IN-SITU IR SPECTROSCOPIC STUDY OF THE ADSORPTION AND DEHYDROGENATION OF ETHENE ON A PLATINUM-ON-SILICA CATALYST BETWEEN 100 AND 294 K

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 catalys t with metal particle sizes predominantly in the range 5-15 nm. Betwee n 129 and 189 K, the spectra of the chemisorbed species were attribute d to the presence of two closely related pi-adsorbed ethene species wi th absorptions at 3022(medium)/1500(medium) and 3016(medium to weak)/1 496(medium to weak) cm(-1), respectively; and to another pair of MCH2C H2M adsorbed species (M = metal atom), designated di-sigma and di-sig ma, respectively, with absorptions at 2922(medium to strong)/2830(weak )lca. 1428(medium to strong, broad) and 2906(medium to strong)/ca. 282 0/ca. 1419(medium to strong, broad)cm(-1). Between 206 and 294 K, the spectrum of the di-sigma species was gradually replaced by new absorpt ions at 2883(strong)/2795(weak)/1342(very strong) cm(-1) attributed to the ethylidyne species CH,CM, that widely occurs on triangular adsorp tion sites of [111] faces. It is deduced that the di-sigma species als o occurs on facets of this type on the metal particles. It is assumed that the more stable di-sigma species occurs on other types of facet such as [100] or [110]. An alternative, but less preferred attribution to the di-sigma designated spectrum is to an ethylylidyne species, M 3CCH2M. A strong shoulder to the 2922/2906 cm(-1) di-sigma/di-sigma a bsorptions at ca. 2958 cm(-1) is tentatively attributed to another v(C H2)-s mode of the pi-adsorbed species. A less-preferred alternative wo uld be to a v(CH3)-as of an ethyl group. Reasons are given for not att ributing this absorption to an ethylidene species, CH3CHM2.