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* E-mail: slin@fzu.edu.cn; dqxie@nju.edu.cn Received July 12, 2012; accepted September 9, 2012. † Dedicated to the 80th Anniversary of Chinese Chemical Society. 2036 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2012, 30, 2036—2040

DOI: 10.1002/cjoc.201200714

Initial Decomposition of Methanol and Water on In2O3(110): A Periodic DFT Study†

Lin, Sen*,a(林森) Xie, Daiqian*,b(谢代前) a Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory

Breeding Base, Fuzhou University, Fuzhou, Fujian 350002, China b Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry, School of

Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210093, China

Pure In2O3 is considered as an efficient methanol steam reforming catalyst. Despite of several studies in the past decades, the mechanism of MSR on In2O3 is still not fully understood. In this work, a periodic density functional theory study of the initial dissociation of methanol and water over the In2O3 (110) surface is presented. The activa-tion energy barriers and thermochemistry for several elementary steps are reported. It is found that the energy barri-ers for O—H bond cleavage of both CH3OH and H2O to produce CH3O and OH species at a surface In-O pair site are very low, indicating that In2O3 (110) can facilely catalyze these two important processes at low temperatures. In addition, the subsequent dehydrogenation of CH3O to CH2O is also found to proceed with a low barrier.

Keywords DFT, methanol, water, decomposition, In2O3 (110)

Introduction Methanol is considered as an important fuel for

needs of future transportation, either in direct fuel cells or for generation of H2 for proton-exchange membrane fuel cells. A possible and efficient way to produce hy-drogen on board is using methanol steam reforming (MSR):[1-3]

CH3OH＋H2O → 3H2＋CO2 ΔH0＝49.6 kJ/mol.

As a hydrogen carrier, methanol has many advantages.[4] For example, the liquid characteristic makes itself read-ily stored and transported using the infrastructure for the existing transportation fuels with minor modifications. In addition, it exhibits a property of high H/C ratio and no sulfur or nitrogen, so that it is a very clean fuel.

Traditionally, MSR can be realized by copper dis-persed on oxide support.[1,2] This catalyst can be used to produce CO2 gas from methanol with high selectivity. An assumption supported by both experimental and theoretical observations shows that the dehydrogenation of methoxyl generated from initial O—H bond cleavage of methanol to formaldehyde, which is known to be a key intermediate in MSR, might be the rate determined step.[5-7] In despite of high selectivity, the experiments found that the copper catalyst had a few disadvantages, such as low thermal stability. Therefore, there comes out a strong desire for more stable and equally active and selective MSR catalysts. Furthermore, in the past dec-

ades, the increasing demand of energy source also stimulated many research activities to design new effi-cient MSR catalysts, such as metal alloys and ox-ides.[8-13] For instance, Iwasa et al. found that Pd sup-ported on In2O3 exhibits high catalytic performance for MSR.[8] Although they attributed the selectivity toward CO2 to PdIn alloy formed when heating Pd/In2O3 com-pound under reductive conditions, the function of oxide support in the MSR process is still unclear. Recently, Lorenz et al. reported very high CO2 selectivity with less than 5% CO formation in MSR reaction by using a pure In2O3 catalyst over a broad temperature range (450 K＜T＜673 K).[12] In contrast to copper, this catalyst possesses high thermal stability, namely no pronounced catalyst sintering was observed below 673 K. In addi-tion, In2O3 catalyst exhibits high catalytic activity for steam reforming of ethanol at low temperature to pro-duce hydrogen with no detectable CO impurity.[10] In-terestingly, Bielz et al. found that, with pure In2O3 cata-lyst, if formaldehyde (CH2O) was used as reactant, 95% selectivity toward CO2 under typical steam reforming conditions and temperatures of ～550 K could be achieved.[13] This similarity with MSR suggested that CH2O species might play as an important intermediate in MSR. In despite of the current experiments, the de-tailed mechanism of the initial steps of MSR on pure In2O3 catalysts is not completely understood, which might hinder the further design of more efficient cata-lysts.

Initial Decomposition of Methanol and Water on In2O3(110): A Periodic DFT Study

Chin. J. Chem. 2012, 30, 2036—2040 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 2037

Compared with extensive theoretical research of MSR on transitional metal or metal alloy catalysts,[14-21] few theoretical studies on the catalytic properties of In2O3 were reported.[22] Due to its complicated surface structure, most theoretical studies reported before fo-cused on the physical properties of In2O3, such as phase stability and optical properties.[23-26] In this work, a pe-riodic density functional theory study of dissociation of methanol and water on In2O3 (110) is presented, aiming to understand the initial story involved in MSR and then help to ultimately resolve the whole mechanism of MSR on the indium oxide-based catalysts.

Theory All calculations were carried out based on the peri-

odic DFT calculations by using the Vienna ab initio simulation package (VASP)[27-29] with the gradient- corrected PW91 exchange-correction functional.[30] For valence electrons a plane-wave basis set was employed with a cut-off of 500 eV and the ionic cores were de-scribed with the projector augmented-wave (PAW) method.[31,32] A 2×2×1 Monkhorst-Pack k-point grid[33] was adopted to sample the Brillouin zone, which was tested to be converged well. The Fermi level was smeared using the Methfessel-Paxton method with a width of 0.1 eV.[34]

The lattice parameter of In2O3 was calculated to be 10.328 Å after bulk optimization, which is in good agreement with the previously reported results.[22] In order to investigate the adsorption and decomposition of methanol and water, we selected In2O3 (110) surface as a slab model, which consists of four atomic layers of a 1×1 unit cell with the top two layers relaxed in all cal-culations. We also used a vacuum spacing of 14 Å to avoid interactions between adsorbates and slab images in z direction.

In this study, we firstly investigated the adsorption of CH3OH/H2O and their dissociative products involving CH3O, CH2OH, CH2O, CH3, OH, and H on the In2O3 (110) surface. The adsorption energy was calculated as: Eads ＝ E(adsorbate ＋ surface) － E(free molecule) －E(free surface). The climbing image nudged elastic band (CI-NEB) method[35,36] was used to determine the transition states with the conventional energy (10－4 eV) and force (0.03 eV/Å) convergence criteria. Stationary points were confirmed by normal mode analysis using a displacement of 0.02 Å and an energy convergence cri-terion of 10－6 eV.

Results and Discussion In2O3 (110) slab model

Among all the polymorphs of In2O3, the body-cen-tered cubic bixbyite crystal structure consisting of eighty atoms in the unit cell is found to be a thermo-dynamically stable phase. Accordingly, we generated (110) surface from this stable crystal structure as the

slab model, which is in very good agreement with the model used by Walsh et al.,[26] but somewhat different from Ye's theoretical structure.[22] The In2O3 (110) sur-face is essentially non-polar and reveals stoichiometric layers of indium and oxygen ions oriented perpendicular to the slab. After relaxation, the indium atoms on the first layer slightly go down. The optimized geometry of the In2O3 (110) surface is shown in Figure 1, which also defines all the correlative sites on In2O3 (110). In order to make each adsorption site legible, the bottom two layers are neglected here and the atoms of first layer are represented by larger balls. As shown in the frame (by green line), each long chain as a repeated unit contains two four-membered In-O-In-O squares together with a short O-In-O chain. Different from the 6-coordinated bulk In atoms and 4-coordinated bulk O atoms, the In and O atoms in the first layer are 4-coordinated (In1) or 5-coordinated (In2, In3, and In4) and 3-coordinated (O5, O6, O7, O8, O9, O10), respectively, indicating that they might serve as the active sites for the adsorption and MSR reactions on In2O3 (110).[22]

Figure 1 Site map (top and side views) for pertinent adsorption sites on the In2O3 (110) surface. 1, 2, 3, 4, and 11 sites represent In sites (dark brown), 5, 6, 7, 8, 9, and 10 sites denote O sites (red).

Adsorption of pertinent species The adsorption energies and geometries of different

adsorption configurations for pertinent species are given in Table 1 and Figure 2, respectively. In general, the oxygen and carbon atoms of these species favor to in-teract with lattice In and O atoms, respectively.

Our calculations show that methanol molecularly adsorbs at the In1 site with its hydroxyl oxygen inter-acting with In1 atom and its hydroxyl hydrogen forming a hydrogen bond with lattice oxygen atom (Olattice). This adsorption pattern is different from the case when methanol weakly adsorbs on metal catalysts.[16-18,20] It is clear that, at the In-O site of this oxide catalyst, the ad-sorption of methanol with a calculated binding energy of －1.312 eV is much stronger than those on pure metals, which might activate the cleavage of O—H bond. Similar to CH3OH*, H2O* adsorbs at the In1 site with its body between two chains over the surface. The

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adsorption energy is found to be －1.314 eV and the stability of this adsorption is also due to its hydrogen bonding interaction with Olattice. However, this adsorp-tion pattern is found to be different from the previous results,[22] in which water adsorbs above a single chain with a smaller binding energy of －0.830 eV.

For the dissociation product CH3O*, it likes to stay at the In1 site with its oxygen bonding with In1 atom. The binding energy is calculated to be －1.257 eV and the distance of O—In is about 2.115 Å. For another de-composition product OH*, it adsorbs strongly between two surface In atoms and points its hydrogen to the op-posite Olattice to form a hydrogen bond. Because of the interaction with two unsaturated In atoms, the binding energy of OH* is expectedly larger than that of CH3O*. After the dehydrogenation of methanol and water, H* is produced. Through extensive search, it was found that Olattice sites are energetically favored for H* adsorption, which is consistent with the previous theoretical re-sults.[22] As shown in Table 1 and Figure 2, the largest adsorption energy is －3.950 eV when H* is on O7 site. Nevertheless, due to its strong interaction with surface, it might have a trouble to migrate from one site to an-other site, Interestingly, Ye et al. theoretically observed that the dissociation of H2 to two H atoms was energeti-cally favorable and H was difficult to migrate from the In site to O site with an activation barrier of 1.32 eV.[22] Although our calculations show that methanol and water can facilely give out their hydrogen atoms to stick on the catalyst at low temperatures (see reaction part for detail), the succedent combination of two H* to form H2 gas on In2O3 (110) surface seems to be energetically adverse and might need to overcome a considerable high activation energy. This is thus consistent with the previous experimental temperature-programmed MSR observations that, until the temperature was increased to 450 K, the signals H2 were strongly enhanced.[12]

The adsorption of CH2OH**, CH2O**, and CH3* species was also investigated. CH2OH** interacts strongly with the surface, preferring In3-O7 site. The hydroxyl hydrogen points to the nearby Olattice, generat-ing a hydrogen bond. The calculated adsorption energy is about －4.837 eV. For CH2O** species, it preferen-tially adsorbs at In1-O6 site with a moderate binding energy of －1.649 eV. A strong and stable CH3* ad-sorption was observed on O6 site with binding energy of －2.806 eV on this surface.

Reactions In this work, we focused on the dissociation of

methanol and water, which represents the initial steps in MSR. It should be noted that, the complete picture of the mechanism is important but often difficult to be yielded, so these initial processes shed valuable light onto the role of the catalyst.

As shown in Figure 3, one can see that the O—H bond scission barriers (0.017 and 0.042 eV, respectively) for both adsorbed CH3OH* and H2O* on In2O3 (110)

Figure 2 Adsorption geometries for CH3OH, CH3O, H2O, OH, H, CH2OH, CH2O and CH3 species (top views). Atoms are color labeled: In (dark brown), O (red), C (grey), and H (white).

are very low. To the contrary, it is well known that methanol and water are often hard to give out their hy-droxyl hydrogen on metal catalysts, such as Cu and PdZn.[18,19] Before dissociation, methanol lies between two chains over In2O3 (110) and its hydroxyl hydrogen forms a hydrogen bond with Olattice. At the transition state, the O—H bond length of CH3OH is elongated from its initial 1.068 Å to 1.216 Å and the distance be-tween lattice O and hydroxyl H was calculated to be

Initial Decomposition of Methanol and Water on In2O3(110): A Periodic DFT Study

Chin. J. Chem. 2012, 30, 2036—2040 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 2039

Table 1 Preferred adsorption sites and binding energies for CH3OH, CH3O, H2O, OH, H, CH2OH, CH2O and CH3 species on the In2O3 (110) surface

Species Adsorption site Adsorption Energy/eV

CH3OH In1 －1.312 CH3O In1 －1.257 H2O In1 －1.314 OH In11 －2.150 H O7 －3.950 CH2OH In3-O7 －4.837 CH2O In1-O6 －1.649 CH3 O6 －2.806

1.249 Å. After dissociation, CH3O* locates at the In1 site and H* moves onto the Olattice. The barrier height and exothermicity of this process were calculated to be 0.017 eV and －0.016 eV, respectively, indicating that it is likely to take place at low temperatures. Similarly, water can also give out its H quite facilely with a slightly higher barrier (0.042 eV). Hence, it is con-cluded that, the In-O pair site on In2O3 catalyst can catalyze CH3OH and H2O to accomplish the first dehy-drogenation efficiently. According to the above discus-sion, we can assume that, on Pd/In2O3 catalyst,[8] as the reactants methanol and water in MSR approach the catalyst, they may have a trend to firstly decompose on the In2O3 surface.

The dehydrogenation of methoxy to formaldehyde was also investigated. Several MSR experimental ob-servations suggested that formaldehyde species often serves as an important intermediate, and the reaction of CH3O dehydrogenation to CH2O and H is the rate-de-termined step in MSR.[17,18] Recently, it was reported that, instead of CH3OH reactant, CH2O could also be transformed to CO2 with 95% selectivity under typical steam reforming conditions.[13] Combining this interest-ing experimental observation with MSR results, it can be speculated that CH3OH might firstly dehydrogenate to CH2O species facilely, and then perform the subse-quent story from CH2O intermediate to the final product CO2 and H2. From our calculated results shown in Table 2 and Figure 4, one can see that the dehydrogenation of CH3O* to CH2O** possesses a very low barrier about 0.084 eV and a very large exothermicity (－1.882 eV). This result indicates that methoxy species can easily decompose on In2O3 (110) and the reaction of CH2O**＋H* → CH3O* is irreversible. In the transition state, the C—H bond length was found to be 1.178 Å and H is ready to break away from carbon. After dehydrogena-tion, CH2O** goes to lie between two surface chains with its oxygen interacting with In atom and the bond between bulk In atom and O6 is found to be ruptured. Therefore, our results support the above assumption that CH2O** might play an important role in the MSR on In2O3-based catalysts. In addition, considering the acti-vation energy (1.016 eV) of MSR by experiment,[12] it

Table 2 Calculated activation and reaction energies (eV) for the dissociation of CH3OH and H2O on the In2O3 (110) surface

Elementary reaction E≠ ∆E

CH3OH* → CH3O*＋H* 0.017 －0.016H2O* → OH*＋H* 0.042 －0.013CH3O* → CH2O**＋H* 0.084 －1.882CH3OH* → CH2OH**＋H* 1.533 －0.494CH3OH* → CH3*＋OH* 2.581 0.173

Figure 3 Energetics and geometries (top views) of the O—H bond scissions of CH3OH (red line) and H2O (blue line) on the In2O3 (110).

Figure 4 Energetics and geometries (top views) of the dehy-drogenation of CH3O on the In2O3 (110).

can be speculated that the rate-determined step should exist in the subsequent reactions.

A new question comes out. What is the exact mechanism of reaction from CH2O** with high selec-tivity toward CO2 rather than CO? Due to the expensive calculation cost of this In2O3 system, studies for the fur-ther dehydrogenation of CH2O** or the reaction be-tween CH2O** and the available surface OH* species to

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produce CO2 have not been established yet and will be carried out in the future in our group.

Beside O—H bond cleavage, CH3OH has another two decomposition ways, namely C—H and C—O bond scission, which leads to the generation of CH2OH**/H* and CH3*/OH*, respectively. Due to the large barriers (1.533 and 2.581 eV, respectively), the cleavage of C—O and C—H bonds was found to be unfavorable, which essentially excludes them from playing a significant role in MSR on the In2O3 (110) surface.

Conclusions We reported a periodic DFT study of the decomposi-

tion of both methanol and water on the In2O3 (110) sur-face. The calculated processes represent the initial steps of methanol steam reforming. Due to the calculated low dissociation barriers, we could conclude that, the reac-tions of O—H bond scissions of CH3OH and H2O, would take place very facilely at an active In—O pair site to yield CH3O and OH intermediates at low tem-peratures. On the other hand, due to the high barriers, C—H and C—O bond cleavages of CH3OH are very difficult, rendering them of minor importance in MSR. The further dehydrogenation of CH3O readily leads to the formation of CH2O, whose further transformations will branch out to various other intermediates and products.

Acknowledgements The authors are grateful for the financial support of

National Basic Research Program of China (973 Pro-gram)(No. 2013CB632405), Science & Technology Development Foundation of Fuzhou University (No. 600617 to SL), Natural Science Foundation of Fujian Province, China (No. 2012J05022 to SL), and National Natural Science Foundation of China (No. 21133006 to DX). We are also grateful to the High Performance Computing Center of Nanjing University for the award of CPU hours to accomplish this work.

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