Browse Publications Technical Papers 2000-01-2962
2000-10-16

An Assessment of the Plasma Assisted Catalytic Reactor (PACR) Approach to Lean NOx Abatement: The Relative Reducibility of NO and NO2 using #2 Diesel fuel as the Reductant 2000-01-2962

The plasma assisted catalytic reactor (PACR) approach to lean NOx abatement is a two step process. The non-thermal plasma oxidizes the engine out NO to NO2, which is then reduced to N2 over a catalyst using a hydrocarbon reductant. Whereas it was once believed that the plasma itself directly reduces NOx to N2, it has been shown that the plasma's principle function is to oxidize NO to NO2. This is accomplished without oxidizing SO2 to SO3, resulting in lower sulfate particulate when compared to standard lean NOx catalysis using platinum or reducible oxide catalysts. We have performed reactor studies comparing the relative reducibility of NO2 and NO in a synthetic diesel exhaust using diesel fuel as the hydrocarbon reductant, with attention to time-on stream behavior and determination of NOx reversibly adsorbed on the catalyst. We find that at 200°C, 50% of the NO2 disappearance over Na-ZSM5 is attributable to reversible adsorption on the catalyst. For a 200 M2/g γ-alumina catalyst, 15% of the NOx disappearance is due to adsorption, with 85% of the NOx disappearance attributable to NO2 conversion to N2. However, the γ-alumina catalyst has limited utility. With increasing time on stream, diesel fuel adsorption on the γ-alumina decreases the catalyst activity, and at temperatures above 220°C, NOx reduction activity over γ-alumina for NO and NO2 feeds are identical. In contrast, H-ZSM5 shows higher reduction activity for NO2 than for NO from 150°C to 500°C. Maximum conversions for monolith supported H-ZSM5 at 14,000/hr space velocity are only 25% at 200°C, and decrease with increasing temperature. We conclude that in the absence of plasma generated hydrocarbon species that increase the reduction efficiency of diesel fuel, these catalysts alone cannot be used in the catalyst - reductant sub-system of a plasma assisted catalytic reactor for lean-NOx abatement.

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