Catalytic consequences of the identity and coverages of reactive intermediates during benzene hydrogenation on Pt, Pd, and Pt-Re catalysts

Abstract

Mechanistic differences in benzene and cyclohexene hydrogenation on Pt, Pd, and Pt-Re catalysts are interrogated with kinetic assessments, temperature programmed surface reactions, infrared absorption spectroscopic study, and DFT calculations. Their effective rate constants are larger on Pt than Pd surfaces either uncovered or covered with sulfur species, due to the weaker H* adsorption and higher SH* acidity on Pt than Pd. Without sulfur species, H* is the only reactive hydrogen species—on Pt, its first insertion on the adsorbed benzene (C6H6*) and its second insertion onto the cyclohexene intermediate (C6H10+1*) restrict turnovers in benzene and cyclohexane hydrogenation, respectively. H2S treatments lead to SH* species bound irreversibly that not only decreases turnovers but also alters the elementary steps and their kinetic relevance within the catalytic cycle—SH* initially attacks C6H6*, before five successive H* additions, where the third addition limits turnovers. Incorporating isolated Re atoms onto sulfur-covered Pt surfaces increases benzene hydrogenation turnovers, because OH* associated with Re sites are more acidic and effective in initiating the reaction.