Hydrocarbon Reactivity in a Plasma-Catalyst System: Thermal Versus Plasma-Assisted Lean NOx Reduction

The steady-state reduction of NO_x at temperatures between 150-300°C has been investigated under simulated lean-burn conditions using a two-stage transient flow reactor system consisting of non-thermal plasma in combination with a sodium Y zeolite catalyst. Reactivity comparisons were made with and without plasma operation in order to identify the plasma-generated hydrocarbon species necessary for the selective catalytic reduction (SCR) of NO_x. With propene as the hydrocarbon in the feed, NO is completely oxidized to NO₂ in the plasma and the formation of oxidized carbon-containing species include formaldehyde, acetaldehyde, carbon monoxide, carbon dioxide, and methanol. Fourier transform infrared (FTIR) measurements indicate a close carbon balance between plasma inlet and outlet gas feed concentrations, signifying the major species have been identified.

Plasma-assisted NO_x conversion over the sodium Y zeolite catalyst is improved with supplemental acetaldehyde injection, primarily through the removal of NO from the feed stream. In addition, the thermal (no plasma) reduction of NO is much greater with acetaldehyde as the reductant compared to propene, demonstrating that the NO_x reduction reaction is controlled by the acetaldehyde present in the gas feed. The thermal catalytic NO_x conversion with acetaldehyde in a NO-containing feed is much higher than that in a NO₂-containing feed, indicating that plasma conversion of NO to NO₂ is not required for NO_x reduction. The higher activity of NO is due to the increased formation of HCN, a partial reduction product that can be removed using a downstream Pt/Al₂O₃ catalyst. Octane, which is more representative of hydrocarbons found in diesel fuel/exhaust, forms acetaldehyde over the catalyst and yields comparable NO_x reduction activity. At similar acetaldehyde concentration levels entering the catalyst, thermal NO_x conversion with the feed containing NO and acetaldehyde is comparable to the plasma-assisted NO_x conversion efficiency with the feed containing propene and NO, suggesting that an appropriate hydrocarbon can reduce NO without the use of non-thermal plasma.