CO2 Activation on Catalyst Surfaces
Contributors:

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides in this scale dimension can catalyze the CO2 transformation reactions. Herein, CO2 activation on metal-based catalyst surfaces and how their structures impact the activation process are highlighted.

  • CO2
  • CO2 activation
  • metal oxide nanoparticles

1. Introduction

CO2 is a chemically inert molecule due to its kinetically stable nature; thus, its conversion by reduction to economically viable products relies on its activation to kinetically vibrant species [1]. The stable nature of CO2 and high activation barrier (1.9 eV) implies that its transformation in the presence of a catalyst should create a unique environment for facilitating activation pathways [2][3][4][5][6]. It is possible to apply either homogeneous or heterogeneous catalysis to transform CO2 into value-added products, the latter through thermo-, electro-, or photocatalysis, to induce the catalytic reactions [7][8]. Each of these processes comes with its shortcomings and benefits. The electrocatalytic and photocatalytic processes have advantages in tuning the reaction products but are difficult to scale up [9]. Scale-up is, however, promoted in the thermocatalytic process due to its fast reaction rate and high efficiency, although thermodynamic balance is an issue. Given the intricacy of the accompanying reaction mechanisms and the dynamics of heterogeneous catalysis under reaction conditions, there is still a lack of very fundamental understanding of the chemistry of CO2 reduction. In the presence of small but reactive molecules such as hydrogen or water, CO2 can be transformed into stable products over heterogeneous catalysts with more favorable thermodynamics. The catalyst must possess activity for activating CO2 in addition to enabling effective conversion reactions.
Adsorption and activation are two important steps that occur during the CO2 reduction on catalyst surfaces. The adsorption (physisorption and chemisorption) of CO2 on the surface of heterogeneous catalysts has become a subject of growing research interest in recent years [10]. The activation of CO2 on the heterogeneous catalyst surface yields CO2-derived species that eventually convert to useful products. Several CO2-derived species, including carboxylate and carbonates, have been identified, irrespective of which approach is adopted for CO2 conversion [3]. The generation of specific activated carbon species determines the kind and selectivity of the product(s). Co-reactants such as water contribute to the overall reduction reaction by promoting adsorption and subsequent activation. As with most chemical conversion technologies, the choice of catalyst is also an important factor, which must have suitable selectivity and activity for activating CO2 under relatively mild conditions. Many studies reported in the literature tried to understand how to promote the chemisorption and subsequent activation of CO2 by focusing on catalysts preparation and structures. Specifically engineered or surface-modified catalysts have shown improved performance for CO2 activation. Catalysts (metal oxides) with rich surface defects or high concentrations of oxygen vacancies have been particularly effective for CO2 activation. Oxygen vacancies can greatly influence the interaction of CO2 with the surface and enhance the adsorption of CO2 molecules. Thus, oxygen vacancies play important roles in CO2 conversion. Surface defects can be created in metal oxide catalysts either during reactions or incorporated by external methods. Understanding the nature of CO2 in the activated form and the catalyst characteristics will be important in developing novel strategies for activating CO2. An appreciable understanding of the reaction of CO2 on pure metal surfaces is well documented; however, less is known about the reactivity of CO2 on metal oxide surfaces, e.g., how the surface defects contribute to the mechanism of CO2 activation. A discussion on the CO2 activation mechanism on metal oxide surfaces and the roles of surface defects in the activation process is put forward in this piece.

2. Activation of CO2 on Heterogeneous Catalyst Surface

The interaction of CO2 molecules with metal-based surfaces has attracted intense research attention for a long time. Reduction of CO2 by H2 has been studied on some transition metals (e.g., Cu, Co, and Ni) and metal oxide catalysts (e.g., TiO2, CeO2, and In2O3). The catalytic surfaces for the activation of CO2 include metal sites, metal-oxide interfaces, and oxygen vacancies. For facile CO2 activation and conversion, efforts should be made to know, design, and optimize functional sites in heterogeneous catalysts, which involves constructing surface sites with a charge density gradient capable of reorganizing the electronic structures of CO2 and polarizing the adsorbed species [11].
Metal-based catalysts are capable of effectively activating CO2 even in the absence of hydrogen [12]. These include pure metal surfaces, doped or promoted metal surfaces, and supported metal nanoparticles. On supported metal surfaces, CO2 activation occurs mainly through acceptance of charge from the metal; the oxide support plays a big part in the activation process through different mechanisms such as acid sites interactions and provision of oxygen vacant sites [13]. The metal surface is also important in the dissociation of H2 in CO2 hydrogenation reactions [3].

2.1. CO2 Activation on Representative Pure Metals

Metal nanoparticles sit on a metal oxide serve as active sites for the electron transfer. Transition metals such as Cu, Ni, and Fe are particularly energetic for activating CO2. Their surfaces possess high binding abilities to CO2. DFT calculations evidenced that on Pt, Rh, Ni, Cu, Ag, and Pd (111) surfaces, the affinity toward oxygen is different for the metals, which selects their reaction pathways for the CO2 activation in the RWGS) reaction. Pt, Ag, and Pd tend to favor the COOH-mediated mechanism, whereas Rh, Ni, and Cu dissociate CO2 into CO and O [14]. This difference underlines the variation in the CO2 dissociation barrier of the different metal groups. Thus, the nature of the interaction between the adsorbed O and the surface is critical for determining the CO2 dissociation barrier. In the activation of CO2 via the charge transfer mode, which is prevalent on metal surfaces, the partial and full charge transfer leads to the formation of CO2δ− and CO2, respectively. The degree of charge transfer can be analyzed on metal surfaces after adsorption. Physisorption and chemisorption of CO2 on single crystal surfaces of various metals have been studied by means of DFT calculations [15]. Chemisorption states are highly dependent on both the metal itself and adsorption sites. Chemisorbed CO2 molecules often have a bent structure with O–C–O angle varying between 121 and 140° compared with the nearly linear coordination of the physisorbed CO2 molecules, and their extent depends on the kind of metal surface. In other words, the degree of CO2 activation varies with metal surfaces. Similar behavior can be observed for the amount of charge transferred. According to Wang et al. [16], who investigated CO2 chemisorption on nine transition metal surfaces (Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, and Au), the adsorption strength is affected by both the d-band center of the metal surface and the charge transfer, which control the activation of the C=O bond. It is possible to promote chemisorption by adjusting the properties of the catalysts, such as the catalyst surface area, surface defects, basic sites, and the addition of promoters [17].
The coordination of CO2 with metals can take different modes via the electron-deficient C atom as the electron acceptor and the C=O bonds or O atoms as the electron donor. Electron-rich metal surfaces such as Ni (110) and Cu (100) generally activate CO2 by attaching to the carbon atom [18][19]. In the process, electrons in the dz2 orbital of the metal transfer to the unpopulated antibonding π* orbital of the CO2 molecule. Consequently, negatively charged surfaces are efficient for CO2 activation via the charge transfer mode. Such charged surfaces can be generated through inserting strong metal-support interaction, bimetallic and ligand effects, etc. [11]. Electron-deficient metal surfaces, such as Ti, Cr, V, and Mn, favor the end-on coordination with CO2 [20]. In this mode, the bending or distortion of the linear CO2 molecule is difficult [11]. For metal centers with combined binding behavior, i.e., possessing both an electron acceptor site and an electron donor site, CO2 adsorption generally prefers the bridge sites. More efficient activation can result through this mechanism and lead to bending of the linear CO2 molecule from different sites [11].
Investigations by DFT calculations revealed the characteristic adsorption and activation of CO2 on Rh, Pd, Pt, Ni, Fe, Cu, Re, Al, Mg, and Ag metals [14][21]. Strong evidence has been provided for the formation of CO2 according to spectroscopic results. Depending on the type of metal, CO2 can also dissociate into CO and O or be transformed into CO32− and CO. The activation of CO2 on Pt, Rh, Ni, Cu, Ag, and Pd followed different elementary steps as a result of the different levels of interaction of the metals with adsorbed oxygen in the RWGS reaction. Metals with high affinities toward oxygen presented lower activation barriers, leading to facile hydrogenation reactions [14]. The presence of preadsorbed oxygen was responsible for forming carbonates of different structures [14][21]. On most metal surfaces, CO2 activation is highly surface orientated, pressure- and particle size-dependent [16][22][23][24]. For e.g., Yu et al. [24] demonstrated through spin-polarized DFT calculations that adsorption and dissociation of CO2 were dependent on the Co particle size. They showed that Co55 nanoclusters had the highest CO2 dissociation activity in comparison to Co13 and Co38. However, Co13 activated CO2 with the smallest O–C–O angle (123°) against 137° for both Co38 and Co55 nanoclusters.
Cu-based materials have gained much attention in the CO2 conversion process due to their wide applicability in the different conversion processes and low cost [9][25]. Despite these and other massive studies, the activation of CO2 on Cu catalysts is still an issue due to the poor understanding of its mechanism. Studies have shown that CO2 interacts weakly on low-index Cu surfaces under UHV conditions [26][27]. However, a recent study found Cu (100) surface to be more active in dissociating CO2 than Cu (111), producing atom oxygen [19]. Ambient pressure X-ray photoelectron spectroscopy (APXPS) and DFT calculations revealed the activation of CO2 on Cu surfaces. APXPS showed that CO2 adsorbed as CO2δ on Cu (111) surface under a pressure of 0.01 mbar at 300 K. With an increase in pressure to 1 mbar, adsorbed CO2δ partially transformed into carbonate as a result of the disproportionation reaction between CO2 molecules. Subsequent annealing at 400 K or higher temperatures led to the dissociation of CO2δ and carbonate and the formation of a chemisorbed oxygen-covered surface. On Cu (110) surface, the CO2δ gradually dissociated into CO and chemisorbed oxygen under the same CO2 pressure at room temperature. On both surfaces, atomic oxygen was generated that catalyzed the self-deactivation of CO2 adsorption. The DFT results, which collaborated experimental findings, further indicated that the Cu (110) surface was more active than the Cu (111) surface in breaking C–O bonds [23]. Comparing the adsorption of CO2 on Cu (111), (100), and (110) surfaces, it was found that CO2 molecules aligned parallel to Cu (111) and (100), whereas a vertical configuration was more stable for the adsorbed CO2 on Cu (110) with one of two oxygen atoms towards the surface. The degree of CO2 activation followed the order: Cu (110) > Cu (100) > Cu (111) [16]. A decrease in the activation energies for CO2 dissociation has been observed when Cu surfaces have step or kink defects in comparison with the flat surface. Chemisorption of CO2 was reported on Cu stepped surfaces [26][27][28][29].
Ni-based catalysts can dissociate and convert CO2 into value-added products, such as methane; thus, a fundamental understanding of the interaction between CO2 and Ni surface at the atomic level is crucial to design even more efficient Ni-based catalysts. According to theoretical studies, the surface orientation of Ni influences the activation of CO2 by altering the energetics for subsequent C–O bond cleavage [30]. Ab initio calculations using slab models have shown that CO2 reactions on model Ni are surface sensitive, with reactivity in the following trend: Ni (110) > Ni (100) > Ni (111) [30][31]. Experimental investigations revealed the capability of Ni (110) surface to molecularly adsorb and subsequently dissociate CO2 at room temperature [32]. By using in situ APXPS, carbonate was identified as the dominating surface intermediate at room temperature upon CO2 adsorption on Ni (111) and Ni (110) surfaces [33][34][35]. However, where there are multiple dissociation products, their distribution depends on the surfaces. Carbonate, CO, and graphitic carbon were all observed on both Ni (111) and Ni (100) surfaces under a CO2 pressure of 0.2 Torr. Ni (111) was predominantly covered with carbonate, whereas adsorbed CO* and graphitic carbon were prevalent on the Ni (100) surface as indicated in Figure 1a–d [35]. The CO2 adsorption and dissociation on ideal Ni (111) and stepped Ni (211) surfaces are shown in Figure 1e. It can be seen that CO2 adsorption is endothermic by 20 kJ·mol−1 on Ni (111) surface, whereas it is exothermic by 40 kJ·mol−1 on Ni (211) surface [36].
Figure 1. Ambient pressure XPS spectra of (a,b) C 1s, and (c,d) O 1s Ni (111) and Ni (100), respectively. CO2 adsorption was performed under 0.2 Torr CO2 at room temperature. Reproduced with permission from [35]. Copyright 2019 American Chemical Society; (e) Energy profile diagram for the CO2 activation on Ni (111) and Ni (211) surfaces, edges of Ni13 particle, and edges and terraces of the Ni55 particle and the activation geometries on the Ni13. Reproduced with permission from [36]. Copyright 2016 American Chemical Society.
Based on the experimental results of the ultrahigh vacuum (UHV), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS), it is difficult for CO2 to adsorb on a clean Pt (111) surface between 110 and 300 K, but the dissociation capability can be improved by doping alkali metals such as potassium [37][38]. Conversely, it was reported that Pt foil treated with CO2 in the gas phase during in situ UHVXPS experiments at 77 K formed chemisorbed CO species [39]. In collaboration with the latter assertion, on the clean Pt (111) surface, CO2 dissociated into adsorbed CO and O at both room and elevated temperatures [40]. The production of adsorbed CO increased upon the introduction of H2 (hydrogenation reaction). This observation was obvious for the CO2 adsorption at all temperatures and led to the deoxygenation (consumption of oxygen) of the surface, cleaning the sites for further CO production and desorption from the surface at elevated temperatures. The Pt surface was active in the RWGS reaction. At low pressure, the RWGS was initiated at 300 °C; on the contrary, at high pressure (H2:CO2 of 150 mtorr: 15 mtorr), a low temperature (200 °C) favored the initiation of the RWGS reaction, and the conversion of CO2 increased with increasing temperatures [40]. Moreover, under a pressure of 40 mtorr of pure CO2 and at temperatures below 150 °C, graphitic carbon has been also observed as a product of the Boudouard reaction. IR spectroscopy revealed the size-dependent nature of CO2 adsorption on Pt clusters. On small Pt clusters anions (Ptn, n = 4–7), CO2 was highly activated but remained molecularly adsorbed on Pt4. On large clusters, dissociative adsorption was observed [10].

2.2. CO2 Activation on Bimetallic/Alloyed Catalyst Surfaces

Bimetallic surfaces are highly unique and active for a wide range of CO2 transformation reactions due to the electronic and geometric alterations within the structure. These alterations could be observed as a change in the morphology of metal, adsorption mode and configuration, and chemical ordering with varying composition and particle size [5][41]. This can help to control the adsorption properties to attain desired adsorption coverages. Numerous studies have discussed both activation of CO2 and the subsequent conversion of CO2 to fuels and chemicals on bimetallic catalysts. Ko et al. [15] studied the CO2 activation and adsorption by performing DFT calculations on a range of bimetallic alloy surfaces. The dissociation energy barriers of CO2 were screened by combining Bronsted–Evans–Polanyi (BEP) relation, scaling relation, and surface mixing rule. It was found that CO2 dissociated into CO and O, which sum of their adsorption energies was linearly related to both the energy for CO2 dissociation and that for CO2δ− adsorption. The activation of CO2 proceeded through a direct dissociation (CO2 → CO + O) mechanism in three successive elementary steps: physisorption of CO2 from the gas phase on the metal surface, chemisorption of CO2δ− from the physisorbed CO2, and direct dissociation of CO2δ− into CO and O.
The predicted activation energies for the CO2 dissociation on bimetallic alloy surfaces are shown in Figure 2, with the row and column indicating the host and solute metals of the bimetallic alloys, respectively. The alloying effect would lead to a reduction in surface reactions when the solute metals are placed in the bulk region. In the figure, the activation energy (Eaact) decreases from right to left and from bottom to top, thus alloys with relatively low activation energies (~0.75 eV) are Fe-, Ru-, Co-, Ni-, Rh-, and Ir-based alloys, whereas Pd-, Pt-, and Cu-based alloys possess high activation energies (~1.51 eV). Still, some metals, including Ru-, Co-, Ni-, Rh-, Ir-, and Cu-based alloys, have activation energies in between the extremes (0.76–1.50 eV). This picture makes sense for fabricating bimetallic catalysts for CO2 conversion reactions by manipulating the activation energies [15].
Figure 2. Screening for Eaact for CO2 dissociation on pure metals and bimetallic alloys. Gray cells indicate the bimetallic alloys which are not preferred to the surface segregation of solute atoms. Eaact were estimated by combining BEP relation, scaling relation, and surface mixing rule. Reproduced with permission from the authors of [15]. Copyright 2016 American Chemical Society.
The mechanisms of CO2 dissociation on bimetallic clusters (Pt3Ni1, Pt2Ni2, and Pt1Ni3) were investigated and characterized by the activation barrier [42]. Compared with the single metallic clusters (Pt4 and Ni4), results showed the activation barrier to be between those of the monometallic clusters. Increasing Ni atoms in the bimetallic clusters moderately raised the activation barrier, which means that such a bimetal combination could have a significant negative impact on the activity of Pt for CO2 reaction [42].
Results from an ab initio study of chemisorption and activation of CO2 on Pt-based transition-metal nanoalloys on 55-atom nanoclusters (PtnTM55−n), where n = 0, 13, 42, 55, and TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Au, indicated a linear correlation between the interaction energy, charge transfer from the nanoclusters toward CO2 and the bent CO2 angle [5]. It was further realized that the interaction energy was enhanced for larger angles and molecular charge. With 55 atoms for Cu, Ag and Au in the Pt alloy, a change from physisorption to chemisorption was observed, whereas the strong interaction energy of CO2 with Os55, Ru55, and Fe55 can be decreased by alloying with Pt [5]. It means that certain metals (Fe, Co, and Ni) activate CO2 more strongly as monometals than in an alloyed form due to weaker adsorption energy in the latter.

2.3. CO2 Adsorption and Activation on Metal Oxide Surfaces

2.3.1. Metal Oxide

Various metal oxides (MOx) or (MxOy) are investigated as supports or as catalysts for CO2 conversion, including In2O3, CeO2, ZnO, ZrO2, TiO2, and CeO2. Metal oxide surfaces consist of both metal (Mn+) and oxygen (O2−) ions, which are effective sites for CO2 activation. The activation can occur by coordination to one or two adjacent metal sites through the terminal oxygen atoms or C atom of the CO2 molecule, forming monodentate or bidentate carbonate species [43]. Interaction of the C atom is on the surface oxygen sites of the metal oxide. The CO2 activation can also occur via the σ-bond and π-bonds activation on metal ions and oxygen ions, respectively, as observed upon chemisorption on metal oxides applied as catalyst supports [44]. Due to the large surface areas, switchable redox properties, and rich oxygen vacancies, metal oxides can act as adsorption and activation sites for small molecules, including O2, H2, and CO2 [45]. The surface oxygen vacancies interact with the carbon and/or oxygen atoms of CO2 through which electron transfer from the oxide defective site to adsorbed CO2 becomes feasible. One example is In2O3, which is rich in oxygen vacancies and has shown a high activity for CO2 activation and methanol synthesis by hydrogenation [45].
On oxide surfaces, generally, CO2 interactions can vary from physisorption to chemisorption, the extent that affects the structure and reactivity of the adsorbed CO2, and the kinetics and mechanisms of surface catalytic reactions [43]. The surface structure is important for CO2 adsorption and activation [46]; thus, the interaction of CO2 with metal oxides can be structure-dependent. It was found that CO2 adsorption on Zn2GeO4 (001) was higher than that on Zn2GeO4 (010) surface [46]. The interaction with (010) surface led to bidentate carbonate species, whereas on the (001) surface, stronger interaction with CO2 resulted in a bridged carbonate-like species. The strongest adsorption based on calculated CO2 adsorption energies was around the surface oxygen vacancy site on both surfaces. Analysis of the LDOS and Mulliken charge for adsorbed CO2 on perfect Zn2GeO4 surfaces revealed that CO2 formed a CO2δ− species upon accepting electrons from the surface. CO2 molecule was found to be activated on the CuO surfaces ((011), (111), and (−111)), with strong adsorption only on the (011) surface. The CuO (111) and CuO (−111) surfaces showed relatively weak adsorption. CO2 activation was characterized by structural transformations and charge transfer that resulted in the formation of bent CO2δ− species with an elongation of the C–O bonds [22].
The ability of metal oxides to bind and activate CO2 depends greatly on several factors, including their preparation methods, physiochemical properties, redox properties, and electronic and geometric structures [36][43][47]. The preparation methods were found to impact the properties of CeO2 nanostructures for the photocatalytic reduction of CO2 [48]. A high surface area was obtained for catalysts synthesized through the sunlight-assisted combustion process, in addition to possessing a small particle size, high concentration of oxygen vacancies, and a narrow bandgap. Compared to that prepared from the conventional combustion process with a spongy-like structure, a porous network consisting of small and uniformed pores was also obtained for the sunlight-assisted process. The superior catalytic properties could be attributed to the novel properties endowed by solar irradiation during the synthesis process. As demonstrated with the CeOx/Cu catalyst, the important roles of metal oxide ions were revealed on the catalytic cycle of H2O and CO2 activation. The Cu phase was reduced into Cu0 that promoted Ce4+ reduction into Ce3+. H2O and CO2 activation occurred on the Ce3+ sites. Without the presence of Cu, Ce3+ would lead to oxidation into Ce4+. However, in contact with Ce4+, Cu0 reacted to form Cu+ and Ce3+, sustaining ceria in the more active state. The cycle is closed when Cu+ reduced to Cu0 [49]. This synergistic effect afforded the catalyst with high reactivity in the RWGS reactions. The chemisorption of CO2 molecules on CeO2 at RT as studied using in situ DRIFTS indicated adsorption at both the Ce3+ and Ce4+ sites, although adsorption was also found at the oxygen sites that resulted in carbonates and bicarbonates species [50]. In the same study, the CO2 chemisorption on TiO2 under similar reaction conditions and instrumentation was observed at both Ti3+ and Ti4+ sites, exhibiting O–C–O vibrations at 1667 and 1248 cm−1 and 2339–2345 cm−1, respectively [50]. The CO2 molecules adsorbed at Ti3+ sites formed CO2 species, which concentration increased with the amount of oxygen vacancies present. Like with CeO2, CO2 chemisorption at the oxygen sites formed carbonates and bicarbonates species. It was observed that the interaction between TiO2 and CO2 molecules is somewhat weak compared with that of CeO2. Such weak interactions can be improved by doping TiO2 with CeO2. CeO2 doping can improve the interaction of TiO2 with CO2 as a result of the introduction of Ce3+, which strengthens the bonding of CO2 with catalyst surfaces and enhances the production of bidentate carbon species that can readily be transformed to surface CO2 in the presence of H2O under solar irradiation. The formation of adsorbed species of CO2 over CeO2/TiO2 could be attributed to the binding of CO2 species to Ti/Ce atoms that have reductive capabilities under photo-irradiation. Furthermore, the Ce3+ availability from CeO2 facilitates photogenerated charge separation; thus, the CO2 adsorption and enhanced charge separation can be tuned for increased activity of CeO2/TiO2 catalyst [51]. The surface area of materials positively correlates with their adsorption capacity. It was found that the Bi12O17Cl2 nanotubes had a higher adsorption capacity for CO2 (~4.3 times) than bulk Bi12O17Cl2 due to the higher BET specific surface area of the former. As a result, the effective adsorption of CO2 on Bi12O17Cl2 nanotubes over bulk Bi12O17Cl2 favored the photocatalytic process [52]. In addition, the high surface area correlated with strong adsorption. Weak chemisorption of CO2 has been reported for CeO2 nanostructures with low exposed surface area [50]. Mesostructured photocatalyst displayed improved activity for CO2 reduction into CH4 due to the presence of high specific surface area and well-developed mesostructure that enhanced adsorption of CO2 [53]. Highly mesoporous In(OH)3 synthesized via the sol-gel hydrothermal treatment exhibited ~20-fold higher efficiency for CO2 reduction in comparison with those lacking mesopores [54]. It is reported that the methanol activity of the In2O3 catalyst could also be improved by increasing the (111) surface area [55].

2.3.2. Characteristic Adsorption of Representative Metal Oxides

Ceria (CeO2) has shown catalytic activity in the reduction of CO2 to liquid fuels and chemicals. It has rich oxygen vacancies and high oxygen storage/release capacity. Several studies demonstrating the interaction of CO2 with high-surface-area ceria catalysts have been reported. As noted in ref [56], CO2 dissociates into CO and an oxygen-containing surface species on the surface Ce3+ ions, which are considered active sites for CO2 activation due to the formation of carbonates or inorganic carboxylates. Graciani et al. reported a highly active CeOx/Cu nanoparticles catalyst for methanol synthesis from CO2 [57]. The catalyst activated CO2 as CO2δ and exhibited a faster methanol production rate than Cu/ZnO, on which CO2 was chemisorbed as CO32. A study on the CO2 adsorption sites of CeO2 (110) surface using DFT was carried out by Cheng et al. [58]. Reduced and stoichiometric ceria (110) surfaces were compared. Results revealed that CO2 adsorption on the reduced ceria (110) surface was thermodynamically favored than on the stoichiometric ceria (110) surface. Furthermore, the most stable adsorption configuration consisted of CO2 adsorbed parallel to the reduced ceria (110) surface at the oxygen vacancy. Upon adsorption, the CO2 molecule distorted out of the plane and formed carbonates with the remaining oxygen anion at the surface [58]. It was suggested that the structural changes in the catalyst after CO2 adsorption were due to charge transfer between the surface and adsorbate molecule. The formation of two different adsorbate species: a carbonate and a weakly bound and linear physisorbed species, were observed upon exposure of reduced CeO2−x (110) substrates to CO2 at low temperatures. There was no evidence for the formation of CO2δ. Furthermore, based on angle-dependent C K-edge NEXAFS spectra, the most preferred orientation of the adsorbate could not be observed. The physisorbed CO2 species and carbonate species were completely desorbed at 250 and 500 K, respectively. The authors remarked that it is most unlikely that the activation of CO2 on the reduced CeO2−x (110) surface was via breaking the C=O bond to form CO and O. However, on fully oxidized CeO2 (110), CO2 adsorbed as a carbonate which was completely decomposed and desorbed as CO2 at 400 K [59]. CO2 adsorbed on the CeO2 (111) surface formed monodentate carbonate species found to be most stable on CeO2 at low coverages [60]. Increasing the CO2 coverage destabilized the formed species, indicating a mixed adsorption mechanism with both carbonate and linearly adsorbed CO2 species. Although CeO2 has been studied for CO2 reduction reactions, the insights into CO2 adsorption, activation, and reaction on ceria surfaces are not yet fully understood.
Titania (TiO2) possesses good photocatalytic properties for many chemical reactions, including CO2 reduction. Since its first demonstration in the photoelectrochemical CO2 reduction to formic acid and formaldehyde by Inoue et al. [61], TiO2-based materials have attracted great research interests in CO2 photoreduction reactions. The adsorption properties of CO2 on both the rutile and anatase phases of TiO2 have been widely studied using various surface science techniques [62][63][64][65]. Sorescu et al. [66] investigated the adsorption and dissociation of CO2 on an oxidized anatase (101) surface using dispersion-corrected DFT and found CO2 to adsorbed at a fivefold coordinated Ti site in a tilted configuration. Based on in situ FTIR experiments, the CO2 adsorption formed CO32 and CO2 bonded to Ti, with absorption bands at 1319, 1376, 1462, 1532, 1579, and 2361 cm−1. The band at 2361 cm−1 was assigned to adsorbed CO2 with Ti–O–C–O adsorption configuration [67]. The 1319 and 1579 cm−1 bands were assigned to bidentate carbonate, while the band at 1461 cm−1 was due to monodentate or free carbonate. Under the vacuum condition, the intensities of all of the bands were reduced at 35 °C. The bidentate carbonate was the predominant species for CO2 on TiO2. The scanning tunneling microscopy (STM) enabled a study of the dissociation of CO2 adsorbed at the oxygen vacancy of TiO2 (110) at the single-molecule level [68]. It was found that the electrons injected from the STM tip into the adsorbed CO2 caused its dissociation into CO and O, and the released O was observed to heal the oxygen vacancy. According to experimental analysis, ~1.4 eV above the conduction band minimum of TiO2 is needed for the electron induction process to dissociate CO2. The formation of a transient negative ion by the injected electron is an important step in the CO2 dissociation, and this can only be possible above the threshold voltage. TiO2 modified with metal oxide nanoclusters possess enhanced activity to adsorb and convert CO2 [69][70]. The Bi2O3-TiO2 heterostructures obtained by modifying TiO2 with Bi had low coordinated Bi sites in the nanoclusters and a valence band edge consisting mainly of Bi–O states due to the presence of the Bi lone pair. Upon interaction of CO2 with the reduced heterostructures, CO or CO2 were observed mainly through electron transfer to CO2, and the Bi2O3–TiO2 heterostructures became oxidized in the process with adsorbed CO2 in carbonate form [70]. In a related study, clean or hydroxylated extended rutile and anatase TiO2 surfaces modified with Cr nanoclusters presented an upshift valence band edge related to the existence of Cr 3d–O 2p interactions, which promoted the CO2 activation. [69]. The activated CO2 molecule reduced its O–C–O angle to 127–132° and increased the C–O bond length to 1.30 Å. It was concluded that the strong CO2–Cr–O interaction induced the structural distortions.
Iron oxides (FeOx) are an important component of catalysts for the conversion of CO2 to hydrocarbons (liquid fuels). The adsorption and activation of CO2 on FeOx have been investigated by researchers [71][72][73]. It is suggested that Fe2+ and Fe3+ cations are crucial for CO2 adsorption. Using TPD, Pavelec et al. [71] observed a weak interaction between CO2 and Fe3O4 (001) surface. On this surface, CO2 molecules existed in the physisorbed state as they desorbed at a low temperature (115 K). However, strong CO2 adsorption was observed on the defects and surface Fe3+ sites. Weak CO2 adsorption has also been observed on Fe3O4 (111) as investigated by various experimental techniques [74]. At different CO2 dosages and temperatures (between 120 and 140 K), TPD experiments suggested CO2 adsorb very weakly on a regular Fe3O4 (111) surface. However, CO2 chemisorption was also observed but at relatively long CO2 exposure times [74][75] via binding to under-coordinated oxygen sites [75]. The formation of chemisorbed species such as carboxylates and carbonates was facilitated by surface imperfections. Conclusively, FeOx exhibit weak interaction with CO2 molecules, and studies are recommended in this direction to adjust its CO2 adsorption strength.
ZrO2 has been demonstrated as catalyst support for the CO2 hydrogenation reactions to a variety of products. In a study on CO2 hydrogenation on Cu/ZrO2 catalyst using the first-principles kinetic Monte Carlo simulations by Tang et al. [76], the authors showed that CO2 prefers to adsorb on the bare ZrO2 nanoparticles surface rather than at the Cu/ZrO2 interface. This led to the bending of the CO2 molecule with a calculated adsorption energy of 0.69 eV. The stretching of the C–O bonds and charge transfer from the ZrO2 surface to the antibonding 2πμ orbital of CO2 were also observed. On the bare ZrO2 surface, bidentate bicarbonate (HCO3) was formed upon CO2 adsorption based on observable IR frequencies at ~1225, ~1620, and ~3615 cm−1 [77].

3. Conclusion

CO2 is a kinetically stable molecule that requires high energy input for the C–O bond breaking. Its proper activation can reduce the high energy barrier substantially, easing conversion by various processes. The CO2 activation is an important step that precedes the conversion of CO2 to chemicals and fuels. It can be effected in the presence of a catalyst by altering the CO2 electronic and molecular properties. Upon accepting an extra electron from a substrate, the neutral CO2 molecule forms an anion with a full charge (CO2–) or partial charge (CO2δ–). Some metals and metal oxides are efficient catalysts for CO2 conversion reactions; thus, they should be good for CO2 activation. In general, metal nanoparticles serve as active sites for electron transfer, with certain factors such as change in morphology of metal particles, nanoparticle size, adsorption mode and configuration, and chemical ordering as the CO2 activation marker. The interaction of CO2 with some pure metals is rather weak but can be improved by incorporating promoters (e.g., alkali metals) with low electronegativity. Metal oxide nanoparticles are utilized as supports or as catalysts for CO2 conversion. Their surfaces comprise both metal (Mn+) and oxygen (O2–) ions, which can act as active sites for CO2 activation. They can activate CO2 by coordinating to one or two adjacent metal sites through the terminal oxygen atoms of the CO2 or by interaction of the carbon atom of CO2 with surface oxygen sites. A particularly interesting feature in metal oxides is the oxygen vacancies that facilitate CO2 adsorption and activation.

This entry is adapted from 10.3390/nano11123265

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