Activation of H2S by Atomic Cr, Mn, and Fe: Matrix Infrared Spectra and Quantum Chemical Calculations

Hydrogen sulfide is toxic and corrosive gas abundantly available in nature. The activation of hydrogen sulfide to produce hydrogen and elemental sulfur is of great significance for possible applications in toxic pollutant control and hydrogen energy regeneration. The activation of H2S by transition metal atoms (M = Cr, Mn, and Fe) has been studied by low-temperature matrix isolation infrared spectroscopy and quantum chemical calculations. Experimental and theoretical results indicate that the reaction between ground-state M atoms and H2S is inhibited by the repulsive interactions between the reactants. After being excited upon photolysis, the corresponding excited-state M atoms react with H2S molecules spontaneously. The produced insertion product HMSH further decomposed to metal sulfides upon full-arc mercury lamp irradiation by the splitting of hydrogen.


■ INTRODUCTION
Toxic hydrogen sulfide originates from nature, and industrial waste gases are responsible for the formation of aerosols and acid rain. The common process utilized to dispose hydrogen sulfide is the Claus process. 1 Meanwhile, the potential for hydrogen generation from hydrogen sulfide is lost in the Claus process. The splitting of hydrogen sulfide to produce hydrogen and sulfur is of great significance for toxic pollution control as well as hydrogen energy regeneration. Numerous materials such as metals, metal oxides, and metal sulfides have been explored as possible catalysts for hydrogen sulfide decomposition. 2−4 The low-temperature matrix isolation technique is useful in the reaction mechanism study. The activation of H 2 S by laser-ablated group 4 5 and group 5 6 transition metal atoms in a low-temperature argon matrix takes place spontaneously, and the produced insertion products decompose to metal sulfides and hydrogen upon photoirradiation. Laser-ablated Th and U atoms react with H 2 S on the annealing processes to produce H 2 ThS and H 2 US, respectively. The products further decompose to metal sulfides and hydrogen on broadband mercury lamp irradiation. 7 For the purpose of obtaining more possible catalysts for the activation of H 2 S, matrix isolation infrared spectroscopy and quantum calculations were employed to study the reaction mechanisms of transition metal atoms (M = Cr, Mn, and Fe) with hydrogen sulfide molecules in this work. We will show that laser-ablated Cr, Mn, and Fe atoms react with H 2 S molecules in solid argon upon photolysis to produce the insertion product HMSH.

■ EXPERIMENTAL AND THEORETICAL METHODS
The experimental setup for laser ablation and matrix isolation infrared spectroscopy has been described in detail previously. 8,9 Briefly, a fundamental Nd:YAG laser (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused on a rotating metal target. The laser-ablated Cr, Mn, or Fe metal atoms were co-deposited with hydrogen sulfide diluted in an argon matrix (typically 0.3%) on a 5 K CsI window for 1 h. After sample co-deposition, the Fourier transform infrared spectra were recorded between 400 and 4000 cm −1 at 0.5 cm −1 resolution using a Bruker 80 V spectrometer with a liquid nitrogen-cooled broadband MCT detector. Then, samples were annealed to the desired temperature and exposed to light with selected wavelengths to induce further reaction. A mercury lamp (75 W, without a globe) was used as a light source in the photolysis process with the aid of band filters to allow light of selected wavelength to pass through.
All the calculations were performed with the Gaussian 09 software package. 10 The def2-TZVPP basis sets were employed for all atoms. Structures of relative species were fully optimized, and harmonic frequencies were calculated analytically on the optimized structures. Transition states were characterized with one imaginary frequency and confirmed to link the corresponding products and reactants by intrinsic reaction coordinate calculations (IRC). Time-dependent density functional theory (TDDFT) at the B3LYP theoretical level was used to calculate the potential energy surfaces for the excited states.

■ RESULTS
Matrix isolation infrared spectroscopy was employed to study the reaction of laser-ablated transition metal atoms M (M = Cr, Mn, and Fe) with H 2 S molecules diluted in the argon matrix (typically 0.3%). Besides the absorptions due to reactants and impurities such as water existing in all our experiments, absorptions due to chromium hydrides (CrH and CrH 2 ) 11 and chromium sulfides (CrS and CrS 2 ) 12 and two new absorptions at 1661.8 and 1683.4 cm −1 were identified in the experiments of Cr + H 2 S in the solid argon matrix. Absorptions contributed from MnH, MnH 2 , 13 and MnS 14 and a new absorption at 1661.6 cm −1 were identified in the experiments of Mn with H 2 S by infrared spectroscopy. In the reaction of Fe + H 2 S in the solid argon matrix, absorptions due to FeH 2 , 15 FeS, and FeS 2 16 and two new absorptions at 1731.6 and 1688.9 cm −1 were identified. The assignments of the absorptions will be discussed in detail below.
HCrSH and HCr(SH) 2 . The spectra in selected regions from reactions of laser-ablated Cr atoms with H 2 S are presented in Figure 1 and Figure s1 in the Supporting Information.
Absorptions due to CrH (1603.3 cm −1 ) and CrH 2 (1614.5 and 1650.9 cm −1 ) were identified. 11 The absorption at 1661.8 cm −1 appeared after co-deposition and greatly enhanced upon >400 nm photolysis. In the reaction of Cr + 0.1% H 2 S + 0.1% D 2 S + 0.1% HDS in the solid argon matrix (Figure 2 Upon full-arc mercury lamp irradiation, absorption at 1661.8 cm −1 was nearly destroyed, accompanied by the appearance of absorption due to CrS (476.4 cm −1 ), 12 indicating the conversion from the complex contributing to the 1661.8 cm −1 absorption to CrS upon photolysis. We assigned this absorption to HCrSH.
Absorption at 1683.4 cm −1 appeared as weak absorption after the annealing of the sample to 25 K and tripled upon the following >400 nm photolysis. The 1683.4 cm −1 absorption is 21.6 cm −1 higher than the Cr−H stretching mode of HCrSH. This absorption is suitable for the Cr−H stretching mode. No other absorption was observed to track with this absorption. It suggests the existence of a Cr−H subunit in the complex. Absorption at 1683.4 cm −1 was nearly destroyed, and absorption due to CrS 2 12 appeared upon full-arc mercury lamp irradiation, indicating the conversion of the complex to CrS 2 . Accordingly, the 1683.4 cm −1 absorption is assigned to HCr(SH) 2 .
Density functional theory calculations at the B3LYP theoretical level were carried out to further prove our assignments (Table 1). HCrSH was calculated to have a 5 A ground state with Cr−H stretching vibration predicted at 1708. HMnSH. Figure 3 presents the infrared spectra in selected regions from the reaction of laser-ablated Mn atoms with H 2 S in solid argon. After annealing to 25 K, weak absorption at 1661.6 cm −1 sharpened and absorptions due to MnH 2 (1592.3 cm −1 ) and MnH (1477.9 cm −1 ) 13 appeared. The 1661.6 cm −1 absorption enhanced by about 10% upon >350 nm irradiation and increased by about 10% again upon >300 nm photoirradiation. After being exposed to full-arc mercury lamp irradiation, the absorption decreased by about 10% accompanied by the enhancement of absorption due to MnS 14 at 507.1 cm −1 . In the reaction with the mixture of H 2 S, HDS, and  DFT calculations predict HMnSH to have a C s symmetric structure at the 6 A′ ground state. The Mn−H stretching vibration predicted at 1695.0 cm −1 overestimates the experimental value by 2.0% ( Table 2). The predicted H/D isotopic ratio (1.4015) is in good agreement with the experimental value of 1.3896.
HFeSH and HFeSHSH 2 . In the reaction of laser-ablated Fe atoms with H 2 S in solid argon ( Figure 5), absorption at 1731.6 cm −1 appeared as weak absorption after co-deposition and doubled on annealing to 25 K. The absorption negligibly changed on >400 nm photolysis but sharply increased (by over 500%) on >350 nm photolysis. Upon full-arc mercury lamp     irradiation (>220 nm), the 1731.6 cm −1 absorption decreased by about 10% along with the enhancement of absorptions due to FeS (523.2 cm −1 ) and FeS 2 (471.1 cm −1 ). 16 It suggests that the composite contributing to the 1731.6 cm −1 absorption converts to FeS upon full-arc mercury lamp irradiation. The 1731.6 cm −1 absorption shifts to 1246.2 cm −1 in the reaction with D 2 S as a reagent (Figure 6 HFeSH is predicted to have a C s symmetric structure at the Cr:a 7 S(3d 5 4s 1 ) with H 2 S releases heat of 23.3 kcal/mol, but the reaction is hindered by a reaction barrier of 12.6 kcal/mol on the ground-state surface. In the low-temperature argon matrix, absorptions assigned to HCrSH exhibited no change but greatly increased upon >400 nm photolysis. The z 7 P ← 7 S transition of Cr atoms occurred at 396 nm in a lowtemperature krypton matrix. 21 The experimental results indicate that the reaction between Cr and H 2 S only takes place after the excitation of chromium atoms to the Cr:z 7 P(3d 5 4p 1 ) excited state on >400 nm photolysis.
Theoretical studies indicate that the first step for the inserting reaction is the formation of the MSH 2 complex by the approaching of transition metal atoms to H 2 S molecules. 6,22 As shown in Figure 9, the interaction energy between groundstate Cr and H 2 S is 1.36 kcal/mol at a Cr−S distance of 3.1 Å, which is assigned to the van der Waals force. A stable complex could be formed between excited-state Cr:z 7 P(3d 5 4p 1 ) atoms   and H 2 S with a binding energy of 31.6 kcal/mol at a Cr−S bond length of 2.35 Å (Figure 9). The formed excited-state MSH 2 rearranges to HCrSH spontaneously on the heptet state surface followed by spin-forbidden surface crossing and nonradiative decay to a quintet ground state. As shown in Figure 8, the quintet ground-state HCrSH( 5 A) is 35.3 kcal/ mol lower in energy compared with heptet ground-state HCrSH( 7 A′). The transition state linking the reactants and insertion products on the quintet state is 10.2 kcal/mol lower in energy compared with that on the heptet state surface. It suggests that the spin-forbidden surface crossing occurred before the transition state.
Reactions of Mn and Fe with H 2 S are spin-conserved. As shown in Figure 9, interactions between ground-state Mn and Fe atoms and H 2 S are totally repulsive. The repulsive interaction hindered the approaching of the two reactants for further reaction. The reaction barriers of 14.0 and 6.2 kcal/mol for the reactions of ground-state Mn:a 6 S(3d 5 4s 2 ) and Fe:a 5 D-(3d 6 4s 2 ) with H 2 S also inhibited the occurrences of the reactions. Absorptions assigned to HMnSH start to increase on >350 nm photolysis. The a 6 S → z 6 P transition of manganese atoms occurred at 397.4 nm. 23 In our experiments, excitedstate Mn:z 6 P(3d 5 4s 1 4p 1 ) atoms produced upon >350 nm photolysis become attractive to H 2 S with a binding energy of 8.5 kcal/mol. The formed MSH 2 complex rearranges to HMnSH spontaneously followed by nonradiative decay to the ground state via conical intersection. Similarly, laser-ablated Fe atoms react with H 2 S upon >350 nm photolysis in the solid argon matrix. The a 5 D → z 5 D transition of Fe atoms occurs at 386.0 nm in the argon matrix. 23 The interaction between Fe atoms and H 2 S molecules is totally repulsive on the ground state and becomes attractive after the excitation of Fe to the Fe:z 5 D(3d 6 4s 1 4p 1 ) state on >350 nm photolysis (with a binding energy of 9.5 kcal/mol). The experimental results indicate that the reaction of ground-state transition metals M (M = Cr, Mn, and Fe) with H 2 S is hindered by the repulsive interaction and the reaction barrier on the ground-state surface. In the solid argon matrix, laser-ablated group 4 5 and group 5 6 transition metal atoms react with H 2 S on the annealing process. Theoretical studies indicate that stable complexes can be formed by the donation of electrons from H 2 S to transition metal atoms (group 4 and group 5). The formed hot complexes rearrange to the insertion products spontaneously. Meanwhile, the activation of H 2 S by group 12 metal atoms 24 and transition metal atoms studied here occurred upon photolysis. Quantum chemical calculations suggest that the interactions between H 2 S molecules and ground-state transition metal atoms M (M = Zn, Cd, Hg, Cr, Mn, and Fe) are repulsive. After being excited to corresponding excited states upon photolysis, the excitedstate transition metal atoms become attractive to H 2 S, leading to the formation of the complex MSH 2 on the excited state. The produced hot complex [MSH 2 ]* can further rearrange to insertion products by surmounting the reaction barrier on the ground-state surface. Experimental and theoretical studies conclude that the formation of a stable complex by the approaching of transition metal atoms to H 2 S molecules is the vital process for the activation of H 2 S. H 2 S can be activated by a metal atom that is attractive to a H 2 S molecule to produce the stable complex MSH 2 for further reaction.  Upon full-arc mercury lamp irradiation, absorptions assigned to HMSH (M = Cr, Mn, and Fe) decreased along with the enhancement of absorptions due to MS. The experimental results indicate that the MS molecules are produced by the elimination of hydrogen from HMSH upon photolysis. As shown in Figure 8, the elimination of hydrogen from HMSH is endothermic. The energy needed for the splitting of hydrogen from HMSH could be supplied by photolysis. In the argon matrix, metal sulfides and hydrogen are produced by the reaction of M atoms with H 2 S upon photolysis, which delivers a possible way to reproduce hydrogen from H 2 S with the participation of transition metal atoms.

■ CONCLUSIONS
Matrix isolation infrared spectroscopy and quantum chemical calculations were employed to study the reaction of laserablated M (M = Cr, Mn, and Fe) atoms with hydrogen sulfide in a 5 K argon matrix. Experimental and theoretical results indicate that the activation of H 2 S by M atoms takes place upon photolysis. The ground-state Mn and Fe atoms are repulsive to hydrogen sulfide molecules, and ground-state Cr atoms were slightly attractive to H 2 S molecules by van der Waals force. The excited-state Cr:z 7 P(3d 5 4p 1 ), Mn:y 6 P-(3d 5 4s 1 4p 1 ), and Fe:z 5 D(3d 5 4s 1 4p 1 ) atoms produced on photolysis become attractive to H 2 S molecules with relatively large binding energies. The activation of H 2 S by Cr atoms takes place upon irradiation at longer wavelength compared with Mn and Fe. It suggests that Cr atoms are more suitable for the photochemical activation of H 2 S among the three kinds of metal atoms investigated in this work. The formed MSH 2 rearranges to HMSH spontaneously. Upon full-arc mercury lamp irradiation, metal sulfides were produced by the elimination of hydrogen from HMSH molecules.