DeNOx Reduction

In this configuration, the role of the noble metal is essential, firstly to oxidize NO in NO2 and secondly to reduce NO2 to N2 [18], From an economical point of view, the use of a strong amount of noble metals into the fuel consumption is not in fevor of this technology and also formation of ammonia can be observed in some conditions [33-35]. 

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Thus zeolite ZSM-5 can be grown (ref. 15) onto a stainless steel metal gauze as shown in Figure 6. Presumably the zeolite crystals are chemically bonded to the (chromium-) oxide surface layer of the gauze. After template removal by calcination and ion exchange with Cu(II) a structured catalyst is obtained with excellent performance (ref. 15) in DeNOx reactions using ammonia as the reductant. 

Abstract A review is provided on the contribution of modern surface-science studies to the understanding of the kinetics of DeNOx catalytic processes. A brief overview of the knowledge available on the adsorption of the nitrogen oxide reactants, with specific emphasis on NO, is provided first. A presentation of the measurements of NO, reduction kinetics carried out on well-characterized model system and on their implications on practical catalytic processes follows. Focus is placed on isothermal measurements using either molecular beams or atmospheric pressure environments. That discussion is then complemented with a review of the published research on the identification of the key reaction intermediates and on the determination of the nature of the active sites under realistic conditions. The link between surface-science studies and molecular computational modeling such as DFT calculations, and, more generally, the relevance of the studies performed under ultra-high vacuum to more realistic conditions, is also discussed. 

This question has to be answered to completely understand the DeNOx process, and design the final efficient catalyst, according to the nature of reductant and the experimental conditions (more particularly, temperature window). 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

Figure 5.1. The three-function model for designing DeNOx catalysts in the presence of methane as reductant [12].

If the preceding requirements are fulfilled, then the DeNOx process (function 3) does not need a large amount of reductant, as it is very often claimed the stoichiometry of 2NO -t-C H CX, = N2 + xCO/CO, + v/2 H20 should be considered. Clearly, it is generally impossible to avoid the competition between the Oads left by NO and the Oads due to 02 dissociation, for the total Q II/l. oxidation on function 3 (this competition corresponds to a kinetic coupling of at least two catalytic cycles, through Oads [13]). Both of them contribute to the total oxidation of reductants. 

I] Prediction of the temperature where DeNOx can take place, by TPD of NO preadsorbed with or without oxygen. In the absence of oxygen, check the formation of N2, N20, N02 and NO during TPD. If N20 and/or N2 are formed, it means that the reaction already took place. If not, the system needs the reductant to take place. This experiment also means that function 3 can work [10,25],... 

The second DeNOx technology, the selective catalytic reduction with ammonia (SCR-NH3) commercially available in heavy-duty vehicles since 2006, seems to present an interesting potential in terms of efficiency, reliability, HC penalties, etc. 

Koebel, M. and Elsener, M. (1998) Selective Catalytic Reduction of NO over Commercial DeNOx-Catalysts Comparison of the Measured and Calculated Performance, Ind. Eng. Chem. Res., 37, 327. 

Reductive elimination and storage of dilute NOx has been an important technology to achieve cleaner exhaust gas from engines. At present, Pt, Pd and Rh are often used for the so-called three-way catalyst. The demand for a high-performance DeNOx catalyst with low cost and non-harmful materials is growing as the environmental issues expand worldwide. 

The Possibility of Modifying Thermal DeNOx Chemistry to suit Diesel Engines The Use of Other Radicals NH2 is not the only kind of radical known to be capable of reducing NO. Hydrogen abstraction from HNCO can form the free radical NCO which is known to react rapidly with NO, reducing it to N20. The initial report of the RAPRENOx process8 claimed that HNCO was capable of reducing NO at temperatures as low as 592°C,but subsequent work showed that reduction of NO by HNCO produces much more N20 than does the reduction with Nt and that the two processes have essentially identical temperature windows. Since the reaction sequence... 

The first of these requirements is obviously a problem. The Thermal DeNOx reaction is exothermic but only weakly so. If we assume that 600 ppm NO is reduced with 900 ppm NH3 and 1800 ppm H2, the rest of the exhaust gas having a composition of 10% 02, 6.9% C02, 6.2% H20, and balance nitrogen, the heat released by the reduction of the NO will provide a temperature increase of 42.5°F. While it is possible to raise the temperature of the exhaust gas by burning more fuel, the amount of fuel this requires is substantial. For an engine using a diesel fuel with an H/C ratio of 1.8, and a heat of formation of -5.5 kcal/mole of C, the additional fuel consumption needed to raise the exhaust gas temperature from 942.5°F to 1300°F, is 14.9% of the primary fuel consumption. 

Another material, DENOX , was introduced commercially in 1997 and is, contrary to the XNOx , a single function NOx reduction additive (no CO oxidation activity) that decreases NOx by more than 50%. The field results are summarized in Figure 5.27. 

The SCR of NOx by NH3 is the best control technology but a new breakthrough would be achieved in power plants by the SCR of NOx using methane as reductant. Regarding deNOx from mobile sources, new concepts are appearing, and NOx trap and plasma-assisted catalytic reduction seem promising. 

The efficiency of the above catalysts for NO reduction depends definitely on the kind of metals and their loadings onto supports, the type of reductants and the feed gas composition employed as well as on the kinds of supports and structure of the parent zeolite and its historical nature during preparation. In particular, the effect of the presence of H2O and SO2 in the exhaust gas from mobile sources is well documented on the maintenance of time-on-stream deNOx activity of SCR catalysts, and their resistance to these co-existing gases is an essential parameter determining successful applications to engine sources. The durability of the documented catalysts under hydrothermal conditions should also be considered to verify if those were applicable to controlling vehicle NOx... 

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