The Preparation And Characterization Of The Ruthenium Catalysts Supported On Active Carbon For Ammonia Synthesis

A serial of activated carbon(AC)-supported Ru catalysts for ammonia synthesis have been prepared by impregnation from aqueous solutions of RuCl3·3H2O precursor. N2-physisorption,CO chemisorption and X-ray diffraction were carried out to characterize the properties of supports and ruthenium catalysts.The ammonia synthesis activities of ruthenium catalysts and the methanation of AC supports were measured at a certain temperature and under a certain pressure.Particular attention was paid to study the effect of ruthenium metal,the promoter,the supports with different pretreatments and the catalytic preparation process on the catalytic properties of ruthenium catalyst for ammonia synthesis. The result showed that the thermal modification of the activated carbon can bring about different degrees of graphitization, but the surface area and pore volume decrease dramatically. But the surface area and pore volume can be recovered by oxidative treatment. It is found that high surface area is the main reason leading to high Ru dispersion and the pore structure of supports is important to high catalytic activity. The optimum results were obtained by oxidative pretreatment till about 15% weight lost of activated carbon. After the pretreatment of AC support by washing with a certain amount of HNO3, a developed mesopore structure of AC support and a great increase of AC surface carboxyl which led to the developed hydrophilicity of AC were found. These resulted in the higher dispersion of Ru and the higher activity of the catalysts by impregnation method using RuCl₃ aqueous solution. But the excessive HNO₃ pretreatment of AC support also led to the great increase of some unstable surface groups which were unfavorable for the catalysts activity and the dispersion of Ru. The optimum results were obtained by pretreated with 5mol/L HNO3. The dechlorination temperature of RuCl3 precursor have a certain impact on the Ru dispersion and ammonia synthesis activity of the Ru/AC ammonia synthesis catalysts.Barium and Potassium were effective promoters for Ru/AC ammonia synthesis catalysts. Barium-promoted Ru/AC catalyst was most effective in improving ammonia synthesis activity than Potassium-promoted Ru/AC catalyst. Furthermore, Barium decrease dramatically the methanation of the AC support and Potassium just on the contrary. The RuCl3 aqueous solution with PEG(400) was used as impregnated solution for Ru/AC ammonia synthesis catalysts. And it result in the higher Ru dispersion of Ru/AC catalysts.But the higher molecule weight PEG(PEG2000,PEG6000) were possible to botted the pore structure of activated carbon or to form the micelle due to its big volume and structural feature.And all these led to the lower Ru dispersion of Ru/AC catalysts. There was a limit of the quantity of surfactant used in the dispersion. This limit corresponds to CMC. CMC is the criterion for the determ-ination of the quantity of PEG400 used in the dispersion.