ISOSTERIC HEAT OF SORPTION OF DIHYDROGEN ON ALUMINA-SUPPORTED PALLADIUM

Pd/Al2O3 samples of different palladium loadings were prepared by an incipient wetness technique. The palladium dispersion (D) of each sample was estimated from its isotherm of hydrogen uptake measured volumetrically at 298 K. The isosteric heat of hydrogen sorption (q, including absorption and adsorption) on palladium was determined from a series of uptake isotherms, measured at temperatures in the range 273-673 K, using the Clausius-Clapeyron equation. The value of q varied with the extent of the uptake and indicated three consecutive stages for sorption of hydrogen into palladium crystallites, i.e. chemisorption on the surface of the crystallites, penetration into sublayers of the crystallites to form the alpha-phase PdH and diffusion into bulk crystallites to form the beta-phase PdH. The dissipated heat of chemisorption (q(c)) decreased with increasing coverage (theta) from q(c) greater than or equal to 22 kcal mol(-1) at low coverages (theta < 0.25), levelling-of momentarily at q(c) approximate to 18 kcal mol(-1) before falling to 8 kcal mol(-1) at high coverages. The decrease in q(c) from zero coverage to the point where it levels off reflects a distribution in the size of palladium crystallites on each sample. The subsequent decrease in q(c) is attributed to weak chemisorption.