Effects of Hydration and Oxygen Vacancy on CO - American

Jan 4, 2010 - hydrated defective surface begin with an HD. The dry ..... Color coding: red, O atoms; green, Ga atoms; white, H atom; sky-blue, C atom...
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Effects of Hydration and Oxygen Vacancy on CO2 Adsorption and Activation on β-Ga2O3(100) Yun-xiang Pan,†,‡ Chang-jun Liu,‡ Donghai Mei,§ and Qingfeng Ge*,† †

‡

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering, Tianjin University, Tianjin 300072, China, and §Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352 Received October 9, 2009. Revised Manuscript Received December 14, 2009

The effects of hydration and oxygen vacancy on CO2 adsorption on the β-Ga2O3(100) surface have been studied using density functional theory slab calculations. Adsorbed CO2 is activated on the dry perfect β-Ga2O3(100) surface, resulting in a carbonate species. This adsorption is slightly endothermic, with an adsorption energy of 0.07 eV. Water is preferably adsorbed molecularly on the dry perfect β-Ga2O3(100) surface with an adsorption energy of -0.56 eV, producing a hydrated perfect β-Ga2O3(100) surface. Adsorption of CO2 on the hydrated surface as a carbonate species is also endothermic, with an adsorption energy of 0.14 eV, indicating a slightly repulsive interaction when H2O and CO2 are coadsorbed. The carbonate species on the hydrated perfect surface can be protonated by the coadsorbed H2O to a bicarbonate species, making the CO2 adsorption exothermic, with an adsorption energy of -0.13 eV. The effect of defects on CO2 adsorption and activation has been examined by creating an oxygen vacancy on the dry β-Ga2O3(100) surface. The formation of an oxygen vacancy is endothermic, by 0.34 eV, with respect to a free O2 molecule in the gas phase. Presence of the oxygen vacancy promoted the adsorption and activation of CO2. In the most stable CO2 adsorption configuration on the dry defective β-Ga2O3(100) surface with an oxygen vacancy, one of the oxygen atoms of the adsorbed CO2 occupies the oxygen vacancy site, and the CO2 adsorption energy is -0.31 eV. Water favors dissociative adsorption at the oxygen vacancy site on the defective surface. This process is spontaneous, with a reaction energy of -0.62 eV. These results indicate that, when water and CO2 are present in the adsorption system simultaneously, water will compete with CO2 for the oxygen vacancy sites and impact CO2 adsorption and conversion negatively.

1. Introduction Catalytic conversion of captured CO2 to liquid fuels or other valuable chemicals has a positive impact on decreasing CO2 concentration in the atmosphere, thereby alleviating the greenhouse effect.1-3 Gallium oxide (Ga2O3) has been found as one of most active catalyst materials which can be used as either catalyst or promoter in catalytic CO2 conversion processes.4-10 Despite the improved performance of Ga2O3-containing catalysts, the interaction between CO2 and the Ga2O3 surface is not well characterized. Teramura et al.4 reported that Ga2O3 has reasonable photocatalytic activity in the reaction of CO2 with H2 under ambient condition. A Pd/Ga2O3 catalyst was also shown to be more active than the classic Cu/ZnO catalyst for CO2 hydrogenation to methanol, by a factor of 2 in yield and 20 in turnover frequency.5 Chiavassa et al.8 found that both the conversion of *Corresponding author. E-mail: [email protected].

(1) Knofel, C.; Hornebecq, V.; Llewellyn, P. L. Langmuir 2008, 24, 7963. (2) Ruckenstein, E.; Hu, Y. H. J. Catal. 1996, 162, 230. (3) Song, C. S. Catal. Today 2006, 115, 2. (4) Teramura, K.; Tsuneoka, H.; Shishido, T.; Tanaka, T. Chem. Phys. Lett. 2008, 467, 191. (5) Fujitani, T.; Saito, M.; Kanai, Y.; Watanabe, T.; Nakamura, J.; Uchijima, T. Appl. Catal., A 1995, 125, L199. (6) Calatayud, M.; Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L. Phys. Chem. Chem. Phys. 2009, 11, 1397. (7) Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L. J. Catal. 2004, 226, 410. (8) Chiavassa, D. L.; Barrandeguy, J.; Bonivardi, A. L.; Baltanas, M. A. Catal. Today 2008, 133, 780. (9) Jochum, W.; Penner, S.; Kramer, R.; Fottinger, K.; Rupprechter, G.; Klotzer, B. J. Catal. 2008, 256, 278. (10) Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 5498.

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CO2 and the synthesis rate of methanol on the Ga2O3-promoted Pd/SiO2 catalyst are significantly higher than those on the unpromoted Pd/SiO2. In CO2 hydrogenation to methanol over Pd/Ga2O3 and Ga2O3-promoted Pd/SiO2 catalysts, CO2 was found to adsorb on the Ga2O3 surface and was hydrogenated to methanol by the hydrogen atoms spilled over from the supported Pd particles.7,8 Ga2O3 has a similar polymorphism to Al2O3, but experimental studies have shown that the surface chemistries of the two oxides are different.11-13 For instance, Vimont et al.12 reported that the Lewis acidity of γ-Ga2O3 is weaker than that of γ-Al2O3, but the concentration of Lewis acid sites on γ-Ga2O3 is higher than that on γ-Al2O3. They attributed the weaker Lewis acidity of γ-Ga2O3 to the smaller polarization ability of the coordinatively unsaturated Ga3þ which has a smaller charge/radius ratio than Al3þ. In contrast to the rich literature on γ-Al2O3, the Ga2O3 surfaces were not studied widely.13-18 A detailed characterization of CO2 interaction with the Ga2O3 surfaces can provide some insights (11) Lavalley, J. C.; Daturi, M.; Montouillout, V.; Clet, G.; Arean, C. O.; Delgado, M. R.; Sahibed-dine, A. Phys. Chem. Chem. Phys. 2003, 5, 1301. (12) Vimont, A.; Lavalley, J. C.; Sahibed-Dine, A.; Arean, C. O.; Delgado, M. R.; Daturi, M. J. Phys. Chem. B 2005, 109, 9656. (13) Arean, C. O.; Bellan, A. L.; Mentruit, M. P.; Delgado, M. R.; Palomino, G. T. Microporous Mesoporous Mater. 2000, 40, 35. (14) Manchado, M. C.; Guil, J. M.; Masia, A. P.; Paniego, A. R.; Menayo, J. M. T. Langmuir 1994, 10, 685. (15) Baltrusaitis, J.; Jensen, J. H.; Grassian, V. H. J. Phys. Chem. B 2006, 110, 12005. (16) Rosynek, M. P. J. Phys. Chem. 1975, 79, 1280. (17) Morterra, C.; Magnacca, G.; Cerrato, G.; Delfavero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1993, 89, 135. (18) Gregg, S. J.; Ramsay, J. D. F. J. Phys. Chem. 1996, 73, 1243.

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into the surface chemistry, especially the acidic and basic properties, of Ga2O3. There have been a number of studies of CO2 adsorption on Ga2O3.10-12 Collins et al.10 studied CO2 adsorption on Ga2O3 using in situ infrared spectroscopy. They reported that carbonate and bicarbonate species were formed upon CO2 adsorption. After outgassing the adsorption systems at 323 K, only the bicarbonate and polydentate carbonate species remained on Ga2O3. Upon heating the adsorption systems under flowing CO2 at 760 Torr, most of the carbonate and bicarbonate species were removed at T > 550 K, except the polydentate carbonate species which stayed on Ga2O3 up to 723 K. Also using FTIR spectroscopic investigation of CO2 adsorption, Lavalley et al.11 found that R-Ga2O3 has a higher surface basicity than γ-Ga2O3. A higher degree of ionicity of R-Ga2O3 may be responsible for its higher surface basicity.11 As we previously demonstrated, the formation of surface carbonate or bicarbonate species indicates an activation of CO2 as the double bonds were broken in the adsorbed state.19 There is a strong binding for the activated (bent) CO2 on the γ-Al2O3 and γ-Al2O3-supported metal dimers. Water adsorption on a metal oxide surface can significantly influence the nature of the surface sites, which in turn affects the subsequent adsorption of other molecules such as CO2.20-27 Quantum chemical studies of the competitive adsorption of NO, NO2 and H2O on BaO demonstrated that adsorbed H2O blocked the surface sites available for NOx adsorption on BaO.23 In our previous work, we showed that H2O adsorption competed with CO2 adsorption for the surface O-Al bridge sites and reduced the number of O-Al sites available for CO2 adsorption on γ-Al2O3 surface.25 Another factor which will affect the surface chemistry of a metal oxide is the defects such as surface oxygen vacancies.28-34 The presence of the oxygen vacancies will make the surface electron-rich, and thereby enhance the electrondonating ability of the surface.29 Gonzalez et al.32 studied the adsorption of hydrogen on the β-Ga2O3(100) surface with oxygen vacancies. Creation of oxygen vacancies made the surface active toward hydrogen adsorption.32 The electrical conductivity of β-Ga2O3 was also attributed to the existence of the oxygen vacancies.34 In the present work, we examined CO2 interaction with the Ga2O3 surface using density functional theory (DFT) slab calculations. Among the five different polymorphs of Ga2O3, i.e. R, β, γ, δ and ε, β-Ga2O3 is the only one which is stable under ambient conditions.10-13,35 The (100) surface is the most stable (19) Pan, Y. X.; Liu, C.-j.; Wiltowski, T. S.; Ge, Q. Catal. Today 2009, 147, 68. (20) Odelius, M. Phys. Rev. Lett. 1999, 82, 3919. (21) Tikhomirov, V. A.; Jug, K. J. Phys. Chem. B 2000, 104, 7619. (22) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys. Rev. Lett. 1998, 81, 1271. (23) Tutuianu, M.; Inderwildi, O. R.; Bessler, W. G.; Warnatz, J. J. Phys. Chem. B 2006, 110, 17484. (24) Szanyi, J.; Kwak, J. H.; Chimentao, R. J.; Peden, C. H. F. J. Phys. Chem. C 2007, 111, 2661. (25) Pan, Y. X.; Liu, C.-j.; Ge, Q. Langmuir 2008, 24, 12410. (26) Rege, S. U.; Yang, R. T. Chem. Eng. Sci. 2001, 56, 3781. (27) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2004, 226, 54. (28) Branda, M. M.; Collins, S. E.; Castellani, N. J.; Baltanas, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 11847. (29) Han, Y.; Liu, C.-j.; Ge, Q. J. Phys. Chem. C 2007, 111, 16397. (30) Binet, L.; Gourier, D.; Minot, C. J. Solid State Chem. 1994, 113, 420. (31) Yamaga, M.; Villora, E. G.; Shimamura, K.; Ichinose, N.; Honda, M. Phys. Rev. B 2003, 68. (32) Gonzalez, E. A.; Jasen, P. V.; Juan, A.; Collins, S. E.; Baltanas, M. A.; Bonivardi, A. L. Surf. Sci. 2005, 575, 171. (33) Cooper, C. A.; Hammond, C. R.; Hutchings, G. J.; Taylor, S. H.; Willock, D. J.; Tabata, K. Catal. Today 2001, 71, 3. (34) Yamaguchi, K. Solid State Commun. 2004, 131, 739. (35) Yoshioka, S.; Hayashi, H.; Kuwabara, A.; Oba, F.; Matsunaga, K.; Tanaka, I. J. Phys.: Condens. Matter 2007, 19, 346211.

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and frequently studied surface.6,32,36 We, therefore, chose the β-Ga2O3(100) surface and studied CO2 adsorption as well as the effect of surface hydration and oxygen vacancy on CO2 adsorption.

2. Methodology and Models All the calculations were performed in the framework of DFT by using the Vienna ab initio simulation package (VASP).37,38 The projector augmented wave method was used to describe the interaction between ions and electrons.39,40 The nonlocal exchange-correlation energy was evaluated using the PerdewBurke-Ernzerhof functional. A plane wave basis set with a cutoff energy of 400 eV and a 4  2  1 k-point grid determined by the Monkhorst-Pack method were found to give converged results. 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 e0.03 eV/A˚. The (100) surface of β-Ga2O3 was modeled by a supercell with a dimension of 6.20 A˚  11.84 A˚  22.28 A˚. Sixteen Ga2O3 molecular units in the slab were distributed in six layers. The vacuum region separating the slabs along the [001] direction was set to 12 A˚. In all calculations, the bottom two layers were frozen in their bulk positions, whereas the top four layers together with the adsorbates were allowed to relax. Transition states along a reaction pathway were determined in two steps: first, the nudged elastic band method was used to locate the likely transition states; second, the likely transition states were relaxed using the quasi-Newton algorithm with the same force convergence criterion. Each transition state was confirmed by vibrational frequency analysis. H2O 2 The adsorption energies of CO2 and H2O, ΔECO ad and ΔEad , were defined as: CO2 ¼ ECO2 =β-Ga2 O3 - Eβ-Ga2 O3 - ECO2 ΔEad

H2 O ΔEad ¼ EH2 O=β-Ga2 O3 - Eβ-Ga2 O3 - EH2 O

where ECO2/β-Ga2O3, Eβ-Ga2O3, EH2O/β-Ga2O3, ECO2 and EH2O represent the total energies of the β-Ga2O3(100) slab with adsorbed CO2, the clean β-Ga2O3(100) slab, the β-Ga2O3(100) slab with adsorbed H2O, a free CO2 molecule and a free H2O molecule in the vacuum, respectively. In the case of CO2 adsorption on the hydrated β-Ga2O3(100) surface, EH2O/β-Ga2O3 was used as the reference to calculate the CO2 adsorption energy, whereas ECO2/β-Ga2O3 was used as the reference to calculate the H2O adsorption energy. According to the above definitions, negative values of adsorption energy indicate the adsorption is thermodynamically favorable whereas positive values correspond to an endothermic process.

3. Results and Discussion 3.1. CO2 Adsorption on the Perfect β-Ga2O3(100) Surfaces. We first explored the adsorption of CO2 on the dry and hydrated perfect β-Ga2O3(100) surfaces. For clarity, we use DP to prefix the CO2 adsorption configurations on the dry perfect surface and HP to prefix those on the hydrated surface. For example, DP-1 represents the first configuration of CO2 adsorbed on the dry perfect surface, whereas HP-2 refers to the second configuration of CO2 adsorbed on the hydrated perfect surface. (36) (37) (38) (39) (40)

Bermudez, V. M. Chem. Phys. 2006, 323, 193. Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169. Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115. Blochl, P. E. Phys. Rev. B 1994, 50, 17953. Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758.

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Article Table 1. Structural Parameters and Adsorption Energies of CO2 Adsorbed in Different Configurations on β-Ga2O3(100)

Figure 1. Optimized structures of the Ga-terminated perfect β-Ga2O3(100) surfaces. The surface sites are labeled. Color coding: red, O atoms; brown, Ga atoms.

Figure 2. Optimized structure of CO2 adsorbed on the dry perfect β-Ga2O3(100) surface. Bond lengths in A˚. Color coding: gray, C atoms; others are the same as in Figure 1.

In the adsorption configurations, the two oxygen atoms of the adsorbed CO2 were labeled as Oa and Ob if they are in unequivalent positions. Similarly, the oxygen and hydrogen atoms from H2O were marked as Ow, Ha and Hb, respectively. The surfaces of β-Ga2O3 have been analyzed in detail by Bermudez.36 Between the two possible terminations of the (100) surface, O- and Ga-terminated, the Ga-terminated surface with a calculated surface energy of 0.62 J/m2 was chosen as a model for the dry perfect β-Ga2O3(100) surface. The relaxed structure of the Ga-terminated β-Ga2O3(100) surface is shown in Figure 1. Among the three types of surface sites, three-fold-coordinated O (O3c) and 5-fold-coordinated Ga (Ga5c) are from the 4-foldcoordinated O (O4c) and 6-fold-coordinated Ga (Ga6c) atoms of the bulk β-Ga2O3, respectively, and are therefore coordinately unsaturated. The 4-fold-coordinated Ga (Ga4c) site is saturated as it is from the Ga4c atom of the bulk β-Ga2O3. Different sites of the dry perfect β-Ga2O3(100) surface, including the Ga5c, O3c-Ga5c, Ga4c, O3c-Ga4c, Ga4c-Ga4c, O3c and Ga5c-Ga5c sites, were examined for CO2 adsorption. Only one stable adsorption configuration, DP-1, shown in Figure 2, was obtained after relaxing 15 initial structures. In DP-1, CO2 adsorbs at the O3c-Ga5c bridge site as a bidentate carbonate species. The C and Oa atoms are bonded with the surface O3c and Ga5c atoms, respectively. Upon adsorption, the Ga5c and O3c atoms are pulled out of the surface plane by 0.18 A˚ and 0.08 A˚, respectively. The adsorption energy of CO2 in DP-1 is 0.07 eV. As shown in Table 1, the adsorbed CO2 in DP-1 is bent, with both C-O bonds being Langmuir 2010, 26(8), 5551–5558

configurations

C-Oa bond (A˚)

C-Ob bond (A˚)

Oa-C-Ob angle (deg)

DP-1 HP-1 HP-2 TS(HP-1) TS(HP-2) FS(HP-1) FS(HP-2) DD-1 DD-2 HD-1 HD-2 HD(TS) HD-3 CO2 molecule CO32-

1.28 1.29 1.28 1.25 1.26 1.25 1.26 1.38 1.32 1.28 1.25 1.25 1.26 1.18 1.28

1.20 1.22 1.22 1.33 1.30 1.33 1.32 1.20 1.22 1.25 1.32 1.35 1.34 1.18 1.28

136 131 132 124 121 124 124 122 123 129 122 122 122 180 120

CO2 ΔEad (eV)

0.07 0.14 0.15 -0.13 -0.01 -0.31 -0.24 0.13 -0.03 0.10

stretched from those in the free CO2 molecule. This structural distortion of adsorbed CO2 from its gas-phase structure indicates that the adsorbed CO2 is activated. The formation of bidentate carbonate species from CO2 adsorption on the Ga2O3 surface has been observed in in situ IR experiments.10 Bader charge analysis41,42 was performed to understand the charge redistribution upon CO2 adsorption in DP-1. The results are summarized in Table 2. As a reference, the Bader charges for the free CO2 molecule are also provided in Table 2. A total of 0.26 |e| is transferred from the surface to the adsorbed CO2 in DP-1, indicating that CO2 acts as a Lewis acid and accepts electrons from the surface. The acidic nature of CO2 is also reflected in its interaction with other metal oxides.15,25,43 3.2. CO2 Adsorption on the Hydrated Perfect β-Ga2O3(100) Surface. Water adsorbs on the perfect β-Ga2O3(100) surface molecularly, resulting in a partially hydrated surface. We determined five stable configurations and chose the most stable one, P(W-1), as the hydrated substrate for CO2 adsorption. The structure of P(W-1) is shown in Figure 3. P(W-1) and the remaining four less stable structures and dissociation of molecularly adsorbed water are provided in Supporting Information. Starting from 15 possible initial structures, we obtained two configurations, shown as HP-1 and HP-2 in Figure 4. Adsorption energies and additional structural parameters are included in Table 1. In HP-1, CO2 adsorbs at the surface O3c-Ga5c bridge site next to the surface Ga5c site, whereas the coadsorbed H2O is located at the surface Ga5c site, resulting in a bridging bidentate carbonate species. A C-O3c bond (1.46 A˚) and an Oa-Ga5c bond (2.02 A˚) are formed. Similar to the DP-1 configuration, adsorption of CO2 in HP-1 causes both the O3c and Ga5c atoms to relax upward, by 0.12 A˚. In HP-2, the adsorbed CO2 also forms a bidentate carbonate, but at the O3c-Ga5c bridge site away from the coadsorbed H2O. The adsorption of CO2 causes the Ga5c and O3c atoms to relax upward by 0.18 A˚ and 0.09 A˚, respectively. Although the adsorption of CO2 in HP-1 and HP-2 are both endothermic, with adsorption energies being 0.14 and 0.15 eV, respectively, the adsorbed CO2 in HP-1 and HP-2 is distorted significantly from the linear structure of the free CO2 molecule in the gas phase. Bader charge analyses show that the adsorbed CO2 in HP-1 and HP-2 configurations are also negatively charged. As shown in (41) Bader, R. F. W. Acc. Chem. Res. 1985, 18, 9. (42) Henkelman, G.; Arnaldsson, A.; Jonsson, H. Comput. Mater. Sci. 2006, 36, 354. (43) Casarin, M.; Falcomer, D.; Glisenti, A.; Vittadini, A. Inorg. Chem. 2003, 42, 436.

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Table 2. Atomic Bader Charges (in |e|) on C and O of Adsorbed CO2 in Different Configurations β-Ga2O3(100) and of a Free CO2 Molecule

C Oa Ob Total

DP-1

HP-1

HP-2

DD-1

DD-2

HD-1

HD-2

HD-3

CO2

þ1.41 -0.84 -0.83 -0.26

þ1.39 -0.83 -0.85 -0.29

þ1.38 -0.81 -0.86 -0.29

þ0.85 -0.83 -0.79 -0.77

þ0.85 -0.80 -0.79 -0.74

þ1.38 -0.81 -0.85 -0.28

þ1.40 -0.81 -0.86 -0.27

þ1.41 -0.83 -0.86 -0.28

þ1.46 -0.73 -0.73 0.00

Figure 3. Most stable adsorption structure of H2O on the dry perfect β-Ga2O3(100) surface. Bond lengths in A˚. Color coding: white, H atoms; others are the same as in Figure 1.

Table 2, the charges on the adsorbed CO2 in HP-1 and HP-2 are slightly more negative than those on the adsorbed CO2 in DP-1. This can be attributed to the enhanced electron-donating ability due to hydration of the surface. Next, we studied the protonation of the adsorbed CO2 species by the coadsorbed water using HP-1 and HP-2 as the starting configurations. The protonation of the adsorbed CO2 in HP-1 went through a transition state TS(HP-1) and led to the formation of FS(HP-1), as shown in Figure 4a. In FS(HP-1), the Hb-Ow bond is completely broken and replaced by a new Hb-Ob bond (0.98 A˚). This protonation is exothermic, by -0.27 eV, with an activation barrier of 0.91 eV. The protonation of the adsorbed CO2 in HP-2 is shown in Figure 4b. In this process, the Ha atom is transferred from the Ow atom of the adsorbed H2O to the Ob atom of the adsorbed CO2 through a transition state TS(HP-2), producing FS(HP-2). This protonation is also exothermic, by -0.16 eV, and has to overcome an activation barrier of 0.95 eV. Our results indicate that the bicarbonate species that was observed experimentally10 should be a result of the protonation of the adsorbed CO2 species in the presence of coadsorbed H2O. Both protonation processes are thermodynamically favorable on the perfect Ga2O3(100) surface, although the activation barriers are high. Similar protonation processes have been observed in CO2 adsorption on γ-Al2O3 surfaces.15,25 3.3. CO2 Adsorption on Defective β-Ga2O3(100) Surfaces. In this section, we will explore CO2 adsorption on the dry defective β-Ga2O3(100) surface with an oxygen vacancy, as well as the effect of water on CO2 adsorption on the defective surface in this section. To distinguish from the CO2 configurations on the perfect surfaces, we designate CO2 adsorption configurations on the dry defective surface to begin with a DD, whereas those on the hydrated defective surface begin with an HD. The dry defective β-Ga2O3(100) surface is created by removing a surface O3c atom from the dry perfect surface. The slab was kept neutral after the O3c atom was removed, making the slab electronrich. The original Ga5c and Ga4c atoms bound to the O3c atom at the vacancy site become four- and three-coordinated, and are denoted as Ga4c(d) and Ga3c(d), respectively, as shown in Figure 5. We point out that the nature of the Ga4c(d) atom may be different from the Ga4c atoms on the perfect surface. Creation of the surface oxygen vacancy site induces relaxation of atoms in its vicinity. For example, Ga3c(d) relaxed upward by 0.11 A˚ with 5554 DOI: 10.1021/la903836v

respect to the original position on the dry perfect surface. The two Ga4c(d) atoms move away from each other along the [100] direction, increasing their distance by 0.16 A˚. This leads to a Ga4c(d)-O3c-Ga4c(d) angle of 110°, significantly larger than the corresponding angle of 99° on the dry perfect surface. The formation energy of an oxygen vacancy is defined with respect to the gas-phase energy of O2 in the triplet state, i.e. EVo = -(Eperfect - Edefective - 1/2 EO2). Accordingly, the calculated oxygen vacancy formation energy on β-Ga2O3(100) is 0.34 eV, indicating that the formation of a surface oxygen vacancy on β-Ga2O3(100) is facile. Various sites on the dry defective β-Ga2O3(100) surface are explored for CO2 adsorption, and configurations DD-1 and DD-2 are found to be stable for CO2 adsorption. The relaxed structures and key structural parameters are illustrated in Figure 6. In DD-1, the Oa atom of the adsorbed CO2 fills in the oxygen vacancy site and binds the two Ga4c(d) atoms. The filling of the oxygen vacancy site by the Oa atom caused the two Ga4c(d) atoms to move toward each other. Consequently, the Ga4c(d)-O3c-Ga4c(d) angle decreases from 110° to 105° upon adsorption. The C atom of the adsorbed CO2 in DD-1 interacts with the Ga3c(d) atom, forming the C-Ga3c(d) bond. As such, the Ga3c(d) atom relaxes back almost to its initial position prior to the creation of the oxygen vacancy. The adsorption energy of CO2 in DD-1 is -0.31 eV. In DD-2, the Oa atom of the adsorbed CO2 is bound to one of the Ga4c(d) atoms, whereas the C atom interacts with the Ga3c(d) atom. Formation of the Oa-Ga4c(d) bond in DD-2 causes the Ga4c(d) atom to relax upward by 0.14 A˚. The position of the Ga3c(d) atom is not perturbed by CO2 adsorption in DD-2 from that on the clean dry defective surface. The calculated adsorption energy of CO2 in DD-2 is -0.24 eV. Similarly, the structures of the adsorbed CO2 in DD-1 and DD2 are distorted with respect to that of the free CO2 molecule. We find that the Bader charges on the adsorbed CO2 in DD-1 and DD-2 are -0.77 |e| and -0.74 |e|, respectively, which are significantly larger (more electrons accepted from the surface) than those on the adsorbed CO2 in DP-1, HP-1 and HP-2. This suggests that the dry defective β-Ga2O3(100) surface has a stronger electron-donating ability than both the dry and hydrated perfect surfaces. 3.4. CO2 Adsorption on the Hydrated Defective β-Ga2O3(100) Surface. Adsorption of a water molecule on the dry defective β-Ga2O3(100) surface results in the hydrated defective β-Ga2O3(100) surface. We obtained six stable adsorption configurations and chose the most stable one, D(W-1) shown in Figure 7, as the hydrated defective β-Ga2O3(100) surface to examine CO2 adsorption. We also explored the possibility of the H atom from H2O dissociation binding the neighboring Ga3c(d). Subsequent geometry optimization from such initial structures resulted in the H atom binding the O site. D(W-1) as well as other stable adsorption configurations of water, both molecular and dissociative, are summarized in the Supporting Information. Configurations HD-1, HD-2 and HD-3 were found to be stable for CO2 adsorption on the hydrated defective surface. The optimized structures and the structural parameters are illustrated Langmuir 2010, 26(8), 5551–5558

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Figure 4. Potential energy profiles for protonation of the adsorbed CO2 species by coadsorbed H2O from (a) HP-1 and (b) HP-2. HP-1 and HP-2 are the initial states prior to protonation. TS(HP-1) and TS(HP-2) are transition states, and FS(HP-2) and FS(HP-1) are the products of protonation. Bond lengths in A˚. See Figures 1, 2, 3 for color coding.

Figure 5. Optimized structure of the dry defective β-Ga2O3(100) surface. The oxygen vacancy site is shown as a black sphere. See Figure 1 for color coding.

in Figure 8. In HD-1, CO2 adsorbs at a surface O3c-Ga5c bridge site, resulting in a bidentate carbonate species. The adsorption Langmuir 2010, 26(8), 5551–5558

energy of CO2 in HD-1 is 0.13 eV. Protonation of the adsorbed CO2 in HD-1 leads to the formation of HD-2 also shown in Figure 8. The conversion from HD-1 to HD-2 was facilitated by transferring the Ha atom from the Ow atom to the Ob atom, and produced a bicarbonate species in HD-2. The CO2 adsorption energy in HD-2 is -0.03 eV. HD-2 can be further converted into HD-3, through a transition state HD(TS). The CO2 adsorption energy in HD-3 is ∼0.10 eV. As illustrated in Figure 8, the conversion from HD-1 to HD-2 is exothermic, by -0.16 eV, without an activation barrier, whereas the conversion from HD-2 to HD-3 is endothermic, by 0.19 eV, with an activation barrier of 0.60 eV. As summarized in Table 1, the adsorbed CO2 in HD-1, HD-2 and HD-3 are greatly distorted from a gas-phase CO2 molecule. Both the C-Oa and C-Ob bonds are stretched, and the Oa-C-Ob angle is decreased to ∼120°, indicating that the adsorbed CO2 is highly activated. Bader charge analyses were performed for HD-1, HD-2 and HD-3 and the results are summarized in Table 2. The electronic charges on the adsorbed CO2 in HD-1, HD-2 and HD-3 are -0.28 |e|, -0.27 |e| and -0.28 |e|, respectively. Compared with the adsorbed CO2 in DD-1 and DD-2, the electronic charges on the adsorbed CO2 in HD-1, HD-2 and HD-3 are much less, indicating that the electron-donating ability of the defective surface was reduced due to the hydration process. 3.5. General Discussion. We calculated vibrational frequencies for the stable CO2 adsorption configurations, including DP-1, DOI: 10.1021/la903836v

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Figure 6. Structures of the CO2 adsorption configurations on the dry defective β-Ga2O3(100) surface. Bond lengths in A˚. (a) DD-1, (b) DD-2. See Figures 1 and 2 for color coding.

Figure 7. The most stable adsorption structure of H2O on the dry defective β-Ga2O3(100) surface. Bond lengths in A˚. See Figures 1 and 3 for color coding.

HP-1, HP-2, FS(HP-1), FS(HP-2), DD-1, DD-2, HD-1, HD-2 and HD-3, and summarized the results in Table 3. We would point out that these frequencies are harmonic frequencies. Neither anharmonic correction nor scaling factor was applied to the calculated frequency values. Although the calculated frequencies are not quantitatively in agreement with the experimental results, they fall in the range of the experimental observations. CO2 adsorption on the perfect β-Ga2O3(100) surface, either dry or hydrated, can produce bidentate carbonate species, as shown in DP-1, HP-1 and HP-2. The adsorption energies of CO2 in DP-1, HP-1 and HP-2 are 0.07 eV, 0.14 eV and 0.15 eV, respectively, which indicate that CO2 adsorption is endothermic, and thereby, thermodynamically unfavorable. Furthermore, the CO2 adsorption energies in HP-1 and HP-2 are more endothermic than that in DP-1, showing that hydration further weakens the interaction of CO2 with the surface. However, in the presence of coadsorbed H2O, the bidentate carbonate species could be protonated to form the bicarbonate species, as shown in configurations FS(HP-1) and FS(HP-2), making the overall process thermodynamically favorable. Although the formation of surface carbonate species is not thermodynamically favorable, the resulting adsorption structure 5556 DOI: 10.1021/la903836v

is highly stable. The stability of the carbonate and bicarbonate species has been confirmed by the selected constant-temperature ab initio molecular dynamics (MD) trajectories. The MD simulation has been performed with the implementation in the VASP code. Each trajectory ran for 5000 steps with a time step of 0.6 fs. Snapshots of two MD trajectories, starting from DP-1 (carbonate) and FS(HP-1) (bicarbonate), respectively, are shown in Figure 9. The trajectory movies are available in Supporting Information. Our results clearly show that bidentate carbonates will lead to CO2 desorption and the event happens well within the simulation time (∼2 ps) at 500 K, whereas the bicarbonate species decomposes and releases CO2 and H2O (∼1 ps) at 750 K. These results are consistent with previous experimental observation.10 In the IR spectroscopic study, the CO2 adsorbed as the bidentate carbonate species vanished after outgassing the adsorption system at 323 K, whereas complete desorption of the CO2 adsorbed as the bicarbonate species needed a temperature higher than 550 K.10 DD-1 and DD-2 are the stable configurations for CO2 adsorption on the dry defective surface. The adsorption energies of CO2 in DD-1 and DD-2 are -0.31 eV and -0.24 eV, respectively, and are both much higher than that on the dry perfect surface (0.07 eV). These results suggested that the presence of oxygen vacancies promoted the adsorption of CO2. In addition, the adsorbed CO2 in DD-1 and DD-2 is distorted more significantly than that in DP-1. A more significant distortion indicates a higher extent of activation. As such, the presence of oxygen vacancies also enhanced the activation of the adsorbed CO2. This might be due to the fact that the defective surface is electron-rich, and therefore, has a higher electron-donating ability.29 The increased CO2 adsorption energy and higher degree of activation on the dry defective β-Ga2O3(100) surface can be attributed to the high electron-donating ability of the surface. As shown in Table 2, the electronic charges on the adsorbed CO2 in DD-1 and DD-2 are -0.77 |e| and -0.74 |e|, respectively, whereas that on the adsorbed CO2 in DP-1 is only -0.26 |e|. Next, we compare CO2 adsorption on the dry defective β-Ga2O3(100) surface with that on the hydrated perfect β-Ga2O3(100) surface. The comparison will help to clarify which, water or surface oxygen vacancy, has a greater effect on CO2 adsorption and activation. Although protonation of the adsorbed CO2 makes the overall adsorption process exothermic, the CO2 adsorption energies in the bicarbonate states (FS(HP-1) and FS(HP-2)) remain small (-0.13 eV and -0.01 eV, respectively). On the other Langmuir 2010, 26(8), 5551–5558

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Figure 8. Structures and potential energy diagram for protonation of the adsorbed CO2 species by dissociatively adsorbed water on the defective β-Ga2O3(100) surface. Bond lengths in A˚. See Figures 1, 2, 3 for color coding. Table 3. Calculated Frequencies (in cm-1) of the Characteristic Vibrational Modes for Different CO2 Adsorption Configurations configuration

ν(OCO)as

ν(OCO)s

free CO2 molecule

2380

1325

DP-1 HP-1 HP-2 FS(HP-1) FS(HP-2) DD-1 DD-2 HD-1 HD-2 HD-3 bidentate carbonate a bicarbonate a carboxylate a a Reference 10.

1768 1755 1775 1595 1586 1753 1674 1655 1602 1591 1587 1630 1750

1292 1206 1203 1421 1448 1002 1139 1233 1341 1417 1325 1455

ν(OHO)as

ν(OHO)s

3576 3650

3423 3552

ν(OH)

δ(COH)

3731 3763

1159 1148

3560 3602 3653

1123 1168 1225 1150

Figure 9. Snapshots from the molecular dynamics trajectories starting (a) from DP-1 (carbonate) and (b) from FS(HP-1) (bicarbonate) configurations. Color coding: red, O atoms; green, Ga atoms; white, H atom; sky-blue, C atom. Full trajectories are provided as movie files in the Supporting Information. Langmuir 2010, 26(8), 5551–5558

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hand, CO2 adsorption on the dry defective surface in DD-1 and DD2 has adsorption energies of -0.31 eV and -0.24 eV, respectively. Therefore, the enhancement of CO2 adsorption by the surface defective site (oxygen vacancy) is greater than that by hydration. We further examined the effect of preadsorbed water on CO2 adsorption on the defective surface. In the most stable adsorption configuration (DD-1), one of the oxygen atoms of the adsorbed CO2 filled in the oxygen vacancy site. We also showed that hydration of the defective surface also occurred at the oxygen vacancy site. Therefore, hydration will compete with CO2 adsorption for the oxygen vacancy sites if both H2O and CO2 are present in gas phase. Since the presence of water is inevitable in CO2 conversion, the adsorption of H2O will decrease the availability of surface oxygen vacancy sites for CO2 adsorption and activation. This indicates that the presence of water will have a negative effect on the catalytic activity of β-Ga2O3-containing catalysts in CO2 conversion. Therefore, separating and removing water from the product stream will help to maintain the activity of the catalysts. The present results help to understand why H2O showed a negative effect on the catalytic activity in CO2 hydrogenation to methanol on the Ga2O3-promoted Pd/SiO2 catalyst.8 We can also examine the effect of preadsorbed CO2 on water interaction with the β-Ga2O3(100) surface. HP-1 and HP-2 can be considered as a result of the molecular adsorption of H2O on the perfect β-Ga2O3(100) surface in the presence of the preadsorbed CO2. The adsorption energies of H2O in HP-1 and HP-2 were calculated to be -0.49 eV and -0.48 eV, respectively. Compared with the adsorption energy of H2O on the dry perfect β-Ga2O3(100) surface (-0.56 eV), the adsorption energy of the molecularly adsorbed H2O decreases. Similarly, HD-1, HD-2 and HD-3 can be viewed as the products from the dissociative adsorption of H2O on the defective β-Ga2O3(100) surface in the presence of the preadsorbed CO2. The adsorption energies of H2O in HD-1, HD-2 and HD-3 are -0.85 eV, -1.00 eV and -0.87 eV, respectively. Compared with the adsorption energy of the dissociatively adsorbed H2O without adsorbed CO2 (-0.62 eV), the presence of preadsorbed CO2 strengthens the interaction of H2O with the surface. Previously, we showed that the strongest binding of CO2 occurs at the O-Al bridge site of the (100) surface of γ-Al2O3, an adsorption energy of -0.80 eV.25 Clearly, the binding strength of CO2 on the β-Ga2O3(100) surface, both perfect and defective, is much weaker than that of CO2 with the γ-Al2O3 surfaces. This is consistent with the weaker basicity of the surface sites on the β-Ga2O3(100) surface than those on the γ-Al2O3 surface.

4. Conclusions In the present work, the effects of water and oxygen vacancy on CO2 adsorption on the β-Ga2O3(100) surface have been studied

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using the DFT slab calculations. On the dry perfect surface, CO2 is adsorbed as a carbonate species with an adsorption energy of 0.07 eV. Molecular adsorption of H2O on the dry perfect β-Ga2O3(100) surface resulted in a hydrated surface. Adsorption of CO2 on the hydrated perfect β-Ga2O3(100) surface as the carbonate species remains endothermic and has an adsorption energy of 0.14 eV. The carbonate species on the hydrated perfect surface can be protonated by the coadsorbed H2O to a bicarbonate species. Formation of the bicarbonate species transformed the CO2 adsorption into an exothermic process, with an adsorption energy of -0.13 eV. A defective β-Ga2O3(100) surface was created by removing a surface O3c atom from the dry perfect surface. Formation of an oxygen vacancy is endothermic by 0.34 eV with respect to a gas phase O2 molecule. Presence of oxygen vacancies promotes CO2 adsorption and activation on β-Ga2O3(100). The oxygen vacancy sites provided active sites for adsorption and activation of CO2. Accordingly, a high oxygen vacancy density should benefit CO2 adsorption and activation, and thereby, improve the catalytic activity of β-Ga2O3-containing catalysts for CO2 conversion. Our results also indicate that H2O competes with CO2 for the oxygen vacancy sites. Separating and removing water from the product stream will help maintaining a high activity of β-Ga2O3-containing catalysts in CO2 conversion. Acknowledgment. We gratefully acknowledge support from the Petroleum Research Fund (PRF-G44103-G10), the National Natural Science Foundation of China (under Contract 20490203), and the Illinois Clean Coal Institute. D.M. acknowledges support of a Laboratory Directed Research and Development (LDRD) project at Pacific Northwest National Laboratory (PNNL). A portion of the computing time was granted by the scientific user projects using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). The EMSL is a DOE national scientific user facility located at PNNL, and supported by the DOE’s Office of Science, Biological and Environmental Research. For the computing time, we also thank the National Energy Research Scientific Computing Center (NERSC) under Project No. m752. Supporting Information Available: Structures and energetics for water adsorption and dissociation on the dry perfect and defective β-Ga2O3(100) surfaces; movie files showing molecular dynamics trajectories starting from the surface carbonate and bicarbonate species, leading to desorption of CO2. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(8), 5551–5558