Transition metal oxides have recently attracted considerable attention as candidate catalysts for the complete oxidation of methane, the main component of the natural gas, used in various industrial processes or as a fuel in turbines and vehicles. A series of novel Co-Ce mixed oxide catalysts were synthesized as an effort to enhance synergistic effects that could improve their redox behavior, oxygen storage ability and, thus, their activity in methane oxidation. The effect of synthesis method (hydrothermal or precipitation) and Co loading (0, 2, 5, and 15 wt.%) on the catalytic efficiency and stability of the derived materials was investigated. Use of hydrothermal synthesis results in the most efficient Co/CeO2 catalysts, a fact related with their improved physicochemical properties, as compared with the materials prepared via precipitation. In particular, a CeO2 support of smaller crystallite size and larger surface area seems to enhance the reducibility of the Co3O4/CeO2 materials, as evidenced by the blue shift of the corresponding reduction peaks (H2-TPR, H2-Temperature Programmed Reduction). The limited methane oxidation activity over pure CeO2 samples is significantly enhanced by Co incorporation and further improved by higher Co loadings. The optimum performance was observed over a 15 wt% Co/CeO2 catalyst, which also presented sufficient tolerance to water presence.
Co-based catalysts were synthesized and studied as novel oxidation catalysts, exploring and optimizing the effect of synthesis method on the redox behavior, the oxygen storage ability and thus the catalytic performance of the derived Co3O4 materials in the complete CH4 and/or CO oxidation reactions. Thus, a series of Co-based catalysts were synthesized applying either the precipitation and/or the hydrothermal method, using different precipitating agents (Na2CO3, NH3, urea or NaOH), Co precursor salt (nitrate or acetate) and finally varying the Co/Na ratio. In addition, the reaction time (6 or 24 h aging) was also investigated for the hydrothermally prepared Co3O4. The best catalysts for the CH4 oxidation are the precipitated Co3O4, using cobalt acetate as precursor salt and NaOH as precipitating agent, presenting the highest surface areas and the lowest Co3O4 particle sizes. On the other side, hydrothermally prepared cobalt oxides reveal higher performance for CO oxidation, with Co3O4 prepared with cobalt acetate, NaOH and low aging time shown as the optimum materials. The best catalysts were further promoted with incorporation of Pd (0.5wt.%) and explored for both reactions. The addition of Pd enhanced the activity of Co3O4 for CH4 oxidation, while Pd did not improve any further the catalyst performance for CO oxidation, presenting thus the same activity with pure cobalt oxides.
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