Oxidation of volatile organic compounds on Al2O3, Pd/Al2O3, and PdO/Al2O3 catalysts

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to study the decomposition and oxidation of methanol, ethanol, acetaldehyde, formic acid, and acetic acid on Al2O3, Pd/Al2O3, and PdO/Al2O3 catalysts. The oxidation and decomposition rates were much higher on Pd/Al2O3 than on Al2O3, even though the volatile organic compounds (VOCs) were adsorbed on the Al2O3 support in both cases. The VOCs surface-diffused to Pd and mostly dehydrogenated during TPD whereas they oxidized to CO2 and H2O in the presence of oxygen. Partial oxidation products also apparently formed on the surface during TPO and they oxidized completely above 550 K. Above 600 K, VOCs oxidation was consistent with the Mars-van Krevelen mechanism, involving the oxidation and reduction of Pd and PdO. On PdO/Al2O3 adsorbed VOCs were oxidized by lattice oxygen from PdO, but PdO was less active than Pd metal for VOC decomposition. Oxidation began at the same temperatures on PdO/Al2O3 whether or not O-2 was present, indicating that extraction of lattice oxygen from PdO was the limiting factor initially. After lattice oxygen was removed, metallic Pd decomposed VOCs and also adsorbed O-2, which was incorporated into the Pd lattice above 600 K. The reduction of PdO during TPD resulted in an autocatalytic oxidation since metallic Pd was more active than PdO. A portion of the VOCs reacted in parallel on Al2O3 since these sites are active for dehydration and dehydrogenation at moderate to high temperatures. (C) 1996 Academic Press, Inc.