Production of ammonia U.S. = 8.2%China = 28.4% Russia = 8.4% India= 8.6%
"I let the discovery of the ammoniasynthesis slip through my hands. It wasthe greatest blunder of my scientificcareer” Henry Le ChatelierWithin less than five years ofLe Chatelier’s accident, Haberstarted ammonia productionon large scale. The Nobel Prizein Chemistry 1918 was awardedto Fritz Haber "for the synthesisof ammonia from its elements" Haber and Bosch
2.1 Osmium:Haber tested osmium, ammonia yield= 8%.Under cyclic performance Os showsactivity equals to Ru.Prepared by depositing K2[Os3(CO)13] onSibunit.
Table 2.1 Ammonia concentration and rate for Os and Ru catalyst.Temperature = 250 to 400⁰C & Pressure = 1 atm
2.2 IronAlwin Mittasch discovered ironcatalyst in 1909.Magnetite from Sweden showed goodactivity. Alwin MittaschIron with alumina and potassiumyielded a catalyst.
2.1.1 Iron catalyst promoted by potassium. Table 2.2 Ammonia concentration & rate for iron catalystPressure = 1 atm.
2.2.2 Catalyst based on Fe l – X O (wustite)Highest activity, easiest reduction.Stronger mechanical strength.Low cost. Used in chinaPrepared by melting magnetite mixture and cooling.
Figure 2.1 Relative activities of the catalysts. Pressure, 15 MPa;space velocity, 30000 h-1. (1) A301 catalyst, (2) conventional catalystl, (3) conventional catalyst 2 (Liu et el., 1996).
2.2.3 Iron catalyst doped with Lithium oxide.Till 2009 Li2O is not an effective activator for Fe catalyst. Figure 2.2 The influence of promoters on the activity of fused iron catalyst for ammonia synthesis (Kuzniecov et al., 2009).
Figure 2.4 Relative activity of the catalysts measured at 450OC(Walerian et al., 2009).
2.3 RutheniumIn 1990 KAAP used ruthenium on graphite support.Higher surface area.Higher activity at low pressure.Prepared by subliming ruthenium-carbonyl onto carboncoated support which is impregnated by rubidium nitrate.
2.3.1 Ruthenium supported on carboncoated alumina.Graphite takes electron from alkali metal promoter andtransport them towards Ru.Prepared by pyrolysis of an alkene on γ-Al2O3– drying– at150⁰C u/vacuum for 8 h.Ru: Cs: support ratio was kept10:51:100.
Table 2.3 BET surface areas of supports end ammoniasynthesis yields of cesium-promoted Ru catalysts (RamaRao et el., 1990). Steady state NH3 BET surface area NH3 yieldCatalysts Supports conc. at 350⁰C (m2 g-1) [cm3 h-1 g Ru-1] [% (v/v)] 1 Carbon (SKT) 1350 0.005 2 2 Carbon (Subunit) 500 0.652 261 3 8% - Al2O3 220 0.670 268 4 24% - Al2O3 230 0.763 305Temperature = 350⁰C & Pressure = 1 atm.Thermodynamic equilibrium NH3 concentration at 350 C is 0.864%(v/v).
Figure 2.5 Effect of reaction temperature onthe steady-state concentrations of ammoniaover cesium promoted supported rutheniumcatalysts. Symbols:( ) catalyst 1: Cs-Ru/carbon (SKT),(ο) catalyst 2: Cs-Ru/carbon (Subunit),( ) catalyst 3: Cs-Ru/8% C-Al2O3 Also,(Δ) catalyst 4: Cs-Ru/24% C-Al2O3(Rama Rao et el., 1990).
2.3.2 Ruthenium catalyst based onsupported K2 [Ru4 (CO)13].The role of electron promoter in these catalysts isplayed by potassium.Replacement of ‘Sibunit’ carbon by usual commercialactive carbons resulted in a sharp decrease in activityand stability of the catalysts.
Table 2.4 Experiment results of ammonia synthesis over Ru supported on different supports (Yunusov et el., 1998).Enhancement in the electron density on ruthenium atoms occurswhich favours the effective dinitrogen activation and, as aconsequence, the efficient work of the catalyst
2.3.3 Ru-Ba/AC (Activated Carbon)Catalyst Promoted by Magnesium.MgO reduce the agglomeration of Ru at hightemperature, which increases available amount of Ru.Size of Ru improved, increase the Ru surface area.magnesium to Ru-Ba-K/AC significantly improved theutilization ratio of the noble metals and the performance-price ratio.
Table 2.5 Effect of Mg promoter on the activity of the Ru-Ba/ACcatalysts for ammonia synthesis (Jun et el., 2011).Reaction conditions: 10 MPa, *KOH content: 18%.
2.3.5 Iron catalyst and Ru catalyst inseries.Iron catalyst followed by Ru used in KAAP.Catalyst life increased.
Table 2.6. Ammonia concentration at different temperatures forRu catalyst and iron catalyst (Chonggenet et al., 2011).Reaction conditions: 10 MPa, 10000 h-1, N2+3H2.
2.4 CobaltHigh ammonia concentration corresponds to highconversion.Prepared by impregnation of cobalt nitrate and ceriumnitrate on AC, followed by evaporation and calcination.Inorganic material to carbon matrix ration is kept auto1.5:1.
Table 2.7 Activity of the promoted cobalt catalysts; T = 400⁰C, p =63 bar, gas (3H2+N2) flow rate = 70 dm3 [STP] h-1, mi = 0.4 gCo3O4(Raróg-Pilecka et el., 2007).a After the additional overheating in 3H2+N2 at 520⁰C for 150 h.
2.5 Photochemical synthesis of ammonia onMg/TiO2 catalyst systemDoping with metal ion improve absorption property ofcatalyst.Mg+-TiO2 enhanced photocatalytic effect compare to aloneTiO2Efficiency of catalyst depend upon doping level ofMg2+, mixing temperature & duration of heating.Yields of ammonia increase with increase in pH.
Figure 2.6 Variation of the ammonia yields with time atdifferent magnesium dopant levels (doping temperature500⁰C, doping time 2 h) (Ileperuma et al., 1990).
3. ConclusionOsmium showed activity for ammonia synthesis but not acceptedbecause of its drawbacks.Iron was the first successful catalyst used. Li2O showed goodresults.Ruthenium is the second generation o catalyst, showed goodactivity at low pressure.Cobalt is good option for replacement of Ru, less costly.Photocatalytic reactions are possible.