Lean NOx Catalyst

W. Addy Majewski

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Abstract: Two major groups of catalysts are known for the reduction of NOx with hydrocarbons: a copper substituted zeolite ZSM-5 catalyst, which is active at high temperatures, and a platinum/alumina catalyst, exhibiting low temperature activity. Both catalysts have narrow operating temperature windows, resulting in a limited NOx reduction efficiency, and exhibit other problems. Some lean NOx catalysts have been commercialized, primarily to provide small DeNOx functionality in diesel oxidation catalysts.

Selective Reduction of NOx by Hydrocarbons

Chemical Reactions

After it became apparent that NO decomposition catalysts had too many shortcomings to produce a robust, commercial catalyst system [164], research turned towards selective reduction of NOx by compounds of combustion gases. It was discovered that several catalysts promoted selective catalytic reduction of NOx by hydrocarbons or other exhaust gas components, including carbon monoxide, or alcohols [168]. Catalysts selectively promoting the reduction of NOx by hydrocarbons have been termed “lean NOx catalysts” (LNC) or “DeNOx catalysts”. We use these expressions as synonyms, but the scope of the term “DeNOx” is not consistent in the literature, with some authors using it also in reference to other types of NOx reduction catalysts.

NOx reduction by HC was found to be less susceptible to sulfur poisoning than NO decomposition and higher conversion efficiencies were demonstrated. In the case of the diesel application, diesel fuel was the obvious source of hydrocarbons necessary for the reaction. Since the enrichment of exhaust gases with diesel fuel seemed more straightforward than carrying on-vehicle ammonia vessels, catalyst research focused on that process producing numerous technical publications. In fact, selective NOx reduction by hydrocarbons became the mainstream in NOx reduction catalyst research over the period of early- to mid-1990s.

In the selective catalytic reduction, hydrocarbons react with NOx, rather than with O2, to form nitrogen, CO2, and water.

(1) {HC} + NOx = N2 + CO2 + H2O

Assuming that a single hydrocarbon species of the formula CmHn reacts with nitric oxide, the above equation can be written as a properly balanced chemical reaction, as follows:

(1a) CmHn + (2m + ½n)NO = (m + ¼n)N2 + mCO2 + ½nH2O

The competitive, non-selective reaction with oxygen is given by the general Equation (2),

(2) {HC} + O2 = CO2 + H2O

or, in the case of the hydrocarbon CmHn, by reaction (2a):

(2a) CmHn + (m + ¼n)O2 = mCO2 + ½nH2O

DeNOx catalysts have to be optimized to selectively promote the desired reaction (1) with hydrocarbons over the undesired reaction (2) with oxygen. The catalyst selectivity is determined by the catalyst formulation, but it also depends on a number of other factors, such as the hydrocarbon species used for the reaction, temperature, exhaust gas oxygen content, and the HC/NOx ratio.

The selectivity of a reactant X for reduction of NOx is defined as the ratio of the number of moles of X which react with NOx to the total number of moles of X consumed. In the case of the hydrocarbon CmHn, its selectivity for the reaction (1a) is given by the following equation:

(3) Selectivity = {CNO·[NO]} / {(2m + ½n)·CCmHn·[CmHn]}

where:
[NO] and [CmHn] - inlet molar concentrations of NO and HC (actual, not as C1)
CNO and CCmHn - fractional conversions of NO and hydrocarbons.

Equations (1) and (1a) present the ideal reaction path, with the formation of elemental nitrogen, N2, as the only nitrogen-containing compound. In practice, selective reduction of NOx by hydrocarbons produces a mixture of nitrogen and nitrous oxide, N2O, which is an undesired product. The proportion of N2O depends, among other parameters, on the catalyst formulation. Catalysts may be characterized by their selectivity towards the formation of nitrogen, as opposed to nitrous oxide. This selectivity, defined as the molar ratio of nitrogen to nitrous oxide that were produced in the reaction, is given by the following equation:

(4) N2 Selectivity = 1 - 2[N2O]out/(CNOx·[NOx])

where:
[N2O]out - outlet molar concentration of N2O (assuming zero at the inlet)
[NOx] - inlet molar concentration of NOx (NO + NO2)
CNOx - fractional conversions of NOx (NO + NO2)

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