Palladium-hydrogen interaction on supported palladium catalysts of different metal dispersions

Palladium-hydrogen interaction on supported Pd catalysts of different metal dispersions has been studied by hydrogen chemisorption and back-sorption, and by temperature-programmed desorption (TPD) of either deuterium or hydro gen from the Pd surface after different initial gas dosages. Pd interacts w ith three hydrogen species whose amounts van, with the metal dispersion and with the dosage pressure. The amount of hydrogen strongly adsorbed on the Pd surface increases with the metal dispersion but is unaffected by the hyd rogen pressure. The amount of hydrogen absorbed in the bulk of Pd is signif icant with a poorly dispersed catalyst, particularly when the hydrogen pres sure is higher than about 10 Ton; but decreases with the metal dispersion. Hydrogen weakly bound with Pd shows two characteristic TPD peaks, whose int ensity depends on the metal dispersion and the initial gas dosage. The peak at 270-330 K, assigned to the recombination of absorbed hydrogen with surf ace hydrogen, is large with a poorly dispersed catalyst and grows with the initial hydrogen dosage. The intensity of the peak at 215-226 K is not affe cted by the metal dispersion or by the hydrogen dosage. It has been conclud ed that an analysis of the intrinsic properties of Pd catalysts should be b ased on an understanding of the hydrogen species associated with the cataly sts.