Ammonia decomposition 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. 

Yin S F, Xu B Q, Zhou X P, Au C T (2004), A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications , AppZ. Catal. A, 111, 1-9. 

By combining such complexes with an ammonia decomposition catalyst (see above) one obtains a very versatile hydrogen source (see Fig. 6.40). Christensen et al. [243] have shown that the kinetics of ammonia adsorption and desorption are reversible and fast, even at moderate temperatures. 

Fig. 20. Proposed simplified diesel exhaust after-treatment system (2010). A diesel oxidation catalyst, wall-flow filter, selective catal5dic reduction with urea injection, and an ammonia decomposition catalyst. All catalysts are deposited on monoliths.

Above pH 9, decomposition of ozone to the reactive intermediate, HO, determines the kinetics of ammonia oxidation. Catalysts, such as WO, Pt, Pd, Ir, and Rh, promote the oxidation of dilute aqueous solutions of ammonia at 25°C, only two of the three oxygen atoms of ozone can react, whereas at 75°C, all three atoms react (42). The oxidation of ammonia by ozone depends not only on the pH of the system but also on the presence of other oxidizable species (39,43,44). Because the ozonation rate of organic materials in wastewater is much faster than that of ammonia, oxidation of ammonia does not occur in the presence of ozone-reactive organics. 

Hot spot formation witliin tlie reactor can result in catalyst breakdown or physical deterioration of tlie reactor vessel." If tlie endothermic cyanide reaction has ceased (e.g., because of poor catalyst performance), the reactor is likely to overheat. Iron is a decomposition catalyst for hydrogen cyanide and ammonia under the conditions present in the cyanide reactor, and e. posed iron surfaces in the reactor or reactor feed system can result in uncontrolled decomposition, which could in turn lead to an accidaital release by overheating and overpressure. 

Fig. 3 gives the conversions for acetic acid and ammonia decomposition over Ti02 and Al-Ti02 in a three-phase fluidized photoreactor. In the case of acetic acid decomposition (Inlet condition of 300 ppm), the conversion increased with alununum addition. In particular, the conversion to CO2 reached about 90% and then it was kept until 600 mins on Al-TiOa catalyst. On the other hand, in b), the anunonia removal (Inlet condition of 80ppm) also enhanced on Al-Ti02 compared to that conventional Ti02 catalyst the conversion to N2 reached above 95% in Al-Ti02. We have also observed that the ammonia conversion in a conventional batch type steady photoreactor could be obtained up to 70%. From this result, we could confirmed that... 

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]. 

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). 

The above procedures for catalyst preparation have generally provided excellent results. Especially important are surface-sensitive reactions. With supported catalysts in which the active components have a narrow particle-size distnbution, the optimum particle size for a demanding reaction can be established. Major improvements of supported catalysts, e.g. with respect to carbon deposition and ammonia decomposition, can be achieved by preparing catalysts with a narrow par-ticle-size distribution. Also, the preparation of catalysts in which the active components have a uniform chemical composition is highly important One instance is the preparation of supported vanadium oxide phosphorus oxide (VPO) catalysts for the selective oxidation of w-butane to maleic anhydride, which has been carried out using vanadium(III) deposition onto silica [31]... 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

Reaction (a) is measurable at about 275°C. The rate of decomposition of Hagg carbide, reaction (b), depends upon the amount of carbidic carbon present. For a completely carbided synthetic-ammonia-type catalyst the reaction, which proceeds approximately according to Equation (2),... 

Table 4.1.1 Illustration of the derivation of the elementary steps. Step 1 Adsorption of NH3 on catalyst surface step 2 first release of H2 step 3 N2-formation of one N atom from catalyst and one from former NH3 steps 4 and 5 filling of one vacancy with another N atom from NH3. Unchanged table adapted from [13], Reprinted from Applied Catalysis A General, Vol. 392, T. Otremba, N. Frenzel, M. Lerch, T. Ressler, and R. Schomacker, Kinetic studies on ammonia decomposition over zirconium oxynitride, 103-110, Copyright (2011), with permission from... 

Soerijanto H, Rodel C, Wild U, et al. The impact of nitrogen mobility on the activity of zirconium oxynitride catalysts for ammonia decomposition. J Catal. 2007 250(1) 19-24. 

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