Hydrogenation of 1,3-butadiene on two ordered Sn/Pt(111) surface alloys

Adsorption and reaction of 1,3-butadiene (C₄H₆) on two ordered Pt-Sn surface alloys precovered with hydrogen adatoms were studied with the use of temperature-programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). The two alloys investigated were the (2×2)Sn/Pt(111) and (3×3)R30°-Sn/Pt(111) surface alloys, with 25 and 33% Sn alloyed in the surface layer, respectively, formed by vapor deposition of Sn onto a Pt(111) single crystal. Alloyed Sn opens a new hydrogenation reaction pathway compared with Pt(111). Butadiene hydrogenation by coadsorbed hydrogen occurs with 100% selectivity to liberate butene (C ₄H₈) in reaction rate-limited peaks in TPD, and no deeper hydrogenation product (butane) was observed. The activation energy barrier for hydrogenation of strongly bound 1,3-butadiene is estimated to be 91 and 72 kJ/mol on the (2×2) and (3×3)R30°alloys, respectively. Butadiene conversion was highest on the (2×2) alloy, reaching 100% at high hydrogen precoverages. Strong site-blocking effects of preadsorbed H adatoms were observed for 1,3-butadiene chemisorption on both alloys under these conditions; butadiene chemisorption was eliminated by θH=0.49 ML on the (2×2) alloy and θH=0.34 ML on the 3 alloy. These studies addressing the influence of alloyed Sn on the reaction barrier to 1,3-butadiene hydrogenation and the effect of surface Sn concentration on hydrogenation activity provide observations of several novel phenomena and may aid in the development of heterogeneous catalysts to selectively remove dienes in alkene streams.