Infrared spectra of chemisorbed molecules: V. Magnetic and infrared measurements on methane, ethane, ethylene, and acetylene adsorbed on silica-supported nickel

Abstract

Magnetic and ir measurements were made on methane, ethane, ethylene, and acetylene adsorbed on a silica-supported nickel catalyst. Adsorption of acetylene at −20 and 25 °C and ethylene at −20 °C at a low surface coverage on a bare surface did not give any ir absorption bands, although the presence of chemisorbed species followed from a decrease in magnetization. The number of bonds attached to the metal was estimated by comparing the magnetization-volume isotherms of the adsorbed hydrocarbons with that observed for hydrogen; this indicated the presence of the same surface species NiCHCHNi for ethane, ethylene, and acetylene adsorbed at 25 °C on a bare nickel surface. At a higher coverage, the spectra of adsorbed acetylene and ethylene indicated the presence of surface alkyl groups, which could not be derived from magnetic measurements. No ir spectra were obtained for methane and ethane. Hydrogenation of adsorbed acetylene was studied by magnetic and ir measurements at 25 °C. At a low surface coverage, the first portion of the hydrogen that was admitted was chemisorbed; hydrogenation resulted from further additions of hydrogen. Adsorption of acetylene and ethylene was also studied on a hydrogen-covered nickel surface. Magnetic measurements indicated a surface reaction accompanied by a decrease in the number of bonds formed between adsorbate and metal surface. Adsorption of acetylene on such a surface gave rise to ir absorption bands at a low surface concentration; except that the intensity at a given surface concentration was higher, the spectrum did not differ from that obtained with a bare surface.