Adsorption of atomic hydrogen on alumina by hydrogen spillover

The surface species formed by adsorption of atomic hydrogen on alumina has been characterized by a desorption peak at 480 °C in the temperature-programmed desorption. The atomic hydrogen has been generated either by a high-frequency discharge or by dissociative adsorption on platinum or nickel. On the alumina-supported metals hydrogen atoms formed on the metal are able to spill over onto the alumina until the alumina surface is equilibrated. From the measurement of the area of the corresponding desorption peak obtained as a function of adsorption time, pressure and temperature the values of the saturation coverage of hydrogen atoms on the alumina (2 × 1012 atoms/cm2), the coefficient of surface diffusion of atomic hydrogen on alumina at 400 °C (0.9 × 10−15 cm2 sec−1) and the activation energy of this diffusion (28.5 kcal/mole) have been evaluated. The data obtained with two platinum on alumina catalysts with different metal dispersion agree with each other within experimental error.