Selective catalytic reduction operating conditions

At present the most effective available after-treatment techniques for NO, removal under lean conditions are ammonia selective catalytic reduction (SCR) [1-3] and NO, storage reduction (NSR) [4—6]. Indeed, three-way catalysts (TWCs) are not able to reduce NO, in the presence of excess oxygen, because they must be operated at air/ fuel ratios close to the stoichiometric value. Also, non-thermal plasma (NTP) and hydrocarbon-selective catalytic reduction (HC-SCR) are considered, although they are still far from practical applications. 

Diesel engines and some gasoline-fuelled engines operate under lean-burn conditions, where there is 10-15% more oxygen than is needed to burn all the fuel.8,9 The most important strategies for NO,c removal under these conditions are to use either (i) NO, storage-reduction catalysts or (ii) selective catalytic reduction (SCR) by an added reductant the term selective implies that the reductant attacks the NO, in preference to the oxygen. 

Van Swaaij and coworkers [5,6] proposed the use of nonpermselective macroporous membranes for gas phase reactions, which by their nature require strict stoichiometric conditions. Such reactions include selective catalytic reduction of NO by NH3, and the Clauss reaction. The principle for the CNMR operation is shown in Fig. 11.7. By creating a sharp reaction front within the membrane one avoids the slip by either reactant (NH3 or NO, SO2 or H2S) on either side of the membrane. Small perturbations in the feed could be accommodated in principle by a shift in the reaction front within the membrane. The success of the CNMR concept depends, as one would expect, on the sharpness of the reaction front created within the membrane. For a non-instantaneous reaction the front created is rather diffuse (see Fig. 11.7) and there is, as a result, a reactant slip. 

In the introduction we mentioned that the two applications for the catalytic methane combustion were in power generation and in the abatement of methane emissions in engine exhausts. In the first application, the amount of NO produced is in fact very small because the presence of a catalyst reduces the operating temperature. However, in the second type of application, the amounts of NO present can be substantial (e.g. several thousand ppm). In addition, the so-called selective catalytic reduction of NO can be effected by using methane as a reductant. In this case, the methane combustion takes place simultaneously with the NO reduction. Therefore, it is important to understand how the presence of NO affects the methane combustion under typical exhaust conditions. In the presence of NO, methane can undergo two different reactions, the reduction of NO and the total oxidation ... 

The method results in less NO reduction than selective catalytic reduction, although higher consumption of chemicals is required. There are now eight commercial installations (1700 MWe) in operation in the FRG and Austria. The first circulating fluidised bed boiler to be equipped with selective non-catalytic reduction began operating in the USA in 1988. This has been followed by other similar installations in the USA. Selective non catalytic reduction is expected to reduce NO by 30-50%. Higher reduction, up to 70-80%, is supposed to be attainable under favourable conditions. 

These examples and many others have provided evidence of significant changes in catalyst structures resulting from changes in operating conditions. Techniques are thus necessary that can be applied to catalysts in the presence of probe molecules, in reactive environments (e.g., when catalysts undergo reduction, oxidation, etc.), and under catalytic reaction conditions. Moreover, the simultaneous determination of catalyst structure and activity or selectivity is needed to establish structure-activity or structure-selectivity relationships, which provide a basis for improvement and development of catalysts (Banares, 2005 Thomas, 1980 Thomas, 1999 Topsoe, 2000 Wachs, 2005). The need for characterization of catalysts during... 

The selective catalytic hydrogenation of 2-butyne-l,4-diol to ciJ-2-butene-l,4-diol over palladiiun represents a common situation involving a multifimctional conpound. Fortunately, the reduction of triple bonds is very selective over palladium [1,2]. Butenediol is an important chemical intermediate due to its use in the production of several insecticides and pharmaceuticals (i.e. endosulfan and vitamin B6). The use of supported palladium [3-8], Raney nickel [9] or nickel [10] catalyst has been reported for this reaction under relatively mild operating conditions. However, due to the possibility of several side reactions [3-4], the problem of selectivity towards c/r-butenediol becomes important. Over Pd/C, it was reported... 

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