Electronic structures and water reactivity of mixed metal sulfide cluster anions

Abstract

The electronic structures and chemical reactivity of the mixed metal sulfide cluster anion (MoWS4(-)) have been investigated with density functional theory. Our study reveals the presence of two almost isoenergetic structural isomers, both containing two bridging sulfur atoms in a quartet state. However, the arrangement of the terminal sulfur atoms is different in the two isomers. In one isomer, the two metals are in the same oxidation state (each attached to one terminal S). In the second isomer, the two metals are in different oxidation states (with W in the higher oxidation state attached to both terminal S). The reactivity of water with the two lowest energy isomers has also been studied, with an emphasis on pathways leading to H2 release. The reactive behavior of the two isomers is different though the overall barriers in both systems are small. The origin of the differences are analyzed and discussed. The reaction pathways and barriers are compared with the corresponding behavior of monometallic sulfides (Mo2S4(-) and W2S4(-)) as well as mixed metal oxides (MoWO4(-)).

Bond Activation and Hydrogen Evolution from Water through Reactions with M3S4 (M = Mo, W) and W3S3 Anionic Clusters

“…The overall reaction of the M 3 S 4 – and W 3 S 3 – clusters with water, X – + H 2 O → XO – + H 2 (g), allows us to eliminate H 2 through the oxidation of the original metal sulfide cluster. Though the reaction of each of the clusters proceeds via essentially the same three-step mechanism, , in which the first step is rate-determining, the M 3 S 4 – and W 3 S 3 – clusters differ noticeably in their reactivities and have significant variations in their paths leading up to the final oxidized metal cluster.…”

“…First, it is important to note that although the lowest free energy barrier for the dissociative adsorption of water for the W 3 S 4 – cluster was much less than zero (−5.1 kcal/mol with respect to the separated reactants), the lowest barrier for this same step for W 3 S 3 – cluster is positive (2.4 kcal/mol). Though the initial addition of water to the W 3 S 3 – cluster has a higher barrier than its M 3 S 4 – analogue, its value of 2.4 kcal/mol is still small and comparable to those for the bimetallic TMS clusters that we have investigated previously. , Another important point here is that, for the initial addition of water, the lowest energy mode for the W 3 S 3 – cluster (mode 3) is not analogous to the lowest energy mode observed in the M 3 S 4 – cluster (mode 1). Mode 1 in the case of the W 3 S 3 – cluster was found to have a higher initial barrier of 7.6 kcal/mol (compared to a barrier of 2.4 kcal/mol in mode 3).…”

“…Although the group VI metal oxides have been studied extensively, − there is less definitive information about transition metal sulfides in the literature. − We have recently studied the reactions of gas phase group VI bimetallic sulfides with water and shown that the energy barriers are very small for bond activation to release hydrogen. , The system that we have chosen to investigate in this study is the M 3 S 4 – anionic TMS cluster (where M = Mo, W). This system is of particular interest because it contains three undercoordinated sulfurs that have been shown to have high reactivity .…”

Insight into ethylene interactions with molybdenum suboxide cluster anions from photoelectron spectra of chemifragments

“…We consider whether the spectrum of MoC 4 H 4 can be fully reconciled with the bowtie structure. In addition to the 3 A ground state, the excited 5 A bowtie state, which has significantly extended Mo− −C bond lengths, should be energetically accessible. The substantial structural differences between the D 2d 4 A 2 anion ground state and the two neutral states would result in the activation of several low-frequency modes, and the harmonic approximation used in our simulation codes is inappropriate.…”

“…Transition metal-organic frameworks, 1 metal oxide and sulfide semiconductors, [2][3][4][5][6] metal nanoparticles, 7 and metalhydrogenase complexes 8 have been explored as photocatalysts for H 2 production from H 2 O. Molybdenum oxides that are modestly substoichiometric, MoO 3x (x < 1) in particular, has shown potential as an electrocatalyst 9 or photocatalyst 10 for hydrogen evolution reactions (HERs). This material is appealing because it is low cost, stable, nontoxic, and environmentally benign.…”