Ammonia oxidation kinetics

Ammonia Oxidation Kinetics in a High Temperature Flow Reactor , Univ California, Berkeley UCB-TS-71-6, AFOSR (1971)... 

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. 

The first observation of sensitivity-stability was reported by Liljenroth (1918) in connection with the autothermal operation of ammonia oxidation reactors. Papers of Damkdhler (1937) and Wagner (1945) went unnoticed. At Union Carbide Corp. Perkins (1938) used zero order kinetics to define a safe range for ethylene oxidation in an unpublished report. His result,... 

Kf., while not quantified experimentally, was recently introduced by Kirby and coworkers on the basis of product formation from O-attack at electrophilic P and C centers, as well as MO calculations incorporating the novel species, ammonia oxide, NH3+-0 . In common with other ambident nucleophiles, factors such as electronic, steric, kinetic and thermodynamic effects will determine actual extant pathways in a given system. Af-substituted hydroxylamines (see Scheme 1) can in principle partake of the equilibria shown in Scheme 2. Again, actual outcomes will be influenced by the aforementioned criteria. 

To describe the NH3 + NO + O2 (standard SCR) reacting system, NH3 adsorption-desorption, ammonia oxidation to nitrogen and standard SCR have been considered with the kinetics already presented in the previous section. 

To describe the NH3 + NO/NO2 reaction system over a wide range of temperatures and NO2 NOxfeed ratios in addition to ammonia adsorption-desorption, ammonia oxidation and standard SCR reaction with the associated kinetics already discussed in Section 2.3.2, the following reactions and kinetics have been considered by Chatterjee and co-workers [79] ... 

Products of the reaction have been identified as ethylenediamine, formaldehyde, formic acid, and ammonia. A kinetic evaluation of rate experiments indicates that for each cobalt (II) ion oxidized either one molecule of ethylenediamine or one molecule of ammonia appears. 

The reaction of ammonia with oxygen over V-based catalysts produces mainly nitrogen, according to the stoichiometry of R5 in Table V. Analogously to the case of the ammonia adsorption-desorption, specific runs were carried out in order to extract the intrinsic kinetics of ammonia oxidation and at the same time to validate the previously fitted kinetics of the ammonia adsorption-desorption process. 

The kinetic parameters of ammonia oxidation were fitted by multiresponse nonlinear regression, while the parameter estimates for the ammonia adsorption-desorption kinetics were kept unchanged with respect to those obtained from the fit in the previous section. Notably, in this case both the NH3 and the N2 outlet concentrations were regarded as regression responses. 

The reactions of adsorption desorption of NH3 and ammonia oxidation to N2 were considered with the kinetic expressions shown in Section V.A.2.b. 

To observe ammonia oxidation on platinum in the kinetic region, the reaction must be carried out at low pressures since the decrease in pressure increases the diffusion coefficient and slows down the reaction, both factors favoring the transition to the kinetic region. 

Ammonia oxidation under low pressures, as a method of transferring this reaction into the kinetic region, is inapplicable in the case of the Co304 catalyst since, at such temperatures as 700°C and low 02 pressures, Co304 decomposes with the formation of CoO. In order to obtain information on the kinetic of NH3 oxidation on Co304, we studied limiting loads at which the catalyst is extinguished (166, 167). The experiments were performed at pressures from 1 to 9 atm. A catalyst pellet was placed in a vertical tube of a diameter such that the cross-section occupied by the pellet comprised one-half of the cross-section of the tube. This was an imitation of conditions in the bed of pellets. The gas mixture at the inlet was at room temperature the stream of the mixture was directed downward. 

Table 16.5 Kinetic parameters of ammonia oxidation on chromium carbides...

Webley PA, Tester JW, Holgate HR. Oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water in the temperature range 530-700°C at 246 bar. Ind Eng Chem Res 1991 30 1745-1754. 

Webley PA, Tester JW, Holgate HR. Oxidation kinetics of ammonia and am-... 

The kinetics of the ammonia oxidation reaction are limited by the mass transfer of the chemical species (NH3, 02, NO and H20) to and from the vicinity of the catalyst surface. If the catalyst area available to reactants is higher than the minimum area permitting the reaction, the mass transfer limits the kinetics. 

Apparently at a temperature above 300 C, the oxidation kinetics of NOx and ammonia gas is so fast that slip of the reactants, when fed from the opposite sides of an alumina or alumina-titania membrane, can be avoided. Vanadium oxide, used as the catalyst for the reaction, is impregnated onto the membrane pore surface. The conversion of NOx reach 70% with the selectivity for nitrogen up to 75% in the temperature range of 300 to 350 C [Zaspalis etal., I991d]. 

Since ammonia does not react with oxygen at room temperature, studies of its oxidation kinetics can be divided into two categories those thermally initiated, and those photochemically initiated. Therefore, an important question which shall be borne in mind during this discussion of the oxidation of the nitrogen-hydrogen compounds is whether O2 reacts at all with ammonia, or whether all the experimental results can be explained by the attack of O2 on the free radical intermediates produced either photochemically or thermally. 

At high pressures the heat evolved is sufficient for adiabatic operation of reactors. Since the normal operating temperatures of industrial converters lie between 800 and 1100°C and pressures between 1 and 10 atm, homogeneous gas-phase reactions may contribute to the observed product distribution. The initiation temperatures for homogeneous oxidation are of course a function of pressure, so that in the laboratory the catalytic reaction may be isolated by working at low pressures under molecular beam conditions. In this way the reaction kinetics can be observed over very wide temperature ranges (usually up to 1200 °C). Studies of ammonia oxidation carried out in this way range from 0.1 to 10 Torr total pressure. ... 

The most recent work on ammonia oxidation by Gland and Korchak is different in that they have used a Pt(lll) stepped single-crystal face (as used in adsorption studies) at much lower pressures (10 —10 Torr) and related kinetic data to quantitative surface composition data obtained by AES their maximum temperature was 700 °C. Under these reaction conditions LEED proved that the Pt(S)-[12(l 11) x (111)] surface was stable throughout the work. The results are of high quality and extremely revealing. 

By contrast, there is no doubt or any contradictions in literature concerning the importance of macro-kinetic factors (heat- and mass-transfer processes) in the reactions discussed in this section. It is generally realized that these processes proceed in spatially distributed systems and generate sharp gradients of parameters (temperature, pressure, density, concentrations, etc). It could be noted here a distinct similarity of alkane oxidation over Pt-group metals at short contact times and well studied and practically implemented ammonia oxidation and cyanic acid synthesis reactions, in which transfer processes play a dominant role (see Satterfield, 1970). 

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

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