Mo Insertion into the H2 Bond in MoxSyâ + H2 Reactions | The Journal

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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Mo Insertion into the H2 Bond in MoxSy− + H2 Reactions Published as part of The Journal of Physical Chemistry virtual special issue “Leo Radom Festschrift”. Ankur K. Gupta, Josey E. Topolski, Kathleen A. Nickson,‡ Caroline Chick Jarrold,* and Krishnan Raghavachari* Department of Chemistry, Indiana University 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, United States

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S Supporting Information *

ABSTRACT: A combined experimental and computational study of H2 reactions with small 98MoxSy− clusters ranging from subsulfide (x ∼ y) to hypersulfide (y > 2x) is presented. Results suggest that the subsulfides react with H2 primarily by insertion of a more reduced Mo center into the H−H bond, forming a dihydride product. We find that this reaction occurs up to Mo oxidation states of +4. For the subsulfides containing a second metal in a sufficiently low oxidation state, a second insertion of H2 occurs, leading to a tetrahydride product. The reaction mechanisms of the sulfides are found to be very similar, albeit slightly higher energetically to those of the analogous oxosulfides that are also observed at low abundances in the experiments. In addition, the experimental results show an overall reduction of hypersulfides in the presence of H2, suggesting loss of H2S neutral molecules.

1. INTRODUCTION MoS2, a material with interesting physical, electronic,1 and catalytic properties,2 has emerged as an important material in applications for energy conversion,3 including the hydrogen evolution reaction (HER).4−12 As a lamellar material that can form interesting nanostructures,13,14 MoS2 has also been explored as a potential H2 storage material.15,16 In addition, H2 annealing of MoS2, which creates Mo0 sites,17 has been shown to increase the activity of the material for HER.18 Therefore, understanding interactions between H2 and MoS2 is relevant for understanding HER19,20 as well as for potential H2 storage applications. Defect and dopant sites are frequently implicated as active sites on catalysts.21−29 Since both bonding and defect sites in metal sulfides are inherently localized, small clusters can provide an informative experimental30−33 and computational34−36 platform, complementary to bulk studies, for determining the molecular-scale interactions involved in catalytic processes.37,38 The molecular bonding and electronic structures of clusters are governed by the same attributes that govern bulk properties. Structures determined for small molybdenum oxide and molybdenum sulfide clusters provide an interesting case in point, in terms of the relative stabilities of terminal MoO bond motifs in the former39,40 compared to MoSMo bridge bond motifs in the latter.41 We recently reported the results of experimental and computational studies on H2O + MoxSy− HER,42 as an extension of our previous HER studies on the analogous reactions of water with group 6 transition metal (TM) suboxide clusters.39,40,43−46 Ablation of MoS2 targets generates © XXXX American Chemical Society

a very broad distribution of cluster sizes and stoichiometries, ranging from subsulfide (y < 2x) to hypersulfide (y > 2x), allowing the observation of oxidation with HER along with water addition reactions. While there were numerous parallels between water reactions with MoxOy− and MoxSy−, several interesting distinctions emerged from both experimental and computational results, including much slower reaction rates for the sulfides, and apparent oxidation of the more profoundly hypersulfides for which no reactivity was observed in analogous hyperoxides. Differences in the lowest energy molecular structures determined for sulfides versus oxides also resulted in differences in the reaction paths. While not observed experimentally, a noteworthy finding from the computational results was the possibility of the more reduced MoxSyO− clusters undergoing reactions with the H2 molecule that had just been produced via MoxSy− + H2O, which warranted further investigation. In this current study on molybdenum sulfide clusters, we determine which electronic and molecular structures may favor H2 addition, Mox Sy− + H 2 → Mox Sy H 2−

(1)

versus reduction, Mox Sy− + H 2 → Mox Sy − 1− + H 2S

(2)

Received: April 30, 2019 Revised: July 24, 2019

A

DOI: 10.1021/acs.jpca.9b04079 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

298.15 K were also computed from the vibrational analysis using the standard rigid-rotor harmonic-oscillator (RRHO) approximation. To obtain reliable relative energies between chemical species, electronic energies of all the optimized structures were calculated using a larger basis set (def2TZVPP), which were then added to the corresponding free energy corrections to obtain total free energies for all chemical species at 298.15 K. Using these raw free energies, the energies for all reacting species relative to the sum of the free energies of electronic ground state of reactants at infinite separation were then calculated (in kcal mol−1) and plotted on a reaction coordinate diagram. For most cases, only the lowest energy isomer was considered in reactions with H2, since other geometric isomers were too high in energy to have any significant population under the experimental conditions. However, a few species do have isomers energetically similar to the lowest energy one, and hence in such cases all structures were considered for the reactivity with H2. Both doublet and quartet pathways were computed in most cases; however, if one of the spin states turned out to be significantly higher in energy, only the lower energy spin pathway was followed.

We further explore several oxosulfide reactions with H2. These comparisons are all the more intriguing because the sulfides and oxides tend to favor different structures and spin states.

2. METHODS 2.A. Experimental Details. Clusters were generated and mass analyzed using a home-built experimental apparatus that has been described previously.47,48 Briefly, 98MoxSy− clusters were generated via ablation of a pressed rotating 98MoS2 [synthesized using 98Mo (Trace Sciences International, 99%+ enrichment) and natural isotope abundances of S via a solid state thermal reaction34 akin to the method developed by Pol et al.49] target using under 4 mJ/pulse energy of the second harmonic output of a Nd:YAG laser operated at a 30 Hz repetition rate. The resulting plasma was entrained in a pulse of either ultrahigh purity (UHP) helium carrier gas or 20% hydrogen/balance UHP He carrier gas (30 psig stagnation pressure) issued from a pulsed molecular beam valve operated at a 30 Hz repetition rate and swept into a 25 mm long, 3 mm diameter reaction channel. Carrier gas mixtures of 10% H2/ balance UHP He and 40% H2/balance UHP He were also used. The relative intensities of H2 addition products increased with H2 content, but not linearly. Reactivity studies were also performed by injecting H2 into the reaction channel, as done in previous cluster reactivity studies, but no further reaction products were observed. The resulting gas mixture then expanded into a vacuum chamber and was collimated by passing through a 3 mm diameter skimmer. The anions were accelerated on the molecular beam axis to 1 keV, re-referenced to ground, and subsequently entered a 1.2 m beam-modulated time-of-flight mass spectrometer. Within the drift tube, the anions were guided by several ion optics, finally colliding with a 25 mm microchannel plate (MCP) detector at the end of the drift tube. The resulting signal from the detector was recorded with a digitizing oscilloscope. The m of the mass spectrometer is Δm 350. In this size regime, the molybdenum sulfide clusters can bind no more than one electron. A comparison of the distribution of clusters formed in both the presence and absence of hydrogen was carried out to determine which clusters undergo reactions with hydrogen. The temperature of the clusters as they undergo H2 reactions is expected to be much higher than room temperature, since nascent clusters typically require thousands of collisions with the buffer gas to reach room temperature. We have determined that the clusters reside in the reaction channel for approximately 25−50 μs. 2.B. Computational Details. In our computational studies, different mechanistic routes were explored for the reactions between MoxSy− (or MoxSyOz−) and hydrogen (H2) using the Gaussian 16 program suite.50 Optimization and vibrational analysis (at 298.15 K) of all chemical species were performed using the unrestricted dispersion-corrected B3LYPD3BJ51−55 hybrid density functional with the def2-SVPP56,57 basis set. The Stuttgart/Dresden (SDD)58 effective core potential was used to model the core electrons of molybdenum. All transition states were recognized by the presence of one (and only one) imaginary frequency, and the reaction coordinate corresponding to that imaginary frequency was followed (by performing an IRC calculation) to confirm the species connected to that particular transition state. The entropies and free energy corrections for all the species at

3. RESULTS AND DISCUSSION As discussed in section 2.A, molybdenum sulfide species of different composition were produced from laser ablation of a pressed 98MoS2 target. Unavoidably, the target is exposed to oxygen during its preparation and ablation giving rise to small quantities of molybdenum oxosulfide (MoxSyOz−) species as well. These species are relatively low in intensity; therefore, we expect very minor dioxide contributions coinciding with the pure sulfide clusters, which is supported by the mass spectral simulations that incorporate the natural isotopic distribution of sulfur. Figure 1a shows an overview of the broad distribution of

Figure 1. (a) Initial mass distribution of 98MoxSy− and 98MoxSyOz− clusters generated via laser ablation of a pressed 98MoS2 target. (b) Cluster mass distribution produced with 20% H2/balance UHP He carrier gas.

MoxSy− and MoxSyOz− clusters, ranging from subsulfide to profoundly hypersulfide. Observation of cluster stoichiometries that are modestly hypersulfide relative to bulk (2x < y < 3x) are not surprising, since every Mo center is comparable to a corner site on a bulk layer. Because the mass of 98Mo and S3 (96 amu being the most abundant isotopomer) are similar, overlapping 98MoxSy− and 98Mox+1Sy′− (y = y′ + 3) cluster series complicate the mass spectrum. However, simulations of the isotopic distributions of 98MoxSy− and 98Mox+1Sy′− (y = y′ + B


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indicated in the simulation by different colors. All addition reactions determined from this analysis are summarized in Table 1. Peaks unmatched by the simulations are attributed to MoxSyOzCn− clusters resulting from trace carbon sources in the vacuum chamber present during cluster formation.

3) allow definitive relative intensity determinations of the individual members of these adjacent cluster series (vide infra). Figure 1b shows an overview of the cluster distribution generated with 20% H2/balance UHP He carrier gas, which generally shows a decrease in the relative intensities of the most hypersulfide species compared to the species in the 2x < y < 3x range of stoichiometries. In addition, new mass peaks appear at 2 or 4 amu higher than several of the subsulfide species, with a similar reduction of the associated subsulfide species. Figure 2 shows the mass spectra obtained with UHP He and 20% H2/bal. UHP He for (a) x = 1, (b) x = 2, and (c) x = 3,

Table 1. Experimentally Observed MoxSyOz− + H2 Reactions x

y+z

reaction

2

2

Mo2S2− + H2 → Mo2S2H2− Mo2S2H2− + H2 → Mo2S2H4− Mo2S2O− + H2 → Mo2S2OH2− Mo2S2OH2− + H2 → Mo2S2OH4− Mo2S3− + H2 → Mo2S3H2− Mo2S3O− + H2 → Mo2S3OH2− Mo2S4− + H2 → Mo2S4H2− Mo2S4H2− + H2 → Mo2S4H4− Mo2S4O− + H2 → Mo2S4OH2− Mo3S3− + H2 → Mo3S3H2− Mo3S3O− + H2 → Mo3S3OH2− Mo3S4− + H2 → Mo3S4H2− Mo3S4H2− + H2 → Mo3S4H4− Mo3S4O− + H2 → Mo3S4OH2− Mo3S5− + H2 → Mo3S5H2−

3

4

3

5 3 4

5

Figure 2a shows that no H2 addition products were observed for the x = 1 series of clusters, though there is clearly a decrease in the relative intensities of MoSx− molecules with x > 3 relative to MoS3−. This effect will be examined more closely in Section 3.C. 3.A. Bimetallic Molybdenum Sulfides. Figure 2b shows the mass spectra and simulations for clusters in the Mo2Sy− (y = 2−7) mass range. Comparing the mass distribution generated with the H2 gas mixture (lower trace) to the pure He carrier gas (upper trace), it is apparent that both H2 addition [eq 1] and reduction [eq 2] of Mo2Sy− clusters can occur, with addition of one and two H2 molecules observed for both Mo2S2− and Mo2S4−. Moreover, addition of two H2 molecules is clearly observed for the Mo2S2O− cluster. In some cases, congeners show slightly different behavior. In particular, Mo2S3− (congener of Mo2S2O−) and Mo2S3O− (congener of Mo2S4−) show smaller peaks for addition of two H2 molecules under the experimental conditions. All the addition reactions observed are summarized in Table 1. A decrease in the relative intensities of species with y > 5 relative to Mo2S5− suggests that hypersulfide reduction might be taking place. Computationally, H 2 adds to the less coordinated molybdenum atom in Mo2S2− in an overall barrierless fashion to form a 1,1-dissociation product, which then undergoes a spontaneous rearrangement to yield a trans-1,2 addition product (Figure 3a). Owing to the availability of a low coordination site on one of the Mo centers in the final product, another H2 reacts with it, consistent with the experimental observations. A further rearrangement through modest reaction barriers leads to a relatively stable bridged hydrogen−sulfur addition product (Figure 3b). H2 can also react with Mo2S2− to form a hydride−thiol product through a relatively higher energy pathway (Figure S1a in the Supporting Information). H 2 dissociates spontaneously at the more reduced molybdenum atom in Mo2S2O− to form a thermodynamically stable 1,1-dihydride, with subsequent steps involving possible intramolecular isomerizations having high energy barriers

Figure 2. Mass spectra of 98MoxSy− obtained with UHP He and 20% H2/bal UHP He for (a) x = 1, (b) x = 2, and (c) x = 3. Mass peaks indicated with the dashed vertical lines indicate 98MoxSyO− species.

along with the simulations based on the relative abundances of the isotopomer distributions for all 98MoxSy− species. Based on the simulations, the relative abundances of the overlapping 98 MoxSy− and 98Mox+1Sy′− (y = y′ + 3) series cluster compositions could be determined unambiguously and are C


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Figure 3. Computed lowest barrier free energy reaction paths (T = 298.15 K) for (a) Mo2S2− + H2 and (b) Mo2S2H2− + H2 reactions. Other pathways explored are included in the Supporting Information.

(Figure 4a). Interestingly, the doublet dihydride is highly spin contaminated (S2 = 1.47), thus lowering the energy of the (broken-symmetry) doublet, which otherwise would have been higher compared to the quartet’s energy. The same trend is observed for Mo2S3− as well (Figure 4b), which has the same overall molybdenum oxidation states as in Mo2S2O−. The quartet form of Mo2S2OH2− may further add another H2 to form a double addition product being in line with the experimental findings (Figure 4c). Not surprisingly, the tetrahydride favors a doublet ground state starting from a quartet precursor as the unpaired spins tend to pair up as the reacting species combine. Contrary to experimental observations, Mo2S3H2−’s reaction with H2 (Figure 4d) provides an energetically stable intermediate. Like Mo2S2OH2−, Mo2S3H2− has a vacant coordination site, which sterically (and electronically) favors further addition of H2. Nevertheless, a lesser amount of tetrahydride is likely to form from this cluster relative to Mo2S2OH2− due to repulsion from the relatively larger sulfur atom at the terminal position in Mo2S3H2−. Mo2S3O−and Mo2S4− follow similar mechanisms while reacting with H2 (Figure 5a,b). The reactions start on the quartet surface but undergo a spin-crossover when forming the 1,1-dissociation product, which has a doublet ground state. This is a typical example of what is called the two-state reactivity (TSR), which has been found to occur commonly in reactions involving transition metals.59,60 Any further attempt in migrating a hydrogen atom to other atom centers requires high activation energies and hence is unlikely to be feasible. Therefore, although a cis-1,2 addition product is thermodynamically more stable than other possible products, due to the high energy pathway required to reach there, the reaction is expected to stop at the first step to yield a 1,1-dihydride. Unlike Mo2S4−, Mo2S3O− is structurally asymmetric due to the presence of an oxygen atom attached to one of the Mo centers.

Figure 4. Computed lowest barrier free energy reaction paths (T = 298.15 K) for (a) Mo2S2O− + H2, (b) Mo2S3− + H2, (c) Mo2S2OH2− + H2, and (d) Mo2S3H2− + H2.

Thus, Mo2S3O−’s reaction with H2 can produce different isomeric addition products depending on where hydrogen attacks. It turns out that the reaction between H2 and the Mo center bonded to oxygen leads to a less crowded, and hence lower energy intermediate. The relatively higher energy pathway for the reaction between H2 and the Mo center with the terminal sulfur is depicted in Figure S2 of the Supporting Information. D


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dihydride, since the associated energy barriers are relatively lower for oxosulfides. This makes the product obtained from the first reaction (cis-1,2-dihydride Mo2S3OH2−) sterically inert, making its further reaction with H2 unlikely, perhaps explaining the lower amount of tetrahydride formation in this case. Unlike Mo2S5−, Mo2S4O− can attain different isomeric forms that differ by a few kcal mol−1 and hence may coexist in the reaction environment (Figure 6). The lowest energy isomer of

Figure 6. Three lowest energy isomers of Mo2S4O−. Doublet states are more stable than the quartet states for all three geometries.

Mo2S4O− (isomer 1) has terminal sulfurs in trans positions, the second energetically lowest structure (isomer 2) is a cis with respect to terminal sulfurs, whereas the third one (isomer 3) has terminal sulfurs on the same Mo atom. Isomer 3, having a lone oxygen (at the terminal position) on one of the Mo atoms, provides a relatively sterically unhindered coordination site available to the hydrogen, which allows for its spontaneous dissociation on the reactive Mo center to form the 1,1dihydride (Figure 7a). H2 may also bind weakly at the only available coordination site on one of the Mo atoms in Mo2S4O− (isomers 1 and 2) and Mo2S5− clusters (Figure 7b,c). The weakly bound hydrogen complex may further transform to a 1,1-dihydride through a moderate energetic barrier. Experimentally, however, the peak corresponding to Mo2S5H2− was weaker in the mass spectra, suggesting that steric repulsion from the terminal sulfur might be responsible for the lower reactivity of Mo2S5−, similar to the case of Mo2S3H2−. 3.B. Trimetallic Molybdenum Sulfides. As summarized in Table 1, several specific subsulfide/oxosulfide (Mo3Sy−/ Mo3SyO−) clusters undergo H2 addition reactions. No addition is observed for species in which the total number of S and O atoms exceeds 5; the average oxidiation state of Mo for reactive species is below 4. While the number of structural isomers in the trimetallic species, compounded by the number of different H2 addition sites, is too vast to include in this report, we do present a representative addition pathway for the Mo3S3O− + H2 and Mo3S4− + H2 reactions in Figure 8. H2 binds on one of the less coordinated Mo atoms of Mo3S3O− to form a weak hydrogen complex, which then transforms to a bridged hydrogen intermediate through a partial hydrogen transfer from one Mo center to the neighboring less coordinated Mo atom. Subsequently, the bridging hydrogen completes its transfer to the second Mo center to form a thermodynamically more stable 1,2-dihydride (Figure 8a). The whole reaction occurs on the ground doublet state without any spin-crossovers between potential energy curves. Since Mo3S4− is structurally similar to Mo3S3O−, it follows the same mechanism while reacting with H2 (Figure

Figure 5. Computed lowest barrier free energy reaction paths (T = 298.15 K) for (a) Mo2S3O− + H2, (b) Mo2S4− + H2, (c) Mo2S3OH2− + H2, and (d) Mo2S4H2− + H2. Other pathways explored are included in the Supporting Information.

Mo2S4H2− may further react with hydrogen (Figure 5d) to form a 1,1-substituted intermediate in accordance with the experimental results. The computations indicate that Mo2S3OH2− is also expected to behave like Mo2S4H2− while reacting with H2 (Figure 5c) owing to its structural and electronic similarity to Mo2S4H2−. This is expected to be the case provided the reaction between Mo2S3O− and H2 stops at the 1,1-dihydride. However, there is an additional possibility for this reaction to terminate with the formation of cis-1,2 E


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Figure 8. Computed lowest barrier free energy reaction paths (T = 298.15 K) for (a) Mo3S3O− + H2 and (b) Mo3S4− + H2.

Figure 7. Computed lowest barrier free energy reaction paths (T = 298.15 K) for (a) Mo2S4O− (isomer 3) + H2, (b) Mo2S4O− (isomer 1) + H2, and (c) Mo2S5− + H2. The energy profile for Mo2S4O− (isomer 2) + H2 is shown in Figure S3 of the Supporting Information.

8b). The reaction barriers and overall profile are comparatively lower in energy for the oxosulfide compared to the sulfide species, a trend that was observed for bimetallic species as well. 3.C. Monometallic Molybdenum Sulfides. While no H2 addition reactions are observed within the MoSy− series, there is an apparent reduction in the hypersulfides that we discuss separately in this section. Figure 2a shows the MoSy− cluster distribution, which is dominated by MoS3−, with significant abundances of the hypersulfide MoSy− (y = 4−6) ions. Comparing the mass distribution generated with UHP He carrier gas (upper trace) and with 20% H2/balance UHP He (lower trace), no H2 addition products are observed; however, the loss of relative intensity of the hypersulfide species suggests that reduction of the hypersulfides via eq 2 might be occurring. The lowest energy structures of experimentally observed monometallic sulfides (viz., MoS3−, MoS4−, MoS5−, and MoS6−) are shown in Figure 9. It is apparent (from Figure 9) that the lone Mo atom is saturated in MoS4−, and most of the spin density is concentrated on sulfur atoms; thus the higher sulfides (hypersulfides) tend to form a structure in which terminal sulfur atoms maintain an end-on orientation (as in MoS5− and MoS6−).

Figure 9. Optimized ground state structures of (a) MoS3−, (b) MoS4−, (c) MoS5−, and (d) MoS6−. All have doublet ground states. Black and red numbers represent bond lengths (Å) and spin densities (def2-TZVPP basis), respectively. Blue spheres represent Mo atoms; yellow spheres represent S atoms.

Unlike bimetallic sulfides, the Mo center in monometallic species does not have sufficient coordination space to accommodate two hydrogen atoms. As shown in Figure 10a, the MoS3−−H2 complex is around 16 kcal mol−1 higher in energy than the separated reactants, which indicates the energetic difficulty in its formation. Therefore, the only plausible route for the reaction to proceed is through a dithiol F


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steric hindrance near the Mo atom, H2 prefers to be dissociated on the (terminal) sulfur atoms to form a dithiol intermediate. Since this step involves breaking a strong H−H bond, it is the most energetically demanding step of the mechanism. One of the thiol units may then transfer its hydrogen to the other thiol to yield H2S and a lower monometallic sulfide. A noticeable difference in the energy barrier between mechanisms of MoS4− and MoS5− for the intramolecular hydrogen transfer between thiols is due to the presence of weakly attached −SH unit to sulfur in MoS5− as opposed to stronger Mo−SH bond in MoS4− and hence is readily available to accept another H atom. Other energetically similar covalently bound isomers of MoS4− and MoS5− encounter similar energy barriers while reacting with H2 and are included in the Supporting Information (Figure S4 and S5).

4. CONCLUSIONS A study of reactions between a distribution of MoxSy− clusters ranging from subsulfide (x ∼ y) to hypersulfide (y > 2x) with H2 using both experimental and computational methods has provided insight into how the electronic and molecular structures influence H2 addition versus H2S evolution beyond simple stoichiometric considerations. Most of the bimetallic molybdenum sulfides (and oxosulfides) with a Mo center in an oxidation state equal to or less than +4 allow for the addition of H2 to form a dihydride product. A few exceptional cases of unexpected molecular inertness (shown by species having an active Mo center as in Mo2S3H2−, Mo2S5−) is mainly attributed to the steric repulsion caused by the terminal sulfur atom. Overall, oxosulfides produce relatively lower energy barriers, and addition products. One of its consequences is the potential formation of inert 1,2-dihydride Mo2S3OH2−, in contrast to the reactive 1,1-dihydride as in its sulfur counterpart. Monometallic sulfides, being saturated and sterically crowded, are inert to H2 addition but may undergo reduction as indicated by experimental data. However, due to Mo being in a high oxidation sate, and lacking coordination space for the H2 to dissociate, the reaction is likely to proceed through H2 dissociation on terminal sulfurs, providing somewhat high reaction barriers that seem difficult to overcome under low energy conditions.

Figure 10. Computed lowest barrier free energy reaction paths (T = 298.15 K) for the (a) MoS3− + H2, (b) MoS4− + H2, and (c) MoS5− + H2 reactions. Other pathways explored are included in the Supporting Information.

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or hydride−thiol intermediate. A thermodynamic analysis of the experimentally inferred reduction reactions for monometallic sulfides (refer Table 2) indicates that such reactions

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b04079. Additional, higher-barrier reactions between the bimetallic subsulfides and oxo-subsulfides, along with reactions with monometallic hypersulfides noted in the text, for comparison with the lower barrier pathways shown in the text; list of coordinates and single-point energies (PDF)

Table 2. Free Energy Change during the Reactions of Monometallic Molybdenum Sulfides with Hydrogen To Evolve H2S free energy change (kcal mol−1)

reaction −

−

MoS4 + H2 → MoS3 + H2S MoS5− + H2 → MoS4− + H2S MoS6− + H2 → MoS5− + H2S

ASSOCIATED CONTENT

S Supporting Information *

9.6 −3.4 −1.7

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are endothermic or only weakly exothermic, which implies that there is a high probability that the mechanisms of these reactions may involve high energy transition states or that metastable hypersulfide structures are populating the ion beam and account for most of the apparent reduction, making the reduction products unachievable under low energy conditions. The reduction reaction of MoS4− and MoS5− may proceed through similar mechanisms (Figures 10b,c). Due to huge

AUTHOR INFORMATION

Corresponding Authors

*C. C. Jarrold, E-mail: [email protected]. Phone: (812) 856-1190. Fax: (812) 855-8300. *K. Raghavachari. E-mail: [email protected]. Phone: (812) 856-1766. Fax: (812) 855-8300. ORCID

Caroline Chick Jarrold: 0000-0001-9725-4581 G


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Evolution Reaction Performance. Electrochim. Acta 2018, 283, 306− 312. (14) Escalera-Lopez, D.; Niu, Y. B.; Park, S. J.; Isaacs, M.; Wilson, K.; Palmer, R. E.; Rees, N. V. Hydrogen evolution enhancement of ultra-low loading, size-selected molybdenum sulfide nanoclusters by sulfur enrichment. Appl. Catal., B 2018, 235, 84−91. (15) Li, X. S.; Xin, Q.; Guo, X. X.; Grange, P.; Delmon, B. Reversible hydrogen adsorption on MoS2 Studied by Temperature-Programmed Desorption and Temperature-Programmed Reduction. J. Catal. 1992, 137, 385−393. (16) Chen, J.; Wu, F. Review of hydrogen storage in inorganic fullerene-like nanotubes. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 989−994. (17) Kiriya, D.; Lobaccaro, P.; Nyen, H. Y. Y.; Taheri, P.; Hettick, M.; Shiraki, H.; Sutter-Fella, C. M.; Zhao, P. D.; Gao, W.; Maboudian, R.; et al. General Thermal Texturization Process of MoS2 for Efficient Electrocatalytic Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 4047−4053. (18) He, M.; Kong, F.; Yin, G.; Lv, Z.; Sun, X.; Shi, H.; Gao, B. Enhanced Hydrogen Evolution Reaction Activity of HydrogenAnnealed Vertical MoS2 Nanosheets. RSC Adv. 2018, 8, 14369− 14376. (19) Hakala, M.; Kronberg, R.; Laasonen, K. Hydrogen Adsorption on Doped MoS2 Nanostructures. Sci. Rep. 2017, 7, 15243. (20) Wang, J.; Liu, J.; Zhang, B.; Ji, X.; Xu, K.; Chen, C.; Miao, L.; Jiang, J. The Mechanism of Hydrogen Adsorption on Transition Metal Dichalcogenides as Hydrogen Evolution Catalyst. Phys. Chem. Chem. Phys. 2017, 19, 10125−10132. (21) Spirko, J. A.; Neiman, M. L.; Oelker, A. M.; Klier, K. Electronic structure and reactivity of defect MoS2 I. Relative stabilities of clusters and edges, and electronic surface states. Surf. Sci. 2003, 542, 192−204. (22) Lobos, S.; Sierraalta, A.; Ruette, F.; Rodríguez-Arias, E. N. Modeling MoS2 Catalytic surface With Simple Clusters. J. Mol. Catal. A: Chem. 2003, 192, 203−216. (23) Ataca, C.; Ciraci, S. Dissociation of H2O at the Vacancies of Single-Layer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 195410. (24) Ghuman, K. K.; Yadav, S.; Singh, C. V. Adsorption and Dissociation of H2O on Monolayered MoS2 Edges: Energetics and Mechanism from ab Initio Simulations. J. Phys. Chem. C 2015, 119, 6518−6529. (25) Huang, Y.; Nielsen, R. J.; Goddard, W. A., III; Soriaga, M. P. The Reaction Mechanism with Energy Barriers for Electrochemical Dihydrogen Evolution on MoS2. J. Am. Chem. Soc. 2015, 137, 6692− 6698. (26) Zhang, J.; Wu, J.; Guo, H.; Chen, W.; Yuan, J.; Martinez, U.; Gupta, G.; Mohite, A.; Ajayan, P. M.; Lou, J. Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2. Adv. Mater. 2017, 29, 1701955. (27) Deng, Y.; Ting, L. R. L.; Neo, P. H. L.; Zhang, Y.-J.; Peterson, A. A.; Yeo, B. S. Operando Raman Spectroscopy of Amorphous Molybdenum Sulfide (MoSx) during the Electrochemical Hydrogen Evolution Reaction: Identification of Sulfur Atoms as Catalytically Active Sites for H+ Reduction. ACS Catal. 2016, 6, 7790−7798. (28) Ting, L. R. L.; Deng, Y.; Ma, L.; Zhang, Y.-J.; Peterson, A. A.; Yeo, B. S. Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction. ACS Catal. 2016, 6, 861−867. (29) Kim, K. Y.; Lee, J.; Kang, S.; Son, Y.-W.; Jang, H. W.; Kang, Y.; Han, S. Role of Hyper-Reduced States in Hydrogen Evolution Reaction at Sulfur Vacancy in MoS2. ACS Catal. 2018, 8, 4508−4515. (30) Baloglou, A.; Oncak, M.; van der Linde, C.; Beyer, M. K. GasPhase Reactivity Studies of Small Molybdenum Cluster Ions with Dimethyl Disulfide. Top. Catal. 2018, 61, 20−27. (31) Baloglou, A.; Oncak, M.; Grutza, M. L.; van der Linde, C.; Kurz, P.; Beyer, M. K. Structural Properties of Gas Phase Molybdenum Sulfide Clusters Mo 3 S 13 (2‑) , HMo 3 S 13 (−) , and H3Mo3S13(+) as Model Systems of a Promising Hydrogen Evolution Catalyst. J. Phys. Chem. C 2019, 123, 8177−8186.

Krishnan Raghavachari: 0000-0003-3275-1426 Present Address ‡

Department of Chemistry, University of Wisconsin Madison, 1101 University Ave, Madison WI 53706. Notes

The authors declare no competing financial interest. The calculated isotopomer distribution and the input scaled intensities for every feasible 98MoxSyOzHn− (x = 1−5; y = 1− 10; z = 0,1; n = 0−2) stoichiometry for before and after H2 addition is available upon request.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, CPIMS Program, under award no. DE-FG02-07ER15889 at Indiana University. K.A.N. was supported by NSF REU program, Grant # CHE1460720.

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REFERENCES

(1) Shi, J.; Hong, M.; Zhang, Z.; Ji, Q.; Zhang, Y. Physical Properties and Potential Applications o Two-Dimensional Metallic Transition Metal Dichalcogenides. Coord. Chem. Rev. 2018, 376, 1−19. (2) He, Z.; Que, W. Molybdenum Disulfide Nanomaterials: Structures, Properties, Synthesis and Recent Progress on Hydrogen Evolution Reaction. Applied Materials Today 2016, 3, 23−56. (3) Xia, D.; Gong, F.; Pei, X.; Wang, W.; Li, H.; Zeng, W.; Wu, M.; Papavassiliou, D. V. Molybdenum and Tungsten Disulfide-Based Nanocomposite Films for Energy Storage and Conversion: A Review. Chem. Eng. J. 2018, 348, 908−928. (4) Dong, S.; Wang, Z. Grain Boundaries Trigger Basal Plane Catalytic Activity for the Hydrogen Evolution Reaction in Monolayer MoS2. Electrocatalysis 2018, 9, 744−751. (5) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (6) Li, G.; Zhang, D.; Qiao, Q.; Yu, Y.; Peterson, D.; Zafar, A.; Kumar, R.; Curtarolo, S.; Hunte, F.; Shannon, S.; et al. All the Catalytic Active Sites of MoS2 for Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 16632−16638. (7) Gao, M.-R.; Xu, Y.-F.; Jiang, J.; Yu, S.-H. Nanostructures Metal Chalcogenides: Synthesis, Modification and Applications in energy conversion and storage devices. Chem. Soc. Rev. 2013, 42, 2986−3017. (8) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (9) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalysts for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296− 7299. (10) Li, H.; Du, M.; Mleczko, M. J.; Koh, A. L.; Nishi, Y.; Pop, E.; Bard, A. J.; Zheng, X. Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy. J. Am. Chem. Soc. 2016, 138, 5123−5129. (11) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. (12) Wang, H.; Lu, Z.; Xu, S.; Kong, D.; Cha, J. J.; Zheng, G.; Hsu, P.-C.; Yan, K.; Bradshaw, D.; Prinz, F. B.; et al. Electrochemical Tuning of Vertically Aligned MoS2 Nanofilms and its Application in Improving Hydrogen Evolution Reaction. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 19701−19706. (13) Li, Z.; Ma, J.; Zhou, Y.; Yin, Z.; Tang, Y.; Ma, Y.; Wang, D. Synthesis of Sulfur-Rich MoS2 Nanoflowers for Enhanced Hydrogen H


Article

The Journal of Physical Chemistry A (32) Pham, C. V.; Zana, A.; Arenz, M.; Thiele, S. Mo3S13(2‑) Cluster Decorated Sulfur-doped Reduced Graphene Oxide as Noble MetalFree Catalyst for Hydrogen Evolution Reaction in Polymer Electrolyte Membrane Electrolyzers. ChemElectroChem 2018, 5, 2672−2680. (33) Rajagopal, A.; Venter, F.; Jacob, T.; Petermann, L.; Rau, S.; Tschierlei, S.; Streb, C. Homogeneous visible light-driven hydrogen evolution by the molecular molybdenum sulfide model Mo2S12(2‑). Sustainable Energy Fuels 2019, 3, 92−95. (34) Ziane, M.; Amitouche, F.; Bouarab, S.; Vega, A. Density functional study of the structural, electronic, and magnetic properties of Mon and MonS (n = 1−10) clusters. J. Nanopart. Res. 2017, 19, 380. (35) Zhang, X. F.; Liu, X. J.; Xu, R. N.; Wu, N.; Huang, X.; Wang, B. Probing Diverse Disulfur Ligands in the Mo2Sn-/0 (n = 4 similar to 8) Clusters: Structural Evolution and Chemical Bonding. Chinese Journal of Structural Chemistry 2018, 37, 497−516. (36) Gushchin, A. L.; Laricheva, Y. A.; Sokolov, M. N.; Llusar, R. Tri- and tetranuclear molybdenum and tungsten chalcogenide clusters: on the way to new materials and catalysts. Russ. Chem. Rev. 2018, 87, 670−706. (37) Böhme, D. K.; Schwarz, H. Gas-Phase Catalysis by Atomic and Cluster Metal Ions: The Ultimate Single-Site Catalysts. Angew. Chem., Int. Ed. 2005, 44, 2336−2354. (38) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley: New York, 1994. (39) Rothgeb, D. W.; Mann, J. E.; Waller, S. E.; Jarrold, C. C. Structures of Trimetallic Molybdenum and Tungsten Suboxide Cluster Anions. J. Chem. Phys. 2011, 135, 104312. (40) Yoder, B. L.; Maze, J. T.; Raghavachari, K.; Jarrold, C. C. Structures of Mo2Oy− and Mo2Oy (y = 2, 3, and 4) Studied by Anion Photoelectron Spectroscopy and Density Functional Theory Calculations. J. Chem. Phys. 2005, 122, 094313. (41) Saha, A.; Raghavachari, K. Hydrogen Evolution from Water through Metal Sulfide Reactions. J. Chem. Phys. 2013, 139, 204301. (42) Topolski, J. E.; Gupta, A. K.; Nickson, K. A.; Raghavachari, K.; Jarrold, C. C. Hydrogen Evolution from Water Reactions with Molybdenum Sulfide Cluster Anions. Int. J. Mass Spectrom. 2018, 434, 193−201. (43) Mann, J. E.; Mayhall, N. J.; Jarrold, C. C. Properties of metal oxide clusters in non-traditional oxidation states. Chem. Phys. Lett. 2012, 525−6, 1−12. (44) Mayhall, N. J.; Rothgeb, D. W.; Hossain, E.; Jarrold, C. C.; Raghavachari, K. Water Reactivity with Tungsten Oxides: H2 Production and Kinetic Traps. J. Chem. Phys. 2009, 131, 144302. (45) Rothgeb, D. W.; Hossain, E.; Kuo, A. T.; Troyer, J. L.; Jarrold, C. C.; Mayhall, N. J.; Raghavachari, K. Unusual Products Observed in Gas-Phase WxOy− + H2O and D2O reactions. J. Chem. Phys. 2009, 130, 124314. (46) Rothgeb, D. W.; Mann, J. E.; Jarrold, C. C. H2 production from Reactions Between Water and Small Molybdenum Suboxide Cluster Anions. J. Chem. Phys. 2010, 133, 054305. (47) Moravec, V. D.; Jarrold, C. C. Study of the Low-lying States of NiO− and NiO Using Anion Photoelectron Spectroscopy. J. Chem. Phys. 1998, 108, 1804−1810. (48) Waller, S. E.; Mann, J. E.; Jarrold, C. C. Asymmetric Partitioning of Metals Among Cluster Anions and Cations Generated via Laser Ablation of Mixed Aluminum/Group 6 Transition Metal Targets. J. Phys. Chem. A 2013, 117, 1765−1772. (49) Pol, V. G.; Pol, S. V.; Gedanken, A. Micro to Nano Conversion: A One-Step, Environmentally Friendly, Solid State, Bulk Fabrication of WS2 and MoS2 Nanoplates. Cryst. Growth Des. 2008, 8, 1126− 1132. (50) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 16, revision A.03; Gaussian, Inc.: Wallingford, CT, 2016. (51) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100.

(52) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti Correlation Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785− 789. (53) Becke, A. D. Density-Functional Thermochemistry. 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (54) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (55) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456−1465. (56) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Fully Optimized Contracted Gaussian-Basis Set for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571−2577. (57) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Fully Optimized Contracted Gaussian-Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (58) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-Adjusted ab initio Pseudopotentials for the 2nd and 3rd Row Transition Elements. Theor. Chim. Acta 1990, 77, 123−141. (59) Schröder, D.; Shaik, S.; Schwarz, H. Two-state reactivity as a new concept in organometallic chemistry. Acc. Chem. Res. 2000, 33, 139−145. (60) Armentrout, P. B. Chemistry of excited electronic states. Science 1991, 251, 175−179.

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