Surface Enrichment in Supported Structured Pt/Ce-Mn-O Particles during Flame Spray Pyrolysis

Abstract

Noble mertal catalyst (Pt, Pd and Ru et. al) made by flame aerosol synthesis are efficient in a variety of reactions, including NOx storage reduction (NSR),1-4 hydrogenation of aromatic compounds,5, 6 photocatalytic mineralization7, 8 and methane catalytic combustion.9-11 Although catalytic activity of the flame made catalyst are found to be dependent on the type of metal and support, noble metal dispersion, particle size and interaction between noble metal and support are reported to have great influence on catalytic performance.12, 13 Process control including temperature and time history during flame aerosol synthesis is critical for catalytic performance, which in sequence determines the catalysts structure and morphology. Many volatile organic compounds (VOCs) catalytic oxidations including benzene are structure-sensitive.14-16 Better catalytic performance requires unique structure and morphology of the catalysts. A variety of smart structure and morphology have been achieved by flame spray pyrolysis.17 Traditionally, metal oxides and noble metal catalysts via FSP were spherical shaped primary particles without porous structure.11, 18, 19 In the process, flame is used to drive chemical reactions of precursors, resulting in the appearance of clusters, which grow to nanoparticles by coagulation and coalescence.20 During the process, many noble metal clusters are embedded in metal oxide particles dynamically, although vapor pressure of noble metal precursor is higher than that of metal oxide precursor thermodynamically. Pd/SiO2 21 could be an illustrations that noble metal clusters were embedded in the matrix. Assuming that noble metal is much more active than metal oxide, surface enrichment of noble metal facilitates the exposure of active phase, which is benefit for better catalytic activity. In our paper22, we developed a novel method (Emulsion Combustion Method, ECM) for supported catalyst preparation based on FSP and emulsion. Pt precursor and metal oxide precursors were dissolved respectively in oil and water phase of emulsion. The advanced element separation is considered to account for the formation of noble metal surface enrichment, which promoted catalytic performance.