Aspects of temperature-programmed analysis of some gas–solid reactions. Part 2.—Hydrogen temperature-programmed desorption from silica-supported platinum

Abstract

Well resolved temperature-programmed desorption spectra have been obtained for hydrogen preadsorbed upon silica-supported platinum; these reveal three maximum rates of desorption (β₁ at T_max1, β₂ at T_max2 and β₃ at T_max3). The total extent of desorption of hydrogen exceeds the amount of hydrogen observed to chemisorb in extrapolation of adsorption data at ambient temperature to zero pressure. As the Pt dispersion increases, the amount of hydrogen associated with the β₃ peak becomes smaller than the sum of the (β₁+β₂) peaks. This may contradict suggestions that integral enthalpies of hydrogen chemisorption on supported platinium are independent of its average particle (Sen et al., J. Catal., 1986, 101, 517). Therefore the temperature required for desorption of a given fraction of adsorbed hydrogen is observed to decrease as the Pt surface area increases and the average Pt particle size decreases, possibly indicating an increase in the concentration of sites binding hydrogen with lower energy. The β₃ peak may arise from desorption from distinct spillover sites upon the support, but the β₁ and β₂ peaks appear associated with Pt-held hydrogen, and interdiffusion between these means that β₁ and β₂ peaks cannot be taken to indicate the populations of different surface Pt sites; indeed Pt surface areas estimated from the sum of the (β₁+β₂) peaks can exceed those determined by hydrogen chemisorption. However, Pt surface areas estimated from hydrogen t.p.d. to the temperature (here 573 K) to which catalysts are evacuated prior to volumetric measurement of chemisorption do agree well with chemisorption measurements. There is no consistent trend of desorption activation energies with Pt dispersion (as reported by Takasu et al., J. Chem. Soc., Chem, Commun., 1983, 1329) or t.p.d. peak temperature (as reported by Foger and Anderson, J. Catal., 1978, 54, 318 and Anderson et al., J. Catal., 1979, 57, 458).