Experimental and Theoretical Study of the Reactions between

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Experimental and Theoretical Study of the Reactions between Vanadium Oxide Cluster Cations and Hydrogen Sulfide Mei-Ye Jia,†,‡ Bo Xu,†,‡ Xun-Lei Ding,† Yan-Xia Zhao,† Sheng-Gui He,*,† and Mao-Fa Ge*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China ABSTRACT: Vanadium oxide cluster cations (Vm16On+ and Vm18On+) are 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 is used to detect the cluster distributions before and after the interactions with H2S. The experiments suggest that, in addition to H2S adsorption to form association products VmOnH2S+, three types of reactions are evidenced in the reactor: (1) VmOn+ + H2S → VmOnH+ + SH, (2) VmOn+ + H2S → VmOnH2+ + S, and (3) VmOn+ + H2S → VmOn−1S+ + H2O. Density functional theory calculations are performed for reaction of VO2+ with H2S, and the results indicate that the above three types of reaction channels are both thermodynamically and kinetically favorable, which supports the experimental observations. This gas-phase cluster study may provide insights into selective oxidation of H2S to elemental sulfur over vanadium-based oxide catalysts.

1. INTRODUCTION Investigation of gas-phase vanadium oxide clusters is very important because these atomic clusters can be suitable models1 for the active sites of vanadium-based oxide catalysts that are widely used in both industrial and laboratory processes, including oxidation of SO2 to SO3 in the production of sulfuric acid,2 partial oxidation of hydrocarbon molecules,3 and selected oxidation of hydrogen sulfide.4 During the past years, structures of vanadium oxide clusters5 and their reactivity toward sulfur dioxide,6 hydrocarbons,7−10 alcohols,11 water,12 and others have been studied extensively and many useful insights into the related reaction mechanisms have been achieved. However, to the best of our knowledge, there is no report on the reactions between vanadium oxide clusters and hydrogen sulfide (H2S), although vanadium-based oxides are effective catalysts4 in the oxidative removal of H2S that can be either in natural gas or in waste gas streams from chemical plants. In this work, we study the reactions of vanadium oxide cluster cations with H2S in the gas phase by time-of-flight mass spectrometry and density functional theory (DFT) calculations. Many condensed-phase studies have been conducted to understand mechanistic details involved with removal of poisonous H2S. The H2S adsorption and dissociation on the surfaces of a series of metal oxides, including Fe2O3,13 Al2O3,14,15 Cr2O3,15 Cu2O,15,16 ZnO,15,17 CeO2,18 and so on, have been explored. It is generally found that H2S mainly interacts with metal centers on the surfaces while molecularlevel mechanisms involved with H2S transformation into water, elemental sulfur, or other sulfur-containing compounds are far from clear. The study of interactions between H2S and metal oxide clusters in the gas phase and under well-controlled conditions may provide new insights into related surface reactions. Note that the reactions of H2O (parent molecule of © 2012 American Chemical Society

H2S) with the gas-phase or matrix isolated metal atoms and metal oxide clusters have been extensively investigated,4 whereas there are a very limited number of studies on the H2S reaction system under similar conditions.20

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Experimental Details. The experiments are conducted by a time-of-flight mass spectrometer (TOF-MS) coupled with a laser ablation cluster source and a fast flow reactor.21 The VmOn+ clusters are generated by laser ablation of a rotating and translating vanadium metal disk (99.9%) in the presence of 0.8% 16O2 or 18O2 seeded in a helium carrier gas with a backing pressure of 5 atm. A 532 nm (second harmonic of Nd3+:yttrium aluminum garnet - YAG) laser with an energy of 5−8 mJ/pulse and a repetition rate of 10 Hz is used. The gas is controlled by a pulsed valve (General Valve, Series 9, Parker Hannifin Corporation, Fairfield, NJ, USA). To prevent residual water in the gas handling system to form undesirable hydroxo species, the prepared gas mixture (O2/He) is 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) is also applied in the use of the reactant gases (5% H2S seeded in He) that is 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 22, the instantaneous total gas pressure in the fast flow reactor is estimated to be around 220 Pa at T = 300 K. The intracluster vibrations are likely equilibrated (cooled or heated, depending on the vibrational temperature Received: January 17, 2012 Revised: March 26, 2012 Published: April 5, 2012 9043

dx.doi.org/10.1021/jp3005458 | J. Phys. Chem. C 2012, 116, 9043−9048

The Journal of Physical Chemistry C

Article

after exiting the cluster formation channel with a supersonic expansion) close 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 the reactor (300 K).23 Our recent experiments indicate that the cluster vibrational temperature in the reactor can be close to 300 K.24 After reacting in the fast flow reactor, the reactant and product ions are skimmed into the vacuum system of a TOF-MS 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 the current experimental setup. More details of the experiments can be found in our previous works.25 2.2. Computational Details. The DFT calculations with the Gaussian 03 program26 are performed to investigate the reaction mechanisms of VmOn+ clusters with H2S, particularly for the reaction of VO2+ + H2S. The hybrid B3LYP functional27 and TZVP basis set28 are used. This choice of method with moderate computational cost has been tested to provide reasonable results to interpret vibrational spectra5b of vanadium oxide clusters and their reactivity with hydrocarbon molecules.7b,8b,9,10 The B3LYP/TZVP calculations can also well reproduce the experimental bond dissociation enthalpies of HS−H and S−H species. The bond dissociation enthalpies of H 2S and SH by experiments are 3.92 and 3.69 eV, respectively,29 and the B3LYP/TZVP calculated values are 3.81 and 3.60 eV, respectively. Geometry optimizations with full relaxation of all atoms are carried out. Vibrational frequency calculations are 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.

Figure 1. Time-of-flight mass spectra for reactions of H2S with V16On+ (a, b) and V18On+ (c−e). Panels (a) and (c) plot the reference spectra without H2S in the reactor. Partial pressures of H2S in the reactor are about 0.8, 0.8, and 1.3 Pa for (b), (d), and (e), respectively. Numbers m,n and m,n,k denote VmOn+ and VmOn(H2S)k+, respectively. Some unlabeled product peaks can be due to secondary reactions of the primary products (see reactions 1−3 in the text) with H2S.

3. RESULTS 3.1. Experimental Results. Figure 1a,b plots the TOF mass spectra for reactions of V16On+ (n = 1−6) with H2S. The natural abundances of 32S, 33S, 34S, and 36S are 95.0, 0.76, 4.22, and 0.02%, respectively.30 Only the primary isotopomer H232S is considered for hydrogen sulfide in this study. Figure 1b shows that the interaction of V16On+ with about 1.0 Pa H2S in the fast flow reactor for about 60 μs generates various product clusters. For example, at each of the high-mass sides of V16On+ (n = 2−5) cluster peaks, products that can be assigned as V16OnH+ and V16OnH2+ are observed. Note that ion molecule reactions often generate simple addition products, so V16OnH2+ peaks may also be assigned as V16On−2H2S+ that are due to association of H2S with V16On−2+. To differentiate VOnH2+ from VOn−2H2S+ and resolve other possibly overlapped mass peaks (32S and 16O2 have the same mass number), the Vm18On+ clusters are generated and reacted with different partial pressures of H2S, and the results are plotted in Figures 1c−e and 2. Figure 1d shows that products that can be assigned as V18OnH2+ as well as V18OnH+ are also generated for n = 2−5. The simple addition products V18OnH2S+ (n = 1−3) are now separately observed. Moreover, at each of the low-mass sides of V18OnH2S+ (n = 1−3) cluster peaks, new product that can be assigned as V18OnS+ is apparently observed. Such peaks are completely overlapped with those of the reactant clusters in the 16O experiments

Figure 2. Time-of-flight mass spectra for reactions of about 0.8 Pa H2S with V2,318On+. Panels (a) and (c) plot the reference spectra without H2S in the reactor. Numbers m,n and m,n,k denote VmOn+ and VmOn(H2S)k+, respectively.

(Figure 1a,b). The above reaction pattern (appearance of hydrogen and sulfur-containing products) can also be found for some of the larger clusters (V2,318On+) in Figure 2. The experiments thus suggests that, in addition to the simple association reaction (VmOn+ + H2S → VmOnH2S+), three types of reaction channels involving H2S dissociation are possible: VmOn+ + H 2S = VmOnH+ + SH

(1)

VmOn+ + H 2S = VmOnH 2+ + S

(2)

VmOn+ + H 2S = VmOn − 1S+ + H 2O

(3)

The observation of reaction 2 in the gas phase is quite surprising as this reaction generates free sulfur atoms under thermal collision conditions. There are various ion molecular reactions explored,1a−f whereas examples for the generation of free atoms of main group elements are seldom reported. It is noticeable that, in the reactor, the concentration of the clusters (VmOn+ as well as VmOn− and VmOn) and their reaction 9044



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products (such as VmOnH+, VmOnH2+, SH, and S from reactions 1 and 2) have negligible concentrations in comparison with the reactant gas H2S. As a result, each of the observed reaction products (Figures 1 and 2) should be considered to be due to the interaction of H2S (rather than SH or S) with the clusters or their products. This means that it is very unlikely that the experimentally observed sulfur-containing clusters VmOn−1S+ (suggested to be from reaction 3) are due to the association of the S atoms (generated from reaction 2) with the VmOn−1+ clusters, although the association process (VmOn−1+ + S → VmOn−1S+) is entirely possible theoretically. The average number of collisions that a cluster (radius ∼ 0.25 nm) experiences with 1.0 Pa H2S (radius ∼ 0.18 nm) in the fast flow (velocity ∼ 1 km/s) reactor for 60 μs is about 8.5. Figure 1e shows that products that can be assigned as S2+ are also observable as the H2S partial pressure is increased to 1.3 Pa in the reactor, indicating that the products VmOn−1S+ from reaction 3 may further react with H2S to generate elemental sulfur species (VmOn−1S+ + H2S → VmOnH2 + S2+). The generation of neutral S2 may also be possible (VmOn−1S+ + H2S → VmOnH2+ + S2), but the S2 cannot be detected in our experiments. The first-order rate constant (k1) in the fast flow reactor can be estimated by using the following equation

k1 =

ln(I0/I ) ρ × Δt

(4)

in which the ratio I/I0 is the percentage of unreacted clusters after the cluster interaction with reactant gas that has a molecular density of ρ in the reactor for a reaction time of Δt.22,31 The estimated rate constant for VO2+ + H2S is 4.2 × 10−11 cm3 molecule−1 s−1 by using the data in Figure 1d. 3.2. Computational Results. To support the experimental results that reactions 1−3 can really take place in the reactor under thermal collision conditions, the DFT calculations are performed for the simplest reaction system in Figure 1 1

+

Figure 3. DFT calculated potential energy profiles (a) and structures (b) of reaction intermediates and transition-state species for (1) VO2+ + H2S (R) → VO2H+ + SH (P1), (2) R → VO2H2+ + S (P2), and (3) R → VOS+ + H2O (P3). The reaction intermediates and transition states are denoted as In and TSn1/n2, respectively. The relative energies of ΔH0K and ΔG298K (in eV) with respect to the separated reactants are given in the parentheses below each geometry. Bond lengths are in picometers.

+

VO2 + 1H 2S → 2 VO2 H + 2SH, ΔH0K /ΔG298K = −0.41/−0.41 eV +

(1.29 eV) for the first hydrogen atom transfer (HAT) from sulfur to one of the oxygen atoms (I1 → TS1/2 → I2). Because the O−H bond is much stronger than the S−H bond, additional energy can be released in the HAT. The high energy (ΔH0K = 2.87 eV) available after formation of I2 can cause (1) fragmentation of the closed-shell complex into two open-shell radicals (I2 → VO2H+ + SH, reaction 5), (2) structure rearrangement for the reaction system (I2 → TS2/4 → I4), and (3) the second HAT that leads to either a second OH moiety (I2 → TS2/3 → I3) or a H2O unit (I4 → TS4/5 → I5). The free sulfur atom (reaction 6) and H2O molecule (reaction 7) can be desorbed from I3 and I5, respectively. It should be pointed out that the reaction intermedieates (I2 and I3) shown in Figure 3 are much lower in energy than the products (P1− P3). Given that the reaction intermediates that carry not only the binding energy beteween VO2+ and H2S but also the centerof-mass kinetic energy as well as the vibrational energy of the reactants are not quickly stabilized by collissions with the helium bath gas in the reactor, it is possible to observe the products. Figure 1d indicates that, for the reaction of VO2+ with H2S, in addition to the products from reactions 5−7, the association complex VO2H2S+ is also observed, implying that a competition between formation of P1−P3 and collisional stabilization of the reaction intermediates, such as I3 in Figure 3, takes place in the experiment.

(5)

+

VO2 + 1H 2S → 3 VO2 H 2 + 3S, ΔH0K /ΔG298K

1

= −0.58/−0.36 eV +

(6)

+

VO2 + 1H 2S → 1VOS + 1H 2O, ΔH0K /ΔG298K

1

= −0.33/−0.31 eV

(7)

in which the superscripts are spin multiplicities of the reactants and products and ΔH0K and ΔG298K are the DFT calculated enthalpies and free energies of reactions, respectively. It indicates that all of the three reaction channels are exothermic; even reaction 5 or 6 involves the generation of two open-shell products from two closed-shell reactants. The potential energy profiles and DFT optimized structures for reaction intermediates and transition-state species in reactions 5−7 are shown in Figure 3. The energies (ΔH0K) for all of the transition states are negative with respect to the separated reactants (VO2+ + H2S), so reactions 5−7 are both thermodynamically and kinetically favorable by the DFT calculations, which is consistent with the experiments (Figure 1). During the reaction of VO2+ with H2S, the energy released (ΔH0K = 2.12 eV) in the formation of the encounter complex (I1 in Figure 3) is high enough to overcome the energy barrier 9045



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be facilitated by the closed-shell species VO2+ and the HAT from sulfur to the oxygen atom in the formal oxidation state (O2−) can be facile. As a result, the atomic oxygen radical (O−•) plays an important role in alkane C−H bond activation over the clusters,35 whereas the activation of the much weaker S−H bond does not necessarily need such reactive oxygen species. In addition to the HAA and O−S exchange reaction channels that can be compared with previous results for the reactivity of vanadium oxide clusters, the generation of the free sulfur atom in the gas phase (reaction 2 or 6) is not expected as the chemical generation of free atoms of main group elements under thermal collision conditions is barely observed not only for the reactions of vanadium oxide clusters6−12 but also for many other ion molecule reactions.1a−f This reaction channel for H2S with VmOn+ can also be traced back to the much stronger bond strength of O−H versus S−H (Figure 3 and reaction 9). 4.2. Consideration of Condensed Phase Reactions. Vanadium-based oxide materials are effective catalysts in many studies4 to facilitate selective oxidation of H2S by O2 to elemental sulfur and water:

Figure 3 shows that all of the transition states are lower in energy (ΔH0K) than both the reactants and the three types of products. As a result, reactions 5−7 can be considered to be driven by the favorable thermodynamics. This may quickly rationalize the experimental indication that similar reaction channels are also observed for other clusters (reactions 1−3). It is important to note that, for larger clusters, such as VO3+ and V2O5+, the mechanisms of reactions with H2S may be different from those of VO2+ + H2S in Figure 3. The DFT studies in the future are necessary to interpret the experimental observations for these larger clusters.

4. DISCUSSION 4.1. Comparison with Related Reactivity of Vanadium Oxide Clusters in the Literature. Step-by-step hydrogen atom migration from H2S to VmOn+ clusters that can finally lead to oxygen−sulfur (O−S) exchange (reaction 3 or 7) between H2S and VmOn+ in this work are comparable with the reactions between H218O and Vm16On+,12 in which the isotopic 16O/18O exchange is observed for most of the studied vanadium oxide cluster cations, including VO2+. Note that the O−S exchange reactions were also identified in a few reports for reactions, such as Sn2O2,3+ + H2S,20d AlO2− + H2S and SiO3H− + H2S,20e and Mn2O2+ + H2S.20f The V−S bond32 can be much weaker than the V−O bond, which disfavors the O−S exchange reactions. For example, for diatomic molecule systems, the energy cost for change of the V−O to the V−S bond is33 VO + S → VS + O, ΔH298K = 1.56 eV

H 2S +

(9)

It can be seen that the energetics consideration for the simple diatomic oxide and sulfide (VO and VS) and triatomic hydrides (H2O and H2S) as well as the DFT study on the VO2+ reaction system (Figure 3) supports the experimentally observed results of the O−S exchange reactions for VmOn+ with H2S (reaction 3). It is noticeable that the VO2+ is the smallest cluster that can be subjected to the O−S exchange reaction with H2S in the experiments, and no evidence of such a reaction for VO+ (Figure 1) is due to unfavorable thermodynamics, as predicted by the DFT calculations: VO+ + H 2S → VS+ + H 2O, ΔH0K = 0.47 eV

(11)

Note that the conversion of H2S into SO2 is usually considered as an unwanted side reaction. The generation of sulfur atoms (reaction 2; see also S2+ in Figure 1e) and water (reaction 3) in this study is thus paralleled with similar behaviors of H2S reactions over vanadium oxide catalysts. In addition, it provides insights into elementary steps for how the elemental sulfur (S6−8 in reaction 11) may be generated: the free sulfur atom (or weakly absorbed surface sulfur atom) can be generated by two consecutive HATs from a reactant molecule (H2S) to surface oxygen atoms, and the aggregation of sulfur atoms finally generates elemental sulfur over the surfaces. Moreover, reaction 3 that generates sulfur-containing species and their further reactions with H2S or S atoms over the surfaces may also generate elemental sulfur. It is noticeable that this cluster study indicates that the generation of SH free radicals (reaction 1 and Figure 3) is also possible. The role of such a reactive intermediate may be considered in practical catalysis.

(8)

However, the sulfur-containing clusters (V 18 O 1−3 S + , V218O3−6S+, and V318O6S+ in Figures 1 and 2) that can be due to the O−S exchange reactions are clearly observed. This means that energy gain involved with transformation of S−H to O−H bonds should be more than the energy cost in reaction 8:29 H 2S + O → H 2O + S, ΔH298K = −2.00 eV

1 1 O2 → Sn + H 2O (n = 6−8) 2 n

5. CONCLUSION The reactions of vanadium oxide cluster cations with hydrogen sulfide are studied by time-of-flight mass spectrometry and density functional theory calculations. Hydrogen atom abstraction, free sulfur atom generation, and oxygen−sulfur exchange are observed as three types of reaction channels in the experiments. The theoretical study for reaction of VO2+ with H2S is in agreement with the experiments and interprets that the three reaction channels are driven by favorable thermodynamics that is due to the much weaker S−H versus O−H bonds. The gas-phase cluster chemistry for vanadium oxide cluster cations with H2S parallels similar behavior in selective oxidation of H2S over vanadium-based oxide catalysts. The free sulfur atom generation and oxygen−sulfur exchange in the cluster reactions provide insights into elementary steps for elemental sulfur production in related surface reactions.

(10)

In the reactions with alkane molecules, such as methane, ethane, or butane, the hydrogen atom abstraction (HAA) channel is observed primarily for vanadium oxide clusters with specific compositions: (V2O5)1−5+8b,9c and (V2O5)1,2O−34, which are open-shell species and contain oxygen-centered radicals (O−•) as the active sites toward C−H bond activation. In the reaction with H2S in this work, not only the O−•containing cluster (V2O5+ in Figure 2b) but also the clusters (such as VO2+ and V2O4+) without O−• radicals are able to abstract one hydrogen atom from H2S. This can also be traced back to the much stronger bond strength of O−H as well as C−H versus S−H. The DFT result in Figure 3 (R → I1 → TS1/2 → I2) also suggests that the S−H bond activation can 9046



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-G.H.), [email protected]. cn (M.-F.G.). Tel: +86-10-62536990. Fax: +86-10-62554518. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences (Knowledge Innovation Program No. KJCX2-EW-01, Hundred Talents Fund), the National Natural Science Foundation of China (Nos. 20933008 and 21173233), and the 973 Program (No. 2011CB932302).

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