Catalytic DeNOx

Thus, special converters and advanced engine control techniques are necessary to meet the upcoming stringent NOx emission limits. Two types of catalytic deNOx systems for mobile applications—NSRC using periodic lean/rich operation and the SCR of NOx by NH3 (urea-SCR)—are discussed in Sections VI and VII, respectively. 

Cu-Exchanged Zeolites. Copper ions and/or complexes exchanged into such commercially manufactured zeolites as MFI, MOR, FAU, FER, BEA and so forth have been shown to be active for deNOx catalysis with HCs. Catalytic deNOx activity for this reaction can be maximized when combining copper with the MFI structure zeolites, representatively ZSM-5, depending mainly on the nature of reductant and physicochemical properties of the zeolite employed. These Cu-based zeolites reveal the peak NOx reduction activity at higher tern-... 

J B Infers, P Lodder, G.D Enoch, Modelling of selective catalytic denox reactors—strategy for replacing deactivated catalyst elements, Chem. Eng Technol. 74 192 (1991). 

The system is isothermal. This assumption is valid in an important projected BSR application, namely, catalytic deNOxing. 

Frache A, Palella B, Cadoni M et al (2002) Catalytic DeNOx activity of cobalt and copper ions in microporous MeALPO-34 and MeAPSO-34. Catalysis Today 75 359-365 Poignant F, Saussey J, Lavalley JC et al (1995) NH3 formation during the reduction of nitrogen monoxide by propane on H-Cu-ZSM-5 in excess oxygen. Journal of the Chemical Society, Chemical Communications 1 89-90... 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

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. 

It is finally easy to understand why this simultaneity of functions 2 and 3 can lead to a complete misunderstanding of the DeNOx reaction. As function 2 is producing RNOx compounds, simultaneously with function 3 which is producing N2, it is rather difficult to discriminate between the elementary steps leading to either RNOx or N2. The present chapter will demonstrate that, in lean conditions, RNOx leads to [CxH),Oz + NO] and N2 is formed in another catalytic cycle. 

Figure 5.4. Catalytic device for DeNOx reaction coupled with non-thermal plasma.

It has been chosen, for presenting the three-function model, to start from the true DeNOx catalytic cycle corresponding to function 3, which leads to the N—N bonding and N2 release from the catalyst. Subsequently, it appears that two other functions are necessary to assist function 3. 

The three cycles have to turn over in the same range of temperature. This catalytic approach of the DeNOx reaction is not new. There is the same process for isomerization of alkanes, where there are also 3 catalytic cycles which have to turn over simultaneously (bifunctional catalysis). The kinetics of isomerization is given by only one cycle, the other two turning over very rapidly and are near equilibrium [13]. 

Figure 5.18. Catalytic assisted DeNOx reaction each cycle corresponds to one function (FI, F2 and F3).

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. 

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. 

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. 

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