Copper-aluminum oxide catalysts for total oxidation of toluene synthesized by thermal decomposition of co-precipitated precursors

• Cu–Al oxides obtained by coprecipitation are active catalysts in toluene combustion.
• Advantage of the catalyst is low copper content and alkali-free precipitating agent.
• The stable oxide form built of CuO and CuAl2O4 is attained at 900 °C.
• The optimum atomic Cu:Al ratio, ensuring maximum toluene conversion, is about 0.6.
• The most active sample contains small CuO crystallites dispersed on the surface.

Copper–aluminum containing precursors with various Cu/Al molar ratios (from 0.32–1.28) were prepared by co-precipitation in the presence of ammonium carbonate. The thermal stability of the obtained materials was investigated by thermal analysis, which revealed three crucial decomposition steps, finally resulting in the formation of mixed Cu–Al oxides. The changes in structure and texture of the samples at each decomposition step were examined by X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy (UV–vis-DRS) and low temperature sorption of nitrogen. It was found that the entire removal of structural carbonates requires a calcination temperature as high as 900 °C. The samples after thermal treatment at this temperature varied in the phase composition of the bulk (determined by XRD) as well as of the surface (determined by X-ray photoelectron spectroscopy). All samples contained the CuAl2O4 phase. Furthermore, an increase in Cu content led to the appearance of an increasing amount of CuO. Copper oxide in the form of relatively small crystallites turned out to be the catalytically active phase in the total oxidation of toluene.