Influence of the preparation method on the activity of copper-manganese oxides for toluene total oxidation

Copper manganese oxides were prepared either by a co-precipitation method using metal nitrates as precursors, tetramethylammonium hydroxide (TMAH) as precipitant or by a redox-precipitation method using manganese acetate and copper nitrate as precursors, permanganate of potassium as oxidant. Copper manganese oxides synthesized by the redox method and calcined at 300 °C were also doped with Pt and Pd (0.5 wt%). The materials were characterized by ICP-OES, X-ray powder diffraction, N2 adsorption/desorption analysis, temperature-programmed reduction, X-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectrometry. The catalyst properties were assessed in total oxidation of toluene and compared with those of the corresponding single oxides and of a commercial Hopcalite catalyst. Copper manganese oxides were proved to be more active than the single oxides whatever the method of preparation used. CuMnOx prepared via redox method were more active than the catalyst prepared by co-precipitation and compared favorably with the commercial Hopcalite. The overall characterization results revealed that the redox method can ensure a good dispersion of copper in close interaction with manganese preserving more active sites at the outermost layers of the catalyst in comparison with the catalyst obtained via co-precipitation. However all the catalysts deactivate to some extent at the earlier stages of the reaction before to reach a steady-state. For redox catalysts calcined at 300 °C, although the dispersion of trace amount of noble metals does not ensure a better activity, adding Pt allows to get a better resistance to deactivation. Additionally it is to be noticeable that the catalyst using redox-precipitation method calcined at 200 °C outperforms the commercial hopcalite overtime on stream.