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

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.