Catalytic Effects of Chemisorbed Sulfur on Pyridine and Cyclohexene Hydrogenation on Pd and Pt Clusters

Kinetic assessments of cyclohexene and pyridine hydrogenation on metallic Pd and Pt surfaces covered with chemisorbed sulfur species (H2S*, HS*, and S*) at controlled sulfur chemical potentials probe quantitatively two distinct types of sulfur and their catalytic involvements in H-addition events. Irreversible sulfur species (Sir,T*) are not chemically equilibrated with H2S(g) and remain on surfaces, even after the complete H2S(g) removal. Reversible sulfur species (Sr,T*) are those in dynamic equilibrium with H2S(g) and desorb completely upon H2S(g) removal. Structure-insensitive cyclohexene hydrogenation shows higher Sir,T* coverages on Pd (56.8%) than Pt (35.1%) because of the larger electron affinity difference between Pd and S. The remaining sites (43.2% on Pd and 64.9% on Pt) are either unoccupied or occupied with reversible sulfur. The reversible sulfur-to-unoccupied site ratios are dictated by the (fH2S)x-to-(fH2)y (x = 1, y = 0–1) fugacity ratios and temperature. Due to the stronger S* and H2S* binding strengths on Pt, the Sr,T* coverages on Pt (56.8–62.8%) are higher than Pd (32.0–39.0%). Pyridine hydrogenation on Pd and Pt exhibits similar intrinsic reactivities, when normalizing rates to sites that are not permanently occupied by sulfur. The reaction requires pyridine adsorption via either σ-interaction through its N or π-bonds through its aromatic ring, five hydridic H* additions at the C atoms, and one attack of the protonic H from the reversible HS* to the N atom, where the third (or fourth) hydrogen addition is kinetically relevant. On surfaces without sulfur, pyridine hydrogenation reactivities increase with increasing metal–sulfur binding strengths (Pd < Pt < Ru). On surfaces covered with sulfur, pyridine hydrogenation and C–N bond hydrogenolysis reactivities exhibit volcano dependences on metal–sulfur binding strengths. The hydrogenolysis-to-hydrogenation rate constant ratios increase with decreasing metal–sulfur binding strengths due to changing metal chemical states, charges of hydrogen species, and their catalytic roles in the hydrogenation turnover.