Selective catalytic reduction reactions

R. T. Yang, J. P. Chen, M. A. Buzanowski, and J. E. Cichanowicz", Catalyst Poisoning in the Selective Catalytic Reduction Reaction", 1989 Joint Symposium on Stationary Combustion Control, San Francisco, CA, 1989. 

Selective catalytic reduction is based on selective reactions of a continuous gaseous flow of ammonia or similar reducing agents with the exhaust stream in the presence of a catalyst. The reaction that occurs is as follows ... 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

NO, however, can only be removed by adding a reductant, ammonia, and using a catalyst. The process is called selective catalytic reduction, or SCR. The catalyst consists of vanadia and titania and works in the temperature interval 600-700 K according to the overall reaction ... 

Figure 10.17. Principle of selective catalytic reduction using, for example, urea or a solution of ammonia urea as the reducing agent for application of the SCR reaction on mobile diesel units such as ferries or trucks. (Courtesy of HaldorTopsoe A/S.)...

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

Long, R.Q. and Yang, R.T. (2002) Reaction mechanism of selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalyst, J. Catal., 207, 224. 

Brosius, R., Bazin, P., Thibault-Starzyk, F. et al., (2005) Operando FTIR study of reaction pathways of selective catalytic reduction of NOx with decane in the presence of water on iron-exchanged MFI-type zeolite, J. Catal., 234, 191. 

The simplest variant of the selective catalytic reduction of NO with NH3 is the standard-SCR reaction, in which NH3 and NO comproportionate in a 1 1 stoichiometry to nitrogen. This reaction is efficiently catalyzed with high activity and selectivity between 300 and 400°C by V205/W03-Ti02 catalysts, which are wide-spread in stationary SCR systems [1],... 

Mok, Y.S., Dors, M. and Mizerazcyk, J. (2004) Effect of reaction temperature on NOx removal and formation of ammonium nitrate in nonthermal plasma process combined with selective catalytic reduction, IEEE Trans. Plasma Sci. 32, 799-807. 

Catalytic reactions The performances of the catalysts in the selective catalytic reduction of NO by NH3 were evaluated as reported by Delahay and coll. [7]. 

Question 5 ("Is combustion with air the only chemistry intended at your facility ") can be answered YES in this case, assuming the "facility" being addressed is limited to the incinerator system. Due to the great number of combustion systems in operation, many other resources are available for ensuring safe design and operation of the combustion part of the incinerator facility. However, it should be noted that many combustors now have effluent treatment systems, such as selective catalytic reduction (SCR) systems, that involve intentional chemistry beyond the combustion reaction. 

Dendrimer templated Pt-Au catalysts are also active for the selective catalytic reduction of NO by propylene in the presence of excess O2. In addition to its commercial importance, this reaction is particularly interesting for the Pt-Au system. Previous work with cluster-derived Pt-Au catalysts has demonstrated that this reaction exhibits structure sensitivity, suggesting that it may be possible to use it as a structural probe for Pt based catalysts. ... 

The use of this enzyme in multi-step synthesis is relatively recent. Clapes et al. have reported the first example of FSA-mediated synthesis of iminocyclitols [53]. The synthetic strategy is similar to the one previously described for DHAP-dependent aldolases without the need for the dephosphorylation step. AldoUc reaction of DHA with N-Cbz-3-aminopropanal catalyzed by FSA followed by selective catalytic reductive aminahon furnishes the naturally occurring imino-sugar D-fagomine (Scheme 4.22). 

Selective Catalytic Reduction (SCR) process is very similar to SNCR with the exception that a catalyst is used to accelerate the reactions at lower temperatures allowing it to be applied to both full and partial-burn regenerators. An SCR system consists of a catalyst bed installed in the flue gas line of a combustion system. Ammonia is injected into the flue gas with air in the presence of a catalyst. The catalyst is typically oxide forms of vanadium and tungsten. The ammonia selectively reacts with NOx to form molecular nitrogen and water via an exothermic reaction that has achieved > 90% reduction in NO when applied to an FCCU. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

For all the oxidation reactions listed in Table 2, it was found that the specific oxide support had a significant effect on the oxidation reaction TOF. The methanol oxidation TOF was the most sensitive to the oxide support ( 104 factor), followed by butane oxidation 20,38 and selective catalytic reduction of NO by NH345 ( 102 factor), and oxidation of CO46 and S0247 ( 10 factor) were the least sensitive. The differences in order of magnitude of the support effect on the... 

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