Influence of single- and double-flame spray pyrolysis on the structure of MnOₓ/γ-Al₂O₃ and FeOₓ/γ-Al₂O₃ catalysts and their behaviour in CO removal under lean exhaust gas conditions

MnOx/Al2O3 and FeOx/Al2O3 samples were prepared by two-nozzle flame spray pyrolysis to minimize the formation of composite phases. For this purpose, manganese(ii) naphthenate or iron(ii) naphthenate and aluminium-sec-butylate were sprayed in separate flames and both the structure and the catalytic performance of the materials in CO oxidation were compared to the corresponding single-nozzle flame spray pyrolysis catalysts. Characterization by X-ray diffraction, diffuse reflectance UV-vis spectroscopy and X-ray absorption near-edge structure unravelled that the phases formed in double-flame spray pyrolysis (DFSP) were significantly different from those in single-flame spray pyrolysis; highly dispersed separate entities of manganese/iron oxide and alumina were identified in this case. Despite a slightly lower BET surface area the DFSP prepared samples performed generally better in catalytic CO oxidation than those derived from one single flame. In addition, the manganese-based catalysts were more effective for CO conversion than the corresponding iron-based samples, even at low concentrations.