Infrared spectra of hydrocarbons chemisorbed on silica-supported metals: I. Experimental and interpretational methods; acetylene on nickel and platinum

Abstract

A description is given of the experimental methods and spectroscopic interpretational procedures used in a series of studies of the infrared spectra of hydrocarbons chemisorbed on silica (pressed CabOSil disc) supported metals. Infrared spectra have been obtained from the surface species formed by initial adsorption, and subsequent hydrogenation, of acetylene on silica-supported nickel and platinum. On nickel, absorption bands from CH, CH 2, and CH 3 groups are obtained on initial adsorption. The former probably correspond to MCHCHM surface species (M = metal), and the latter to surface alkyl groups formed by self-hydrogenation and polymerization processes. After hydrogenation the spectra indicate the presence of surface attached n-butyl groups, M(CH 2) n CH 3, ( n = 3.) On platinum a weak initial spectrum is obtained and a large intensity increase occurs on hydrogenation. This indicates that the dissociation of acetylene to form a surface carbide together with MH bonds is greater on platinum than on nickel. The surface species after hydrogenation are alkyl groups with an average value of n ≥ 4. i.e. the degree of surface polymerization is somewhat greater on platinum. The published spectra from acetylene on palladium and copper are similar to those on platinum except that the initially adsorbed species have a higher proportion of olefinic CH bonds. It is suggested that the relatively smaller activity of nickel as compared with platinum for the acetylene hydrogenation reaction is related to a higher proportion of hydrocarbon surface residues (and a smaller proportion of MH bonds) formed by adsorption on nickel.