Platinum ammonia decomposition

Ammonia selectivity of platinum and platinum-nickel catalysts for NOx reduction varies with the nature of the supporting oxide. Silica, alumina, and silica-alumina supports on monolithic substrates were studied using synthetic automotive exhaust mixtures at 427°-593°C. The findings are explained by a mechanism whereby the reaction of nitric oxide with adsorbed ammonia is in competition with ammonia desorption. The ease of this desorption is affected by the chemistry of the support. Ammonia decomposition is not an important reaction on these catalysts when water vapor is present. 

We explored the mechanistic implications of the apparent importance of acidity to ammonia selectivity, beginning with the assumption that ammonia is an intermediate in the reduction of nitric oxide over precious metal catalysts. (Attempts to explain our observations on the alternative assumption that chemisorbed isocyanate is the active intermediate (7) were fruitless since nickel should not change isocyanate s behavior. However, an acidic surface would probably lower isocyanate stability.) Since the total conversion of NO increased with nickel addition, it is likely that some conversion takes place on the nickel, probably by the reaction of NO with hydrogen spilled over from the platinum. Disappearance of this ammonia could occur by ammonia decomposition (Reaction 1) as hy-... 

We also observed, with nickel-promoted palladium catalysts not reported on here, that ammonia decomposition was poisoned by CO as well as water. If the same occurs on platinum, then the high NH3 formation at high CO levels could result from competition of NO and CO for the active sites. We believe that this happens with our platinum-nickel catalysts although measurements of this inhibition were obscured by reactions which consumed CO. 

The first attempt to use the simultaneous solution approach for ammonia synthesis over iron catalysts gave estimated reaction rates several orders of magnitude lower than those found in practice. This is discussed in the chapter on iron kinetics, but can be attributed to inadequacies in the underlying database, in particular the use of low-coverage adsorption data. The closest approach to a mechanistically acceptable model has come from studies into ammonia decomposition on platinum. " The characteristics of this reaction over many transition metals are ... 

Ammonia decomposition over Fe, Cu, Ag, Au, and Pt Hydrolysis of starch to glucose catalyzed by acids Mixture of coal gas and air makes a platinum wire white hot Measurements on the rate of H2O2 decomposition Selective oxidation of ethanol to acetic acid over platinum Comprehensive paper on the H2 + O2 reaction on platinum foils, including reaction rates, deactivation, reactivation, and poisoning Definition of catalysis, catalyst, and catalytic force First quantitative analysis of reaction rates Systematic studies on the concentration dependence of reaction rates First concise monograph on chemical kinetics Definition of order of reaction Arrhenius equation k = u exp (-Ea/RT)... 

Heating metallic lithium in a stream of gaseous ammonia gives lithium amide [7782-89-0] LiNH2, which may also be prepared from Hquid ammonia and lithium in the presence of platinum black. Amides of the alkaH metals can be prepared by double-decomposition reactions in Hquid ammonia. For example... 

The decomposition of ammonia on platinum has a rate expression of this form. The reaction is first order in ammonia and inverse first order in hydrogen. 

Hinshelwood and Burk [J. Chem. Soc., 127 (1105), 1925] have investigated the decomposition of ammonia on tungsten and platinum... 

At low pressures the rate of decomposition of ammonia on a hot platinum wire is proportional to the partial pressure of ammonia and inversely proportional to the partial pressure of hydrogen. Some total pressure data were obtained at 1348 C (Schwab Schmidt, 2 physik Chem B 3 344, 1929). Check the proposed rate relations. 

Nitric oxide is commercially produced by the catalytic oxidation of ammonia using a platinum catalyst 4NH3(g) + 50 —> 4NO(g) + 6H20(g). Nitrous oxide is produced by the thermal decomposition of ammonium nitrate at approximately 240°C NH4N03(g) —> N O + 2H . ... 

This limiting case is almost realized in the decomposition of ammonia on the surface of a heated platinum wire, in the region of 1,000° C. In the first few instants the reaction velocity appears to be extremely high, but falls off as soon as the smallest traces of hydrogen make their appearance ... 

The equation expressing the retarding action of the products is not always of such a simple form. While the decomposition of ammonia on platinum is well enough represented by the simple equation... 

The decomposition of ammonia on the surface of platinum takes place at a speed which is inversely proportional to the pressure of the hydrogen present. The combined influence of the two terms E and A produces an apparent heat of activation of more than 100,000 calories. This is in striking contrast with the value of E true, 39,000 calories for the unretarded reaction on the surface of tungsten. The decomposition of ammonia on molybdenum is of zero order, but retarded by nitrogen the value of E according to Burk is 53,200 calories. Kunsman finds 32,000 calories Trans. Faraday Soc., 1922, 17, 621. 

Another instance of the same curious phenomenon can be cited. Hydrogen has a most pronounced inhibiting effect on the thermal decomposition of ammonia at the surface of a heated platinum wire. On the other hand, it is found to be almost without influence on the decomposition of hydrogen iodide on the surface of the same wire, Compt. Rend., 1922, 175, 277. 

When arsine is passed over a heated metal, such as the alkali and alkaline earth metals, zinc or tin, the decomposition of the gas is accelerated and the arsenide of the metal is formed. If platinum is used, the removal of arsenic from the gas is complete.3 The action of sodium or potassium on arsine in liquid ammonia yields 4 the dihydrogen arsenide (MHgAs). Heated alkali hydroxides in the solid form quickly decompose the gas, forming arsenites, and at higher temperatures arsenates and arsenides of the metals.5 The aqueous and alcoholic solutions have no appreciable action.6 When the gas is passed over heated calcium oxide the amount of decomposition is not more than that due to the action of heat alone. Heated barium oxide, however, is converted into a dark brown mixture of barium arsenite and arsenate, hydrogen being liberated.7 The gas is absorbed by soda-lime.8... 

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