| Partial oxidation of methane is one of the important ways for the conversion and utilization of methane,which efficiently utilizes the methane and subsequently yields the high-quality syngas.This syngas has become very attractive due to its role as a starting material for the production of important chemicals and liquid fuels through the indirect conversion methods.As a well-known fact,the reduction of operation temperature could minimize the energy expenditure during the reaction process.The application of chemical looping technology in the partial oxidation of methane(POM)is considered to be a low-cost strategy with the least impact on the environment.On the one hand,the oxygen carriers are used to store the oxygen molecules from the air.The redox reaction between methane and high-valent metal oxides is achieved by the release of lattice oxygen of metal oxides instead of molecular oxygen.On the other hand,the use of this pathway can reduce the temperature of traditional oxidation reactions and further minimize the energy consumption.In this study,due to the characteristics of fast lattice oxygen transport rate and stable structure,we developed and designed the functional oxygen carriers based on?-CeZrO4.Successively,we employed these oxygen carriers in the chemical looping partial oxidation of methane.The co-precipitation method was used to synthesize the?phase CeZrO4 composite oxide.The phase,oxidation performance and surface properties of the oxygen carrier were systematically analyzed by using XRD,SEM,H2-TPR,XPS and other characterization methods.The oxygen supply and the mechanistic action of oxygen carrier in the studied methane oxidation were discussed.The characterization results of H2-TPR and H2-TG indicate that the release of oxygen begins at 660?,the oxygen release rate(0.438%/min,less than or equal to 300°C)and the oxygen recovery ability are relative faster and easier.By investigating the effect of loaded active components on the crystallinity,morphology,reduction performance and catalytic oxidation activity of the?phase oxygen carrier,the results show that the appropriate loading of NiO is 2wt.%.The addition of NiO can change the distribution of oxygen species on its surface,the adsorption and activation performance of methane,and further reduce the oxygen release temperature of the oxygen carrier(660?to345?).The evaluation results show that the addition of NiO in Ce-based oxygen carrier leads to the formation of high-quality syngas during the chemical looping partial oxidation of methane(CLPOM)process.A higher methane conversion rate(52.38%)and syngas selectivity(88.44%)could be achieved at a reaction temperature as low as 700?,and interestingly,the oxygen carrier remains stable after 20 cycles.Based on the above research,the present work is further extended to carry out the quasi-in-situ CH4-XPS analysis which unveils the reason for the high selectivity of CO,and the oxygen supply mechanism of lattice oxygen in CLPOM reaction.This study also proposes the reaction mechanism of partially oxidized methane over NiO/?-CeZrO4 at low and medium temperatures(500-700?).In addition,the quasi-in-situ CH4-XPS analysis shows that the addition of NiO changes the relative content of OA(adsorbed oxygen)/OL(lattice oxygen)on the surface of oxygen carrier.The oxygen vacancy adjustments lead to the modification in the chemical state of element and the energy barrier of each elemental reaction on the interface.These changes are significant because they alter the reaction pathway and enhance the selectivity of syngas at low and medium temperatures.The interaction of NiO and?-CeZrO4 promotes the partial oxidation of CH4via CLPOM and as a result,the high-quality syngas is produced with the higher methane conversion rate(52.38%)and syngas selectivity(88.44%).The interaction between NiO and?-CeZrO4 suppresses the formation of carbon deposits during the redox process.This study has developed an oxygen carrier(NiO/?-CeZrO4)possessing the high stability,activity,oxygen storage capacity and fast oxygen release performance.NiO/?-CeZrO4realizes the oxygen replacement averting O2preparation from the air separation and improves the CH4 conversion in CLPOM process at low and medium temperatures.The experiments disclose the interaction mechanism between the active component and?-CeZrO4,providing a theoretical basis for the further development and fabrication of functional oxygen carrier materials with high activity and low cost. |
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