Kinetics study of hydrogen adsorption over Pt/MoO 3

Abstract

The rate controlling step and the energy barrier involved in the hydrogen adsorption over Pt/MoO 3 were studied. Rates of hydrogen adsorption on Pt/MoO 3 were measured at the adsorption temperature range of 323–573 K and at the initial hydrogen pressure of 6.7 kPa. The rate of hydrogen uptake was very high for the initial few minutes for adsorption at and above 473 K, and reached equilibrium within 2 h. At and below 423 K, the hydrogen uptake still continued and did not reach equilibrium after 10 h. The hydrogen uptake exceeded the H/Pt ratio of unity for adsorption at and above 423 K, indicating that hydrogen adsorption involves hydrogen atom spillover and surface diffusion of the spiltover hydrogen atom over the bulk surface of MoO 3 followed by formation of H x MoO 3. The hydrogen uptake was scarcely appreciable for Pt-free MoO 3. The rate controlling step of the hydrogen adsorption on Pt/MoO 3 was the surface diffusion of the spiltover hydrogen with the activation energy of 83.1 kJ/mol. The isosteric heats of hydrogen adsorption on Pt/MoO 3 were 18.1–16.9 kJ/mol for the hydrogen uptake range 2.4–2.8 × 10 19 H-atom/g-cat. Similarities and differences in hydrogen adsorption on Pt/SO 4 2−–ZrO 2, Pt/WO 3–ZrO 2 and Pt/MoO 3 catalysts are discussed.