(265e) Chemical Identity of Hydrogen Species on Sulfided Ruthenium: A Combined Theoretical and Experimental Work

Transition metal sulfides (TMS) play a central role in industrial catalysis, including hydroprocessing. Despite the extensive studies of the catalytic properties of TMS in the context of hydrogenation reactions [1-6], some key aspects of their reactivity are still not well understood. For example, the chemical identity of surface hydrogen atoms on TMS surfaces is still under debate. This important point is directly related to mechanistic details of hydrogenation reactions. Indeed, the chemical identity of hydrogen, characterized by its charge and surface environment, can determine its involvement in catalytic molecular events.

In this study, we probe the chemical identity of reactive hydrogens on sulfided Ruclusters (RuS_x) by constructing ab initio surface phase diagrams. A combined experimental, kinetic and density functional theory (DFT) study is then used to shed light on the reactive events occurring during pyridine and pyrrole hydrogenation.

Results suggest the co-existence of three chemically distinct surface hydrogen atoms, acting either as protons or hydrides at the operating reaction conditions. The acidity of each reactive hydrogen is quantified by computationally obtained descriptors, namely the differential binding energy and surface deprotonation energy. Starting from this information, corroborated by the calculation of reaction energies for pyrrole and pyridine hydrogenation, and kinetic modeling of reaction kinetics experiments, we propose a mechanism for hydrogenation for pyrrole and pyridine on RuS_x. We demonstrate that the simultaneous presence of hydride and protons on the surface is required for pyrrole and pyridine hydrogenation and that the identity of the kinetically relevant step is determined by the pyridine and pyrrole proton affinity. Hydrogenation of pyridine, which has a higher proton affinity than pyrrole, is kinetically limited by the second hydrogen addition. Instead, the hydrogenation of pyrrole is kinetically limited by the first hydrogen addition.

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