Kinetic study of hydrogen adsorption on Pt/WO3-ZrO2 andWO3-ZrO2

Rates of hydrogen adsorption on Pt/WO3-ZrO2 and WO3-ZrO2 were measured in the adsorption temperature range 323-573 K and the hydrogen pressure range 20-70 Torr (I Torr = 133 Pa) to elucidate the rate-control ling step involved in the hydrogen adsorption, the corresponding energy barrier, and the heat of adsorption. The hydrogen adsorption was faster on Pt/WO3-ZrO2 than on WO3-ZrO2, On both Pt/WO3-ZrO2 and WO3-ZrO2, the adsorption continued for a long time. The adsorption continued for more than 10 h for Pt/WO3-ZrO2 below 373 K and for WO3-ZrO2 below 473 K. The adsorption of hydrogen involved the surface diffusion of hydrogen atoms for both adsorbents. For WO3-ZrO2, each hydrogen molecule was dissociated on a specific site Of WO3-ZrO2 to form hydrogen atoms, which undergo surface diffusion over the surface Of WO3-ZrO2. 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/WO3-ZrO2, two routes were operating. One route was same as that for WO3-ZrO2, and the other route involved dissociation of hydrogen molecules on the Pt sites to form hydrogen atoms that undergo spillover onto the WO3-ZrO2 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/WO3-ZrO2 were obtained as 11.6-8.0 kJ/mol for the hydrogen uptake range 2-3 x 10(19) atom H/g cat. Similarities and differences in hydrogen adsorption between SO42--ZrO2 and WO3-ZrO2 types are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.