Calorimetric heat of adsorption measurements on palladium: I. Influence of crystallite size and support on hydrogen adsorption

Abstract

A modified differential scanning calorimeter was used to measure integral heats of adsorption of hydrogen, Q ad , at 300 K on unsupported Pd powder and on Pd dispersed on SiO 2, SiO 2Al 2O 3, Al 2O 3, and TiO 2. The supports were found to have no significant effect on Q ad , and although reduction of Pd TiO 2 samples at 773 K sharply decreased the amount of hydrogen chemisorbed on these samples, the Q ad values measured on these samples were comparable to the other catalysts. In contrast, Pd crystallite size had a very pronounced effect on Q ad . On all these catalysts the heat of adsorption for hydrogen remained constant at 15 ± 1 kcal mole −1 as the average Pd crystallite size decreased from 1000 to 3 nm, but it increased sharply as the size dropped below 3 nm. The highest value, 24 kcal mole −1, was obtained on one of the most highly dispersed samples. Heats of formation of bulk Pd hydride showed a similar behavior, remaining constant at 8.7 ± 1.0 kcal mole −1 for samples with low Pd dispersions and then increasing noticeably as the crystallite size dropped below 3 nm. Most of this variation in Q ad is attributed to changes in the electronic properties of small Pd crystallites because the differences in Q ad values reported on single crystal surfaces are not sufficient to explain the enhanced bond strength.