Catalytic Effects of Chemisorbed Sulfur on Pyridine and Cyclohexene Hydrogenation on Pd and Pt Clusters
Haiting Cai, Ya-Huei Cathy Chin
Abstract
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.