Abstract

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 NiMn_2 − 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.