Ammonia oxidation mechanism

Data, not reported, indicated that ammonia does not appreciably adsorb onto the PGM catalyst. However, literature reviews on the ammonia oxidation mechanism over PGM catalysts unanimously consider the adsorption of both reactant molecules (NH3 and O2) [16], with ammonia adsorbing in on-top position on Pt [17]. Indeed, the lack of a detectable storage capacity for ammonia during our runs is not sufficient to mle out the adsorption of the same species on the catalyst surface accordingly, notwithstanding the experimental evidence, ammonia adsorption/desorption steps were included in the developed kinetic model (R.16 and R.17 in Table 18.2). 

The oxidation of NO to NO2, which is an important step in the manufacture of nitric acid by the ammonia-oxidation process, is an unusual reaction in having an observed third-order rate constant (k o in ( rm) = kso Oc02) which decreases with increase in temperature. Show how the order and sign of temperature dependence could be accounted for by a simple mechanism which involves the formation of (NO)2 in a rapidly established equihbrium, followed by a relatively slow bimolecular reaction of (NO)2 with O2 to form NO2. 

Another copper catalyst, prepared by treating a NaY zeolite with copper nitrate, for ammonia oxidation (160—185°C) has been studied by Williamson et al. [349], The reaction is first order in NH3 and zero order in oxygen. The mechanism here is based on a Cu(II)(NH3)4+ complex formed in the large cavities of the zeolite. The rate-determining step is the reduction of Cu(II) by ammonia. 

The enc cluster contains four genes (encH, encl, encJ, encP) involved in the biosynthesis of the unusual benzoyl-CoA (185) starter from phenylalanine (186) via a plant-like p-oxidation mechanism (Fig. 32) [208-210]. This pathway is initiated by the unique phenylalanine ammonia-lyase EncP [211], which catalyzes the generation of cinnamic acid (187) from 186. The cinnamate-CoA ligase EncH... 

If water contains ammonia, hypobromous acid reacts with it to finally yield a monobromamine, and bromate formation is reduced. In systems where hydroxyl radicals are not scavenged by organic compounds, bromate can be formed in high concentration. In Table 11, a scheme of the 03/Br oxidation mechanism is presented. 

In the presence of an adequate amount of water, aliphatic amines are generally dealkylated by anodic oxidation. llius, a tertiary amine is successively dealkylated to a secondary amine, a primary amine and finally to ammonia, llie mechanism involves initial removal of one electron ftom the lone pair electrons of nitrogen leading to a cation radical, though a variety of mechanisms have been proposed depending on the structures of the amines and the reaction conditions. 

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. 

The electrochemistry of amino acids has been studied in strong acid solutions. In general, the compounds are decomposed to carboxylic acids, aldehydes, ammonia, and carbon dioxide. The results are reviewed by Weinberg [35]. The anodic oxidation mechanism has been studied in pH 10 buffer solution. Decarboxylation accompanied by C-N bond cleavage is the main reaction process [182]. The synthetically interesting Hofer-Moest decarboxylations of A/ -protected amino acids and a-amino malonic half esters under the formation of A/ -acyliminium ions is treated in the following section. 

In this work, a Cu0/Ti02 catalyst (10wt% CuO, specific surface area = 120 m /g) was studied in NH3 oxidation. The catalyst was also characterized by NH3 TPD technique and FT-IR spectroscopy in order to obtain information about the nature of active sites for SCO and the occurrenece of competitve adsorption of water on such sites. A mechanism of ammonia oxidation is proposed. 

As previously said, water does not inhibit the formation of species I at high temperatures (where species I can be formed by decomposition of species III), and this agrees with the absence of any detectable effects of water in the formation of NO by ammonia oxidation. According to the above mechanism, it is also expected that the reaction NO + NH3 will not be affected by the presence of H2O. In previous work it was found that water inhibits the SCR reaction on metal oxide based catalysts [2,16], but to a lower extent in comparison with ammonia oxidation. However, the SCR reaction occurs at lower temperatures with respect to ammonia oxidation to NO, so that at these conditions species III and species I are still in competition. 

Fig. 1.7. A schematic presentation of the mechanisms by which the ammonia-oxidizing bacteria acquire energy and biosynthesize organic compounds...

Fig. 3.3. A scheme presenting the oxidation mechanism of ammonia to nitrous acid by Nitro-somonas europaea (prepared mainly on the basis of Lees, 1952 Aleem, 1966 Yamanaka and Shinra, 1974 Yamanaka and Sakano, 1980 Suzuki and Kwok, 1981 Andersson and Hooper, 1983 Yamazaki et al., 1985 Igarashi et al., 1997). Dashes with arrows, unverified Cyt, cytochrome Pi, phosphate...

Although the oxidation mechanism of nitrite to nitrate in the heterotrophic nitrifiers has not been known at all on the enzyme level, the oxidation mechanism of ammonia to nitrite has been partially clarified. Ammonia is oxidized to nitrite through hydroxylamine also in the heterotrophic bacteria. The oxidation of ammonia to hydroxylamine is catalyzed by ammonia monooxygenase as in the enzyme of Nitrosomonas europaea. The enzyme purified from Paracoccus pantotropha GB17 (formerly Thiosphaera pantotropha GB17 or Paracoccus denitrificans GB17) catalyzes the oxidation of ammonia to hydroxylamine and contains copper, but its activity is not inhibited by acetylene (Moir et al., 1996), unlike the enzyme of Nitrosomonas europaea. 

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

The mechanism of oxidative deamination foiiows a pathway simiiar to that of N-deaikyiation. initiaiiy, oxidation to the imminium ion occurs, foiiowed by decomposition to the carbonyi metaboiite and ammonia. Oxidative deamination can occur with a-substituted amines, exempiified by amphetamine (Fig. 10.13). Disubstitution of the a-oarbon inhibits deamination (e.g., phentermine). Some secondary and tertiary amines as weii as amines substituted with buiky groups can undergo deamination directiy, without N-deaikyiation (e.g., fenfiuramine). Apparentiy, this behavior is associated with increased iipid soiubiiity. 

The starting point in development of an ammonia flame mechanism was a mechanism previously used to model ammonia oxidation in a flow tube near 1300 K ( ). Additional reactions were added that were thought to be important at the higher flame temperatures. Calculations with this mechanism produced profiles in marked disagreement with our data. The predictions were slower than observed decay of NH species was much too slow, and OH peaked too late by about 2.5 mm. To make matters worse, far too much NO was formed. The NO problem was especially troublesome in that attempts to increase the rate of NH decay only served to produce even more NO, since NO was the primary decay channel for the NHi species. A possible resolution of this dilemma involves reactions of the NHi species with each other to form N-N bonds. These complexes could then split off H atoms to ultimately form N2. 

The PEP technique enables one to measure concentration profiles of labelled molecules (reactants and/or products) within reactors as a function of axial position and time. Once these profiles have been obtained, they provide substantial information on the reaction kinetics and involved intermediates. We will illustrate this with our research on the understanding of the mechanism of the ammonia oxidation on platinum based catalysts. 

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