Study Of Active Carbon Properties And Ruthenium Based Catalysts For Ammonia Synthesis

The industrial synthesis of ammonia is carried out at high pressure and temperature, which is connected with remarkable energy consumption. The promoted Ruthenium-based catalysts are expected to be second-generation catalysts for ammonia synthesis for its much more active at low pressure and temperature than fused iron catalysts. Active carbon (AC) as support seems to be particularly promising. In the early nineties, British Petroleum, Kellog and Engelhard group worked out an extremely active ruthenium catalyst, supported on thermally modified carbon (so-called high surface area graphite). The new catalyst was successfully introduces to the industrial practice. However, detailed data concerning the chemical composition, preparation procedure, and operating properties of the catalyst have not been disclosed so far. In this thesis, the properties and treatments of AC, catalysts preparation methods, promoters and operating properties of ruthenium-based ammonia catalysts supported on active carbon were studied and the catalyst activity has increased from about 15% to higher than 21%.1. The effect of surface characteristics of different activated carbons on the catalytic activities of ruthenium catalysts was studied. When the AC has low ash content, proper pH value and pore distribution, high surface area and total pore volume, the catalyst supported on this AC has high activity. Furthermore, oxygen-groups present in high temperature were good for ruthenium dispersion.2. There are two kinds of oxygen-groups on AC's surface. One is oxygen-groups present in low temperature, and the other is oxygen-groups present in high temperature. The former is harmful for catalyst activity, and the latter is good for catalyst activity because it maybe the adsorbing center of Ru3+. HNO3 treatment can reduce the ash content of AC and increase oxygen-groups on surface greatly. In addition, gas-oxidation of carbons after HNO3 treatment can change their textile and surface groups significantly, which is favorable for catalyst activity.3 The influence of preparation methods of catalysts on activities was examined. The results show that the impregnation sequence affects the catalytic activity obviously. Preferred impregnation sequence is Ba-Ru-K and the best drying temperature of Ba(NO3)2 is 110℃. Catalyst is more active if the precursor of ruthenium or promoter is impregnated in two times. Reduction of RuCl3 at 200℃ for 12 hours is a proper condition.4. At 10Mpa, l0000h-1, H2/N2=3 conditions, when reaction temperature is higher than 400℃, the ammonia concentration of reactor is close to equilibrium. When reaction temperature is lower than 400℃, catalyst activity increases with the increasing of ruthenium content.5. The effect of promoter on activity of ruthenium catalyst was studied. Results indicate that optimal molar ratio of alkaline earth metal to ruthenium is 1.4-1.5 and alkaline metal to ruthenium is 16. Ce or Sm add into the catalyst as third promoter can maximize catalyst activity but La Sr and Cs have the opposite effect.6. At last, operating conditions of ruthenium catalyst were studied. Results indicate that the ruthenium catalyst has high activity at low pressure and low GHSV. It is found that the best H2/N2 ratio is different with the change of reaction temperature and the effect of H2/N2 ratio on catalyst activity become more visible at high GHSV or low reaction temperature. So ruthenium catalyst can exert its advantage when it is used at low pressure, low temperature, low H2/N2 valure and high GHSV.