Study On La,Ba Modified Nickel-Al₂O₃ Catalysts Used For Partial Oxidation Of Methane

Ni/Al₂O₃ catalysts exhibit much high activity for the partial oxidation of methane, its initial activity is comparable to noble metal catalysts. In order to inhibit the formation of NiAl₂O₄, Ni/Al₂O₃ prepared by conventional impregnation is mostly calcinated at low temperature. The weak interaction between carrier and Ni will lead the aggregation and sintering, which result in the coke formation and deactivation of catalysts at high temperature. The conversion of crystal of Al₂O₃ is another reason for deactivation. In this paper, in light of principal problems mentioned above we have carried on research in preparation method and components of catalysts, we drow the following conclusions. For the first place, we use Na₂CO₃-NaOH as solvent to precipitate mental ions. The results of XRD and BET reveal that this method has better ability of controlling content and distributing of components than impregnation. The catalysts prepared by coprecipitation method have many characters such as small partical,high dispersion and strong interaction between antive component and carrier. The second place, we add La(NO3)3.6H2O,Ba(NO3)2 into this system respectivily. The results of BET and XRD reveal that the adding of La(NO3)3.6H2O can inhibit the formation of NiAl2O 4 in large size when the catalyst is calcinated at high temperature, it maintains a certain proportion of active components to insure high activity for POM. The adding of Ba(NO3)2 is unfavorable to enhance the SBET of catalysts, but it has better ability to inhibit the formation of NiAl₂O₄ in large size than the adding of La(NO3)3.6H2O, It also has high activity for POM. At the same time the adding of La(NO3)3.6H2O,Ba(NO3)2 enhances the temperature that the crystal type of Al₂O₃ changes. The third place, after calcinated at higher temperature than 800℃, there is a strong interaction between Ni and carrier to inhibit the coking reaction for which the stability of catalysts is improved. We drow the conclusion from the results of activity test and TPR that the non-stoichiometric nickel alumina is the major active component for POM . At 800℃, the XCH4,SCO,SH2 of 20%Ni-based Al₂O₃ modified by La,Ba which are prepared by coprecipitate method are 100%,95.95%,99.87% and 100%,92.19%,98.05%. The activities of the two catalysts hardly decline in 15h. At last we discuss the mechanism of POM according to the experimental data and results of analysis. We believe that there is key temperature that the mechanism of POM changes, the temperature is affected by the temperature of calcination, reduction and starting reaction and the content of Ni of the catalyst and so on.