Catalytic removal of methane over thermal-proof nanostructured catalysts for CNG engines

Five spinel-type catalysts AB₂O₄ (Co_0.8Cr₂O₄, CoCr₂O₄, MnCr₂O₄, MgFe₂O₄ and CoFe₂O₄) were prepared and characterized by XRD, BET and FESEM techniques. The activity of these catalysts towards the combustion of methane was evaluated in a Temperature Programmed Combustion (TPC) apparatus. The half conversion temperature of methane over the Co_0.8Cr₂O₄ catalyst was 369 °C with a W/F = 0.12 g s/cm³. On the basis of Temperature Programmed Desorption (TPD) of oxygen as well as of catalytic combustion runs, the prevalent activity of the Co_0.8Cr₂O₄ catalyst could be explained by its higher capability to deliver suprafacial chemisorbed oxygen species. This catalyst, promoted by the presence of 1 wt% of Pd, deposited by wet impregnation, was lined on cordierite monoliths and then tested in a lab-scale test rig. The combination of Pd and Co_0.8Cr₂O₄ catalysts enables half methane conversion at 340 °C (GHSV = 10,000 h⁻¹), a performance similar to that of conventional 4 wt% Pd-γ Al₂O₃ catalysts but guaranteed with just a four-fold lower amount of noble metal. Both the catalysts in powder and the monolith hosting the Co_0.8Cr₂O₄ + 1 wt% Pd catalyst, submitted to a thermal ageing treatment in air at 700 °C for 12 h, displayed a negligible deactivation.