Kinetic study of hydrogen adsorption on sulfated zirconia-supported platinum

Rate of hydrogen adsorption on sulfated zirconia-supported platinum (Pt/SO42--ZrO2) was measured in the temperature range 323-523 K to elucidate the slowest step involved in the hydrogen adsorption and the corresponding energy barrier which the hydrogen species should cross over. At a nearly constant hydrogen pressure (50 Torr (6.7 kPa)), the hydrogen uptake continued more than 24 h, and the uptake in 24h increased with an increase in the adsorption temperature. Above 473 K, the hydrogen uptake far exceeded the H/Pt ratio of unity, indicating that the hydrogen adsorption involves hydrogen spillover. Without Pt, the hydrogen uptake was scarcely appreciable. The hydrogen adsorption rate followed the Fick's equation for surface diffusion. Activation energy for the diffusion constant was obtained as 84 kJ/mol. The rate could not be expressed by the rate equation derived on the assumption that the rate controlling step is the spillover step. (C) 2000 Elsevier Science B.V. All rights reserved.