CHAPTER 1: A Fundamental Approach Towards Carbon Dioxide Conversion to Chemicals and Fuels: Current Trends for CO₂ Utilization Technologies

Energy management is one of the central strategies for the socio-economic development of any country. The rising demand for energy has increased the consumption of fossil fuels and the abundant release of CO₂ into the atmosphere. This leads to undesired climate change and destruction.¹  Carbon is a foremost component of earth’s environment, and exists in the core, crust and also in the atmosphere as CO2. It is the backbone of life on Earth. The carbon atom is an important element of the atmosphere, seawater, rocks, and all living things and can move in between these realms as a part of the biogeochemical carbon cycle. The carbon cycle exchanges carbon between each reservoir of the Earth and the atmosphere as shown in Figure 1.1. The carbon is assimilated from the atmosphere by plants through the process of photosynthesis. CO₂ is converted to food for plant growth. Later, carbon moves from plants to animals through the food chain. When these living organisms die, they decay and deposit carbon into the earth. The carbon transfers to the atmosphere in the form of CO₂ from living things when they exhale and from fossil fuels when they are burnt. Any imbalance in the cycle disturbs the balance of the carbon cycle.²  This cycle had worked in equilibrium over millions of years until the Industrial Revolution. As a result, the rate of CO2 release increased by less than 1 ppm y-1 during the 1960s. In the subsequent years the rate increased by more than 2ppm y-1, which has alarmed researchers.³ 

The combustion of any carbonaceous fossil fuels like oil, diesel, petrol, biogas, and natural gas releases a large amount of CO₂ leading to a substantial increase in its emission, which ultimately contributes to 72% of all greenhouse gases emitted. These greenhouse gases cause an increase in temperature across the world leading to global warming. This association between global temperatures and CO₂ concentrations has been true throughout Earth's history.⁴  However, CO₂ emissions in the power sector have reduced in 2019 on a global basis after the preceding two years of increase. This indicates the effect of switching to renewable energy sources like solar and wind from conventional fossil fuels. The Belt and Road Initiative (BRI) is an initiative by the Chinese government to increase the economic growth in developing countries in Asia. The substantial economic growth has resulted in increased CO₂ emission among BRI countries as shown in Figure 1.2.⁵  Currently, the global coronavirus pandemic in 2020 reduced CO₂ emission as a response to the imposed lockdowns and economic crises of the affected countries.⁶  The stringent measures taken by world governments such as the closure of mass gatherings, schools, colleges, and the ‘work from home’ strategy have reduced transportation requirements, leading to a decline in energy use and reduced CO₂ emissions.

There have been intense efforts to regulate greenhouse gas emissions as well as scavenge existing greenhouse gases from the atmosphere. The mitigation of CO₂, which is a major contributor to greenhouse gases, is a serious global concern. Owing to its abundance in the atmosphere through uncontrolled anthropogenic causes, it is extremely desired to find a way to utilize CO₂ to generate valuable energy fuels and chemicals in large volumes. Researchers are using scientific tools to develop mature technologies to slow the increase and alleviate the current issues. Scavenging atmospheric CO2 and converting it to valuable fuel is a dual benefit approach that mitigates the concerns on global warming and energy crisis respectively. In the chemical literature it can be seen that researchers all over the world have been attempting to make use of this waste CO₂ as a chemical feedstock for the enlargement of innovative state-of-the-art chemical products with unique properties and greater technical performance as shown in Figure 1.3.^7–10  The foremost difficulty associated with the conversion of small molecules like CO₂ is their high capital costs, thermodynamic stability, and kinetic limitations.⁷  Despite such encounters, a large number of approaches for CO₂ capture, and conversion techniques like adsorption, electrochemical, thermal, and photocatalysis have been initiated, investigated, and developed so far. The conventional technologies that are used in these progressions often suffer from low conversion, poor energy efficiency, and selectivity. The current research strategies need to consider appropriate process intensification techniques based on reactors, materials, and process development to combine emerging technologies with innovative explorations to make this ideal reaction a reality. Currently, there exist very limited intensifying technologies that are potent for industrial applications in consideration for sustainability. This gap can be filled by intensive research that evaluates the potency of various technologies that are capable of making the world more climate-friendly.¹¹ 

This chapter aims at contributing a very intense discussion on various methodologies that have been adapted to convert harmful CO₂ to valuable fuels and chemicals. This chapter consists of three sections: the first section starts with the effects of CO₂ and the need to mitigate it for a greener future. A brief introduction to the potential of CO₂ to act as a valuable chemical feedstock is also presented in this section. Section 2 comprises various technologies like electrochemical, photochemical, thermal, and biological methods highlighting their working principle, conversion efficiency, technological advancements, and the products formed. This chapter will provide deeper insight and also validate the evolution of current technology as a remediation tool for the rapid utilization of CO₂ as a chemical feedstock. Finally, an overall conclusion and summary of the discussions made in the preceding sections along with prospects to upgrade this technology as an industrially viable one are presented in Section 3.

Carbon assimilation or its fixation is the process in which inorganic carbon is transformed into an organic compound by living plants. Carbon mitigation is a technique developed to continue the balance of the carbon cycle by opening a new avenue on carbon capture, storage, and utilizing it to generate energy fuels and other chemical products of commercial significance using an environmentally safe, economically viable, and sustainable method.^12–15  In parallel, the other mitigation strategies like increasing transportation efficiency, increased use of renewable resources also play their role.

CO₂ is a colorless and odorless gas possessing linear geometry with zero dipole moment. The structure of CO₂ shows a highly stable linear and centrosymmetric structure with a carbon atom attached to highly electronegative oxygen at its two linear ends. The highly electronegative oxygen pulls electron density from the carbon atom towards itself. This facilitates an electrophilic attack on partial positive carbon atoms and a nucleophilic attack on partial negative oxygen atoms. In general, during the activation of CO₂, the elongation of the molecular axis leads to polarization of atomic charges converting a highly stable linear shape of CO₂ into an unstable bent geometry.⁷  Figure 1.4 illustrates five possible models of CO₂ adsorption on a rutile TiO₂ [110] surface.¹⁶  However, the transformation of a small and stable molecule like CO₂ suffers from high thermodynamic stability and kinetic inertness inhibiting it to be an easier technique and demands a high amount of additional energy to drive the process.¹⁷  Most of the conversion methods of CO₂ are endothermic reactions including natural photosynthesis. This clearly illustrates the need for an additional driving force in terms of energy for these reactions to occur. The conversion of CO₂ can be achieved by providing an inorganic catalyst like a natural leaf, applied potentials, reaction of CO₂ with other reactants of higher Gibbs energy at a higher temperature, and artificial enzymes to lower the activation energy.^18,19  Here, we summarize different methods like electrochemical, photocatalytic, thermal, and biological that are adopted to scavenge CO₂ and convert it to usable forms.

As mentioned above, CO₂ is a particularly stable molecule. The high C–O bonding energy (750 kJ mol⁻¹) is much greater than its association with other chemical bonds such as C–H (430 kJ mol⁻¹) and C–C (336 kJ mol⁻¹).²⁰  The electrochemical use of CO₂ to generate valuable synthetic energy sources is an important approach to reduce the CO₂ concentration in the atmosphere as it results in higher product formation rates and exhibits good efficiencies under ambient conditions.²¹  A graphic representation of the electrochemical CO₂ reduction is given in Figure 1.5.

The reaction typically takes place in a fabricated electrochemical cell with two partitions, which is separated by a Nafion membrane to elude the oxidation product. The electrochemical CO₂ reduction like any other electrochemical process proceeds at the electrode–electrolyte interface, where a saturated CO₂ solution acts as an electrolyte and the electrode acts as electrocatalysts in the process. Figure 1.6 displays a graphic representation of an electrochemical cell for CO₂ reduction. Later, to address various issues like low solubility of CO₂ and low current density flow cell designs like liquid phase electrolyzer, gas-phase electrolyzer and solid-phase electrolyzer are fabricated to meet the industrial requirement.^22–24 

The mechanism of CO₂ reduction has always been paid very little attention due to the great intricacy of the method. However, it is very important to recognize the mechanism to progress high-performance catalytic material endowed with higher activity, stability, and product selectivity. The tuning of selectivity of products like HCOOH, CO, CH₄, and CH₃OH towards voltage would make the process highly attractive to introduce a balance in the carbon cycle.¹⁸  Typically electrochemical reduction of CO₂ comprises three basic steps.²⁵ 

Electron transfer from electrocatalyst and proton passage from the electrolyte towards CO₂ results in dissociation of C–O bonds as well as the generation of C–H bonds, Reorganization of product configuration that leads to their desorption from the electrocatalyst surface towards the electrolyte surface.

Electrochemical reduction is a combination of electrochemistry and catalysis reactions. This combination utilizes electricity to drive the chemical reduction of CO₂ to valuable fuels at a higher rate by making use of an efficient electrocatalyst surface. The overall reactions consist of cathodic and anodic reactions in two different compartments separated by a membrane. Presently, the preferred cathodic electrochemical reactions are taking place at the cathode–electrolyte interface. The ability of an electrocatalyst to insert electrons into the antibonding of CO₂ and dissociate the proton source like H₂ or H₂O to H* facilitates a proton mediated multiple electron pathway with a moderately lower energy barrier technique that facilitates CO₂ activation and conversion. Depending upon the number of electrons and protons reacting with CO₂, different products can be generated at various potentials as given in Table 1.1.²⁶ 

During the reaction conditions, the CO₂ is chemisorbed on the electrode surface. The chemisorbed CO₂ adheres to specific sites and the orientation of HOMO and LUMO regarding the surface decides the nature of the bonding. However, the surface phenomenon of electrochemical reaction does not predict a generalized reaction pathway towards the selectivity of product formation. In detail, the mechanism of the reaction is essentially being influenced by the nature, structure, and morphology of the catalyst, selectivity, the number of electrons transferred, and the essential energy barrier for the electron transfer.²⁷ 

Despite several signs of progress achieved, this area still possesses certain confines that are discussed below.

The overpotential is the potential difference between the standard reduction potential of a half-reaction that would occur in thermodynamic equilibrium conditions and the potential at which the actual redox reaction takes place. The presence of large overpotential infers that energy additional to that which is thermodynamically needed is essential to drive this reaction. This voltage inefficiency makes the process energetically less efficient. Single-electron reduction of CO₂ to CO₂^˙− radical demands a higher negative potential of −1.9 eV. This makes the method to be exactly unfavorable, owing to the great intake of energy and is responsible for overpotential during the electrochemical reduction of CO₂.²⁸  The conversion of a linear molecule of CO₂ to their bent structure promotes the reaction with a promising yield of product formation. Despite this, the onset potential for product formation over the metal electrode is more positive than the equilibrium potential of −1.9 V vs. NHE.²⁹  Hori et al.³⁰  observed that the bent structure of adsorbed CO₂^˙− drops the energy barrier enabling a faster electron passage, and also favors the hydrogenation of the O⁻ atom interacting with the electrocatalyst surface. The hydrogenation of the O⁻ atom results in the proton-assisted multi-electron reduction process of CO₂^˙– at lower potential prominent to an inclusive diversity of product formation. Declined electron transfer kinetics results in low exchange current density. It is necessary to compute the exchange current density and charge transfer coefficient to define the activation energy and the rate-limiting step of electrochemical reduction of CO₂.³¹  The highly competitive H₂O splitting, which is a single-electron reduction process, takes place at a lower potential of −0.83 V in comparison to the multi-electron reduction process of CO₂ hydrogenation as illustrated in Table 1.1. To win over this highly competitive reaction successfully, the CO₂ reduction reaction needs to take place at low potential and display high current efficiency with good selectivity of product formation. This drives the research towards finding suitable electrocatalysts with high adsorption of CO₂ and lower affinity to H₂O reduction.

A lack of selectivity in product generation results in the generation of an assortment of products, which makes the process less valuable in economic terms. The product selectivity is highly influenced on the nature of the electrodes and the reaction medium. Several signs of progress in the research have been made during the design of electrocatalysts and the process to switch the product selectivity.³² 

The quicker deactivation of electrodes in less than 100 h confines its potency in practical and technological use. The presence of electrolyte impurities and intermediates that are produced during the electrochemical reduction of CO₂ reacts with active metal electrodes and converts them to electrochemically inactive metal oxides or metal hydroxides. This limits the re-use of electrodes or restricts the operational durability of the electrode.³³ 

Therefore, the precise engineering of an effective electrocatalyst with lower overpotential, high current efficiency, stability for a long period, and product selectivity plays a vital part in the progress of electrochemical CO₂ reduction.

The CO₂ molecule serves as a building block and produces valuable products like CO, HCHO, HCOOH, CH₃OH, C₂H₅OH, and C₂H₂ during its electrochemical reduction process. The simplest CO₂ hydrogenation process would be a two-proton-assisted two-electron transfer step to produce CO and HCOOH as verified in the majority of the reported studies. Numerous metallic elements such as Cu, Zn, Ag, Au, In, Sn, Pb, Pd, Bi, and carbon-based materials like graphene, carbon nanotube, and fullerenes are also functional electrocatalysts, possessing high potential towards electrochemical CO₂ reduction in aqueous electrolytes. Product selectivity is influenced by the nature of the electrodes and the reaction medium. Depending on the nature of the electrodes, metal electrodes are categorized into different groups according to the adsorption strength of CO₂^˙– anion. In, Sn, Hg, and Pb, which do not adsorb CO₂^˙–, mostly generate HCOOH, conversely, Zn, Ag, Au, adsorb CO₂^˙–, and mostly generate CO. Cu electrodes specifically generate hydrocarbons and alcohols. When chemical reactions are carried out on the surfaces of solid electrodes, among various factors that impact the performance of the heterogeneous electrocatalysis, the nature of the electrodes takes the predominant role. Hence, the rational design of electrocatalysts has been a very important aspect of research. Different metals are recognized as efficient electrocatalysts for electrochemical CO₂ reduction over the competition for water-splitting reactions as the binding energy of the CO* intermediate is relatively weaker on their surface than that of the H* intermediate. This arises as the intended selective electrocatalytic CO₂ reduction to CO rather than the competitive water splitting to produce H₂ on these metal electrocatalysts. In general, there are two main modes of manipulating strategies that are described as (i) increasing the number of active sites that mainly include nanostructuring and nanoporous materials, and (ii) the intrinsic activity of the material by introduction of dopants, defects, and the construction of heterostructures.³⁴  The recent studies fundamentally focus on a blend of these approaches like size reduction from bulk to the nanoscale, design of high surface area porous hierarchical assembly, doping, the introduction of defects in the system, construction of heterostructured nanocomposites are developed to alter the geometry and electronic properties of electrocatalyst to tune a promising route for the high rate of electrocatalytic CO₂ reduction as displayed in Figure 1.7.^27,35 

The low dimensional material can be altered by various reformation strategies as the newly added inclusions like dopants would be placed near to the surface and can play an active role in catalysis reactions.³⁶  In the case of bulk materials, these inclusions are trapped inside their structure which displays a reduced effect. The recent exploration of low-dimensional materials for catalysis has been inspired by their rising role of confinement effects in optics and electronics. The tuning of ideal particle size strongly influences their electrocatalytic behavior by increasing values of current density and faradic efficiency. Zhu et al.³⁷  have reported that Au nanoparticles of 8 nm size show enhanced faradic efficiency up to 90% at −0.67 V as shown in Figure 1.8(a). The size-dependent electrocatalytic reduction performance is largely associated with their edge-to-corner ratio. The edge sites are more favored than corner sites of noble metal nanoparticles for CO generation as shown in Figure 1.9(a) and 1.10(a). Liu et al.³⁸  have demonstrated a shape-dependent electrocatalytic activity of triangular Ag nanoplates as shown in Figure 1.8(b). The triangular Ag plates maintained a high edge to corner ratio in comparison with the similarly sized Ag nanoparticles and bulk Ag nanoparticles. The faradic efficiency value shows that optimum surface and edge sites are essential to display high electrocatalytic activity towards CO₂ reduction reactions as shown in Figure 1.9(b). The low free-energy barriers and formation of crucial intermediates by computational studies assist to confirm the active site of engineered photocatalysts for CO₂ reduction.³⁹ 

With reference to the above discussion, it is well understood that the exploitation of earth-abundant transition metal electrocatalysts is dominant among published works for CO₂ reduction. The need for the progress of cost-effective materials with appreciable activity opens up the avenue for the development of conducting polymers, organic frameworks, and carbon-based electrocatalysts. Among these, the highly favorable properties of electronically conducting polymers, i.e., they are economical, effortless large scale synthesis, high conductivity after sufficient incorporation of functional components, and unique electrochemical and optical properties validate their potent application in the electrochemical reduction of CO₂.⁴⁰  Coskun et al.⁴¹  developed polydopamine, wherein the structure integrates hydrogen-bonded designs as established in enzymes to suggest the blend of proficient electrical conduction, with less active catalytic sites, and possibly enhance CO₂ reduction reactions. The higher electrocatalytic activity exhibited by metal catalysts like Au, Fe, and Ni tends to decrease over time as these nanoparticles tendency to agglomerate grows during their periodic use. This would decrease their overall catalytic active area, which eventually deteriorates its activity. The incorporation of covalent organic frameworks (COFs) with the macrocyclic cluster is an effective approach to increase its activity and stability. Two-dimensional (2D) COFs consist of porous and crystalline phases with excellent thermal stability and large surface area. In comparison with the conventional polymer structure, COFs possess a variety of advantages that are highly favorable for electrocatalytic reduction of CO₂ due to their ability to tune heteroatom to precisely control 2D nanostructures. This hybrid strategy boosts its catalytic activity by facilitating an efficient conduction channel and enhancing the active surface area with controlled pore size. The highly beneficial structure permits reactants to be in good contact with the active catalyst sites and support the mass transfer of the reactants or products, thereby enhancing the rate of electrochemical reduction of CO₂.⁴²  For instance, Kung et al.⁴³  developed a hybrid strategy by embedding Cu into Zr-based organic MOF. The electrocatalyst displayed high selectivity towards CO₂ reduction with increased faradic efficiency of about 2–3 times. The declined faradic efficiency toward H₂ evolution reaction up to 30% significantly enhances their selectivity towards CO₂ reduction reactions as shown in Figure 1.10(b). Recently, Zhang et al.⁴⁴  described the role of polyethyleneimine as co-catalyst on nitrogen-doped CNT electrocatalyst to enhance the electrocatalytic reduction of CO₂. The polyethyleneimine specifically enhances the catalytic role of N-doped CNT and does not play many roles on undoped CNT. The synergetic effect of the N-doping and PEI layer creates a local environment that facilitates enhanced electrocatalytic reduction of CO₂ to HCOOH with lower overpotential and greater current density and faradaic efficiency. The development of a hybrid catalyst essentially benefits its catalytic performance, but a deeper insight into the role of chemical coupling interaction towards an efficient electrocatalytic reduction of CO₂ residues is unclear. Zhang et al.⁴⁵  have validated the influence of porous In₂O₃ nanobelts and reduced graphene oxide (rGO) to progress electrical conductivity and stabilize the key intermediate HCOO–* and bring about higher faradaic efficiency and specific current density as shown in Figure 1.9(c). The synergetic effect displayed between diverse constituents in a hybrid material establishes a high prospect to improve their electrocatalytic properties when compared to their independent performance. The composition engineering of nanocomposites electrocatalysts has received greater attention after the work of Chen et al.,⁴⁶  wherein copper-based binary catalysts like Cu–Au and Cu–Pd alloys are used for electrocatalytic reduction of CO_2. The Cu–Pd nanoalloys with spherical geometry displayed higher electrocatalytic activity towards CO₂ conversion with 93% faradaic efficiency when compared with dendritic Cu–Pd nanoalloy, which displayed higher selectivity towards hydrogen evolution reaction. Li et al.⁴⁷  have further considered an attractive strategy by the development of molecular-catalyst, carbon-based material by the polymerization of cobalt phthalocyanine on 3D GCN with an interconnected network. This expands the electrocatalytically active surface area and enriches the structural and functioning strength as shown in Figure 1.8(c). Therefore, it is highly necessary to evaluate scientific findings that are reported in the literature to design economical, capable and robust electrocatalyst for the process. The unique structures of hybrid materials and their highly tunable electronic properties design make them fascinating in catalysis.

The use of an ample amount of sunlight as a source of energy for photocatalytic CO₂ reduction with the H₂O molecule is more economically viable over costlier resources of energy like high temperature for thermal reduction and electroreduction by applied voltage.^48,49  The benefit of being renewable, economical, and more importantly, sustainable, the photocatalytic reduction of CO₂ makes it more advanced and responds equally to global warming and progressing energy demands.⁵⁰  The revolutionary work by Halmann⁵¹  in 1988 on the photoelectrochemical reduction of CO₂ which was followed by Inoue et al.⁵²  in 1979 has given rise to the development of a series of semiconductors such as TiO₂, ZnO, SrTiO_3, CdS, SiC, and GaP as shown in Figure 1.11.

When light falls on a semiconductor system, it functions as a photocatalyst after receiving light energy that is equal to or higher than that of its bandgap energy. This results in the photoexcitation of electrons from its valence band (VB) to the conduction band (CB), leaving back holes in the VB and generating electrons in CB. Therefore, the CB and VB potential edge of the semiconductor is an important criterion to determine the probability of thermodynamically unfavorable uphill CO₂ reduction reactions taking place. It was well recognized that for a perfect photocatalyst, the CB potential edge must be more negative than the CO₂ reduction potential to facilitate the capture of photogenerated electrons from CB to CO₂, enabling its reduction.

Little consideration has been given to their mechanism due to the extreme intricacy of the process. The elevated electron affinity and high bandgap energy of CO₂ make it less susceptible to photocatalytic reduction reactions. The suitable adsorption of CO₂ on an ideal photocatalyst surface constructively reduces its energy barrier after the conversion of its linear form of CO₂ to a bent form revealing high reactivity.¹⁶  In general, the mechanism of photocatalytic reduction of CO₂ consists of three basic steps as described in electrocatalytic reduction of CO₂ after absorption of light energy^53,54 . A schematic illustration of the proton-assisted multi-electron reduction process of CO₂ to generate various products at different reduction potentials by transfer of a different number of photogenerated electrons respectively is illustrated in Figure 1.12.¹  The highly complex process of photoreduction of CO₂ involves intricate pathways and intermediates. The need to tune the selectivity of product formation necessitates an ultimate understanding of the mechanism of the reaction.

Despite the beneficial effects offered by instantaneous photoexcitation, the swift recombination of these charge carriers takes place immediately and impedes the efficiency of the process.^14,15  These findings have placed the focus of the research on the bandgap engineering of the photocatalyst. The wide bandgap semiconducting nanomaterials possess a larger distance between the VB and CB electronic levels, which declines the rate of excited electron–hole recombination reactions. However, these wide bandgap materials usually get excited in the UV range of the sunlight and make it less effective in practical terms as UV is 3–5% of total spectral content.⁵⁵  Therefore, the design of an efficient photocatalyst with enriched photocatalytic activity and cyclability is the preliminary stage.

The photocatalytic reductions of CO₂ are typically carried out using inefficient lab-made reactors. The batch scale photoreactor typically used, employed to estimate the potential of the developed photocatalyst, is highly unsuitable for viable application. The photocatalyst remained extremely dispersed in a CO₂ enriched reaction media. This enabled the reactant molecules to interact with the well-dispersed photocatalyst.⁵⁶  The significant design strategies of an efficient photocatalytic reactor increase the photocatalytic efficiency by trapping the incident energy towards the surface of the photocatalyst. The product generation rate displays a robust relationship on the diameter of the reactor, liquid phase volume, lamp position to the reactor, light distribution, and uniform distribution of catalyst.⁵⁷  The various modifications for the development of new reactor geometries generally include two methodologies such as increasing the active area by using monolith support, novel twin, membrane reactors, and intensifying the light penetration towards the reactor by utilizing solar concentrators and concentrators like a compound parabola, Fresnel lens and optical fibers.^58–63  Figure 1.13 illustrates different structural engineering strategies and an efficient photoreactor design that are exploited to uplift a conventional inefficient photocatalytic system to a highly potent industrially valuable proficient photocatalytic system.

The assortment in the structural and morphological characteristics of the photocatalyst enhances their light extraction property. The wide bandgap of nanomaterials declines the rate of recombination reactions, enabling a large amount of photoexcited electrons to reduce CO₂ to suitable products. The exceptional properties of nanomaterials like increased surface area, and reduction in path length of photoexcited charge carriers, has increased their photocatalytic performance. Further, intensification of light-harvesting can be attained with 1D nanostructures, hollow, and mesoporous nanostructures.^50,64  The occurrence of a huge number of active sites over these photocatalysts enhances diffusion, adsorption, and reaction displaying improved photocatalytic performance. The materials lower than 100 nm may impose a quantum confinement effect on the electronic density of states that offers beneficial properties when compared to their bulk counterparts. The nanomaterials possess the additional benefit of high bandgaps, as the electrons are confined and occupy less space in comparison to the bulk matter and hence a shift in VB edge and CB edge occurs. The increased bandgap of nanomaterials is responsible for lower electron–hole recombination.⁶⁵  Previously, we have reported that the fascinating CdS nanorods developed on porous anodic alumina supports proficiently entrap light between the nanorods creating a light-intensive area as shown in Figure 1.14(a).⁵⁰  The permeation of reactant molecules towards electron-rich catalytic active sites drastically increases the rate of reaction. The size of mesopores tunes an enhanced photocatalysis. The 2D materials act as an efficient platform due to their large specific surface area, and large absorption ability.⁶⁶  Apart from these structures, 2D and 3D photocatalysts largely boost the rate of CO₂ reduction from their interesting optical and electronic characteristics.^67,68  The fascinating favorable features like large surface area promotes mass transportation, light-harvesting, active area, and diminish electron–hole recombination towards a value-added photocatalysis.⁶⁹  The 2D graphitic carbon nitride (GCN) is a metal devoid photocatalyst with a bandgap of 2.7 eV and acts as an economical and strong visible light active photocatalyst.^70,71  Apart from these beneficial effects, this material acts as an exceptional template for the fabrication of different heterostructures.⁷²  The integration of co-catalysts, carbon materials, metal oxides, and metal sulfides enhances the photocatalytic performance of GCN based materials.⁷³ 

The growth or incorporation of photocatalyst on suitable porous supports like mesoporous silica, molecular sieve, membranes, and montmorillonite (MMT) generates huge active sites for enhanced CO₂ reduction reaction.^74,75  The exploration of the pores that are retained, enhances the adsorption of CO₂ molecules and harvests light in the vicinity of the photocatalyst.⁵⁰  The current progress of these materials are highly exploited to increase the availability of vital conditions around the photocatalyst surface.^76,77  Li et al.⁷⁸  has deposited Cu/TiO₂ photocatalysts on a molecular sieve support as shown in Figure 1.14(b). The uniform dispersion of TiO₂ and adsorption of CO₂ has resulted in high photogeneration of CO and CH₄ as products.

To further tune the photocatalyst, the use of conducting supports can scavenge the photogenerated electrons generated by the excitation of the photocatalyst. Later, these energetic electrons can be circulated within a conducting support that ultimately permits a large number of free electrons to be available for photocatalytic reduction reactions. Carbon-based hybrid materials like graphene, CNT, and conducting polymers are known to be highly efficient for enhancing photocatalytic activity of the supported photocatalysts.⁷⁹  The photocatalyst after absorbing suitable light energy corresponding to their bandgap acts as a charge generator. The Fermi level of RGO and CNT is less than the CB of mostly used photocatalyst enabling a rapid scavenging of photogenerated electron and circulation within their π–π conjugated structure. The improved adsorption of CO₂ reactant molecules on carbon-based conductive supports escalates the photocatalytic CO₂ reduction rate to obtain CH₄. The CNT−TiO₂ interfaces as shown in Figure 1.14(c) act as photocatalytic sites with enhanced photon absorption and synchronized photogeneration of electrons into CB of TiO₂, enabling an increased rate of CO₂ reduction.⁸⁰  The conducting carbon support not only captures photogenerated electrons but also enhances the surface area, CO₂ adsorption, generates a visible light response, and retards photocorrosion of the photocatalysts.⁶²  However, most commercially used wide bandgap TiO₂ photocatalysts usually get excited in the UV range of the sunlight and make it less effective in practical terms as UV is 3–5% of total spectral content. Tan et al.⁵⁵  have synthesized a visible light active RGO–TiO₂ nanocomposite that displays improved photocatalytic reduction of CO₂ to CH₄. The other conducting forms of carbon display comparable beneficial effects as shown in Figure 1.15.

The cationic and anionic doping of photocatalysts increases the visible light activity and stability of the photocatalysts. The addition of cationic dopants like transition metal ions, rare earth metals, and other metals generates novel composites with significantly improved photochemical properties.^81–86  The anionic doping incorporates non-metals like N, C, and S and extends visible light activity. The new energy levels that are generated below CB photocatalyst and near to VB after cationic and anionic doping respectively is schematically represented in Figure 1.16.⁸  These mid-states alter the path of an excited electron, which ultimately increases the lifetime of photoexcited electrons and also leads to a change in their absorption behavior from UV light towards enhanced visible light.

The emerging contribution of plasmonic photocatalysis is highly significant in enhanced photocatalysis. It makes use of plasmonic metals like Au, Ag or Cu, which is smaller (10–100 nm) than the wavelength of the prominent features like (i) the formation of Schottky barrier, (ii) surface plasmon resonance (SPR),⁸⁷  and (c) alter the surface properties of the photocatalyst.⁸⁸  The recognition of light waves by plasmonic active materials generates resonant oscillations of free electrons, identified as surface plasmons causing plasmonic photocatalysis as shown in Figure 1.17.⁸ 

The efficient exploitation of the exciting properties of these attractive constituents that comprise structural design, use of porous and conducting support, the insertion of dopants and plasmonic photocatalysis would raise this process to be an industrially viable technology. In general, the incredible properties revealed by these nanocomposites like large accessible area, visible light performance, a greater lifetime of photogenerated charge carriers, demonstrate its potential to serve as an artificial leaf that displays enhanced photoreduction of CO₂.

Thermal catalysis is one of the prominent technologies that has been widely used in industries due to its high efficiency and applicability to large-scale processes. The thermodynamic point of view demands a large energy consumption, and an active catalyst as neither entropy nor the enthalpy of the reactions seems to be favorable to drive the reaction. However, this does not signify that the CO₂ reduction reaction will not take place. Indeed, a substantial amount of energy is a prerequisite of this reaction and the reaction seems to be inefficient from an economical point of view.⁸⁹  The research into the thermal reduction of CO₂ can be further classified into the production of three classes of products like CO, CH₃OH, and other hydrocarbons as discussed below. A graphic representation of the process is given in Figure 1.18.

Thermocatalytic conversion of CO₂ to CO through reverse water gas shift (RWS) reaction as shown in reaction (1.1) is regarded as an extremely valuable process as CO can be used as the preliminary material for the generation of methanol and other fuels through established syngas (CO and H₂) conversion technologies, such as Fischer–Tropsch (FT) synthesis and methanol synthesis. This reaction is highly contested by other feasible thermal conversion reactions like the Sabatier reaction or CO₂ methanation and methanol formation, which decreases CO yield. The strongly exothermic methanation reaction is thermodynamically more favorable to take place when compared to mild endothermic RWGS reaction.⁹⁰  According to Le Chateliers principle, endothermic reactions are favored at higher temperatures. Hence, the reaction that converts extremely stable CO₂ to energetic CO needs to be carried out at higher temperatures to avoid any side reactions. This makes the process more energy-intensive.

This is a reversible process and the catalyst that drives the forward reaction is also active for the backward reaction. To compete with a highly exothermic reaction of methanation reaction at lower temperatures, the activity and selectivity of the thermal catalyst is a very significant challenge. Iron and copper are the most active catalyst for RWGS reaction during its operation at higher and lower temperatures respectively. These common catalysts suffer from poor thermal stability and form methane, which is an undesired product in the present process. Therefore, it is important to develop a more interesting catalyst with fewer drawbacks. The combination of Cu and Fe based catalysts like CuFe₂O₄ with good activity and durability along with the largest amount of weak basic sites acts as an excellent precursor when compared with conventional copper-based and iron-based thermal catalyst.⁹¹ 

CH₃OH is a highly desirable product as it can serve as initial starting material for olefins and higher aromatics. This reaction competes with RWGS under low-pressure conditions and is considered to be less selective and unfavorable. To win over this competition, high-pressure conditions and low temperatures (473–523 K) are applied to suppress the RWGS reaction as in reaction (1.2).⁸⁹ 

The main reaction routes for CO₂ conversion to CH₃OH include:

formate route, wherein the key intermediate HCOO* is hydrogenated to H₂COO* or HCOOH*, and further hydrogenation to H₃CO* or H₂COH*, and finally, CH₃OH is formed. The extensive commercialization of this technology involves the development of proficient catalytic systems that are capable of high conversion and product selectivity.^92,93  Larmier et al.¹⁹  have developed zirconia supported copper nanoparticles as an exceedingly efficient thermal catalyst for CO₂ reduction. Zirconia increases selectivity and enhances the CH₃OH production rate. It was observed that the interface of Zirconia–Cu facilitates a more favorable adsorption mode, like the bent structure of CO₂. This adsorption mode facilitates hydrogenation of CO₂ to CH₃OH and CO at a rapid rate.

In the carboxylate route, CO₂ is first hydrogenated to COOH* with O–H bond formation, then COOH* is converted to COHOH*, and finally producing CH₃OH, and the CO* formed during the RWGS route is considered to be the key intermediate, followed by subsequent hydrogenation reactions.⁹⁴ 

The carbon capture and its conversion by CO₂ methanation is a highly effective method. However, this method was not found to be commercially viable. Methanation reaction involves the conversion of CO₂ and CO to CH₄ through their hydrogenation. The methanation reactions of CO₂ were initiated by Sabatier and Senderens in 1902 as an important step for the creation of synthetic natural gas. CO₂ methanation is an exothermic reaction, wherein H₂ and CO₂ react to produce CH₄ and H₂O. The reaction is represented as reaction (1.3). Diverse reaction mechanisms have been suggested, generating various reaction intermediates with numerous guiding factors.

The extensively accepted mechanism involves a combination of an endothermic RWGS and an exothermic CO methanation, as shown in reactions (1.4) and (1.5) respectively.

The overall methanation reaction is favored at a lower temperature. The rate of reaction increases with an increase in temperature. However, the RWGS reaction is favored at increasing the temperature above 500 °C. Consequently, the methanation reaction is not explored above 500–600 °C to avoid the hindrance of competing for RWGS reaction. Agreeing with Le Chatelier's principle, the CO₂ methanation is preferred at elevated pressures up to a certain point and further increase of pressure is less effective.⁹⁵  Typically, the catalytic CO₂ methanation is extensively carried out in a fixed-bed reactor because of various advantages like high surface area, the low-pressure fall in the column, and improved regulation on various reaction parameters. The enhanced reactivity of Ni (15%)/TiO₂-DP in a fixed bed reactor is due to the great dispersion of Ni catalysts on the TiO₂ support.⁹⁶  The supported Ni catalysts are the most extensively considered as catalysts for the Sabatier reaction. However, the reaction of Ni particles with CO results in the formation of mobile nickel subcarbonyls that leads to the deactivation of Ni-based catalyst even at low temperatures. This has resulted in the use of noble metals, like Pd, Pt, and Ru that are stable at operational conditions and express higher catalytic activity. Supported ruthenium catalysts has been established as the active one as they are suitable to operate even at lower temperatures like less than 200 °C, where CO formation is withdrawn due to both constrained kinetics and the endothermic type of the corresponding RWGS reaction as shown in Figure 1.19.⁹⁷  Martins et al.⁹⁸  have observed that the high surface area developed by hydrotalcite-derived metal makes them most promising for CO₂ methanation. Several challenges such as finding a suitable catalyst with appropriate support or a promoter that operates at low temperature were encountered. Numerous good combinations of the catalyst with less explanation on their intricate mechanism and intermediates have been reported with increased efficiency.

The current rapid climate change and energy crisis have prompted the research community to utilize CO₂ in a green and proficient way. Biological mitigation approaches include the use of bacteria and algae to produce high calorific fuels like biodiesel, fatty acids, and pigments through a sustainable route as shown in Figure 1.20. The high stability (∆G⁰), optimized reaction conditions, considerable energy input, and catalysts with large activity and stability are essential to convert CO₂ into valuable chemicals. The enzymes are the extraordinary molecular devices that efficiently regulate the configuration of such chemical transformations.⁹⁹  The metabolic pathways in living organisms are catalyzed by enzymes and they act to be highly potent for practical application studies when compared with microorganisms as they do not require any vigilant conditions to keep them alive.¹⁰⁰  The different CO₂ reduction catalyzing enzymes are discussed in the following sections.

Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly abbreviated as “Rubisco” assists plants and other photosynthetic organisms to assimilate atmospheric CO₂ to energy-rich molecules like glucose. The structure of the Rubisco enzyme shows that it comprises two types of protein subunits, a major chain of about 55 000 Da and a minor chain of about 13 000 Da.¹⁰¹  The enzymatically active Rubisco substrate binding sites are located in the major chains that produce dimers in which amino acids from the respective major chain add to the binding sites. The activation of Rubisco enzymes and the correct position of Mg²⁺ ions are required for enzymatic activity. The Rubisco facilitates the binding of metal ions after the active site is carbamylated. The activation of Rubisco by CO₂ is a principal step in catalyzing the carboxylation or oxygenation of RuBP.¹⁰²  The biochemical origin of this activation is carbamylation of an active site lysine residue. The concept of a specific protein called activase for activating Rubisco was initially presented in 1985 based on genetic and biochemical studies of a high CO₂-requiring mutant of Arabidopsis.¹⁰³  The accumulation of an activating CO₂ molecule to a lysine residue in its active site forms carbamate and involves deprotonation of lysine residue causing it to rotate by 120° to the trans conformer. This decreases the distance between the nitrogen of Lys and the carbon of CO₂ facilitating a closer interaction that leads to the generation of a covalent bond, resulting in the generation of carbamate species. This is stabilized by monodentate coordination to an Mg²⁺ ion.¹⁰⁴  Therefore, carbamylation is a necessity for Rubisco activity and displays a large amount of mechanical integrity in the absence of carbamylation as shown in Figure 1.21.¹⁰⁵ 

The use of dehydrogenase enzyme utilizes two possible pathways of either CO₂ reduction to CO utilizing carbon monoxide dehydrogenase and oxidation of CO₂ to CH₃OH by a three-step enzyme cascade that involves initially to HCOOH by formate dehydrogenase, HCHO by formaldehyde dehydrogenase, and alcohol by dehydrogenase enzyme as shown in Figure 1.22.¹⁰⁶ 

The foremost task of the dehydrogenase enzymes is to transfer the electron/H₂ from an oxidant to a reductant that acts as an acceptor. The dehydrogenase enzyme specifically endows suitable catalytically active sites for CO₂ reactant molecules and intermediates that are formed. The majority of the CO₂ reducing dehydrogenases possess a cofactor such as nicotinamide adenine dinucleotide-H₂ (NADH), nicotinamide adenine dinucleotide phosphate-H₂ (NADPH) that possess appropriate binding sites to donate electrons or H₂. Therefore, for each reduction step, one NADH molecule is irreversibly oxidized to NAD⁺.¹⁰⁷ 

The CO dehydrogenase enzyme catalyzes the reverse water–gas shift reaction, wherein CO₂ is converted to CO in the presence of an appropriate electron donor like NADH, NADPH, methyl viologen (MV²⁺).¹⁰⁸  CO dehydrogenase has an ancient origin and was established in primitive microbes prior to diversification into the Bacteria and Archaea domains.¹⁰⁹  CO dehydrogenases are identified as two groups, the MoSCu containing enzymes from aerobes, that catalyze the CO oxidation to CO₂ only, and serves to be less interesting in the present topic. The Ni–Fe enzymes from anaerobes are highly exploited. Homodimeric Ni CO dehydrogenase contains five metal complexes as clusters.¹¹⁰  The active site of [NiFe] CO dehydrogenase known as the C cluster consists of Ni and Fe centers bound to three sulfur ligands that cluster these two metal centers in close proximity. Figure 1.23 represents the DFT calculated system.¹¹¹ 

Formate dehydrogenases are a class of enzymes that catalyze the oxidation of formate to CO₂ by donating the electrons to a second substrate (S_ox) and their reverse reaction of CO₂ by donating the electrons to a second substrate (S_red). The second substrate of formate dehydrogenase is classified into two categories, NAD-dependent and cytochrome dependent.¹¹²  The NAD-dependent ones are mainly in methylotrophic yeast and bacteria and are dynamic in the C1 compounds catabolism like CH₃OH.¹¹³  The cytochrome-dependent ones are mainly found in anaerobic metabolism in prokaryotes.¹¹⁴  The conversion reactions are represented in eqn (1.6) and (1.7):

NAD-dependent reaction¹¹² 

Cytochrome-dependent reaction

Formaldehyde dehydrogenase is used to catalyze the generated formate from CO₂ to formaldehyde using NADH as the terminal electron donor represented as reaction (1.8). This intermediate step is the bottleneck reaction, as this enzyme displays less activity and is profound to pH, and substrate/product concentration.¹¹⁵  Therefore, a great deal of research effort is concentrated in improving the activity and stability of the enzyme.¹¹⁶ 

Alcohol dehydrogenase is an enzyme-containing Zn medium-chain and catalyzes the oxidation of alcohol-containing groups into aldehydes in the existence of NAD⁺. In this enzyme cascade, the final role of alcohol dehydrogenase is to convert HCHO to the desired CH₃OH product.¹¹⁷  The vitamin co-factor, NAD, accepts reducing equivalents like electrons or hydrogen from the alcohol. As a result, the alcohol is oxidized to the aldehyde product and, NAD+ is reduced to the NADH and ADH.

Although the reduction of CO₂ by various methods such as electrochemical, photocatalytic, enzymatic, and thermal methods are strong in their principle and a number of research advances have been achieved, there are still many challenges that remain unresolved.

In this chapter, we opened the discussion by addressing the global concern to mitigate the expanding level of CO₂ in the atmosphere and later expanded it to the various methodologies that efficiently convert harmful and abundant CO₂ into valuable chemicals and energy fuels. CO₂, being an extremely stable molecule with high bond energy makes it highly unfavorable to participate in reduction reactions. This does not mean that the CO₂ reduction would not take place; instead, it is important to develop an efficient catalyst that would reduce the energy barrier in the process enabling higher energy efficiency.

It was identified that the various efforts that have been built by the research community can be basically categorized into electrochemical, photocatalytic, thermal, and enzymatic methods. The widely developed CO₂ reduction approaches that are discussed in this chapter ensure that they will play a dual beneficial role in terms of CO₂ mitigation as a response to global warming and the generation of valuable fuels to meet the rising energy demand.

Among the several methods, electrochemical and thermal methods provide maximum output as they involve high energy input in the form of electricity and heat energy. The use of an expensive energy source makes the process less feasible in terms of practical use. However, CO₂ reduction using photocatalytic and biological methods is highly advantageous in terms of renewable, ecofriendly, and more importantly, sustainability of the process.

Although the reaction mechanism seems to be highly complex and elusive, basic knowledge of surface phenomena has focused the entire research on the engineering of the catalyst.

Although a great number of catalysts have been developed to achieve selective conversion, certain confines make it hard to satisfy commercial needs. The drive towards high surface area and enhanced CO₂ adsorption have created porous nanomaterials as efficient catalysts. Advanced nanocomposites when compared with conventional composites possess various advantages such as better electrical conductivity, thermal conductivity, a higher lifetime of charge carriers, and greater recyclability.

Finally, all the processes discussed have still failed to meet large-scale integrated reactor development. Therefore, this process remains in its infancy. The present study has significantly evaluated the potential of these CO₂ reduction techniques in becoming established to meet the rising energy demand and CO₂ neutralized economy.

CO₂ capture and sequestration techniques with several pilot plants are currently in operation around the world. The acceptance level of mature technologies related to the recycling of CO₂ that are widely being developed by various researchers depends on certain principal barriers that are precisely being influenced by the efficiency and economic issues during their scale-up. The consideration of developing an industrially viable technology in research works should essentially avoid the use of costlier noble metals, and incorporate more sustainable approaches, which would reduce the later scale-up risk. Apart from these, social issues like acceptance of CO₂ storage sites and CO₂ reduced products need to be addressed more proficiently. For the presently developed mature technologies, CO₂ is a reactant that is abundantly available in nature, and its conversion to value-added fuels would be highly profitable.

MS sincerely acknowledges the department of science and technology for the award of the DST-Inspire Faculty research grant (IFA-CH286).