Ammonia oxidation at

The ammonia oxidation reaction proceeds in the first part of the catalyst bed [Fig. 16(a)]. This part is subsequently deactivated, mainly by nitrogen species. The high activity of the catalyst is maintained due to the movement of the reaction front to the next positions in the catalyst bed. When [ Nj-NH3 is injected at the moment that the reaction was already 20 seconds on-stream, labelled N species adsorb further on in the catalyst bed. Thus, in time to come, the deactivation front moves to the end of the catalyst bed. When this front reaches the end of the bed, the catalyst is covered with reaction species and the deactivation is observed in the concentration of the products. An experiment with half an amount of the catalyst also supports this reaction front movement. This experiment showed the formation and concentration of the products in the same manner, however, the catalyst remained active for half the time of the normally applied catalyst bed. Thus, below 413 K, the catalyst remains initially active because the reaction zone moves to the next bed positions, after the previous positions became fully covered with the adsorbed reaction species. Injection of a [ N]-NH3 or [ 0]-02 pulse after the initial deactivation, confirmed that the platinum surface is fully covered and that conversion of ammonia and oxygen is low. No significant amount of nitrogen or oxygen species remains adsorbed at the catalyst surface. 

The atomic oxygen reacts further with NHx abstracting hydrogen. Two possible routes for the OH involvement in the formation of N2 are proposed, either from NH or NH2. At low temperature, the reaction of NH with OH to form water is favoured with a reaction energy of —11 kcal/mol.I l The reaction energy of the two step reaction of NH2 (via NH) with 

The third nitrogen peak with a maximmn at 483 K most probably due to an NO(a) intermediate already formed at much lower temperatures. A relatively high surface coverage promotes the NO formation, but the desorption of NO is slow at these relatively low temperatures. 

The most important results from the TPO experiment are the evolution of one N2 peak, together with one H2O peak and additional NO evolution at higher temperature (Fig. 18). Nitrogen is formed at 383K, followed by the water production with a peak maximum at 403 K. The production of N2O is clearly much higher than in the TPD experiment. This can be explained by the reaction of NH with 0(a) or OH(a), which gives atomic nitrogen and 

02(g) will adsorb and dissociate on vacant sites that becomes available. This will lead to production of NO(a) 

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It has been reported that titanium supported vanadium catalyst is active for ammonia oxidation at temperatures above 523 K [2,3]. Also, supported vanadium oxides are known to be efficient catalyst for the catalytic reduction of nitrogen oxides (NO ) in the presence of ammonia [4]. This work investigates the nanostructured vanadia/Ti02 for low temperature catalytic remediation of ammonia in air. 

Ammonia oxidation was a prototype system, but subsequently a number of other oxidation reactions were investigated by surface spectroscopies and high-resolution electron energy loss spectroscopy XPS and HREELS. In the case of ammonia oxidation at a Cu(110) surface, the reaction was studied under experimental conditions which simulated a catalytic reaction, albeit at low... 

Figure 5.2 Oxygen states present at the ends of -Cu-O-Cu-O- chains are established as the active sites in ammonia oxidation at Cu(110) from a Monte Carlo simulation of the growth of the oxygen adlayer. The reactivity (the experimental curve) is best fitted to the atoms present at chain ends. (Reproduced from Ref. 7).

In view of the spectroscopic evidence available, particularly from coadsorption studies (see Chapter 2), ammonia oxidation at Cu(110) became the most thoroughly studied catalytic oxidation reaction by STM. However, a feature of the early STM studies was the absence of in situ chemical information. This was a serious limitation in the development of STM for the study of the chemistry of surface reactions. What, then, have we learnt regarding oxygen transient states providing low-energy pathways in oxidation catalysis ... 

Heat and Mass Transfer.—As already stated both processes operate adiabatically, the catalyst temperature being maintained by the heat of reaction. The steady state temperature reached depends mainly on the pressure and flow regime. Oele has calculated that for ammonia oxidation at 1 atm the convective heat transfer accounts for 95% of the heat losses. Values found for exit gas temperatures differ by only 20 °C from the calculated values. 

Fig. 15. Ammonia oxidation at 373 K until the catalyst is deactivated followed by a temperature programmed reaction. TP-reaction part is only shown (lOK/min, GHSV = 5000hr i, NH3/O2 = 2/1.5, flow = 46.5cm /min.).

Fig. 17. Formation of N2, N2O, and H2O measured by oniine mass spectrometry in a temperature programmed desorption experiment after ammonia oxidation at 323 K (lOK/min, He flow of 40cm /min).

A basic problem for the production of nitric acid from ammonia can be found in [1,7]. However, typically dual processes have become preferred in order to take advantage of the physicochemical principles so that it is possible to enhance the conversion of ammonia oxidation at low pressure and improve the gases absorption downstream at a higher pressure. The problem that we address is presented as follows. The production of HNO3 fro ammonia starts with atmospheric air. The ratio between oxygen and ammonia to be fed to the converter must be 2.11, and the... 

In a typical process, approximately stoichiometric quantities of propylene, ammonia and oxygen (as air) are fed into a reactor containing a fluidized catalyst, such as a bismuth, molybdenum or uranium containing compound. The reaction is conducted at 400-500°C and 1-3 atmospheres. The exit gases are scrubbed with water and acrylonitrile is obtained from the aqueous solution by a series of distillations. In a variation of this process, propylene is treated with nitric oxide (which may be regarded as a product of ammonia oxidation) at about 700°C in the presence of a silver catalyst ... 

As discussed in Section 6.4 for ammonia oxidation at a single Pt wire, that is, where the cylindrical wire is heated by an exothermic chemical reaction, the variation of temperature around a cylinder can nowadays be modeled by computer programs, for example, by the finite element method. The geometric structure is approximated by a meshing procedure that is used to define and break the model up into small elements. The differential equations of heat transfer and of the fluid dynamics (Navier-Stokes equations) are then numerically solved. The temperature gradients at the surface of the cylinder (Tcyi = const. = at... 

The NH3 into NO conversion efficiency increases with decreasing pressure, whereas the conversion of NO into NO2 and the subsequent absorption is favored by high pressures. Thus, modern nitric acid piants are duai pressure processes, that is, the product gas of ammonia oxidation (at 6 bar) is compressed to 12 bar and then fed to the absorption tower for NO oxidation and for NO2 absorption. 

An example of structure sensitivity in deactivation is found in the work of Ostermaier et al. (3A) for 2-15 nm Pt/Al203 and Pt black in ammonia oxidation at 368-473 K. The effects of crystallite size and temperature in deactivation were investigated it was found that the extent of deactivation increased with decreasing temperature and there was a difference in the Arrhenius behavior between sintered and unsintered materials. Deactivation was more severe with smaller crystallites, but the surface could be completely reactivated by H2 at 673 K. It was suggested that PtO was the deactivated surface, and an excellent correlation of activity was provided by ... 

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