The structure and activity of supported metal catalysts: III. Desorption of carbon monoxide- 14C from platinum/ silica impregnated catalysts

Abstract

The desorption of carbon monoxide labeled with carbon-14 from Pt/silica catalysts was followed with a Geiger-Müller tube mounted directly over the catalyst sample. Varying amounts of CO could readily be desorbed depending on the Pt content of the catalyst; the remainder was more strongly adsorbed. Platinum crystallite size distributions were determined from X-ray line-broadening, the proportion of Pt detectable by X-ray diffraction, and electron micrographs of ultramicrotome sections. The percentage of readily desorbed CO was greatest on small crystallites occurring in catalysts with low Pt content. By comparison with previous infrared work, the two types of CO identified from these desorption studies are correlated with linear and bridged species, the latter being more strongly bonded. Present quantitative measurements of the relative amounts of each type make it possible to calculate the number of exposed Pt atoms from CO adsorption results. The variation of the relative numbers of linear and bridged CO molecules is explained in terms of the fractions of the platinum surface composed of edge and plane atoms (with low and high coordination numbers). The necessary crystallite-size variation of these fractions is found by considering the crystallites to be perfect cubo-octahedra exposing (100) and (111) faces with additional incomplete layers of Pt atoms on the faces placed either randomly or as close-packed islands. Comparisons are made between these models and experimental results.