Formula omitted]: I. Percentage exposed and its effect upon the reactivity of adsorbed oxygen

Abstract

This paper reports the preparation of a set of platinum catalysts on wide-pore silica gel by impregnation with H 2PtCl 6 aq and by ion exchange with Pt(NH 3) 4 2+. Some catalysts were reduced directly and others after preliminary calcination. Percentages exposed (platinum dispersion) measured at 25 °C by hydrogen chemisorption, hydrogen desorption, and hydrogen-oxygen titration vary from 6.3 to 81%. Hydrogen-oxygen titration gives values which are about 88% of those given by hydrogen chemisorption. Upon storage in air at 25 °C, the content in surface oxygen increases substantially beyond that resulting from the short exposure to oxygen employed in the hydrogen-oxygen titration. Exposure to oxygen at 300 °C results in still a further increase in the content in surface oxygen. The surface oxide resulting from the short exposure to oxygen employed in hydrogen-oxygen titration is rapidly reduced by hydrogen at 25 °C but coverages by oxygen much in excess of those resulting from the short exposures result in much reduced rates of reaction with hydrogen. The effect is structure sensitive: The degree of reduction in rate is much larger on the catalysts with a higher percentage exposed. The percentage exposed of the catalysts with the lowest percentage exposed appears to be larger when stored catalyst exposed to oxygen at 300 °C is reduced at 25 or 100 °C than when it is reduced at 300 °C and swept with argon at 450 °C.