Lean NOx Catalyst (HC-SCR)

W. Addy Majewski

Abstract: Several types of catalysts have been known to promote the reduction of NOx with hydrocarbons under lean exhaust conditions—including copper exchanged zeolites, platinum/alumina, and silver/alumina catalysts. However, HC-SCR catalysts tend to have narrow operating temperature windows, resulting in a limited NOx reduction efficiency, and exhibit other problems. Active HC-SCR systems with HC enrichment must be used to achieve meaningful levels of NOx conversion efficiency. Lean NOx catalysts have found only limited commercial use.

Introduction
Selective Reduction of NOx by Hydrocarbons
HC-SCR Catalysts
HC-SCR Emission Systems
Diesel Engine Experience

Introduction

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 (HC-SCR), other exhaust gas components, including carbon monoxide, or alcohols [168]. These HC-SCR, alcohol-SCR, and related catalysts have been referred to by the customary—even if vague—term lean NOx catalysts (LNC).

In the diesel engine application, diesel fuel is 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 urea (or ammonia) tanks, catalyst research focused on that process producing numerous technical publications. In fact, selective reduction of NOx by hydrocarbons became the mainstream in NOx reduction catalyst research in the 1990s. The research work was not limited to diesels—the LNC technology was also considered for NOx reduction from lean-burn gasoline engines [5611].

Notwithstanding the vast amount of research, the lean NOx catalyst technology never matured to provide—in a durable manner—the levels of NOx reduction required by various stringent NOx emission standards. The major issues and unsolved problems include:

HC enrichment: Hydrocarbon levels in diesel exhaust are too low for any significant NOx reduction using simple, passive HC-SCR systems. HC enrichment is necessary, which may require additional hardware and penalizes fuel economy.
NOx conversion efficiency: Even with HC enrichment, the maximum conversion efficiency at realistic HC/NOx ratios and acceptable levels of fuel economy penalty is insufficient to meet stringent NOx emission standards.
Temperature window: The temperature window of known catalysts is too narrow and does not always correspond to the exhaust gas temperature range at which most NOx is emitted from the engine, particularly during real driving.
N₂O emissions: Some types of HC-SCR catalysts have low selectivity to N₂ and may produce high levels of N₂O emissions.
Hydrocarbon slip: Systems with HC enrichment produce HC slip. An additional DOC may be required to oxidize these secondary HC emissions.
Catalyst volume: High space velocity significantly reduces NOx conversion efficiency. Therefore, achieving high NOx reduction requires high HC-SCR catalyst volumes.
Catalyst durability: Many HC-SCR catalysts are poisoned by such species as SO₂ or heavy hydrocarbons, and/or suffer from insufficient hydro-thermal stability.

Selective Reduction of NOx by Hydrocarbons

Chemical Reactions

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

{HC} + NOx = N₂ + CO₂ + H₂O(1)

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

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

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

{HC} + O₂ = CO₂ + H₂O(2)

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

C_mH_n + (m + ¼n)O₂ = mCO₂ + ½nH₂O(2a)

HC-SCR 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 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 C_mH_n, its selectivity for the reaction (1a) is given by the following equation:

Selectivity = {C_NO·[NO]} / {(2m + ½n)·C_CmHn·[C_mH_n]}(3)

where:
[NO] and [C_mH_n] - inlet molar concentrations of NO and HC (actual, not as C₁)
C_NO and C_CmHn - 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 can produce a mixture of nitrogen compounds, including elemental nitrogen, nitrous oxide (N₂O), as well as ammonia (NH₃). If ammonia is formed, it can be oxidized over a downstream DOC or it can be utilized to further reduce NOx in a downstream passive NH₃-SCR catalyst.

Nitrous oxide, on the other hand, is an undesired product that, once formed, cannot be easily eliminated. 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:

N₂ Selectivity = 1 - 2[N₂O]_out/(C_NOx·[NOx])(4)

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

Passive and Active LNC Systems

The most attractive source of hydrocarbons for reducing NOx is the diesel exhaust gas itself. Systems utilizing exclusively the native diesel HCs are referred to as the passive LNC systems.

Passive systems, due to their simplicity, reliability, and lower cost, are always the preferred emission control option. However, due to the selective character of the reaction, HC-SCR catalysts show increasing NOx conversion rates with increasing hydrocarbon concentrations. The limited supply of native diesel hydrocarbons—often insufficient considering the NOx emission levels—may present a barrier in achieving higher NOx conversions, especially if the catalyst selectivity is low. Enrichment of the exhaust gases with additional HC material has been perceived as a solution to this problem. In general, such enrichment could be realized by two methods:

Injection of hydrocarbons (such as diesel fuel) into the exhaust system upstream of the catalyst, or
Late in-cylinder injection in a common rail fuel system (or late injection timing in mechanical fuel systems [435]).

Catalysts with HC enrichment are referred as active systems. The concept of passive and active HC-SCR catalyst systems is illustrated in Figure 1.

Figure 1. Passive and active HC-SCR catalyst configurations

Systems for injecting hydrocarbons into the exhaust gas can reliably provide the required quantity of hydrocarbons to the HC-SCR catalyst, but they add to the system complexity and cost. By using common rail post-injection, the cost of implementation of an active system can be fairly low. The biggest problem, in view of the continuous character of fuel enrichment, is cylinder wall wetting by the fuel spray and its impact on the lubrication oil film and engine wear [196]. Both types of HC enrichment involve a certain fuel economy penalty.

As shown in Figure 1, active systems typically require an additional oxidation catalyst to oxidize the hydrocarbon slip from the HC-SCR catalyst.

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