Consecutive H2 Oxidation Mediated by Au2VO4+ Clusters - The

Article pubs.acs.org/JPCC

Consecutive H2 Oxidation Mediated by Au2VO4+ Clusters Xiao-Hong Zhou,† Zi-Yu Li,*,‡ Tong-Mei Ma,*,† and Sheng-Gui He‡ †

School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510641, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: Laser-ablation generated gold−vanadium oxide cluster cations Au2VOx+ (x = 3, 4) were mass-selected by a quadrupole mass filter and interacted with H2 in an ion trap reactor. The reactions were characterized by mass spectrometry and density functional theory calculations. The Au2VO4+ cluster can oxidize two H2 molecules consecutively to produce Au2VO3+ and then Au2VO2+, the latter of which can react with one O2 molecule to regenerate Au2VO4+. Therefore, a catalytic cycle for H2 oxidation by O2 mediated with the Au2VO2+ clusters can be proposed. The theoretical studies predict that the gold atoms in both Au2VO4+ and Au2VO3+ are the active adsorption sites and facilitate the heterolytic cleavage of H2 in collaboration with the separate O2− ions of the vanadium oxide support. The consecutively oxidative reactivity of Au2VO4+ is attributed to the activation and dissociation of the superoxide or peroxide species (O2•− or O22−) into two reactive atomic oxygen species (O2−) with the promotional effect of gold atoms acting as electron donors. This study provides molecular-level insights into the elementary mechanisms of the H2 catalytic oxidation on the surface. extensively studied.35−38 It has been revealed that the atomic oxygen radical anions (O−•) are identified as the active species to activate H2 in a homolytic manner in the reactions of OsO4+ and V4O10+ clusters with H2.35,36 Moreover, in the TaO4 + H2 reaction systems, the M−O unit was also viewed as the active site to induce heterolytic cleavage of the H−H bond.37 Recently, the lattice oxygen ions (O2−) in the gold-containing oxide clusters have also been demonstrated to be reactive for H2 oxidation in collaboration with the separate gold species, in which the H−H bond cleavage follows a heterolytic cleavage manner.38 It is noteworthy that the oxygen species in the above-mentioned reactive clusters are O•− and O2− ions, and only one H2 molecule can be oxidized. Herein, we report the study of consecutive H2 oxidation by the Au2VO4+ cluster which contains superoxide or peroxide species (O2•− or O22−). This study provides molecular-level insights into the elementary mechanisms of the catalytic H2 oxidation reactions on the surface.

1. INTRODUCTION The catalytic oxidation of H2 is of great interest due to its role in H2O2 synthesis, catalytic oxidation of hydrocarbons, and the selective removal of CO from hydrogen streams1−5 as well as for its simplicity which makes it ideal for fundamental bond making and breaking studies.6 Supported gold catalysts have attracted much attention because of their excellent catalytic performance including the reactions involving hydrogen.7−10 Despite the hot topics regarding the nature of active sites, the catalytic role of gold, as well as the oxide supports,11−16 the mechanism about the activation of molecular oxygen17,18 and the catalytically active oxygen species19,20 is still a crucial issue for better understanding of the H2 oxidation catalysis. Superoxide radicals (O2•−), peroxides (O22−), atomic oxygen radical anions (O•−), and lattice oxygen (O2−) are four typical oxygen species involved in the O2 activation and dissociation processes: O2 → O2−• → O22− → 2O−• → 2O2−.21,22 It has been proposed that the oxygen species is usually supplied by the oxide supports in gold catalysis and shows high reactivities in the oxidation reactions of H2 molecule. However, owing to the complexity of the catalyst surface and the difficulty to characterize the oxygen species on the surface, the mechanistic details for the reaction of H2 with oxygen species in the surface reactions are still far from clear. Therefore, it is very necessary to adopt alternative ways to uncover the mechanism of H2 oxidation by the oxygen species at the molecular levels. Gas-phase atomic clusters that can be studied under wellcontrolled and reproducible conditions serve as ideal models that can be used to reveal the nature of active site and the mechanism of catalysis at a molecular level.23−34 The reactions of H2 molecules with atomic clusters including homonuclear oxide clusters and heteronuclear oxide clusters have been © 2016 American Chemical Society

2. METHODS 2.1. Experimental Methods. The experiments were conducted by a time-of-flight mass spectrometer (TOF-MS)39 coupled with a laser ablation cluster source, a quadrupole mass filter (QMF),36 and a linear ion trap (LIT) reactor.40 The Au2V18Ox+ clusters (x = 3, 4) were generated by laser ablation of a mixed-metal disk compressed with Au and V powders (molar ratio Au/V = 1/1) in the presence of 0.5% O2 seeded in Received: March 10, 2016 Revised: April 22, 2016 Published: April 22, 2016 10452

DOI: 10.1021/acs.jpcc.6b02515 J. Phys. Chem. C 2016, 120, 10452−10459

Article

The Journal of Physical Chemistry C

Restricted calculations are employed for the singlet states and unrestricted calculations are adopted for the triplet states in this work. To find out the cross point in the spin conversion, relaxed potential energy curve (PEC) of the unrestricted triplet electronic state is obtained by fixing RAu−Au at different values and optimizing all other degrees of freedom of the cluster. The partially optimized geometries from the triplet PEC were used for singlet-point energy calculations of the restricted singlet electronic state. The position of cross point represents an upper limit of the minimum energy required for conversion between two different electronic states. (Note that two multidimensional potential energy surfaces cross at multidimensional seems rather than a single point.)

a He carrier gas with a backing pressure of 6.5 standard atmospheres. A 532 nm (second harmonic of Nd3+: yttrium aluminum garnet-YAG) laser with energy of 5−8 mJ/pulse and repetition rate of 10 Hz was used. The gas was controlled by a pulsed valve (General Valve, series 9). To prevent the formation of undesirable hydroxo species by residual water in the gas-handing system, the prepared gas mixture (O2/He) was passed through a 10 m long copper tube coil at low temperature (T = 77 K) before entering into the pulsed valve. The clusters formed in a gas channel were expanded with a supersonic jet into the quadrupole mass filter through a 4 mm diameter hole in an electric shielding plane. To mass-select the clusters of interest, appropriate combinations of dc (direct current) and rf (radio frequency) potentials were applied to the quadrupole rods. To well resolve Au2VOx+ (x = 3 and 4) from AuV3Ox+6 and increase the ion transmittance of the QMF, the 18 O2 was used as the oxygen source to generate the clusters. The mass-selected cluster ions passed through an ion focusing assembly and a 4 mm diameter hole and entered into LIT formed by a set of hexapole rods and two cap electrodes. Slightly before the cluster ions enter into the trap, a pulse of helium gas is delivered through a second valve into the trap so that the clusters can be cooled in terms of translation and vibration. It is noteworthy that the cooling gas (He, maximal instantaneous pressure around 5 Pa) helps very much to confine the ions in the trap. After confining the cluster ions in the trap for about 0.9 ms, the reactant gas molecules (H2 or O2) are delivered through a third pulsed valve to interact with the clusters. Note that higher pressure of the cooling gas and longer cooling time (>0.9 ms) did not affect the reaction efficiency,40,41 indicating that the Au2VO3−4+ ions had been thermalized before they reacted with hydrogen. The partial pressures of the reactant molecules ranged from about 1 to 10 mPa, depending on the nature of reaction systems. The temperature of cooling gas (He), reactant gas H2, and the LIT reactor was around 298 K. After interacting for a period of time, the reactant and product ions are ejected from the trap and focused into the homemade reflectron TOF-MS for mass and abundance measurements. 2.2. Theoretical Methods. Density functional theory (DFT) calculations using the Gaussian 0942 program were carried out to investigate the mechanistic details of catalytic H2 oxidation mediated by Au2VOx+ clusters (x = 2−4). The TPSS functional43 has been proved to perform well for the Au−V−O system;44,45 thus, the results by TPSS were given throughout this work. The TZVP basis set46 for V, O, H atoms and a D95 V basis set47 combined with the Stuttgart/Dresden relativistic effective core potential (denoted as SDD in Gaussian software) for the Au atom were used in all the calculations. A Fortran code based on a genetic algorithm48 was used to generate initial guess structures of Au2VO2−4+. The reaction mechanisms were studied for Au2VO3,4+ + H2 and Au2VO2+ + O2 systems. The relaxed potential energy surface scan was used extensively to obtain good guess structures for the intermediates and the transition states along the pathways. The transition states were optimized using the Berny algorithm.49 Intrinsic reaction coordinate (IRC) calculations50,51 were performed so that each transition state connects two appropriate local minima. Vibrational frequency calculations were carried out to check that intermediates and transition states have zero and only one imaginary frequency, respectively. The zero-point vibration corrected energies (ΔH0) in units of electronvolts (eV) are reported in this work.

3. RESULTS 3.1. Experimental Results. The TOF mass spectra for the interactions of the mass-selected Au2V18O3,4+ cluster ions with H2 and D2 are shown in the Figure 1. It is noticeable that

Figure 1. TOF mass spectra for interactions of mass-selected Au2V18O4+ (a) and Au2V18O3+ (d) with H2 (b and e) and D2 (c and f) in an ion trap reactor for about 1.6 ms. The reactant gas pressures are given, and the AuxVy18Oz+ is labeled as x, y, z. Three signals marked with ∗,⧫, and ◊ can be assigned to Au2V18O4H2O+, Au2V18O2H2O+, and Au2V18O3H2O+, respectively.

Au2VO4+ can react with a trace amount of H2O impurity in the gas-handing system to generate Au2VO2H2O+ (Au2VO4+ + H2O → Au2VO2H2O+ + O2) and Au2VO4H2O+ (Au2VO4+ + H2O → Au2VO4H2O+) or dissociate into Au2VO2+ (Au2VO4+ → Au2VO2+ + O2) directly (Figure 1a), which indicates that the prepared Au2VO4+ clusters contain the O−O unit. Upon the interaction of Au2VO4+ with H2, the coappearance of Au2VO3+ and Au2VO2+ can be identified (Figure 1b). This indicated that Au2VO4+ may oxidize two H2 molecules consecutively to produce Au2VO3+ and Au2VO2+. In addition to major products Au2VO3+ and Au2VO2+, other minor products Au2VO2H2+, Au2OH+, Au2H+, and VO3H2+ were also observed in the reaction of Au2VO4+ with H2. The Au2VO3+ cluster is also reactive with H2 to generate Au2VO2+, Au2H+, and VO3H+ (Figure 1e). According to the experimental results of Au2VO3+ + H2 and the pressure-dependent experiments, it can be concluded that the formations of Au2VO2+, Au2H+, and VO3H+ 10453


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of a least-squares fitting procedure,52 the pseudo-first-order rate constants (k1) for the reactions of Au2VO3+ + H2 and Au2VO4+ + H2 were estimated to be (11.21 ± 0.25) × 10−10 cm3 molecule−1 s−1 and (11.24 ± 0.26) × 10−10 cm3 molecule−1 s−1, respectively. The theoretical collision rate constants [kcollision = 2π (e2α/μ)1/2, in which e is the charge of the cluster ion, α is the electric polarizability of the reactant molecule, and μ is the reduced mass,53] of reactions (1) and (2) are calculated to be both 1.5 × 10−9 cm3 molecule−1 s−1, corresponding to the reaction efficiencies (Φ = k1/kcollision) of (74.9 ± 1.7)% and (74.7 ± 1.7)%, respectively. The kinetic isotope effects (KIE) of reactions 1 and 2 are 1.3 and 1.4, respectively. 3.2. Theoretical Results. 3.2.1. Structures of Au2VO2−4+. Density functional theory calculations are performed to predict the ground-state structures of Au2VO2−4+ and understand the mechanisms of reactions 1 and 2. The low-lying isomeric structures of Au2VO2−4+ are presented in Figure 3. The lowest-

shown in Figure 1b are due to the secondary reaction of product ions Au2VO3+ with H2. The following primary reaction channels are suggested: Au 2VO4 + + H 2 → Au 2VO3+ + H 2O +

(86 ± 2)%

(1a)

→ Au 2OH + VO3 H

(13 ± 2)%

(1b)

→ Au 2VO2 H 2+ + O2

(4 ± 1)%

(1c)

Au 2VO3+ + H 2 → Au 2VO2+ + H 2O

(66 ± 2)%

(2a)

→ Au 2H+ + VO3 H +

→ VO3 H 2 + Au 2

(21 ± 1)% (13 ± 1)%

(2b) (2c)

The above reaction channels were also confirmed by using the isotopic labeling experiment with D2 (Figure 1, parts c and f). According to the assumed reaction models shown in Figure 2, parts a and b, the relative ion intensities of the reactants and products in reactions of Au2VO3+ + H2 and Au2VO4+ + H2 with respect to the H2 pressures P can be well fitted, and the results are plotted in Figure 2, parts c and d, respectively. On the basis

Figure 3. DFT-calculated isomers of (a) Au2VO4+, (b) Au2VO3+, and (c) Au2VO2+. The relative energies in electronvolts are given under each structure isomer. Some bond lengths in picometers are given. The superscripts indicate the spin multiplicities for the isomers.

lying isomer of Au2VO4+ (3IS1, Figure 3a) has an open-shell electronic structures which contains a superoxide unit (O2•−) and a gold dimer. Furthermore, another isomer 1IS6 with a closed-shell electronic structure and separate gold atoms was also determined as candidates for the ground-state structures of Au2VO4+ since it has close energy to 3IS1. The previous study45 has determined the ground-state structure of Au2VO3+ cluster (1IS4, Figure 3b), which has a closed-shell electronic structure with two terminal gold atoms and one terminal oxygen atom (Ot). The gold atoms terminally bonded with bridging oxygen atoms are separated from each other. For Au2VO2+, the lowestlying isomer (1IS10) has a closed-shell electronic structure with one gold dimer and two terminal oxygen atoms (Ot) directly bonded to V atom, as shown in Figure 3c. 3.2.2. Reaction Mechanism for Au2VO3,4+ + H2. The potential energy profiles for Au2VO3,4+ + H2 are given in Figures 4 and 5, respectively. Both 3IS1 and 1IS6 are considered

Figure 2. Fitting models for reactions of Au2VO3+ + H2 (a) and Au2VO4+ + H2 (b) are shown. Variations of the relative ion intensities with respect to the H2 pressures in the reactions of Au2VO3+ (c) and Au2VO4+ (d) with H2 by adopting the reaction models in panels a and b. 10454


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barriers occurs, leading to a more stable encounter complex 3I4 (ΔH0 = −0.78 eV). Note that the intermediate I4 is more stable in the singlet state than that in the triplet state. Thus, the Au− Au bond-breaking step may involve in a spin conversion from the triplet state to the singlet state between the 3TS3 and 1I4. The crossing point (CP) occurring along 3TS3 → 1I4 is lower in energy than the separate reactants by 0.88 eV (see CP in Figure 6). As a result of the spin conversion, a more stable

Figure 6. DFT-calculated potential energy curves (PECs) for spin conversions occurring in the Au−Au breaking step (3TS3 → 1I4) in Figure 4. The filled circle line is the relaxed PECs obtained by IRC calculations starting from 3TS4 to 1I4. The optimized geometries from the filled circle line (triplet) were used for single-point energy calculations of the singlet (empty circle line). The energy of the crossing point (CP) relative to the separate reactants (3Au2VO4+ + 1 H2) is given.

Figure 4. DFT-calculated potential energy profiles (a) and structures (b) for 3Au2VO4+ + 1H2 → 1Au2VO3+ + 1H2O (P1), 1Au2OH+ + 1 VO3H (P2), and 3Au2VO2H2+ + 3O2 (P3). A triplet-to-singlet crossing point (CP) is marked with an asterisk in panel a, and details are shown in Figure 6. The zero-point vibration corrected energies (ΔH0 in eV) with respect to the separate reactants are given. See Figure S1 in the Supporting Information for detailed information on the complete mechanism.

intermediate in the singlet state (1I4, ΔH0 = −1.21 eV) is formed. For another ground state of Au2VO4+ (1IS6), one terminally bonded gold atom can trap one H2 molecule to form 1 I4 directly, releasing the binding energy of 1.21 eV. Therefore, these two candidates would exhibit the uniform reactivity toward H2 due to the same intermediate (1I4) formed in the reaction pathway. After the formation of 1I4, the H−H bond can be easily activated and split by Au+ species in collaboration with O2− ion which is bonded with another Au atom, following the Lewis pair mechanism proposed in previous studies.38 The above process is subject to an energy barrier of 0.93 eV (1TS4). The reaction proceeds through the gold atoms transfer to the peroxide unit, leading to the cleavage of O−O bond (1I5 → 1I6 → 1I7 → 1I8) and the formation of an extremely stable intermediate 1I8 with two OH groups. After the formation of 1 I8, the system has enough energy (4.15 eV) to transfer one hydrogen atom from one OH group to the other OH group to produce H2O unit (1I8 → 1I9). Then the reaction terminates through the evaporation of H2O. Note that the crucial transition state 1TS8 is much lower in energy (−2.69 eV) than the separate reactant, indicating the easy formation of H2O. To generate Au2OH moiety, the intermediate 1I8 undergoes the formation of gold dimer and subsequent OH group transfer (1I8 → 1I10 → 1I11 → 1I12). After an arrangement of Au2OH moiety (1I12 → 1I13), reaction 1b proceeds with the dissociation of 1I13 into Au2OH+ and VO3H (P2). The energy of P2 (ΔH0 = −1.31 eV) is higher than that of P1 (ΔH0 = −2.14 eV), indicating that the formation of P1 is

Figure 5. DFT-calculated potential energy profiles (a) and structures (b) for 1Au2VO3+ + 1H2 → 1Au2VO2+ + 1H2O (P4), 1VO3H2+ + 1Au2 (P5), and 1Au2H+ + 1VO3H (P6). The zero-point vibration corrected energies (ΔH0 in eV) with respect to the separate reactants are given. See Figure S2 in the Supporting Information for detailed information on the complete mechanism.

as reactant candidates in the reaction of Au2VO4+ + H2. For 3 IS1, when a H2 molecule approaches to the Au2VO4+ (3IS1) cluster, the H2 is trapped by the gold dimer to form the intermediate 3I1 with the binding energy of 0.34 eV (Figure 4). Before the activation of H−H bond, the isomerization involving the cleavage of Au−Au bond from 3I1 to 3I4 with negligible 10455


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theory suggested that O2−−H+−H−−Au is a transition state structure for H2 dissociation on Au/TiO2, and the O2− ion on TiO2 surface, not directly connecting with Au atom, is more active for H2 dissociation than the perimeter O2− ion in contact with Au directly.54 The activation of H2 by other sites was also explored by theoretical calculations. However, the results indicate that the superoxide species O2•− and peroxide O22− in Au2VO4+ reaction system are inert toward H2 activation even in collaboration with Au+ species. The overall barriers of 0.83 and 0.05 eV to split the H−H bond from intermediates 3I1 and 1 I4 are predicted, respectively. The lower reactivity of O2•− and O22− may be attributed to their weaker basicity compared to the lattice oxygen species when they are viewed as Lewis bases in the mechanism of H2 activation by frustrated Lewis pairs. Although the O2•− and O22− species act as spectators in the activation and cleavage of the H−H bond, they indeed play crucial roles in the consecutive H2 oxidation processes. It is noticeable that only one H2 molecule can be oxidized in the previously investigated oxidation reactions between oxide clusters and H2. These reactions are all initiated by atomic oxygen species, either by the oxygen-centered radicals or by the lattice oxygen atoms. In this study, the dissociation of the superoxide or peroxide species (O2•− or O22−) into two atomic oxygen species to supply enough reactive oxygen species (O2−) is the key factor for the consecutive oxidation of H2 molecules. This is consistent with the result of the catalytic CO oxidation by O2 mediated with AuAl3O3−5+, in which the oxygen molecule is dissociated into two reactive atomic oxygen species O2− and O•−.52 In contrast, the molecular oxygen species were not dissociated in the previous study on the oxidation of CO by gold-doped titanium oxide cluster anions AuTi3O8−.56 As a result, the subsequent CO oxidation by the generated AuTi3O7− which contains an O22− unit was indeed not observed. This study reveals the importance of the further activation and dissociation of the superoxide or peroxide species (O2•− or O22−) in consecutive oxidation reactions of small molecules, such as CO44,52,56 and H2. 4.2. Catalytic Mechanism. The catalytic formation of water from hydrogen and oxygen is one of the most important catalytic reactions to be well-recognized. However, in spite of numerous subsequent investigations, the mechanism for this catalytic reaction is still far from clear. To explore the catalytic reactivity of the Au2VOx+ system, we performed the experiments and theoretical calculations about the reactivity of Au2VO2+ toward the oxygen molecule, and the results are shown in Figure 7, parts a and b. Upon the interaction of Au2V18O2+ with about 5.2 mPa 16O2 in the reactor for about 7.9 ms, a weak peak that can be assigned as Au2V18O216O2+ appears, indicating that the reaction Au2VO2+ + O2 → Au2VO4+ occurs. The DFT calculations indicate that the oxygen molecule can be trapped by the vanadium atom and activated to superoxide species (O2−•) through the cleavage of the Au−V bond and the formation of the Au−O bond. The small amount of Au2VO4+ observed experimentally may be attributed to the back-dissociation of Au2VO4+ into the reactants. Depending on the experimental and theoretical results of Au2VO2+ + O2, we proposed a catalytic cycle for H2 oxidation by O2 mediated with Au2VO2+, as shown in Figure 7c. The identified model catalysis follows a typical redox mechanism. The fundamental scientific problem involved with the redox mechanism is the activation of molecular oxygen: O2 → O2−• → O22− → 2O−• → 2O2−. At least four electrons are required

much more favorable than the formation of P2, which is pretty consistent with the experimentally observed branching ratio for reaction 1a of 83% and that for reaction 1b of 13%. To achieve the H2/O2 exchange channel (reaction 1c), the intermediate 3I3 has to overcome a higher barrier (3TS14, ΔH0 = 0.01 eV) to form 3I14. This process involved in the H−H bond activation by the Au2 dimer. After one hydrogen atom transfer to the terminal O atom (3I14 → 3TS14 → 3I15), a more stable species, 3I15, is generated, which has enough energy to release a free O2 molecule. The single-point energies of R (Au2VO4+ + H2) and 3TS14 in the DFT-calculated geometries were also calculated with the high-level quantum chemistry method of restricted coupled-cluster method with single, double, and perturbative triple excitations [RCCSD(T)]. In this case, the energy of 3TS14 is changed to −0.01 eV. Compared to the other reaction channels, the possibilities of the intermediate 3I3 surmounting the high energy barrier (3TS14) is small, suggesting that the H2/O2 exchange channel is the least favorable channel, which is in agreement with the lowest branching ratio (4%) for reaction 1c observed experimentally. The reaction mechanism of Au2VO3+ + H2 is similar to that of Au2VO4+ + H2, as shown in Figure 5. Once the hydrogen molecule is trapped by the terminally bonded gold atom, the H−H bond can be easily activated and split by the synergistic effect of two separate ions, Au+ and O2−, following the Lewis pair mechanism. After the formation of I20 which contains two hydroxyl moieties and a gold dimer, the water molecule can be easily formed and evaporated (I20 → I21 → P4) or the loss of gold dimer takes place (I20 → I21 → P5). Moreover, I19 can also rearrange into I22 along with the formation of a Au−Au bond to form the Au2H moiety, and then the dissociation of Au2H+ and VO3H from I22 occurs (I22 → P6). The DFT calculation indicates that all the intermediates and transition states are lower in energy than the separate reactants (Au2VO3+ + H2), which is consistent with the experimental observation of reactions 2a−2c. Note that the formation of P4 is slightly more favorable thermodynamically than the formations of P5 and P6, which well reproduce the experimental results of the higher branching ratio of reaction 2a than that of reactions 2b and 2c. The comparative reactivity of Au2VO3+ and Au2VO4+ is ascribed to the similar reaction mechanism of Lewis acid− base pairs (Figures 4 and 5). The DFT-determined transition states for H−H activation (TS4 and TS17) in the reactions of Au2VO3,4+ + H2 are both located below the separate reactants, which is in agreement with KIE values [k1(Au2VO4+ + H2/ k1(Au2VO4+ + D2) = 1.3 and k1(Au2VO3+ + H2/k1(Au2VO3+ + D2) = 1.4] that are slightly larger than 1 by the experiment (Figure 1).

4. DISCUSSION 4.1. Role of Reactive Oxygen Species in H2 Oxidation Reactions. The direct activation of H2 molecule by the oxygen-centered radicals35,36 and by the lattice oxygen in collaboration with Au+ species38 has been reported previously. The investigation of the reactions between Au2VO3,4+ and H2 in this study provides a further evidence for the synergistic effect of O2− and separate Au+ in the cleavage of H−H bond. The reaction mechanism presented in this study has also been uncovered in the previous study on the reactivity of AuCeO2+ toward H 2 , 38 which parallels similar behaviors of H 2 dissociation by gold catalysts over metal oxide supports in the condensed-phase systems.11,14,54,55 The analysis of Hückel 10456


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Additionally, the making and breaking of the Au−O and Au−V (Au−M) bonds is the driving force for the oxidation of H2 by Au2VO4+ and Au2VO3+, which is consistent with the oxidation of some small molecules by gold-containing heteronuclear oxide clusters, such as the oxidation of methane and ethane by AuNbO3+,59,60 the oxidation of CO by AuAl3O5+,52 Au(TiO2)yOz− (y = 2, 3; z = 1, 2),56 and Au2VOx− (x = 3, 4).44

5. CONCLUSION The reactions of mass-selected gold−vanadium heteronuclear oxide cluster cations Au2VO4+ and Au2VO3+ with H2 have been investigated by mass spectrometry in conjunction with density functional theory calculations. Consecutive H2 oxidation by Au2VO4+ to generate Au2VO3+ and subsequently Au2VO2+ is identified. With the regeneration of Au2VO4+ from Au2VO2+ + O2, a catalytic cycle for H2 oxidation by O2 mediated with Au2VO2+ is proposed. Theoretical calculation results predict that the dissociation of the superoxide or peroxide species (O2•− or O22−) into atomic oxygen species is crucial for consecutive H2 oxidation reactions. In the catalytic cycle, gold atoms not only act as an active site to split the H−H bond in collaboration with O2− ion, but also promote the reduction of the oxygen molecule by storing and releasing electrons. This study provides molecular-level insights into the elementary mechanism of the H2 catalytic oxidation on the surface.

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Figure 7. (a) TOF mass spectra for the reaction of mass-selected Au2V18O2+ with 16O2. The relative signal magnitudes are amplified by a factor of 5 for m/z > 510. (b) DFT-calculated potential energy profiles for Au2VO2+ + O2 → Au2VO4+. The zero-point vibration corrected energies (ΔH0 in eV) of the reaction intermediates, transition states, and products (Au2VO4+) with respect to the separate reactants are given. (c) Proposed catalytic cycle for H2 oxidation with O2.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02515. Figures giving additional DFT-calculated reaction mechanisms, the Cartesian coordinates of all structures, and the imaginary frequencies and normal coordinates of the transition states on display (PDF)

to reduce the O2 molecule into two lattice oxygen ions (O2 + 4e− → 2O2−). Two electrons that can be used to reduce O2 are stored in the Au2−V fragment in Au2VO2+ produced from the oxidation of the H2 molecule (O2− + H2 → H2O + 2e−). One of the electrons participates in the reduction of the O2 molecule to O2•− in the formation process of Au2VO4+ (O2 + e− → O2•−), and the other is released to reduce the O2•− to O22− (from I1 to I4) along with the cleavage of the Au−Au bond (O2•− + e− → O22−). Two additional electrons originate from the redox reaction of H2 and are used to reduce the O22− in I4 to two lattices oxygen ions in I9 (O22− + 2e− → 2O2−). The crucial role of gold species playing in the activation of molecular oxygen species, including O2, O2•−, and O22−, has been demonstrated and emphasized in both gas-phase and condensed-phase investigations. In previous studies on the CO oxidation by AuAl3O4,5+ and Au2VO3,4−, the promotional effect of gold atoms in the dissociation of O2, O2•−, and O22− was revealed. In condensed phase, electron-rich Au nanoparticles were reported to adsorb O2 more strongly and to activate the O−O bond via charge transfer from Au by forming a superoxolike species.57,58 Gold atoms play an important role in the catalytic cycle of H2 oxidation mediated by Au2VO2−4+. On the one hand, gold atoms act as the active sites to capture H2 molecules (I1, Figure 4; I17 Figure 5) and split the H−H bond in collaboration with O2− ions (I1 → I2 → I3 → I4 → I5, Figure 4; I17 → I18, Figure 5). On the other hand, gold atoms can promote the reduction of the oxygen molecule by acting as electron donors. The electrons stored in the Au−V and Au−Au bonds can be used to reduce the oxygen molecule into atomic oxygen species.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-62423763. Fax: +86-10-62559373. *E-mail: [email protected]. Fax: +86 20-22236337. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21273247, 21107026, 21325314, and 91545122) and the Natural Science Foundations of Guangdong Province (No. 1614050002151).

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REFERENCES

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