Tracer studies of retained hydrogen and surface residues on nickel catalysts. Isotopic exchange and reactivity towards ethylene

Abstract

[²H]₄ethylene, [¹⁴C]ethylene and tritium have been used as tracer molecules to study the reactivity of adsorbed species on Ni/Al₂O₃ and Ni/SiO₂ catalysts. The amount of hydrogen associated with the catalyst has been determined by exchange with tritium at several temperatures in the range 293–673 K and is of an amount consistent with it being present mainly on the support. In the absence of gas-phase hydrogen, ethylene was found to undergo self-hydrogenation at all temperatures in the range 293–623 K, but with participation of support hydrogen only at 423 K and above. Incorporation of hydrogen from previously accumulated surface residues into ethane and ethylene was observed above 423 K, but carbon from this source did not appear in these products at any temperature. Methane was obtained as a product only at 523 K and above and was comprised of hydrogen derived from the support and of carbon partly from the surface residues.