20674
J. Phys. Chem. C 2009, 113, 20674–20682
Effect of Pt Clusters on Methanol Adsorption and Dissociation over Perfect and Defective Anatase TiO2(101) Surface You Han,†,‡ Chang-jun Liu,‡ and Qingfeng Ge*,† Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901, Key Laboratory of Green Chemical Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 30072, China ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: October 7, 2009
Methanol adsorption and dissociation on the perfect and defective anatase TiO2(101) surfaces with and without Pt clusters have been studied using density functional theory periodic calculations. On the clean perfect anatase TiO2(101) surface, the energies of molecularly and dissociatively adsorbed methanol are almost equal. The dissociation via O-H scission with an activation barrier of 0.51 eV is the most favorable route among three possibilities: breaking the O-H, C-O, or C-H bond. In contrast, the activation barrier for C-O bond cleavage is as high as 2.56 eV. In the presence of a Pt cluster on the perfect surface, both molecular adsorption and dissociative adsorption via C-O bond scission were enhanced. At low temperatures, methanol prefers to adsorb molecularly at the interface of Pt/TiO2. As the temperature increases, methanol begins to dissociate. The immediate products after breaking the C-O bond are more stable than those formed from breaking either the O-H or C-H bond. On the clean defective surface, methanol prefers dissociative adsorption at the oxygen vacancy site, with its oxygen atom occupying the vacancy site and proton on the two-coordinated oxygen site. Once the oxygen vacancy site is occupied by a Pt particle, it will not be available for methanol dissociation. Consequently, dissociation of methanol will be forced to take place at less active sites. The results were discussed in the context of catalytic production of hydrogen from methanol over the Pt/TiO2 catalyst. 1. Introduction Methanol is widely used as a feedstock to produce many industrial chemicals. Methanol has also been proposed as a source of hydrogen due to its relatively high hydrogen content, making it attractive to a future hydrogen economy.1,2 Hydrogen can be produced from methanol through a catalytic dehydrogenation process, in particular, a photocatalytic process over the titania-supported metal catalysts.3-12 A series of studies on the photocatalytic activities of Pt/TiO2 for methanol dissociation has been reported by Yamakata and co-workers.13-15 These studies showed that Pt played important roles in the methanol dissociation reaction. The importance of Pt was also demonstrated recently by Chen et al.,6 who studied methanol adsorption and dissociation on Pt/TiO2 using the Fourier transform and time-resolved IR spectroscopic methods. These experimental studies demonstrated that Pt is essential for the photocatalytic activity of Pt/TiO2 exhibited in methanol decomposition. However, it is not yet clear how the supported Pt modifies methanol interaction with the TiO2 surface and further influences the methanol dehydrogenation process. In the present paper, we use the density functional theory method to characterize the effect of Pt clusters on the energetics of methanol adsorption and dissociation over the perfect and defective anatase TiO2(101) surfaces. The present study is built on our previous detailed characterization of Pt clusters supported on the perfect and defective anatase TiO2(101) surfaces.16,17 Despite the immense interest in photocatalytic dehydrogenation of methanol, there have been no direct theoretical studies * Corresponding author. E-mail:
[email protected]. † Southern Illinois University. ‡ Tianjin University.
of the heterogeneous photocatalytic process, mainly due to the electronically excited states involved in the process. Previous theoretical studies have been focused on methanol adsorption and dissociation over the surface of metal oxides, including TiO2,18-22 SnO2,23 MgO,24,25 SrTiO3,26 R-Al2O3,27 R-Cr2O3,28 and β-Ga2O3,29 as well as on the surface of metals, including Pt(111),30-34 Cu(111),35,36 and Pd(111).37 Selloni and co-workers examined methanol chemistry on anatase TiO2(101) using the first-principles total energy calculations and Car-Parrinello molecular dynamics simulations.18 They reported that the adsorption energy of molecularly adsorbed methanol was higher than the dissociatively adsorbed species on the perfect surface. In contrast, the dissociatively adsorbed species became energetically favorable on the defective surface. These authors also investigated methanol adsorption on the anatase TiO2(001)-1 × 1 surface and found that dissociative adsorption was favored on the perfect surface at both low and high methanol coverages.20 Bates et al.19 studied methanol adsorption on the perfect rutile TiO2(110) surface using first-principles static and dynamic calculations. Their results showed that the adsorption energies of molecularly and dissociatively adsorbed methanol at both full and half monolayer coverage have only a small difference. Oviedo et al.22 reported that the dissociative adsorption via O-H bond breaking is more favorable, by 0.5 eV, than the molecular one for methanol adsorption on the defective rutile TiO2(110) surface. Breaking the O-H bond was reported to be energetically favorable over the C-O scission on both perfect anatase TiO2(101)18 and rutile TiO2(110)19 surfaces. On the other hand, C-O bond scission becomes favorable over breaking the O-H bond on perfect SnO2(110).23 The metal may also impact on the preference of which bond is to be broken. For example, cleavage of the C-H bond is thermodynamically favored over
10.1021/jp907399j CCC: $40.75 2009 American Chemical Society Published on Web 11/04/2009
Effect of Pt Clusters on Methanol the O-H scission on Pt(111),30,32 whereas breaking the O-H bond becomes energetically favorable over C-O scission on Pd(111).37 These studies suggest that methanol dissociation may be selectively controlled by tuning either the oxide or the metal component of a catalyst. Our present paper aims at understanding the effect of Pt clusters on methanol adsorption and dissociation over the perfect and defective anatase TiO2(101) surface. As a reference, the methanol adsorption and dissociation on the clean anatase TiO2(101) surface was also studied. 2. Methodology The Vienna ab initio simulation program (VASP)38 with ultrasoft pseudopotentials39 and PW91 functional40 were employed in our calculations. To avoid spurious interactions between the adsorbed methanol molecule and the bottom side of the next periodic image, the vacuum space was increased to ∼17 Å in the present calculations. All other parameters, including cutoff energy, lateral dimension of the surface unit cell, and the number of TiO2 layers, were kept the same as those used in our previous studies.16,17 Spin-polarization was included in the relaxation of all the structures presented here. Again, atoms in the lower half of the TiO2 slab were fixed at their bulk positions, and those in the top half of the slab together with the adsorbed Ptn clusters and atoms of adsorbed methanol were allowed to relax. Our test calculations and previous studies showed that Γ point sampling of the surface Brillouin zone is sufficient for the systems studied here.16,17 Adsorption energies, Ead, of methanol on the clean TiO2 surface and the Pt/TiO2 system were defined as
J. Phys. Chem. C, Vol. 113, No. 48, 2009 20675
Figure 1. Structure of molecularly adsorbed methanol on the clean perfect anatase TiO2(101) surface. Possible hydrogen-bonding interactions are labeled by green lines. The Ti atoms are in purple, the O atoms are in red, the C atom is in gray, and the H atoms are in white. Both side (left) and top (right) views are provided.
Ead ) - (Emethanol/TiO2 - ETiO2 - Emethanol(g)) and
Ead ) - (Emethanol-Ptn/TiO2 - EPtn/TiO2 - Emethanol(g)) respectively, with Emethanol/TiO2, Emethanol-Ptn/TiO2, ETiO2, EPtn/TiO2, and Emethanol(g) being the total energies of methanol on the TiO2 slab, methanol on Ptn/TiO2, the clean TiO2 slab, Ptn/TiO2, and a methanol molecule in gas phase, respectively. Transition states for the initial bond-breaking step of methanol dissociation were determined in two steps: first, the nudged elastic band method41-43 as implemented in the VASP code was used to locate the likely transition state; second, the likely transition state was relaxed using the quasi-Newton method until maximum force acting on the movable atoms was
