Nitrogen oxide , catalytic decomposition

Removal of NOx from stack gas presents some formidable problems. Possible approaches to NOx removal are catalytic decomposition of nitrogen oxides, catalytic reduction of nitrogen oxides, and sorption of NOx by liquids or solids. 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

The exceptional activity exhibited by ion-exchanged copper ZSM-5 zeolite catalysts for nitric oxide (NO) decomposition, and for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the presence of excess oxygen is well documented [1-10]. The nature of the active copper species in the SCR reaction however still remains uncertain. We and others have recognised that there are two different types of copper species within the ZSM-5 zeolite channels [11]. Isolated copper ions exist in low symmetry environments, and small clusters, where the copper atoms are linked by extra-lattice oxygen species such as [Cu(II)-0-Cu(n)] dimers, are also present. Recent studies have also suggested that the isolated copper ions in ZSM-5 occupy two types of sites [11], which may have different SCR reactivity. It is likely... 

The decomposition of the thermodynamically unstable nitrogen oxides is in theory the best way to eliminate them from contaminated gas streams. N2O is relatively easy to decompose catalytically while NO is much harder. In fact, the NO transformation is always limited by the presence of oxygen over all the catalysts assayed so far. 

Another example for the check of the stoichiometric consistency is the catalytic decomposition of nitrogen oxide (NO) with hydrogen (H2) over alumina supported Rli monolith. Besides the main reaction, the formation of nitrous oxide (N2O) and self-decomposition also take place. The overall reactions are given below 2N0+2H2=>N2+H20, 2NO+H2=>N20+H20 2N20=>2N2+02. The system consists of three linearly independent reactions. The vector of chemical symbols is a = [ A NO 02 N20 H2 ] and consequently, the stoichiometric matrix becomes... 

Figure 10.20. Primary experimental data in the catalytic decomposition of nitrogen oxide (NO) with hydrogen.

The most efficient method for NO removal from stationary and mobile sources is catalytic reduction with ammonia, hydrocarbons, CO or H2. Modified zeolites are active catalysts in these processes. For Cu-ZSM-5 especially high activity and stability have been reported. In this work the properties of copper-containing ZSM-5 zeolites prepared by wet or solid state ion-exchange have been investigated. The Bronsted acidity of the Cu -exchanged samples was much lower than that of the parent zeolites, and they had high activity in selective reduction with ammonia, propene or propane. A comparison of Cu-ZSM-5 activity in the decomposition of NO and in the reaction of NO with propene or propane revealed that the hydrocarbons as well as the nitrogen oxides play important role in the performance of NO reduction catalysis. 

The catalytic reduction or decomposition of nitrogen oxides, particularly NO to N2, is very important from the viewpoint of environmental air pollution. The selective reduction by hydrocarbons in the presence of air has been extensively studied as a potential process in NO emission control for Diesel and lean-bum engines (see certain recent publications [61]). 

Figure 9.57 perfectly illustrates the amelioration brought to the sensor in the matter of carbon monoxide selective detection. The rhodium acts here as a catalytic agent furthering the decomposition of the nitrogen oxides. 

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