KINETICS OF PDO COMBUSTION CATALYSIS

The kinetics of the catalytic combustion of methane by supported palla dium oxide catalysts (2 wt.-% Pd/La2O3 . 11Al(2)O(3) and 5 wt.-% Pd/ga mma-Al2O3) were examined for several oxygen partial pressure levels ov er the temperature range from 40-900 degrees C using temperature-progr ammed reaction and slow ramp and hold temperature-time transient techn iques. Combustion rates were measured by differential reaction in a fi xed bed of powdered catalyst at lower temperatures (200-500 degrees C) . Also, by preparing the catalysts as thin (ca. 10 mu m) coatings on a n alumina tube and conducting the experiments with very high flows of dilute methane and oxygen in helium, the rate measurements were extend ed up to 900 degrees C without significant contribution from gas phase reactions. The specific combustion activity of supported PdO shows a persistent hysteresis between 450 and 750 degrees C, i.e., the rate of combustion between these temperature limits depends strongly on wheth er the catalyst is cooling from above 750 degrees C or heating from be low 450 degrees C. This region is also notable for negative apparent a ctivation energy in the rate of methane oxidation, i.e., the rate incr eases with decreasing temperature during reoxidation of the Pd metal a nd decreases with increasing temperature (especially with low oxygen p artial pressure) prior to decomposition of the bulk oxide. Detailed ti me-temperature transient kinetic analyses were performed for supported PdO catalysts within the 450-750 degrees C temperature range. The hys teresis in methane combustion rate is caused by a higher activation en ergy for reduction of oxygen chemisorbed on metallic Pd and by suppres sed reoxidation of Pd metal relative to PdO decomposition.