Adsorption and reaction of 1,3-butadiene on Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys

Temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) were used to study the chemistry of 1,3-butadiene (H2CCHCHCH2, C4H6) on Pt(1 1 1) and p(2 × 2)-Sn/Pt(1 1 1) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys. All chemisorbed 1,3-butadiene completely dehydrogenated to H2 and surface carbon on Pt(1 1 1). Alloying Sn on Pt(1 1 1) can completely inhibit this decomposition and 1,3-butadiene reversibly adsorbs and desorbs from the two Sn/Pt(1 1 1) alloys under UHV conditions. The desorption activation energy of 1,3-butadiene on the (2 × 2) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys is 88 and 75 kJ/mol, respectively. These values are good estimates of the adsorption energies, and also place lower limits on the activation energy barrier for dissociating vinylic C–H bonds on the (2 × 2) and √3 surface alloys. Even though 1,3-butadiene is much more strongly chemisorbed than 1-butene (H2CCHCH2CH3, C4H8) on the (2 × 2)-Sn/Pt(1 1 1) alloy, 1,3-butadiene is less reactive than 1-butene because there are no allylic β-CH bonds in 1,3-butadiene as there are in 1-butene.