Selectivity in hydrogenation reactions on Pt and Pt-Sn alloys: Examining the influence of coadsorbed hydrogen on hydrocarbon chemistry

A key aspect to an improved understanding of catalytic selective hydrogenation is elucidating the influence of coadsorbed hydrogen on hydrocarbon chemistry on transition metal surfaces. The effects of H adatom preadsorption on the adsorption, desorption, and reaction of ethylene and 1,3-butadiene on Pt(111) and two Sn/Pt(111) ordered surface alloys were studied primarily by using TPD and AES. Preadsorbed H adatoms are effective site-blockers, decreasing the amount of chemisorbed ethylene and 1,3-butadiene on these surfaces. In contrast to extensive ethane evolution from H and ethylene coadsorbed on Pt(111), no hydrogenation products desorb from reactions of H and 1,3-butadiene coadsorbed on Pt(111). Alloying Pt with Sn strongly alters this site blocking by H adatoms and subsequent reaction chemistry. These results have broad implications for understanding hydrogen coadsorption phenomena in general, and provide additional basic information on the surface science of selective catalytic hydrogenation of alkenes and dienes.