Isosteric heat of sorption of dihydrogen on alumina-supported palladium

Abstract

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 α-phase PdH and diffusion into bulk crystallites to form the β-phase PdH. The dissipated heat of chemisorption (q_c) decreased with increasing coverage (θ) from q_c 22 kcal mol^–1 at low coverages (θ < 0.25), levelling-of momentarily at q_c≈ 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.