Selective reduction of NOX with hydrocarbons

Burch, R., Breen, J. and Meunier, F. (2002) A Review of the Selective Reduction of NOx with Hydrocarbons under Lean-Burn Conditions with Non-Zeolitic Oxide and Platinum Group Metal Catalysts, Appl. Catal. B Environ., 39, 283. 

It is well known that Cu-HZSM5 zeolites are much better catalysts for the direct decomposition of NO than other catalysts based on different zeolites. However, the lack of large differences in the catalytic performances of the two Cu-loaded zeolites investigated must be related to the specific features of the selective reduction of NOx with hydrocarbons, especially in the real conditions of the engine exhaust gas. Moreover the activity exhibited by the H form of ZSM5 and Y zeolites in the selective reduction of NOx [6, 13] requires the role of the transition metal to be understood, in comparison to that played in the direct decomposition of NO [2, 6,14]. 

Breen, J.P. and Burch, R. (2006) A review of the effect of the addition of hydrogen in the selective catalytic reduction of NOx with hydrocarbons on silver catalysts. Top. Catal, 39, 53-58. 

Traa Y, Burger B and Weitkamp J 1999 Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons Mioroporous Mesoporous Mater. 30 3-41... 

The activity of the CoAPOs catalysts in the oxidation of NO to NO2 [2 NO + O2 2 NO2] was primarily evaluated in view of the possible applications in the reduction of NOx with hydrocarbons or CO [28-30]. NO2 is known, in fact, to be intermediate in the NO selective reduction with hydrocarbons on copper-exchanged zeolites [31-33]. 

The reoxidation of the reduced copper phase was accomplished by temperature programmed oxydafion (TPO). All the studied catalysts could be almost completely reoxidized to CuO, as indicated by the values of oxidation-percent calculated (>60 %). Quite the same TPO profiles were observed on all the catalysts with two oxidation steps at. first, oxidation of surface copper species and at higher temperature, bulk oxidation. The effective redox properties of CuO allow it to find application in several reaction processes demanding cyclic reduction and reoxidation process with the reactants (i.e., selective catalytic reduction of NOx with hydrocarbons as reducing agent, [20-22]). 

The NOx reduction for the exhaust from lean-burn engine is one of the greatest challenges in environmental protection, and a lot of researchers have strived to develop more effective catalysts by many ways. Their efforts could be possibly categorized into four approaches (1) NOx direct decomposition, (2) selective catalytic reduction on NOx with hydrocarbons (HC SCR),... 

Metals other than A1 have been successfully introduced in numerous zeolite frameworks. Aluminum substitution by other metals, such as Fe, Ga, Zn, Co or Cu in the zeolite framework results in modified acidity, and subsequently modified catalytic activity, for certain reactions such as selective catalytic reduction of NOx by hydrocarbons. For example, a calorimetric and IR spectroscopic study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [92]. The active sites for both N2O and CO are Cu (I) ions, which are present on the surface as a result of the pre-treatment in vacuum at 673 K. The amounts of chemisorbed species adsorbed by the investigated systems and the values of the differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mof ) and carbon monoxide (between 140 and 40 kJ mol ) demonstrate the dependence of the adsorption properties on the copper content. 

In order to elucidate the reasons for the dependence of the catalytic properties of these samples on their preparation method, we studied the acid surface properties of cobalt- and chromium-modified Zr02 catalysts by ammonia thermoprogrammed desorption and IR-spectroscopy. Our results again indicated that the activity of these catalysts in the SCR of NOx by hydrocarbons is a function both of the surface acidity and content of the active metal. The acid site concentration of the starting Zr02 samples prepared by various methods is significant (0.13 and 0.23 mmol/g) but these samples are inactive, while 10% CriOilZtOi prepared by the sol-gel method displays considerable activity in the reaction studied with lower surface acidity. The acid site concentration of the sample with the same composition prepared by the precipitation method is reduced by a factor of 2.5 and, thus, this catalyst has much lower activity in the selective catalytic reduction. 

Selective catalytic reduction (SCR) of NOx employing hydrocarbons (HC s) as reduct ants constitutes a promising catalytic approach in dealing with... 

Examples of reactions of industrial and environmental importance that run according MvK mechanism are alcohol selective oxidations, VOC total oxidations, selective catalytic reductions of NOx by ammonia or hydrocarbons, and more recently photocatalytic oxidation of hydrocarbons [5] etc. Revisitation of MvK mechanism for various reactions appeared in Ref. [6] For each one of these reactions, a metal oxide-based catalyst with well-developed redox properties has been optimized. 

In the selective catalytic reduction, the hydrocarbon selectively reacts with NOx rather than with O2 to form nitrogen, C02 and water. Traa et al. (54) have classified all the mechanisms proposed in the SCR of NOx by HC over TM-zeolites within three groups ... 

A great effort is underway to develop reliable aftertreatment systems for lowering NOx emissions from diesel and LB engines. A variety of approaches have been proposed for NOx aftertreatment of advanced vehicles including lean NOx catalysts (LNC), NOx storage and reduction (NSR) catalysts, selective catalytic reduction with urea (urea-SCR), and plasma-assisted catalysis (PAC). Lean NOx catalysts are mainly designed to reduce NOx with unburned hydrocarbons already included in the exhaust stream in the presence of O2 but result in... 

The third strategy for minimizing NOx, known as posttreatment, involves removing NOx from the exhaust gases after the NOx has already been formed in the combustion chamber. Two of the most common methods of posttreatment are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).7 Wet techniques for posttreatment include oxidation/absorption, oxidation/absorption/reduc-tion, absorption/oxidation, and absorption/reduction. Dry techniques for posttreatment, besides SCR and SNCR, include activated carbon beds, electron beam radiation, and reaction with hydrocarbons. 

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