H2S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster

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H2S Reactivity on Oxygen Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions Debashis Adhikari, and Krishnan Raghavachari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10899 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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The Journal of Physical Chemistry

H2S Reactivity on Oxygen Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions Debashis Adhikari*,†,‡ and Krishnan Raghavachari*,† † ‡

Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

Department of Chemistry and the Institute of Catalysis for Energy Processes, Northwestern University, Evanston, Illinois 60208, United States

Abstract Understanding the mechanistic aspects of heterogeneous reactions on supported metal catalysts is challenging due to several disparate factors, among which the dynamic nature of the surface is a major contributor. In this study, the dynamic aspect of a surface has been probed by choosing small metal clusters as illustrative models. Two anionic heterotrimetallic clusters, namely W2TcO6 ‒ and W2OsO6 ‒ were reacted with H2S gas to exhibit splitting of the gas molecule and complete oxygen‒sulfur exchange in the metal core. During this atom-exchange process, the core exhibits remarkable fluxionality to augment a thiol proton migration from one metal center to another, as well as a rapid interchange of the terminal and bridging oxygens. The fluxional nature of the core is further evidenced by two oppositely oriented oxo groups working in concert to accomplish the proton transfer, upon introduction of sulfur inside the core. These fluxional processes in the small heterotrimetallic cores closely resemble the dynamic nature of the surface in a heterogeneous reaction. Throughout the fluxional processes investigated in this study, two-state reactivity and multiple instances of spin crossover are observed in our computational studies. Interestingly, the neutral heterotrimetallic cores can also undergo complete oxygen‒sulfur exchange reaction with H2S. The investigated metal clusters are promising materials, since they not only can liberate dihydrogen from water (reported in J. Phys. Chem. A, 2014, 118, 11047), but also completely strip the sulfur from environmentally hazardous H2S gas.

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Introduction The metal/metal oxide interface has garnered considerable attention in recent years due to its major role in heterogeneous catalysis, responsible for providing 90% of the world’s industrial output of materials from fertilizers to plastics. In supported catalysts, often made of small metal clusters supported on oxide substrates, it is well documented that the oxide support often plays a beneficial role, e.g., vanadia submonolayers over titania display superior catalytic activity compared to unsupported vanadia for partial oxidation of hydrocarbons and selective catalytic reduction of nitric oxide.1,2 The observed reactivity is usually a convolution of the properties of the metal cluster and the support, hence changes associated with the substrate and supported metal clusters are both important. Not surprisingly, drawing a sound correlation between surface composition and reactivity is hindered by the highly heterogeneous nature of the surface morphology of the catalyst. Moreover, the support is subjected to changes due to adsorbatesurface interactions and often undergoes surface reconstruction as a result of chemisorption. In this context, a detailed scanning tunneling microscopy (STM) study has been conducted focusing on structural studies of TiO2 surfaces at the atomic scale revealing oxygen vacancies and reconstruction of the surface.3 In chemisorption studies on metal/TiO2 systems, it has been reported that the surface defects on TiO2 heavily influence the adsorption and dissociation of adsorbates such as CO, O2, H2, or H2O. It has also been shown that the dissociative adsorption of H2O and H2 increases the number of oxygen defects on the nearly perfect surface. To imitate the behavior in such surface reactions, gas-phase studies along with high-level calculations have turned out to be powerful tools to elucidate the mechanistic details of complex metal oxide surfaces and catalytic processes at the molecular level.4-14 In this present work we choose heterotrimetallic clusters as a small model for the surface, investigate its reactivity with hydrogen sulfide gas (H2S), and present some fundamental insights regarding the dynamic nature of these clusters. Hydrogen sulfide, H2S, is prevalent in natural gas, crude oil, urban and agricultural sewage, and waste gas steams released from chemical plants. It is also a common chemical component of many gaseous steams such as liquefied petroleum gas (LPG), off-gases from industrial catalytic hydrodesulfurization (HDS) and upgrading heavy petroleum and bitumen. Since H2S is a strongly corrosive gas causing major health hazards, H2S adsorption and recovery of elemental sulfur is very important from both an environmental and economic standpoints.15-17 Many metal oxides, Al2O3,18,19 TiO2,20 Fe2O3,21,22 V2O5,23,24 Cr2O325and others, have been tested as H2S sorbents based on both thermodynamic and kinetic criteria and ZnO is found to be the most effective.26 To understand the desulfurization reactions, spectroscopic and kinetic studies have been conducted in tandem with theoretical studies.26,27 In a recent study, Ge and coworkers have shown how vanadium cluster anions react with hydrogen sulfide to perform consecutive oxygen‒sulfur exchange.28 In addition, H2S has been used widely as a precursor to produce the S-doped


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titania surface where the oxygen‒sulfur exchange mechanism might be operative.29 Previously, we have shown that anionic oxygen-deficient heterotrimetallic cores have advantages over analogous homotrimetallic cores in eliciting fluxionality, a process involving a series of efficient proton hops that effectively results in the interchange of bridging and terminal oxygens in the cluster core. These fluxional processes result in structural intermediates that greatly facilitate the liberation of dihydrogen from water.30 In this work, we extend the reactivity studies of heterotrimetallic cores with H2S and show that the behavior of clusters closely resembles the dynamic aspects of surface chemical reactions. The stripping of sulfur atom from the substrate gas and release of water is also reminiscent of desulfurization reactions which are relevant to environmental catalysis aimed towards the destruction of sulfur containing molecules.25 Computational Methods All calculations have been performed with the B3LYP hybrid density functional.31,32 The Stuttgart‒ Dresden (SDD) relativistic pseudopotentials were used to replace the 28, 60, 60 core electrons of technetium, tungsten, and osmium, respectively. The remaining valence electrons were treated with a double-ζ basis set augmented with a set of polarization functions of l+1 angular momentum along with a set of diffuse functions needed to describe the extended anionic electron densities (s, p, d and f functions on W, Tc, Os, s and p functions on O, S). These diffuse and polarization functions were added to the SDD basis set for W, Os, and Tc, and the D95 basis set for O, S. This augmented basis set is denoted as “SDDplus”. Optimization of the geometries and vibrational calculations were carried out at the B3LYP/SDDplus level of theory. All minima were confirmed by vibrational analysis to have no imaginary frequencies. Transition states were characterized by the presence of a single imaginary frequency. Single point calculations were carried out using an augmented triple-ζ quality basis set at the B3LYP level of theory to get more reliable relative energies. To get the triple-ζ quality basis set for the metal part, two f-type functions and one g-type function for W, Tc, and Os, were added to the basis set, as recommended by Martin and Sundermann.33 The large polarized aug-cc-pVTZ basis set was used for S, O, H in these calculations. For all calculations, the ultrafine grid was used to obtain numerically stable DFT results. All reported energies are zero-point corrected. Finite temperature effects on enthalpies and Gibbs free energies are estimated within the rigid rotor-harmonic oscillator and ideal gas approximations. All the relative energies discussed in this article represent the free energies (∆G) at 298 K. Open-shell systems were investigated by spin unrestricted formalism and values were checked to ensure that spin contamination effects are small. The Gaussian09 program suite has been used for all calculations.34



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Results and Discussion (a) H2S Splitting and Proton Migration Through W2TcO6‒ and W2OsO6‒ Cores In our previous study we found that when one of the tungsten atoms in a C3V symmetric oxygen deficient anionic W3O6 ‒ core is replaced by technetium, the core W2TcO6 ‒ (1) becomes Cs symmetric and can shuttle a proton effectively through all metal centers, so that the core can be a viable candidate for liberating H2 from H2O.30 To study the reactivity of the core with H2S as an effective medium for desulfurization, we follow a similar pathway as was observed for H2O. H2S binds to the W(1) atom with a moderate binding strength; in the triplet geometry of the complex, the free energy of complexation is ‒1.7 kcal mol−1. The corresponding electronic binding of H2S is exothermic by ‒10.7 kcal mol−1, corroborating the fact that metal oxides bind polar molecules more strongly than metal surfaces due to the electrostatic interaction with the surface cation and anion sites.35 The W(1) ‒S distance in this encounter complex 1a’ is 2.66 Å which is shorter than the Ti‒S distance, 2.82 Å, observed in the case of H2S binding to anatase (101) surface by plane wave DFT calculations.36 Hydrogens of the bound H2S are oriented towards the bridging oxygens in the core to attain modest hydrogen-bonding stability, having an average distance of 2.70 Å. As depicted in Figure 1, H2S splits at the metal center W(1), protonates the bridging oxygen and generates a thiol functionality attached to W(1).37 The splitting of H2S in the cluster closely resembles the dissociative adsorption of the same on a surface and it produces a S‒H stretching mode that is decreased slightly compared to that of the free molecule. The associated enthalpy of binding for thiol upon H2S splitting is ‒25.5 kcal mol−1 which is very close to similar adsorption on a tricoordinate Al site on the alumina(110) surface (~26 kcal mol−1).38 A similar observation has been reported for H2S splitting on a MgO(100) surface.39 The bridge oxide between W(1) and W(2) in the intermediate 1c further opens to accept the proton migrated from the thiol, generating structure 1e. In the proton migration TS, the S---H and O---H distances are 1.50 and 1.46 Å, respectively. It is important to remember that hybrid DFT functionals such as B3LYP underestimate the hydrogen bonding strength with respect to the MP2 or CCSD(T) results,40 hence the transition state (TS) energy may even be lower than that depicted on the potential energy surface. However, overstabilization of the higher spin state by B3LYP,41 compensating for the underestimation of hydrogen bond strength is also possible. In the intermediate 1g, S forms a double bond to W(1), as seen from the W(1)=S bond length of 2.21 Å.


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Figure 1. Free energy surface for H2S reactivity with W2TcO6 ‒ (1) is shown. The singlet spin surface is shown in red (solid line), and the triplet surface is shown in blue (dashed line). The * symbol designates that the transition state for water detachment was not located, but shown for the purpose of illustration only.

Followed by the proton migration step, the terminal sulfide bridges between W(1) and W(2) via traversing a TS at ‒14.4 kcal mol−1 on the singlet surface. Most notably, there are multiple spin surface crossing points along the H2S splitting and proton migration pathways. The intermediate 1i is an important species along the fluxionality pathway, as complete detachment of sulfur from H2S has been achieved in it. The W(1) ---S and W(2) ---S distances are obtained as 2.29 and 2.50 Å, respectively, which also makes the core quite asymmetric. The incorporation of sulfur inside the core in the form of sulfide is also relevant to the process of S-doping to the metal oxide surfaces by complete exchange of oxygen to narrow the band gap of the material.42‒44 For example, CsTaWO6 is a better photocatalyst for hydrogen production from water under visible light when it is doped with sulfur.43 To achieve sulfur doping to the semiconductor, gaseous H2S may be an adequate source, as exemplified by the growing peaks typical of metal sulfides in the S K‒edge spectrum when Cr2O3 powder is exposed to H2S at 350 K.44 In the same vein, Chen et al. have employed H2S as a precursor to prepare S-doped TiO2 surface42 where sulfur doping can efficiently change its optical absorbance edge into the visible light region, further enhancing its photocatalytic activity.45 As we shall see below, an oxygen-sulfur exchange reaction is very facile in the chosen anionic core, possibly augmented by its fluxional character. The intermediate 1i readily converts to a highly stable



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new type of intermediate, 1k, where the bridging hydroxo group between Tc(3) and W(1) opens and becomes terminally connected to W(2). We have previously observed that a similar family of intermediates plays an important role in bringing the energy of subsequent intermediates and TSs down during H2O reaction with heterotrimetallic cores.30 As expected, similar features are also observed for the reaction with a different substrate, H2S. The second proton-shuttling pathway from W(2) to Tc(3) is equally facile and follows the same sequence of steps to complete the migration. Interestingly, inclusion of sulfur in the core makes it sufficiently asymmetric to further help in attaining significant dynamic behavior. In our earlier studies on both homo- and heterotrimetallic M3O6 ‒ cores, we observed that the three terminal oxo groups occupy the opposite face of the three bridging oxides with respect to a plane passing through three metal centers. In the intermediate 1m (see Figure 2), this distinction between two faces is entirely lost (considering the orientation of the oxo in 1i, Figure 2) and an upward bridging oxide ends up as a downward terminal oxo functionality.

W(1)

B

W(2) A

B

B

Tc(3) B 1i

A 1l

A 1m

A 1n

Figure 2. Preparation for proton migration as assisted by two optimally oriented oxo groups, A and B. When bridge oxo group A between tungsten and technetium opens, oxo group B attains the right orientation to accept the migrating proton. Double arrow designates multiple structures involved in between 1i and 1l. 1n is the TS for the proton migration event. Overall, two oxo groups, A and B work in concert to complete the process.

As demonstrated in the Figure 2, the bridging oxo (labeled as A) opens up and the terminal oxo (labeled as B) at the technetium center becomes ready to accept the migrating proton. Upon closer analysis it is noticed that the oxo groups A and B change their orientation in the TS 1l, where Tc(3)‒O bonds are very similar in lengths (1.74 and 1.72 Å, respectively). In the intermediate 1m, the oxo group B changes the W(2) ‒Tc(3) ‒O(B) angle from 150.9o to 128.6o, thus prepares itself for the right orientation to accept the proton. Overall, the oxo groups A and B become indistinguishable and their specific facial orientation is lost. We note on passing that the homotrimetallic anionic core46, W3O6‒, also exhibits indistinguishability of oxides during the second proton migration through it (please see Supporting Information, SI). The identical observation for both homo- and heterometallic cores signifies that such a geometry likely originates due to the asymmetry induced by the introduction of the bulky sulfur.


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After the second proton migration step, the W(2)=O bridges between W(2) and Tc(3) to result in the intermediate structure 1q, where the two hydroxo groups attached to W(1) and Tc(3) are hydrogenbonded. At this stage, subsequent proton migration from the W(1)‒OH functionality to the Tc(3)‒OH is barrierless (TS at ‒14.0 kcal mol−1, singlet surface) to form water. The resulting W(1)=O coordinates further Tc(3) to form a bridge with subsequent release of water, thus accomplishing a complete oxygensulfur exchange in the core. The free-energy for this desulfurization reaction by core 1 is ‒19.1 kcal mol−1, favorably comparable to ‒20.8 kcal mol−1 known for a well-performing desulfurization catalyst ZnO.26 Overall, the H2S splitting, sequential proton migration, and dehydration to fulfill complete oxygen‒sulfur exchange through the core is a very facile process and the core elicits remarkable dynamic character toward this process. Notably, en route to the process there are several instances of spin surface hopping throughout the trajectory for the lowest-energy reaction coordinate, and the full potential surface resembles two-state reactivity. Since small metal clusters are often modeled to mimic and understand the nature of the bulk surface, similar aspects may play a crucial role in heterogeneous catalysis on supported metal surfaces. Upon achieving the desulfurization of H2S by the core 1, we examined another heterotrimetallic anionic core W2OsO6 ‒, 2; also considered previously for H2 liberation from water. This core 2 largely behaves the same way as noticed in core 1 when exposed to H2S. The computed binding free energy of the complex between H2S to tungsten is slightly higher, 1.34 kcal mol−1 on the doublet surface. Perhaps larger basis set or electron correlation effects may decrease the energy of this complex.47 The rate determining steps for this reaction involve the splitting of H2S along a bridge oxygen and W(1) as well as the proton migration for the first step. Further processes are relatively facile considering spin crossover between high (quartet) and low spin (doublet) states is operative. As expected, the intermediate after the first migration of a proton undergoes a significant structural change to generate a family of low-energy intermediates which makes the subsequent processes reasonably low in energy. The behavior of the two oxo groups in losing the facial orientation to work in tandem is also observed in this core 2. The similarity of the reactive processes for the cores 1 and 2 prevents us in detailing the individual steps further. The similarity in behavior for both core 1 and 2 underscores the fact that such fluxionality might be occurring from the asymmetry created in the core due to the presence of sulfur.



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Figure 3. Free energy surface for the H2S reactivity with W2OsO6‒ (2) is shown. The doublet surface is shown in red (solid line) and the quartet is shown in blue (dashed line). Note that there are multiple spin surface crossing points along the H2S splitting and proton migration pathways. The * symbol designates that the transition state for water detachment was not located, but shown for the purpose of illustration only

(b) Fluxionality Observed in Oxygen Deficient Neutral Cores There have been many previous reports describing the dependence of cluster reactivity on cluster size, structure, and charge state.48-52 Castleman and co-workers studied the oxidation of CO to CO2 over cationic and anionic FeO3 clusters to prove that charge has a dramatic effect on the oxidation behavior.53 We wanted to consider whether the facile dynamic nature of the oxygen deficient anionic cores upon the exposure of H2S is a function of their charge states.


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Figure 4. a) Geometric structural comparison between the neutral (3) and anionic cluster (1) of the mixed metal cluster W2TcO6 b) SOMO and LUMO of 3.

Figure 5. Free energy surface for H2S reactivity with the neutral cluster W2TcO6 (3) is shown. Only the doublet spin surface has been presented for the path from 3a to 3q. Two consecutive spin inversions as described by # symbol take place after intermediate 3q to complete water release via the lowest energy pathway. The TS for water release, designated with a * symbol is for illustration only.



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To address this issue, we considered the neutral analog of 1, W2TcO6 (3). The ground state of 3 is found to be a doublet which is more stable than its excited quartet state by 9.22 kcal mol−1. A careful comparison between the geometric structures of 1 and 3 exposes that the W(1)---W(2) distance elongates (2.55 Å in 1, whereas 2.61 Å in 3) when the charge is detached. On the contrary, terminal oxo bond lengths shrink slightly (Tc(3)‒O in 1 is 1.71 Å compared to 1.68 Å in 3). Such an elongation and contraction of bonds can be explained from the nature of molecular orbitals for the neutral core 3. The LUMO of the neutral core 3 shows antibonding nature in metal-oxo bonds, but a bonding nature in the metal-metal interaction (Figure 4). The above trend of bond elongation/contraction is intuitive, since during formation of the anion, the LUMO of the neutral species is populated by a single electron. Upon exposure to H2S, the neutral core demonstrates very similar behavior to that has been documented for anionic core 1 except for the H2S binding step. A striking difference in the H2S complexation step is the binding free energy (‒9.6 kcal mol−1) for neutral core 3 compared to that in anionic core 1 (‒1.7 kcal mol−1). This phenomenon indicates that just the electrostatic interaction between the adsorbate and the core is not responsible for greater binding ability; rather a proper orbital overlap is the primary regulating factor. Similar observation has been reported by Rodriguez and co-workers for H2S binding on MgO (100) surface where the orbital overlap with the valance band of metal oxides has been invoked to explain the greater binding strength.39 In our case, the anionic nature of 1 makes the d-orbitals more diffuse to decrease significant overlap with the frontier orbitals of the adsorbate H2S. Conversely, the neutral core 3 has relatively contracted orbitals facilitating good overlap with the H2S orbitals which is reflected in more exothermic binding. This proposition is further supported from comparison of the H2S binding with anionic (1.34 kcal mol−1) and neutral (‒11.5 kcal mol−1) W2OsO6 cores (please see SI). The splitting of H2S on a tungsten center demands slightly higher energy barrier (3.44 kcal mol−1) and can be judged as the rate determining step for the entire process. Similar steps to display fluxional behavior leads to intermediate 3i containing a sulfur bridge which upon slight rearrangement forms a low-energy intermediate 3k. The subsequent processes remain low in energy to promote proton migration for the second time, leading to the formation of intermediate 3q. At this point, a spin surface crossover is imminent, since the proton migration TS for the formation of water (bound to Tc) appears to be significantly higher in energy on the doublet surface. Despite our multiple attempts, we were unable to converge to a low energy TS featuring water formation on the doublet surface. On the contrary, the TS for water formation is easily located at ‒7.70 kcal mol−1 on the quartet surface. The oxide bridge formation between W and Tc followed by water release undergoes another spin inversion to obtain complete oxygen‒sulfur exchanged


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product, 3w. In the same vein, we examined the neutral analogue of anionic 2, W2OsO6 (4), which also displays the similar H2S splitting and fluxional behavior to that is observed in 2 (see SI).

Conclusions The reactivity of the environmental and industrial pollutant H2S gas with two anionic cores towards desulfurization and dehydration has been detailed. The oxygen deficient anionic clusters have been chosen as a small molecule mimic of the metal surface, and their reactivity may provide significant insight for the actual heterogeneous catalyst. The dynamic nature of the cores is conspicuous from the interchangeability of the bridge and terminal oxides. Moreover, the facial selectivity of terminal and bridging oxo groups is entirely lost when the core becomes substantially asymmetric owing to the presence of bulky sulfur. Both cores considered in this study show similar behaviour upon exposure to H2S except slight differences in the barriers for H2S splitting and proton migration. Additionally, we report that both the anionic and neutral clusters are competent in H2S splitting, proton migration, and eliciting fluxional behaviour. Since surface structures in a heterogeneous catalyst are often dynamic in nature, small molecule mimic with these clusters provide molecular-level insight and mechanistic details of how substrates are activated and take part in reactions on a surface.

In addition, our investigation provides

fundamental molecular-level insight on the details of dehydration by an oxygen‒sulfur exchange reaction at the gas-solid interface. Such an understanding will help to develop efficient catalytic or adsorptive materials for H2S remediation. Finally, this knowledge may also advance understanding of other technologically important reactions over metal oxide surfaces. Author Information: Corresponding authors DA, E-mail: [email protected] KR, E-mail: [email protected] Notes: The authors declare no competing financial interest. Supporting Information: Figures, basis set exponents, enthalpies and free energies, and cartesian coordinates of the investigated structures. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgments This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-FG02-07ER15889 at Indiana University. D.A. gratefully acknowledges the generous financial support from Northwestern University.



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H2S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster

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