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Jan 3, 2014 - Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Instit...
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Consecutive Oxygen-for-Sulfur Exchange Reactions between Vanadium Oxide Cluster Anions and Hydrogen Sulfide Mei-Ye Jia,†,‡ Bo Xu,†,‡ Ke Deng,§ Sheng-Gui He,*,† and Mao-Fa Ge*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Vanadium oxide cluster anions Vm16On− and Vm18On− were prepared by laser ablation and reacted with hydrogen sulfide (H2S) in a fast flow reactor under thermal collision conditions. A time-of-flight mass spectrometer was used to detect the cluster distributions before and after the interactions with H2S. The experiments suggest that the oxygen-for-sulfur (O/S) exchange reaction to release water was evidenced in the reactor for most of the cluster anions: VmOn− + H2S → VmOn−1S− + H2O. For reactions of clusters VO3− and VO4− with H2S, consecutive O/S exchange reactions led to the generation of sulfur containing vanadium oxide cluster anions VO3−kSk− (k = 1−3) and VO4−kSk− (k = 1−4). Density functional theory calculations were performed for the reactions of VO3−4− with H2S, and the results indicate that the O/ S exchange reactions are both thermodynamically and kinetically favorable, which supports the experimental observations. The reactions of VmOn+ cluster cations with H2S have been reported previously (Jia, M.-Y.; Xu, B.; Ding, X.-L.; Zhao, Y.-X.; He, S.-G.; Ge, M.-F. J. Phys. Chem. C 2012, 116, 9043), and this study of cluster anions provides further new insights into the transformations of H2S over vanadium oxides at the molecular level. During the past decades, structures27−31 of vanadium oxide clusters and their reactivity32−34 toward hydrocarbons,35−44 halohydrocarbons,45−47 water,48−51 sulfur dioxide,52 alcohols,53−56 etc. have been studied extensively and many useful insights into the related reaction mechanisms have been achieved. It is noteworthy that the Castleman group is among the pioneers to study the structures and reactivity of vanadium oxide clusters.31−37,45−47 In our recent work on the reactions between VmOn+ cluster cations and H2S,57 three reaction channels were observed for most of the cluster cations: H-atom abstraction to generate the SH radical, consecutive H-atom abstractions to derive the S atom, and O/S exchange reaction to release water. As is reported, the charge state of atomic clusters plays an important role in their reactivity.23,33,35,58−62 For example, the charge state dependent reactivity of metal oxide clusters in C−H bond activation of hydrocarbons has been extensively investigated.23,58,59 A general conclusion is that the cations are usually more reactive than their counterpart anions. When it comes to the charge state dependent reactivity of vanadium oxide clusters toward H2S, one expects that VmOn− anions can be less reactive and possibly more selective than VmOn+ cations. It is of great interest to find out which of the

1. INTRODUCTION Hydrogen sulfide commonly exists in natural gas and waste gas steams originated from chemical plants.1,2 The selective oxidation of H2S to recover elemental sulfur is an important reaction no matter from environmental or economic point of view.3−5 In the process of H2S oxidation, many metal oxides are used as catalysts, such as Al2O3,6 TiO2,7 Fe2O3,8−10 Cr2O3,11,12 V2O513,14 and so on. To investigate the interactions of H2S with these metal oxide surfaces, spectroscopic and kinetic measurements as well as theoretical calculations have been performed and many insights have been obtained.15−18 As is reported, in a detailed comparison of the performance of metal oxides in the selective oxidation of H2S, vanadium oxides are found to be most active and selective,14 so there have been many condensed phase studies about the mechanism of the selective oxidation of H2S over vanadium oxides.13,14 However, the detailed reaction mechanism, especially for the nature of the initial stages of the molecular interaction between H2S and vanadium oxides at the gas−solid interface is still unclear. Because of the complexity of metal oxide surfaces, alternative ways are needed to investigate the interactions between H2S and vanadium oxides. It has been proposed that the surface structure of bulk materials can be envisioned as a collection of atomic clusters with varying sizes and structures so clusters can be used as models of catalytic surfaces.19−26 The studies of the reactions between vanadium oxide clusters with H2S can be conducted to investigate the molecular level mechanisms of H2S oxidation over vanadium oxide surfaces. © XXXX American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: December 6, 2013 Revised: December 30, 2013

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Figure 1. Time-of-flight mass spectra for reactions of V16O3−5− (a, b) and V18O3−5− (c−e) with H2S. (a) and (c) plot the reference spectra without H2S in the reactor. Partial pressures of H2S are about 0.5, 0.5, and 1.0 Pa for (b), (d), and (e), respectively. Labels m,n, m,n,x, and m,n,xH2S denote VmOn−, VmOnSx−, and VmOnSxH2S−, respectively. Unlabeled peaks in (c−e) can be assigned to Vm16O18On−1−.

K.66,67 After reacting in the fast flow reactor, the reactant and product ions were skimmed into the vacuum system of a TOFMS for mass and abundance measurements. The uncertainties of the reported relative ion signals are about 15% and mass resolution (M/ΔM) is 400−500 with current experimental setup. More details of the experiments can be found in our previous works.43,68 2.2. Computational Details. The DFT calculations with the Gaussian 09 program69 were performed to investigate the structures of clusters VO3−5− and the reaction mechanisms of VO3−4− with H2S. The hybrid B3LYP functional70,71 and TZVP basis set72 were used. This choice of method with moderate computational cost has been tested to provide reasonable results to interpret vibrational spectra27−30,73 of vanadium oxide clusters and their reactivity toward hydrocarbon molecules35−44 and H2S.57 The B3LYP/TZVP calculations can also well reproduce the experimental bond dissociation enthalpies of HS−H and S−H species. The experimental bond dissociation enthalpies of HS−H and S−H are 3.95 and 3.57 eV, respectively;74 and the B3LYP/TZVP calculated values are 3.81 and 3.60 eV, respectively. Geometry optimizations with full relaxation of all atoms were performed. Vibrational frequency calculations were performed to check that the reaction intermediates and transition state species have zero and one imaginary frequency, respectively. The DFT calculated energies reported in this study are the zero-point vibration corrected (ΔH0K) or the relative Gibbs free energies (ΔG298K) under a temperature of 298.15 K and pressure of 1 atm. Structures and vibrational frequencies for all of the optimized structures are available upon request.

three channels identified for VmOn+ + H2S is most favorable for VmOn− + H2S. In this work, we explore the reactions between VmOn− clusters and H2S through mass spectrometry and density functional theory (DFT) calculations, aiming to find a common channel for the reactions of VmOn+ and VmOn− with H2S to interpret the interactions of H2S with bulk vanadium oxides.

2. METHODS 2.1. Experimental Details. The experiments were conducted by a time-of-flight mass spectrometer (TOF-MS) coupled with a laser ablation cluster source and a fast flow reactor.63 The VmOn− clusters were generated by laser ablation of a rotating and translating vanadium metal disk (99.9%) in the presence of 1% 16O2 or 18O2 seeded in a helium carrier gas with backing pressure of 5 atm. 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 residual water in the gas handling system from forming undesirable hydroxo species, 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. Similar treatment (T = 215 K) was also applied in the use of reactant gases (5% H2S seeded in He) that was pulsed into the reactor 20 mm downstream from the exit of the narrow cluster formation channel by a second pulsed valve (General Valve, Series 9). By using the method in ref 64, we estimated the instantaneous total gas pressure in the fast flow reactor to be around 220 Pa at T = 300 K. The intracluster vibrations were likely equilibrated (cooled or heated, depending on the vibration temperature after exiting cluster formation channel with a supersonic expansion) to the bath gas temperature before reacting with the diluted H2S. It may be safe to assume that the bath gas has the temperature of the wall of reactor (300 K).65 Our recent experiments indicate that the cluster vibrational temperature in the reactor can be close to 300

3. RESULTS 3.1. Experimental Results. The natural abundances of 32S, 33 S, 34S, and 36S are 95.0%, 0.76%, 4.22%, and 0.02%, respectively. 75 Only the primary isotopomer H 2 32 S is considered for hydrogen sulfide in this study. The TOF mass spectra for the reactions of clusters VmOn− with H2S are plotted B

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Figure 2. Time-of-flight mass spectra for reactions of V218O5−8− (a, b) and V318O7−9− (c, d) with H2S. (a) and (c) plot the reference spectra without H2S in the reactor. The partial pressure of H2S in the reactor is about 1.0 Pa. See also the caption of Figure 1.

Figure 2b,d, the O/S exchange reactions can also be observed for V2O5−6− and V3O7−9−. In addition, H-atom abstraction product V2O6H− was generated for V2O6−. The pseudo-first-order rate constant (k1) of the reactions between VmOn− clusters and H2S can be estimated by using the following equation:

in Figures 1 and 2. Parts a and b of Figure 1 show the mass spectra of clusters V16O3−5− before and after interactions with H2S, respectively. Because 32S and 16O2 have the same mass number, the 16O experiment cannot identify O/S exchange products Vm16On−1S− for which the mass peaks are overlapped with those of Vm16On+1−. For example, the peak at m/z = 117 (Figure 1b) can be assigned to V16O4− or V16O2S−, and the peak at m/z = 131 can be assigned to V16O5− or V16O3S−. For the reason mentioned above, in this work, the generation of clusters VmOn− was also conducted by using 18O2 as oxygen source to differentiate the overlapped peaks. The mass spectra for reactions of V18O3−5− with different partial pressures of H2S are plotted in Figures 1c−e. Figure 1d indicates many product peaks that can not be identified in the 16O experiment (Figure 1b) are now observed. For V18O3−, under 0.5 Pa H2S, the corresponding consecutive O/S exchange products V18O2S−, V18OS2−, and VS3− appear at m/z = 119, 133, and 147, respectively. For V18O4−, the O/S exchange products V18O3S− and V18O2S2− are located at m/z = 137 and 151, respectively. When the partial pressure of H2S is increased to 1.0 Pa (Figure 1e), the cluster VO3− and its corresponding O/S exchange products VO3−kSk− (k = 1−3) can associate one H2S molecule to generate adsorption products VO3−kSkH2S−. In addition, the peak intensities of V3−kSk− (k = 1−3) and VO4−kSk− (k = 1−2) increase and the new peaks of VOS3− (m/z = 165) and VS4− (m/z = 179) appear. The experiments indicate that the following sequential O/S exchange reactions occurred in the reactor: VO3 − k Sk − + H 2S = VO2 − k Sk + 1− + H 2O

k1 = −

(3)

in which the ratio I/I0 is the percentage of unreacted clusters after the cluster interaction with reactant gas that has molecular density of ρ in the reactor for reaction time of Δt (see refs 50, 76, and 77 for details of rate constant determination). Because it is hard to determine accurately the ρ and Δt values in a pulsed experiment, the absolute k1 value can be systematically under- or overestimated. Uncertainties of relative values are about 30%. The rate constants k1 estimated by using the data in Figure 1e for reactions of VO3−, VO4−, and VO5− with H2S are listed in Table 1. The reaction efficiency is defined as ϕ = (k1/ Table 1. First-Order Rate Constants (k1 and kT in 10−11 cm3 molecule−1 s−1) for the Reactions of Clusters VO3−5− with H2S reactions

experimental (k1)

theoretical (kT)

k1/kT (%)

VO3− + H2S VO4− + H2S VO5− + H2S

2.9 1.3 2.5

85.9 84.4 83.2

3.4 1.5 3.0

k = 0−2 (1)

VO4 − k Sk − + H 2S = VO3 − k Sk + 1− + H 2O

ln(I /I0) ρ × Δt

kT) × 100% and kT is the theoretical rate of collision that is calculated with kT = 2π(e2α/μ)1/2 in which e is the charge of the cluster ion, α is the electric polarizability of the reactant molecule H2S, and μ is the reduced mass (see ref 78 for details). 3.2. Computational Results. To gain insights into the reactions of VmOn− clusters with H2S, the sequential O/S exchange reaction pathways were calculated for VO3−kSk− (k = 0−2) and VO4−kSk− (k = 0−3) with H2S. In addition, the

k = 0−3 (2)

Besides the O/S exchange reactions, the generations of VO4H2− (m/z = 125) and VO5H− (m/z = 142) by H-atom abstraction were also observed for clusters VO4− and VO5−, respectively. Figure 2 plots the mass spectra for reactions of V218O5−8− and V318O7−9− clusters with H2S. As is shown in C

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Figure 3. DFT calculated reaction pathway for 1VO3− + H2S (R1) → 1VO2S−+ H2O (P1). The reaction intermediates and transition states are denoted as IMn and TSn, respectively. The relative energies of ΔH0K/ΔG298K (eV) with respect to the separated reactants are given in parentheses. Bond lengths are in picometers.

structures of VO5− and the reaction mechanism of H-atom abstraction is presented. 3.2.1. Reactions of VO3−kSk− (k = 0−2) with H2S. The DFT calculated potential energy profile for reaction of VO3− with H2S is plotted in Figure 3. Upon the approach of H2S to VO3−, an encounter complex IM1 (VO3H2S−) is formed and this process (VO3− + H2S → IM1) can release 0.48 eV (ΔH0K) heat of formation that is high enough to overcome the energy cost for the cleavage of one of the S−H bonds, and the energy is recouped by the formation of O−H and V−SH bonds (IM1 → TS1 → IM2). The energy released (ΔH0K, 2.64 eV) during the formation of IM2 (VO3H−SH−) is high enough to supply the energy needed for the subsequent formation of H2O molecule (IM2 → TS2 → IM3) and the dehydration process (IM3 → VO2S− + H2O). The transition states (TS1 and TS2) and the products (VO2S− + H2O) are well below in energy than the separated reactants (VO3− + H2S), so the O/S exchange reaction is both thermodynamically and kinetically favorable. The DFT result is thus consistent with the experimentally observed reaction between VO3− and H2S (Figure 1). The potential energy profiles for reactions of VO2S− and VOS2− with H2S are shown in Figures S1 and S2 in the Supporting Information. They indicate that the mechanisms for the second and the third O/S exchange are very similar to that of the first one, which interprets the experimental generations of VOS2− and VS3− in Figure 1d,e. 3.2.2. Reactions of VO4−kSk− (k = 0−3) with H2S. The DFT calculated potential energy profiles for reactions of VO4− with H2S are plotted in Figure 4. There are two types of oxygen in the lowest energy structure of VO4−: the terminal bonded O2− and the η2-O22− moiety. For the O/S exchange reaction, there are two possibilities: the terminal O2− or one of the two O in

O22− moiety is substituted. In the reaction pathways of VO4− with H2S, the process for the cleavage of one of the S−H bonds and formation of the O−H and V−SH bonds (IM10 → TS7 → IM11) is similar to that in the reactions of VO3− with H2S. After the first H-atom transfer, there are two pathways leading to dehydration: (1) the H atom of SH moiety transfers to the OH moiety forming the water molecule (IM11 → TS8 → IM12) and then the dehydration process takes place (IM12 → P4 in which P4 = 1[V5+O22−O2−S2−] + H2O); (2) the O−O bond of O22− moiety dissociates and the O−S bond is formed along with the cleavage of V−SH bond (IM11 → TS9 → IM13), which eventually leads to formation of 1[V5+(O− S)2−(O2−)2] and a H2O molecule (IM13 → TS10 → IM14 → P5). From IM13, the H atom of SH moiety can also transfers to one of the two O2− ions to form the second OH moiety (IM13 → TS11 → IM15), which can lead to the generation of P7 (1VO4H2− + 3S) by the process of desulfuration (IM15 → P7). It is noteworthy that there is a spin inversion79,80 during the process of desulfuration from IM15. As is shown in Figure S3 (Supporting Information), from IM15 to P7, the spin inversion of reaction species VO4H2S− from singlet to triplet can occur at a structure (CP) that is 1.93 eV (ΔESCF) lower in energy with respect to the separated reactants. The energy released (ΔH0K, 1.44 eV) during the formation of IM11 (VO3H−SH−) is high enough to supply the energy needed for the cleavage of the O− O bond (IM11 → TS9 → IM13) and further higher energy (ΔH0K, 2.88 eV) can be released after formation of the O−SH bond in IM13. The energy released during the formation of the stable intermediate IM13 is high enough to overcome the energy cost for the desulfurization process (IM13 → TS11 → IM15 → P7). The transition states (TS7−TS11) and products P4 (1[V5+O22−O2−S2−] + H2O), P5 (1[V5+(O−S)2−(O2−)2] + D

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Figure 4. DFT calculated (a) reaction pathways and (b) structures of reaction intermediates and transition states for 1VO4− + 1H2S (R4) → 1VO3S− + 1H2O (P4 and P5) and 1VO4− + 1H2S (R4) → 1VO3S− + 3S (P7). See also the caption of Figure 3.

mechanisms for the consecutive O/S exchanges of VO3S−, VO2S2−, and VOS3− are very similar to that of VO4−, which interprets the experimental observations of VO2S2−, VOS3−, and VS4− in Figure 1d,e. 3.2.3. Structure of VO5− and Reaction of VO5− with H2S. To the best of our knowledge, the very-oxygen-rich cluster VO5− has not been well studied in the literature. The structures of the lowest and some low-lying energy isomers of VO5− are

H2O), and P7 (1VO4H2− + 3S) are lower in energy than the separated reactants (VO4− + H2S), so both the O/S exchange reaction and the generation of S atom are thermodynamically and kinetically favorable. The DFT results are thus consistent with the experimentally observed reaction between VO4− and H2S. The potential energy profiles for O/S exchange reactions of VO3S−, VO2S2−, and VOS3− with H2S are shown in Figures S4−S6 in the Supporting Information. It indicates that the E

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Figure 5. B3LYP/TZVP optimized isomer structures of VO5−, which are denoted as I1−I5, etc. The symmetry, electronic state, and energy (eV) with respect to the most stable isomer I1 are given below each isomer. Bond lengths are given in picometers.

pathway dominates the reactions of VmOn− with H2S (Figures 1 and 2) and the H-atom abstraction channels are only observed for a few anions, namely VO4−5− and V2O6−. For VO3−4− with H2S, consecutive O/S exchange leads to the generation of sulfur containing vanadium oxide clusters VOn−kSk− (n = 3, k = 1−2; n = 4, k = 1−3) initially and the vanadium sulfide clusters VS3−4− at the end. It should be noted that V atoms in clusters VO2+ and VO3− have the same oxidation state of five. Compared with VO2+ + H2S that generates VO2H+, VO2H2+, and VOS+ as product ions, VO3− + H2S does not generate Hatom abstraction products and only O/S exchange product VO2S− is observed (Figure 1). The DFT calculations indicate that the following two reaction pathways are endothermic, VO3− + H2S → VO3H− + SH (ΔH0K = 0.58 eV) and VO3− + H2S → VO3H2− + S (ΔH0K = 1.80 eV), which is consistent with the experiment. According to the aformentioned, it comes to a conclusion that the anionic clusters VmOn− are more selective than the counterpart VmOn+ clusters and the O/S exchange reaction is the major channel for the anionic system. The common reaction channel (O/S exchange) for both cations and anions may well take place during the interaction of H2S with bulk vanadium oxide surfaces. 4.2. Reactivity of Peroxide Species O22− toward H2S. In oxidation reactions involving molecular oxygen, the O2 molecule is considered to dissociate in the following scheme: O 2 (molecular oxygen) → O 2 −• (superoxide) → O 2 2− (peroxide) → 2O−• (mononuclear oxygen-centered radical) → 2O2− (lattice oxygen),96 among which the O2−•, O22−, and O−• are reactive oxygen species. According to the DFT results, the VO4− cluster contains one unit of peroxide O22− and two unit of “lattice” oxygen (O2−). Both the O2− ions and the peroxide moiety O22− can be directly substituted by the S atoms from H2S (Figure 4). Studies on reaction pathways of VO4− + H2S (Figure 4) suggest that the reaction of peroxide species O22− is thermodynamically more favorable (ΔH0K = −1.80 eV) than the reaction of O2− (ΔH0K = −0.21 eV). In the reaction channel involving O22− (IM11 → TS9 → IM13), the dissociation of O−O bond and formation of O−S bond take place simultaneously. The gas-phase reaction is also kinetically favorable as the energy of TS9 (ΔH0K = −0.35 eV) is negative with respect to the separate reactants. It is noteworthy that many oxide clusters were produced23,40−43,58,60,61 and their reactions with small molecules such as CO and hydrocarbon molecules have been studied but there has been no good evidence for direct participation of O22− in the reactions (most of the reactions are attributed to O−• and O2−). This reaction of 1[V5+(O−O)2−(O2−)2] + H2S (R4) → [V5+(O−S)2−(O2−)2] + H2O (P5) provides a first good evidence for direct participation of O22− in the reactions of metal oxide clusters with small molecules. It means the molecular oxygen O2 does not have to dissociate completely (O2 → 2O2− or 2O−•) to oxidize small molecules, such as H2S over vanadium oxides.

shown in Figure 5. For the most stable isomer I1, which is a triplet, it contains one superoxide (O2−•) radical, one oxygen radical (O−•), and two terminal bonded oxygen atoms O2−. The triplet VO5− can be considered to be generated by the adsorption of a triplet O2 molecule on the singlet VO3−, which is similar to our previous works on La3O5− + O2 → La3O7− 81 and Zr2O6− + O2 → Zr2O8−.82 The generation of reactive oxygen species O−• and O2−• is through the following electron transfer mechanism: O2− + O2 → O−• + O2−•. Photoelectron spectroscopy (PES) experiments and DFT calculations also suggested that M2O7 (M = Nb or Ta) containing one unit of O−• and one unit of O22−• can be considered to be generated by the adsorption of an O2 molecule on M2O5.83 For the reactions of VO5− with H2S, two reaction channels are observed in experiments: O/S exchange to release water and H-atom abstraction to generate SH radical. The H-atom abstraction is facilitated by the high reactivity of the O−• radical23,58 and the mechanism is given in Figure S7 (Supporting Information). We speculate that the O/S exchange pathway of VO5− + H2S is similar to that of VO3−4− + H2S and the detailed reaction pathways are omitted.

4. DISCUSSION 4.1. Charge State Dependent Reactivity of Vanadium Oxide Clusters toward H2S. The reactivity of atomic clusters can be closely correlated with the cluster sizes and compositions,60,65,84 charge states,62 and details of the electronic structures.60 As is reported, the thermal activation of C−H bonds of alkane molecules can only be accomplished by metal oxide clusters with specific compositions that containing oxygen-centered radicals23,58,85 and the reactivity is further controlled by the local charge59 and local spin60 environments around the radical centers.57,86 So far, to the best of our knowledge, the reactions of metal oxide cluster anions with H2S have been investigated for MmOn− (M = Mn, Fe, Co, Ni, Cu, and Zn),87,88 AlO2−/SiO2−,89 SnmOn−,90 and ComOn−.91 In addition, the reactions of cluster cations VmOn+,57 FeO+,92 Fe2O2+,93 and MnmOn+ 94,95 with H2S have also been studied. In these reaction systems, the O/S exchange reactions to generate H2O were identified. In our recent study on the gas-phase reactions of VmOn+ with H2S,57 in addition to H2S adsorption to form association products VmOnH2S+, three types of reactions were identified for most of the cluster cations: (a) VmOn+ + H2S → VmOnH+ + SH (Hatom abstraction), (b) VmOn+ + H2S → VmOnH2+ + S (consecutive H-atom abstraction), and (c) VmOn+ + H2S → VmOn−1S+ + H2O (O/S exchange). For reactions of VO2+ with H2S, the above three types of reaction channels are both thermodynamically and kinetically favorable. The pseudo-firstorder-rate constant of VO2+ + H2S is 4.2 × 10−11 cm3 molecule−1 s−1, which is higher than that of VO3− + H2S in this study. Nevertheless, in this work, the O/S exchange F

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5. CONCLUSION The reactions between vanadium oxide cluster anions VmOn− and H2S under thermal collision conditions were studied by mass spectrometry and density functional theory calculations. The oxygen-for-sulfur (O/S) exchange is the main reaction channel for the majority of the cluster anions. Reaction mechanism studies indicated that the consecutive O/S exchange reactions of clusters VO3− and VO4− with H2S are both thermodynamically and kinetically favorable. Further, the high selectivity for reactions of VmOn− clusters with H2S (O/S exchange channel) is different from the multichannel reactions of VmOn+ with H2S. It indicates that the charge states play a very important role in the reactions of vanadium oxide clusters with H2S. The common O/S exchange channel for both VmOn− anions and VmOn+ cations may well take place during the interaction of H2S with bulk vanadium oxide surfaces. The complete substitution of four O atoms in VO4− by four S atoms from H2S to generate VS4− provides the first good evidence for direct participation of peroxide unit in the gas-phase oxidation reactions.



ASSOCIATED CONTENT

S Supporting Information *

Complete ref 69 and density functional theory calculated reaction pathways of VO2S−, VOS2−, VO3S−, VO2S2−, and VOS3− with H2S as well as the spin inversion diagram for VO4− + H2S. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Authors

*S.-G. He: e-mail, [email protected]; tel, +86-1062536990; fax, +86-10-62559373. *M.-F. Ge: e-mail, [email protected]; tel, +86-10-62554518; fax, +86-10-62558682. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by Chinese Academy of Sciences (Knowledge Innovation Program KJCX2-EW-H01), the National Natural Science Foundation of China (Nos. 20933008 and 21325314), Major Research Plan of China (Nos. 2011CB932302, 2012CB933001, and 2013CB834603), and ICCAS (CMS-PY-201306).



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