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(2×2)-Sn/Ni(110) single-crystal surfaces. On Ni, propylene forms a π-bonded structure with ν(C=C) at 1500 cm⁻¹, which desorbs at 170 K, and a di-σ-bonded structure with ν(C=C) at 1416 cm⁻¹, which desorbs at 245 K. The di-σ-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-σ- and π-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 π-bonded structure on Ni−Sn with ν(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.