Mechanism and Site Requirements of Thiophene Hydrodesulfurization Catalyzed by Supported Pt Clusters

Hydrodesulfurization (HDS) processes remove sulfur atoms from hydrocarbons by chemical reduction to hydrogen sulfide using H2 with transition metals and their sulfides as catalysts. Mechanistic and structural insights, concerning reaction pathways and catalysts, respectively, are required to design more effective HDS catalysts, with higher rates at lower H2 pressures, so as to meet emerging requirements for ultra-low sulfur fuels without excessive costs. Extensive studies of HDS pathways and site requirements have been performed on commercial Co(Ni)Mo(W) sulfide compositions. Nevertheless, the atomic structure of active sites and its consequences for surface reactivity remain controversial, to some extent because of challenges in establishing the number and type of surface structures in non-uniform and anisotropic Co(Ni)Mo(W) sulfides with lamellar structures. Other transition metals and their sulfides are also active HDS catalysts and the observed periodic trends have led to several proposals for electronic and sulfur binding energy effects as descriptors of HDS reactivity and selectivity. 8,13,16, 18–23] Some reports suggest an inverse relation between intrinsic reactivity and metal–sulfur bond energies in the corresponding sulfides, while others report a volcano-type dependence with maximum rates at intermediate metal–sulfur bond energies. 13, 20–23] These conclusions are supported by rates only occasionally measured as turnover rates on materials of uncertain bulk structure and phase during catalysis, and at conditions and sulfur chemical potentials that led to significant axial gradients in reactivity, sulfur coverages, and even thermodynamically stable phases. Here, we report the intrinsic reactivity of supported Pt clusters in HDS reactions, provide a rigorous mechanistic interpretation for these properties from kinetic and isotopic data, and contrast the behavior of Pt clusters with that of Ru-based materials. Pt and Ru are among the most active HDS catalysts and form well-defined isotropic structures as both metals and sulfides; as a result, they are more amenable to structural and functional assessment by experiment and theory. Our recent studies have shown that Ru clusters are Kinetic, isotopic, and chemical analysis methods are used to examine the identity and kinetic relevance of elementary steps and the effects of Pt cluster size on thiophene hydrodesulfurization (HDS) turnover rates. Quasi-equilibrated H2 and H2S heterolytic dissociation steps lead to sulfur chemical potentials given by the prevalent H2S/H2 ratio and to cluster surfaces with a metallic bulk, but near-saturation sulfur coverages, during steady-state catalysis. Sulfur-vacancies on such surfaces are required for h(S) or h thiophene adsorption modes and for H2 and H2S dissociation steps. H-assisted C S bond cleavage of h(S) thiophene and H-addition to h thiophene limits rates of direct desulfurization and hydrogenation sulfur removal pathways, respectively. These steps, their kinetic relevance, and the prevalent sulfur-saturated surfaces resemble those on Ru clusters ; they are also consistent with the observed kinetic effects of reactants and products on rates, with the rapid isotopic exchange in H2/D2/H2S mixtures during HDS catalysis, and with measured H2/D2 kinetic isotope effects. Small Pt clusters exhibit lower turnover rates, stronger inhibition by H2S, and a greater preference for desulfurization pathways than those of large clusters. These effects reflect the prevalence of coordinatively unsaturated corner and edge sites on small clusters, which bind sulfur atoms more strongly and lead to lower densities of vacancies and to a preference for h(S)-bound thiophene species. Sulfur binding energies and their concomitant effects on the number of available vacancies also account for the higher turnover rates measured on Pt clusters compared with Ru clusters of similar size. These data and their mechanistic interpretation suggest that the concepts and steps proposed here apply generally to hydrogenation and direct desulfurization of organosulfur compounds. Taken together with similar observed effects of oxygen binding strength, metal identity, and cluster size for oxidation reactions of NO, hydrocarbons, and oxygenates, which also require vacancies in their respective kinetically relevant steps, these data also indicate that low reactivity of small clusters may reflect in most instances their coordinative unsaturation and the concomitant kinetic and thermodynamic preference for low vacancy concentrations on nearly saturated surfaces.