English

• 1. INTRODUCTION

  Water (H₂O) is one of the most studied adsorbates on well-defined metal oxide single crystal samples. The adsorption of H₂O at solid surfaces shows a variety of applications in electrochemistry (The formation of water molecules near a metal electrode plays a key role in electrochemical reactions) and in heterogeneous catalysis^[1-3]. It has been generally accepted that the interaction of H₂O with metal oxide surfaces has important consequences on their catalytic behaviors. The adsorption of H₂O on a metal oxide surface may result in different adsorption modes, or combinations of them, depending on the measurement temperature, the intrinsic reactivity of the surface, and the number of oxygen-vacancy sites on the surface.

  Vanadium pentoxide (V₂O₅) is an important transition metal oxide and has been widely used as electrode materials for rechargeable aqueous Zn-ion battery and catalysts in many industrial applications^[4-6]. Since Zn-ion storage and heterogeneous catalytic reactions take place in aqueous solutions or in a humid environment, it is significant to understand the interaction mechanisms of H₂O on V₂O₅. Meanwhile, in the application of photocatalysis, water not only provides the reaction environment but also is the key participant, as it can decompose to produce hydrogen, oxygen, and hydroxyl radicals. Therefore, a detailed investigation of H₂O adsorption and dissociation on V₂O₅ surfaces is also very important, which can help to elucidate the mechanism of catalytic processes occurring on V₂O₅ catalyst.

  Experimental studies based on Fourier transform infrared (FTIR) spectroscopy showed that H₂O is adsorbed on unsupported V₂O₅ and surface metal centers acting as Lewis acids are the favorable sites^{[7, 8]}. Costa et al. observed that water is molecularly adsorbed on V₂O₅(001) based on the observation of the ambient atomic force microscopy on V₂O₅(001) surface^[9]. Moshfegh et al. have carried out temperature programmed desorption (TPD) studies and found that H₂O adsorbs dissociatively on the V₂O₅ surface through the action of two neighboring vanadium sites of -V⁵⁺-O-V^4+[10]. The analyis results of some important reactions show that the structures of surface V₂O₅/supports are generally altered in the presence of water, and water effect is connected with the formation of the Brönsted acids V-OH. Sadovskaya et al.^[11] and Lee et al.^[12] proposed that the lattice oxygen of surface vanadia species VO_x undergoes isotope exchange with that of adsorbed water rather than gas-phase oxygen. Recently, Ma et al.^[13] have studied the reactions of vanadium oxide cluster cations with heavy water (D₂O) in a fast flow reactor. Their results suggested that (V₂O₅)_1-3⁺ clusters are reactive toward water resulting in (V₂O₅)_1-3D⁺ and (V₂O₅)_1-3D₂O⁺ through the deuterium atom abstraction and adsorption reactions. The work of Goclon et al. showed that under low value of the oxygen partial pressure conditions, the V₂O₅ phase is unstable and may form oxygen-vacancy surface that is more active than stoichiometric V₂O₅ surface^[14].

  Compared to experimental works, the computational chemistry methodologies have been used to give further insight into the structural, electronic, magnetic and catalytic properties of the V₂O₅ surfaces, but the theoretical studies for H₂O/V₂O₅ interfaces and H₂O/V₂O₅ interactions are relatively limited. Yin et al.^[15] and Witko et al.^[16] studied the adsorption and dissociation of water at low-index V₂O₅ surfaces by cluster models. However, they did not study the water monolayers with V₂O₅ and a systematical investigation of adsorption and dissociation of water on different V₂O₅ surfaces including oxygen-vacancy surface by periodic density functional theory. In this study, the first-principles calculations based on density functional theory and the periodic slab models are used to investigate water monolayers with V₂O₅ and single-molecule adsorption and dissociation at three low-index surfaces ((001), (010) and (100)) of V₂O₅ and oxygen-vacancy V₂O₅(001) surface.

  2. COMPUTATIONAL METHODS

  Geometry optimizations and density of states calculations were performed using the Cambridge Serial Total Energy package (CASTEP)^[17]. CASTEP is a first principles density functional theory plane wave pseudopotential simulation code. The calculations were performed in the frame of the generalized gradient approximation (GGA, Perdew-Burke-Enzerhoff (PBE)) for the exchange and correlation effects^{[18, 19]}. Ultrasoft pseudopotentials corresponding to the optimization scheme of Vanderbilt were employed^[20]. Surfaces were simulated by slabs separated by a vacuum region (14 Å). The cutoff energy is 380 eV. The number of k-points for Brillouin-Zone integration (BZ) was chosen according to the Monkhorst-Pack scheme^[21]. For bulk V₂O₅, a 5 × 5 × 5 k-points mesh was used, whereas for the supercells representing (001), (010), (100) and defective surfaces 4 × 4 × 1 grids were applied. All atoms in the three surfaces were allowed to relax. The convergence criteria for the structure optimization and energy calculation were set to (a) a SCF tolerance of 2.0 × 10^-6 eV/atom, (b) an energy tolerance of 2.0 × 10^-5 eV/atom, (c) a maximum force tolerance of 0.05 eV/Å, and (d) a maximum displacement tolerance of 0.002 Å. Spin-polarized calculations were also included in the DFT investigations. We have considered the effect of Hubbard U on the adsorption energies of water on V₂O₅(001) surface, and found that the effect is small (less than 1 kJ/mol), so the Hubbard U parameter was not included in the calculations.

  V₂O₅ is a layerlike material with a no-covalent interaction between layers, making it easy for cleavage along the (001) plane, as shown in Fig. 1. V₂O₅ is an orthorhombic lattice (space group Pmmn) and its optimized unit cell parameters (a = 11.512, b = 3.564, c = 4.368 Å) are within +0.2%, +0.1% and –2.8% errors, respectively of the experimental determined lattice constants^[22], suggesting that the calculations are reliable. Three possible low-index surfaces of V₂O₅, namely the (010), (001) and (100), are shown in Fig. 2. In this work, two layers of V₂O₅ are used in the (001), (100), (010) and oxygenvacancy (001) surfaces.

  Figure 1

  

  Figure 1.  Crystal structure of V₂O (Red and grey spheres are O and V atoms, respectively)

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  Figure 2

  

  Figure 2.  Three low-index surfaces of V₂O₅. (a) V₂O₅(001), (b) V₂O₅(100), and (c) V₂O₅(010) (Red spheres are O atoms and the gray ones are V atoms)

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  There are three types of oxygen atoms differing by their coordination number with the vanadium atoms: (ⅰ) singly coordinated terminal oxygen, O(1), which is a vanadyl oxygen (V=O); (ⅱ) two-coordinated oxygen, O(2); and (ⅲ) three-coordinated oxygen, O(3). The O(2) and O(3) bridge two and three vanadium atoms, respectively.

  For the adsorption of water molecule, 5-fold coordinated vanadium atoms and the surface oxygen atoms are the active sites. 5-Fold coordinated vanadium atoms are considered as Lewis acid site, which can interact with oxygen atom (O_w) of water with free electron pairs. Oxygen atoms at the surface can interact with hydrogen atoms of water through hydrogen bonds. For the adsorption of H₂O, both vanadium and oxygen sites of V₂O₅ surfaces are considered.

  The adsorption energy (E_ads) has been calculated according to the expression:

  ${{E}_{\text{ads}}}={{E}_{(\text{adsorbate}-\text{substrate})}}-({{E}_{adsorbate}}+{{E}_{\text{substrate}}}), $

  (1)

  where E_{(adsorbate-substrate)} is the total energy of the adsorbate/substrate system, E_adsorbate is the total energy of the isolated adsorbate at its equilibrium geometry, and E_substrate is the total energy of the substrate. A negative E_ads value corresponds to a stable adsorbate/ substrate system.

  All transition states (TSs) are located by using the complete LST/QST method^[23-25]. It starts with linear synchronous transit (LST) maximization, followed by energy minimization in directions conjugated to the reaction pathway. These approximated TS are then used to perform quadratic synchronous transit (QST) maximization. From that point, another conjugate gradient minimization is performed. The cycle is repeated until a stationary point is located. A transition state is identified when (1) the forces on the atoms vanish and (2) the energy is a maximum along the reaction coordinate but a minimum with respect to all of the other degrees of freedom. The energy barrier is the energy difference between the transition state (TS) and the intermediate (IM).

  3. RESULTS AND DISCUSSION

  3.1 H₂O/V₂O₅ interface structures and adsorption of H₂O on the stoichiometric V₂O₅(001) surface

  Firstly, the adsorption of H₂O on the V₂O₅(001) surface was considered since the V₂O₅(001) surface is the thermodynamically most stable^[26]. H₂O is initially placed on different adsorption sites, i.e., O(1), O(2), O(3) and V on the V₂O₅ surface, as shown in Fig. 3. In the calculation process, all possible adsorption orientations were considered including parallel and vertical (O-down and H-down modes) to get all possible adsorption models for water on surfaces. The final stable configurations are obtained on the surface for each parallel and vertical orientations. The calculated adsorption energies and bond lengths for stable configurations are listed in Table 1.

  Figure 3

  

  Figure 3.  Scheme view of the four studied cases of H₂O adsorption on V₂O₅(001) surface. (a) Side view and (b) Top view. Three different oxygen atoms and surface V atoms are available for reactivity (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Table 1

  

  Table 1.  Optimized Parameters for H₂O Adsorption on the V₂O₅(001) Surface

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  Sites	Eads (kJ/mol)	Rsurface-H2O (Å)	ROw-H (Å)	θH-Ow-H (°)	QH2O (e)	Msurface-H2O
  O(1)	–15.60	2.231	0.978/0.978	106.8	0.01	0.03
  O(2)	–14.33	2.240	0.978/0.976	105.9	0.00	0.02
  O(3)	–8.01	3.010	0.981/0.981	105.2	0.00	0.00
  V	–14.53	2.674	0.980/0.980	107.1	0.02	0.04
  Eads denotes adsorption energies. R is bond length. For O(1), O(2), and O(3) sites, the Rsurface-H2O means the distance between surface O atoms and H atom of H2O. For V site, the Rsurface-H2O stands for the distance between surface V atoms and Ow atom of H2O. ROw-H represtents the two bond lengthes of adsorbed H2O. Q shows charge transfer from H2O to surface. And M denotes the bond population.

  The adsorption energies are in the range of –8.01 to –15.60 kJ/mol, and adsorption abilities decrease in the order of O(1) > V ≈ O(2) > O(3). The surface-H₂O distances are changing from 2.231 to 3.010 Å. The highest adsorption energy of H₂O calculated was –15.60 kJ/mol, which constitutes a weak interaction. The O(3) site is inactive with adsorption energy of –8.01 kJ/mol, which is different and is roughly 7.39 kJ/mol. The adsorption mechanism of H₂O on stoichiometric V₂O₅(001) surface is physisorption. The Mulliken population analysis is performed to understand the charge trend between adsorbate and substrate. As listed in Table 1, the Mulliken charges for adsorbed H₂O in the four adsorption configurations are below 0.02 e, suggesting few electrons withdrawing from H₂O to the surface during adsorption. The bond populations between surface and H₂O are almost zero. The above analyses suggest that the adsorption of water on V₂O₅(001) surface is physisorption, which supports the previous experimental identifications^{[7, 9, 10]}.

  Furthermore, we also give the most stable H₂O/V₂O₅ interface structure (Fig. 4) after an optimization of various structures. We suggest whe the ratio of H₂O and V₂O₅ is 1 (i.e, the same number of H₂O molecule and V₂O₅ unit), H₂O/V₂O₅ interface structure can still maintain stable (a negative value relative to isolated H₂O molecule). The adsorption energy for H₂O in H₂O/V₂O₅ interface is –0.75 eV, where the negative energy means the adsorption is exothermic. The other H₂O/V₂O₅ interface structures are shown in Fig. S1. Fig. 4 shows that hydrogen binding interactions are generally found in H₂O/V₂O₅, which contributes to the stability of H₂O/V₂O₅ interface. More importantly, the right inset of Fig. 4 indicates the water forms a weakly ordered overlayer with V₂O₅(001) surface. Therefore, there is a locally ordered (2 × 2) superstructure of molecular water in H₂O/V₂O₅ interface. The formation of ordered overlayers has been observed on some oxide surfaces like TiO₂(101)^[27].

  Figure 4

  

  Figure 4.  Scheme view of the most stable H₂O/V₂O₅ interface structure (8·H₂O/8·V₂O₅) (Red spheres are O atoms, gray spheres are V atoms, and dashed lines are hydrogen bonding interaction)

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  To determine if the dissociation of H₂O occurs on the V₂O₅(001) surface, the partial dissociation of H₂O is proposed, in which products are one hydroxyl group and one hydrogen atom. Hydrogen atom approaches different oxygen sites of the surface including O(1), O(2) and O(3) sites, and the hydroxyl group attaches to a V site, as shown in Fig. S2. The calculated adsorption energies for water dissociation at O(1), O(2) and O(3) sites displayed in Fig. S2 are positive with 171.73, 197.78 and 209.36 kJ/mol, suggesting that the three types of H₂O dissociative adsorption are energetically unfavorable. Therefore, the dissociation of the water hardly occurs on the stoichiometric V₂O₅(001) surface.

  3.2 Adsorption and dissociation of H₂O on oxygen-vacancy V₂O₅(001) surface

  Oxygen-vacancies play an important role in catalytic reactions, and thus it is necessary to study the adsorption and dissociation of water on the oxygen-vacancy V₂O₅(001) surface. Considering the interlayer interaction effect on the geometrical structure and electronic characterization of the oxygen-vacancy surface, a two-layer-slabs model was built, as shown in Fig. 5. In general, water prefers to interact with the surface atoms with high electron deficiency. Thereby, the favorable site for H₂O adsorption is V_cus (cus stands for one-fold coordinately unsaturated site). The optimized parameters for H₂O adsorption on oxygen-vacancy surface are presented in Table 2.

  Figure 5

  

  Figure 5.  H₂O adsorption on the V site of oxygen-vacancy V₂O₅(001) surface (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Table 2

  

  Table 2.  Optimized Parameters for H₂O Adsorption on the Oxygen-vacancy V₂O₅(001), V₂O₅(100) and V₂O₅(010) Surfaces

  DownLoad: CSV

  Surfaces	Eads (kJ/mol)	RV-Ow (Å)	ROw-H (Å)	θH-Ow-H (°)
  Oxygen-vacancy V2O5(001)	–98.41	2.071	0.981/1.091	110.3
  V2O5(100)	–108.06	2.174	0.977/0.978	107.4
  V2O5(010)	–84.90	2.275	0.980/0.991	106.1
  Eads, R and θ denote the adsorption energies, bond lengths and the angle of H2O, respectively.

  After adsorption, as shown in Fig. 5, the H₂O is strongly adsorbed on the surface with an adsorption energy of –98.41 kJ/mol, forming two bonds: the O_w atom of H₂O forms a covalent bond with V atom with a distance of 2.071 Å, and one H atom generates a H-bond with surface O(1) atom with a distance of 2.082 Å. The strong H-bonding gives rise to a significant OH bond stretching with the O–H distance elongating from 0.973 to 1.091 Å, suggesting the bond is somewhat activated.

  In addition, the dissociative chemisorption of H₂O is investigated to understand the properties and reactivity of H₂O on oxygen-vacancy V₂O₅(001) surface. The strong adsorption of the pre-dissociation state of H₂O allows the O_w–H bond to be activated, leading to the migration of H atom to the surface O atom to form an adatom and a hydroxyl species. Thus, the energy profile of water dissociation via O_w–H scission is examined and the intermediate, transition state, and final state (FS) along the pathway are also located, as shown in Fig. 6. Relative to the adsorption configuration, the dissociation of H₂O is exothermic by 37.69 kJ/mol. The energy barrier is 93.72 kJ/mol, suggesting the dissociation might be difficult at low temperature. During the reaction process, the O_w–H bond length increases gradually: 1.091 Å in IM1, 1.501 Å in TS1, and finally breakes in FS1, indicating the O_w–H bond broken process. At the same time, the H–O(1) and O_w–V bond lengths reduce gradually: 2.082 and 2.071 Å in IM1, 1.620 and 1.970 Å in TS1, and 1.096 and 1.874 Å in FS1. The changes of bond lengths indicate H–O(1) and O_w–V are gradually strengthened.

  Figure 6

  

  Figure 6.  Energy profile for H₂O dissociation on oxygen-vacancy V₂O₅(001) surface with the relative intermediate, transition state and final state. The asterisks here denote the adsorption species (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  From the above analyses, H₂O can undergo dissociative chemisorption on the oxygen-vacancy V₂O₅(001) surface forming the surface hydroxyl group and adsorbed H atom, which is consistent with the experimental result that the lattice oxygen of V₂O₅ undergoes isotope exchange with the adsorbed water^{[11, 12]}. In addition, H₂O dissociative adsorption may alter the structures of V₂O₅ surface, thereby affecting the reactivity of V₂O₅-based catalysts and the redox cycle of V₂O₅-based heterogeneous reactions, which can be applied to guide the formation of surface lattice oxygen in isotope exchange experiments.

  3.3 Adsorption and dissociation of H₂O on the V₂O₅ (010) and (100) surfaces

  The side ((010) and (100) planes) surfaces contribute about 15% of the total surface area of a V₂O₅ crystallite, indicating a non-negligible role in the catalytic activity of V₂O₅^[28]. Therefore, in this study, the adsorption and dissociation of water on the (010) and (100) planes are also studied.

  The optimized parameters of water adsorption on the side V₂O₅ surfaces, i.e., V₂O₅(100), V₂O₅(010), are summarized in Table 2 and the optimized configurations are shown in Fig. 7a and 8a. In Table 2, the H₂O is strongly adsorbed on the V sites of V₂O₅(100) and (010) surfaces with the adsorption energies to be –108.06 and –84.90 kJ/mol and the V–O_w bond lengths of 2.174 and 2.275 Å, respectively. In comparison with the adsorption energy of H₂O on stoichiometric V₂O₅(001) surface, molecular H₂O adsorption on the side of V₂O₅ ((100) and (010)) surfaces is stronger than that on the stoichiometric V₂O₅(001) surface, suggesting that the side V₂O₅ surfaces are chemically more active and may play an important role in the activity of V₂O₅ as an oxidation or reduction catalyst.

  Figure 7

  

  Figure 7.  (a) Optimized structures of H₂O adsorbed on the V₂O₅(100) surface; (b) Possible dissociative types of H₂O on the V₂O₅(100) surface (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 8

  

  Figure 8.  (a) Optimized structures of H₂O adsorbed on V₂O₅(010) surface; (b) Possible dissociative types of H₂O on the V₂O₅(010) surface (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  Next, the water dissociative chemisorption on the side V₂O₅ surfaces is investigated. The possible dissociative types of H₂O on the side V₂O₅ surface are shown in Figs. 7b and 8b. The calculated energy profiles for (100) and (010) surfaces, along with intermediates, transition states and final states, are shown in Figs. 9 and 10, respectively.

  Figure 9

  

  Figure 9.  Energy profile for H₂O dissociation on V₂O₅(100) surface with the relative intermediates, transition states and final states (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 10

  

  Figure 10.  Energy profile for H₂O dissociation on V₂O₅(010) surface with the relative intermediates, transition states and final state (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  For the V₂O₅(100) surface, two pathways are examined, i.e., IM2 → TS2 → FS2 and IM2 → TS3 → FS3, namely pathways A and B, as shown in Fig. 9. The energies of the optimized structures are relative to the intermediate. For pathway A, H₂O adsorbs on the surface to form intermediate IM2, then the O_w–H bond is broken through the transition state TS2 to form FS2 with independently adsorbed hydroxyl group on the V site and H atom on the O(1) site. The reaction barrier is 71.50 kJ/mol and the process is exothermic by 41.56 kJ/mol. During the process, the O_w–H bond length increases gradually: 0.991 Å in IM2, 1.391 Å in TS2, and finally breaks in FS2, indicating the O_w–H bond broken process. At the same time, the H–O(1) and O_w–V bond lengths reduce gradually: 2.390 and 2.174 Å in IM2, 1.421 and 2.067 Å in TS2, and 0.978 and 1.840 Å in FS2. The changes of bond lengths suggest H–O(1) and O_w–V are gradually strengthened. For pathway B, the energy barrier is 78.25 kJ/mol, which is about 6.75 kJ/mol higher than that of pathway A. The reaction is exothermic by 22.32 kJ/mol which is smaller than that of pathway A (41.56 kJ/mol). These results indicate that pathway A is kinetically and thermodynamically more favored than pathway B. Furthermore, the energy barriers of pathways A and B are 71.50 and 78.25 kJ/mol, respectively. This difference should result from different calculated models and transition state searching methods.

  In the same way, the energy profile of H₂O dissociation on V₂O₅(010) surface is also examined, as shown in Fig. 10. Two pathways are examined, i.e., IM3 → TS4 → FS4 and IM3 → TS5 → FS5, namely pathways C and D. For pathway C, the dissociation process is exothermic by 22.36 kJ/mol with an energy barrier of 91.79 kJ/mol, while for pathway D, the corrponding values are 19.33 and 105.30 kJ/mol, respectivley. Comparing the dissociation pathway of H₂O on V₂O₅(100) and V₂O₅(010) surfaces found the H₂O dissociation on V₂O₅(100) surface is more favorable both thermodynamically and kinetically than that on the V₂O₅(010) surface. Based on the above analyses, the side ((010) and (100) planes) V₂O₅ surfaces are more active than the stoichiometric V₂O₅(001) one for H₂O adsorption and dissociation, which suggests that V₂O₅(100) and (010) surfaces show more strong reactivity for surface reactions.

  4. CONCLUSION

  The water/V₂O₅ interface, adsorption and dissociation of H₂O on different V₂O₅ surfaces are investigated using periodic density functional method. Adsorption of H₂O on stoichiometric V₂O₅(001) surface follows the physisorption mechanism and the dissociation of H₂O hardly occurs due to the thermodynamic analysis. The most stable H₂O/V₂O₅ interface structure shows that H₂O adsorbs as an intact monomer with a locally ordered superstructure of molecular water. On the oxygen-vacancy V₂O₅(001) surface, H₂O is strongly adsorbed on V site and can undergo dissociative chemisorption with a moderate energy barrier, forming a surface hydroxyl and a H adatom. For (100) and (010) of V₂O₅ surfaces, H₂O is also strongly adsorbed on both surfaces. The energy profile analyses show that H₂O can undergo dissociative chemisorption with low energy barriers, indicating (100) and (010) surfaces are more active than the V₂O₅(001) surface. The formation of O–V bond from the hydroxyl group adsorbed on V sites on oxygen-vacancy V₂O₅(001) and side V₂O₅ surfaces suggests that V₂O₅ catalysts can take the oxygen from H₂O, which is consistent with the experimental results that the lattice oxygen of V₂O₅ undergoes isotope exchange with that of H₂O.

  
  Conflicts of interest: The authors declare no conflict of interest.
  
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  Figure 1  Crystal structure of V₂O (Red and grey spheres are O and V atoms, respectively)

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  Figure 2  Three low-index surfaces of V₂O₅. (a) V₂O₅(001), (b) V₂O₅(100), and (c) V₂O₅(010) (Red spheres are O atoms and the gray ones are V atoms)

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  Figure 3  Scheme view of the four studied cases of H₂O adsorption on V₂O₅(001) surface. (a) Side view and (b) Top view. Three different oxygen atoms and surface V atoms are available for reactivity (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 4  Scheme view of the most stable H₂O/V₂O₅ interface structure (8·H₂O/8·V₂O₅) (Red spheres are O atoms, gray spheres are V atoms, and dashed lines are hydrogen bonding interaction)

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  Figure 5  H₂O adsorption on the V site of oxygen-vacancy V₂O₅(001) surface (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 6  Energy profile for H₂O dissociation on oxygen-vacancy V₂O₅(001) surface with the relative intermediate, transition state and final state. The asterisks here denote the adsorption species (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  Figure 7  (a) Optimized structures of H₂O adsorbed on the V₂O₅(100) surface; (b) Possible dissociative types of H₂O on the V₂O₅(100) surface (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 8  (a) Optimized structures of H₂O adsorbed on V₂O₅(010) surface; (b) Possible dissociative types of H₂O on the V₂O₅(010) surface (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  Figure 9  Energy profile for H₂O dissociation on V₂O₅(100) surface with the relative intermediates, transition states and final states (Red spheres: O atoms, gray spheres: V atoms, purple spheres: O_w atoms of H₂O, and blue spheres: H atoms)

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  Figure 10  Energy profile for H₂O dissociation on V₂O₅(010) surface with the relative intermediates, transition states and final state (Red, gray, purple and blue spheres are O, V, O_w of H₂O and H atoms, respectivley)

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  Table 1.  Optimized Parameters for H₂O Adsorption on the V₂O₅(001) Surface

  Sites	Eads (kJ/mol)	Rsurface-H2O (Å)	ROw-H (Å)	θH-Ow-H (°)	QH2O (e)	Msurface-H2O
  O(1)	–15.60	2.231	0.978/0.978	106.8	0.01	0.03
  O(2)	–14.33	2.240	0.978/0.976	105.9	0.00	0.02
  O(3)	–8.01	3.010	0.981/0.981	105.2	0.00	0.00
  V	–14.53	2.674	0.980/0.980	107.1	0.02	0.04
  Eads denotes adsorption energies. R is bond length. For O(1), O(2), and O(3) sites, the Rsurface-H2O means the distance between surface O atoms and H atom of H2O. For V site, the Rsurface-H2O stands for the distance between surface V atoms and Ow atom of H2O. ROw-H represtents the two bond lengthes of adsorbed H2O. Q shows charge transfer from H2O to surface. And M denotes the bond population.

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  Table 2.  Optimized Parameters for H₂O Adsorption on the Oxygen-vacancy V₂O₅(001), V₂O₅(100) and V₂O₅(010) Surfaces

  Surfaces	Eads (kJ/mol)	RV-Ow (Å)	ROw-H (Å)	θH-Ow-H (°)
  Oxygen-vacancy V2O5(001)	–98.41	2.071	0.981/1.091	110.3
  V2O5(100)	–108.06	2.174	0.977/0.978	107.4
  V2O5(010)	–84.90	2.275	0.980/0.991	106.1
  Eads, R and θ denote the adsorption energies, bond lengths and the angle of H2O, respectively.

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