Propane Dehydrogenation to Propylene and Propylene Adsorption on Ni and Ni-Sn Catalysts

Temperature programmed reaction (TPR) measurements with propane over silica-supported Ni, Ni Sn and Sn catalysts show that the reaction products change significantly from mostly methane, hydrogen and surface carbon over Ni to propylene and hydrogen over Ni Sn. Propylene formation over Ni Sn starts at a moderate temperature of 630 K. Since the activity of Sn by itself is low, Sn serves as a promoter for Ni. The promoter effects are attributed to a lower adsorption energy of molecularly adsorbed propylene and suppression of propylidyne formation on Ni Sn based on temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) measurements as well as density functional theory (DFT) calculations for propylene adsorption on Ni(110) and c(2x 2)-Sn/Ni(110) single-crystal surfaces. On Ni, propylene forms a p-bonded structure with.(C=C) at 1500 cm(-1), which desorbs at 170 K, and a di-s-bonded structure with.(C=C) at 1416 cm(-1), which desorbs at 245 K. The di-s-bonded structure is asymmetric, with the methylene C atom being in the middle of the Ni Ni bridge site, and the methylidyne C atom being above one of these Ni atoms. Therefore, this structure can also be characterized as a hybrid between di-s- and p-bonded structures. Only a fraction of propylene desorbs from Ni because propylene can convert into propylidyne, which decomposes further. In contrast, propylene forms only a p-bonded structure on Ni Sn with.(C=C) at 1506 cm(-1), which desorbs at 125 K. The low stability of this structure enables propylene to desorb fully, resulting in high reaction selectivity in propane dehydrogenation to propylene over the Ni Sn catalyst.