Cerium-based Composite Oxide, Preparation And Partial Oxidation Of Methane To Synthesis Gas Performance Study

A novel process of methane partial oxidation using lattice oxygen instead of gaseous oxidant to participate in methane partial oxidation to synthesis gas has been recently proposed.In this process a suitable oxygen storage compound(OSC)is circulated between two reactors.In one reactor,methane is oxidized to synthesis gas by the lattice oxygen of OSC,and in the other,the reduced OSC are re-oxidized by air to restore its initial state.This technology has many advantages,when compared with the partial oxidation of methane(POM).First,it can avoid the risk of explosion due to the premixed CH₄/O₂ mixture within the ignition and explosion limits.Second,the selectivity of desired product can be enhance,because the product is not easy to be deeply oxidized in the absence of molecular oxygen.Third,it can save oxygen supply by the cryogenic distillation of air needs additional investment and operational expense,because which does not need using pure oxygen.In this thesis,we incorporate ceria and transition metal oxides such as Fe₂O₃,CuO, MnO₂ and Co₃O₄ aimed at increasing the oxygen storage capacity and the oxygen mobility in the oxygen storage compound for this redox cycle process.Their catalytic activities in the direct conversion of methane to synthesis gas in a fixed-bed reactor are investigated in the absence of gas-phase oxygen by temperature programmed surface reaction,continuous reactions,and sequential redox cycles.The Ce-Fe-O sample was found to be suitable for partial oxidation methane to synthesis gas.Then,the effect of n(Ce):n(Fe),calcination temperature and doped Zirconia on catalytic activity was measured.The OSC with good performance and the reaction mechanism between oxygen carriers and methane are desirable to be obtained.The Ce-Fe-O sample exhibits the highest selectivity to synthesis gas,and it is the best oxygen carrier among the tested Ce-M-O oxides for synthesis gas production.The phase cooperation between CeO₂ and Fe₂O₃ is responsible for the better activity.First,the solid solution based upon ceria-ferric oxide system can enhance the lattice oxygen mobility of oxygen carrier.Second,dispersed Fe₂O₃ was firstly returned to original state and then virtually form Fe or Fe₃C species on the catalyst which could be considered as the active site for selective CH₄ oxidation.The appearance of carbon formation is significant and the oxidation of carbon appears to be the rate-determining step.CeO₂ mainly provides selective lattice oxygen which is the necessities for synthesis gas production.Too high content of Fe₂O₃ seems to be disadvantageous to the catalytic activity enhancement and favor deep oxidation of methane.There is a suitable atom ratio exhibits highest degree of interaction between Ce and Fe species.Comparison of seven types of complex oxide systems Ce-Fe-O-X(X was the cerium iron Molar ratio,X=9/1.8/2.7/3.6/4.5/5.4/6.2/8)made in this work,Ce-Fe-O-7/3 shows the best catalytic activity.The calcination temperature exerts an important influence to the performance of the ceria-ferric oxide system,the concerned results have shown that there exist a best temperature range which is at about 800℃.A better crystallinity could improve the syngas selectivity,but actually would lead to decreased activity of oxygen carrier.After a long-term redox cycle,the selective methane oxidation performance of Ce-Fe-O-7/3(800) decreased appreciably,mainly caused by the sintering of CeO₂.However,the oxygen storage capacity of Ce-Fe-O-7/3(800)has not declined but increased slightly through redox cycle.It is shown that introduction of ZrO₂ into the Ce-Fe-O-7/3(800)framework with formation of cerium-zirconium solid solution strongly modifies the reduction behaviour in comparison to that seen with Ce-Fe-O-7/3(800)alone.Moreover,it also results in more active catalysts with enhanced synthesis gas selectivity and promotes the value of n(H₂):n (CO)more close to 2.Remarkably,ZrO₂ enhances the thermal stability and the oxygen storage capacity of Ce-Fe-O-7/3(800),resulting in better redox capacities for partial oxidation of methane at moderate temperatures.