Effects of metal-support interactions on the hydrogenation of CO over Pd/SiO2 and Pd/La2O3

Abstract

A study of CO hydrogenation over Pd SiO 2 and Pd La 2 O 3 has been carried out for the purpose of identifying the effects of Pd dispersion, Pd morphology, and support composition on the catalytic activity of supported Pd. The specific activity of each catalyst for methanol and methane synthesis was determined from microreactor studies carried out at a fixed set of reaction conditions. Palladium dispersion was measured by H 2 O 2 titration, and the morphology of the Pd crystallites, as expressed by the distribution of Pd(100) and Pd(111) planes, was determined from in situ infrared spectra of adsorbed CO. The crystallite morphology of the Pd SiO 2 catalysts is the same, independent of Pd weight loading: 90% of the surface is comprised of Pd(100) planes and 10% of the surface is comprised of Pd(111) planes. By contrast, the crystallite morphology of the Pd La 2 O 3 catalysts changes with Pd loading. Primarily Pd(100) planes are exposed at low-weight loadings while Pd(111) planes are exposed at high-weight loadings. The Pd dispersion has little effect on the methanol turnover frequency over both Pd SiO 2 and Pd La 2 O 3 , for dispersions between 10 and 20%. On the other hand, the methane turnover frequency is independent of Pd dispersion over Pd SiO 2 , but increases with decreasing dispersion over Pd La 2 O 3 . It is further observed that the Pd morphology influences the specific activity of Pd La 2 O 3 for methanol synthesis: Pd(100) is nearly threefold more active than Pd(111). For a fixed morphology, the specific methanol synthesis activity of Pd La 2 O 3 is a factor of 7.5 greater than that of Pd SiO 2 .