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°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.