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.