Adsorption of H2O on the V2O5(010) Surface Studied by Periodic

Adsorption of H2O on the V2O5(010) Surface Studied by Periodic Density ... (O2)6,8 or the tricoordinated (O3)13 oxygen to be an active site, also exis...
0 downloads 0 Views 89KB Size
Download PDF
3218

J. Phys. Chem. B 1999, 103, 3218-3224

Adsorption of H2O on the V2O5(010) Surface Studied by Periodic Density Functional Calculations Xilin Yin, Adil Fahmi,† Huanmei Han, Akira Endou, S. Salai Cheettu Ammal, Momoji Kubo, Kazuo Teraishi, and Akira Miyamoto* Department of Materials Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aoba-yama 07, Sendai 980-8579, Japan ReceiVed: August 11, 1998; In Final Form: December 2, 1998

We have investigated the molecular and dissociative adsorption of a water molecule on the V2O5(010) surface by means of periodic boundary models and density functional approach. It has been observed that molecular adsorption of water on the V2O5(010) surface occurs favorably, whereas the dissociation hardly occurs on the stoichiometric surface due to the significant Coulombic repulsion of the lattice oxygens around the exposed vanadium center to the approaching oxygen of the hydroxyl species. For molecular adsorption at the surface oxygen sites, it has been confirmed that hydrogen bonding plays a crucial role, and the adsorption abilities of the surface oxygens correlate with the electron-donating ability from the surface oxygen sites to the water molecules, and with the ratio of the accumulated charge on the adsorption site and the adsorbed water species. As for molecularly adsorbed water species at the exposed vanadium site, the coordination interaction and hydrogen bonding are the important contributions. For both the molecular and dissociative adsorption, it has been elucidated that the vanadyl oxygen plays the most important role among the three surface oxygens and acts as the most favorable adsorption site.

1. Introduction Vanadium pentoxide (V2O5) is an important active compound in heterogeneous catalysis, such as the selective oxidation of hydrocarbons1 and the selective catalytic reduction of nitrogen oxides by ammonia.2 The discovery of its excellent heterogeneous catalytic properties stimulated the numerous studies on the interaction of its surfaces with various adsorbates.3-8 It has been known for many years that the interaction of H2O with metal oxide surfaces has important consequences for their catalytic behaviors.9 Taking into account that catalysts are exposed under the ambient conditions, and generally water exists in both reactants and products, interaction of water with metal oxide surfaces has significant influence on the catalytic behaviors. On the contrary, to the numerous investigations on the adsorption states of water on titanium dioxide,10,11 very few studies concerning the adsorption states of water on V2O5 have been undertaken, although V2O5 is often supported upon TiO2 in catalysts. On the other hand, since it is believed that the catalytic properties of V2O5-based catalyst depend strongly on their ability to provide lattice oxygen as a reactant, investigation of the active site on the V2O5 surfaces is another interesting topic in both experimental and theoretical sides. So far, the majority of the studies have shown that the singly coordinated oxygen (O1), which is the vanadyl oxygen (VdO species), acts as an active site;3,4,12 however, different proposals, concerning either the dicoordinated (O2)6,8 or the tricoordinated (O3)13 oxygen to be an active site, also exist. Busca et al.5 have investigated the surface properties of unsupported V2O5 using FT-IR spectroscopy. The XRD spectrum of the V2O5 sample shows good crystallinity and the absence of the reduced phases. The spectrum of a pressed disk † Present address: Condensed Matter Group, International School for Advanced Studies (SISSA), via Beirut 2/4, I-34114, Trieste, Italy.

of V2O5 evacuated at room temperature contains bands at 1620 cm-1 due to the adsorbed water molecule and at 3645 cm-1, probably due to ν(OH) of surface V-OH groups. Thus, the presence of adsorbed molecular water at ambient condition has been observed. However, a coherent picture with respect to the orientation of the molecularly adsorbed water is missing. By means of atomic force microscopy, Da Costa et al.6 have not observed the dissociation of water molecule on V2O5(010) surface in ambient conditions. Instead, they have found the two rows on the surface that are considered to be rows of vanadyl oxygens and a row between the two rows of vanadyl oxygens that is thought to be the row of molecularly adsorbed water. Further, they have pointed out that the O2 seems to play an important role as an active site for water adsorption based mainly on the following assumptions: (1) a water molecule bridges either two O2 sites or two O3 sites through its hydrogen atoms, (2) O2 is more negatively charged than O3, and (3) the distance of ca. 3.5 Å between two O2 sites matches the required distance of ca. 3 Å for the water adsorption between two oxygen sites, whereas it is 4.3 Å for the other sites. In fact, the distance between either the two O1 sites or the two O3 sites is also on the order of ca. 3.5 Å on the (010) surface, and even shorter distance of about 3.3 Å between the two O1 sites exists. Moreover, the proposal concerning that the O2 is more negatively charged than the O3 is contrary to the chemical intuition and the theoretical studies.4,12,14 Therefore, the conclusion inferred by them seems not so reliable. In addition, Moshfegh and Ignatiev7 have reported that water adsorbs dissociatively on the V2O5 surface through the action of two neighboring vanadium sites of -V5+-O-V4+- by means of the temperature-programmed desorption technique, and the activation energy is measured to be ca. 11.7 kcal/mol for desorption of H2O from the V2O5 surface. On the theoretical side, Witko et al.8 have reported that molecular adsorption of

10.1021/jp9833395 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999

Adsorption of H2O on the V2O5(010) Surface water takes place on the O2 site of the (010) surface rather than the vanadyl oxygen by means of the cluster approach. So far, adsorption states of the water molecule have been hardly elucidated completely. However, it seems on balance that the dissociation is promoted by particular structural features, such as the presence of defects on the surface.15 It has been known that the presence of defect lattice sites and “nonlattice” oxygen atoms at oxide surfaces appears to play a key role in dissociation and hydroxylation of H2O over these surfaces.7 Although it has been revealed that the dissociation of H2O occurs on the defect and/or reduced V2O5 surface rather than the defectfree and/or oxidized surface,7,15 as for the theoretical reasons regarding the nondissociation of water on the stoichiometric V2O5 surface, as far as we know, no reports have been found. Overall, the experimental studies show that the V2O5(010) surface exhibits a crucial role in catalysis16,17 and that the supported (010) surface monolayer has the excellent catalytic activity.18,19 In the present study, we have performed the periodic boundary density functional calculations and investigated the molecular and dissociative adsorption of a water molecule on the V2O5(010) surface monolayer, to elucidate the adsorption states and corresponding origin, as well as to extend our knowledge in understanding the active site present in the V2O5 surface and the atomistic mechanism behind the reactions that occur on the surface of V2O5. 2. Methodology 2.1. Computational Details. The periodic first-principles density functional calculations have been performed by using CASTEP and DSolid (from MSI) programs. For the investigation of adsorption states with respect to transition-metal-oxide catalysts, our earlier study has confirmed that this methodology is reliable, and it gets rid of the restrictions of cluster approach.4 The geometries for all the adsorption systems are optimized by CASTEP, which uses a conjugated gradient technique in a direct minimization of the Kohn-Sham energy functional20 and employs pseudopotentials to represent core electrons. Basis sets used are plane-wave functions. In this approach HellmannFeynman forces on ions can be easily evaluated, and therefore geometry optimizations can be performed to get stable structures. Exchange and correlation effects are included within both the Ceperley-Alder local density approximation (LDA)21 and the generalized gradient approximation (GGA) of Perdew and Wang.22 Gradient-corrected energies are computed selfconsistently. We use a plane-wave cutoff energy of 650 eV, for which vanadium and oxygen pseudopotentials are well converged. For hydrogen, we have used its pure potential, as many researchers from their experience found that using a pseudopotential for H does not necessarily have real advantage.23 DSolid within the Kohn-Sham formalism,24 has been used to perform Mulliken population analysis. In this program, oneelectron Schro¨dinger equations are solved only at the k ) 0 wave vector point of the Brillouin zone. The Vosko-WilkNusair local type functional (LDA)25 is employed since the atomic charges obtained by this methodology are very close to the earlier reports,4,12 and the order of their magnitude is also the same. Molecular orbitals are expanded into a set of numerical atomic orbitals by double-numerical basis functions together with polarization functions (DNP) provided by the software package. The DNP is comparable in quality to Pople’s splitvalence 6-31G** basis set and usually yields the most reliable results.26-28 To eliminate convergence problems in some cases, we allowed partial occupancies for the high-lying orbitals using smear values of less than 0.02 eV. To reduce the necessary CPU time, the core orbitals of the atoms are frozen.

J. Phys. Chem. B, Vol. 103, No. 16, 1999 3219

Figure 1. The periodic boundary model of the V2O5(010) surface32 used in the present calculations indicated by broken lines, projection onto (a) the (a, c) plane and (b) the (a, b) plane. The closed small circles represent the exposed V atoms on the surface, and the opened small ones designate the V atoms beneath the singly coordinated oxygens.

Our approach is as follows: The CASTEP program was used to optimize the geometries of both the stoichiometric (010) surface and adsorption systems at the GGA level, and then the electronic structure calculations with respect to the equilibrium geometries obtained by CASTEP were performed using the DSolid program at LDA. The adsorption energy (Eads) has been calculated according to the expression

Eads ) E(adsorbate+substrate) - (Eadsorbate + Esubstrate) where E(adsorbate+substrate) is the total energy of the adsorbate/ substrate system, Eadsorbate is the total energy of the isolated adsorbate at its equilibrium geometry, and Esubstrate is the total energy of the substrate. A negative Eads value corresponds to a stable adsorbate/substrate system. 2.2. Model. The V2O5(010) face is the thermodynamically most stable single-crystal surface. Experimental29,30 and theoretical12,31 studies demonstrate that the (010) surface has very similar physical properties and stability with its bulk crystal. Therefore we considered the V2O5(010) surface for our present study and the surface has been modeled by the periodic slab composed of monolayer, as shown in Figure 1. The successive slabs along the b direction are separated by a vacuum region. The shortest distance between atoms belonging to successive slabs is greater than 8.3 Å, and the interlayer interaction is not significant at such a distance. Therefore it can be considered as a realistic model of V2O5(010) surface. For the dissociation systems, we had tested various models in which the H dissociated from H2O goes to the lattice oxygen sites (for example, O1 site) closer to the exposed vanadium. However, the obtained equilibrium geometry shows that the water molecule is formed from the dissociated H and OH species. The same happened also for O2 and O3 sites. Instead, we have used the models as shown in Figures 4a-c, which may prevent the dissociated H and OH species from forming H2O. For all the molecular and dissociative adsorption systems, both the adsorbates and corresponding adsorption sites are fully relaxed.

3220 J. Phys. Chem. B, Vol. 103, No. 16, 1999

Yin et al.

TABLE 1: Bond Lengths (dHads-O, dO-H) and Bond Angles (∠H-O-H) of the Adsorbed and Free Water Molecule, as Well as the Distances (dsite-H2O) between the Water Molecule and Corresponding Adsorption Sites with Respect to the Molecular Adsorption Systems adsorption sites dHads-O (Å) dO-H (Å) dsite-H2O (Å) ∠H-O-H (deg)

V

O1

O2

O3

0.974 0.974 2.921 104.4

0.976 0.974 2.015 102.2

0.975 0.974 2.068 103.0

0.975 0.974 2.089 103.7

H2O molecule 0.971 (0.957)32 104.4 (104.5)32

3. Results and Discussion 3.1. H2O Molecule. To check the reliability of our method, we first calculated the equilibrium geometry and electronic structure of the free water molecule. Since our techniques use periodic boundary conditions, the calculations are actually performed on periodic arrays of molecules, with the repeating cell chosen to be a cube of side R. We found that the choice R ) 8 Å is large enough to render the interactions between periodic images negligible. Comparison of the calculated and experimental32 structural parameters of H2O, as shown in Table 1, confirms that the molecule is described accurately by the present method. The charges on the H and O atoms of free molecule are +0.280 and -0.560, respectively. For the dimer, a cube with side R ) 10 Å has been used. The calculated stabilization energy is -8.03 kcal/mol, resulting from the hydrogen bonding, which is very close to the literature value of -7.28 kcal/mol.10 The length of the hydrogen bond O-H‚‚‚‚O formed between the two water molecules is 1.938 Å, and the bond angle ∠O-H‚‚‚‚O is 167.7°. The lone pair electrons of the oxygen of one water molecule were donated to the O-H antibonding orbital of another water molecule, and consequently, changes in the geometrical parameters of both molecules are observed. The amount of the net donated or accepted electrons is found to be 0.043 per water molecule. 3.2. Stoichiometric (010) Surface. In our earlier work,12 we have optimized the geometry of the (010) surface containing only monolayer. The obtained a and c lattice parameters of the surface (a ) 11.535 Å, c ) 3.603 Å) remain almost the same to the bulk values.33 However, V-O bonds show an elongation ranging from 0.01 to 0.04 Å relative to the bulk, whereas the vanadyl bond shows an abstraction less than 0.01 Å. Among the three kinds of surface oxygens, the singly coordinated oxygen shows the greatest vertical expansion of 0.13 Å, whereas the O2 and O3 sites relax by 0.08 and 0.05 Å, respectively. Projected DOS (density of states) analysis12 shows that the vanadium atom on the surface seems to be more active than that on the bulk, and the activity order regarding the three kinds of surface oxygens can be expected as O1 > O2 > O3. In the present study, the calculated atomic charges of the stoichiometric (010) surface are shown in Table 2. 3.3. Molecular Adsorption. The water molecule has a large dipole moment and lone-pair electrons on oxygen and thus is a good electron donor, and the hydrogens of the water molecule can interact with the surface oxygens by hydrogen bonding. The adsorption on the V2O5 surface may be considered as a Lewis acid-base reaction, as a consequence, the atoms H (Lewis acid) and O (Lewis base) of a water molecule are able to interact with the Lewis base sites (surface oxygens) and Lewis acid sites (surface vanadiums), respectively. 3.3.1. At the Exposed Pentacoordinated V Site. Vanadium centers can be considered as Lewis acid sites that can interact with atoms having free electron pairs through the formation of coordination bonds, hence molecular adsorption of water takes

Figure 2. Equilibrium interatomic distances (Å) and hydrogen bond angles (deg) of molecularly adsorbed water at (a) exposed V, (b) O1, (c) O2, and (d) O3 sites on the V2O5(010) surface.

TABLE 2: Atomic Charges on Adsorbate (qHads, qH, and qO) and Adsorption Sites (qsite) with Respect to the Molecular Adsorption Systemsa adsorption sites qHads qH qO (H2O)x+ qsite ∆qsite

V

O1

O2

O3

+0.292 +0.289 -0.496 +0.085 +1.240 (+1.261) -0.021

+0.297 +0.330 -0.489 +0.138 -0.456 (-0.316) -0.140

+0.324 +0.323 -0.495 +0.152 -0.621 (-0.527) -0.094

+0.338 +0.319 -0.505 +0.152 -0.747 (-0.681) -0.066

a The values in the brackets are corresponding charges on the clean surface atoms, ∆qsite represents the difference in charge on the adsorption site before and after the adsorption, and (H2O)x+ denotes the increased charge on water molecule due to the adsorption.

place on the exposed 5-fold coordinated vanadium site and leads to the formation of a V-OH2 bond (Figure 2a). Furthermore, hydrogen bonds have also been formed by the interaction of the two hydrogens of H2O molecule with the nearest neighbors of the O1 and O2 atoms of the surface with lengths of 2.542 and 2.832 Å, respectively. In addition, the surrounding surface oxygens of the V site give rise to the Coulombic repulsion to prevent the H2O molecule from approaching due to the recession of the V site into the surface. The adsorption energy is calculated to be -15.5 kcal/mol, indicating a rather strong interaction

Adsorption of H2O on the V2O5(010) Surface

J. Phys. Chem. B, Vol. 103, No. 16, 1999 3221

TABLE 3: Molecular (Eads,mol) and dissociative (Eads,diss) Adsorption Energies of the Water Molecule on the (010) Surface adsorption sites Eads,mol (kcal/mol) Eads,diss (kcal/mol)

V

O1

O2

O3

-15.5

-23.1 +58.8

-12.9 +62.9

-9.69 +66.0

(Table 3). In contrast to the adsorption of H2O on TiO2 surfaces where the preferred Ti-OH2 geometry is planar,10 the V-OH2 geometry is found to be nonplanar. The vanadium-water distance of 2.921 Å is rather long and can be described as an ionic bond, which does not significantly affect the internal geometry of the adsorbate. Indeed, it is confirmed that the geometry of the adsorbed water species did not show any significant change. Another argument in favor of the ionic interaction is that the water molecule is adsorbed in an sp3 configuration, the dihedral angle between the HOH plane and the O-V axis is observed to be 116.5°. Thus, the adsorbed water molecule possesses only one lone pair of electrons and, therefore, it is expected to have an electronic and geometric structure very similar to that of the free ammonia molecule. Actually, we have observed that the geometry of this adsorbed species is consistent with that of ammonia with the bond angles of 104.4°, 105.6°, and 113.7°, respectively. The average bond angle (107.9°) of the adsorbed water species is very close to the angle ∠HNH of 106.7° in ammonia.32 Water molecule donates the lone-pair electrons located on its oxygen to one of the empty d-orbitals of the vanadium site; it can be expected that the positive charge on this vanadium site will decrease accordingly. Indeed, as shown in Table 2, the water molecule did donate 0.085 electrons to the surface. Accordingly, charge on the vanadium site was slightly decreased by 0.021 which is consistent with the former prediction. This indicates that the vanadium site transferred part of the electrons accepted from the water molecule. At the same time, it was observed that the charges on both O1 and O2 atoms, interacting with the water molecule by H-bonds, increased by 0.068 and 0.007. Very similar to the latter descriptions with respect to the molecular adsorption of water at O1, O2, and O3 sites, here also the O1 and O2 atoms donate their lone-pair electrons to the antibonding orbitals of the O-H groups of water, and simultaneously there is a back-donation of electrons from the O-H groups to the O1 and O2 atoms that results in the increase in negative charges on the two atoms. Thus, the surface is observed to be polarized. The increase in coordination number owing to the adsorption of water makes the exposed surface vanadium more bulklike and more ionic, and therefore more stable. Since the interaction contributed by the coordination and hydrogen bonds is much stronger than the Coulombic repulsion as mentioned above, the adsorption of H2O at the V atom, Lewis acid site, makes the coordinately unsaturated surface more stable when compared to the isolated one. 3.3.2. At the O1 Site. As described above, molecular adsorption of H2O also takes place on the Lewis base sites by means of hydrogen bonding (see Figure 2b-d). For the O1 site, the adsorption energy is calculated as -23.1 kcal/mol (Table 3). As shown in Table 1, the H-bond distance of O-H‚‚‚O1 is 2.015 Å and it is nonlinear with a bond angle of 158.8°. Accordingly, the O1 site is relaxed upward from the surface by 0.010 Å, while the bond angle of the adsorbed H2O molecule was decreased by 2.2°. According to the nature of charge transfer observed in the formation of the hydrogen bond in water dimer, here also it is expected that the adsorption site O1 donates its lone-pair

electrons to the O-H antibonding orbital of water to form the H-bond. Surprisingly, charge on the O1 site was negatively increased by 0.140 compared with that of the stoichiometric surface, while the charges on the other surface atoms are scarcely changed. In addition, the adsorbed water molecule donates 0.138 electrons to the surface, which is almost the same with the increased charge on the O1 site (Table 2). This phenomenon clearly indicates that the interaction of the substrate with the water is mainly contributed by the adsorption site O1 rather than the other surface atoms. However, an increase in the negative charge on the O1 site and corresponding increase in the positive charge on the H2O molecule show that the nature of the H-bonding present in this system is different from that of the water dimer. There might be a donation of lone pair of electrons from the O1 site to the water molecule, as in the case of the water dimer, but the net result shows that this donation is rather weak and it is dominated by the back-donation of O-H bonding electrons from the water molecule to the O1 site of the surface. At the same time, the positive charge on the vanadium atom bounded to the O1 site is increased by 0.042 while that of other vanadiums on the surface essentially remain the same compared to the clean surface. This indicates that there is a charge flow from vanadium to oxygen that happens only when there is a donation of electrons from the surface oxygen to the water molecule. All such processes result in the weakening of the O-H and V-O1 bonds and they are elongated by 0.005 and 0.010 Å, respectively. But the donation from the surface oxygen to the water molecule is less compared to the back-donation, as this oxygen is connected to the transition metal atom (V) and has less negative charge. 3.3.3. At the O2 Site. As shown in Figure 2c, molecular adsorption of H2O is also observed at the O2 site by the H-bond interaction, with the adsorption energy of -12.9 kcal/mol (Table 3). The length and angle of the H-bond formed at the O2 site are 2.068 Å and 168.1°, respectively, which are greater than that of the H-bond formed at the O1 site. The O2 site moves slightly downward on the surface by 0.002 Å; also, the two identical V-O bonds have not changed significantly. In contrast to the case of the O1 site, the O2 site accumulates only 0.094 electron, which is much less than the donated electron (0.152) from the adsorbate (Table 2), while the remaining 0.058 electron (38.2%) was transferred to the other surface atoms. The negative charge on the O2 site in clean surface is relatively larger than that of O1 site, and so the accumulation of charge on the O2 site is found to be smaller though the net charge transfer from the H2O molecule is larger compared to that of the O1 site. Similar to the adsorption at the O1 site, the O2 atom also donates its lone-pair electrons to the water molecule and the water molecule donates back the O-H bonding electrons to the surface. In contrast to the O1 case, charges on the two vanadiums bonded to the adsorption site O2 were decreased slightly. This indicates that the vanadium d-orbitals only accepted the electrons from the water molecule rather than transferring the electrons, and so the donation from the O2 site to the water molecule is less compared to the O1 site. 3.3.4. At the O3 Site. The interaction of the water molecule at the O3 site has also been studied and the formation of the hydrogen bond similar to the other two sites is observed, as shown in Figure 2d. The hydrogen bond length and the angle of O-H‚‚‚‚O3 are found to be 2.089 Å and 170.6°, respectively, which are larger than that of O-H‚‚‚‚O1 and O-H‚‚‚‚O2, indicating that this H-bond is weaker than the other two. The adsorption energy is calculated as -9.69 kcal/mol. It is observed that the two O-H bonds of the adsorbed H2O species are

3222 J. Phys. Chem. B, Vol. 103, No. 16, 1999 elongated by 0.004 and 0.003 Å, and the bond angle ∠HOH is larger than that of the O1 and O2 sites by 1.5° and 0.7° respectively. The site O3 is moved downward on the surface by 0.092 Å. Similar to the adsorption at both the O1 and O2 sites, O3 also accumulates 0.066 electron, but it is pronouncedly less than the former two cases. From Table 2, we can see that the O3 site retains only 43.4% of the donated electrons from the adsorbed water molecule. The interaction of the O-H group with the O3 site is therefore weakened and the stability of this adsorption system becomes very low compared to the other two sites. Moreover, charges on the three vanadiums connected to the O3 site are found to be lower than in the case of the O2. This demonstrates that these vanadiums accommodated more electrons donated from the water molecule, indicating that the donation from the O3 site to the water molecule is less compared to the cases of O1 and O2 sites. 3.3.5. Comparison of the Molecular Adsorption Systems. As described above, molecular adsorption of water at three kinds of surface oxygens and exposed surface vanadium are energetically stable, and their adsorption abilities decrease in the order O1 > V > O2 > O3. This implies that the surface vanadyl oxygen is the most active, which is in accordance with the earlier studies.3,4,12 As shown in Figure 2, the H-bond distance between the proton and corresponding adsorption site increases in the order H‚‚‚O1 < H‚‚‚O2 < H‚‚‚O3, indicating that the interaction of the O-H group of water with the adsorption site decreases accordingly. That is, the most active adsorption site of the O1 formed the strongest H-bond, while the least active adsorption site of the O3 produced the weakest one. The implied meaning behind these phenomena is that the hydrogen bonding plays a key role for molecular adsorption of water at the surface oxygens of V2O5. Since the vanadyl oxygen forms the strongest H-bond, it therefore acts as the preferable adsorption site rather than the O2 or O3 site. However, this conclusion is different from the reports of Da Costa et al.6 and Witko et al.8 The main reasons for this disagreement are probably due to the unreasonable assumptions6 as described in section 1 and due to the artifacts of cluster approach as discussed in ref 4. In the adsorption process, the adsorbate of water donated its electrons to the substrate and became positively charged; the substrate became polarized accordingly (Table 2). Such a redistribution of the charge donated to the surface is believed to be a very important factor.34-36 To find out the quantitative correlation between the redistribution of the charge and the adsorption ability, we focus mainly on both the difference in charge on the adsorption site before and after the adsorption (∆qsite), and the increased charge on the water molecule [(H2O)x+] due to the adsorption. It is observed that the adsorption abilities of the surface oxygens with respect to the molecular adsorption correlate with the ratio of ∆qsite/(H2O)x+, as shown in Figure 3. Essentially, the curve demonstrates a linear relationship between the adsorption energies and the ratio of ∆qsite/(H2O)x+, since the correlation coefficient (r) is calculated to be 0.996 by linear regression analysis. To be precise, the higher this ratio, the more favorable the corresponding surface oxygen is. This trend has not been observed for the adsorption of H atoms on the same surface, where the adsorption of H atoms at all the three different surface oxygens is very stable energetically, and the vanadyl oxygen acts as the preferable adsorption site.4 This is probably due to the difference of the bonding properties: the interaction of the molecularly adsorbed water with the surface oxygen is mainly contributed by the hydrogen bonding, whereas the interaction of H atoms with these oxygens is attributed to covalent force.

Yin et al.

Figure 3. Correlation between the adsorption energy (Eads) and the ratio of ∆qsite/(H2O)x+ (×100%). ∆qsite: the difference in charge on the adsorption site before and after the adsorption. (H2O)x+: the increased charge on the water molecule due to the adsorption. Linear regression equation: y ) 0.235x - 0.982. Correlation coefficient: r ) 0.996.

Furthermore, the change in the charges on the vanadium atoms connected to O1, O2, and O3 sites reveals that the donation from the surface oxygen to the water molecule is more in the case of the O1 site than the other two. The net charge transfer is found to increase in the order O1 < O2 ≈ O3 while the stability order decreases in the order O1 > O2 > O3. This reverse trend proves that the stability of the adsorption system is decided mainly by the donating ability of the surface oxygen sites rather than the acceptance of electrons from the water molecule. The bond lengths and bond angles of all the adsorbed water molecules are found to be changed from that of the isolated one. As shown in Table 1, the O-H bond lengths of the adsorbed water species are increased in all the cases. Furthermore, it is confirmed that the H2O molecule adsorbed at the most favorable site O1 possesses the smallest bond angles of ∠HOH and ∠O-H‚‚‚‚O1, while the H2O molecule adsorbed at the least favorable site O3 takes the largest bond angles of ∠HOH and ∠O-H‚‚‚‚O3. This suggests that the bond angles of both the adsorbed water species and the H-bonds correlate with the adsorption abilities of their corresponding adsorption oxygen sites and again demonstrates the important role of hydrogen bonding. 3.4. Dissociative Adsorption. We have also examined the dissociative adsorption of water on the (010) surface. Since molecular adsorption takes place on the surface as discussed above, next, we assume that the adsorbed water molecule dissociates and the proton is transferred to an oxygen anion of the surface, which are of three kinds, and the hydroxyl group attaches to an exposed pentacoordinated vanadium site. For hydroxyl species OH-, which is the most basic ligand, the exposed V atom is the favored site for adsorption. More precisely, totally three types of orientations have been considered, i.e., the protons attack the O1, O2, and O3 sites to form new surface OH species, whereas the hydroxyl species dissociated from the water molecule binds to the exposed V atom in all the cases, as shown in Figure 4. The three types of dissociative adsorption of the H2O molecule have been investigated and all of them are found to be unfavorable energetically. They are endothermic by 58.8, 62.9, and 66.0 kcal/mol for the dissociation at O1, O2, and O3

Adsorption of H2O on the V2O5(010) Surface

J. Phys. Chem. B, Vol. 103, No. 16, 1999 3223 TABLE 5: Atomic Charges on Adsorbate (qHads, qH, and qO) and Adsorption Sites (qOsite, qV, qO1b and qO3b) for the Dissociative Adsorption of Water at (a) O1, (b) O2, and (c) O3 Sites on the V2O5(010) surfacea adsorption sites qHads qH qO qOsite qV qO1b qO3b

O1

O2

O3

+0.314 +0.333 -0.387 -0.484 (-0.316) +1.178 (+1.261) -0.405 (-0.316) -0.628 (-0.681)

+0.387 +0.334 -0.393 -0.588 (-0.527) +1.171 (+1.261) -0.353 (-0.316) -0.618 (-0.681)

+0.450 +0.340 -0.398 -0.685 (-0.681) +1.174 (+1.261) -0.368 (-0.316) -0.640 (-0.681)

a The values in the brackets are corresponding charges on the clean surface atoms. b Notation as in Table 4.

Figure 4. Equilibrium interatomic distances (Å) of dissociatively adsorbed water at (a) O1, (b) O2, and (c) O3 sites on the V2O5(010) surface.

TABLE 4: Equilibrium Geometries of Dissociatively Adsorbed Water at (a) O1, (b) O2, and (c) O3 Sites on the V2O5(010) Surfacea geometry

O1

O2

O3

dH-Osite (Å) dH-OV (Å) dHO-V (Å) dO1-HOb (Å) dO3-HOb (Å) ∠H-O-V (deg)

0.978 0.980 2.398 2.296 2.495 118.3

0.984 0.980 2.417 2.295 2.504 117.8

0.988 0.981 2.454 2.293 2.527 117.2

a Selected bond length of the newly formed H-O species (d H-Osite), interatomic distances (dH-OV and dHO-V) and bond angles (∠H-OV) of the V-OH species, as well as distances between the H atom of the V-OH species and its closest singly and triply coordinated surface oxygen (dO1-HO,b dO3-HOb). b The O1 and O3 shown here represent the closest singly and triply coordinated surface oxygens of the O-H group attached to the exposed V site.

sites, respectively (Table 3). This indicates that the dissociative adsorption of the H2O molecule on the stoichiometric V2O5 surface hardly takes place, which supports the experimental observations.6 The bond lengths of newly formed O-H species in the current study vary from 0.98 to 0.99 Å (see Figure 4 and Table 4). It is observed that the protonation of the three surface oxygens is essentially identical to that of our previous study,4 which reveals that the three surface oxygens show very strong binding abilities to hydrogens, along with the O-H bond lengths ranging from 0.97 to 0.99 Å. This is also justified by the pronounced change of charge on the protons and their adsorption sites (Table 5), which demonstrate again the strong interaction. Thus, the origin regarding the nondissociation of water on the stoichiometric (010) surface must have been attributed to the weak attractive interaction between the OH species and the V site. Indeed, it is revealed that the steric hindrance is the

dominant factor. The V2O5 surface shows the cleaved octahedron structure, where the plane shared by four surface O atoms is higher than their center V atom (see Figures 1 and 4). Precisely, Lewis acid site V is recessed into the plane. The angles between these four oxygens and the axis perpendicular to the oxygens’ plane are 70.5°, 72.5°, 72.5°, and 76.3°, respectively. The adsorbed oxygen of the OH group is located on the axis where an O1 lattice oxygen bound to the center V atom is essentially oriented in the case of bulk. The equilibrium interatomic distances between the approaching O atom of the OH species and the four surface oxygens are identified to range from 2.478 to 2.789 Å for all the dissociation systems. At such orientations, the approaching O atoms bear strong Coulombic repulsion from these surface oxygens. As a result, the OH species are oriented over the vanadium sites at a distance of ca. 2.5 Å in the equilibrium geometries (Table 4). On the other hand, the lonepair electrons of the OH group interact with one of the empty d-orbitals of the V atom; it is also found that the hydrogen of the O-H group interacts with its closest O1 and O3 sites with bond lengths varying from 2.293 to 2.527 Å. However, these attractive forces are so weak that they cannot offset the significant Coulombic repulsion, which is well justified by the nonstability of the dissociation systems. This is also supported by only a minor change in the atomic charges on the OH group and its adsorption site V as well as its closest neighbors, as demonstrated in Table 5. All these facts show that the repulsion between the lattice oxygens around the exposed V atom and the O of the hydroxyl group hinders the dissociation of water on the stoichiometric V2O5 surface. On the other hand, our results revealed that the adsorption ability trend of these three kinds of oxygen sites decreases in the order: O1 > O2 > O3, which indicates that the vanadyl oxygen is the most favorable with respect to the water dissociation despite their instabilities. The order of bond lengths of the O-H and V-OH species also corresponds to the above adsorption ability trend. In addition, studies on the adsorption states of ammonia, oxides of nitrogen, and carbon as well as sulfur on the (010) surface are in progress. 4. Conclusions We have performed the periodic first-principles density functional calculations and investigated the molecular and dissociative adsorption of water on the V2O5(010) surface. Our results show that molecular adsorption of H2O takes place at the exposed pentacoordinated vanadium and the three types of structurally different surface oxygens, which supports the experimental identifications. For molecular adsorption at the exposed vanadium site, coordination interaction and hydrogen bonding are found to play the key role. Regarding molecular

3224 J. Phys. Chem. B, Vol. 103, No. 16, 1999 adsorption at the surface oxygen sites, it has been confirmed that hydrogen bonding plays a crucial role, and the donation from the surface oxygen sites to the water molecule is the dominant contributor; in addition, the adsorption abilities of the surface oxygens correlate linearly with the ratio of the increased charge on the adsorption site and the adsorbed water species. On the other hand, dissociation of the water molecule hardly occurs on the stoichiometric (010) surface, mainly due to the pronounced Coulombic repulsion of the lattice oxygens around the exposed vanadium center to the oxygen of the hydroxyl species, which is also consistent with the experimental observations. For both molecular and dissociative adsorption, it is found that the vanadyl oxygen plays the most important role and acts as the most favorable site, although dissociation of water discussed in the present study is unfavorable energetically. This supports the generally accepted conclusion indicating the highest activity of the vanadyl oxygen. The present study has revealed that the periodic first-principles density functional methodology is reliable to investigate the adsorption states with respect to transition-metal-oxide catalysts. References and Notes (1) Grzybowska-Swierkosz, B. Appl. Catal. A 1997, 157, 263. (2) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (3) Tarama, K.; Yoshida, S.; Ishida, S.; Kakioka, H. Bull. Chem. Soc. Jpn. 1968, 41, 2840. (4) Yin, X.; Han, H.; Endou, A.; Kubo, M.; Teraishi, K.; Chatterjee, A.; Miyamoto, A. J. Phys. Chem. B 1999, 103, 1263. (5) Busca, G.; Ramis, G.; Lorenzolli, V. J. Mol. Catal. 1989, 50, 231. (6) Da Costa, A.; Mathieu, C.; Barbaux, Y.; Poelman, H.; DalmaiVennik, G.; Fiermans, L. Surf. Sci. 1997, 370, 339. (7) Moshfegh, A. Z.; Ignatiev, A. Surf. Sci. Lett. 1992, 275, L650. (8) Witko, M.; Hermann, K.; Tokarz, R. Book of Abstracts of 215th ACS National Meeting; COMP 078, Dallas, 1998. (9) Henrich, V. E. Rep. Prog. Phys. 1985, 48, 1481. (10) Fahmi, A.; Minot, C. Surf. Sci. 1994, 304, 343. (11) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. Surf. Sci. 1994, 302, 329. (12) Yin, X.; Fahmi, A.; Endou, A.; Miura, R.; Gunji, I.; Yamauchi, R.; Kubo, M.; Chatterjee, A.; Miyamoto, A. Appl. Surf. Sci. 1998, 130132, 539.

Yin et al. (13) Ramirez, R.; Casal, B.; Utrera, L.; Ruiz-Hitzky, E. J. Phys. Chem. 1990, 94, 8960. (14) Michalak, A.; Witko, M.; Hermann, K. Surf. Sci. 1997, 375, 385. (15) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994; Chapter 6. (16) Miyamoto, A.; Yamazaki, Y.; Inomata, M.; Murakami, Y. J. Phys. Chem. 1981, 85, 2366. (17) Inomata, M.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1981, 85, 2372. (18) Saleh, R. Y.; Wachs, I. E.; Chan, S. S.; Chersich, C. C. J. Catal. 1986, 98, 102. (19) Bond, G. C. Appl. Catal. A: General 1997, 157, 91. (20) Teter, M. P.; Payne, M. C.; Allan, D. C. Phys. ReV. B 1989, 40, 12255. (21) Perdew, J. P.; Zunger, A. Phys. ReV. B 1981, 23, 5048. (22) Perdew, J. P. In Electronic Structure of Solids ‘91; Ziesche, P., Eschrig, H., Eds.; Akademie Verlag: Berlin, 1991. (23) CASTEP Version 3.0; MSI: San Diego, 1997. (24) Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133. (25) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (26) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (27) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797. (28) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (29) Poelman, H.; Vennik, J.; Dalmai, G. J. Electron. Spectrosc. Relat. Phenom. 1987, 44, 251. (30) Zhang, Z.; Henrich, V. E. Surf. Sci. 1994, 321, 133. (31) Lambrecht, W.; Djafari-Rouhani, B.; Vennik, J. J. Phys. C (Solid State Phys.) 1981, 14, 4775. (32) Lide, D. R. Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, FL, 1993; reprinted from: Kagaku Benran, 3rd ed., Vol. II, 1984. (33) In this paper, we index V2O5 using the conventional setting, a ) 11.519 Å, b ) 4.373 Å, and c ) 3.564 Å. We note that there is a frequent inversion of the b and c axes in different studies. (34) Nalewajski, R. F.; Ko¨ster, A. M.; Bredow, T.; Jug, K. J. Mol. Catal. 1993, 82, 407. (35) Nalewajski, R. F.; Korchowiec, J.; Tokarz, R.; Broclawik, E.; Witko, M. J. Mol. Catal. 1992, 77, 165. (36) Nalewajski, R. F.; Korchowiec, J. J. Mol. Catal. 1991, 68, 123.