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, acetald ehyde, formic acid, and acetic acid on Al2O3, Pd/Al2O3, and PdO/Al2O3 catalysts. The oxidation and decomposition rates were much higher on P d/Al2O3 than on Al2O3, even though the volatile organic compounds (VOC s) were adsorbed on the Al2O3 support in both cases. The VOCs surface- diffused to Pd and mostly dehydrogenated during TPD whereas they oxidi zed to CO2 and H2O in the presence of oxygen. Partial oxidation produc ts also apparently formed on the surface during TPO and they oxidized completely above 550 K. Above 600 K, VOCs oxidation was consistent wit h the Mars-van Krevelen mechanism, involving the oxidation and reducti on 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 decompo sition. Oxidation began at the same temperatures on PdO/Al2O3 whether or not O-2 was present, indicating that extraction of lattice oxygen f rom PdO was the limiting factor initially. After lattice oxygen was re moved, metallic Pd decomposed VOCs and also adsorbed O-2, which was in corporated into the Pd lattice above 600 K. The reduction of PdO durin g TPD resulted in an autocatalytic oxidation since metallic Pd was mor e active than PdO. A portion of the VOCs reacted in parallel on Al2O3 since these sites are active for dehydration and dehydrogenation at mo derate to high temperatures. (C) 1996 Academic Press, Inc.