OXIDATION-KINETICS OF CALCIUM-DOPED PALLADIUM POWDERS

The oxidation kinetics of submicron Ca-containing Pd powders produced by spray pyrolysis were studied in the temperature range 600 to 675 de grees C using thermogravimetric analysis. The oxidation of pure Pd pow der had an activation energy of similar to 230 kJ/mol in the region 27 % <oxidation <70% and 65 kJ/mol for oxidation <70%. The activation ene rgies for Pd particles containing 0.01 weight percent (w/o) and 0.4 w/ o Ca in the region 27% <oxidation <70% were similar to 230 kJ/mol and similar to 50 kJ/mol, respectively. Transmission electron microscopy s uggested that the conversion of Pd to (PdTO)-O-11 (stoichiometric PdO) proceeds from the particle surface into the interior and not homogene ously throughout the particle. The predictions of a variety of models and rate laws (shrinking core, parabolic, cubic, logarithmic, and inve rse logarithmic) were compared with the data. The comparison suggested a mechanism in which oxidation ol pure Pd proceeds by chemisorption a nd diffusion of oxygen to form a substoichiometric oxide, followed by the conversion of substoichiometric PdO to (PdO)-O-11. Oxidation of pu re Pd is then probably limited by the diffusion Of oxygen through the substoichiometric PdO and/or (PdO)-O-11. The addition of Ca increased the oxidation resistance of Pd most likely by inhibiting oxygen diffus ion through the metal oxide layers surrounding the Pd.