Reactions of Ti, Zr, and Hf Atoms with Hydrogen Sulfide: Argon Matrix

Article pubs.acs.org/JPCA

Reactions of Ti, Zr, and Hf Atoms with Hydrogen Sulfide: Argon Matrix Infrared Spectra and Theoretical Calculations Qiang Wang,†,‡ Jie Zhao,† and Xuefeng Wang*,† †

Department of Chemistry, Tongji University, Shanghai 200092, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China

‡

ABSTRACT: Laser-ablated Ti, Zr, and Hf atoms have been codeposited at 4 K with hydrogen sulfide in excess argon. The metal atoms insert into the S− H bond of hydrogen sulfide to form the HMSH, H2MS, and H2M(SH)2 molecules (M = Ti, Zr, Hf), which were identified on the basis of the D2S and H234S isotopic substitutions. The observed vibrational frequencies of these species were reproduced by B3LYP functional calculations. The reaction mechanisms have been proposed on the potential energy surface of the studied system to account for the formation of these molecules. We have made a theoretical prediction about the H2MS complexes dehydrogenation, which can provide a novel proposal for generating hydrogen from H2S.

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INTRODUCTION Metal hydrosulfides are of great importance in catalytic processes such as hydrogenation and hydrodesulfurization. For example, titanocene dihydrosulfide [Cp2Ti(SH)2] has been prepared from [Cp2TiCl2] and H2S in the presence of NEt3.1 The tucked-in zirconocene complex [ZrI{η5:η1-C5Me4(CH2)}Cp*] was reacted with H2S to give [Cp*2Zr(SH)I], which afforded the highly reactive [Cp*2ZrS] species.2 The decomposition of H2S with photocatalysis has been investigated,3−7 and a direct splitting of H2S to produce H2 and S using solar energy is most desirable to produce clean hydrogen energy.8 During past few years, experimental workers were also interested in designing different fluorescent probes for the detection of H2S in blood and biological systems.9−11 Earlier researchers have explored the oxidation of H2S both experimentally and theoretically.12−18 Recent studies of group IV metal atoms with small molecules have provided insight into activation of bonds such as H−H, C−O, H−O, C−H, B−F, and S−O.19−29 Laser-ablated thorium and uranium atom reactions with H2S have been studied in excess noble gas matrixes and H2MS (M = Th, U) have been identified.30 Here we report a combined argon matrix isolation infrared spectroscopy and density functional theory (DFT) calculations on the laser-ablated Ti, Zr, and Hf atom reactions with H2S molecule.

The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a rotating metal target, which gave an energetic atomic beam to react with reagent gas mixtures. After sample deposition, infrared spectra of the resulting samples were recorded on a Bruker 80 V spectrometer at a 0.5 cm−1 resolution between 4000 and 400 cm−1 using a liquid nitrogen cooled broad band HgCdTe (MCT) detector. Samples were later irradiated for 15 min periods by a mercury arc lamp (175 W) with the globe removed and annealed to allow reagent diffusion and further reaction. Quantum chemical calculations were performed to determine the molecular structures and to help the assignment of vibrational frequencies of the observed reaction products. The calculations were performed with the Gaussian 09 package33 at the level of density functional theory with the B3LYP functional.34,35 The 6-311++G(3df,3pd) basis sets were used for C, H, and Ti, and the SDD pseudopotential and basis set was used for Zr and Hf atoms.36,37 The geometries of various reactants, intermediates, and products were fully optimized, and the harmonic vibrational analysis was carried out to verify each stationary point on the potential energy surface (PES). All of the optimized structures were characterized to be either local minima or transition state on the PES by performing vibrational analysis. The intrinsic reaction coordinate (IRC) calculations were further carried out to ensure that obtained TSs connect the desired reactants or initial states and products or final states.38

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EXPERIMENTAL AND COMPUTATIONAL METHODS The technique for investigating reactions of laser-ablated metal atoms with small molecules in low temperature matrices has been described in detail previously.29,31 Laser-ablated Ti, Zr, and Hf atoms were reacted with H2S, H234S, and D2S in excess argon (4 K) during condensation by means of a closed-cycle helium refrigerator (SHI, SRDK-408D2). The H2S, H234S, and D2S samples were prepared by previously described method.32 © XXXX American Chemical Society

Special Issue: Markku Räsänen Festschrift Received: June 2, 2014 Revised: July 4, 2014

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dx.doi.org/10.1021/jp5054106 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. IR spectra from the laser-ablated Ti atom reactions with H2S in excess argon. (a) Ti + 0.2% H2S in Ar, codeposited for 1 h, (b) after annealing to 30 K, (c) after photolysis (λ > 500 nm), (d) after annealing to 25 K, (e) after broadband irradiation, (f) Ti + 0.1% H2S + 0.05% HDS + 0.1% D2S in Ar, codeposited for 1 h, and (g) after annealing to 30 K.

Figure 2. IR spectra from the laser-ablated Zr atom reactions with H2S in excess argon. (a) Zr + 0.2% H2S in Ar, codeposited for 1 h, (b) after annealing to 25 K, (c) after broadband irradiation, (d) after annealing to 30 K, (e) Zr + 0.1% H2S + 0.05% HDS + 0.1% D2S in Ar, codeposited for 1 h, and (f) after annealing to 30 K.

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RESULTS

respectively. Common species, such as metal sulfides of MS2 and MS (M = Ti, Zr, Hf) were observed in our experiments.39 Ti + H2S. In the Ti + H2S reaction, as shown in Figure 1 absorptions at 1636.7, 1615.4, and 625.8 cm−1 (group A) can be grouped together by their consistent behavior upon annealing and photolysis. These bands appeared upon deposition, increased by about 50% upon annealing to 30 K, but decreased by 20% upon visible irradiation (λ> 500 nm), increased again by 100% upon annealing to 25 K, and decreased by 20% broad-band irradiation. Notice the band at 563.2 cm−1 due to diatomic TiS increased at the expense of

Infrared Spectra. Infrared spectra for the reactions of laserablated Ti, Zr, and Hf atoms with H2S molecules in excess argon in the selected regions are illustrated in Figures 1−3, respectively, and the observed absorptions are listed in Table 1. The stepwise annealing and photolysis behaviors of these product absorptions are also shown in the figures. D2S and H234S samples were employed for product identification through isotopic shift and splitting, and the representative spectra in selected regions are shown in Figures 1−4, B



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Figure 3. IR spectra from the laser-ablated Hf atom reactions with H2S in excess argon. (a) Hf + 0.02% H2S in Ar, codeposited for 1 h, (b) after annealing to 25 K, (c) after broadband irradiation, (d) after annealing to 25K, (e) Hf + 0.1% H2S + 0.05% HDS + 0.1% D2S in Ar, codeposited for 1 h, and (f) after annealing to 30 K.

cm−1 (group C) in the Ti +H2S experiments appeared on deposition and kept a trend of increasing in the process of annealing and irradiation. Zr + H2S and Hf + H2S. The absorptions at 593.6, 1569.7, and 1598.8 cm−1 tracking together in Zr atom reactions with H2S in solid argon were observed upon deposition and increased by 4-fold on annealing to 25 K, but decreased 50% upon broad-band irradiation and recovered upon further annealing to 30 K as shown in Figure 2. Two strong absorptions at 1613.3 and 1586.1 cm−1 were observed upon deposition, increased 3-fold upon annealing to 25 K, and decreased slightly upon further annealing to 30 K. Similarly Hf atom reactions with H2S gave absorptions at 1666.8, 1640.2, and 634.8 cm−1 (Figure 3). These bands appeared weakly on deposition, increased markedly on annealing to 25 K, but decreased by 10% upon broad-band irradiation and increased by 20% upon annealing to 25 K. Calculations. Calculations at the B3LYP level of theory were done for three isomers of (M)(H2S), namely, M−SH2 complex, inserted HMSH, and H2MS molecules (M = Ti, Zr, Hf). The geometries of the stationary points at the triplet and singlet states are depicted in Figure 5, and the calculated potential energy profiles are presented in Figure 6. The calculated vibrational frequencies and intensities are listed in Tables 2−5 for observed molecules. The TiSH2, ZrSH2 and HfSH2 with planar Cs symmetry were predicted to have 3A″ ground state, and all three H2MS molecules were computed to have a 1A′ ground state with planar Cs symmetry. Notice the singlet state of H2TiS and triplet state of HTiSH molecules are almost energetically identical (Figure 6). However, for Zr and Hf the 1A′ state H2MS molecules are more stable than the HMSH molecules. Calculations were also done for H2M(SH)2 (M = Ti, Zr, Hf) molecules and geometry parameters are shown in Figure 5.

Table 1. Infrared Absorptions (cm−1) Observed for Products of the Reaction of Ti, Zr, and Hf Atoms with H2S Molecules in Argon H2S

D2 S

1642.4 1636.7 1624.7 1615.4 1522.4 1489.1 625.8 540.2

1180.8 1178.5 1175.6 1170.8 1099.8 1073.5 590.7 539.6

1613.3 1598.8 1586.1 1569.7 1558.2 1501.3 593.6 504.6

1155.2 1145.1 1137.6 1129.2 1119.6 1078.4 539.4 504.0

1681.3 1666.8 1655.4 1640.2 1624.3 634.8 520.5

1202.1 1191.8 1185.9 1175.4 1161.3 518.6

H234S Ti 1642.4 1636.7 1624.7 1615.4 1522.4 1489.1 621.4 530.4 Zr 1613.3 1598.8 1586.1 1569.7 1558.2 1501.3 591.3 494.4 Hf 1681.3 1666.8 1655.4 1640.2 1624.3 634.5 507.6

assignment H2Ti(SH)2, Ti−H str H2TiS, Ti−H str H2Ti(SH)2, Ti−H str H2TiS, Ti−H str HTiS, Ti−H str HTiSH, Ti−H str H2TiS, Ti−S str HTiS, Ti−S str H2Zr(SH)2, Zr−H str H2ZrS, Zr−H str H2Zr(SH)2, Zr−H str H2ZrS, Zr−H str HZrS, Zr−H str HZrSH, Zr−H str H2ZrS, Zr−S str HZrS, Zr−S str H2Hf(SH)2, Hf−H str H2HfS, Hf−H str H2Hf(SH)2, Hf−H str H2HfS, Hf−H str HHfS, Hf−H str H2HfS, Hf−S str HHfS, Hf−S str

group A bands, which decreased by annealing but increased by irradiations. In the same experiment, two strong bands at 1642.4 and 1624.7 cm−1 (group B), presented on sample deposition, always maintained a growth trend on the process of annealing and photolysis. Other bands at 1522.4 and 540.2 C



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Figure 4. IR spectra in 1690−1560 and 650−460 cm−1 region, (a) the laser-ablated Ti atom reactions with H2S (0.2%) in excess argon, after annealing to 30 K; (b) the laser-ablated Ti atom reactions with H234S (0.2%) in excess argon, after annealing to 25 K; (c) the laser-ablated Zr atom reactions with H2S (0.2%) in excess argon, after annealing to 25 K; (d) the laser-ablated Ti atom reactions with H234S (0.2%) in excess argon, after annealing to 25 K; (e) the laser-ablated Hf atom reactions with H2S (0.2%) in excess argon, after annealing to 25 K; (f) the laser-ablated Hf atom reactions with H234S (0.2%) in excess argon, after annealing to 25 K.

Figure 5. Optimized stationary structures in the experiments of Ti, Zr, and Hf with H2S. Parameters (distances in Å and angles in °) are given for B3LYP.

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at 625.8 cm−1 tracking absorptions at 1636.7 and 1615.4 cm−1 shifted to 621.4 cm−1 with the H234S sample giving a 32S/34S isotopic ratio of 1.0071, and shifted to 590.7 cm−1 with the D2S sample, and showed triplet distribution at 625.8, 606.8, and 590.7 cm−1 with the mixed H2S + HDS + D2S sample, indicating that this band is a Ti−S stretching vibration perturbed by hydrogen atoms. Accordingly, this group of bands is assigned to the H2TiS molecule, in which 1636.7 and 1615.4 cm−1 bands are appropriate for symmetric and antisymmetric Ti−H stretching modes and the 625.8 cm−1 band is due to a terminal Ti−S stretching vibration. Notice our

DISCUSSION H2MS. In the Ti + H2S reaction, absorptions at 1636.7 and 1615.4 cm−1 showed no sulfur-34 shift with the H234S sample but shifted to 1178.5 and 1170.8 cm−1 with D2S, giving isotopic H/D ratios of 1.3888 and 1.3797, respectively, suggesting these two bands are due to Ti−H stretching vibrations. In the mixed H2S + HDS + D2S experiments, two triplets were observed at 1636.7, 1624.7, and 1615.4 cm−1 in the upper Ti−H stretching region and 1178.5, 1175.6, and 1170.8 cm−1 in the lower Ti-D stretching region, which clearly show that two equivalent H atoms are involved in this molecule. In concert, a weaker band D



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Table 2. Calculated Frequencies of the H2MS Molecules (M = Ti, Zr, Hf) in Cs Symmetrya H2S

H234S

D2S

1731.3(282) 1703.8(341) 665.2(181) 561.3(26) 458.3(34) 385.6(144)

1233.2(149) 1223.6(180) 625.0(113) 425.6(30) 333.4(14) 282.4(79)

1685.6(280) 1657.7(383) 613.7(170) 526.5(45) 463.6(32) 400.2(102)

1197.4(144) 1181.9(199) 550.9(101) 415.2(35) 335.4(13) 291.1(59)

1716.7(241) 1690.2(311) 645.5(106) 506.9(74) 476.4(29) 420.1(81)

1217.2(122) 1200.3(159) 518.4(83) 443.8(21) 343.1(12) 304.7(53)

Ti 1731.3(282) 1703.8(341) 660.4(180) 555.4(26) 457.9(34) 385.3(143) Zr 1685.5(280) 1657.7(383) 611.5(166) 517.4(50) 463.2(32) 399.5(100) Hf 1716.7(241) 1690.2(311) 644.9(105) 495.9(80) 476.0(30) 418.7(75)

description Ti−H sym str Ti−H antisym str Ti−S str HTiH bending (in) HTiH bending (out) HTiH rocking Zr−H sym str Zr−H antisym str Zr−S str ZrH2 bending (in) ZrH2 bending (out) ZrH2 rocking Hf−H sym str Hf−H asym str Hf−S str HfH2 bending (in) HfH2 bending (out) HfH2 rocking

a Frequencies and intensities are in cm−1 and km/mol. B3LYP functional, the 6-311++G(3df, 3pd) basis set, was used for S, H, and Ti, and the SDD was used for Zr and Hf.

Table 3. Calculated Frequencies of the H2M(SH)2 Molecules (M = Ti, Zr, Hf) in C1 Symmetrya H2S

D2S

2622.9(5) 2604.4(7) 1753.8(234) 1730.6(273) 579.5(96) 567.4(109)

1882.8(2) 1869.4(4) 1249.7(128) 1242.0(141) 503.5(145) 461.7(31)

2621.5(4) 2620.6(6) 1701.9(306) 1677.7(371) 599.9(114) 532.5(69)

1881.8(2) 1881.1(3) 1208.4(160) 1196.4(191) 438.9(79) 424.7(132)

2632.2(4) 2621.0(6) 1732.1(282) 1705.9(337) 628.1(100) 542.4(42)

1889.5(2) 1881.4(3) 1227.7(144) 1211.7(171) 447.8(56) 408.9(23)

H234S

description

2620.6(5) 2602.2(7) 1753.8(234) 1730.6(273) 578.5(91) 566.6(108) Zr 2619.2(4) 2618.3(6) 1701.9(306) 1677.7(371) 599.9(114) 532.2(68) Hf 2629.9(4) 2618.7(6) 1732.1(282) 1705.9(337) 628.1(99) 542.1(42)

S−H str S−H str Ti−H sym str Ti−H antisym str TiH2 rocking TiH2 scissoring

Ti

Figure 6. Potential energy surfaces for reactions of Ti, Zr, and Hf with H2S.

two observed Ti−H stretching modes are very close to the same modes at 1646.8 and 1611.9 cm−1 for H2TiO molecules in solid argon;23 however, the antisymmetric Ti−H stretching mode for H2TiS is located 180 cm−1 higher than the same mode for the TiH2 molecule in solid argon.21 The H2TiS assignment is strongly supported by DFT calculations. Our B3LYP computation predicted the Ti−H symmetric and antisymmetric stretching modes at 1731.3 and 1703.8 cm−1, which are overestimated by 5.5% and 5.8%, respectively, in good agreement with group 4 metal hydride frequency calculations. The predicted Ti−S stretching mode at 665.2 cm−1 is overestimated by 6.3% and the calculated 32S/34S isotopic frequency ratio for this mode is 1.0071, which matches the observed value at 1.0073 very well. In solid argon, the reactions of Zr atom with H2S gave a set of bands at 1598.8, 1569.7, and 593.6 cm−1, which tracked together in the whole reaction process. The 1598.8 and 1569.7 cm−1 bands showed no sulfur-34 shift with H234S sample, but gave deuterium counterparts at 1129.2 and 1145.1 cm−1, defying H/D ratios of 1.3901 and 1.3962, respectively, indicating that these two bands are due to Zr−H stretching modes. Using a mixed H2S + HDS + D2S sample, two triplet distribution were observed at 1598.8, 1583.6, 1569.7 cm−1 and 1145.1, 1137.4, 1129.2 cm−1 (Figure 2), suggesting that two equivalent H atoms were involved. The 593.6 cm−1 band

S−H str S−H str Zr−H sym str Zr−H antisym str ZrH2 rocking ZrH2 scissoring S−H str S−H str Hf−H sym str Hf−H antisym str HfH2 rocking HfH2 scissoring

Frequencies and intensities are in cm−1 and km/mol. B3LYP functional, the 6-311++G(3df, 3pd) basis set, was used for S, H, and Ti, and the SDD was used for Zr and Hf. a

shifted to 539.4 cm−1 with D2S and to 591.3 cm−1 with H234S, which characterizes a Zr−S stretching mode perturbed by hydrogen atoms. On the basis of the above spectroscopic information, this set of bands is appropriate for the H2ZrS molecule. The assignment to H2ZrS is confirmed by B3LYP harmonic frequency calculations, which predicted a Zr−S E



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Triplet distributions (1666.8, 1652.4, and 1640.2 cm−1 in upper region and 1191.8, 1184.8, and 1175.4 cm−1 in low region) were observed in the mixed H2S + HDS + D2S experiment (Figure 3), suggesting that two equivalent hydrogen atoms were involved. The observed 634.8 cm−1 band shifted to 518.6 cm−1 with D2S substitution, but only shifted 0.3 cm−1 with H234S substitution. The Hf−H antisymmetric and symmetric stretching modes are predicted at 1690.2 and 1716.7 cm−1, respectively, which are each overestimated by 3.0%. The observed 26.6 cm−1 symmetric−antisymmetric Hf−H stretching mode separation is matched by the calculated 26.5 cm−1 separation. The predicted 645.5 cm−1 band exhibited a very small (0.6 cm−1) sulfur-34 shift, which matches the observed value (0.3 cm−1 shift) very well. The M−H, M−S, and M−O stretching modes of H2MS and H2MO (M = Ti, Zr, Hf, Th) are listed in Table 6 for

Table 4. Calculated Frequencies of the HMSH Molecules (M = Ti, Zr, Hf) in Cs Symmetrya H2S

H234S

D2S

2665.8(3) 1586.6(420) 425.2(79) 380.2(69) 296.1(70) 100.8(115)

1913.8(2) 1134.2(221) 382.2(90) 307.7(11) 212.0(43) 76.3(64)

2660.8(6) 1613.1(394) 466.2(19) 360.3(76) 283.0(26) 208.6(59)

1910.3(3) 1147.5(202) 356.3(63) 338.1(1) 203.6(20) 151.6(32)

2673.1(14) 1703.7(321) 495.3(6) 345.2(45) 330.1(1) 301.4(16)

1919.3(8) 1208.5(163) 360.6(9) 338.2(28) 237.1(6) 217.9(8)

Ti 2663.4(3) 1586.6(420) 424.6(75) 373.9(73) 295.8(67) 100.7(115) Zr 2658.4(5) 1613.1(394) 465.8(19) 353.3(76) 282.3(24) 208.3(59) Hf 2670.7(14) 1703.7(321) 494.8(6) 340.7(39) 326.1(5) 301.1(16)

description S−H str Ti−H str TiH−SH rocking Ti−SH str TiH−SH scissoring TiH−SH wagging S−H str Zr−H str ZrH−SH rocking Zr−SH str ZrH−SH scissoring ZrH−SH wagging

Table 6. Comparison of M−H, M−S, and M−O Stretching Frequencies of H2MS and H2MO Molecules (M = Ti, Zr, Hf, Th) in Solid Argon

S−H str Hf−H str HfH−SH rocking Hf−SH str HfH−SH scissoring HfH−SH wagging

molecule H2TiSa H2ZrSa H2HfSa H2ThSb H2TiOc H2ZrOc H2HfOc H2ThOd


Table 5. Calculated Frequencies of the HMS Molecules (M = Ti, Zr, Hf) in Cs Symmetrya H2S

D2S

1613.7(268) 596.7(125) 378.2(70)

1154.4(143) 589.9(95) 277.9(37)

1668.3(286) 548.8(110) 471.2(25)

1187.0(147) 539.4(74) 345.9(25)

1692.9(224) 515.9(86) 461.4(10)

1201.0(114) 506.2(53) 337.9(17)

H234S

description

1613.7(268) 587.2(124) 377.2(68)

Ti−H str Ti−S str Ti−H bending

1668.3(286) 539.4(113) 468.6(21)

Zr−H str Zr−S str ZrH bending

1692.8(224) 506.5(89) 457.8(6)

Hf−H str Hf−S str HfH bending

a

M−H str 1636.7; 1598.8; 1666.8; 1468.6; 1646.8; 1577.6; 1646.4; 1443.5;

1615.4 1569.7 1640.2 1435.3 1611.9 1539.4 1615.6 1435.5

M−S str

M−O str

625.8 593.6 634.8 454.5 1010.5 924.7 921.0 --

This work. bReference 30. cReference 23. dReference 47.

comparison. Notice the frequencies decrease from Ti to Zr, but increase from Zr to Hf, which can be contributed from the relativistic effect for this group of metals. For Hf, the 5d orbitals undergo a relativistic expansion while the 6S orbital contracts, resulting in more 5d character in bonding and enhanced Hf−H, Hf−O, and Hf−S bonds. A similar trend was discussed in our previous paper.30 However, for Th bonding, the observed frequencies are much lower since 5f orbitals are involved in bonding. H2M(SH)2. In laser-ablated Ti atom reactions with H2S in solid argon, two strong new Ti−H stretching vibrations were observed at 1642.4 and 1624.7 cm−1, which shifted to 1180.8 and 1175.6 cm−1 with the D2S sample, giving H/D isotopic ratios of 1.3909 and 1.3820, respectively. These two bands exhibited no sulfur-34 shift for the H234S sample, but triplet distributions were observed at 1642.4, 1632.6, and 1624.7 cm−1 in the Ti−H stretching region and 1175.4, 1184.8, and 1191.8 cm−1 in the Ti−D stretching region with mixed H2S, HDS, and D2S sample as shown in Figure 1. These two bands are assigned to the Ti−H stretching vibrations of the H2Ti(SH)2 molecule. Notice other absorptions for this molecule were not observed. The H2Ti(SH)2 assignment was further supported by B3LYP calculations. As listed in Table 3, the calculated symmetric and antisymmetric TiH2 stretching vibrations were computed at 1753.8 and 1730.6 cm−1, which require scaling factors of 0.936 and 0.939 to fit the observed values. Laser-ablated Zr atom reactions with H2S in solid argon gave 1613.3 and 1586.1 cm−1 bands, which exhibited no sulfur-34 shift but shifted to 1155.2 and 1137.6 cm−1 with deuterium substitution. The H/D ratios of 1.3966 and 1.3943 are appropriate for Zr−H stretching modes. In the mixed H2S +

Ti

Zr

Hf


stretching mode at 613.7 cm−1 and strong Zr−H symmetric and antisymmetric stretching modes at 1685.6 and 1657.7 cm−1, respectively. The calculated frequencies are in very good agreement with experimental values (overestimated by 3.4%, 5.6% and 5.4%). Notice that the Zr−S stretching mode is predicted to red shift 2.2 cm−1 upon 34S substitution, which is in excellent agreement with the observed shift value of 2.3 cm−1. The symmetric and antisymmetric Hf−H stretching vibrations due to H2HfS were found at 1666.8 and 1640.2 cm−1, which are very close to the same modes for HfH2 and H2HfO in solid argon.19,23 With D2S substitution, two Hf-D stretching modes were observed at 1175.4 and 1191.8 cm−1, giving H/D isotopic ratios of 1.3954 and 1.3986, respectively. F



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shifted to 1073.5 cm−1 with D2S, giving an isotopic H/D ratio of 1.3871. The isotopic shift suggests assignment to a Ti−H stretching mode. With mixed H2S, HDS and D2S sample, we observed Ti−H stretching mode at 1489.1 cm−1 and Ti-D stretching mode at 1073.5 cm−1. This absorption is assigned to the Ti−H stretching vibration of the HTiSH molecule. The Ti−H stretching mode was calculated at 1586.6 cm−1, which is overestimated by 6.5% with very good agreement. Another TiSH bending mode was computed at 425.2 cm−1 with medium intensity; however, this mode must fall below 400 cm−1 in experiment, which cannot be observed since our measurement is above 400 cm−1. The Zr−H stretching mode for HZrSH was found at 1501.3 cm −1 , which shifted to 1078.4 cm −1 with deuterium substitution, defying the 1.3922 H/D isotopic frequency ratio. The effect of deuterium and sulfur-34 substitution on the spectra is very similar to the Ti case, so the assignment is straightforward. As listed in Figure 4, our B3LYP calculation reproduced frequencies very well. Notice HHfSH was not observed in our experiments.

HDS + D2S experiments, two additional bands at 1593.7 and 1146.9 cm−1 were observed, which are the typical pattern for Zr−H stretching vibration of the ZrH2 subunit. The behavior of this set of bands is very similar to that of the H2Ti(SH)2 molecule, which is assigned to the analogous H2Zr(SH)2 molecule. The symmetric and antisymmetric Zr−H stretching modes for H2Zr(SH)2 molecule are predicted at 1701.9 and 1677.7 cm−1, which are overestimated by 5.5% and 5.8%. The H/D ratios for two modes are predicted to be slightly higher (1.4084 and 1.4023). In the Hf + H2S experiments, absorptions at 1681.3 and 1655.4 cm−1 are assigned to the H2Hf(SH)2 molecule. These two bands appeared on the deposition, increased upon annealing and irradiation, which is analogous to the trend of the H2Ti(SH)2 molecule. Two absorptions showed no 34S shift, and the deuterium counterpart at 1202.1 and 1185.9 cm−1 defined the 1.3986 and 1.3959 H/D ratios. In the mixed H2S + HDS + D2S experiments, two intermediates at 1669.8 and 1197.2 cm−1 were observed, which are typical distributions for symmetric and antisymmetric Hf−H stretching vibrations with two equivalent hydrogen atoms. This assignment is also supported by theoretical calculations. The observed 25.9 cm−1 symmetric−antisymmetric HfH2 stretching mode separation is matched very well by the calculated 26.2 cm−1 separation. HMS. Two bands at 1522.4 cm−1 (Ti−H stretching) and 540.2 cm−1 (Ti−S stretching) track together. The 1522.4 cm−1 band showed no shift with H234S sample, but moved to 1099.8 cm−1 with the D2S sample, giving a H/D isotopic ratio of 1.3843. In the mixed H2S + HDS + D2S experiments, two bands at 1522.4 and 1099.8 cm−1 were observed, suggesting only one hydrogen atom was involved. The 540.2 cm−1 band shifted to 530.4 cm−1 with H234S and 539.6 cm−1 with D2S, respectively, which is characteristic of a Ti−S stretching mode. The triatomic molecule HTiS comes to mind for this group band assignment. Absorptions at 1558.2 and 504.6 cm−1 in the Zr +H2S experiments appeared upon deposition, increased 50% upon annealing to 25 K, and slightly increased on further annealing and irradiation. The deuterium counterparts of two bands were observed at 1119.6 and 504.0 cm−1. In the H234S experiment, the upper 1556.7 cm−1 band showed no shift, but the lower 504.6 cm−1 band shifted to 494.4 cm−1, suggesting Zr−S stretching vibration. Accordingly, the HZrS molecule is assigned. The 1624.3 and 520.5 cm−1 bands in the Hf +H2S experiments exhibited very similar behavior to the bands of HTiS and HZrS molecules. With the D2S sample, the 1624.3 cm−1 band shifted to 1161.3 cm−1, defying the 1.3987 H/D ratio. The 520.5 cm−1 band shifted to 507.6 cm−1 with H234S experiment, but the 1624.3 cm−1 band showed no shift. These group bands are due to the HHfS molecule. The B3LYP functional frequency calculations support these HMS assignments. The M−H stretching modes of the HMS molecules were calculated at 1613.7 cm−1 (Ti), 1668.3 cm−1 (Zr), and 1692.9 cm−1 (Hf), which are overestimated by 5.9%, 7.2%, and 5.8%, respectively. In addition, M−S stretching modes were predicted at 596.7 cm−1 (Ti), 548.8 cm−1 (Zr), 515.9 cm−1 (Hf), which match observed values reasonably well. HMSH. The 1489.1 cm−1 band in the Ti +H2S experiments increased 10-fold on 30 K annealing, but remained unchanged upon visible irradiation (λ> 500 nm), and slightly decreased on broad-band photolysis. This band showed no 34S shift, but was

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REACTION MECHANISM Our theoretical calculations indicate that the H2S molecule attaches to the Ti, Zr, Hf atom to form metal−hydrogen sulfide complexes at first stage, which are exothermic by 6.7 (Ti), 8.3 (Zr), and 3.9 (Hf) kcal/mol, respectively (reaction 1). 1 3 M(3F) + H 2S(A 1) → M(SH 2)( Α″)

(1)

The hydrogen atom is transferred from sulfur to the metal to form the HMSH intermediate, exothermic by 47.5 (Ti), 53.7 (Zr), and 54.2 (Hf) kcal/mol (reaction 2). M(SH 2)(3A ″) → HMSH(3A ″)

(2)

Furthermore, HMSH molecules can rearrange through second hydrogen transfers from sulfur to form the H2MS, exothermic by 0.9 (Ti), 26.7 (Zr), and 28.6 (Hf) kcal/mol (reaction 3). Notice that the H2TiS and HTiSH are very close in energy. 1 HMSH(3A ″) → H 2MS(A ′)

(3)

The H 2M(SH) 2 absorptions increased on annealing, suggesting that HMSH molecules can further react with hydrogen sulfide through reactions 4, which were calculated to be exothermic by 40.0 (Ti), 70.7 (Zr), and 77.8 (Hf) kcal/ mol. 1 1 HMSH(3A ″) + H 2S(A 1) → H 2M(SH)2 (A)

(4)

Figure 6 illustrates the computed potential energy surfaces for the reactions starting from the separated M + H2S to the insertion products of H2MS. Notice that the formations of MSH2, HMSH, and H2MS are thermodynamically favorable. Our calculation shows that the processes from the ground state H2TiS to TiS (1Σg) + H2 are endothermic by 35.2 kcal/ mol; however, the process from the excited state H2TiS to TiS (3Δ) + H2 is exothermic by 21.7 kcal/mol. Meanwhile, the triplet state TiS is more stable by 24.7 kcal/mol than the singlet state. The direct decomposition of H2TiS into TiS and H2 is a very interesting chemical reaction if the ground molecule H2TiS absorbs additional energy. From our experiment, the H2TiS absorptions show a 20% decrease upon visible photolysis, but the band at 563.2 cm−1 increases in this process. As the H2TiS absorptions increased upon annealing, the band at 563.2 cm−1 G



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decreased simultaneously. In a previous experiment, 583.8 cm−1 was tentatively assigned to TiS,39 and a vibrational frequency of 558.3 cm−1 was for the X3Δ state of the TiS molecule in the gas phase.40 The 563.2 cm−1 in argon is assigned to the TiS molecule. These intensity variations of the two absorption groups upon photolysis and annealing suggest H2TiS can dissociate to TiS + H2, and TiS can also absorb molecular hydrogen to yield H2TiS. Therefore, the system TiS + H2 is expected to be selective for the reversible hydrogen storage. As can be seen in Figure 6, H2ZrS and H2HfS exhibit behavior very similar to that of H2TiS. Formation of ZrS/HfS + H2 along this pathway is calculated to be exothermic on the triplet state potential surface by 6.7/3.2 kcal/mol. Various strategies for converting H 2 S into hydrogen and sulfur have been proposed.41,42 Hydrogen sulfide splitting to H2 and S is an uphill reaction and thermodynamically unfavorable. It needs the standard Gibbs free energy change ΔGθ of 33 kJ/mol.43 The conversion of H2S with photocatalysis has been investigated.44−46 In the above discussion, we found that these H2MS species are the minimum energy overall PES and could produce a hydrogen molecule provided additional energy. In contrast, the H2TiS complex dehydrogenation is the most favorable and can provide a novel proposal for generating hydrogen from H2S.

Trapping Reactions with Unsaturated Organic Molecules and Dative Ligands. J. Am. Chem. Soc. 1990, 112, 6426−6428. (3) Stephanopoulos, M. F.; Sakbodin, M.; Wang, Z. Regenerative Adsorption and Removal of H2S from Hot Fuel Gas Streams by Rare Earth Oxides. Science 2006, 312, 1508−1510. (4) Baykara, S. Z.; Figena, E. H.; Kale, A.; Nejat Veziroglu, T. Hydrogen from Hydrogen Sulphide in Black Sea. Int. J. Hydrogen Energy 2007, 32, 1246−1250. (5) Villasmil, W.; Steinfeld, A. Hydrogen Production by Hydrogen Sulfide Splitting Using Concentrated Solar Energy − Thermodynamics and Economic Evaluation. Energy. Convers. Manage. 2010, 51, 2353− 2361. (6) Wang, H. Hydrogen Production from a Chemical Cycle of H2S Splitting. Int. J. Hydrogen Energy 2007, 32, 3907−3914. (7) Zong, X.; Han, J. F.; Seger, B.; Chen, H. J.; Lu, G. Q.; Li, C.; Wang, L. Z. An Integrated Photoelectrochemical−Chemical Loop for Solar-Driven Overall Splitting of Hydrogen Sulfide. Angew. Chem., Int. Ed. 2014, 53, 4399−4403. (8) Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295. (9) Liu, C. R.; Pan, J.; Li, S.; Zhao, Y.; Wu, L. Y.; Berkman, C. B.; Whorton, A. R.; Xian, M. Capture and Visualization of Hydrogen Sulfide by a Fluorescent Probe. Angew. Chem., Int. Ed. 2011, 123, 10511−10513. (10) Peng, H. J.; Cheng, Y. F.; Dai, C. F.; King, A. L.; Predmore, B. L.; Lefer, D. J.; Wang, B. H. A Fluorescent Probe for Fast and Quantitative Detection of Hydrogen Sulfide in Blood. Angew. Chem., Int. Ed. 2011, 123, 9846−9849. (11) Xuan, W. M.; Sheng, C. Q.; Cao, Y. T.; He, W. H.; Wang, W. Fluorescent Probes for the Detection of Hydrogen Sulfide in Biological Systems. Angew. Chem., Int. Ed. 2012, 51, 2282−2284. (12) Tursi, A.; Nixon, E. R. Infrared Spectra of Matrix isolated Hydrogen Sulfide in Solid Nitrogen. J. Chem. Phys. 1970, 53, 518−521. (13) Smardzewski, R. R.; Lin, M. C. Matrix Reactions of Oxygen Atoms with H2S Molecules. J. Chem. Phys. 1977, 66, 3197−3204. (14) Tso, T.-L.; Lee, E. K. C. Formation of Sulfuric Acid and Sulfur Trioxide/Water Complex from Photooxidation of Hydrogen Sulfide in Solid Oxygen at 15K. J. Phys. Chem. 1984, 88, 2776−2781. (15) Woodbridge, E. L.; Tso, T.; McGrath, M. P.; Hehre, W. J.; Lee, E. K. C. Infrared Spectra of Matrix Isolated Monomeric and Dimeric Hydrogen Sulfide in Solid O2. J. Chem. Phys. 1986, 85, 6991−6994. (16) Carpenter, J. D.; Ault, B. S. Matrix Isolation Study of the Reaction of Diborane with Hydrogen Sulfide: Spectroscopic Characterization of Mercaptoborane, H2BSH. J. Phys. Chem. 1992, 96, 7913−7916. (17) Isoniemi, E.; Pettersson, M.; Khriachtchev, L.; Lundell, J.; Räsänen, M. Infrared Spectroscopy of H2S and SH in Rare-Gas Matrixes. J. Phys. Chem. A 1999, 103, 679−685. (18) Thompson, S. J.; Goldberg, N.; Ault, B. S. Infrared Matrix Isolation Study of the Oxidation of H2S by CrCl2O2. Phys. Chem. Chem. Phys. 2006, 8, 856−861. (19) Chertihin, G. V.; Andrews, L. Reactions of Laser-Ablated Zr and Hf Atoms with Hydrogen. Matrix Infrared Spectra of the MH, MH2, MH3, and MH4 Molecules. J. Phys. Chem. A 1995, 99, 15004−15010. (20) Chertihin, G. V.; Andrews, L. Infrared Spectra of ZrH4 and HfH4 in Solid Argon. J. Am. Chem. Soc. 1995, 117, 6402−6403. (21) Chertihin, G. V.; Andrews, L. Reactions of Laser Ablated Ti Atoms with Hydrogen during Condensation in Excess Argon. Infrared Spectra of the TiH, TiH2, TiH3, and TiH4 Molecules. J. Am. Chem. Soc. 1994, 116, 8322−8327. (22) Zhou, M. F.; Andrews, L. Reactions of Zirconium and Hafnium Atoms with CO: Infrared Spectra and Density Functional Calculations of M(CO)x, OMCCO, and M(CO)2− (M = Zr, Hf; x = 1−4). J. Am. Chem. Soc. 2000, 122, 1531−1539. (23) Zhou, M. F.; Zhang, L. N.; Dong, J.; Qin, Q. Z. Reactions of Group IV Metal Atoms with Water Molecules: Matrix Isolation FTIR and Theoretical Studies. J. Am. Chem. Soc. 2000, 122, 10680−10688.

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CONCLUSIONS The reactions of Ti, Zr, and Hf atoms with hydrogen sulfide have been investigated at 4 K in excess argon and theoretical calculations. The metal atoms were observed to insert into the S−H bond of hydrogen sulfide to form the HMSH (M = Ti, Zr, Hf) molecules. From HMSH molecules, the second hydrogen transferred from sulfur to form the H2MS molecules. The HMSH molecules could further react with the other hydrogen sulfide to form the H2M(SH)2 molecules. These molecules were identified on the basis of the D2S and H234S isotopic substitutions and the comparison with theoretical predictions. In addition, the HMS species were also observed and identified. Qualitative analysis of the possible reaction paths on the singlet and triplet state potential energy surface leading to the observed products are proposed. We have made a theoretical prediction about the H2MS complex dehydrogenation. The results show that these H2MS species could produce hydrogen molecules provided additional energy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86 134 8228 7768. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21173158 and No. 21373152) and the Ministry of Science and Technology of China (No.2012YQ220113-7).

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REFERENCES

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Reactions of Ti, Zr, and Hf Atoms with Hydrogen Sulfide: Argon Matrix

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