and roles of defect centers inherent in the solid catalysts in the activation

and reduction of CO2?

5. How to identify the selectivity of a reaction based on the catalyst functionalities and also

reaction conditions in the case of conversion of CO2?

6. It is better to quote from Micheal Schmidt “Further improvements in CO2 catalysis are possible, not only for the elimination of the CO2

- intermediate but also for eliminating the more stable formate, CO, and formaldehyde intermediates for the synthesis of useful methanol and methane fuels. It must be remembered, however, that no matter what achievements are made in catalysis, the fundamental thermodynamics of the overall problem will not be changed and a substantial input of energy will be needed for conversion of CO2 to more reduced carbon products”. 

7. Carbon di oxide capture and storage (CCS) is another important and much pursued area [12]. These later two aspects may be important on their own.

8. In the reduction of CO2 the first electron transfer to generate CO2- requires nearly -1.9 V vs

NHE and it is an uphill process with great over potential. Still experimentally one realizes

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multiple electron reduction products as shown in Table 10.3. How and why these routes are preferred?

It appears that thermodynamically, proton coupled multiple-electron reduction of CO2 is easier

than single electron reduction. But to manage multiple proton coupled multiple-electron

processes is a huge challenge kinetically. Why? Thermodynamically, proton coupled multiple-

electron reduction of CO2 is easier than single electron reduction. But to manage multiple proton coupled

multiple-electron processes is a huge challenge kinetically [13, 14].

9. First electron reduction generates the carbon dioxide radical anion.  There is no detailed information on the anion radical and its reactivity. The information is scattered in literature. The relevance of this species and participation of this species in CO2 reduction process is not yet clear. 

10. The product analysis in CO2 reduction and the established procedures for the analysis of the products are not unequivocally established. For instance, whether the analysis has to be done in gas or aqueous phase or in both and what sampling techniques have to be followed..

11. Multiple products are formed in the reduction of CO2. The role of competitive reactions in the photo-catalytic reduction of carbon dioxide is not fully established. 

12. The effect of carbon artifacts on the reduction of CO2 has not yet been clearly understood. Even though some studies deal with it, established concern has not yet evolved.

13. During the past few decades, although many achievements have been made in the fields of photochemical, electrochemical, and photo-electrochemical reduction of CO2 , many key scientific issues remain to be solved. In all three fields, the development of inexpensive, stable catalysts that are highly selective for a specific product and operate at minimal or no over potential is the main issue.

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14. The current densities and TONs of catalysts must also be greatly improved in order to produce economically viable systems. Possibly, catalysis is the key to reducing such over- potential, maximize CO2 reduction rates with highest selectivity desired.

15. Individually these three methodologies, namely, photochemical, electro-chemical and photo-electrochemical, will have advantages and disadvantages, but the combination of knowledge from each of these may lead to postulation of an efficient system.

16. Between photo-electrochemical reduction and electrochemical reduction, the latter is possibly advantageous since the necessary differences in the over potential required. The concept of coupling the advantages of both of them in a single device means development of new solution based catalysts, which at present appears to be infancy stage. Today the field of research in this area is in the domain of material science not only concerned with energy efficiency.

16. Homogeneous photochemical systems can be alternates but the issues of concern are low reduction rates, and the use of expensive metals that is not viable on a large scale, and the need for sacrificial reagents.

17. Though a number of studies on two electron reduction pathways for CO2 to yield formate and CO have been reported, the studies on further reduction to methanol, methane and higher species are limited. This is an area where more studies must be focused to make these processes economically viable and easily adaptable.[15]

18. Water decomposition to produce fuel hydrogen, synthesize ammonia under ambient conditions and convert carbon dioxide to useful products, are natural processes and there is much to be learnt to mimic them from nature. But the attempts in this direction should not simply copy the natural processes but adopt them to chemically feasible processes. In all these three processes the man-made attempts have not been successful yet. The reasons and the path to be followed should be identified and implemented.

19. Another major issue in the experimental studies on the reduction of CO2 is the absence of standardized reactor design to compare the results obtained at various scientific laboratories. Since the products formed can be simultaneously present in more than one phase and when the products formed are so low, it is necessary to have a standardized reactor design so that results can be directly compared.

20. Another issue often suspected in the results so far reported in the literature is the origin of carbon source in the products formed. 13C Labeling is the suggested remedy for this issue. However, not all laboratories are capable of carrying out experiments with labeled atoms. In this situation alternate methodology has to be evolved.

These twenty points are only indicative ones. There can be more such questions possible and some of them can be more valuable than the suggested ones. In science, personal bias should not cloud the thinking process and also it should not hamper the progress of scientific inquisitiveness. Even though, the hurdles to be surmounted to make the CO2 reduction process

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economically feasible and industrially-viable, are many, there appears to become feasible in these directions and it is only a matter of time to realize these. The motivations that drive these efforts are the increasing cost of the fuel with simultaneous concern for the environment. All that remain to be realized mainly are designing feasible multiple electron coupled proton transfer catalysts based on earth abundant sources. It is hoped that this is within realizable time scale. This compilation is a small step towards realizing this goal.

References

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