Enhanced Oxidation Reactivity of WO3(001) Surface through the

Article pubs.acs.org/JPCC

Enhanced Oxidation Reactivity of WO3(001) Surface through the Formation of Oxygen Radical Centers Hua Jin,† Jia Zhu,† Wenjie Chen,† Zhenxing Fang,† Yi Li,†,‡ Yongfan Zhang,*,†,§ Xin Huang,*,† Kaining Ding,† Lixin Ning,⊥ and Wenkai Chen† †

Department of Chemistry, Fuzhou University, Fuzhou, Fujian, 350108, China State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou, Fujian, 350002, China § Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, 361005, China ⊥ Department of Physics, Anhui Normal University, Wuhu, Anhui, 241000, China ‡

S Supporting Information *

ABSTRACT: The γ-WO3(001) surfaces doped by a series of group VB elements have been investigated by means of firstprinciples density functional theory (DFT) calculations combined with a slab model. Our results show that the doping of VB element is preferential under O-rich growth conditions and that the replacement of tungsten by Ta atom is energetically favorable among three group VB elements. The introduction of a group VB atom into the surface results in the downward shift of the Fermi level, and in most cases, the 2p states derived from the in-plane oxygen atom are still the dominate components at the Fermi level as before doping. However, the substitution of Ta dopant for 6-fold-coordinated tungsten atom (W6f) at the top layer is a special case in which the 2p states of the top terminal oxygen atom just above Ta become the primary compositions at the Fermi level. Only in this model, the spin densities are mainly located on the terminal oxygen atoms near the Ta site, and the oxygen radical center observed in the gas-phase W3O9+ cluster is reproduced. Therefore, the formation of radical oxygen center in the condensed phase depends on not only the substituent site but also the type of the dopant. Moreover, additional calculations are performed to study the oxidation reaction of CO molecule on the above Ta doped surface, and results indicate that the energy barrier for CO oxidization is obviously reduced compared to the undoped one, which implies that the introduction of Ta at W6f site can efficiently improve the oxidation reactivity of the WO3(001) surface. presence of radical oxygen centers (W−O·) in the W3O9+ cluster (see Figure 1a). Similar phenomena of radical oxygen centers in governing the reactivity of the cluster are also observed in a series of cationic or anionic zirconium oxide clusters, namely, (ZrxO2x)+ and (ZrxO2x+1)− (x = 1−4).32−34 Interestingly, in addition to the above positively or negatively charged systems, such radical oxygen centers also can be found in the neutral binary metal oxide clusters with the same number of total valence electrons. Recent theoretical calculations performed by Nöβler et al. reveal that,35 after replacing a Zr atom in the zirconium oxide cluster with an atom that has one more or one less electron, the resulting neutral binary clusters, namely, ZrScO4 and ZrNbO5 clusters, show similar arrangements of the radical oxygen center as observed in the corresponding isoelectronic charged species, Zr2O4+ and Zr2O5−, respectively. Consequently, the predicted reactivity characteristics of these neutral binary clusters and isoelectronic charged species are effectively the same.35 For the tungsten

1. INTRODUCTION Tungsten trioxide (WO3) has many industrial applications and has been widely used in different fields including chemical sensors, electrochromism, photochromism, and so forth.1−3 In particular, tungsten oxides are important acid−base and redox catalysts, and they show excellent activity for many catalytic reactions.4−18 Therefore, considerable effort has been made to design new catalysts that are based on tungsten oxide with a high degree of selectivity and activity.19−26 As a first step to get insight into the properties of tungsten oxide catalysts, a variety of gas-phase tungsten oxide clusters have been used to model the geometry, electronic structure, and reactive behavior of the surface of actual catalysts.27−32 Among them, special efforts have been paid to W3O9 cluster because it is the major species in the vapor phase, and theoretical calculations indicate that W3O9 can be considered as the smallest molecular prototype for bulk WO3.28 Although W3O9 shows unusual stability, the experimental results of mass spectrometry show that the corresponding cation, W3O9+, exhibits enhanced activity and selectivity for the transfer of a single oxygen atom to CO or propylene (C3H6) molecules.32 This improvement of the oxidation reactivity is due to the © 2012 American Chemical Society

Received: October 23, 2011 Revised: February 8, 2012 Published: February 8, 2012 5067

dx.doi.org/10.1021/jp210171f | J. Phys. Chem. C 2012, 116, 5067−5075

The Journal of Physical Chemistry C

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Figure 1. The spin density distributions for the ground state of (a) the positively charged W3O9+ cluster and (b) the binary neutral MW2O9 (M = V, Nb, Ta) clusters. The results shown in the figures are obtained by using the B3LYP method, and Stuttgart+2flg and aug-cc-pVTZ basis sets are employed for metal and oxygen atoms, respectively.36 The W and O atoms are denoted by blue and red spheres, respectively.

structure of the WO3 surface is different from that of the W3O9 cluster. For the W3O9 cluster, the highest occupied molecular orbital is localized on the terminal oxygen atoms.29 The removal of a single electron to form W3O9+ cation results in the localization of a single unpaired electron on two terminal O atoms (see Figure 1a). However, according to our previous study, in the case of WO3(001) surface,38 the in-plane oxygen (like the bridging oxygen in cluster) has the dominant contribution for the highest occupied crystal orbital. Correspondingly, the unpaired electrons might tend to be located on the in-plane oxygen when one electron is moved from the WO3(001) surface by doping one group VB atom, which is quite different from the gas-phase cluster. Therefore, it seems that things will become more complicated, and theoretical calculations are required to verify the existence and effect of radical oxygen centers in the group VB atom-doped WO3(001) surface. Prompted by the important role of radical oxygen centers in influencing the reactive activity and also motivated by the recent experimental advances in the preparation of group VB transition metal−tungsten mixed oxides,39,40 in this paper, density functional theory (DFT) calculations are carried out to explore the electronic structures and reactivity of the V, Nb, and Ta doped WO3(001) surfaces. We first study the surface configurations and thermodynamic stabilities of WO3(001) surfaces after introducing a different kind of VB atom, and the result shows that the addition of Ta atom to WO3 surface is energetically favorable among three group VB elements. Next, the electronic structures of four Ta doped surfaces are carefully investigated, and by examining the distributions of the unpaired electrons, the radical oxygen centers found in the cationic W3O9+ or neutral MW2O9 clusters are reproduced when one 6fold-coordinated W atom at the top layer is replaced by a Ta

oxide clusters, recently we have studied the geometries and electronic structures of bimetallic MW2O9 (M = V, Nb, Ta) clusters,36 and as displayed in Figure 1b, the locations of the radical oxygen center (M−O·) in these neutral clusters are also similar to the positively charged W3O9+ cluster. Like the zirconium oxide cluster, it can be expected that the high activity toward the oxidation reaction observed in the W3O9+ cluster will remain in the binary neutral MW2O9 cluster. Therefore, the introduction of radical oxygen centers may provide an effective way to design new tungsten oxide catalysts with high activity. Considering that the final objective for the study of the gasphase clusters is to reveal the fundamental reactive behavior happening on the surface of heterogeneous catalysts,37 it may be wondered whether the above findings derived from the gasphase clusters are still preserved on the surface of the corresponding bulk oxides. To answer this question, in the present work, we turn our attention to the WO3 surface that is modified by introducing group VB transition-metal atoms (M = V, Nb, and Ta) into the surface. Since the dopant has one less electron than the tungsten atom, just like the corresponding binary neutral MW2O9 cluster, the M doped WO3 surface can be seen as a kind of (WO3)n+ species. However, the finding relative to the gas-phase cluster may not be directly applicable because there are some differences between the WO3 surface and the small W3O9 cluster. First, as presented in Figure 2, the top oxygen and the in-plane oxygen atoms at the surface correspond to the terminal and bridging oxygen atoms in the gas-phase cluster, respectively. From the structural point of view, the WO3 surface has more possible W sites involved in the doping with respect to the W3O9 cluster in which the M atom can replace the W atom at the top layer and also below the surface, and in addition, even for the top layer, two types of tungsten sites need to be considered. Second, the electronic 5068



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spacing between the adjacent slabs was set about 10 Å, and a (5 × 5 × 1) Monkhorst−Pack k-point mesh49 was used for integration in the reciprocal space. Our previous study on the WO3(001) surface indicated that the above settings of calculations could produce reasonable results.38 In addition, using the above model, the predicted adsorption energy (0.44 eV) and C−W distance (2.51 Å) for CO molecule adsorbed on the WO3(001) surface were also in good agreement with the results (0.37 eV and 2.47 Å) of a recent work by Oison et al. that a larger supercell and PBE functional were employed.50 Concerning the reactions of CO oxidation on the clean and group VB atom-doped WO3(001) surfaces, the climbing image nudged elastic band (CINEB) method51,52 was used to determine the minimum energy path (MEP). The NEB method has been proven to be an efficient method for finding the reaction pathways when both the initial and final states are known,53 and in the modified CINEB procedure, the image with the highest energy corresponds to the transition state (TS). The vibration frequency analysis was also performed to ensure the stationary points of the potential energy surface as local minimum or first-order saddle point in the reaction path. The values of frequencies were calculated from the diagonalization of the mass-weighted Hessian matrix constructed by the finite difference process.54 To help the subsequent discussion, we use symbols W6f, W5f to represent 6-fold- and 5-fold-coordinated tungsten atoms at the first layer (see Figure 2), respectively. The W atoms below W6f and W5f atoms are labeled W6i and W5i, respectively. Furthermore, the top, the in-plane, and the bridge oxygen atoms below W6f and W5f are denoted by Ot, Oi, O6f, and O5f, respectively.

Figure 2. Schematic side view of the WO3(001) surface with γmonoclinic phase. The blue and red spheres indicate the W and O atoms, respectively. To avoid the dipole perpendicular to the surface, half of the top oxygen atoms are moved to the bottom of the slab. In the picture, the W6f, W5f, W6i, and W5i symbols represent four types of surface tungsten atoms that may be replaced by group VB transitionmetal atom, and Ot, Oi, O6f, and O5f labels are denoted the top terminal oxygen, the in-plane oxygen and the bridging oxygen atoms below W6f and W5f atoms. When a W6f atom is replaced by a VB group atom, there are two kinds of Ot atoms which sit above W6f and the VB dopant.

3. RESULTS AND DISCUSSION 3.1. Structures and Thermodynamic Stabilities of the M Doped WO3(001) Surfaces. For each dopant, there are four possible doping configurations, namely, M−W6f, M−W5f, M−W6i, and M−W5i models, which correspond to the replacements of a W atom at W6f and W5f sites on the top layer and W6i and W5i sites on the sublayer (Figure 2), respectively. When a W6f atom is replaced by a group VB atom, there are two kinds of Ot atoms that just sit above W6f and the dopant, respectively. To study the thermodynamic stability of the WO3(001) surface after introducing VB atom, the doping energy (Edoping) for each model is calculated, which is defined as follows:55−57

atom. Finally, to verify the activation of the surface terminal oxygen, the oxidation reaction of CO molecule on WO3(001) surfaces is investigated, and the improvement of the oxidation performance is observed for the doped surface that contains radical centers around the top terminal O atoms.

2. COMPUTATIONAL DETAILS The spin-polarized DFT calculations within the generalized gradient approximation (GGA) were performed in Vienna ab initio simulation package (VASP).41−44 The Perdew−Wang type (PW91)45 exchange correlation functional was employed to study the energies and structures of the doped WO3(001) surfaces. Vanderbilt ultrasoft pseudopotentials46,47 were used to describe the interaction between the ion cores and the valence electrons for all atoms, and the kinetic cutoff energy was set to 400 eV. In the calculations, the convergence energy threshold for self-consistent iteration was set to 10−4 eV/atom along with the residual atomic forces that were smaller than 0.03 eV/Å. A five-layer periodic slab model with dimensions of 7.50 Å × 7.66 Å (including 20 W and 60 O atoms) was adopted to simulate the WO3(001) surface with γ-monoclinic phase, and each layer consists of three atomic planes (see Figure 2), namely, the O−WO2−O layer. As confirmed by experiments,48 half of the terminal oxygen atoms on the top layer were transferred to the bottom to avoid residual charges on the surface. During the structural optimization, the top three layers were fully relaxed in all directions while the atoms on the remaining two layers were fixed to their bulk position. The

Edoping = EM − doped − E pure + μW − μM

(1)

where EM‑doped and Epure are the total energies of the WO3(001) surface with and without dopant, respectively, and μW and μM are the chemical potentials of the tungsten and dopant atoms. In eq 1, the doping energy is not fixed but depends on the growth condition, including W-rich and W-poor conditions. Under the W-rich (or O-poor) condition, μW is assumed as the energy of bulk W, while the chemical potential of O atom, μO, can be calculated by the thermodynamic equilibrium relation μW + 3μO = μbulk WO3

(2)

Under the W-poor (or O-rich) condition, μO is obtained from the ground-state energy of the O2 molecule, that is, μO = 1/2μO2, and the chemical potential of W is calculated by eq 2. With respect to the dopant atom, the chemical potential μM is 5069



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enhancement of the surface relaxation is predicted in the Ta− W6f model in which the outward movements along the [001] direction for the top oxygen (Ot) and for the in-plane oxygen (Oi) near the doping site increase significantly (more than 0.18 Å). As presented in Table 2, the Ot and Oi atoms near Ta relax outward by 0.319 Å and 0.254 Å, respectively, while the small values of 0.132 Å and 0.062 Å are observed for the undoped WO3(001) surface. For the other three models, the variations of the surface relaxation introduced by the addition of Ta atom are relatively small with respect to the Ta−W6f doped surface. We think that the obvious change of the surface configuration may be the reason why the Ta−W6f model exhibits poor stability among four Ta doped surfaces. For the V and Nb doped systems, the optimized structural parameters are provided in the Supporting Information (Tables S1 and S2). 3.2. Electronic Structures of the Ta Doped WO3(001) Surfaces. Now let us discuss the electronic structure of four Ta doped surfaces and focus on the effects introduced by the doping of Ta atom. Figure 3 presents the total density of states (DOSs) and some atomic partial DOSs of the pure and Ta doped WO3(001) surfaces. Except for the undoped case, the DOS curves displayed in Figure 3 are the sum of the spin-up and spin-down densities of states obtained from the spinpolarized calculations (see Figure S1 in the Supporting Information for the individual spin-up and spin-down total DOSs). For the total DOS of pristine WO3(001) surface, the peaks near −18 eV are dominated by 2s states of different O atoms, and the states in the region from −7.5 to −2 eV are mainly derived from the covalent interactions between tungsten and oxygen atoms. The top valence band of undoped surface is mainly originated from the 2p orbitals of the in-plane oxygen atom (Oi), while the states of the top oxygen atom (Ot) distribute in the regions somewhat far from the Fermi level. Thus, before the addition of dopant, the Oi atom is more active than Ot, and as predicted in our previous work, the Oi atoms are the preferred sites for the nucleophilic reactions.38 For the

estimated from the corresponding metal oxide (namely, M2O5) bulk according to the formula μM = 1/2(μM − 5/2μO2). 2O5 Table 1 lists the doping energies of various M doped WO3(001) surfaces (M = V, Nb, Ta) under the W-rich and OTable 1. Doping Energies (eV) of the Group VB TransitionMetal (M = V, Nb, Ta) Doped WO3(001) Surfacea M−W6f M−W5f M−W6i M−W5i

model model model model

V

Nb

Ta

9.615(1.024) 9.471(0.880) 9.786(1.195) 9.680(1.089)

9.463(0.872) 9.231(0.640) 9.444(0.853) 9.425(0.834)

8.582(−0.009) 8.103(−0.488) 8.262(−0.329) 8.189(−0.402)

a

Values without and within parentheses are the doping energies under the W-rich (O-poor) and O-rich (W-poor) growth conditions, respectively.

rich conditions. According to the values of Edoping, it can be seen clearly that, for a certain dopant, the M atom occupies preferentially the 5-fold-coordinated W (W5f atom) site on the surface for both W-rich and O-rich conditions. Because under the O-rich growth condition the smaller values of Edoping are obtained, the introduction of group VB element is preferential under O-rich conditions. Moreover, our results indicate that the Edoping declines from V to Ta, and the negative values of Edoping are obtained for Ta doped WO3(001) surfaces under O-rich conditions. Therefore, it seems that the replacement of tungsten atom by Ta atom is energetically favorable among three group VB elements, and in the following sections, we will pay close attention to the structural and electronic properties of the Ta doped WO3(001) surfaces. The results of geometry optimization for four Ta doped models are shown in Table 2, and the data of the undoped WO3(001) surface are also given for comparison. Compared to the pure WO3(001) surface, the substitution of Ta for W atom has some influences on the surface relaxation especially when the doping occurs on the W6f site. Although the atomic radii of Ta and W atom are similar (2.09 Å vs 2.02 Å), the obvious

Table 2. Optimized Structural Parameters and the Surface Work Function Change with Respect to the Undoped Surface for Different Ta Doped WO3(001) Surfaces undoped model

Ta−W6f model

Ta−W5f model

Ta−W6i model

Ta−W5i model b

Ota M6f O6f M6i Oi M5f O5f M5i Ot−M6f M6f −O6f M6i−O6f M5f −O5f M5i−O5f work function change (eV)

0.132 0.160 0.014 0.018 0.062 −0.053 −0.033 −0.051 1.712 2.308 1.773 1.749 2.177

Displacement (Å) along the [001] Direction Compared to the Ideal WO3(001) Surface 0.136(0.319)c 0.235 0.195(0.140) 0.190 0.201(0.341) 0.288 0.243(0.197) 0.237 0.005(0.044) 0.033 0.063(0.189) 0.091 0.013(0.064) 0.048 0.072(0.127) 0.095 0.039 (0.254) 0.144(0.131) 0.130(0.168) 0.143(0.121) −0.032 0.008(−0.065) 0.000 0.000(0.003) −0.016 0.019(−0.091) 0.029 0.029(0.021) −0.019 −0.038(0.000) −0.031 −0.016(−0.077) Bond Length (Å) 1.708(1.743) 1.708 1.711(1.720) 1.713 2.350(2.463) 2.416 2.342(2.163) 2.308 1.762(1.756) 1.758 1.765(1.821) 1.771 1.761 1.749(1.800) 1.743 1.744(1.742) 2.171 2.212(2.071) 2.223 2.207(2.247) +0.74 +0.51 +0.50 +0.40

a The meaning of each symbol can be found in Figure 2, and the symbol M stands for metal atom, namely, W or Ta atom. bThe negative and positive values indicate that the atom moves toward the bulk and vacuum sides, respectively. cThe values shown in parentheses correspond to the results of Ta or its neighboring atom.

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Figure 3. (a) Total density of states (DOSs) of the undoped and four Ta doped WO3(001) surfaces, (b) partial DOSs of different oxygen atoms on the Ta doped surfaces, (c) partial DOSs of different O atoms on the Nb−W6f doped surfaces, and (d) partial DOSs of different O atoms on the V− W6f doped surfaces. In b−d, the black, red, and blue lines correspond to the DOSs of the in-plane oxygen (Oi), top oxygen (Ot), and another top terminal oxygen atom just above the dopant, respectively. The vertical dashed line indicates the position of the Fermi level taken as zero energy. The DOSs shown are the sum of the corresponding spin-up and spin-down states obtained from the spin-polarized calculations except for the case of undoped surface.

the vacuum potential at the middle of the vacuum region and the Fermi level. As reported in Table 2, the value of the work function of four Ta doped models is raised more than 0.4 eV with respect to the pure WO3(001) surface. On the other hand, the increase of the work function also implies that the electronaccepting ability and the oxidation capacity of the WO3(001) surface are improved after doping. Moreover, as the most obvious increase of the surface work function (about 0.7 eV) is observed for the Ta−W6f model, it would possess the best oxidation performance among the four doping models. As mentioned above, the top valence band of pure WO3(001) surface mainly consists of the 2p orbitals of Oi atom, and so after losing one electron, the Oi 2p states can be expected to be the main compositions at the Fermi level. However, by analyzing the atomic partial DOSs of different oxygen atoms presented in Figure 3b, comparing to the pure WO3(001) surface, there is a trend that the DOS peaks of Ot atom for the Ta doped surfaces move toward the regions closer to the Fermi level. Especially, the Ta−W6f model can provide a direct comparison for the movement of Ot states because it

bottom of the conduction band, the states of W6f and W5f atoms are the main compositions. Compared with the undoped WO3(001) surface, the most distinct feature of the total DOS of the Ta doped WO3(001) surface (Figure 3a) is that the DOS peaks tend to move toward higher energy level. To estimate the value of the shifting of DOS, we use the localized O 2s states of one terminal oxygen atom at the bottom of the slab (Figure 2) as a reference to measure the movement of DOS. The predicted energy level shifts of the Ta−W6f, Ta−W5f, Ta−W6i, and Ta−W5i models are about 0.30, 0.29, 0.29, and 0.26 eV, respectively, indicating that for the Ta−W6f doped model the movement of the total DOS seems to be most pronounced. The above variation of the DOS is consistent with the downward shift of Fermi level because the system loses one electron after replacing a tungsten atom with a tantalum atom. By considering that the reducing of the Fermi level means the increasing of the surface work function, we also determine the work function of Ta doped surface to confirm the change of the position of the Fermi level. Here, the work function is calculated as the deviation between 5071



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Figure 4. The top (left) and side (right) views of the distribution of spin densities for (a) the Ta−W6f doped WO3(001) surface and (b) the Nb− W6f doped WO3(001) surface. Two kinds of radical oxygen centers which are located at the top oxygen and the in-plane oxygen atoms can be clearly observed, and in the side view, only two layers are presented.

doping occurs at W6f site, and now a sharp peak at −14.5 eV (see Figure 3b) is composed by 2s states of the Ot that is just above Ta atom. Because the 2s peaks of Ot are entirely separated from the bulk states and may be determined accurately by technologies of photoemission spectroscopy, we expect that these peaks could be used as fingerprints to distinguish the doping sites in experiments. The above results indicate that the doping has significant influence on the electronic characteristic of WO3(001) surface, in particular, the addition of the tantalum atom at the W6f site. Figure 4a shows the distribution of the spin density for the Ta−W6f model, and interestingly, the spin densities are mainly located on the top oxygen just above Ta atom, which also can be confirmed by comparing the spin-up and spin-down DOSs of this Ot atom (see Figure S2 of the Supporting Information). Therefore, only when a Ta atom substitutes a W6f atom at the top layer the radical oxygen centers found in the cationic W3O9+ or neutral MW2O9 clusters are reproduced. If using

contains two kinds of Ot atoms, which sit above W6f and Ta dopant (Figure 2). It is quite clear that the Ot atom above Ta shows a partially filled strong peak just at the Fermi level (see the region circled in Figure 3b); while similar to the undoped case, the 2p states of another Ot atom above the W6f site are located in the region below the Fermi level. As a result of the most obvious movement, instead of Oi atom, the 2p contributions of the Ot atom that directly bonds with Ta dopant become the dominated components at the Fermi level for the Ta−W6f doped surface. For the other three doping models, namely, Ta−W5f, Ta−W6i, and Ta−W5i, the main components at the Fermi level are still mainly originated from Oi atoms. In addition, accompanying the variation of the composition at the Fermi level, the position of the 2s peak of Ot atom is also sensitive to the doping site. When a W5f, W5i, or W6i atom is replaced by a Ta atom, the 2s level of Ot is shifted toward the Fermi level about 1.0 eV relative to the undoped surface. This value is further enlarged to about 2.0 eV when the 5072



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vanadium or niobium to replace the W6f atom, the distribution of the spin density is entirely different from the case of the Ta dopant. For instance, as presented in Figure 4b, for the Nb− W6f model, the spin densities are clearly still localized at the inplane oxygen atoms but not at the Ot atoms. Such divergence in the arrangement of the spin density is directly related to the components appearing at the Fermi level. After introducing niobium atom at the W6f site, although the 2p states of Ot atom bonded with Nb also move obviously toward the Fermi level, the corresponding DOS peaks are fully occupied and the compositions at the Fermi level are still dominated by the 2p states of Oi atoms (Figure 3c). Hence, the spin densities are mainly around the Oi atoms. The same reason also exists in the case of the V−W6f model in which the variation of the peak position of the Ot atom bonded with the V atom is small (Figure 3d). In fact, if we use the shift of the Ot 2s level as an indicator to measure the movement of the Ot atom just above the doping site, as labeled in Figure 3, the values in V−W6f, Nb−W6f, and Ta−W6f models are 0.64, 1.14, and 1.44 eV, respectively, indicating that the degree of the movement for the states of this Ot atom is increased in the same sequence. Consequently, the Ta−W6f doped surface exhibits the most significant upward shift of the states of the Ot atom above the doping site, and the corresponding DOS peak appears just at the Fermi level and becomes partially occupied. According to the above results, the electronic structure of the doped WO3(001) surface depends on not only the substituent site but also on the type of the dopant atom. Only when a Ta atom is used to replace a W6f atom the Ot atom close to the doping site is activated because of the formation of Ta−Ot· radical centers. The activation of this Ot atom can be verified in two ways. First, we have determined the energy required to remove a top oxygen atom from the surface (namely, the formation of the defect WO3(001) surface), and the result shows that the energy for eliminating an Ot atom belonging to Ta−Ot· radical center in Ta−W6f doped surface is about 2.27 eV smaller than in the case of the undoped surface. In other words, it is much easier to remove this Ot atom from the surface when a Ta atom is introduced in the W6f site. Second, as will be discussed in the next section, the Ta−W6f doped surface shows enhanced activity for the transfer of an Ot atom of Ta− Ot· radical center to the CO molecule with respect to the undoped surface. 3.3. Oxidation of CO Molecule on the Ta−W6f Doped WO3(001) Surfaces. To further investigate the improvement of the surface activity after Ta doping, carbon monoxide is selected as a probe molecule to study the difference in oxidation reactivity between the pristine and Ta−W6f doped WO3(001) surfaces. Before exploring the MEP for the reaction of CO on the surface, we first optimize the structures of the initial and final states in MEP. As displayed in Figure 5, in the initial configuration, the CO molecule tends to be weakly attached to the W5f atom at the top layer through the carbon atom for both undoped and Ta−W6f doped surfaces, and the distance between C and W5f atoms is more than 2.5 Å. The final state corresponds to the physisorption of CO2 product in which the CO2 molecule prefers to weakly interact with the Ta or W6f atom. From Figure 5, it is clear that the oxidation of CO on WO3(001) surface is quite exothermic, and the magnitudes of the reaction enthalpy for the pure and Ta−W6f doped surfaces are about 1.1 and 3.6 eV, respectively, indicating that the

Figure 5. Minimum energy paths (MEP) for the CO oxidation on the undoped (black line) and Ta−W6f doped (blue line) WO3(001) surfaces. The configurations of top layer and some bond distances (Å) of the initial and final states as well as the transition state (TS) are also shown.

thermodynamic driving force for the reaction on the Ta doping surface is stronger than on the undoped one. The calculated energy profiles for oxidation of CO on two surfaces by using CINEB method are presented in Figure 5. For the pure WO3(001) surface, a transition state (TS) is identified with an energy barrier of 0.91 eV, and the vibrational frequency calculation shows that there is only one imaginary frequency (273i cm−1) for this configuration. In this structure, the CO molecule is bonded with the Ot atom, and the length of the C− Ot bond is 1.561 Å. As a result of the formation of the C−Ot bond, the configuration of the WO3(001) surface is changed remarkably; especially, the W6f−Ot bond is elongated to 1.859 Å, which is about 0.14 Å longer than that in the initial state. Because of the considerable variation of the surface geometry with respect to the initial state (Figure 5), an obvious energy barrier is required to produce CO2 on the undoped surface. However, for the Ta−W6f doped WO3(001) surface, an entirely different structure is obtained for the TS for which also only one imaginary frequency of 376i cm−1 is predicted. Comparing to the initial image, although the CO molecule shows a tendency of moving from the position above W5f to near the Ot atom belonging to the Ta−Ot· radical center, the large distance (2.72 Å) between carbon and this Ot atom indicates that the interaction between CO and substrate is still weak in the TS. Correspondingly, the surface configuration is essentially unchanged, and the reaction barrier associated with this TS is only 0.27 eV, obviously lower (about 0.64 eV) than that of the undoped surface. The above obvious difference in the TS structures is related to the variation of the electronic structure after introducing Ta at W6f site. As mentioned in section 3.2, in Ta−W6f doped surface, the electronic states of Ot atom just above Ta are shifted toward the regions closer to the Fermi level with respect to the undoped surface (Figure 3b), which makes the transfer of this Ot atom to the CO molecule easier. Therefore, in the TS of Ta−W6f doped model, the CO molecule can directly move close to the Ot atom above Ta, and 5073



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the surface work function is increased compared to the undoped surface. In most cases, after doping, the 2p states derived from the Oi atom are still the dominate components at the Fermi level as before, and consequently, the unpaired electrons mainly distribute in the regions around the Oi atoms. However, the substitution of Ta for W6f atom is an exception in which the 2p states of the Ot atom connected with the Ta dopant become the primary component at the Fermi level owing to the most obvious upward shift of the electronic states of this oxygen atom. Correspondingly, the spin densities are now mainly located on the Ot atom just above the Ta atom, and the oxygen radical centers found in the gas-phase W3O9+ or in the neutral MW2O9 clusters are reproduced. It is interesting that unlike the gas-phase clusters, such arrangement of spin density is not observed if the other dopants, such as Nb or V, are employed. Thus, our results show that the variations of electronic structure introduced by doping are sensitive to not only the doping site but also the type of the dopant atom. In other words, compared with the small gas-phase clusters, it is more difficult to form specific radical oxygen centers on the solid surface because of its different configuration and electronic structure. Furthermore, the formation of the radical oxygen center on the top oxygen for the case of the W6f atom being replaced by a Ta atom also means the activation of the Ot atom, and the WO3(001) surface is expected to have much better performance for the oxidation reactions. As a typical example, considering the oxidation of the CO molecule, the predicted energy barrier for the formation of CO2 product is decreased obviously from 0.91 eV of the pure surface to 0.27 eV of the Ta−W6f doped surface. Therefore, similar to the experimental observation for the W3O9+ cluster,32 through controlling the generation of radical oxygen centers, the oxidation reactivity of the WO3(001) surface can be enhanced by introducing Ta dopant at the suitable site. Finally, although the actual preparation of Ta−W6f doped surface might be difficult because this doping model shows the relatively poor stability among four Ta doped surfaces, the radical center located at the surface top oxygen atom can be expected near the region with high concentration of tantalum atoms on the surface of the Ta−W mixed oxide.39

the small energy barrier associated with this TS is mainly used to overcome the weak interaction between C and W5f atoms at the top layer. However, for the undoped surface, the strong W6f−Ot bond means that the transfer of this Ot atom becomes more difficult, and consequently, in the corresponding TS the short bond distance is required for the newly formed C−Ot bond, and in the meantime, unlike the Ta−W6f doped model the weak C−W5f bond is still preserved to stabilize the structure. Therefore, our results confirm that because of the formation of surface Ta−Ot radical centers, the substitution of Ta atom at W6f site can effectively prompt the oxidation of CO molecule on the WO3(001) surface from both thermodynamical and kinetic points of view. In addition, a possible full catalytic cycle is proposed to make the Ta−W6f doped WO3(001) surface as catalyst for CO oxidation. As can be seen from the schematic representation of the catalytic cycle shown in Figure 6, the

Figure 6. Schematic representation of a possible full catalytic cycle to use Ta−W6f doped WO3(001) surface as catalyst for CO oxidation.

surface terminal oxygen is consumed during the oxidation of CO, and after the formation of CO2, the WO3(001) surface becomes defective with one less Ot just above the Ta atom (right side of Figure 6). When the reduced WO3(001) surface is exposed to the air, the O2 molecule can be adsorbed on the site above the Ta atom, and a kind of active O2− species is formed on the surface (see Figure S3 of the Supporting Information for the structure of the lower part of Figure 6), which may further react with the CO molecule, and the initial surface can be subsequently regenerated. However, more work is needed to understand the energy profile of the overall process.

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ASSOCIATED CONTENT

S Supporting Information *

The results of structural optimizations, including the surface relaxations and some bond lengths of Nb and V doped WO3(001) surfaces, the spin-up and spin-down total DOSs of different Ta doped surfaces, and atomic DOSs of different oxygen atoms in the Ta−W6f doped model. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In this paper, the structure, the electronic properties, and the oxidation activity of γ-WO3(001) surface doped by a series of group VB transition metal atoms have been investigated by means of first-principles DFT calculations combined with a slab model. A total of 12 configurations of doping models is considered, including the substitutions of VB metal atom for tungsten atom at the top layer and the sublayer. Our results indicate that the group VB element replaces tungsten atom preferentially under O-rich growth conditions and that the addition of Ta atom is energetically favorable among three group VB elements. Besides the surface relaxation, with respect to the pristine surface, the doping has some effects on the electronic structures. Because the surface loses one electron after replacing a tungsten atom with a group VB atom, the downward shift of the Fermi level is observed, and as a result,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Z.); [email protected] (X.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Y.-J. Zhao for valuable discussions. This work was supported by National Natural Science Foundations of China (grant nos. 21073035, 21071031, 90922022, 11174005, 21171039, 20773024, and 20771026). Y.Z. and W.C. also 5074



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would like to thank the programs for New Century Excellent Talents in University of Fujian Province (grant nos. HX200697, HX2006-103). We are grateful for the generous allocation of computer time at the high performance computer center of Fujian Province.

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Enhanced Oxidation Reactivity of WO3(001) Surface through the

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