Periodic Density Functional Theory Study of Propane

Dec 4, 2008 - Propane dehydrogenation over perfect Ga2O3(100) was studied in detail by periodic density functional theory (DFT) calculations. It was f...
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20382

J. Phys. Chem. C 2008, 112, 20382–20392

Periodic Density Functional Theory Study of Propane Dehydrogenation over Perfect Ga2O3(100) Surface Yan Liu, Zhen Hua Li,* Jing Lu, and Kang-Nian Fan* Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Center for Theoretical Chemical Physics, Department of Chemistry, Fudan UniVersity, Shanghai, 200433, China ReceiVed: September 04, 2008; ReVised Manuscript ReceiVed: October 28, 2008

Propane dehydrogenation over perfect Ga2O3(100) was studied in detail by periodic density functional theory (DFT) calculations. It was found that the initial C-H bond activation mainly follows a radical mechanism that the two-coordinated surface oxygen site (O(2)) abstracts a hydrogen atom from propane with the formation of propyl radical and hydroxyl group (O(2)H). Physically adsorbed propyl radical can easily form propoxide or propylgallium intermediate. Subsequently, propene is formed by a second H abstraction from propyl, propoxide, or propylgallium by surface oxygen and Ga sites. H abstraction by O(2) site always has low energy barrier. However, it is difficult for the hydrogen atoms in the hydroxyl groups to leave the surface in the form of either H2 or H2O. In addition, propene formed through H abstraction by oxygen site has high adsorption energy and is prone to further dehydrogenation or oligomerization, leading to fast deactivation of the catalyst. On the other hand, the formation of H2 from GaH and hydroxyl group is much easier, although the formation of GaH has to overcome high energy barrier. Thus, there is a shift of rate-determining step for propane dehydrogenation: at the initial stage of the reaction, the rate-determining step is H abstraction by oxygen sites and then it shifts to H abstraction from various propyl sources by Ga sites to form gallium hydrides after the surface oxygen sites are consumed. Our results also indicate that dehydrogenation of propane mainly follows a direct dehydrogenation mechanism (DDH), whereas oxidative dehydrogenation (ODH) is energetically less feasible but cannot be ruled out in the presence of mild oxidant such as CO2. 1. Introduction Given the ever-increasing worldwide demand for olefins, alternative inexpensive ways to produce light olefins in industry are highly desired.1 Propene is a useful raw material for polymers and a number of chemical products, for example, polypropene, polyacrylonitrile, acrolein, and acrylic acid. Direct dehydrogenation of propane is an endothermic process which requires relatively high temperatures to obtain a high yield of propene. However, increasing temperatures would favor thermal cracking of light alkanes to carbon deposition and always lead to decrease of the product yield. Hence, the development of highly efficient catalyst has been of interest recently. Besides, considering the overoxidation in the presence of O2, CO2 also becomes a promising mild oxidant introduced in the alkanes dehydrogenation reactions2-10 because CO2 would partially suppress the catalyst deactivation by carbon deposition through Boudouard reaction.11,12 In the presence or absence of CO2, a series of metal oxide catalysts have been studied in the dehydrogenation of alkanes, and among them Ga2O3 is a highly effective one.13 Many experimental studies have shown that Ga2O3-based catalysts are highly active and selective in the dehydrogenation of alkanes.11-23 On the gallium oxide catalyst, the selectivity of alkene products is usually above 90%, and the activity is also good. Furthermore, Ga2O3 has been widely used as support or promoter in alkanes aromatization,24,25 nitrogen oxides reduction,26,27 and methanol synthesis.28,29 Therefore, gallium oxide is a very important oxide and is of general interest to chemists. However, the high activity of gallium oxide in light alkane dehydrogenation reaction was * To whom correspondence should be addressed. E-mail: lizhenhua@ fudan.edu.cn; [email protected].

observed only at the initial stage of the reaction. The activity of catalyst would decrease quickly in 1 h. Micgorczyk30 suggested that carbon deposition on the Ga2O3 surface may contribute to the decrease in propene yield and introduced CO2 in the reaction to remove carbon deposition and improve propene yield. Some results also show that CO2 has a promoting effect on dehydrogenation activity over Ga2O3/TiO2, but a negative effect over Ga2O3/ZrO2 and Ga2O3/Al2O3.15 In order to understand the rapid decrease of the catalytic activity, a detailed mechanism of alkane dehydrogenation on gallium oxide surface is necessary. To pinpoint alkane activation mechanism on gallium oxide is a great challenge to both experiment and theory. So far, there is no agreement on the dehydrogenation mechanism of alkanes on gallium oxide and gallium-containing catalysts. It is generally accepted that alkane dehydrogenation is a three-step reaction composed of two consecutive C-H activation steps and a final step of hydrogen desorption from the surface. However, it is still not very clear in what form, mainly in H2 or H2O, hydrogen desorbs from the surface at the initial stage of the reaction. If it is H2, the reaction mechanism is a direct dehydrogenation (DDH) mechanism, and if not, the reaction mechanism is an oxidative dehydrogenation (ODH) mechanism. ODH reaction is thermodynamically favorable but leads to the reduction of the surface and thus oxidant is needed to reoxidize the catalyst to continue the catalytic cycle, or the activity of the catalyst would decrease with the consuming of oxygen in the catalyst. O2 is widely used as oxidant but it leads to deep oxidation of alkane and decrease of alkene selectivity. Mild oxidant such as CO2 can be used. Noticing that propane dehydrogenation reaction could react in the presence or absence of CO2, Yue

10.1021/jp807864z CCC: $40.75  2008 American Chemical Society Published on Web 12/04/2008

Propane Dehydrogenation over Perfect Ga2O3(100) Surface and co-worker14,15 proposed that ODH mechanism is inapplicable to gallium oxide or supported gallium oxide because gallium oxide is hard to reduce.24 However, recent TPR experiment found that on the gallium oxide 30% of O was removed as H2O during the reduction in H2 at 823 K, a typical reaction temperature for alkane dehydrogenation on Ga2O3.23 In addition, DDH mechanism cannot explain some experimental results; for example, in the reoxidation of gallium oxide by CO2 after treatment with H2 and C3H8, the amount of CO after H2 treatment is much lower than that after C3H8 treatment.17 Theoretically, only van Santen et al. have studied the possibility of forming H2O from the dehydrogenation of ethane over isolated gallyl ions (GaO+) as a model for the active sites in oxidized Ga/ZSM-5 zeolite.31 They found that the formation of H2 is highly endothermic and requires high activation energy, while the recombination of hydroxyl and hydride leading to reduction of [HO-Ga-H]+ to Ga+ via H2O elimination is easier. Another unsettled issue in the reaction mechanism is the role of Ga sites in the C-H bond activation. It is well-known that the acid or basic property of the active sites is of crucial importance for the activity of metal oxide catalysts in many reactions. For some acid oxide catalysts, for example Ga2O3, ZnO, CuO, Fe2O3, W2O3, and Al2O3, the Lewis acid sites, i.e., Ga, Zn, Cu, Fe, W, and Al sites, are considered to be more active in dehydrogenation reactions.13,23,32-40 The role of gallium sites in the dehydrogenation reactions on supported or unsupported Ga2O3 catalysts have been examined by various studies, but no agreement has been reached. Some authors14,15,41 proposed that the C-H bond in propane is polarized by gallium-oxygen pairs. It is the gallium atom that abstracts hydrogen atom to form gallium hydride and gallium alkoxide, and then propene is formed by a second H abstraction by neighboring oxygen sites. However, an IR experiment suggested that the C-H bond of ethane is initially activated by unsaturated gallium atom and results in the formation of hydroxyl and carbon-gallium bond.42 Some experimental results also showed that gallium hydride can only be detected at elevated temperatures42-44 and Kazansky et al.42 suggested that it is formed due to subsequent decomposition of gallium-alkyl species by the gallium sites. In addition, in the study of alkane dehydrogenation on Ga-doped H-ZSM5 catalyst, Guisnet et al. proposed that gallium sites act as promoter to recombinative hydrogen removal during alkane dehydrogenation.45 Theoretically, Broclawik et al.46 suggested that gallium sites play important roles in C-H bond activation. van Santen et al.31,47 and Nascimento et al.48 have studied the role of gallium sites in the alkane dehydrogenation catalyzed by gallium-exchanged zeolite using DFT methods. However, they concluded that gallium atoms act just as alkyl acceptors. Fu et al.49-51 have systematically studied the initial C-H activation of light alkanes on MoO3, WO3, CrO3, and V2O5 metal oxides employing cluster models. They found that on MoO3, CrO3, and V2O5 the C-H activation by the MdO acid-base pair is an unfavorable pathway due to very high energy barrier, but on WO3 it is a favorable pathway with a low enthalpy barrier of 43.6 kcal/mol at 873 K due to the high polarity of the WdO bond. Since Ga-O bonds in Ga2O3 are also highly polar bonds,42 could the C-H activation on the Ga-O acid-base pair be a favorable pathway? To date, there is, to our knowledge, no systematic theoretical work on the mechanism of alkane dehydrogenation on gallium oxide surface. Aiming to gain a clear insight into the dehydrogenation mechanism of alkanes on the Ga2O3 catalysts, here we have studied the conversion of propane to propene on pure

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Figure 1. (a) Unit cell of β-Ga2O3; (b) unit cell of the Ga2O3(100) surface.

gallium oxide surface. It is generally accepted that on metal oxides propene formation is through the second C-H bond activation of propoxide. Alternatively, we propose that propene can be formed not only from chemical adsorbed propyl intermediate (propylgallium or propoxide) but also from physically adsorbed propyl radical. Hence, in the present study we show that (i) the initial C-H bond can be activated by the active sites on the surface, resulting a propyl radical physically or chemically absorbed on the surface; (ii) the propoxide or propylgallium species or propyl radical releases a H atom to form propene; and (iii) the last step is hydrogen desorption from the surface in the form of H2 as the major product to complete the catalytic cycle or in the form of H2O as a minor product leading to the reduction of the catalyst. 2. Model and Computational Details Among the four polymorphs (R-, β-, γ-, and δ-Ga2O3) of gallium oxide, β-Ga2O3 exhibits the highest dehydrogenation activity, high thermal stability, and relatively high specific surface area.14,52,53 Monoclinic β-Ga2O3 belongs to the C2/m space group with lattice parameters a ) 12.23 Å, b ) 3.04 Å, c ) 5.80 Å, and β ) 103.70°.54 Four Ga2O3 units exist in a crystallographic unit cell which consists of two primitive unit cells (Figure 1a). The lattice is composed of two inequivalent Ga sites and three inequivalent O sites.55 Half of the gallium sites are forms of slightly distorted tetrahedron with four oxygen ions, named Ga(t), and the other half are forms of highly distorted octahedron with oxygen ions, named Ga(o). As for oxygen, two different oxygen sites are 3-fold coordinated (one is shared by one octahedron and two tetrahedron, named O(2), the other is shared by two octahedron and one tetrahedron, named O(3)), and the third oxygen sites is 4-fold coordinated which lies at the corner of three octahedrons and one tetrahedrons, named O(4). β-Ga2O3(100) surface have been studied in previous studies.56-60 Bermudez had studied the structure and properties of four different monoclinic β-Ga2O3 surface, showing that (100) surface is the most stable surface.61 In the present study, Ga2O3(100) surface is simulated by a unit cell (p(4×2), 12.37 Å × 11.79 Å) with an 20 Å vacuum region between slabs. β-Ga2O3(100) surface is terminated in rows of unsaturated O(2) with each back-bonded to two tetrahedral gallium Ga(t) sites and of unsaturated gallium Ga(o) sites with each bonded to two O(3) sites and O(4) sites (Figure 1b). In the present study, Ga2O3(100) surface is routinely modeled by a one-layer slab with half of atoms allowed to relax. It is expected that unsaturated sites are more active than saturated ones, and thus Ga(t) sites and O(4) sites have little activity. Hence, we focus on the Ga(o) (named

20384 J. Phys. Chem. C, Vol. 112, No. 51, 2008 Ga without specific mentioning hereafter), O(2), and O(3) active sites on the Ga2O3(100) surface. All total energy density functional theory calculations were carried out using the SIESTA package with numerical atomic orbital basis sets and Troullier-Martins norm-conserving pseudopotentials.62,63 The exchange-correlation functional utilized is the generalized gradient approximation PBE method, known as GGA-PBE.64 A double-ζ polarization basis set (DZP) was employed. The orbital-confining cutoff radii were determined from an energy shift of 0.01 eV except for hydrogen, for which a DZP basis set optimized for H2 was used.65 The energy cutoff for the real space grid used to represent the density was set as 150 Ry. To further speed up calculations, the Kohn-Sham equations were solved by an iterative parallel diagonalization method that utilizes the ScaLAPACK subroutine pdsygvx with two-dimensional block cyclically distributed matrix.36 The Broyden method was employed for geometry relaxation until the Cartesian forces on each relaxed atom were all less than 0.1 eV/Å. Only Γ-point was used to sample the Brillouin zone in our calculations. Spin polarization was considered during all the calculations. A constrained minimization scheme was employed to search the transition states (TS) on the potential energy surface.66,67 A TS is identified when the forces on the atoms vanish and the energy is a maximum along the reaction coordinate but a minimum with respect to all of the other degrees of freedom. Energy barrier is determined as the energy difference between the saddle point and the initial state. Adsorption energy (Eads) is calculated as

Eads ) E(adsorbate+substrate) - (Eadsorbate + Esubstrate) where a process with a negative Eads is exothermic. 3. Results and Analysis 3.1. Molecular Adsorption of Propane. We have first examined the molecular absorption of propane on various active sites. The potential energy surface for the adsorption is found to be quite flat. The most favorable adsorption site for propane is near the unsaturated Ga site, and the adsorption energy is calculated to be -0.7 kcal/mol. The results show that propane undergoes preliminary weak adsorption on the surface prior to dissociation. Indeed, it was concluded that hydrocarbon molecules adsorb weakly on metal sites.46 Considering that the dehydrogenation of propane is carried out at high temperatures, propane in the gas phase is more stable than it is on the surface due to the large entropy contribution of the gas phase molecules. In agreement with theoretical findings, no molecular adsorption of propane has been detected by IR spectroscopy.42 3.2. First Stepsthe Activation of the First C-H Bond. The C-H bond of the methyl or methylene group of propane will be broken with n-propoxide or i-propoxide formed on the surface. It is well-known from thermodynamics that the C-H bond in the methyl group is stronger than the C-H bond in the methylene group. Experimental results indicated that the C-H bond cleavage from the methylene group was more feasible than that from the methyl group both thermodynamically and kinetically.68-71 Several theoretical studies also showed that the energy barrier for breaking the C-H bond of the methyl group was higher than that of the methylene group.49,72 We have calculated the energy barriers for hydrogen abstractions by O(2) site through a radical mechanism and found that the energy barrier for breaking the methyl C-H bond is 9.2 kcal/mol higher than that for breaking the methylene C-H bond, which tallies with previous theoretical study.72 Therefore, in the present study,

Liu et al. we will also focus on the breaking of the methylene C-H bond as other theoretical studies have done on the dehydrogenation of propane on V2O572 and MoO349,73 catalysts. Previous studies suggest that there are two possible mechanisms for the initial C-H bond activation of propane on metal oxide.49,74 One is heterolytic dissociation mechanism, in which the acid-base pair (Ga-O) polarizes and then breaks the C-H bond. Two distinct processes have been considered in the present study for this mechanism:

C3H8 + O-Ga f OH + C3H7Ga

(1)

C3H8 + Ga-O f GaH + C3H7O

(2)

In process (1) the proton of propane and the negatively charged propyl fragment C3H7δ- form bonds with the oxygen and gallium sites, respectively, while in process (2) the negatively charged H atom forms bond with the gallium site, and the positively charged propyl group C3H7δ+ forms bond with the oxygen site. The other is homolytic dissociation mechanism, in which the active site pair is composed of two O or two Ga active sites, and the C-H bond is polarized and activated by O or Ga sites. There are also two distinct processes:

C3H8 + O-O f OH + C3H7O

(3)

C3H8 + Ga-Ga f GaH + C3H7Ga

(4)

Since there are three types of available active sites (O(2), O(3), Ga) on the surface, the activation of the initial C-H bond in propane could result in nine product combinations, namely, O(2)-Ga,O(3)-Ga,Ga-O(2),Ga-O(3),O(2)-O(2),O(2)-O(3), O(3)-O(2), O(3)-O(3), and Ga-Ga, where the H atom is always bonded to the first atom, the propyl group is bonded to the second atom, and to make the discussions brief we have omitted them. For each product combination, there are two possible types of pathways. One belongs to a concerted mechanism (C): active site pair Ga-O, O-O, or Ga-Ga activates the C-H bond of propane and then in one elementary reaction step the propane reacts with two adjacent atoms on the surface in a concerted way. These pathways feature cyclic TSs, involving 4e in these processes.49 The other one belongs to a radical mechanism (R): the oxygen or gallium active site activates the C-H bond of propane, and abstracts an H atom directly from propane, involving 2e in these processes. Subsequently, the propyl radical either physically adsorbs on the surface, or rebinds to an adjacent surface atom, or desorbs into gas phase. To further simplify the discussion, the pathways leading to the nine product combinations could be classified into three groups according to the active site bonded to hydrogen in the products: (i) O(2) site pathways O(2)-X-C and O(2)-X-R; (ii) O(3) site pathways O(3)-X-C and O(3)-X-R; (iii) Ga site pathways Ga-X-C and Ga-X-R, where X ) O(2), O(3), and Ga. 3.2.1. O(2) Site Pathways. (a) O(2)-X-C. The energy profiles of these pathways and the structures of the cyclic TSs are depicted in Figure 2. The O(2)-Ga site pair can be regarded as a Lewis acid-base pair since the active site pair is formed by a positively charged gallium atom and a negatively charged oxygen atom. This step is 9.9 kcal/mol endothermic with an energy barrier of 25.3 kcal/mol (TS1, Figure 2), which is lower than the barrier for the initial methane activation (28.7 kcal/ mol)46 but is higher than the barrier for the initial ethane activation (16.8 kcal/mol)47 on the zeolite-supported gallium atom. For the homolytic activation on the O(2)-O(2) site pair, the process is -8.9 kcal/mol exothermic with an energy barrier

Propane Dehydrogenation over Perfect Ga2O3(100) Surface

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Figure 2. Potential energy profiles and TS structures for C-H bond activation on O(2) sites.

of 38.8 kcal/mol (TS2). For the activation on the O(2)-O(3) site pair, a cyclic transition state could not be located by any means. In addition, forming the O(2)-O(3) product combinations is 20.1 kcal/mol endothermic. Therefore, compared to the O(2)-Ga and O(2)-O(2) site pairs the activation on the O(2)-O(3) site pairs is unfavorable. Overall, for the concerted mechanism the activation on the O(2)-Ga site pairs is more favorable than that on the O(2)-O(2) site pairs. (b) O(2)-X-R. As for the radical mechanism (R), all pathways share the same initial step, i.e., H abstraction by the same O(2) site. As can be seen from Figure 2, the energy barrier for the first H abstraction from propane is 18.1 kcal/mol. A physically adsorbed propyl radical is formed and its adsorption energy is 4.5 kcal/mol. Then the physically adsorbed propyl radical may become chemically adsorbed by forming chemical bond with adjacent active sites, i.e., O(2), O(3), Ga, and the newly formed O(2)H group. The bonding with the newly formed O(2)H will be discussed later. Here we will focus on the other three sites. The energetics indicate that the propyl radical prefers to form bond with the nearest Ga and O(2) sites, since the energy barriers to form chemically adsorbed propyl on the two sites are just 4.4 and 5.5 kcal/mol, respectively, whereas on the O(3) site the barrier is 12.0 kcal/mol. Another possibility is the desorption of the physically adsorbed propyl radical into the gas phase with a desorption energy of 4.5 kcal/mol. Since forming the O(2)-O(3) product combination is most endothermic and needs to overcome very high energy barrier, propyl is unlikely to form a bond with the O(3) site. On the other hand, the adsorption of propyl on the O(2) site is strongly exothermic and thus O(2) is a stable adsorption site for propyl. 3.2.2. O(3) Site Pathways. The O(3) site pathways are similar to the O(2) site pathways except that some TSs could not be located and the energy profiles for pathways with TS located are shifted to higher energies since O(3)H group is less stable than O(2)H group. The energy barrier of the O(3)-Ga-C pathway is 16.4 kcal/mol (Figure 3) higher than that of the O(2)-Ga-C pathway. In addition, this reaction is highly

Figure 3. Potential energy profiles and TS structures for C-H bond activation on O(3) sites.

endothermic with a reaction energy of 31.1 kcal/mol, and the resulting product is much less stable than that formed through the O(2)-Ga pathways. For the O(3)-O(2)-C and O(3)-O(3)-C pathways, the cyclic TS structures could not be located. This may be due to the lower ability of the O(3) site to abstract hydrogen and the lower stability of the O(3)H group or C3H7O(3) intermediate. Our previous study on V2O5-catalyzed propane dehydrogenation also showed that the 3-fold coordinated O(3) sites are the most inert sites.72 For the O(3)-X-R pathways, the energy barrier of the H-abstraction step is 39.4 kcal/mol, which is also much higher than that of the corresponding O(2) pathways. To form C3H7Ga intermediate, an additional energy barrier of 20.6 kcal/mol has to be overcome. The TS structure for forming C3H7O(2) intermediate could not be located. Although the energy barrier for forming C3H7O(3) is just 6.5 kcal/mol, the formed C3H7O(3) intermediate is very unstable with a reverse energy barrier of just 0.9 kcal/mol. This is similar to the formation of C3H7O(3) intermediate from O(2)H + C3H7(ad) pair (Figure 2) where the reverse energy barrier from O(2)H + C3H7O(3) to O(2)H + C3H7(ad) is also very low. Therefore, we are unlikely to observe C3H7O(3) intermediate in the experiment. On the other hand, C3H7O(2) and C3H7Ga intermediates are more likely to be captured by experimental techniques since they fall into deeper potential energy wells. 3.2.3. Ga Site Pathways. All the TSs for the activation of the initial C-H bond by the Ga site to form gallium hydride, either via concerted or radical mechanism, could not be located. However, experiments have observed the existence of the Ga-H bond.42 van Santen and co-workers have also failed to locate the TS to form gallium hydride on the GaO+ species stabilized on zeolite,31,47 and they proposed indirect pathways to form gallium hydride.35 Here, we have also studied a similar pathway to form gallium hydride via a radical mechanism. As mentioned above when discussing the O(2) site pathways, the physically

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Figure 4. Potential energy profiles and TS structures for the formation of gallium hydride via an indirect pathway.

adsorbed propyl radical can become chemically adsorbed by forming bond with the newly formed O(2)H group, leading to an alcohol-like intermediate. The energy barrier for this process is 11.2 kcal/mol (in Figure 4). Then the adjacent Ga site abstracts the hydrogen from the hydroxyl group of the alcohol-like intermediate. The energy barrier of this hydrogen migration step is 40.5 kcal/mol. Since this hydrogen migration step is the ratedetermining step to form gallium hydride, the overall energy barrier to form gallium hydride from the gas phase propane is the relative energy between the H-migration transition state (TS12) to the initial reactants, and it is 58.9 kcal/mol, a little too high. Therefore, gallium hydride may not be formed by this mechanism due to the high hydrogen-migration barrier. The above results indicate that the first C-H bond of propane is mainly activated by the O(2) site through a stepwise radical mechanism. Recent DFT study on MoO3-catalyzed light alkane dehydrogenation using cluster models also found that the radical mechanism is energetically preferred for both methane50 and propane49 dehydrogenations. However, our previous study of propane activation on V2O5(001) surface indicates that an oxoinsertion mechanism, i.e., the oxygen atom inserts into the C-H bond forming an alcoholic intermediate and then the H atom of the hydroxyl group migrates to adjacent sites, is energetically preferred. We have also considered this mechanism and have optimized a transition state structurally very similar to the one optimized on the V2O5(001) surface. However, careful analysis indicates that this transition state is not really the transition state connecting the physically adsorbed propane reactant and the alcoholic intermediate, but the transition state of the second step of the radical mechanism (TS11 in Figure 4) to form the alcoholic intermediate as mentioned above when discussing the Ga site pathways. We further confirmed this by changing the geometry of this transition state slightly toward the reactants and then doing a full geometry optimization. (It is worth mentioning here that IRC analysis has not been implemented in most periodic software codes yet.) The optimization results in a propyl radical physically adsorbed on the O(2)H group, i.e., the intermediate of the H abstraction by the O(2) site

Liu et al. through a radical mechanism (IM1), not the physically adsorbed propane. It seems that the oxygen sites on the Ga2O3(100) surface and the V2O5(001) surface behave differently in the initial C-H activation of propane. In addition, the initial C-H bond cleavage of propane on the O(3) sites is less feasible compared to the O(2) sites both thermodynamically and kinetically. However, since the first step is not the ratedetermining step in the reaction (see the discussions below), the possibility of the O(3) sites as the reaction center of propane activation cannot be ruled out. It is very interesting to see that both the concerted and the stepwise radical mechanisms predict an easy formation of alkyl-gallium species and hydroxyl groups. This is in good agreement with recent experimental results that strong DRIFTS (diffuse reflectance infrared Fourier transform spectrum) bands of ethyl-gallium and hydroxyl groups have been observed after heating Ga2O3 in ethane atmosphere at 423 K.42 The same experiment also indicates that dissociative adsorption of ethane starts already at room temperature. Previous theoretical study51 have shown that the initial activation of the C-H bond by the MdO acid-base pair via a concerted mechanism is governed by the polarity of the MdO bond. Our results show that C-H bond activation via the concerted mechanism is energetically more feasible on the gallium oxides than on other metal oxides due to the high polarity of the GaO bond, although it is energetically less favorable than the activation via the radical mechanism.42 3.3. Second StepsPropene Formation. There are two distinct pathways to form propene after the breaking of the first C-H bond: the losing of a hydrogen atom from either the physically adsorbed propyl radical or the chemically adsorbed propyl species, i.e., propoxide or propylgallium species. The formed propene then desorbs from the surface. Different from the initial C-H bond activation, the second hydrogen atom can be abstracted not only by nearby oxygen active sites, i.e., O(2), O(3), and the newly formed O(2)H groups, but also by the unsaturated Ga sites. 3.3.1. From Physically Adsorbed Propyl Radical. The energy profiles and all TSs for the second H abtraction from propyl radical with the first hydrogen atom abstracted from propane bonded to O(2) are shown in Figure 5. Those for the H abtraction from propyl radical with the first hydrogen atom bonded to O(3) site show similar trend except that the energy profiles are significantly lifted to higher energies (by more than 20 kal/mol) due to the fact that O(3)H groups are much less stable than O(2)H groups. These energy profiles and TS structures are available in the Supporting Information. The results show that the abilities of different sites to abstract H atom from propyl are in the order of O(2) > O(3) > O(2)H > Ga. For the V2O5 surface, Fu et al.72 found that the H-abstraction abilities of the lattice O sites and surface OH groups are similar. However, on the Ga2O3(100) surface the activity of the O sites decreases a lot once they have become hydroxyl groups. We have also tried to search the TSs for the initial C-H bond activation of propane by hydroxyl groups. However, all our efforts have failed. Therefore, the partially reduced Ga2O3(100) surface has lower activity than the unreduced surface. It can be seen from Figure 5 that propene formation through H abstraction by O(2) has the lowest energy barrier (4.5 kcal/mol), and the highest one is through H abstraction by Ga (29.6 kcal/mol). Taking all the three steps to form the gaseous propene product into consideration, i.e., the formation of propyl radical, the formation of physically adsorbed propene, and the desorption of propene from the surface, it can

Propane Dehydrogenation over Perfect Ga2O3(100) Surface

Figure 5. Potential energy profiles for propene formation through H abstraction from propyl by surface oxygen, gallium, and OH group. All energies are relative to the initial reactants C3H8(g) + Ga2O3(100) surface.

be seen that the most energetically preferred pathway is H abstraction by the O(2) sites at both the first and second steps of the C-H bond activation. The second energetically preferred pathway is first H abstraction by O(2) sites and then second H abstraction by O(3) sites. The least feasible pathway is first H abstraction by the O(2) sites and then second H abstraction by Ga sites. Therefore, it can be concluded that the lowest-energy pathway to form propene is on O(2) sites, but it can also occur on Ga sites at high temperatures leading to the formation of gallium hydrides. The formation of gallium hydrides via this mechanism is energetically much more feasible than the indirect mechanism discussed above. This conclusion is in agreement with the recent experimental results for ethane adsorption on Ga2O3 that gallium hydrides are detected by IR spectrum at high temperatures.42 Electron-affluent molecules donate electrons easily to surface active sites and are thus strongly adsorbed on the surface.75 It can be seen that propene formed by O(3) abstraction or O(2)H group has high desorption energy (more than 9 kcal/mol). Charge analysis indicates that there is significant electron transfer from propene to surface hydroxyl groups. Hence the obtained propene on the O(3) sites and OH group may lead to deep dehydrogenation products such as carbon deposition and results in fast deactivation of the Ga2O3 catalyst17 or the formation of aromatic byproducts.23 This is not surprising because it has been found that the acid sites of the catalyst is favorable for coke formation76 and further oligomerization of the adsorbed propene.77 3.3.2. From i-Propoxide Intermediates. C3H7O(2) is the most stable form of the chemically adsorbed propyl on the surface. C3H7O(3) is less stable. To form propene, the adjacent active

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20387 sites, for example O(2), O(3), Ga, and surface O(2)H group must abstract one methyl hydrogen from either C3H7O(2) or C3H7O(3). However, our searches for TSs starting from C3H7O(3) either failed or all the optimized TSs have exactly the same geometries as those optimized starting from the physically adsorbed propyl radical. It seems that the C-O bond of the C3H7O(3) intermediate has to be completely broken before C3H7O(3) further reacts with surface active sites to produce propene since the energy barrier for breaking C-O(3) bond is low (see Figures 2 and 3). For those starting from C3H7O(2), the located TS structures for H abstraction by O(3) and O(2)H group are also exactly the same as those from physically adsorbed propyl radical as discussed above. Thus, in Figure 6a only two pathways, H abstraction by O(2) and Ga, are presented. Both pathways start from the O(2)H and C3H7O(2) product pair resulting from the initial C-H bond activation of propane on O(2) site. Those pathways starting from the O(3)H + C3H7O(2) product pair show similar trend and are available in the Supporting Information. As expected, the most feasible pathway to produce propene is H abstraction by adjacent O(2) site, with an energy barrier of 19.8 kcal/mol. This process is exothermic and leads to one more surface O(2)H group. On the other hand, the energy barrier for H abstraction by Ga site is much higher, reaching to 57.6 kcal/mol. This process is endothermic and leads to gallium hydride on the surface. We have examined the possibility of the reaction of propoxide with adjacent O(2)H, O(3)H, and Ga(2)H groups to form propene and H2 in one step. For the O(2)H and O(3)H groups such concerted TS structures could not be located. As discussed above, it is difficult to generate gallium hydride from H abstraction from propyl, propoxide, or propylgallium (see below). However, once it is produced, we did locate the concerted transition state for its reaction with propoxide to form propene and H2. The TS structure is shown in Figure 5. In this transition state, the C-O bond is elongated from 1.50 Å in propoxide to 2.97 Å, the positively charged β-hydrogen of the propoxide approaches the negatively charged “hydride” hydrogen bound to Ga with an H-H distance of 1.11 Å, and the Ga-H bond is elongated to 1.79 Å. This is an endothermic process with a reaction energy of 24.6 kcal/mol. The energy barrier for this process is 31.7 kcal/mol. Since the subsequent desorption of propene and H2 from surface requires just a small amount of energy, the overall energy barrier to form propene and H2 from the initial reactants through this mechanism is about 48.6 kcal/mol. van Santen et al. have also optimized a similar transition state for the activation of ethane catalyzed by Gaexchanged zeolite, and the energy barrier calculated is 39.6 kcal/ mol.47 3.3.3. From i-Propylgallium Intermediate. Unlike those pathways starting from the i-propoxide intermediate, the transition state for the concerted H2 formation from H abstraction by GaH could not be located. The energy profiles of the pathways starting from the O(2)H + C3H7Ga pair are shown in Figure 7. Those starting from the O(3)H + C3H7Ga product pair show similar trend and are available in the Supporting Information. The results show that H abstraction by the nearest O(2) site has the lowest-energy barrier of just 13.6 kcal/mol, and this process is -9.6 kcal/mol exothermic. H abstraction by Ga sites is less feasible and has to overcome an energy barrier of 38.8 kcal/mol. Summarizing the above results, O(2) sites, Ga sites, and GaH sites are efficient active sites in propene formation. The lowestenergy pathway for propene formation is through H abstraction by O(2) sites. At high temperatures, propene formation on Ga

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Figure 6. Potential energy profiles for propene formation through H abstraction from C3H7O(2) intermediate by (a) O(2) site and (b) GaH site. All energies are relative to the initial reactants C3H8(g) + Ga2O3(100) surface.

TABLE 1: Energy Barriers for H Migration from Nonadjacent Sites to Adjacent Sites initial state

final state

energy barrier (kcal/mol)

+ + + +

O(2)H + GaH O(2)’H+ O(2)H O(2)H + Ga’H O(2)’H + GaH

76.9 37.2 37.0 39.2

O(2)H O(2)H O(2)H O(2)H

Figure 7. Potential energy profiles and transition states for propene formation through H abstraction from i-propylgallium by surface oxygen or gallium sites. All energies are relative to the initial reactants C3H8(g) + Ga2O3(100) surface.

sites and GaH sites cannot be ruled out. Besides, it is obvious that energy barriers for H abstraction by O(3) sites and O(2)H groups are not high, but the desorption energy of the formed propene is very high, indicating that these two pathways are easy to produce side products and lead to fast deactivation of the catalyst. 3.4. Third StepsH2 or H2O Formation. To complete the catalytic cycle, the two H atoms abstracted from propane must eventually desorb from the surface. Zheng et al. suggested that H2 is produced from adjacent hydroxyl group and gallium hydride.14 Using in situ transmission infrared spectroscopy, Collins et al. also reached the same conclusion that when T > 650 K H2 is released from surface Ga-H and Ga-OH species.44 However, Takahara et al. found that on the gallium oxide 30%

O(2)H O(2)H GaH GaH

of O was removed as H2O during the reduction in H2 at 823 K.23 Here we will first study the migration of H atoms between different surface sites and then examine all the possible pathways to form H2 and H2O on the Ga2O3 (100) surface. 3.4.1. Migration of H Atom on the Surface. In order to form H2 or H2O, nonadjacent H atoms have to move to adjacent sites. The energy barriers for the H migration from nonadjacent sites to adjacent sites are listed in Table 1. As shown in Table 1, H migration between different surface sites has to overcome high energy barriers, especially H migration from O(2) site to Ga site to form gallium hydride, for which the energy barrier is 76.9 kcal/mol. On the contrary, the energy barrier for the H abstraction (TS14, 29.6 kcal/mol) from physically adsorbed propyl radical by gallium to form gallium hydride is much lower than the energy barrier for the H migration from hydroxyl group to gallium site. Thus, the experimentally observed gallium hydride is most likely formed through direct H abstraction from various propyl intermediates by Ga sites, not through H migration from adjacent hydroxyl groups. It is also likely that surface hydrogen atoms cannot migrate to their energetically preferred bonding sites before forming H2 or H2O since the energy barriers for the latter processes are relatively low (see below). 3.4.2. H2 Formation Ws H2O Formation. The catalytic cycle is completed via the release of H2 or H2O from adjacent hydroxyl groups or hydride groups on the surface, and if it is H2O formation, then it has to be followed by reoxidization of the catalyst. Energy profiles from various surface hydrogen sources to form H2 or H2O are shown in Figure 8. For the O(2)H-O(3)H pair, the transition state forming H2 could not be located because the distance between O(2) and O(3) is very

Propane Dehydrogenation over Perfect Ga2O3(100) Surface

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Figure 8. H2 formation vs H2O formation through different adjacent pairs: (a) O(2)H-O(2)H pair; (b) GaH-O(2)H pair; (c) O(3)H-GaH pair; and (d) O(2)H-O(3)H pair. All energies are relative to the initial reactants C3H8(g) + Ga2O3(100) surface, and C3H6(g) is included when the relative energies are calculated.

long and the two transition states located are both for forming surface water (one for H2O(2) and one for H2O(3)). For the O(3)H-O(3)H pair, no transition states for forming either H2 or H2O could be located. H2 formation from GaH-GaH pair could not be located either, probably due to the very long distance between Ga sites. For all other three pairs, O(2)H-O(2)H (Figure 8a), O(2)H-GaH (Figure 8b), and O(3)H-GaH (Figure 8c), forming H2 always has lower energy barrier than forming H2O. Forming H2 from the O(3)H-GaH pair has the lowest energy barrier of just 9.5 kcal/mol, followed by forming H2 from the O(2)H-GaH pair (14.7 kcal/mol). On the other hand, the lowest energy barrier for the formation of surface water from two adjacent hydrogen atoms is 20.3 kcal/mol (TS27). As discussed above (see Figure 5), for the formation of H2O there is a low-energy pathway through H abstraction from physically adsorbed propyl by surface O(2)H groups. Through this pathway, forming surface H2O needs only an overall energy of 43.1 kcal/mol. However, the adsorbed H2O is strongly bonded to the surface by two O-Ga bonds and our accurate calculation indicates that its desorption energy is more than 13 kcal/mol. On the other hand, the desorption energy of H2 is quite small, less than 1 kcal/mol (see Figure 6b). Thus, the overall energy barrier for the formation of H2 is about 44 kcal/mol since processes forming GaH are rate-determining steps (see also Figure 6). We can conclude that formation of gaseous H2 is

easier than the formation of gaseous H2O. However, the formation of surface H2O is easier than the formation of surface H2. At high reaction temperatures, H2O can desorb from the surface, creating oxygen vacancies and leading to the reduction of the surface. If no oxidant is used to reoxidize the surface, this process will cause the decrease of the activity of the Ga2O3 catalyst. This is in agreement with experimental results that in the presence of CO2 the activity of the Ga2O3 decreases much slower at the initial stage of the reaction.14,15,78 The finding could also explain the experimental result that the amount of CO with the treatment by H2 is much lower than with C3H8 in the reoxidation of gallium oxide by CO2 after treatment with H2 and C3H8.17 4. General Discussion We now have a whole picture on the dehydrogenation of propane. The low-energy pathways are presented in Figure 9. 4.1. Role of Active Sites. From Figure 9 and the discussions above, it can be seen that the activation of the first C-H bond of propane is mainly through the direct H abstraction by the O(2) sites via a radical mechanism but not by gallium sites as proposed by some experimental studies.14,15,41 C-H activation by Ga-O acid-base pair via a concerted mechanism does have a low energy barrier comparable to that of C-H activation by

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Figure 9. Energy profiles of the low-energy pathways for the dehydrogenation of propane.

the WdO acid-base pair.51,73 However, this energy barrier is significantly higher than that of the direct H abstraction by the O(2) site. Experimentally observed gallium-carbon bonds42 are thus formed from the bonding of the physically adsorbed propyl radicals to the gallium sites and also from direct C-H bond activation by Ga-O via a concerted mechanism at elevated temperatures. H abstraction from propane by O(2) leads to the formation of surface O(2)H groups and physically adsorbed propyl radical. Physically adsorbed propyl radical can form chemical bond with O(2) or Ga site to form C3H7O(2) or C3H7Ga intermediate. Subsequently, O(2) site further abstracts a second hydrogen atom from the propyl radical, C3H7O(2), or C3H7Ga intermediate, and propene is then formed. These processes have relatively low energy barriers and the lowest-energy pathway is through H abstraction from propyl radical by O(2) site with the formation of propene and O(2)H group. Figure 2 also indicates that the formation of surface O(3)H groups via H abstraction from physically adsorbed propyl radical also has relatively low energy barrier. Thus, at the initial stage of the reaction, surface hydroxyl groups are easily formed until most O(2) and O(3) sites become hydroxyl groups. However, the formed hydroxyl groups are very stable. From them, the formation of H2 needs to overcome very high energy barriers while the formation of surface H2O either needs to overcome very high energy barriers or is an endothermic process with high reaction energy. We would expect that at the initial stage of reaction, formation of propene is very quick and the catalyst exhibits high activity. As the surface oxygen sites are consumed and Ga2O3 is partially reduced, the activity would decrease quickly. In addition, propene formed from these pathways is hard to desorb from the surface and may be further dehydrogenated, leading to fast deactivation of the catalyst. This conclusion is in good agreement with experimental studies.17 Since it is hard for the surface hydrogen atoms to leave the surface in the form of water, they must leave mainly in the form of H2. Formation of H2 requires the formation of GaH species because the energy barrier to form H2 from GaH-O(2)H pair or GaH-O(3)H is lower than the barriers of other pathways. GaH is formed through H abstraction from physically adsorbed propyl radical or chemically adsorbed propyl (C3H7O(2) or C3H7Ga) by Ga sites. Thus, as reaction goes on, Ga sites become

the active sites. With the formation of H2 from GaH and surface hydroxyl groups, Ga, O(2), and O(3) sites are released and the catalytic cycle is completed. 4.2. Rate-Determining Step. The above discussions indicate that there is a change of active sites from O(2) sites at the initial stage of the reaction to Ga sites at the steady stage of the reaction. Consequently, there is also a change of rate-determining step. At the initial stage of the reaction, the rate-determining step is the H abstraction by O(2) site. At the steady stage of the reaction, the rate-determining step is the H abstraction from physically or chemically adsorbed propyl intermediates to form GaH species. The energy barrier for the lowest-energy pathway, i.e., H abstraction from propyl radical by Ga site, is 19.6 kcal/ mol. For the dehydrogenation of ethane on the galliumexchanged zeolite modeled by cluster models, Frash and van Santen have calculated the rate constants of the three main reaction steps using transition state theory and found that there is also a change of rate-determining step from low temperatures to high temperatures.47 At low temperature (500 K), the third step, ethane formation with H abstraction by Ga site, is slowest due to its high activation energy. However, the first step, ethyl formation with H abstraction by O site, appears to be the slowest at higher temperatures between 700 and 900 K, due to its low pre-exponential factor, which is a result of a large entropy loss in the surface activated complex with respect to the gas-phase ethane molecule. However, our first step belongs to a radical mechanism, not a concerted mechanism. The transition state of a radical mechanism should have more freedoms than that of a concerted mechanism. In addition, the reactant of our first step is the physically adsorbed propane molecule, not the gas-phase propane molecule. Thus, there is little entropy loss from the physically adsorbed propane to the surface activated complex. Therefore, the pre-exponential factor of the rate constant of our first step should not be so low that it can lead to a change of rate-determining step from low temperatures to high temperatures. The change of rate-determining step in the present study is a result of the change of reaction mechanism from low temperatures to high temperatures. 4.3. DDH or ODH? The calculation results indicate that formation of gaseous H2 is easier than the formation of gaseous H2O. However, formation of surface water is slightly easier than

Propane Dehydrogenation over Perfect Ga2O3(100) Surface the formation of surface H2, and the formation of hydroxyl groups is much easier with very low energy barriers. In addition, formation of gaseous H2O cannot be ruled out at high temperatures since the reaction energy to form gaseous H2O is not significantly higher than the energy barrier to form gaseous H2. Formation of hydroxyl groups, surface water, or gaseous water all lead to the reduction of the oxide and decrease of the catalytic activity. Thus, if no oxidant is introduced in the reaction gas mixture, the dehydrogenation of propane on the Ga2O3(100) surface would follow an ODH mechanism at the initial stage of the reaction while completely a DDH mechanism at the steady stage of the reaction. If oxidant such as CO2 is added, the dehydrogenation mainly follows a DDH mechanism. The adding of CO2 has compound multifold effects in the dehydrogenation of propane: (a) to reoxidize the surface; (b) to remove H2 through a reverse water-gas-shift reaction; and (c) to remove surface aggregated carbon via the Boudouard reaction.11,12,14,17,19,30 At high temperatures H2O can also desorb from the surface. This process can be suppressed by increasing the pressure of gaseous H2O. Experimentally, it was found that for the dehydrogenation of ethane the activity of the catalyst decreases slower if steam is introduced into the reaction system while the catalyst performs best if both steam and CO2 are introduced.19 Nakagawa et al. suggested that H2O can react with carbon deposition to improve the stability of the catalyst.19,79 Our calculation indicates that H2O may have compound promoting effects on the dehydrogenation of light alkanes. 5. Conclusions To recap, we have presented a comprehensive study of propane dehydrogenation over various sites on perfect Ga2O3 (100) surface using periodic DFT methods. Our results are as follows: (i) Two mechanisms for the initial C-H bond activation of propane, a concerted mechanism and a stepwise radical mechanism, have been explored. The results show that the radical mechanism is energetically preferred. According to our calculation, at the initial stage of reaction the O(2) sites on the surface are the main active sites to activate the first C-H bond of propane due to rather low energy barrier (18.1 kcal/mol) of this step. (ii) Propene can be formed by two distinct pathways. One pathway is H abstraction from physically adsorbed propyl radical, and the other one is H abstraction from propoxide or propylgallium intermediates. For both pathways, propene formation on O(2) sites has lower energy barrier. However, surface hydroxyl groups formed are rather stable since formation of either H2 or H2O from them is difficult. In addition, propene adsorbed on these hydroxyl groups is hard to desorb and is prone to further dehydrogenation or oligomerization. Thus, once surface oxygen sites are covered by hydrogen and the surface is partially reduced, the activity of the catalyst decreases fast. (iii) Gallium sites are less active than oxygen sites. However, once surface oxygen sites are consumed, H abstraction from various propyl intermediates by Ga sites becomes the main pathway to form propene. This is because, although H abstraction by Ga has to overcome high energy barriers, the formation of H2 from GaH and O(2)H or O(3)H hydroxyl group is much easier than the formation of H2 or H2O from GaH + hydroxyl group or from hydroxyl groups. Therefore, there is a change of rate-determining step from H abstraction by oxygen sites at the initial stage of reaction to H abstraction by Ga sites to form GaH species. (iv) Dehydrogenation of propane mainly follows a direct dehydrogenation (DDH) mechanism. Since at high temperatures

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20391 it is also possible to form gaseous H2O, creating oxygen vacancies on the Ga2O3(100) surface and the reduction of the catalyst, the activity of the catalyst would decrease without adding oxidant. Our results show that adding mild oxidant CO2 and steam into the reaction atmosphere has compound promoting effects on the performance of the Ga2O3 catalyst such as suppression of the equilibrium to form gaseous H2O, removal of carbon deposition, reverse water-gas-shift reaction to remove H2, and reoxidization of surface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20433020, 20633030, 20673024, 20828003), the National High Technology Research & Development Program of China (No. 2006AA03Z336), and National High Performance Computing Center of China (No. 00510). We are grateful to the Shanghai supercomputer center and supercomputer center of the Fudan University for their allocation of computer time. Supporting Information Available: Potential energy profiles for propene formation from propyl radical, propoxide, or propylgallium with the first H atom abstracted from propane being bonded to O(3) site. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Vansanten, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637–660. (2) Valenzuela, R. X.; Bueno, G.; Corberan, V. C.; Xu, Y. D.; Chen, C. L. Catal. Today 2000, 61, 43–48. (3) Mimura, N.; Takahara, I.; Inaba, M.; Okamoto, M.; Murata, K. Catal. Commun. 2002, 3, 257–262. (4) Wang, S. B.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Catal. Lett. 1999, 63, 59–64. (5) Wang, S. B.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Appl. Catal., A 2000, 196, 1–8. (6) Wang, S. B.; Zhu, Z. H. Energy Fuels 2004, 18, 1126–1139. (7) Shimizu, K.; Satsuma, A.; Hattori, T. Appl. Catal., B 1998, 16, 319–326. (8) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Nishitani-Gamo, M.; Ando, T.; Suzuki, T. Catal. Today 2003, 84, 149–157. (9) Michorczyk, P.; Ogonowski, J. React. Kinet. Catal. Lett. 2007, 92, 61–68. (10) Kocon, M.; Michorczyk, P.; Ogonowski, J. Catal. Lett. 2005, 101, 53–57. (11) Michorczyk, P.; Gora-Marek, K.; Ogonowski, J. Catal. Lett. 2006, 109, 195–198. (12) Li, H. Y.; Yue, Y. H.; Miao, C. K.; Xie, Z. K.; Hua, W. M.; Gao, Z. Catal. Commun. 2007, 8, 1317–1322. (13) Nakagawa, K.; Okamura, M.; Ikenaga, N.; Suzuki, T.; Kobayashi, T. Chem. Commun. 1998, 1025–1026. (14) Zheng, B.; Hua, W. M.; Yue, Y. H.; Gao, Z. J. Catal. 2005, 232, 143–151. (15) Xu, B. J.; Zheng, B.; Hua, W. M.; Yue, Y. H.; Gao, Z. J. Catal. 2006, 239, 470–477. (16) Xu, B. J.; Li, T.; Zheng, B.; Hua, W. M.; Yue, Y. H.; Gao, Z. Catal. Lett. 2007, 119, 283–288. (17) Michorczyk, P.; Ogonowski, J. Appl. Catal., A 2003, 251, 425– 433. (18) Davies, T.; Taylor, S. H. J. Mol. Catal. A: Chem. 2004, 220, 77– 84. (19) Nakagawa, K.; Kajita, C.; Okumura, K.; Ikenaga, N.; NishitaniGamo, M.; Ando, T.; Kobayashi, T.; Suzuki, T. J. Catal. 2001, 203, 87– 93. (20) Saito, M.; Watanabe, S.; Takahara, I.; Inaba, M.; Murata, K. Catal. Lett. 2003, 89, 213–217. (21) Davies, T.; Taylor, S. H. Catal. Lett. 2004, 93, 151–154. (22) Meitzner, G. D.; Iglesia, E.; Baumgartner, J. E.; Huang, E. S. J. Catal. 1993, 140, 209–225. (23) Takahara, I.; Saito, M.; Inaba, M.; Murata, K. Catal. Lett. 2004, 96, 29–32. (24) Guisnet, M.; Lukyanov, D. Stud. Surf. Sci. Catal. 1994, 90, 367– 378. (25) Carli, R.; Mao, R. L.; Bianchi, C. L.; Ragaini, V. Catal. Lett. 1993, 21, 265–274.

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