Effect of alternate CH4-reducing/lean combustion treatments on the reactivity of fresh and S-poisoned Pd/CeO2/Al2O3 catalysts

This work investigates the effect of treatments under different CH4-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al2O3/CeO2 catalysts for methane combustion. Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH4-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H2O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO2 markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle. Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO2-containing atmosphere required very high temperatures (above 750 degrees C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH4- reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550-600 degrees C) due to the reduction of sulphates by CH4 activated on the surface of Pd metal. A protecting role of CeO2 on Pd poisoning due either to exposure to SO2-containing atmosphere or to spill-back of support sulphates species was also evidenced. (C) 2007 Elsevier B.V. All rights reserved.