CHARACTERIZATION OF SILICA-SUPPORTED NICKEL-CATALYSTS BY THE TEMPERATURE-PROGRAMMED DESORPTION OF HYDROGEN ADSORBED AT VARIOUS TEMPERATURES

The temperature-programed desorption (TPD) of hydrogen adsorbed at various temperatures and at nearly saturated coverage has been used to characterize the surface of nickel particles dispersed on silica gel. The TPD spectra have two major peaks, the first and second peaks, at about 375 and 560 K. The second peak has a strong functional relation with the nickel weight loading and the heat-treatment temperature. It decreases with decreasing metal loading and increasing treatment temperature. As the adsorption temperature is increased the amount of hydrogen desorbing in the second peak is mainly increased; this is more remarkable for smaller metal loading. The two observed major peaks suggest the presence of two types of adsorption sites on nickel particles. The adsorption of hydrogen can easily occur on sites for the first peak, while it should go over some energy barrier on sites for the second one. It is assumed that these two types depend on the surface morphology of nickel particles, which is affected by their size and thermal history.