Active sites on Pt containing sulfated zirconia

Abstract

The influence of the Pt and sulfate concentration on the activity of Pt containing sulfated zirconia for n-heptane conversion was investigated. Pt was deposited on the support by impregnation and by photocatalytic deposition. The amount deposited was 2.5 and 0.4 wt% respectively. For comparison a hybrid catalyst consisting of sulfated zirconia and Pt on SiO2 was prepared. As supports a commercial sulfated zirconia with a fixed sulfate concentration, a commercial and self synthesized Zr(OH)4 were used. The sulfate content varied between 20 and 60% of a monolayer. The shifts to higher frequency in the IR spectra of CO adsorbed on Pt correlate with the increasing amounts of sulfates on zirconia and are attributable to the changes in the electron density of the supported metal, i.e. the electron deficiency of Pt increases with increasing concentration of acid sites. After activation in air and reduction in hydrogen two SO2 peaks were detected by a temperature programmed heating procedure (TPE—temperature programmed evolution). The lower the desorption temperature of the first SO2 peak, the higher the activity. The shift to lower temperature is connected with a higher Pt and sulfate concentration, furthermore with the proximity of the metal to acid sites. The catalysts with a low sulfate concentration possess only Lewis acid sites and are inactive for n-heptane conversion. At higher sulfate concentration, Bronsted acid sites are present and the catalysts are active. The concentration of these acid sites is related to the concentration of sulfates, which desorb at lower temperature.