Stoichiometry ammonia oxidation

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 situation is different in the case of ammonia oxidation. Both on platinum (156) and nonplatinum (157) catalysts under the conditions of a commercial process, the reaction occurs in the external diffusion region. Diffusion of ammonia rather than of oxygen is determining the rate since the reaction is conducted with oxygen in excess with respect to stoichiometry, as given by (397). Concentration of ammonia at the surface of the catalyst is so small as compared to its concentration in the gas flow that the difference of concentrations that determines the rate of diffusion virtually coincides with the ammonia content in the flow. 

The ammonia molecules occupy trigonal prismatic sites between the dichalcogen layers and NMR measurements show that they are oriented with the threefold axis parallel to the dichalcogenide layers, indicating only weak Lone Pair interactions with the layers. Careful study of the reaction stoichiometry, prompted by this observation, led to the conclusion that ammonia oxidation was involved and that the overall mechanism of reaction conld be summarized by equation (11). The reaction product has x in the range 0.1 -0.3 and contains ammonium ions solvated by nentral ammonia molecules. 

Rare earth oxides are prepared from the hydroxides by calcination in air or by out-gassing at high temperatures. The hydroxides are obtained from aqueous nitrates by hydrolysis with aqueous ammonia. Except for those of three elements (Ce, Pr, and Tb), rare earth oxides thus prepared are stable in sesquioxide (M2O3) stoichiometry. The oxides of the three exceptions are stable in the nominal compositions, Ce02, PrbOii, and Tb40 . 

Some nitrate is also formed, thus the HOCl/NH stoichiometry is greater than theoretical, ie, - 1.7. This reaction, commonly called breakpoint chlorination, involves intermediate formation of unstable dichloramine and has been modeled kinetically (28). Hypobromous acid also oxidizes ammonia via the breakpoint reaction (29). The reaction is virtually quantitative in the presence of excess HOBr. In the case of chlorine, Htde or no decomposition of NH occurs until essentially complete conversion to monochloramine. In contrast, oxidation of NH commences immediately with HOBr because equihbrium concentrations of NH2Br and NHBr2 are formed initially. As a result, the typical hump in the breakpoint curve is much lower than in the case of chlorine. 

The oxidation of ammonia can produce nitric oxide, nitrous oxide and nitrogen according to the stoichiometries... 

For example, the works cited in the review [11] showed that the mentioned processes are very sensitive to the deviations of the component ratio Me(II) Me(III) in solution from the stoichiometry corresponding to the composition of the final product M"M" 204. When the composition deviates from stoichiometry, the co-precipitated hydroxides appear to be mechanical mixtures only. On contrary, when the composition exactly corresponds to the stoichiometry of the final product, chemical interaction occurs resulting in the formation of nano-sized X-ray amorphous product. This is evident by the data on the loss of chemical individuality of co-precipitated hydroxides. For example, in co-precipitated mixtures, like Cu(OH)2 - Me(OH)j, copper hydroxide becomes insoluble in ammonia and is not transformed into CuO under heating. For some oxides bound in this manner, the braking of dehydration is observed. X-ray amorphous product obtained by coprecipitation can be crystallized in the form of double hydroxide or even as a complex oxide. 

The SCR process consists of the reduction of NO (typically 95% NO and 5% NO2 v/v) with NH3. The reaction stoichiometry is usually represented as 4NO + 4NH3 + 02 4N2 + 6H2O. This reaction is selectively effected by the catalyst, since the direct oxidation of ammonia by oxygen is prevented In the case of the treatment of sulfur-containing gas streams, the DcNO reaction is accompanied by the catalytic oxidation of SO2 to SO3 Oxidation of SO2 is highly undesirable because SO3 is known to react with water and residual ammonia to form ammonium sulfates, which can damage the process equipment. 

Von Sacken and Dahn [89], using TG-MS, showed that below 523 K the thermal decomposition of AMV proceeds by simultaneous release of ammonia and water in a constant proportion, regardless of whether the atmosphere is inert or oxidizing. Above 523 K, residual ammonia reduces the solid products so that the stoichiometry of the final residue is dependent upon the sample size, gas flow, heating rate and reactor design. 

While this equation represents a close approximation of the stoichiometry of the process, the detailed chemistry is complex and poorly understood. The exotherm of the oxidation causes a temperature rise of about 70°C for each 1% of ammonia in the mix. Prewarming of one or both component streams is used to keep the alloy gauze at close to the optimum 900°C [40]. Operating at a gauze temperature of 500°C produces mostly unreactive nitrous oxide as the oxidation product (Eq. 11.36) which would be lost to acid production. 

Despite its brilliant results, it seems unlikely that the Solutia process can become a major source of phenol. Nitrous oxide availability is quite limited and its production on-purpose (by the conventional ammonium nitrate decomposition, which enables nitrous oxide of high purity to be produced for medical anesthetic applications, or even by selective oxidation of ammonia) would result too expensive. Therefore, the only reasonable scenario to exploit the Solutia process is its implementation close to adipic acid plants, where nitrous oxide is co-produced by the nitric oxidation of cyclohexanol-cyclohexanone mixtures and where it could be used to produce phenol instead of being disposed of However, the stoichiometry of the process is such that a relatively small phenol plant would require a world-scale adipic acid plant for its nitrous oxide supply. In fact, a pilot plant has been operated using this technology, but its commercialization has been postponed. 

Vanadium phosphates (VPO) of different structure are suitable precursors of veiy active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Va H2O is transformed into (V0)2P207 applied as the n-butane oxidation catalyst. Otherwise, if VOHPO4 V2 H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(V0)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(V0)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product ( 4.11 [7]) a second, mixed-valent (V 7v ) vanadium-rich phase must be formed. 

Table VI Ammonia—Nitric Oxide Reaction Stoichiometry ...

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