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

Methane combustion was performed over a series of copper chromites (CuCr 2 O 4 ) catalysts prepared by the sol–gel process (SG). The results were compared with those obtained over commercial CuCr 2 O 4 . The samples were characterised by elemental analysis, thermal analysis, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The as-synthesised CuCr 2 O 4 sample exhibited higher specific surface area (248 m 2 g −1 ) with respect to commercial solids (42 m 2 g −1 ). The surface properties were established using XPS. Simultaneously, an increase in the atomic Cr 6+ /Cr 3+ 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 Cu 2+ /(Cu° + Cu + ) ratio of copper species remained constant (ca . 5) in both catalysts. Structure transformations of CuCr 2 O 4 under reduction-reoxidation conditions showed that the reduction of copper and/or chromium cations from the CuCr 2 O 4 and from delafossite (CuCrO 2 ) structure (CuCr 2 O 4 ↔CuCrO 2 + Cu + Cr 2 O 3 ↔Cu + Cr 2 O 3 ) 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 CuCr 2 O 4 tested under the same conditions. Complex hysteresis behaviour for CuCr 2 O 4 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. Graphical Abstract