Experimental and Theoretical Study of the Reactions between MO2

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Experimental and Theoretical Study of the Reactions between MO2¯ (M = Fe, Co, Ni, Cu, and Zn) Cluster Anions and Hydrogen Sulfide Mei-Ye Jia, Xun-Lei Ding, Sheng-Gui He, and Maofa Ge J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp4044623 • Publication Date (Web): 12 Aug 2013 Downloaded from http://pubs.acs.org on August 15, 2013

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The Journal of Physical Chemistry

Experimental and Theoretical Study of the Reactions between MO2‾ (M = Fe, Co, Ni, Cu, and Zn) Cluster Anions and Hydrogen Sulfide Mei-Ye Jia,†,‡ Xun-Lei Ding,† Sheng-Gui He,*,† 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

*E-mail: [email protected] (S.-G. He), [email protected] (M.-F. Ge); Tel.: +86-10-62536990; Fax: +86-10-62554518. 1

ACS Paragon Plus Environment


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Abstract: Transition metal oxide cluster anions Mm18On‾ (M = Fe, Co, Ni, Cu, and Zn) were prepared by laser ablation and reacted with 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 reveal a suite of oxygen/sulfur (O/S) exchange and oxygen/sulfydryl (O/SH) exchange reactions. The O/S exchange reaction to release water was evidenced for all of the MO2‾ cluster anions: MO2‾ + H2S → MOS‾ + H2O; while the O/SH exchange reaction to derive MOSH‾ and OH species was only observed for reactions of NiO2‾, CuO2‾, and ZnO2‾. Density functional theory calculations were performed for reaction mechanisms of MO2‾ + H2S (M = Fe, Co, Ni, Cu, and Zn). The computational results are generally in good agreement with the experimental results. This gas-phase study provides an insight into the metal dependent reactivity in the removal of H2S over metal oxides.

Keywords: Transition metal oxide; oxygen/sulfur exchange; oxygen/sulfydryl exchange; reaction mechanisms; mass spectrometry.

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1. Introduction Removal of poisonous H2S from natural gas and from petroleum refinery gases is of great importance.1-2 Metal or metal oxide catalysts3-5 can be used to recover elemental sulfur in the selective oxidation of H2S:6-7 H2S + 1/2O2 → 1/n (Sn) + H2O

(1)

The oxides of aluminum,8 titanium,9 vanadium,10-12 chromium,13-14 manganese,15-16 iron,17-18 cobalt,19 nickel,15-16 copper,15-16 zinc,15-16 and rare earth elements20 have been demonstrated to be active catalysts for reaction (1). In condensed phase, interactions of H2S with oxide surfaces of FeO/Fe2O3,15-16, 21-22 CoO,15-16 NiO,15-16 CuO,15-16, 23-25 ZnO,15-16, 26-27 and so on have been explored. It is generally found that H2S mainly interacts with metal centers on the surfaces while molecular level reaction mechanisms are still unclear, especially for the details of dehydration by oxygen/sulfur (O/S) exchange reaction at the gas-solid interface:15-16 MxOy + H2S → MxOy-1S + H2O

(2)

To disclose mechanistic details in the condensed phase reactions, an effective approach is to study gas phase metal oxide clusters under well controlled conditions.28-39 In our recent study of the +

−

gas phase reactions of clusters VmOn and MnmOn with H2S,40-41 the O/S exchange reactions to generate H2O were identified, which is consistent with the oxidative removal of H2S by metal oxides in condensed phase systems (reaction 2). Furthermore, the homolytic S−H bond cleavage to +

generate S atom in the cluster reactions of VmOn with H2S provide molecular level insights into the selective oxidation of H2S to element sulfur and water over vanadium oxide based catalysts.10-12 To the best of our knowledge, the reactivity of metal oxide cluster anions toward H2S have been only investigated for MnmOn‾,41 AlO2‾/SiO2‾,42 SnmOn‾,43 and ComOn‾.44 In these investigations, the O/S exchange reactions to generate H2O were identified and the generation of OH radical was also reported in the reactions of SnO2‾with H2S.43 In addition, the reactions of cluster cations 3



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FeO+,45 Fe2O2+,46 Mn2O+/Mn2O2+

47

with H2S have also been studied. In order to compare the

reactions of H2S with various metal oxides, we have studied the reactions of MmOn‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S. The MO2‾ anions are the focus of this work and their reactions with H2S were characterized by mass spectrometry and density functional theory (DFT) calculations. This gas-phase study provides a chance to understand the metal dependent reactivity in H2S removal with the cluster approach.

2. Experimental and Theoretical 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.48 The MmOn‾ (M = Fe, Co, Ni, Cu, and Zn) clusters were produced by laser ablation of a rotating and translating metal disk (> 99.9%) in the presence of 1%

18

O2 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 gas handling system to form 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 the 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 49, the instantaneous total gas pressure in the fast flow reactor was estimated to be around 220 Pa at T = 300 K. The intra-cluster vibrations were likely equilibrated (cooled or heated, depending on the vibration temperature after exiting cluster formation channel with a supersonic expansion) to close to the bath gas temperature before reacting 4


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with the diluted H2S. It may be safe to assume that the bath gas has the temperature of the wall of reactor (~300 K).50 Our recent experiments indicate that the cluster vibrational temperature in the reactor can be close to 300 K.51-52 After reacting in the fast flow reactor, the reactant and product ions were 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/∆M) is 400−500 with current experimental setup. More details of the experiments can be found in our previous works.53-54

2.2. Computational Details The DFT calculations with Gaussian 09 program55 were performed to investigate the structures of MO2‾ (M = Fe, Co, Ni, Cu, and Zn) cluster anions and the reaction mechanisms of MO2‾ + H2S. We have tested several density functionals by calculating the bond enthalpies of M−O, M−S, HS−H, and S−H species. The experimental56-59 and calculated bond enthalpies of these species are shown in Table 1. It can be seen that B3LYP functional60-61 is generally better than other ones when calculating the M−O and M−S bond enthalpies except for Zn−S. The newly developed M06-2X functional62-63 has been tested to provide reasonable results for bond enthalpies of Cu−O, Cu−S, Zn−O, HS−H, and S−H species. However, the M06-2X is generally poor than B3LYP for other M−O and M−S bond enthalpies. The results of functionals M06HF64 and MP265 are bad for most of the M−O and M−S species. The B3P86 functional66-67 is not suitable for Fe−O as well as most of the M−S species and the BPW91 functional68-69 can not reproduce the bond enthalpies for most of the M−O species. The results of X3LYP functional70 are very close to those of B3LYP. Thus, the B3LYP functional with basis set 6-311++G**71 was selected to investigate the structures of MO2‾ and reaction mechanisms of MO2‾ with H2S. In addition, the M06-2X functional with basis set 6311++G** was applied to calculate the reaction mechanism of CuO2‾ + H2S and reaction enthalpy of ZnO2‾ + H2S. Geometry optimizations of all the reaction intermediates and transition states on 5



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the potential energy surfaces were carried out with full relaxation of all atoms. 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 vibrational energy (ZPE) corrected (∆H0K) and the relative Gibbs free energies (∆G298K) under temperature of 298.15 K and pressure of 1.0 atm. Structures and vibrational frequencies for all of the optimized species are available upon request. Table 1. The experimental and calculated bond enthalpies of M−O, M−S, HS−H, and S−H species by different functionals with the basis set 6-311++G**. Bonds

Bond enthalpies / eV M06-2X M06HF MP2 3.57 3.89 4.16 2.93 2.44 1.03 2.44 1.85 2.40 2.34 1.97 5.02 1.32 1.81 1.16

Fe−O Co−O Ni−O Cu−O Zn−O

Expt 4.17 ± 0.08 3.94 ± 0.14 3.87 ± 0.03 2.85 ± 0.15 1.61 ± 0.04

B3LYP 4.29 3.52 3.58 2.67 1.28

Fe−S Co−S Ni−S Cu−S Zn−S

3.51 ± 0.22 3.56 ± 0.22 3.73 ± 0.22 2.95 ± 0.18 2.13 ± 0.13

3.89 2.58 3.20 2.65 1.20

3.96 2.38 2.31 2.52 1.33

3.89 2.25 1.95 2.31 1.76

HS−H S−H

3.95 ± 0.04 3.57 ± 0.12

3.82 3.62

3.83 3.57

3.79 3.58

B3P86 4.97 3.91 3.75 2.78 1.46

BPW91 5.27 4.66 4.59 3.08 1.71

X3LYP 4.78 3.47 3.53 2.66 1.28

2.98 2.14 1.27 5.11 0.93

4.07 2.78 3.43 2.83 1.46

4.06 3.96 4.04 2.92 1.50

3.89 2.55 3.16 2.66 1.21

3.66 3.25

3.97 3.65

3.80 3.58

3.82 3.62

3. Results 3.1. Experimental Results The natural abundances72 of 32S, 33S, 34S, and 36S are 95.0%, 0.76%, 4.22%, and 0.02%, respectively. Only the primary isotopomer H232S is considered for hydrogen sulfide in this study. Because and

16

O2 have the same mass number, the

16

32

S

O experiment cannot identify O/S exchange products

Mm16On−1S‾ of which the mass peaks are overlapped with those of Mm16On+1‾, as were shown in our 6


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recent work for VmOn+ and MnmOn‾ with H2S.40-41 For the reason mentioned above, the generation of MmOn‾ (M = Fe, Co, Ni, Cu, and Zn) cluster anions in this work were conducted by using 18O2 as oxygen source. Fig. 1 plots the TOF mass spectra of M18On‾ (n = 2−4) before (parts 1a, c, e, g, and i) and after (parts 1b, d, f, h, and j) the reactions with H2S at a partial pressure of about 1.2 Pa in the reactor, respectively.

Fig. 1 The time-of-flight mass spectra for reactions of M18On‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S. Parts a, c, e, g, and i plot the reference spectra without H2S in the reactor. Partial pressure of H2S in the reactor is about 1.2 Pa for b, d, f, h, and j. Symbols 1,n, 1,n,S, and 1,n,SH denote MOn‾, MOnS‾, and MOnSH‾, respectively. The asterisk marked peaks are those of the metal isotopomers. 7



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In Fig. 1, the product peaks that can be assigned as M18OS‾ (M = Fe, Co, Ni, Cu, and Zn) and M18OSH‾ (M = Ni, Cu, and Zn) are apparently observed. The experiments thus suggest the O/S and O/SH exchange reactions: MO2‾ + H2S → MOS‾ + H2O (M = Fe, Co, Ni, Cu, and Zn)

(3)

MO2‾ + H2S → MOSH‾ + OH (M = Ni, Cu, and Zn)

(4)

Peaks with two S atoms (FeS2‾, CoS2‾, and NiS2‾) are also observable, indicating that products MOS‾ (M = Fe, Co, and Ni) from reaction (3) can undergo further O/S exchange reactions. As is can be seen from Fig. 2, O/SH exchange channel in the reactions of MO2‾ with H2S becomes more and more favorable than O/S exchange when M changes from Fe to Zn. For FeO2‾, CoO2‾, and NiO2‾, the O/S exchange reaction is more favorable than O/SH exchange reaction, while there is an inverse relationship of O/S and O/SH exchange reactions for CuO2‾ and ZnO2‾.

Fig. 2 Branching ratios of O/S and O/SH exchange channels in the reactions of MO2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S.

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The pseudo first order rate constant (k1) in the fast flow reactor can be estimated by using the following equation: k1 =

ln( I 0 /I ) ρ × ∆t

(5)

in which the ratio I/I0 is the percentage of un-reacted clusters after the cluster interaction with reactant gas that has molecular density of ρ in the reactor for reaction time of ∆t (see ref 73−74 for details of rate constant determination). The estimated rate constants for M18O2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S are listed in Table 2. It can be seen that the clusters FeO2‾, CoO2‾, NiO2‾, CuO2‾, and ZnO2‾ are very reactive (k1 = 1~5 × 10−10 cm3 molecule−1 s−1).

Table 2. The pseudo first order rate constants (k1/cm3 molecule-1 s-1) for the reactions of MO2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S. MO2‾ FeO2‾ CoO2‾ NiO2‾ CuO2‾ ZnO2‾ 1010 k1 2.7 1.6 4.7 2.3 2.5

The mass spectra of reactions between larger clusters Mm18On‾ (M = Fe, Co, and Ni; m = 2−6) and H2S at different partial pressures are shown in Figs. 3−5. The O/S exchange (Fem18On‾, Com18On‾, and Nim18On‾) and O/SH exchange (Nim18On‾) reactions are apparently observed for most of the clusters. The metal dependent selectivity between O/S and O/SH exchange reactions of those lager clusters Mm18On‾ toward H2S is generally in agreement with that of MO2‾ (M = Fe, Co, and Ni). It is noticeable that ion molecule reactions often generate simple addition products75-76 while such products Mm18OnH2S‾ were rarely generated in the interactions of Mm18On‾ with H2S.

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Fig. 3 The time-of-flight mass spectra for reactions of Fem18On‾ with H2S. Parts a1 and a2 plot the reference spectra without H2S in the reactor. Partial pressures of H2S in the reactor are about 0.8 and 1.2 Pa for b1−b2 and c1−c2, respectively. Symbols m,n, m,n,S, m,n,S2, and m,n,H2 denote FemOn‾, FemOnS‾, FemOnS2‾, and FemOnH2‾, respectively.

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Fig. 4 The time-of-flight mass spectra for reactions of Com18On‾ with H2S. Parts a1 and a2 plot the reference spectra without H2S in the reactor. Partial pressures of H2S in the reactor are about 0.8 and 1.2 Pa for b1−b2 and c1−c2, respectively. Symbols m,n, m,n,S, and m,n,S2 denote ComOn‾, ComOnS‾, and ComOnS2‾, respectively. 11



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Fig. 5 The time-of-flight mass spectra for reactions of Nim18On‾ with H2S. Parts a1 and a2 plot the reference spectra without H2S in the reactor. Partial pressures of H2S in the reactor are about 0.8 and 1.2 Pa for b1−b2 and c1−c2, respectively. Symbols m,n, m,n,S, m,n,S2, and m,n,SH denote NimOn‾, NimOnS‾, NimOnS2‾, and NimOnSH‾, respectively. 12


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3.2. Computational Results The B3LYP/6-311++G** calculated structures and profiles of spin density distributions for MO2‾ (M = Fe, Co, Ni, Cu, and Zn) are shown in Fig. 6. The FeO2‾ is of a V-shaped structure (C2v) and the spin density of the cluster is localized on the iron atom. The CoO2‾, NiO2‾, CuO2‾, and ZnO2‾ are linear with high symmetry (D∞h). The spin densities of CoO2‾ and NiO2‾ are delocalized over the metal and oxygen atoms with high fractions on metal atoms. In CuO2‾ and ZnO2‾, the spin densities are highly localized on oxygen atoms.

Fig. 6 B3LYP/6-311++G** calculated structures and profiles of spin density distributions for MO2‾ (M = Fe, Co, Ni, Cu, and Zn) clusters. The spin density values over metal and oxygen atoms in µB are given.

The DFT calculated potential energy profiles and reaction pathways for MO2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S are given in Figs. 7 and 8.

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Fig. 7 The DFT calculated potential energy profiles (∆H0K in eV) of MO2‾ + H2S → MOS‾ + H2O (O/S exchange) and MO2‾ + H2S → MOSH‾ + OH (O/SH exchange) (B3LYP for MO2‾ + H2S and M06-2X for CuO2‾ + H2S). The reaction intermediates and transition states are denoted as MIn and M

TSn.

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Fig. 8 Structures and energies of the reaction species in Fig. 7, the relative energies of ∆H0K and ∆G298K (in eV) with respect to the separated reactants are given in the parentheses. Bond lengths are in pm. 15



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3.2.1. FeO2‾ + H2S The B3LYP/6-311++G** calculated ground state of FeO2‾ is 4B2 and results for FeO2‾ + H2S are as follows: 4

FeO2‾ + 1H2S → 4FeOS‾ + 1H2O,

∆H0K = −1.29 eV

(6a)

4

FeO2‾ + 1H2S → 5FeOSH‾ + 2OH,

∆H0K = −0.12 eV

(6b)

in which the superscripts indicate the spin multiplicities for the reactant and product species, and the ∆H0K values are the DFT calculated enthalpies of the reactions. According to the computational results, reaction 6a is thermodynamically more favorable than reaction 6b, which is consistent with the experimental observation of FeOS‾ rather than FeOSH‾ (Fig. 1b). The DFT calculated potential energy profiles and reaction pathways for FeO2‾ with H2S are given in Figs. 7a and 8a. As is shown in Fig. 8a, upon the approaching of H2S to FeO2‾, cleavage of one of the S−H bonds and formation of the O−H and Fe−SH bonds take place immediately. This process FeO2‾ + H2S → FeO2H−SH‾ (FeI1) can release 2.52 eV (∆H0K) heat of formation that is high enough to supply the energy needed for the cleavage of the second S−H bond FeO2H−SH‾ (FeI1) → Fe(OH)2−S‾ (FeI2). The energy released (∆H0K, 3.09 eV) during the formation of the stable intermediate Fe(OH)2−S‾ (FeI2) is high enough to overcome the energy cost for the subsequent transformation of structural isomers for reaction intermediates Fe(OH)2−S‾ (FeI2) → Fe(OH)2−S‾ (FeI3) and the dehydration process Fe(OH)2−S‾ (FeI3) → FeOS‾−H2O (FeI4) → FeOS‾ + H2O (FeP1). The transition states FeTS1, FeTS2, Fe

TS3 and the products FeOS‾ + H2O are well below in energy than the separated reactants FeO2‾ +

H2S, so the O/S exchange reaction is both thermodynamically and kinetically favorable. The DFT results are thus consistent with the experimentally observed fast reaction between FeO2‾ and H2S (k1 = 2.7 × 10−10 cm3 molecule−1 s−1, see Table 2).

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3.2.2. CoO2‾ + H2S The B3LYP/6-311++G** calculated ground state of CoO2‾ is 5A1 and results for CoO2‾ + H2S are as follows: 5

CoO2‾ + 1H2S → 5CoOS‾ + 1H2O,

∆H0K = −1.48 eV

(7a)

5

CoO2‾ + 1H2S → 4CoOSH‾ + 2OH,

∆H0K = −0.51 eV

(7b)

The computational results indicate that reaction 7a is thermodynamically more favorable than reaction 7b, which is consistent with the experimental observation of O/S exchange product CoOS‾ rather than O/SH exchange product CoOSH‾ (Fig. 1d). As is shown in Figs. 7b and 8b, the reactions of CoO2‾ + H2S are quite similar to that of FeO2‾ + H2S. The transition states

Co

TS1,

Co

TS2, CoTS3 and the products CoOS‾ + H2O are well below in energy than the separated reactants

CoO2‾ + H2S, so the O/S exchange reaction is both thermodynamically and kinetically favorable. The DFT results are also consistent with the experimentally observed fast reaction between CoO2‾ and H2S (k1 = 1.6 × 10−10 cm3 molecule−1 s−1, see Table 2).

3.2.3. NiO2‾ + H2S The B3LYP/6-311++G** calculated ground state of NiO2‾ is 4B1 and results for NiO2‾ + H2S are as follows: 4

NiO2‾ + 1H2S → 4NiOS‾ + 1H2O,

∆H0K = −1.46 eV

(8a)

4

NiO2‾ + 1H2S → 3NiOSH‾ + 2OH,

∆H0K = −0.83 eV

(8b)

The computational results show that both reaction 8a and 8b are thermodynamically favorable, and the reaction 8a is still thermodynamically more favorable than 8b, which is consistent with the experimental observations that NiOS‾ is more abundant than NiOSH‾ (Fig. 1f). The DFT calculated potential energy profiles and reaction pathways for NiO2‾ with H2S are given in Figs. 7c and 8c. As is shown in Fig. 8c, upon the approaching of H2S to NiO2‾, cleavage of one of the S−H bonds and formation of the O−H and Ni−SH bonds take place immediately. This process NiO2‾ + 17



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H2S → NiO2H−SH‾ (NiI1) can release 2.17 eV (∆H0K) heat of formation that is high enough to supply the energy needed for the transformation of structural isomers for reaction intermediates NiI1 (NiO2H−SH‾) → NiI2 (NiO2H−SH‾) and the cleavage of the second S−H bond NiO2H−SH‾ (NiI2) → Ni(OH)2−S‾ (NiI3). The energy released (∆H0K, 2.76 eV) during the formation of Ni(OH)2−S‾ (NiI3) is high enough to overcome the energy cost for the formation of H2O molecule and the dehydration process Ni(OH)2−S‾ (NiI3) → NiOS‾−H2O (NiI4) → NiOS‾ + H2O (NiP1). It also contributes to the generation of O/SH exchange products by de-hydroxyl process Ni(OH)2−S‾ (NiI3) → NiOSH‾ + OH (NiP2). The transition states NiTS1, NiTS2, NiTS3 and the products NiOS‾ + H2O and NiOSH‾ + OH are well below in energy than the separated reactants NiO2‾ + H2S, so both the O/S and O/SH exchange reactions are thermodynamically and kinetically favorable. The DFT results are thus consistent with the experimental observations.

3.2.4. CuO2‾ + H2S The B3LYP/6-311++G** calculated ground state of CuO2‾ is 3B1 and results for CuO2‾ + H2S are as follows: 3

CuO2‾ + 1H2S → 3CuOS‾ + 1H2O,

∆H0K = −1.76 eV

(9a)

3

CuO2‾ + 1H2S → 2CuOSH‾ + 2OH,

∆H0K = −1.13 eV

(9b)

Both reaction 9a and 9b are thermodynamically favorable, the calculated potential energy profiles and reaction pathways are shown in Figs. 7d and 8d. When CuO2‾ meets H2S, cleavage of one of the S−H bonds and formation of the O−H bond take place CuO2‾ + H2S → CuO2H…SH‾ (CuI1), which is different to that of the Fe, Co, and Ni systems. Then, the Cu−SH bond formed CuO2H…SH‾ (CuI1) → CuO2H−SH‾ (CuI2) and the subsequent steps CuO2H−SH‾ (CuI2) → Cu(OH)2−S‾ (CuI3) → CuOSH‾ + OH (CuP2) and

Cu

I2 →

Cu

I3 → Cu(OH)2−S‾ (CuI4) →

CuOS‾−H2O (CuI5) → CuOS‾ + H2O (CuP1) are very similar to those of NiO2‾ with H2S. The energy released (∆H0K, 2.77 eV) in the formation of Cu(OH)2−S‾ (CuI3) is high enough to overcome 18


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the energy cost for the cleavage of one of the Cu−OH bonds, leading to the O/SH exchange products CuOSH‾ + OH (CuP2). It is also high enough to supply the energy cost for the subsequent transformations of reaction intermediates Cu(OH)2−S‾ (CuI3) → Cu(OH)2−S‾ (CuI4) and the dehydration process Cu(OH)2−S‾ (CuI4) → CuOS‾−H2O (CuI5) → CuOS‾ + H2O (CuP1). The products (CuOS‾ + H2O and CuOSH‾ + OH) and the transition states

Cu

TS1,

Cu

TS2, CuTS3,

Cu

TS4

are well below in energy than the separated reactants CuO2‾ + H2S, so both the O/S and O/SH exchange reactions are thermodynamically favorable. The DFT results are in general consistent with the experiment. However, the calculated energetics can not explain why the O/S exchange channel is less favorable than the O/SH exchange channel (Figs. 1h and 2). The M06-2X/6-311++G** calculated potential energy profiles and reaction pathways of CuO2‾ + H2S are shown in Figs. 7f and 8f. As can be seen in Figs. 8d and 8f, except for the large difference in the encounter complex, the M06-2X calculated reaction pathways of CuO2‾ + H2S are quite similar to that of B3LYP. However, the enthalpy of reaction 9a (∆H0K = −2.16 eV) is less negative than that of reaction 9b (∆H0K = −2.35 eV). This means that better agreement with the experimental observations (CuOSH‾ is more abundant

than CuOS‾ in Figs. 1h and 2) can be

obtained by the M06-2X functional.

3.2.5. ZnO2‾ + H2S The B3LYP/6-311++G** calculated ground state of ZnO2‾ is 2B2 and results for ZnO2‾ + H2S are as follows: 2

ZnO2‾ + 1H2S → 2ZnOS‾ + 1H2O,

∆H0K = −2.01 eV

(10a)

2

ZnO2‾ + 1H2S → 1ZnOSH‾ + 2OH,

∆H0K = −1.61 eV

(10b)

Both reaction 10a and 10b are thermodynamically favorable, the calculated potential energy profiles and reaction pathways are shown in Figs. 7e and 8e. After the encounter of ZnO2‾ with H2S and the formation of ZnO2H−SH‾ (ZnI1), the subsequent steps are ZnO2H−SH‾ (ZnI1) → Zn(OH)2−S‾ (ZnI2) → ZnOSH‾ + OH (ZnP2) and 19

Zn

I1 →

Zn


I2 → Zn(OH)2−S‾ (ZnI3) →


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ZnOS−H2O‾ (ZnI4) → ZnOS‾ + H2O (ZnP1). The energy released (∆H0K, 3.62 eV) in the formation of Zn(OH)2−S‾ (ZnI2) is high enough to overcome the energy cost for the cleavage of one of the Zn−OH bonds, leading to the O/SH exchange products ZnOSH‾ + OH (ZnP2). It is also high enough to supply the energy cost for the subsequent transformations of reaction intermediates Zn(OH)2−S‾ (ZnI2) → Zn(OH)2−S‾ (ZnI3) and the dehydration process Zn(OH)2−S‾ (ZnI3) → ZnOS−H2O‾ (ZnI4) → ZnOS‾ + H2O (ZnP1). The products (ZnOS‾ + H2O and ZnOSH‾ + OH) and the transition states Zn

TS1, ZnTS2, ZnTS3 are well below in energy than the separated reactants ZnO2‾ + H2S, so both the

O/S and O/SH exchange reactions are thermodynamically favorable. The B3LYP results are in general agreement with the experiment while it can not explain the observations that O/SH exchange channel is more favorable than the O/S exchange channel, which is similar to the case of CuO2‾ + H2S. The M06-2X/6-311++G** calculated enthalpies of reaction 10a and 10b are ∆H0K = −2.80 eV and ∆H0K = −2.61 eV, respectively; which is in better (with respect to the results of B3LYP) agreement with the experimental observations that ZnOSH‾ is more abundant than ZnOS‾ (Figs. 1j and 2). As is indicated in Figs. 1−2 and 7−8, the O/SH exchange channel in the reactions of MO2‾ with H2S becomes more and more favorable when M changes from Fe to Zn and the differences for the enthalpies of O/S and O/SH reactions also decrease (Fig. 9). This means the B3LYP functional predicts a qualitatively correct trend that O/SH exchange becomes more and more favorable than O/S exchange when M changes from Fe to Zn.

20


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Fig. 9 Thermodynamical competitions between MO2‾ + H2S → MOS‾ + H2O (O/S exchange) and MO2‾ + H2S → MOSH‾ + OH (O/SH exchange) (M = Fe, Co, Ni, Cu, and Zn) with B3LYP (a) and M06-2X (b) functionals.

4. Discussion 4.1. Metal dependent reactivity and selectivity of MO2‾ toward H2S Many gas phase study of atomic clusters have obtained that the reactivity of metal oxide clusters can be very sensitive to the cluster size and composition,49,77 charge state,78 and detail of the electronic state.79 For example, it is reported that thermal activation of C−H bonds in alkane molecules can only take place over metal oxide clusters with specific compositions that contain oxygen-centered radicals80 and the reactivity is further controlled by the local charge81 and local spin79 environments around the radical centers.82-83 In this work, the reactions of MO2‾ with H2S are substantially different for different metals: (I) FeO2‾ and CoO2‾ reacted with H2S by consecutive O/S exchange reactions to eliminate H2O molecule, first forming MOS‾ and then MS2‾; (II) For the reactions of NiO2‾with H2S, besides the above mentioned consecutive H2O elimination to form NiOS‾ and NiS2‾, the O/SH exchange 21



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reaction to generate NiOSH‾ and OH radical was also observed apparently; (III) For reactions of CuO2‾ and ZnO2‾with H2S, the O/SH exchange products MOSH‾ become more abundant than the O/S exchange products MOS‾, and no CuS2‾ and ZnS2‾ are generated (Fig. 1). Such a metal dependent reactivity and selectivity is a new piece of information for chemistry of metal oxide clusters. The number of d electrons of metal centers Fe, Co, Ni, Cu, and Zn increases as the atomic number increases. The increased number of d electrons tends to favor the M−S bond formation (Fig. 10) that is very important for both the O/S and O/SH exchange reactions, especially for the O/SH exchange reactions. It turns out that the O/SH exchange can be more favorable than the O/S exchange for CuO2‾ and ZnO2‾ systems, which is correctly predicted by the M06-2X calculated results for the CuO2‾ system (Figs. 7f and 9b).

Fig. 10 The experimental bond enthalpies (data from Table 1) for diatomic oxides MO and sulfides MS (M = Fe, Co, Ni, Cu, and Zn).

22


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4.2. Considerations of the B3LYP and M06-2X computational results The computational results under B3LYP level confirm the reactivity of MO2‾ (M = Fe, Co, Ni, Cu, and Zn) toward H2S, and give reasonable explanations to the O/S and O/SH exchange reactions. However, the O/S exchange reactions in these studied systems are always thermodynamically more favorable than the O/SH exchange reactions. As a result, the B3LYP results can not interpret the experimental observations that the O/SH exchange product is more abundant than the O/S exchange product for CuO2‾ + H2S and ZnO2‾ + H2S. As is shown in Fig. 9, under different DFT functionals, the enthalpies of the O/S and O/SH exchange reactions differ largely. For CuO2‾ + H2S, under the M06-2X level, the O/S and O/SH exchange products CuOS‾ + H2O and CuOSH‾ + OH are 2.16 and 2.35 eV (∆H0K) below in energy than the separate reactants; while the values are 1.76 and 1.13 eV under B3LYP level, respectively. Thus the results under M06-2X are in better agreement with the experimental observations that O/SH exchange reaction is more favorable than O/S exchange reaction (Figs. 1h and 2). For NiO2‾ + H2S and ZnO2‾ + H2S, the results under M06-2X level are also more reasonable than those under B3LYP. For the reactions of FeO2‾ or CoO2‾ with H2S, the results under M06-2X level are also in agreement with the experiment (Figs. 1 and 9). However, the M06-2X is generally poor than B3LYP (Table 1) for M−O and M−S bond enthalpies. It implies that there can be error cancellation in the M06-2X calculations for the MO2‾ + H2S reactions. It is known that the late transition metals are difficult for quantum chemistry calculations. For example, for FeO2‾, the ground state of this triatomic system is extremely difficult to be determinated. Many studies84-86 agree that the most stable isomer for the FeO2‾ is either a bent C2v or a closely related linear D∞h dioxide structure, which possesses two equivalent iron-oxygen bonds with no direct O−O bond. Because of the small energy differences between the low-lying electronic states of FeO2‾, the DFT calculations can only tentatively predict a 4B2 state (bent C2v structure) as the ground state. In this work, the B3LYP/6-311++G** calculated energies of the sextet or doublet states of FeO2‾ are 0.09 and 0.33 eV higher than the ground state (4B2). While, 23



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under M06-2X/6-311++G** level, the ground state for FeO2‾ is 6∑g. Zhou et al85 performed a combined matrix isolation infrared spectroscopic and systematic theoretical investigation on the FeO2‾ anion. They found that all the single-reference-based methods including various DFT and post-HF methods are not suitable for the FeO2‾ anion due to the strong multi-reference character of the molecule and the symmetry-breaking problems in the single-reference wave functions. However, the state-averaged multi-reference MRCI method predicted that the FeO2‾ anion has a linear doublet ground state,85-86 which is consistent with the experimental observations. Thus, the above discussions indicate that DFT methods (B3LYP and M06-2X) might be unreliable for MO2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S in details, although the results based on these functionals can qualitatively interpret the experiments. Thus, it comes to the conclusion that some modifications of these DFT methods are needed to fit complex situations and systems.

5. Conclusion The reactions of metal oxide cluster anions Mm18On‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S under thermal collision conditions were studied by mass spectrometry and density functional theory calculation (MO2‾ + H2S). The O/S exchange reaction to release water was identified for all of the MO2‾ anions; and the O/SH exchange to generate OH radical was also observed for NiO2‾, CuO2‾, and ZnO2‾. Reaction mechanism studies indicate that the first O−H bond and the M−SH bond formation in the initial stages for reactions of MO2‾ with H2S can release enough energy to drive the O/S exchange and O/SH exchange reactions. Further, the competitions between the O/S exchange and O/SH exchange channels in MO2‾ + H2S lead to more and more favorable formation of the OH radical (MO2‾ + H2S → MOSH‾ + OH) along the series from Fe to Zn, which is qualitatively predicted by the DFT calculations. The M06-2X functional is general better than B3LYP when predicting the thermodynamic data for the O/S exchange versus the O/SH exchange 24


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channels of MO2‾ + H2S while there can be error cancellation in the M06-2X calculations. The generation of OH radical may be considered in the reactions of H2S with copper or zinc oxides in condensed phase studies.

Acknowledgements This work was supported by Chinese Academy of Sciences (Knowledge Innovation Program No. KJCX2-EW-H01), the National Natural Science Foundation of China (Nos. 20933008 and 21173233), the 973 Program (Nos. 2011CB932302 and 2013CB834603).

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7. TOC

The competitions between O/S and O/SH exchange reactions of MO2‾ (M = Fe, Co, Ni, Cu, and Zn) with H2S.

35


Experimental and Theoretical Study of the Reactions between MO2

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