Infra-red study of interaction between hydrogen and supported platinum catalysts

Abstract

Adsorbed hydrogen on platinum-silica gives infra-red bands at 2040 and 2120 cm^–1, and on platinum alumina at 2040 and 2110 cm^–1. The 2110–2120 band is much enhanced by treatment by oxygen, and adsorption-desorption behaviour together with deuterium shift establish the band as due to weakly adsorbed hydrogen. It is proposed that the hydrogen is adsorbed on platinum oxide patches, possibly on the Pt⁴⁺ surface ions. The effect of oxygen pretreatment is to reduce or remove the 2040 band, which, however, slowly appears in the presence of water, more quickly for hydrogen plus water, in either order, for platinum silica. On platinum alumina the addition of water is not necessary to observe this band. It is suggested that the 2040 band is due to hydrogen atoms chemisorbed on Pt⁴⁺ and hydrogen bonded to an adjacent OH. This may be formed by dissociative chemisorption of water, or even by hydroxyl ion diffusing from the adjacent support for alumina. Experiments on carbon monoxide rule out any possible impurity effects due to this gas. While the bands observed agree with the observations of Eischens and Pliskin on platinum-alumina, the new results on the effects of oxygen and water have led us to change considerably their interpretations (which involved hydrogen chemisorbed atomically and interstitially on metallic platinum).