An exploration of the surfaces of some Pt/SiO2 catalysts using CO as an infrared spectroscopic probe

Abstract

Infrared spectra in the bond-stretching, ν CO, region have been measured for CO adsorbed on an impregnated Pt/SiO2 catalyst (16% Pt) with the aim of characterizing the adsorption sites present. This catalyst has previously been widely used for the study of the spectra from adsorbed hydrocarbons. It has relatively large metal particles, typically in the diameter range of 5 to 15 nm. Samples were studied which were hydrogen-covered, hydrogen-depleted, oxygen-covered and “mature”, the latter in the sense that the catalyst had undergone a number of repeated adsorption/desorption/re-reduction cycles. The spectra were interpreted in terms of the adsorption sites available by using recent correlations between ν CO wavenumbers and different patterns of CO metal interactions in metal clusters of structures known from X-ray crystallography. The strongest ν CO absorptions were from linearly adsorbed OCPt species, but these were relatively uninformative (maxima in the region 2087 to 2084 cm−1 in all cases at saturation coverage) because strong dipolar coupling causes intensity-distortion and the merging-together of absorptions from different types of sites. Small proportions of sites, probably of an atomically rough nature from curved areas of the crystallites, gave weaker absorptions near 2050 and 2030 cm−1. On the oxygen-covered surface a sharp absorption at 2099 cm−1 denoted CO adsorption adjacent to sites of adsorbed oxygen. The weaker absorption bands in the ν CO bridged region were more informative. The strongest, near 1850 cm−1 was correlated with 2-fold bridged species adsorbed on (111) surfaces. Weaker, overlapping features near 1880 and 1795 cm−1 (separated from an overall contour with the aid of computer analysis) were correlated, respectively with 2-fold bridged species on (100) or (110) planes, and 3-fold bridged species on (111) planes. Weaker absorptions near 1700 cm−1 were considered to correspond to different types of adsorption sites involving unequal interactions of CO with at least two metal atoms. The “mature” Pt/SiO2 samples gave notably different spectra in both regions and this was tentatively attributed to the effect of residual carbon atoms on or near the Pt surface. A comparison was made of the present spectra with those previously published from similar experiments on a small-particle (ca 2 nm) Pt/SiO2 catalyst, EUROPT-1. The spectral differences could be well accounted for in terms of the reported raft-like (111)-based morphology of the small metal particles of the EUROPT-1 catalyst.