Particle Size Effect on CH4 Oxidation Over Noble Metals: Comparison of Pt and Pd Catalysts

Intrinsic catalytic activities (TOF values) in CH4 complete oxidation under lean conditions were estimated as a function of Pt and Pd particle sizes (d(m)) for two series of Pt/Al2O3 and Pd/Al2O3 catalysts. Comparison of TOF similar to f(d(m)) relationships revealed significant difference between Pt and Pd catalysts. For Pt catalyst TOF showed tendency to increase by 2-3 times with increasing particle size from 1 to ca 3 nm and remained constant, when Pt particles became larger than 3 nm. On the other hand, for Pd catalyst TOF increased almost linearly when particle size grew from 1 to 20 nm. These different tendencies were attributed to the different mechanisms of CH4 oxidation over Pt and Pd catalysts: Langmuir-Hinshelwood and Mars-Van Krevelen respectively.