Effect of Pt/Pd ratio on catalytic activity and redox behavior of bimetallic Pt-Pd/Al2O3 catalysts for CH4 combustion

In the present work CH4 combustion activity and reduction/oxidation behavior of bimetallic Pt-Pd/Al2O3 catalysts with constant Pd loading (2%, w/w) and different Pt/Pd atomic ratios (0, 0.10, 0.25 and 1) are investigated in the presence of alternated CH4 lean combustion/CH4-reducing pulses at 350 degrees C. In the fresh samples, according to XRD and CH4-TPR measurements, Pd is always totally present as PdO and the CH4 combustion activity is progressively promoted by Pt addition. On the other hand the reactivity scale is substantially changed (Pt/Pd = 0.10 >= Pt/Pd = 0 > Pt/Pd = 0.25 >> Pt/Pd = 1) after a conditioning treatment consisting of several reduction/oxidation cycles in CH4-containing atmosphere, which has a progressively positive effect on decreasing the Pt content. Indeed, such a treatment results in a 20-fold catalytic activity enhancement for the monometallic sample (Pt/Pd = 0), whereas it has different effects on the bimetallic catalysts depending on the Pt/Pd ratio: for Pt/Pd = 0.10 the activity markedly increases up to the highest level among the tested catalysts (fresh and conditioned); for Pt/Pd = 0.25 the activity is substantially unchanged while for the Pt/Pd = 1 it is completely suppressed after the first CH4-reducing pulse. Such different behavior is mainly related to the influence of Pt on bulk reduction/re-oxidation properties of palladium; TPO data indicate a strong inhibition of Pt on Pd oxidation, which is completely suppressed for the Pt/Pd-1 catalyst, thus explaining the wide loss of activity after reduction for Pt/Pd-1 and confirming that PdO is the most active phase. In the case of the samples with Pt/Pd = 0.25 and Pt/Pd = 0.10 the inhibiting effect of Pt on Pd oxidation is progressively reduced, resulting in a fraction of PdO formed at the end of the conditioning process equal to 35% and 85% of total Pd, respectively. In line with a Mars van Krevelen redox mechanism controlled by PdO surface reduction by CH4, for the monometallic sample the activity enhancement upon conditioning is associated with an increase of bulk PdO reducibility, as determined by CH4-TPR experiments. Such a correlation is not observed in bimetallic samples possibly due to the ability of metallic Pt to activate CH4 under net reducing conditions (CH4-TPR), which is suppressed under net oxidizing conditions (lean combustion). (C) 2010 Published by Elsevier B.V.