Formation, ammonia oxidation over

The temperature dependence of the conversion and product formation in the ammonia oxidation over a platinum sponge catalyst is investigated. Positron emission profiling experiments demonstrate that below 413K, the catalyst deactivates due to the poisoning of the catalyst surface, mainly by... 

At such a high degree of complexity of modern exhaust after treatment systems, modeling and simulation of the catalyst performances play an important role as part of the total system simulation in the automotive development process. The processes occurring on the SCR catalysts are already well understood and modeled [6-12], whereas this is not the case for the ASC, for which only a few literature surveys exist [2, 3, 13-15]. Scheuer et al. [3] presented a mechanistic kinetic model for ammonia oxidation over a PGM catalyst. Such a model was derived from previous literature works [16, 17] and includes the following reactions NH3, O2 and NO adsorption/desorption from the catalytic sites, NH3 activation and N2, NO and N2O formation, with the last three species being the main NH3 oxidation products. The model consists thus of seven reactions that are assumed to proceed... 

The reaction of the ammoxidation of toluene has also been investigated. Ammonia adsorbs over the V205/Ti02 monolayer catalysts in two forms, as ammonium ions over Brpnsted acidic sites as well as in coordinated molecular form over Lewis acid sites. Benzylamine appears as an intermediate, likely formed through the reaction of amide species, which is a common intermediate in ammonia oxidation over oxide catalysts and benzyl species. Benzylamine is further oxidatively dehydrogenated to benzonitrile, well evident in the IR spectra. A later IR study of the same reaction performed over V-P oxides showed similar intermediates but suggested that, over these catalysts, the main reaction path consists of the formation of benzaldehyde and benzimine. ... 

Ammonia oxidation reactions (4-6) are not desired because they imply the consumption of ammonia and result in a net reversal of the removal of NO and in the formation of N2O as a by-product. Although these reactions are observed over the SCR catalysts in the absence of NO in the feed, they become negligible in the presence of NO. The ability to react selectively with NO in excess oxygen has not been observed in the case of other simple reagents such as carbon monoxide and hydrocarbons. This motivates the choice of ammonia as the unique reducing agent in the SCR process. 

The ammonia oxidation reaction, which is catalyzed by a Pt + complex, proceeds via the rapid formation of a Pt(NH3)3NO complex. This complex is generated by the oxidation of Pt(NH3) in which an H+ is accepted by a basic lattice oxygen atom in the zeolite and H2O is generated. The zeolite lattice oxygen atom takes over the role of the basic ligands in organometaUic chemistry or the role of the proton-accepting water molecules. 

Isotopic experiments on a Ba-Na-Y zeolite catalyst [15] have shown that the formation of N2 takes one nitrogen atom from NOx and another nitrogen atom from ammonia. One important reaction for ammonia-SCR over Fe-zeolite is the NO oxidation to NO2, because the presence of NO2 can significantly improve the SCR performance, especially at low temperature [25, 39 1]. The reaction rate of the ammonia-SCR can be described by the following simple power law ... 

This chapter is a review of the state of the art in kinetic modeling of ammonia/urea SCR over copper containing zeolites. Both fundamental detailed kinetic models as well as more globalized models are discussed. Several submodels are studied for the SCR system (i) ammonia adsorption and desorption, (ii) NO2 adsorption and desorption, (iii) water adsorption and desorption, (iv) ammonia oxidation, (v) NO oxidation, (vi) standard SCR, (vii) rapid SCR, (viii) slow NO2 SCR, (ix) N2O formation, and (x) urea decomposition and hydrolysis to produce ammonia. As can be seen from this large number of steps, this is a complex system. 

At temperatures below 250 °C, Sjbvall et al. [133] observed a beneficial effect of oxygen on the activity, contrary to higher temperatures. Ammonia slip is also affected by temperature. For instance, equal amounts of nitrogen oxides and ammonia are required at 175 °C. In fact, over Cu-ZSM-5 the NOx conversion is achieved by the reaction between NO2 and adsorbed NH3. At higher temperature, ammonia oxidation occurs. However, if exposing the catalyst to equimolecular amounts of NO and NO2 increases the NOx conversion, N2O formation is furthermore observed. 

Industrial fertilizer synthesis starts from ammonia synthesis, and ammonia is then easily oxidized in a separate reactor to nitric oxide over PtRh wire gauze catalyst. Formation of nitric acid requires further oxidation of nitric oxide to nitrogen dioxide (NO2) and absorption of the nitrogen dioxide in water. Overall, three different chemical process plants are used for the synthesis of nitric acid. The ammonia synthesis reaction takes place in a high-tem-perature, high-pressure reactor that requires recycling of products due to the thermodynamic limitations of chanical conversion. The ammonia oxidation reaction is very fast and takes place over a very small reactor length. Finally, nitric acid synthesis takes place in absorption columns. 

Addition of a base, such as ammonia, to propane-oxygen mixture (propane ammoxidation over Ga/H-ZSM-5) has a significant effect by enhancing the formation of valuable products (propene, acetonitrile, and acrylonitrile) over total combustion (Table 13.14) [105]. However, acetonitrile (C2 molecule) prevails over acrylonitrile (C3 molecule) in a similar manner as acetic prevails over acrylic acid in propane oxidation over HFGs. The increase in Si/Al ratio, meaning the reduction of the number of Brpnsted acid sites, benefits the selectivity to propene and acrylonitrile at the expense of acetonitrile. COx selectivities are not sensitive either to change in acidity or Ga content, which could suggest that those are... 

Thermal oxidation is another alternative for destroying cyanide. Thermal destruction of cyanide can be accomplished through either high-temperature hydrolysis or combustion. At temperatures between 140°C and 200°C and a pH of 8, cyanide hydrolyzes quite rapidly to produce formate and ammonia.23 Pressures up to 100 bar are required, but the process can effectively treat waste streams over a wide concentration range and is applicable to both rinsewater and concentrated solutions22 ... 

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