(717b) Controlled Synthesis of Ce-Zr-Mn Based Solid Solution Catalysts Using Flame Spray Pyrolysis

Novel catalysts are required to reduce methane slip from natural gas operated internal combustion engines at low catalyst operating temperatures, e.g. 400. Herein Pd supported on Ce-Zr-Mn oxide solid solution catalysts synthesized by flame spray pyrolysis (FSP) are employed for low temperature methane oxidation. Various catalyst formulations are systematically synthesized at 9 different Ce_xZr_yMn_z element ratios (where x,y,z=0,1,2), precursor molarities and equivalence ratios (ERs), and their activity and stability for methane oxidation are evaluated.

Samples produced at ER-0.8 show solid solution support and a smaller crystallite size (6.1nm), whereas multiple phases (tetragonal ZrO₂ and orthorhombic Mn₃O₄) and a larger crystallite size (14.1nm) are shown in samples at ER-1.2. For the Ce-Zr and Ce-Mn supports, Ce⁴⁺ radius is larger than the substituted cations so the supports with 67% of Ce are in single phase and their lattice constants are similar to the pure CeO₂. However, for Ce-Zr-Mn supports, Ce₁Zr₂Mn₁ shows single phase whereas Ce₂Zr₁Mn₁ has multiple phases. This is because the flash point of Cerium(III) 2-ethlhexanoate precursor is higher than that of other precursors, thereby the formation of Ce and other phases may be asynchronous. The high Ce concentration makes this phase separation apparent to observe in Ce₂Zr₁Mn₁. Raman analysis shows that higher oxygen vacancy concentration is shown in the samples produced at ER-1.2 compared to ER-0.8, which indicates an oxygen deficit synthesis condition. Single-phase samples relatively have high oxygen vacancies, which are expected to have a low-temperature methane oxidation activity as oxygen diffusion are promoted by oxygen vacancies. Therefore, both the phase information and the percentage of oxygen vacancies will be related to the methane oxidation activity. These predications can provide guidance on rational synthesis of catalysts using FSP.