DFT Study of CO2 Adsorption and Hydrogenation on the In2O3

Article pubs.acs.org/JPCC

DFT Study of CO2 Adsorption and Hydrogenation on the In2O3 Surface Jingyun Ye,†,‡ Changjun Liu,† and Qingfeng Ge*,‡ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States

‡

ABSTRACT: Catalytic conversion of CO2 to liquid fuels or valuable chemicals is an attractive alternative to geological sequestration. In the present study, we applied density functional theory slab calculations in the investigation of the adsorption and hydrogenation of CO2 on the (110) surface of In2O3. Our results indicate that the adsorbed CO2 is activated, forming a surface carbonate species by combining with surface oxygen, and has an adsorption energy of −1.25 eV. Heterolytic dissociative adsorption of H2 results in a surface hydroxyl from H binding the surface O site and a hydride from H binding the In site. The migration of H from the In site to the neighboring O site is energetically favorable but has a significant activation barrier of 1.32 eV. Water may adsorb on the surface either molecularly or dissociatively, with adsorption energy of −0.83 eV and −1.19 eV, respectively. Starting from CO2 coadsorbed with the H adatoms on the In2O3 surface, we examined two possible conversion pathways for CO2: (a) CO2 is hydrogenated by the H adatom on the In site to form a surface formate species (HCOO); (b) CO2 is protonated by the H adatom on the O site to form a surface bicarbonate species (COOH). Reaction a is endothermic by +0.33 eV, whereas b is exothermic by −0.78 eV. Although the formation of the bicarbonate species is energetically favorable, the subsequent step to form CO and OH is highly endothermic, with a reaction energy of +1.07 eV. Furthermore, the bicarbonate species can react with a surface hydroxyl easily, resulting in coadsorbed H2O and CO2. These results indicate that hydrogenation of CO2 to the formate species is favorable over protonation to the bicarbonate species on the In2O3 surface. These results are consistent with the experimental observations that the indium oxide based catalyst has a high CO2 selectivity and H2O resistance.

1. INTRODUCTION Increased consumption of fossil fuels due to economic development and global population growth has resulted in a significant increase in anthropogenic greenhouse gases, mainly CO2. Carbon capture and storage have been suggested as the ultimate means to manage the global CO2 level.1−6 However, catalytically recycling CO2 into liquid fuels and valuable chemicals will help to reduce the net CO2 emissions into the atmosphere, and thereby alleviate the greenhouse effect caused by CO2.7−9 Although starting from CO2 and H2O would be the most ideal, the thermodynamic ladder that the reaction needs to climb makes the reaction energetically prohibitive.10 In fact, CO2 reacting with a more active species is often used in practice. For example, CO2 has been used as a reforming reagent for methane in hydrogen production11−16 as well as in methane and syngas production.17−24 Direct synthesis of methanol from CO2 and hydrogen (CO2 + 3H2 → CH3OH + H2O) is moderately endothermic, with a reaction enthalpy of 49.4 kJ/mol. In fact, CO2 was commonly cofed with CO in methanol synthesis.25−28 The versatility of methanol makes it an attractive target for practical application: it can be used as a fuel directly or as a commodity chemical to produce more valuable compounds. © 2012 American Chemical Society

Activation and hydrogenation of CO2 on catalysts based on γ-Al2O3 and β-Ga2O3 have been studied both experimentally and theoretically.29−41 In contrast, studies on the In2O3-based catalyst are not as common, although In, Al, and Ga are in the same group in the periodic table. However, the unique physical properties of In2O3, including its optical transparency and electrical conductivity42−46 as well as high sensitivity to oxidizing (e.g., NO2, Cl2, and O3),47−49 reducing (e.g., H2, CO, and CH4),49−52 acidic (e.g., CO2),53 and basic (e.g., NH3)54 gases, attracted attention in applications in electronic devices, solar energy conversion, as well as gas sensing. In recent years, the interesting catalytic properties of In2O3 have also been recognized and highlighted. Umegaki et al. reported a high CO2 selectivity without CO impurity from an ethanol steam reforming reaction over a worm-like In2O3 catalyst.55 The same authors speculated that the reverse water-gas shift (RWGS) reaction was suppressed due to the presence of In2O3. Similarly, Lorenz et al. reported an almost 100% CO2 selectivity with less than 5% CO formation in methanol steam reforming reaction by using a pure In2O3 catalyst.56 The latter authors Received: January 15, 2012 Revised: February 23, 2012 Published: March 16, 2012 7817

dx.doi.org/10.1021/jp3004773 | J. Phys. Chem. C 2012, 116, 7817−7825

The Journal of Physical Chemistry C

Article

The adsorption energies of CO2, H2, and H2O were defined

attributed the observed selectivity to the presence of active redox surface centers and both relatively weak acidic and rather strong basic sites, which suppress the RWGS reaction. The high CO2 selectivity may be because the oxygen vacancies in In2O3 surfaces created by CO and H2 reduction can be replenished by reoxidizing with oxygen or water but not with CO2.57,58 Recently, Chen et al. found that introducing CO2 into the feed gas could increase the yield of propane on the In2O3−Al2O3 mixed-oxide catalyst and attributed the increase to coupling of the dehydrogenation reaction with the bulk In2O3-catalyzed RWGS reaction.59,60 The seemingly conflicting results may originate from different temperatures used in the experiments: the experiments of Umegaki et al. and those of Lorenz et al. were conducted in the temperature range of 473−673 K, whereas those by Chen et al. were at temperatures >700 K. These results suggest that the reactivity of In2O3 is sensitive to the operating temperature. Most previous theoretical studies have focused on the physical properties of bulk In2O3, such as phase stability, electronic structure and optical properties.61−64 Because of the complicated surface structure of In2O3, few theoretical studies on the catalytic properties are reported.65,66 In the present work, we studied adsorption and hydrogenation of CO2 on the In2O3 surface using density functional theory (DFT) slab calculations, aiming to understand the elementary steps involved in methanol formation from CO2 and hydrogen on the indium oxide-based catalysts. Among the polymorphs of In2O3, we selected the thermodynamically stable body-centered cubic bixbyite structure and created the (110) surface.61 We first examined the adsorption of CO2, H2, and H2O on the surface individually. We then studied coadsorption and reaction between CO2 and H2 as well as CO2 and H2O. On the basis of the reaction energetics and activation barriers of individual elementary steps, we mapped out possible reaction pathways for CO2 hydrogenation on the In2O3 surface.

as ΔEad(M) = EM/In2 O2 − E In2 O2 − EM

where M represents CO2, H2, or H2O as well as their corresponding products upon adsorption. EM/In2O2, EIn2O2, and EM represent the total energies of the surface slab with the adsorbates, the clean surface (110) slab, and a free molecule (CO2, H2, or H2O), respectively. In the case of coadsorption and reaction on the surface, the relative energies were computed with respect to the sum of the total energies of the corresponding free molecules. According to the above definition, a negative value indicates that the process is exothermic, whereas positive values are for endothermic processes.

3. RESULTS AND DISCUSSION 3.1. The (110) Surface. The (110) surface is one of the most stable surfaces with a calculated surface energy of 0.969 J/ m2, close to the value reported by Walsh at al.61 The slab simulating the surface is essentially nonpolar and consists of stoichiometric layers of In and O ions oriented perpendicular to the slab. The relaxed structure of the In2O3(110) surface is shown in Figure 1a. We use the ball-and-stick model in the side

Figure 1. (a) Side and top views of the In2O3 (110) surface. The surface site, together with the corresponding coordination number, is labeled. The In−O chain unit was depicted in the blue box in the top view. The numbering of the atoms within the In−O chain unit was shown in panel b. Red, O atoms; Brown, In atoms.

2. METHODOLOGY AND MODELS All the calculations were performed in the framework of DFT by using the Vienna ab initio simulation package (VASP).67,68 The projector augmented wave method was used to describe the interaction between ions and electrons.69,70 The nonlocal exchange correlation energy was evaluated using the Perdew− Burke−Ernzerhof functional.71 The semicore 4d10 states together with the 5s and 5p states of In were treated explicitly as valence states within the scalar-relativistic projector augmented wave approach.69 A plane wave basis set with a cutoff energy of 400 eV and a 2 × 2 × 1 k-point grid generated by the Monkhorst−Pack method were found to give converged results for the surface calculations The atomic structures were relaxed using either the conjugate gradient algorithm or the quasi-Newton scheme as implemented in the VASP code until the forces on all unconstrained atoms were ≤0.03 eV/Å. The In2O3(110) surface was created based on the optimized bulk unit cell with a cell parameter of 10.1814 Å, which is in good agreement with the experimental value.72 The surface was modeled using a supercell with a dimension of 10.18 Å × 14.40 Å × 17.96 Å. The surface slab consists of 48 O atoms and 32 In atoms, distributed in four layers and separated by a vacuum of ∼10 Å. In all calculations, the bottom two layers were frozen at their equilibrium bulk positions, whereas the top two layers together with the adsorbates were allowed to relax.

view of the slab. In the top view, only the top layer was represented with ball-and-stick. The bottom three layers were shown in the line model. After relaxation, all the In atoms in the first layer relaxed downward slightly, whereas the O atoms relaxed upward. The top layer of the In2O3(110) surface consists of chains of In and O atoms, with the repeating unit shown in the blue box in Figure 1a. The numbering of the atoms in the unit along the chain is provided in the detailed structure depicted in Figure 1b. Each chain consists of two 4membered O−In squares strung together by a short O3−In3− O4 chain symmetrical with respect to In3. Although, each chain is symmetrical, the environment where each atom is located varies, making the atoms in the symmetrical positions not entirely equivalent. For example, In2 is 5-coordinated, while In4 is 4-coordinated. In the top layer, both the 5-coordinated In (In5c) and the 4-coordinated In (In4c) are from the 6coordinated In atoms in the bulk phase of In2O3 and therefore are coordinately unsaturated. All the 3-coordinated O atoms (O3c) in the top layer originate from the bulk 4-coordinated O atoms and are unsaturated too. These unsaturated In and O sites on the surface provide the active sites for the adsorption 7818



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two In−O−In chains, forming a carbonate species. A C atom is inserted in the O2−In2 bond and breaks the ring, leading to C− O2 formation, whereas Oa binds In2, and Ob binds an In atom of the neighboring chain with bond lengths of 1.36 Å, 2.28 Å, and 2.28 Å, respectively. Consequently, the 4-membered ring is replaced by a new 5-membered ring, which links the two O− In−O chains. The adsorption energy of CO2 in C-1 is −1.14 eV. In C-2, Oa−C−Ob and In3−O4−In4 are almost in the same plane, also resulting in carbonate species with an adsorption energy of −1.25 eV. The distances of the newly formed C−O4, Oa−In3, and Ob−In4 bonds are 1.34 Å, 2.19 Å, and 2.20 Å, respectively. Adsorbed CO2 in C-3 is in the form of bidentate carbonate with adsorption energy only −0.70 eV. The Oa−C bond bridges the In4−O5 site, breaking the In4−O5 bond and forming the Oa−In4 and C−O5 bonds. On the basis of the adsorption energies, the In3−O4−In4 site is the most stable site for CO2 adsorption. We note the In3−O4−In4 site close to the symmetric center of the chain. Although C-1 and C-3 are in symmetrical location along the chain with respect to In3 (see Figure 1b), they resulted in very different adsorption structures and energies. The sites to the left of the symmetrical center are more active than the sites to the right toward CO2 adsorption. This correlates directly with the degree of unsaturation of In3 and In4, which are higher than other sites. Table 1 shows the structure parameters, charges, and adsorption energies of CO2 on In2O3(110). In all the configurations, the C−Oa and C−Ob bonds are stretched and the Oa−C−Ob is bent from the linear form of a free CO2 molecule. The total Bader charges on the adsorbed CO2 in C-1, C-2, and C-3 are −0.23 |e|, −0.14 |e|, and −0.19 |e|, respectively, indicating electrons were transferred from the surface to the adsorbed CO2. However, these charges are smaller than those of adsorbed CO2 on the dry perfect γAl2O337 and β-Ga2O332 surfaces. 3.2.2. Dissociative Adsorption of H2. Dissociative adsorption of H2 on the In2O3 (110) surface is exothermic. Figure 3a shows three configurations as the result of heterolytic dissociation of H2 on the surface, resulting in H atoms binding the surface In and O site, respectively. In both H-1 and H-3, H2 dissociates across one In−O of the In−O square, causing the In−O bond to break. H-2 corresponds to hydrogen dissociating across the In−O bond close to the symmetrical center. The newly formed O−H bonds have the same bond length (0.98 Å) in all three structures, whereas the In−H bonds (1.78 Å, 1.72 Å, and 1.75 Å, respectively) are slightly different in each configuration. The adsorption energies in H-1, H-2, and H-3 are −0.40 eV, −1.11 eV, and −1.05 eV, respectively. Again, the sites close to the symmetrical center, O3−In3, are most active for dissociative adsorption of H2. The dissociative adsorption energy shows an interesting correlation with the In−H bond length: the shorter the In−H, the stronger the adsorption. We also examined the possibility of dissociating H2 homolytically, either over two O sites or two In sites. Our results showed that the O sites are energetically preferred for H

and reaction on the In2O3 surface. The similarity and difference will impact the reactivity of these sites toward approaching molecules. 3.2. Adsorption and Dissociation of CO2, H2, and H2O. We first studied the adsorption of CO2, H2, and H2O on the perfect In2O3(110) surface by optimizing the structures individually in either molecularly or dissociatively adsorbed state. In the following presentation, we use C, H, and W to prefix the configurations resulted from CO2, H2, and H2O adsorption, either molecular or dissociative, on the In2O3(110). For example, C-1 represents the first configuration of CO2 adsorption on the surface, and H-1 represents the first configuration of hydrogen adsorption on the surface. In each adsorption configuration, the two oxygen atoms of the adsorbed CO2 are labeled as Oa and Ob if they are in nonequivalent positions. Similarly, the hydrogen atoms from H2 were labeled as Ha and Hb, the oxygen and hydrogen atoms from H2O were marked as Ow, Ha, and Hb, respectively. Different sites of the perfect In2O3(110) surface, including the In1, In1−O1, In2−O2, In2, In2−O1, In2−O2, In2−O3, In3, In3− O3, In3−O4, In4−O4, In4−O5, In4−O6, In1−O5, In1−O6 sites, were examined for CO2, H2, and H2O adsorption. Homolytic sites such as O1−O2, In1−In2, and O3−O4, were also probed. We scanned all the sites from the left end of the chain unit to the right and arranged our results of the stable structures accordingly. 3.2.1. CO2 Adsorption. Three stable adsorption configurations C-1, C-2, and C-3 were obtained after optimizing initial structures from all the sites. The optimized structures and some key structural parameters are shown in Figure 2. The

Figure 2. Optimized adsorption structures of CO2 on the In2O3(110) surface. Black, C atoms; others, see Figure 1.

adsorption energies (ΔEad(CO2)), charges, and vibrational frequencies as well as selected structural parameters of those configurations are summarized in Table 1. In C-1, CO2 bridges

Table 1. Structure, Charges, and Adsorption Energies of CO2 on In2O3(110) charge (|e|) configuration

C−Oa (Å)

C−Ob (Å)

∠Oa−C−Ob (deg)

Oa

Ob

C

CO2

ΔEad (eV)

CO2 C-1 C-2 C-3

1.18 1.27 1.29 1.32

1.18 1.29 1.28 1.23

180 125 126 127

−1.06 −1.15 −1.11 −1.11

−1.06 −1.16 −1.11 −1.12

+2.12 +2.08 +2.08 +2.04

0 −0.23 −0.14 −0.19

−1.14 −1.25 −0.70

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configurations are kinetically stabilized. In the transition state of H adatom migration, the Ha−In bond is elongated to 1.81 Å from 1.75 Å, and the distance between Ha and O6 is 1.61 Å. The Hb−O5 remains at 0.97 Å, the same as that in H-3. However, homolytic dissociative adsorption of H2 can be easily achieved once the activation barrier was overcome as the process is highly exothermic (−1.89 eV). Consequently, we anticipate that dissociative adsorption of H2 produces predominantly surface hydrides on the In sites and surface hydroxyls on the O sites. The consequence of the H adatom migration to transform H-3 to H-6 will be examined in the study of the CO2 hydrogenation reaction pathway (section 3.3). The fact that H2 dissociation is energetically favorable may have important implications for its catalytic activity. We showed previously that dissociation of H2 is not energetically favorable on the perfect β-Ga2O3 surface. Our present results indicate that In2O3 is more active than β-Ga2O3 toward H2 dissociation. In fact, it has been reported that In2O3 can be easily reduced by hydrogen at temperatures T ≥ 673 K, resulting in a mixture of In2O3 and In.42,73 Furthermore, surface reduction by hydrogen at temperatures as low as 300 K has been reported by Bielz et al.74 As we discussed above, heterolytic dissociation of H2 produces a mixture of surface hydride (H adatom on In site) and hydroxyl (H adatom on O site) with a 1:1 ratio, whereas homolytic dissociation only produces surface hydroxyls. The coverage of different surface hydrogen species will affect the reactions involving them. For example, in the WGS reaction, surface hydrides and hydroxyls participate in the process through different elementary reaction steps, altering the reactivity and selectivity. The relative abundance of these surface hydrogen species is also expected to affect the hydrogenation reactions of CO2. 3.2.3. Adsorption of H2O. Water adsorbs both molecularly and dissociatively on the In2O3(110) surface. For molecular adsorption, the structure shown in Figure 5a is the most stable configuration among several initial structures. In this configuration, H2O adsorbs through the formation of the Ow−In3 bond with a bond length of 2.30 Å. The adsorption of H2O on the In3 site causes the O3−In3 bond to elongate to 2.17 Å from 2.07 Å, whereas one of the O−H bonds is increased to 1.03 Å. The adsorption energy of the molecularly adsorbed

Figure 3. Optimized dissociative adsorption structures of H adatoms on the In2O3(110) surface: (a) heterolytic; (b) homolytic.

adatoms. Figure 3b shows the H adatoms on the surface as a result of homolytic dissociation of H2. In H-4, Ha and Hb are bound to the O2 and O1 site of the O−In square, with an adsorption energy of −2.06 eV. In H-5, Ha and Hb bind the O3 and O4 sites, respectively, with an adsorption energy of −2.04 eV. In both configurations, the O−Ha and O−Hb bonds are 0.98 Å and 1.00 Å, respectively. In H-6, Ha and Hb bind the O6 and O5 sites, with an adsorption energy of −1.62 eV and bond lengths of 0.98 Å and 0.99 Å, respectively. Clearly, H-6 is significantly less favorable energetically than H-4 and H-5. We note that even the least favorable configuration, H-6, is energetically more stable than any of the heterolytically bound H adatoms. Therefore, we explored the migration of the H adatom from the In site to the neighboring O site, transforming from the heterolytic to homolytic state. The potential energy profile for transition from H-3 to H-6 is shown in Figure 4. In the transition state, the Ha−In bond was stretched to 1.81 Å, while Ha is approaching O6, reaching a Ha−O6 distance of 1.61 Å. The Hb−O5 remains at 0.97 Å. The activation barrier for H adatom to migrate from the In site to the O site is 1.32 eV. This result indicates that although forming H adatoms homolytically bound on the O sites is energetically favorable, the heterolytic

Figure 4. Structure and potential energy profile for H adatom to migrate from an In site to an O site. 7820



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catalysts suppress the RWGS reaction in the low temperature range,58 making indium oxide an attractive ingredient in catalysts for methanol synthesis. We will analyze the reactions between coadsorbed CO2 and H adatoms and map out the reaction pathways. Such an analysis will help to understand the reaction mechanism of methanol synthesis over the indium oxide based catalysts. We examined ten stable coadsorption structures of CO2 and H2 and determined that two configurations (CH-1 and CH-2 shown in Figure 6) are the most probable configurations for adsorbed CO2 reacting with the H adatoms from dissociative adsorption of H2. In CH-1, Ha binds to In4 and Hb binds to O5 forming InH and OH, respectively. The occupation of the In4 site by Ha broke the original Ob−In4 bond and led to a bidentate carbonate species next to Ha. In this structure, the Ha adatom is positioned to attack the C atom of the adsorbed CO2. The overall adsorption energy with respect to free CO2 and H2 in CH-1 is −1.52 eV. In CH-2, H2 dissociates over In1− O2 across from CO2 adsorbed on the next chain. The overall adsorption energy is −1.59 eV, similar to that of CH-1. In CH2, the Hb adatom has easy access to the Oa atom of adsorbed CO2. In these coadsorption structures, the adsorbed CO2 and H adatoms largely maintained the adsorption structures of the individual species. Starting from CH-1, we indeed obtain a formate (HCOO) species by allowing Ha to attack the C atom of the adsorbed CO2. The formate species produced from this reaction is held to the surface through the Oa−In bond, with an energy of −1.19 eV relative to free CO2 and H2. This pathway is shown in the upper trace (red) of Figure 6. As shown in Figure 6, this step of CO2 hydrogenation is endothermic by 0.33 eV with an activation barrier of 0.65 eV. In the transition state, Oa−C−Ob is almost linear and is held on to the surface through an Oa−In3 bond. The H−In4 bond is stretched to 1.82 Å, while the H−C bond starts to form at a distance of 1.64 Å. As we showed earlier, the activation barrier for H adatom to migrate from the In site to the O site is 1.32 eV. This barrier is considerably larger than the activation barrier and overall reaction energy of the hydrogenation to the formate species. Combining with

Figure 5. Optimized adsorption structures of H2O on the In2O3(110) surface: (a) molecular; (b) dissociative.

water is −0.83 eV. We note that the distance between Ha and the surface O is 1.67 Å and anticipate that the hydrogen bonding interaction will contribute to the stability of the molecularly adsorbed H2O. Hydrogen bonding interaction between coadsorbed methanol molecules has been shown to contribute to the increased adsorption strength of methanol.75 The configuration of the dissociatively adsorbed H2O is shown in Figure 5b. The adsorption energy in this configuration is −1.19 eV. Clearly, the dissociative adsorption of water is energetically more favorable than the molecular adsorption. In this configuration, the Ow−Ha bond in H2O is replaced by the O3−Ha bond, with the hydroxyl (Ow−Hb) binding the In3 site. The O3−Ha and Ow−Ha bonds are 0.98 Å and 0.97 Å, respectively. The hydroxyl interacts with the In3 site more strongly than molecularly adsorbed water, as reflected in the shorter Ow−In3 bond (2.05 Å) and longer O3−In3 bond (2.34 Å). Comparing with β-Ga2O3, the In2O3(110) surface is much more active toward water. This is reflected in the higher adsorption energy of molecular H2O on In2O3 than that on βGa2O3 (−0.56 eV)32 as well as the fact that dissociative adsorption of water is energetically more favorable. The results of CO2, H2, and H2O adsorption on the perfect In2O3(110) surface help us to understand the activity of the In2O3 surface. Among the surface sites, O3−In3−O4 at the center of the chain is more active than the sites either on the left or on the right. 3.3. Coadsorption and Reaction of CO2 and H2. RWGS reaction (CO2 + H2 → CO + H2O) is the major side reaction in methanol synthesis from CO2 hydrogenation (CO2 + 3H2 → CH3OH + H2O). Previous studies indicated that In2O3-based

Figure 6. Potential energy profile for CO2 reaction with the coadsorbed H adatoms on the In2O3(110) surface. 7821



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Figure 7. Potential energy profile for CO2 and H2O coadsorption and reaction on the In2O3(110) surface.

Table 2. Calculated Frequencies (in cm−1) of Characteristic Vibrational Modes for Different CO2 Species on In2O3(110) configuration

υas(OCO)

υs(OCO)

C-2 CH-h CH-p1 CW-p

1537 1740 1592 1541

1302 1231 1421 1460

υ(CH)

υ(OH)surf

υ(CO−H)

δ(CO−H)

2258

3611 3712 3754

3653 3677

1179 1177

Consequently, water plays important roles in the overall selectivity of CO2 hydrogenation to produce methanol. As such, we examined coadsorption of water with CO2 and possible reactions between them. In Figure 7, CW-1 represents a stable coadsorption configuration of CO2 and H2O on the In2O3 (110) surface. Similar to CH-1 and CH-2, CO2 is adsorbed at the O4−In4 site in a bidentate configuration, and H2O is adsorbed molecularly on the In3 site. The overall adsorption energy is −1.77 eV. Treating CO2 adsorption energy as a constant, we could calculate the adsorption energy of the molecularly adsorbed water on the In2O3(110) surface in the presence of adsorbed CO2 and obtained a value of −0.53 eV. Comparing with the adsorption energy of the molecularly adsorbed water on the clear surface, −0.83 eV, we conclude that the water−surface interaction was significantly weakened. This result is in contrast to that of γ-Al2O3 on which the water− surface interaction was strengthened in the presence of adsorbed CO2.37 In CW-p, Hb of H2O is transferred to the Oa atom of CO2 and forms a Oa−Hb bond, accompanied by the breaking of the Oa−In3 bond and relaxation of the O4 atom outward on the surface. After Ha was transferred, the OH that was left behind binds primarily to the In3 site. The conversion from CW-1 to CW-p, viewed as the forward reaction of CO2 protonation by H2O, is endothermic by +0.74 eV and has an activation barrier of 0.84 eV. More importantly, this transformation results in a bicarbonate species that is almost identical to the bicarbonate species formed from the protonation by a surface OH from heterolytic H2 dissociation (shown in CH-p1). This reaction provides an alternative channel to consume the surface bicarbonate species in the reverse reaction, which converts CW-p to CW-1. The reverse reaction is exothermic by −0.74 eV with an activation barrier of only ∼0.10 eV. This makes the reverse reaction both thermodynamically favorable and kinetically feasible. This reverse step will compete with and dominate the decom-

these results, we can conclude that hydrogenation of CO2 to the formate species is the preferred route and will dominate H adatom migration at moderate temperatures. Starting from CH-2, the adsorbed CO2 can be protonated by OH, resulting in a bicarbonate species, shown as CH-p1 in Figure 6. We point out that the transformation from CH-2 to CH-p1 is not a one-step process. The process went through two steps: (1) Hb is transferred from the surface O2 atom to Oa to form a bicarbonate (COOH) species and (2) the surface O2 atom rebinds the In1 atom to form the In1−O2 bond after the proton transfer, allowing Ha to migrate from In1 to O2. These steps are depicted in CH-2 of Figure 6 as two curve-arrows. This protonation process is exothermic, with an overall reaction energy of −0.78 eV. A possible reaction following bicarbonate formation is disproportion of the OH in COOH with an OH on the surface. We examined this possibility and found the product state (CH-p2 in Figure 6), i.e., coadsorbed CO and H2O, is −1.30 eV relative to free CO2 and H2. This corresponds to a reaction energy of +1.07 eV with respect to the bicarbonate (CH-p1) state. The CH-p2 is the product state for the CO2 hydrogenation to follow the RWGS pathway. As shown in this mechanism, surface hydroxyls are the active species facilitating the reaction to proceed along this pathway. Our results so far indicate that the product states from CO2 hydrogenation and protonation have very similar overall reaction energies with respect to free CO2 and H2. The difference is only ∼0.1 eV. However, the steps leading to these product states are very different. The hydrogenation pathway is endothermic (+0.33 eV) and has a moderate activation barrier (+0.65 eV), whereas the protonation pathway goes through a significantly stable bicarbonate intermediate. These differences are expected to impact the reactivity and selectivity, as we will demonstrate in the follow section. 3.4. Coadsorption and Reaction of CO2 and H2O. Water is one of the products in methanol synthesis from CO2. 7822



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Figure 8. Schematic mechanism of the initial steps of CO2 hydrogenation on In2O3. H adatoms on In site and on O site are colored in purple and green, respectively.

position of bicarbonate to CO and coadsorbed H2O, returning the bicarbonate species to CO2 and coadsorbed water. 3.5. General Discussion. In addition to the adsorption structures and relative energies, we also computed the frequencies of important intermediates, including C-2, CH-h, CH-p1, and CW-p, and summarized the results in Table 2. The results reported in Table 2 are harmonic frequencies directly from the calculations without applying any scaling factor. For C-2, the modes at 1537 and 1302 cm−1, corresponding to υas(OCO) and υs(OCO) of carbonate species, respectively, are close to the experimental observed bands at 1550 and 1320 cm−1, which were assigned to υas(OCO) and υs(OCO) of bidentate carbonate species.74 The modes at 1740 and 1231 cm−1 of CH-h were identified as the υas(OCO) and υs(OCO) and 2258 cm−1 as υ(CH). The bands at 3611 cm−1 is clearly due to υ(OH) of the surface hydroxyl. These results are in good agreement with Haneda’s observation of formate species on the surface, which exhibits bands at 1784 and 1237 cm−1 and υ(OH) mode at 3609 cm−1.76 The frequencies of bicarbonate species in CH-p1 and CW-p are very similar. For example, the asymmetrical and symmetrical stretching frequencies of OCO are 1592 cm−1 and 1421 cm−1, respectively, in CH-p1 and 1605 cm−1 and 1424 cm−1, respectively, in CW-p. Again, they are in agreement with the experimentally observed band at 1647 and 1485 cm−1.76 The frequencies of bicarbonate species in CH-p1 and CW-p are very close, further confirming that they are the same species although they were formed following different reaction pathways. Combining the results of CO2 reacting with coadsorbed H adatoms and with H2O, we mapped out possible reactions involved in the initial steps of methanol synthesis from CO2 and H2 on the In2O3 surface and showed the results schematically in Figure 8. As shown in Figure 8, CO2 can be hydrogenated by the H adatom on the In site to produce a formate species or protonated by the H adatom on the O site to form a bicarbonate species. The former is endothermic by +0.33 eV with an activation barrier of 0.65 eV, whereas the latter reaction is exothermic by −0.78 eV. The formate species may be further hydrogenated to form methanol as the surface formate species has been considered as an intermediate in methanol formation. In contrast, the bicarbonate species is likely to react with a surface hydroxyl and produce CO2 and H2O. This analysis is based on the fact that although CO2

protonation by a surface hydroxyl is facile, the subsequent step to form CO and H2O is highly endothermic (+1.07 eV). Consequently, the RWGS reaction (CO2 + H2 → CO + H2O) will be blocked. In the meantime, disproportion of the bicarbonate species with a surface hydroxyl is both energetically favorable and kinetically feasible. Since the steps leading to bicarbonates eventually returns to CO2 and effectively do not consume CO2, the net contribution to CO2 conversion is primarily the hydrogenation pathway. As the In−O−In chain shown in Figure 1 is also the basic unit on the In2O3(111) surface, the qualitative picture is expected to be valid for the (111) surface. As (111) and (110) are the two surfaces with significantly low energies, these surfaces should the dominant surfaces of a In2O3 particle. We would, therefore, anticipate a high selectivity toward CO2 hydrogenation on the indium oxide-based catalysts. In comparison with catalytic CO 2 hydrogenation on γ-Al2O3 supported Ni catalyst,31 we find that the hydrogenation pathway on In2O3 is analogous to the hydrogenation route on the dry surface supported Ni catalyst, whereas the protonation pathway on In2O3 is similar to the reaction over the hydroxylated surface supported catalyst. The main difference between the two systems is that the conversion of the surface bicarbonate species to CO is not favorable on In2O3, thereby blocking the RWGS reaction.

4. CONCLUSIONS In the present work, CO2 adsorption and hydrogenation on the (110) surface of In2O3 have been studied using density functional theory slab calculations. On the surface, the O3− In3−O4 part of the chain is more active than either of the O−In rings it connects due to the low coordination number of its constituent atoms. CO2 is activated upon adsorption, and H2 adsorbs dissociatively whereas H2O adsorbs molecularly and dissociatively. The activated CO2 on the In2O3(110) surface forms surface carbonate species with the participation of a surface O atom. The adsorption energy of CO2 is −1.25 eV. The dissociative adsorption of H2 can occur either heterolytically or homolytically, resulting in surface hydrides and hydroxyls. Heterolytic dissociative adsorption of H2 is less favorable as compared to the homolytically bound H adatoms. Both molecularly and dissociatively adsorption of H2O can occur on the In2O3(110) surface, with the dissociative mode being more stable. 7823



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We examined two possible pathways for CO2 to react with the coadsorbed H adatoms: (1) CO2 is hydrogenated by the surface hydrides on the In site, forming a formate species (HCOO); (2) CO2 is protonated by the surface hydroxyl, leading to a surface bicarbonate species. Our results revealed that although bicarbonate formation is energetically favorable, its subsequent conversion to CO and H2O, COOH + InH → CO + H2O, is highly endothermic. However, disproportion reaction of the bicarbonate and the surface hydroxyl (COOH + OH → CO2 + H2O) is highly exothermic. Consequently, this step is expected to consume the surface bicarbonate species and reproduce CO2 together with H2O. As such, an In2O3-based catalyst for CO2 hydrogenation is expected to suppress the RWGS reaction and increase the selectivity toward formate and its subsequent products.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the supports from the National Natural Science Foundation of China ((#20990223) and from the U.S. Department of Energy, Basic Energy Science program (DE-FG02-05ER46231). The computations were performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), which is a U.S. Department of Energy national scientific user facility located at PNNL in Richland, Washington.

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DFT Study of CO2 Adsorption and Hydrogenation on the In2O3

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