Role of Pd loading and dispersion on redox behaviour and CH4 combustion activity of Al2O3 supported catalysts

In this work the effect of palladium load (1–4%, w/w) and particle size (2–6 nm) on the catalytic activity in CH4 combustion and on the reduction/reformation behaviour of PdO during alternate CH4-reducing/ CH4-lean combustion pulses was investigated over Al2O3 supported catalysts. PdO-reduction/ reformation cycles occurring during the tests were confirmed to be beneficial with respect to deactivation phenomena observed during prolonged exposure to lean combustion atmosphere. The results showed that the catalyst with lower Pd load and higher metal dispersion exhibits a lower specific catalytic activity in CH4 combustion, which is likely associated with a lower reducibility of smaller PdO particles as evidenced by CH4-TPR experiments. The more dispersed system also showed a slower reactivation dynamics upon exposure to lean combustion atmosphere. Such behaviour has been tentatively associated with the presence of ultradispersed species that during oxidation form poorly active aluminate complexes first, which are then slowly transformed into reducible PdO.