Plasma-Enhanced Catalytic Reduction of NOx in Simulated Lean Exhaust

NOx reduction efficiency in simulated lean exhaust conditions has been examined for three proprietary catalyst materials using a non-thermal plasma discharge as a pretreatment stage to the catalyst. Using propene as the reducing agent for selective catalytic reduction, 74% reduction of NOx has been observed in the presence of 20 ppm SO₂. For sulfur-free simulated exhaust, 84% NOx reduction has been obtained. Results show that the impact of sulfur on the samples examined can vary widely from virtually no effect (< 5%) to more than 20% loss in activity depending on the catalyst. Any loss due to sulfur poisoning appears to be irreversible according to limited measurements on poisoned catalysts exposed to sulfur-free exhaust streams.

Catalysts were tested over a temperature range of 473-773K, with the highest activity observed at 773K. Examination of this large temperature window has shown that the optimum C₁:NOx ratio changes with temperature. In general, as temperature increases more propene is required to reach maximum conversion. There are two factors which contribute: (i) substantial plasma oxidation of propene takes place at lower specific energy, and (ii) the catalyst exhibits increased propene demand at higher temperature. In contrast, as temperature increases, the energy required to obtain plateau conversion decreases for a given hydrocarbon level. This mix of observations leads to a hypothesis that as temperature increases there is a tradeoff between electrical energy required for the plasma and the hydrocarbon levels required to maximize selective catalytic reduction.