Synthesis, Characterization And Catalytic Activity Of Mixed Metal Oxides And Noble Metal Catalysts For Methane Combustion

Methane is the major component of natural gas, and its combustion temperature is ratherhigh (above1500°C). At such a high temperature, N2in the air can be easily oxidized intoNOx, resulting in a great environment pollution; in addition, incomplete combustion ofmethane produces CO, and CH4has also a considerable greenhouse effect. Fortunately,catalytic combustion, which is regarded as an ideal combustion mode, can significantlyreduce the ignition temperature of methane, improve its conversion, and contribute to exhaustpollution abatement. Until now, the catalysts for methane catalytic combustion are mainlydivided into two categories, one is supported noble metal catalyst with excellent activity atlow temperature, and the other is non-noble metal oxide catalyst showing better stability athigh temperature. In this paper, Co-based metal oxides and supported noble catalysts havebeen studied in methane catalytic combustion. The effects of preparation method and elementmodification on catalytic performance were investigated. The catalysts were characterized bymeans of TPR, XRD, BET, Raman, etc. The main research results are as follows.(1) The Co3O4supported rare earth oxides were prepared by an impregnation method.The results showed the best methane catalytic combustion activity on CeO2/Co3O4catalyst.When the volume ratio of CH4to O2was1:2.2, and the space velocity was72000mL·g-1·h-1,the methane conversion was~83%at500°C, and~99%at800°C, without any CO detected.(2) Modified CeO2were employed as carriers to support Co3O4. Among the as preparedCo/Ce-M-O(M=La, Zr, Ca, Mg, Bi) catalysts, the Co/Ce-La-O one showed the best catalyticactivity. On this catalyst, when the volume ratio of CH4to O2was1:2.2, and the spacevelocity was72000mL·g-1·h-1, the methane conversion was~87%at450°C. The operatingtemperature window ranged to800°C, and no CO was detected. After the reaction at800°Cfor20h, the methane conversion still stayed above96%. A series of characterizationssuggested that the doping of La increased CeO2crystal defects and improved the dispersion ofCo species. Furthermore, oxygen species reaction progress was analysed to explore themethane catalytic combustion mechanism on the Co/Ce-La-O catalyst.(3) La or Ce was introduced into γ-Al2O3support, and supported noble metal (Pd, Ir, Ru)catalysts were prepared. The results showed that the methane catalytic combustion activityimproved obviously on Pd/Ce/Al2O3catalyst. The dual metal catalysts of Pd-Co and Pd-Mnwere also investigated in this paper. The catalytic activity on Pd-Co/CeO2was much betterthan that on Pd/CeO2and Co/CeO2, and the supported Pd further improved the catalytic activity of Co/Ce-La-O catalyst. Moreover, Pd-Mn/Al2O3catalyst also performed outstandingactivity for methane catalytic combustion. When the volume ratio of CH4to O2was1:2.2, andthe space velocity was72000mL·g-1·h-1, the methane conversion was~87%at350°C.Pd-Mn/Al2O3catalyst showed a good reuseability, only the first reduction pretreatment wasneeded for the first-time use, hereafter, the catalyst could maintain an unchanged high activityrepeatedly, showing the promising potential for practical application.