DFT+U Study of Properties of MoO3 and Hydrogen Adsorption on

Article pubs.acs.org/JPCC

DFT+U Study of Properties of MoO3 and Hydrogen Adsorption on MoO3(010) Yan-Hua Lei and Zhao-Xu Chen* Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: MoO3 is an important catalytic material, and there exist controversial viewpoints about its surface structure, oxygen vacancy, and hydrogen adsorption, which are crucial for rationalizing the catalytic properties and reaction mechanism. To clarify these disputes, we adopted the density functional plus U (DFT+U) method to investigate properties of MoO3 bulk and surfaces and examined atomic hydrogen adsorption. Analyses reveal that the vibration peak at 820 cm−1 previously assigned to the vibration of asymmetrical oxygen is due to the vibration of symmetrical oxygen. On the other hand, the previously unassigned weak peaks at 899 and 723 cm−1 are caused by the asymmetrical oxygen stretching. Single hydrogen atom adsorbs favorably at asymmetric oxygen, while the terminal oxygen becomes the favorable position for accommodating two hydrogen atoms. The H atoms occupy preferentially asymmetrical oxygen at low coverage, whereas at high coverage they favorably reside on the terminal one. Our calculations indicate that different from the previous viewpoint, water binds to the terminal oxygen defective site relatively strongly. Furthermore, the controversial viewpoints about the stability ordering of oxygen vacancy under oxidation and reduction conditions is discussed on the basis of the formation energy of oxygen vacancy and water desorption energy on defect sites. of Mo(110) films under ultrahigh vacuum conditions exclusively generated bridging oxygen species, and only deeper oxidation can result in terminal oxygen.17,18 Although oxidation of Mo(110) may not produce a structure similar to the MoO3(010) surface, this result seems to indicate that the most stable defect sites, instead of the bridging oxygen sites, are the terminal oxygen. Consistent with the controlled oxidation experiment, PBE+U19 and PBE20 calculations on periodic models indicated the terminal oxygen defects to be the most stable, while the symmetric oxygen defects were the least favorable. The different stability ordering of oxygen defect sites in the TPR experiment and the oxidation condition is likely due to hydrogen adsorption. In fact, the interaction of hydrogen atoms with the surface of MoO3 is another important factor that affects the catalytic properties of MoO3.21,22 DFT-LDA calculations23−25 on hydrogen adsorption at the MoO3(010) surface show that hydrogen atoms adsorb most favorably at terminal oxygen on both perfect and terminal oxygen defective surfaces, followed by the asymmetric oxygen; the most unfavorable adsorption site of hydrogen is the symmetric oxygen. At variance, the DFT-PW91 functional predicts the asymmetric oxygen to be the most favorable site for H adsorption, followed by the terminal oxygen; the symmetric oxygen was the least favorable.26 It is worthy to mention that in

1. INTRODUCTION Molybdenum trioxide MoO3 is an important material and has been widely used in lithium batteries1 and information storage2 and as electrochromic components3 and heterogeneous catalysts.4 As a catalyst, it is often employed to activate the C−H bond of alkanes, producing selectively formaldehyde.5−8 When mixed with other oxides, MoO3 takes part in partial oxidation reactions such as the industrial production of acrolein, acrylonitrile, and acrylic acid.9−11 The catalytic properties of MoO3 are closely related to its surface structure and oxygen defects, which play important roles in catalysis processes. Oxygen defects are formed when oxygen atoms are transferred from the MoO3 surface to an adsorbate in oxidation reactions which follow the well-known Mars−van Krevelen mechanism.12 There are three distinct types of oxygen atoms in bulk MoO3: terminal (Ot), asymmetrical (Oa), and symmetrical (Os) oxygen atoms. The latter two are also called bridging oxygen atoms. Many experimental and theoretical investigations have been performed to study the stability (in this paper “stability” always refers to “thermodynamic stability”) sequence of different oxygen defects. Temperature programmed reduction (TPR) experiments indicate that the bridging oxygen atoms were the first to be reduced,13,14 implying that the defects at the bridging oxygen sites might be easily formed or bridging oxygen defects are more stable. Consistently density functional theory (DFT) RPBE calculations using cluster model showed that the most/ least stable defect sites are the symmetric bridging oxygen/ terminal oxygen.15,16 On the other hand, controlled oxidation © 2012 American Chemical Society

Received: April 29, 2012 Revised: November 11, 2012 Published: November 19, 2012 25757

dx.doi.org/10.1021/jp304122n | J. Phys. Chem. C 2012, 116, 25757−25764

The Journal of Physical Chemistry C

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Figure 1. Illustration of the structure of MoO3. Indigo spheres, Mo atoms; red spheres, O atoms.

predicted the total atomic spin density of 1.98e on the Mo atom when the bound terminal oxygen is removed, compared to no spin density with the PBE functional only. The electron−ion interaction was described by the projector augmented wave (PAW) method.35,36 The pseudopotential radii for O are 1.20 au for the s states and 1.52 au for the p states; the radii for Mo are 2.60 au for the s states, 2.75 au for the p states, and 2.50 au for the d states. The Kohn−Sham equations were solved using a plane-wave basis set with a cutoff energy of 400 eV. The Gaussian smearing method was adopted with a width of 0.2 eV to determine how the partial occupancies are set for each wave function. Dipole correction was considered where necessary. For bulk optimization, the sampling of the Brillouin zone was performed using a Monkhorst−Pack scheme37 of (5 × 3 × 5). The optimized lattice parameters with that grid are 3.91, 13.80, and 3.73 Å, which agree well with the experimental results27 and differ somewhat from the reported results in ref 19, likely due to the different optimization scheme used. For surface defects and hydrogen adsorption, we used (3 × 3) surface cells and (3 × 3 × 1) grids. A vacuum spacing of 12 Å was adopted. Previously a one metal oxide layer model was adopted for MoO3(010)23,26 and V2O5(001).38 To check the effect of slab thickness, we calculated the binding energy (Eads) of hydrogen atom on one side of the one-layer and two-layer slabs modeling perfect MoO3(010) surface. The averaged binding energy Eads is defined as Eads = [(n × Eh + Es) − Ehs]/n, where Eh is the energy of the isolated H atom, Es is the total energy of the clean slab, Ehs is the total energy of the slab covered with H atoms, and n is the number of H atoms per unit cell. With this definition, a positive binding energy denotes a favorable adsorption. The calculated binding energy difference between one-layer and two-layer models is less than 0.03 eV; we further calculated the vacancy formation energy at three different oxygen atom sites with one-layer and two-layer slabs. The vacancy formation energy, Ed (Ed = Edef + EO − Eperf, where Edef is the energy of defective surface, EO is the energy of a triplet oxygen atom in gas phase, and Eperf is the energy of perfect surface), differs by less than 0.08 eV. These results are not unexpected as MoO3 is a layered structure along the b direction and the interaction between layers is weak. Therefore, we used a one-layer slab model in our calculations. For vibrational

these studies H only singly adsorbs on an oxygen atom (i.e., only one H atom on an oxygen atom). In view of the importance of MoO3 and the existing discrepancies described above, we recently studied the stability of various defective MoO3(010) surfaces and the adsorption of hydrogen on the perfect and defective MoO3(010) surfaces. We show that different environments may result in different stability sequence of defective surfaces. In oxidization or inert situations, the terminal oxygen defect seems to be most stable, while in H2 atmosphere, the bridging oxygen defects are likely most favorable.

2. COMPUTATIONAL DETAILS The most stable structure of MoO3 is orthorhombic, belonging to the Pbmn space group with the experimental lattice parameters a = 3.964, b = 13.863, and c = 3.699 Å.27 It has layered structure along the [010] direction (Figure 1). In bulk the terminal oxygen only bonds to one Mo atom with a distance of 1.67 Å. The asymmetric oxygen atom links to two Mo atoms with 1.74 and 2.25 Å, respectively. The symmetrically bridging oxygen atom (symmetric oxygen) binds to three Mo atoms: two (named horizontal bond) with equal distance of 1.95 Å and one (called vertical bond) with a longer length of 2.33 Å (Figure 1). All the calculations were performed using the Vienna ab initio simulation program28,29 (VASP). Widely used approximations for the exchange and correlation energy in density functional theory (LDA or GGA) are mainly based on parametrization of nearly homogeneous electron gas. For the systems such as transition metal oxides, the electronic band gap may be underestimated and even a qualitatively wrong metallic ground state may be predicted. The DFT+U approach30,31 has proved to be able to study a large variety of strongly correlated compounds with considerable improvement with respect to LDA or GGA results. In this work, the DFT (Perdew, Burke, and Ernzerhof (PBE) functional32)+U approach of Dudarev et al.33 was applied to the molybdenum centers. A value of 6.3 eV was chosen for the U-J parameter.19 Using this value we calculated a reaction energy of 149 kJ/mol (corrected to 298 K) for MoO3 to MoO2 + 1/2O2 with U-J of 6.3, compared to the experimental result of 156 kJ/mol (at 298 K).34 PBE+U 25758



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frequency calculations, a (2 × 2 × 2) supercell and finite difference method were employed. A displacement of 0.02 Å was used for each degree of freedom.

vacancy is easier than that of asymmetric oxygen vacancy, which is easier than creating symmetric bridging vacancy. This ordering is the same as reported previously by Coquet and Willock,19 although there is a deviation of about 0.7 eV between the present results and the corresponding ones in ref 19. The reason for the difference is not very clear. We note, however, that different pseudopotentials and bulk parameters were used. However, RPBE calculations on the cluster models predicted the formation of terminal vacancy to be more difficult than the symmetric, which are more difficult than symmetric vacancy.15,16 The corresponding formation energies are 7.64, 7.09, and 6.81 eV, respectively.15 According to ref 19, this discrepancy is due to the inappropriate treatment of the Madelung potential using the cluster model, especially for the Os atom that has more ionic character. One might also suspect that the different ordering is due to the different types of basis sets (localized basis set in refs 15 and 16, and plane wave in refs 19 20 and the present work). However, the recent calculations using the localized basis set yielded an ordering consistent with that from plane wave calculations.42 Comparison of the computational details reveals that the different ordering is very likely because the structure was not relaxed in refs 15 and 16. As a support to our PBE+U result, we mention that terminal oxygen defects are most stable under oxidative atmosphere, compared to bridging oxygen defects, as evidenced by the experiment of controlled oxidation of metal Mo.17,18 It is worthwhile to point out that the TPR experiment shows that the bridging oxygen defects are formed preferentially,13,14 and this TPR result was used to corroborate the RPBE calculated results. Clearly the controlled oxidation experiment contradicts the TPR. We will discuss the discrepancy in subsection 3.3.2. Terminal sites and bridging oxygen seem to play different roles in catalytic reactions.13,14 For detective surfaces, we focus on terminal defect surface and only briefly discuss bridging oxygen defect surfaces for which some information is placed in the Supporting Information. The calculated bond lengths in Table 1 indicate that formation of the terminal oxygen defects has negligible influence on the Mo−Ot distances, no matter how close the Mo−Ot bonds are from the defect site. Notable changes caused by the terminal defect are inward displacement of the Mo1 (Mo1 refers to the Mo atom losing the terminal oxygen atom) and outward shift of the two asymmetric oxygen atoms Oa1 and Oa2 (Figure 2), which leads to elongated Mo1−

3. RESULTS AND DISCUSSION 3.1. Bulk and (010) Surface. As mentioned above, there are three types of oxygen atoms: terminal (Ot), asymmetric (Oa), and symmetric (Os) oxygen atoms in bulk and perfect (010) surface. Our calculated bond length of Mo−Ot in bulk is 1.71 Å, 0.04 Å longer than the experimental value.27 The horizontal Mo−Os bonds are 1.96 Å, and the vertical Mo−Os is 2.36 Å (Table 1, Figure 1). Two Mo−Oa bond lengths are 1.79 Table 1. Mo−O Bond Lengths of MoO3 Bulk and (010) Surfaces bond length (Å)

Ot defective (010)

bond type

bulka

(010)

bond type

bond length (Å)

Mo−Ot Mo−Os (horizontal) Mo−Os (vertical) Mo−Oa

1.71(1.67) 1.96(1.95) 2.36(2.33) 1.79(1.74) 2.14(2.25)

1.71 1.96 2.40 1.79 2.18

Mo3−Ot1 Mo1−Os1 Mo2−Os2 Mo1−Oa1 Mo2−Oa1 Mo1−Oa2 Mo3−Oa2

1.71 2.00 2.00 1.82 2.20 2.19 1.80

a

Values in parentheses are experimental results.

and 2.14 Å, respectively. All above calculated bond lengths are in good agreement with the experimental values within an accuracy of 5%.27 Formation of the surface hardly affects the Mo−O distances, as shown by the almost unchanged Mo−Ot, Mo−Oa, and the horizontal Mo−Os bond distances with respect to the corresponding ones in the bulk. Only the longer Mo−Oa and vertical Mo−Os bonds are stretched by about 0.04 Å (Table 1). Bader charge39 analyses (Table 2) reveal that the terminal oxygen Ot has the least negative charges, −0.64|e| in bulk. The Table 2. Formation Energies (Ed) of an Oxygen Vacancy and Bader Charges for MoO3 Bulk and (010) Perfect Surface charges (|e|) species

Ed (eV)

bulk

(010) surface

Mo Ot Oa Os

5.59 6.04 7.50

+2.42 −0.64 −0.82 −0.96

+2.38 −0.59 −0.81 −0.98

charges on symmetric oxygen Os are the most, −0.96 |e|, followed by the asymmetric oxygen atom Oa, −0.82|e|. Compared with the bulk, the negative charges on Os in the perfect MoO3(010) surface increase slightly while those on Ot and Oa atoms decrease with a maximum reduction of 0.05|e| on Ot (Table 2). The above calculated Bader charges show that the Mo−Os bond possesses more ionic character whereas the Mo− Ot has more covalent feature, consistent with the bond order analysis.40,41 The formation energies of oxygen defect on the (010) surface are calculated to be 5.59, 6.04, and 7.50 eV for the terminal, asymmetric, and symmetric oxygen defects, respectively (Table 2). According to the formation energies of oxygen defect, which are positive, formation of the terminal oxygen

Figure 2. Terminal oxygen defective surface. Indigo spheres, Mo atoms; red spheres, O atoms.

Oa1 and Mo1−Oa2. On the other hand, Oa3 that is away from the defect is essentially unaffected, as evidenced by the two almost unaltered Mo−Oa3 distances (not shown). The most pronounced change is calculated with Os, which shows the largest Mo−Os bond elongation of 0.04 Å, compared to that in bulk. In fact, Os atoms, even several Mo−O bonds away from the defect site, still display significant changes. For example, the 25759



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calculations. This group cannot reflect the symmetry with respect to the symmetrical center. Especially the coupling between the MoO5 group and the neighboring atoms surrounding the group were not treated appropriately because these surrounding atoms were fixed in the calculation. They reported 1023, 898, and 711 cm−1 for the experimental peaks at 996 (Ag), 820 (Ag), and 668 (B3g) cm−1, respectively, which are believed to be related to Ot, Oa, and Os atoms, respectively. It is notable that for the vibration of Oa, 898 cm−1 differs significantly from 820 cm−1, while the other two values agree with the experimental ones well, with a deviation of less than 43 cm−1. Thus we adopted the supercell technique, one of the main methods to calculate phonon dispersion,48,49 to recalculate the frequencies of bulk MoO3. We obtained 975 (Ag) for 996 cm−1 and 682 (B3g) for 668 cm−1. The frequencies involving the motion of Oa are computed at 899 (Ag) and 723 (Ag) cm−1, which differ much from the experimental 820 cm−1 previously ascribed to Oa vibration.47 We did calculate a vibration close to 820 cm−1, at 813 cm−1 (Ag). However, this vibration belongs to the antisymmetry stretching of Os, not Oa. In a word, our results suggest that the peak at 820 cm−1 is not related to Oa vibration as previously assigned, but to Os motion; the vibrational frequencies involving Oa are at 899 and 723 cm−1. As a support to our results, we mention that a weak peak at 894 cm−1 is observed,50 and when the samples were treated at high temperatures two additional peaks close to our calculated 723 and 899 cm−1 were detected.51,52 3.3. Hydrogen Adsorption on Perfect MoO3(010) Surface. 3.3.1. At 1/9 Coverage. Table 4 compares the

Mo2−Os2 (Figure 2) bond is elongated from 1.96 to 2.00 Å (Table 2). The above Mo−O bond variation induced by the defect can be rationalized with the ionic characters: the more ionic a Mo−O bond is, the more pronounced the influence is. Asymmetric oxygen defect has pronounced effect on the vicinal Mo−Oa and Mo−Os bond length (Table S1 and Figure S1). For example, Oa1, which binds to the Mo atom closest to the defect site, links to two Mo atoms with essentially equal distance, 1.88 and 1.86 Å, compared to 1.79 and 2.18 Å in the perfect surface. Formation of symmetric oxygen defect negligibly affects the vicinal Mo−O bond length. All the bond lengths except the vertical Mo−Os bond distance change less than 0.02 Å. 3.2. Raman Spectra. The Raman spectra of MoO3 are often used to distinguish different types of oxygen atoms,43−46 because they are associated with the vibrations of terminal, asymmetric, and symmetric oxygen atoms.19,47 Figure 3 depicts

Table 4. Calculated Binding Energy of Hydrogen Adsorbed at Terminal (T), Asymmetrical [in (010) Plane, Ai, and Perpendicular to the Plane, Ap], and Symmetrical (S) Oxygen Atoms on MoO3(010) Surface at 1/9 Coverage and PBE+U Calculated Mo−O (dMo−O) and O−H Bond Length (dO−H) and H−O Stretching Frequency (Freq)

Figure 3. Calculated vibration modes with 2 × 2 × 2 supercell. Only one layer is shown. The crystal direction of C, E is the same as that of A; the crystal direction of D is the same as the one for B. Indigo spheres, Mo atoms; red spheres, O atoms.

type

Eads (eV)a

dMo−O (Å)

dO−H (Å)

Freq (cm−1)

Eads (eV)b

Ai Ap T S

2.99/3.23/2.68 2.82/3.19/2.63 2.77/2.86/2.37 2.11/2.38/2.03

2.08, 2.21 2.11, 2.21 1.92 2.09, 2.09, 2.49

0.981 0.977 0.978 0.995

3562 3668 3646 3333

2.91 2.67 2.45 2.10

some of our calculated vibrational modes using a (2 × 2 × 2) supercell. The calculated frequencies are listed in Table 3, a

a/b/c represent the binding energies calculated with PBE+U, LDA, and PBE methods, respectively. bReference 26

Table 3. Calculated Frequencies for Vibrational Modes in Figure 3 and the Corresponding Experimental Frequencies (cm−1)

a

vibration mode

calcd this work

exp

A B C D E

975 899 813 723 682

996 − 820 − 668

a

b

c

calculated binding energies per H atom on perfect MoO3(010) surface with different methods. The coverage is defined as the ratio of the number of hydrogen atoms to the number of Mo atoms per surface unit cell. The binding energies are given relative to free hydrogen atom. To help readers, we provide the bond energy of H2 here, 4.54 eV, which is comparable to the experimental one of 4.75 eV. Not unexpected, LDA predicts the largest binding energies. GGA-PBE produces the smallest ones. Values from PBE+U are intermediate between those of LDA and GGA. In all cases, our calculated results show that the most stable site for atomic H adsorption is Oa, followed by Ot. Our predicted ordering is consistent with that obtained with the PW91 method,26 but different from the LDA results of Chen et al.23 As PBE+U better describes the systems, following discussions are referred to PBE+U results. Two adsorption configurations are determined at Oa: Ap, in which the formed Oa−H bond is normal to the (010) surface,

d

exp

exp

calcd

993 894 (weak) 822 − 664

990 888 818 724 (weak) 660

1023 898 711 746 520

Reference 47. bReference50. cReference52. dReference19

together with the experimental results. The symmetry of the point group of MoO3 is D2h,16 and symmetry operations include E, 3C2, i, and 3σ. The vibrations with gerade symmetry with respect to the center of symmetry i (Figure 1) are Raman active, while those having ungerade symmetry are IR active. In the previous calculation,19 only atoms in the MoO5 group in the slab model were allowed to relax in the course of frequency 25760



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and Ai, where the Oa−H bond lies in the (010) plane. The binding energies are computed at 2.82 (Ap) and 2.99 (Ai) eV. Upon adsorption, the shorter Mo−Oa bond is elongated from 1.79 to 2.11 (2.08 Å) for Ap (Ai), while the long Mo−Oa bond extends to 2.21 Å (Table 4). The O−H bond lengths in Ap and Ai configurations are 0.977 and 0.981 Å, respectively. Adsorption on the terminal oxygen, labeled as T in Table 4, gives a binding energy of 2.77 eV. The Mo−Ot bond, 1.92 Å, is 0.21 Å longer than that of the clean surface. The O−H bond length is 0.978 Å. The least stable site for the adsorption of atomic H is Os, labeled as S in Table 4. The binding energy, 2.11 eV, is more than 0.6 eV lower than those on asymmetric and terminal oxygen atoms. This site also gives the longest observed O−H bond length, 0.995 Å, which is consistent with the low binding energy. The above calculated binding energies indicate that the bonding ability of the three types of oxygen atoms follows the sequence of Oa > Ot > Os. This sequence can be rationalized with the centroid of p bands of the oxygen atoms, an extended concept of d band centroid.53 Our calculated p band center for Oa, Ot, and Os is at −2.33, −2.35, and −2.75 eV, respectively (Figure S2 and S3), in agreement with the binding energy ordering of H atoms. The computed frequencies of the O−H stretching range from 3333 to 3668 cm−1 (Table 4). The highest frequency 3668 cm−1 is calculated for Ap; H−Os has the lowest frequency of 3333 cm−1. The stretching frequencies of O−H for T and Ai configurations are 3646 and 3562 cm−1, respectively. The magnitudes of the frequencies correlate linearly with the O−H bond length with a correlation coefficient 0.995. Nevertheless, the magnitude of the O−H frequencies does not correlate with the hydrogen binding energy because vibrational frequency reflects a local property of the O−H bond while the binding energy embodies a global stability of the system. For example, the binding energy of Ai, 2.99 eV, is larger than that at the terminal oxygen site, 2.77 eV. The frequency of the O−H stretching for the former, 3562 cm−1, is smaller than 3646 cm−1 for the latter (Table 4). 3.3.2. At 2/9 Coverage. At 2/9 coverage there are two H atoms in one unit cell. The two H atoms may occupy two separate oxygen atoms or share one oxygen atom. We first consider the adsorption on two separate oxygen atoms. We found if hydrogen atoms adsorb on the two oxygen atoms that do not bind to the same Mo atom, the mean binding energy per H atom is essentially the averaged value at 1/9 coverage (i.e., the lateral interaction is negligible). For example, the binding energy per H atom on two neighboring terminal oxygen is 2.76 eV (T+T in Table 5), compared to 2.77 eV (Table 4). On the other hand, if adsorption occurs on the two oxygen atoms bonding to the same Mo atom, the averaged binding energy is larger than the averaged result at 1/9 coverage. For instance, the binding energy of 2.98 eV for the H coadsorption on asymmetric and terminal oxygen (Ai+T in Table 5) is larger than 2.88 eV (= 0.5 × (2.99 + 2.77) eV; 2.99 and 2.77 eV are binding energies at Ai and T, Table 4). Concerning two H atoms sharing one oxygen atom, three situations exist, depending on the type of oxygen atoms: namely, coadsorption on Ot, Oa, and Os. Coadsorption on Ot, labeled as T−2H in Table 5, yields a binding energy per H atom of 3.00 eV, 0.23 eV higher than that for separate adsorption at 1/9 coverage, indicating that coadsorption enhances the adsorption. Compared with the Mo−Ot bond of 1.92 Å for one hydrogen atom adsorption at the Ot atom, adsorption of additional H atom elongates the Mo−Ot further

Table 5. Calculated Binding Energy of Hydrogen Adsorbed on MoO3(010) Surface at 2/9 Coverage, Mo−O (dMo−O) and O−H Bond Length (dO−H), and O−H Stretching Frequency (Freq)a type

Eads (eV)

T+T Ai+T Ai+S T+S T−2H A−2H S−2H

2.76 2.98 2.60 2.52 3.00 (1.19) 2.53 (0.80) 1.87 (0.84)

dO−H (Å) 0.981, 0.981, 0.982, 0.982, 0.982, 0.991, 0.982,

0.984 0.985 0.994 0.996 0.984 0.996 0.989

Freq (cm−1) 3615, 3561, 3558, 3604, 3656, 3459, 3660,

3579 3578 3366 3344 3563, 1605 3333, 1543 3478, 1529

dMo−O (Å)

2.22 2.24, 2.29 2.41, 2.41

a Ai+T denotes that two H atoms each adsorb at Ai and T sites; T−2H means that two H atoms share one terminal atoms. Similar notations can be interpreted accordingly. The value in parentheses is the binding energy of water on the corresponding defective surface.

to 2.22 Å. The distance of two Ot−H bonds is 0.98 Å with the angle of H−Ot−H being 107°. On Oa the averaged binding energy is 2.53 eV, compared to 2.99 eV for separate adsorption at Ai and 2.82 eV at Ap. The two Mo−Oa bonds become 2.24 and 2.29 Å (Figure 4). Coadsorption of two H atoms on an Os

Figure 4. Two hydrogen atoms adsorbed on one oxygen atom. Indigo atoms, Mo; red atoms, O; white atoms, H.

makes the oxygen atom displace outward significantly, which elongates the horizontal Mo−Os bond from 1.96 to 2.41 Å, and the two adjacent terminal oxygen atoms tilt toward two sides (Figure 4). The Mo−Os−Mo angle was 140°on the clean surface but inverts to 122° after the outward displacement of the bridging oxygen. The binding energy per H atom is only 1.87 eV (Table 5), compared to 2.11 eV for the separate adsorption. 25761



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presence of hydrogen the polar oxygen terminated surface is more stable.62 It is worthy to point out that this conclusion is solely based on the calculated reaction energies of H2(MoO3)n to (MoO3)n−1MoO2 + H2O at 0 K, which is a thermodynamic property. The TPR process is a complex process that may involve adsorption and dissociation of hydrogen, water desorption, and lattice oxygen diffusion, as well as different surfaces. To reliably rationalize the dependence of defect site stability on the environment such as hydrogen, discussion should be based on free energies (instead of the total energies at 0 K as used here) and activation energies, which are beyond the scope of the present paper. 3.4. Hydrogen Adsorption on Defect Surfaces at 1/9 Coverage. Binding energies of hydrogen on oxygen defect surfaces are listed in Table 6. Hydrogen adsorbed at Mo1 atom

Our calculated binding energies for two H atoms on one Ot (Oa and Os) are larger (smaller) than the separate adsorption. To unravel the reason behind this phenomenon, we performed Wiberg bond order analysis.54 It is found that the total Mo−O bond orders (thus Mo−O bond strength) increase when one H adsorbs. When one more H atom comes to the terminal oxygen Ot, the Mo−O bond order increases further (with respect to one H adsorption). On the other hand, when the coadsorption site is one Oa or Os atom, the total Mo−O bond orders decrease. These results show that the increased/reduced binding energies for H coadsorption on terminal/bridging oxygen atoms are due to the increased/decreased total Mo−O bonding. Above results show that with the increase of surface H content the most favorable adsorption site of H atom changes from Oa site at low H coverage to Ot position at high H coverage. Experimentally, three thermodynamically stable structures of HxMoO3 were identified: structure I (0.23 < x < 0.40), structure II (0.85 < x < 1.04), and structure III (1.55 < x < 1.72). When the content of hydrogen atom was relatively small (structure I), H atoms are mainly located at asymmetric Oa oxygen site,55 forming a zigzag structure. On the basis of thermochemical data of structure I, the binding energy per hydrogen atom is estimated to be about 2.8 eV.56,57 At high content of hydrogen (structure III), the structure with two hydrogen adsorbed on terminal oxygen site dominates,58,59 consistent with our calculations. Note coadsorption of two H atoms on one oxygen atom can also be viewed as a water molecule adsorbing at the corresponding oxygen defect sites. According to our calculations, the energies needed to remove a water molecule from Oa and Os defect sites are 0.80 and 0.84 eV, respectively (Table 5). In disagreement with previous conclusion that water binds to Ot defect site weakly,60 our calculated binding energy of water at Ot defect site is 1.19 eV, 0.3 eV more than the ones for bridging oxygen defect site. The present results of hydrogen adsorption can shed light on the different stability ordering of oxygen defects. We have pointed out that in oxidation or inert atmosphere, the terminal oxygen atom is the easiest to be removed (in other words, terminal oxygen defect site is the most stable), which is in accord with the smallest vacancy formation energies of 5.59 eV (Table 2). However, in the TPR experiments, defect site occurs favorably on bridging oxygen sites (the bridging oxygen defects are more stable).13,14 The different stability ordering of oxygen defects in the TPR and oxidation condition can be rationalized as follows. Coadsorption of H atoms on one oxygen atom leads to formation of water. In this case the formation sequence of defect sites is determined by desorption of the formed water (i.e., the defect stability ordering depends on the binding energy of water molecule). Since the binding energy of water is H2Oa (H2Oa indicates the water at the defective asymmetrical oxygen site) ≈ H2Os < H2Ot, formation of bridging oxygen defect is preferred over the terminal one. Hence in the TPR experiment, formation of bridging oxygen defects is easier than that of terminal sites. In fact, the energy to break the initial Mo−O bond in the TPR experiments was estimated to be 0.83 eV,61 in nice agreement with our calculated binding energy of H2Oa and H2Os, 0.80 and 0.84 eV. In oxidation condition, the ordering is determined by the formation of Mo−O bonding. The relationship between surface stability and environmental condition is well-known. For example, the nonpolar (0001) surface of α-Al2O3 is stable in UHV condition whereas in the

Table 6. Binding Energies of Hydrogen Adsorbed on Oxygen Defective Surface Eads (eV)

site

terminal vacancya Mo1 Ot1 Ot2 Os1 Os2 Oa1-i Oa1-p Oa2-i Oa2-p Oa3-i Oa3-p

1.83 2.71 2.78 2.40 2.48 3.28 3.25 3.20 3.07 2.94 2.86

site

Eads (eV)

asymmetric vacancyb Mo Ot Oa-i Os

2.28 3.22 2.73 2.29

symmetric vacancyb Mo 2.88 Ot 3.15 Oa-i 3.09 Os 2.48

a

See Figure 2 for the labeled atoms. bOnly binding energies at the sites nearest to the vacancy are listed.

(Figure 2) of terminal defect surface gives the smallest binding energy of 1.83 eV, and the Mo−H bond length is 1.71 Å. The stretching frequency of Mo−H is 1904 cm−1. It should be noted that hydrogen cannot adsorb at Mo atom in perfect surface. Our calculations show that the terminal oxygen defect has negligible influence on hydrogen adsorption at terminal oxygen atoms as shown by the binding energies, 2.71 and 2.78 eV (Table 6), which are close to 2.77 eV on the perfect surface (Table 4). Contrarily, adsorption at bridging oxygen atoms is significantly affected by terminal defects. For example, the calculated binding energies are 2.40 (Os1) and 2.48 (Os2) eV, compared to 2.11 eV (Table 4) on the perfect surface. The computed binding energies on Oa1 and Oa2 range from 3.07 to 3.28 eV, larger than 2.99 and 2.88 eV for the perfect surface (Table 4). On Oa3, the calculated binding energies, 2.94 and 2.86 eV, are close to values on the perfect surface. The variation of H binding energy at Oa site with respect to the values on the perfect surface can be rationalized with the distance of the adsorption site from the defect site: the closer to the defect site, the larger the binding energy. On the terminal oxygen defective surface, the sequence of binding energy is similar to that on the perfect surface: Oa-i > Oa-p > T > S. Previously LDA predicted a different sequence T > Oa > S for H adsorption on the terminal oxygen defective surfaces.25 This discrepancy may be due to the different computational methods. Table 6 also shows the results of H adsorption on bridging oxygen defect surfaces. Some differences for hydrogen adsorption can be noted between the bridging oxygen defect 25762



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surfaces and the terminal oxygen defective surface. The first is that hydrogen adsorption becomes stronger to Mo atoms on the bridging defect surfaces, as indicated by the binding energies that are 0.45/1.05 eV larger on the Mo sites of asymmetry/symmetry oxygen defect surfaces than on the terminal defect surface. The second is the most favorable adsorption site changes from Oa on terminal defect surfaces to Ot on bridging oxygen defect surfaces.

ASSOCIATED CONTENT

S Supporting Information *

One additional table and two figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS The DFT+U method has been used to study the properties of bulk MoO3(010) surface and hydrogen adsorption on it. Our investigations imply that the different ordering of surface oxygen defects in oxidization or inert atmosphere and reduction condition is likely due to different formation mechanisms: in the oxidation or inert atmosphere the ordering depends on the removal of oxygen atom while in H2 reduction condition, removal of water molecule plays the role. We examined the Raman spectra of MoO3. Different from previous assignment, our analyses indicate that the experimental peak at 820/668 cm−1 is attributed to the vibration of the two symmetric oxygen atoms along opposite/same directions (relative to the Mo atom). Our study calls for further experimental identification of the Raman spectra of MoO3. Single H atom adsorbs most stably on asymmetric oxygen atom, whereas terminal oxygen atom is the most favorable site for accommodating two H atoms. The average binding energies of two H atoms sharing one terminal oxygen atom are larger than that for one H atom on the terminal site. On the other hand, the binding energies for two H atoms on one bridging oxygen atom are reduced with respect to that for one H adsorption. Our analyses show that this phenomenon is related to the variation of the Mo−O bonding strength. According to our results, atomic H preferentially resides on Oa in low coverage and Ot in high coverage. The sequence of binding energies for single hydrogen on terminal oxygen defective surface is similar to that on the perfect surface. Defects enhance H adsorption at bridging oxygen that are close to the defect significantly, while defects exhibit negligible influences on H adsorption at terminal oxygen.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel. +86-25-83593353; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Natural Science Foundation of China No.20973090 and the National Key Basic Research Development Program of China (973 Program) 2011CB808604 and 2009CB623504. All calculations were done in the high performance calculation center of Nanjing University. 25763



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DFT+U Study of Properties of MoO3 and Hydrogen Adsorption on

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