Ammonia, decomposition synthesis

The breadth of reactions catalyzed by cobalt compounds is large. Some types of reactions are hydrotreating petroleum (qv), hydrogenation, dehydrogenation, hydrodenitrification, hydrodesulfurization, selective oxidations, ammonoxidations, complete oxidations, hydroformylations, polymerizations, selective decompositions, ammonia (qv) synthesis, and fluorocarbon synthesis (see Fluorine compounds, organic). 

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

Ammonia Synthesis, Ammonia Decomposition and Hydrazine Decomposition... 

Equation (305) describes the ammonia synthesis rate not only on iron catalysts, but also over molybdenum catalyst (105), tungsten (106), cobalt (95), nickel (96), and other metals (107). Equation (300) describes ammonia decomposition on various metals (provided that there is enough H2 in the gas phase). 

Data on the rate of synthesis or decomposition of ammonia on a number of metals give activation energies of ammonia decomposition, E, close to 40 kcal/mol, as in the case of iron catalysts, and m = 0.5 (107). 

Because the decomposition rates are relatively insensitive to temperature, Eq. (18.3) is operated at 130°C to 150°C and 3.0 MPa132 to speed up the ratedetermining step. Excess ammonia, at a ratio of 40 1, is used to minimize the hydrazine-chlorine decomposition. Synthesis efficiency favors a dilute system although the increase in operating cost due to the low concentration may ultimately become inhibiting. The Raschig process is shown in Figure 18.1132. 

Figure 14. The rate constants of ammonia decomposition (A) on and the ammonia synthesis capacities (B) of metals as a function of-A // J. (Mol. denotes molecule, mol denotes mole)...

Ammonia decomposition, on the other hand, may be carried out under more favorable conditions. Stoichiometry favors low pressure, 0 normal atmospheric-pressure equipment is sufficient. Equilibrium yields increase with temperalun and kinetic rates are measured with precision. This is why ammonia decomposition, which is less interesting, has historically received so much attention in the search for improved synthesis catalysts. ... 

Boisen A, Dahl S, Nprskov J K, Christensen C H (2005), Why the optimal ammonia synthesis catalyst is not the optimal ammonia decomposition catalyst , J. Catal., 230, 309-312. 

The carbides and nitrides were also found to be active in ammonia decomposition. The activities of vanadium carbides were 1-2 orders of magnitude lesser than that of Mo nitride catalyst, but 1-2 orders of magnitude higher than that of a Pt/C catalyst (138). Vanadium carbides were 1 order of magnitude more active than vanadium nitrides. Both the synthesis and decomposition of NH3 were found to be structure-sensitive reactions over carbide and nitride catalysts with activities depending strongly on the particle sizes of the catalysts. 

It is obvious that osmium and iron are the most effective elements under the conditions studied by Haber. On the other hand, ruthenium is the most active metal in ammonia decomposition. Since the two reactions are forward and backward steps of the same reaction, the most active metals should be the same, at least near equilibrium. The difference disclosed above is probably caused by some discrepancy in the reaction conditions. The ratio of H2/NH3 in the ammonia synthesis is much higher than that in the ammonia decomposition. Adsorbed N2 and H2 usually become inhibiting species, which depend on the ratio of H2/NH3 ratio. 

Li ZL. Ammonia Synthesis on Ru Supported on Carbon Nanotubes and TAP Study of Ammonia Decomposition, Dissertation, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 2004. 

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