Study On Modifying Of Nickel-Based Catalysts For Methane Reforming With Carbon Dioxide

Nickel-based catalysts for methane-carbon dioxide reforming reaction have a broad application prospect, because of good catalytic activity, and low cost of nickel, But nickel-based catalysts have the tendency of carbon deposition and sintering, which result in short service life. Thus its industrial application is greatly hindered.Alumina-magnesium composite oxide was prepared by citric acid sol-gel method, which magnesium highly dispersed in alumina phase. By means of optimizing the dosage of citric acid, aging time, gelating temperature etc., a high specific surface area composite oxide carrier was prepared. Various nickel-based catalysts were prepared with aluminum magnesium composite oxide as carrier and doping Ce or B, by the one step and multiple steps isometric impregnation method. The influence of magnesium content in the composite oxide, additive sort and dosage were investigated. The surface structure of the carrier was characterized by scanning electron microscopy (SEM). The content of the various elements in the supports and catalysts was determined by EDS. The calcination temperature for composite oxide precursor and the quantity of carbon deposited on nickel-based catalyst were determined by thermoanalysis (TG-DSC). The phase structure of samples was characterized by X-ray diffraction (XRD). The specific surface area of the supporters, fresh and used catalysts was analyzed by BET method. The catalyst reduction temperature and nickel grain size were checked by multifunctional adsorption instrument. The catalytic activity of methane reforming was tested by an online multifunctional catalytic evaluation device.The results showed that the amount of citric acid, sol aging time, gelating time and temperature could affect the specific surface area of the composite oxide. With the molar ratio of metal atoms to citric acid20:1, sol was aged for24h, followed by gelating at50℃and calcining at800℃for3h, the support with the highest specific surface area (113.8m2·g-1) was obtained. With the same quality of nickel, the increase of magnesium content in magnesium aluminum composite oxide could improve the resistance to carbon deposition and reduce the specific surface area of carriers. The perfect dosage of Ce promoter for nickel-based catalyst was about16.6wt%, which resulted in the best carbon deposition resistance. After reaction16h, the carbon deposit quantity on the catalyst was14.08%, which lower than that on the other doping-Ce or undoping catalysts. The best modifying amount of boron doped catalyst was1.4wt%, in which the Ni grain size was about5.2nm and5.5nm after reacting16h. It indicated that the boron could improve the initial dispersion of active component on the catalyst and the carbon deposition resistance. The optimum boron modified catalyst had the less amount of carbon deposited than other samples with different B-modified amount. The optimized catalyst, with support which had high specific surface area and the best amount of promoter (cerium and boron), were tested at the gas hourly space velocity (GHSV) of22000ml·gcat.1·h-1, CH4:CO2=1:1, reaction temperature750℃, reaction pressure atmospheric pressure. The initial conversion rate of methane and carbon dioxide were80.8%and91.5%, respectively. Either of the reaction conversion rate decreased about2%after has reacted10h while the particulate size of Ni in catalyst before and after reaction were5.1nm and5.3nm, respectively, and carbon deposition amount was about9.7wt%. The result illustrated that the optimized catalyst had excellent resistance to carbon deposition and sintering.According to this study, the conclusion can be drawn out that the Ce and B codoped nickel-based catalyst, prepared via the one step and multiple steps isometric impregnation method with the magnesium aluminum composite oxide as support, has the improved catalytic activity of CH4-CO2reforming, better resistance to carbon deposition and higher stability, as well as advantages of simple process and lower requirement for equipment. The optimized process parameters of the citric acid sol-gel method were as follows:the molar ratio of metal atoms to citric acid was20:1, the sol aging time was24h, the gelating temperature was50℃and the calcination condition was800℃for3h. The increase of magnesium content in composite oxide could improve the resistance to carbon deposition of the catalysts, but reduce the specific surface area of supports and activity of the catalysts. Certain doping amount of cerium could increase the catalysts resistance to carbon deposition greatly, and lightly improve the sintering resistance. The optimized doping amount of B improved the dispersion of active component on the catalyst and the carbon deposition resistance. The optimized catalyst, with support which had high specific surface area and the optimized amount of promoter (cerium and boron) has better resistance to carbon deposition and sintering.