APCATA 250-2003-65

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

Rates of hydrogen adsorption on Pt/WO3-ZrO2 and WO3-ZrO2 were measured in the adsorption temperature range
323–573K 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/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 373K 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 × 1019 atom H/g cat. Similarities and differences in hydrogen adsorption
between SO4
2−-ZrO2 and WO3-ZrO2 types are discussed.