from first principles PP-I-1

OP-I-1Particle Size Effect in CH 4 Oxidation over Noble Metals: Comparisonof Pt and Pd CatalystsStakheev A.Yu. 1 , Batkin A.M. 1 , Beck I.E. 2 , Teleguina N.S. 1 , Bragina G.O. 1 ,Zaikovsky V.I. 2 , Larichev Yu.V. 2 , Bukhtiyarov V.I. 21 Zelinsky Institute of Organic Chemistry RAS, Leninsky Prosp. 47, Moscow, 119991 Russia2 Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russiast@ioc.ac.ruOne of the most important factors influencing the efficiency of noble metal catalyst is the sizeof metal particles. High metal dispersion improves utilization of noble metal. On the otherhand, turnover rates of oxidation reactions may depend significantly on the size of metalparticles (particle size effect). As a result, overall activity of the catalyst becomes a functionof metal dispersion (fraction of exposed atoms) and the particle size effect, and carefulmanagement of metal particle size is required for achieving optimal performance and/orminimization of noble metal loading. In this study, we carried out the detailed comparativestudy of the particle size effects for Pt and Pd catalyst in the total CH 4 oxidation. Catalytictests were complemented by in situ XPS study of the oxidation state of Pt and Pd particlesunder reaction conditions for revealing the nature of the observed catalytic effects.Catalytic studies revealed different dependencies of TOF on metal particle size for Pd and Ptcatalysts. Particle size effect for Pd catalysts is much more pronounced than for Pt catalysts:TOF increases by more than order of magnitude with increase in d Pd from 1 to 22 nm.Therefore, the most favorable performance is observed for the catalyst with Pd particle size ~4 nm. For Pt catalysts TOF increases only by ~ 2-3 times with increasing particle size from 1to 3-4 nm, and remains essentially constant when d Pt exceeds 4 nm. As a result, the samplewith maximal Pt dispersion (d Pt = 1.2 nm) demonstrates the best overall activity in CH 4oxidation among Pt catalysts.Results of in-situ XPS studies indicate that Pd particles remain in oxidized state in the courseof the reaction within the whole range of particle sizes (1 – 22 nm). For Pt catalysts it wasfound that 1-3 nm Pt particles are easily oxidized under reaction conditions. Note, that thepronounced particle size effect is observed for the particles of this size range. On the otherhand, for Pt catalysts with bigger metal particles, XPS data showed that Pt remains mainly inthe metallic state. These data suggest that the observed dependence of the catalytic activity onmetal particle size may be associated with the change of the reaction pathway from Mars–vanKrevelen mechanism to Langmuir–Hinshelwood kinetics.29