Calorimetric study on interaction of dioxygen with alumina supported palladium

Pd/Al2O3 samples of various palladium loadings were prepared with the incipient-wetness method. With an increase in the palladium loading, the average diameter (d) of palladium crystallites on these samples was estimated from chemisorption of hydrogen to vary from 10 to 80 Angstrom. Also, the phenomena of oxidizing these dispersed palladium crystallites with dioxygen were studied by a simultaneous TG-DSC instrument. The extent of oxidation was found to increase in three steps with temperature: adsorption on the surface of Pd crystallites at ambient or lower temperatures, penetration of oxygen into the sublayer when raising the temperature over 300 K, and formation of palladium oxide structure at 800 K. The heat evolved during oxygen adsorption on palladium crystallites depended on their average diameter (d) and displayed a relationship of - Q(ad) [kJ (mol O-2)(-1)] = 373 - 14d (nm). Evolved heat upon palladium oxidation decreased with the extent of oxidation (N-O/N-Pd ratio in the product). Upon the complete oxidation at 800 K, a diameter independent Delta(f)H(298)degrees(PdO) of - 187 +/- 2 kJ (mol O-2)(-1) was subsequently obtained. This value differs significantly from the literature Delta H(298)degrees(PdO) values (- 220 kJ (mol O-2)(-1)) obtained from the Clausius-Clapeyron equation. The discrepancy should have resulted from a partial desorption of oxygen from PdO during measurement.