Spinel-type oxide catalysts for low temperatureCO oxidation generated by use of an ultrasonic aerosol pyrolysis process
The aim of this study was to generate spinel-type oxides by a continuous ultrasonic aerosol pyrolysis procedure and to evaluate the chemical and physical properties of the product powders. The ultrasonic aerosol pyrolysis process consists of three steps, which include “atomisation” of an aqeous metal nitrate solution, transport to the furnace system and controlled thermal decomposition of the precursor aerosol. The synthesis method allows the production of pure CuMn2O4, LiMn2O4 and CuCo2O4 spinels, whereas NiMn2 − xO4 contains small quanitities of α-Mn2O3. The powders are finely dispersed, mesoporous products exhibiting a uniform morphology. Scanning electron microscopy reveals that the product is made up of hollow spheres. XRD, TA and XPS results reveal that CuMn2O4 exists in a metastable state. It segregates in air already at temperatures of 300°C, whereas in an inert atmosphere the spinel is stable up to 600°C. XPS measurements indicate the presence of a redox system in the form of , which is one of the main features for the high turnover rates of CO with O2 of this catalyst already at room temperature. XPS measurements, after treatment of CuMn2O4 with CO and O2, confirm the highly reactive surface. NiMn2O4 and CuCo2O4 also show high activities in the catalytic CO oxidation. LiMn2O4 is rather inactive. A comparison of CuMn2O4 with LiMn2O4 points to the crucial influence of the copper cations on the conversion rates of the CO oxidation. In the synthesis of CuMn2O4 the introduction of glucose into the starting metal nitrate solution revealed changed properties of the product powders: as a consequence, the distribution of the cations on the surface is changed and markedly smaller conversion rates for the catalytic CO oxidation are observed.