Effect of Sulfur Chemical Potentials on Identities and Involvements of Hydrogen Species in Hydrogenation of Heteroarenes and Carbonyl Compounds on Sulfided Transition Metal Clusters

Hydrogenation mechanisms of heteroarenes and aliphatic carbonyls on transition metal or metal sulfide catalysts depend on metal chemical states, reactive hydrogen species, and thermochemical properties of hydrocarbons. This thesis examines the structures of Ru/SiO2, Pd/SiO2, and Pt/SiO2 catalysts in the presence of H2S (1.44 × 10-3–2.64 × 10-2 H2S:H2 ratio), where Ru nanoparticles transform into metallic cores covered with one monolayer of S-deficient RuS2, whereas Pd and Pt nanoparticles retain their metallic states with surfaces covered with reversible and irreversible sulfur. On Ru sulfides, the Run+-(Hδ–)-S2–-(Hδ+) proton-hydride pairs and Run+-(SH2)-S2– protons are the reactive hydrogen species. Pyridine has a high proton affinity; its hydrogenation is initiated by a proton addition from either Run+-(Hδ–)-S2–-(Hδ+) or Run+-(SH2)-S2– at its N, followed by a kinetically relevant hydride addition from Run+-(Hδ–)-S2–-(Hδ+) at the α-C. Pyrrole has a lower proton affinity; its hydrogenation is initiated by a kinetically relevant attack of a S2–-(Hδ+) proton from Run+-(Hδ–)-S2–-(Hδ+) at β-C, followed by an addition of a Run+-(Hδ–) hydride at α-C. On Pd and Pt, the protonic, reversible HSr* species and hydridic H* adatoms are the reactive hydrogen species. Pyridine adsorbs on metallic sites either via σ-interaction through the N lone electron pair, or via π-interaction through its aromatic ring. Hydrogenation of both σ- and π-bonded pyridine requires five H* additions at C atoms and one HSr* addition at N, where the third or fourth H* addition at C limits the rates for σ-bonded pyridine, and the third or fourth HSr* attack to N limits the rates for π-bonded pyridine. The comparison of Ru sulfides, S-covered Pd and Pt shows that both hydrogenation and C-N bond hydrogenolysis reactivities have volcano dependences on metal-sulfur binding strengths, and the hydrogenolysis-to-hydrogenation rate constant ratio increases with the decreasing metal-sulfur binding strength. Finally, this thesis establishes the reactivity trends of heteroarenes and aliphatic carbonyls on Ru sulfides. For heteroarenes, the hydrogenation reactivities relate to proton affinities, because the initial proton addition limits the rates. For aliphatic carbonyls, the reactivities relate to hydride affinities, because the initial hydride addition limits the rates.Ph.D