Adsorption studies on catalysts: II. Application to rapid adsorbers — oxygen on Europlatinum 1

Abstract The uptake of oxygen on a 6% Pt on SiO2 catalyst has been measured using temperature programmed desorption and follows on from the previous study of oxygen adsorption on a supported Ag catalyst. For the latter the intrinsic sticking probability could be accurately determined because (i) it had a very low value (10-6) and (ii) the catalyst was one with low metal surface area and large pores. For the present case the situation is the opposite — the Pt catalyst has small pores, high metal area and is likely to have a high intrinsic sticking coefficient (∼ 0.1). It is shown that these factors result in a severe pressure drop down the pores of the catalyst which is in contrast to the Ag catalyst where the pressure was constant and equal to that outside the catalyst. For the Pt catalyst these effects should result in a much lower than expected coverage within the pore and therefore a low result for the sticking coefficient. This is confirmed by the experimental results which give an apparent sticking coefficient of 2 × 10-6. The desorption spectra from the catalyst resemble those from a stepped Pt(111) single crystal surface.