Kinetic study of hydrogen adsorption on Pt/WO 3-ZrO 2 and WO 3-ZrO 2

Abstract

Rates of hydrogen adsorption on Pt/WO 3-ZrO 2 and WO 3-ZrO 2 were measured in the adsorption temperature range 323–573 K and the hydrogen pressure range 20–70 Torr (1 Torr = 133 Pa) to elucidate the rate-controlling step involved in the hydrogen adsorption, the corresponding energy barrier, and the heat of adsorption. The hydrogen adsorption was faster on Pt/WO 3-ZrO 2 than on WO 3-ZrO 2. On both Pt/WO 3-ZrO 2 and WO 3-ZrO 2, the adsorption continued for a long time. The adsorption continued for more than 10 h for Pt/WO 3-ZrO 2 below 373 K and for WO 3-ZrO 2 below 473 K. The adsorption of hydrogen involved the surface diffusion of hydrogen atoms for both adsorbents. For WO 3-ZrO 2, each hydrogen molecule was dissociated on a specific site of WO 3-ZrO 2 to form hydrogen atoms, which undergo surface diffusion over the surface of WO 3-ZrO 2. The rate-controlling step was the surface diffusion of hydrogen atoms. The activation energy was 25.9 kJ/mol for the surface diffusion. For Pt/WO 3-ZrO 2, two routes were operating. One route was same as that for WO 3-ZrO 2, and the other route involved dissociation of hydrogen molecules on the Pt sites to form hydrogen atoms that undergo spillover onto the WO 3-ZrO 2 surface, followed by surface diffusion. For the latter route, the rate-controlling step was the spillover step; its activation energy was 35.5 kJ/mol. The isosteric heats of adsorption of hydrogen on Pt/WO 3-ZrO 2 were obtained as 11.6–8.0 kJ/mol for the hydrogen uptake range 2–3×10 19 atom H/g cat. Similarities and differences in hydrogen adsorption between SO 4 2−-ZrO 2 and WO 3-ZrO 2 types are discussed.