Methane Combustion Over Copper Chromites Catalysts Prepared by the Sol–Gel Process

Abstract

Methane combustion was performed over a series of copper chromites (CuCr2O4) catalysts prepared by the sol–gel process (SG). The results were compared with those obtained over commercial CuCr2O4. The samples were characterised by elemental analysis, thermal analysis, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The as-synthesised CuCr2O4 sample exhibited higher specific surface area (248 m2 g−1) with respect to commercial solids (42 m2 g−1). The surface properties were established using XPS. Simultaneously, an increase in the atomic Cr6+/Cr3+ ratio (0.56 for SG catalyst vs. 0.39 for commercial sample) and a decreasing surface copper concentration (8.3%, for SG specimen vs. 17.6% for commercial catalyst) are observed. XPS study revealed also that Cu2+/(Cu° + Cu+) ratio of copper species remained constant (ca. 5) in both catalysts. Structure transformations of CuCr2O4 under reduction-reoxidation conditions showed that the reduction of copper and/or chromium cations from the CuCr2O4 and from delafossite (CuCrO2) structure (CuCr2O4↔CuCrO2 + Cu + Cr2O3↔Cu + Cr2O3) were different. This has lead to a difference in catalytic properties of the catalysts. Catalytic activity of SG catalysts was superior to that of commercial CuCr2O4 tested under the same conditions. Complex hysteresis behaviour for CuCr2O4 catalysts ramped over a temperature range of 220–850 °C where the stables active phases were obtained only after the first ramp of heating under reactants. No catalysts deactivation was further observed after several cycles of heating and cooling. The stabilisation of catalytic activity was attributed to the formation of mixed crystalline phases containing both copper and chromium species.