Sorption and temperature-programmed desorption of hydrogen from palladium and from palladium on activated carbon

The chemisorption of hydrogen on palladium has been studied by measuring pulse-wise adsorption and temperature-programmed desorption (TPD) on various palladium samples and on palladium-on-activated-carbon catalysts. The desorption of weakly bound (“type C”) hydrogen is found to be first order in coverage, and the enthalpy of adsorption is about 34 kJ/mole. Its population is proportional to the amount of weakly adsorbed hydrogen (“type C” hydrogen), and a desorption mechanism is postulated in which a subsurface hydrogen recombines with a surface hydrogen atom (“breakthrough” mechanism). The desorption of strongly bound hydrogen is second order in coverage, and the enthalpy of adsorption on Pd on activated carbon is 90 ± 5 kJ/mole, a value about equal to that found by E. Conrad, G. Ertl, and E. E. Latta [Surface Sci. 41, 435 (1974)], for palladium (111). Its population is equal to the amount of hydrogen pulsed, minus half the amount of “type C” hydrogen desorbed, in accordance with the proposed “breakthrough” mechanism. Large amounts of electropositive contaminations in palladium, like Zn, Pb, and Ca, suppress “type C” adsorption, whereas the enthalpy of adsorption of strongly bound hydrogen decreases to lower values.