Calorimetric heat of adsorption measurements on palladium: III. Influence of crystallite size and support on O 2 adsorption

Abstract

Integral, isothermal heats of adsorption were measured for O 2 adsorption on a family of Pd catalysts at 300 K using a modified differential scanning calorimeter. On Pd crystallites between 3 and 1000 nm, Q ad values exhibited no obvious trend and lay within 50 ± 6 kcal/mole O 2, regardless of the support. As the crystallite size decreased below 3 nm, Q ad increased sharply to values near 80 kcal/mole O 2, which is equal to the maximum initial Q ad values obtained in ultrahigh vacuum systems. Thus O 2 adsorption appears to parallel the behavior reported earlier for H 2 and CO although the trend is not so clear due to greater scatter in the data. No obvious influence of the support was observed for Pd dispersed on SiO 2, η-Al 2O 3, and SiO 2Al 2O 3, but higher than expected values were obtained on TiO 2-supported Pd, which was attributed to oxygen adsorption on sites on the titania surface that were created during the reduction step. This change in the O 2 heat of adsorption is believed to be due primarily to an electronic effect, and the higher values on very small crystallites could be responsible for the enhanced CO dissociation recently reported on highly dispersed Pd.