Infra-red studies of carbon monoxide and hydrocarbons adsorbed on silica-supported nickel

Abstract

Infra-red studies of NH₃, HCl, CO, C₂H₄, and butene adsorbed on transparent aerogels were made to elucidate the surface chemistry of nickel/silica and the nature of adsorbed species. In addition to the hydroxyl band found on silica at 3750 cm^–1, most “dry” nickel/silica aerogels exhibited a band near 3620 cm^–1, apparently caused by silanol groups near nickel ions. Reduction of all nickel ions by H₂ treatment proved very difficult. Nickel ions remaining on “reduced” gels were apparently the acid sites that adsorbed NH₃ strongly. Chemisorption of HCl showed that reactive oxide ions, not found on silica, were also present. Bands between 1800 and 2210 cm^–1 showed at least 7 types of adsorbed CO, varying in relative importance with the extent of reduction. Three distinct ionic sites for CO adsorption appeared to exist. Bands near 2200 cm^–1 arose from CO on the sites which held NH₃ most strongly, implying binding to a nickel ion. Reaction of HCl with these sites, however, showed that they also contain oxide ions.

Adsorption of ethylene on “reduced” nickel/silica produced 1- and 2-butenes which were quickly converted to butane on subsequent addition of H₂. Spectra ascribed by others to associatively adsorbed ethylene ([graphic omitted]H₂—[graphic omitted]H₂) and to ethyl groups on reduced nickel probably instead show butenes and butane.