Low and high activity states in the oxidation of m-xylene on pallidum catalysts

Low and high activity states in the carbon dioxide vs. inlet temperature transient curves in the catalytic combustion of m-xylenes over Pd2Ni50Nb48 amorphous alloy ribbons (I), Pd/stainless steel flakes (II) and Pd/clinoptilolite (III) samples have been investigated. in addition, in both states hysteresis, in high activity state self-sustained oscillation in the CO2 vs. time at full conversion and damped oscillation at low conversion were observed over samples (II) and (III). It was established that hysteresis is a general phenomenon in both states and is not affected by the catalyst structure, but it can be ascribed to Pd as active component. Transition between the low and high activity state may be due to change in the PdO structure as was indicated by XPS studies. Oscillation is indicative of the partial accumulation of carbonaceous deposits over the catalysts at full conversion, while damped oscillation might be a result of the change of local concentration due to overheating the catalyst sample during combustion.