The Preparation And Application Of The Ruthenium Catalyst For Ammonia Synthesis At Low Temperature And Low Pressure

A serial of alkali- and/or alkali-earth-promoted, carbon-supported Ru catalysts for ammonia synthesis have been prepared by impregnation from acetone solutions of RuCl3·3H2O precursor. The article investigated the effect of support, Ru precursor, promoter and preparation process on catalytic properties of ruthenium catalyst for ammonia synthesis by means of various characterization methods performed by N2 physical adsorption, CO chemisorption and X－ray diffraction. Meanwhile, the application condition of Ru catalyst was investigated under near industrial reaction condition.It was suggested that activated carbon with higher purities, higher surface area and fitting pore size distribution as a support might disperse ruthenium and promoter sufficiently in Ru catalyst, which would manufacture high activity ruthenium for ammonia synthesis. The ammonia synthesis activity of Ru catalyst was greatly improved by using the activated carbon as support which was heated at 1800 ℃ in an inert atmosphere followed by moderate oxidizing treatment in 10 % O2/N2, which can make carbon support with better conductivity, higher purities and fitting pore structure for dispersing metal Ru.Reductive dechlorination of RuCl3/TCOX at lower temperature cannot remove negative-effected chlorine, but the methanation of carbon support is induced at extremely high dechlorination temperature and even lead to the sintering of Ru particles in catalyst. Activity results showed that a moderate dechlorination condition of 400～450 ℃ is advisable. Moreover, optimal ruthenium loading is 4～6 wt% when the ratio of performance and price of catalyst is considered. Promoting role of promoter is in line with electronegativity of compounds. Barium and cesium proved to be most effective in the alkaline-earth metal or alkaline metal promoter accordingly. But Barium-promoted Ru catalyst is more effective in ammonia synthesis than cesium-promoted Ru catalyst. The promoters can play a much promoting role for ruthenium catalysts, only when part of promoters contact with ruthenium effectively. The optimal promoter loading of Barium-promoted Ru catalyst or Cesium-promoted Ru catalyst is (Ba/Ru) mol = 5, (Cs/Ru) mol = 10 respectively. The reason why much more Cesium is needed than Barium for promoted-ruthenium catalysts is attributed to their different morphological models. The needed hydrogenolysis temperature of catalysts is higher than decompose temperature of promoter precursor, which can produce much more activity sites in Ru catalyst. However, the extremely high hydrogenolysis temperature can lead to the sintering of Ru particles in catalyst and the decreasing of Ru catalyst. The hydrogenolysis temperature of alkaline metal promoted catalyst or alkaline metal promoted catalyst is ought to be controlled at 470 ℃, 500 ℃ respectively.The activity of 4 wt% Ru-Ba-Cs/TCOX is higher obviously not only than 4 wt% Ru- (Ba+Cs)/TCOX, but also than 4 wt% Ru-Cs-Ba/TCOX. The barium promoters distribute uniformly on the ruthenium surface while the Cesium promoters mainly distribute at the C/Ru contact points to act as effective promoter. The Cesium promoter is considered to precede via electron transfer from the alkali to the active metal surface and barium promotion acts as a structural and/or electronic promoter. The catalytic activity of the supported Ru catalyst is 26～30 % higher than that of Fe catalyst under 425 ℃,13.0 MPa reaction condition.