InsituIR spectroscopic study of the adsorption and dehydrogenation of ethene on a platinum-on-silica catalyst between 100 and 294 K

Abstract

A new cell capable of obtaining infrared spectra of molecules adsorbed on catalysts within a wide temperature range has been described. The cell was used to obtain spectra between 100 and 294 K from physically and chemically adsorbed ethene on a 16% w/w platinum-on-silica catalyst with metal particle sizes predominantly in the range 5–15 nm.

Between 129 and 189 K, the spectra of the chemisorbed species were attributed to the presence of two closely related π-adsorbed ethene species with absorptions at 3022(medium)/1500(medium) and 3016(medium to weak)/1496(medium to weak) cm^-1, respectively; and to another pair of MCH₂CH₂M adsorbed species (M=metal atom), designated di-σ* and di-σ, respectively, with absorptions at 2922(medium to strong)/2830(weak)/ca. 1428(medium to strong, broad) and 2906(medium to strong)/ca. 2820/ca. 1419(medium to strong, broad) cm^-1.

Between 206 and 294 K, the spectrum of the di-σ species was gradually replaced by new absorptions at 2883(strong)/2795(weak)/1342(very strong) cm^-1 attributed to the ethylidyne species CH₃CM₃ that widely occurs on triangular adsorption sites of [111] faces. It is deduced that the di-σ species also occurs on facets of this type on the metal particles. It is assumed that the more stable di-σ* species occurs on other types of facet such as [100] or [110]. An alternative, but less preferred attribution to the di-σ* designated spectrum is to an ethylylidyne species, M₃CCH₂M.

A strong shoulder to the 2922/2906 cm^-1 di-σ*/di-σ absorptions at ca. 2958 cm^-1 is tentatively attributed to another ν(CH₂)-s mode of the π-adsorbed species. A less-preferred alternative would be to a ν(CH₃)-as of an ethyl group. Reasons are given for not attributing this absorption to an ethylidene species, CH₃CHM₂.