Mechanistic Studies of the Catalytic Chemistry of NOx in Laboratory Plasma-Catalyst Reactors

Several reactor systems have been used to study the catalytic chemistry of a particular proprietary zeolitic catalyst in conditions that mimic those found in light-duty diesel exhaust after a non-thermal plasma generator. Very similar catalytic results were obtained with NO + plasma or NO₂ as the source of NOx using propene (C₃H₆) as the reductant. The formation of nitrogen, carbon dioxide, and other products were studied from 150°C to 250°C using a He balance gas and NOx in the form of NO₂. The results demonstrate that nitrogen is formed by the selective catalytic reduction of NO₂ by propene. The highest activity for N₂ formation from NO₂ was near 50% conversion at 200°C for a space velocity of 12,600 h^-1. The NOx conversion by adsorption and by catalytic reduction was quantified. By performing studies with and without the presence of water, a clear separation in behavior between adsorption processes and catalytic reaction was observed. Good carbon and nitrogen balances were obtained in these experiments. While much still needs to be done to demonstrate the viability of a plasma-catalyst system in light-duty vehicles, mechanistic studies should help in the optimization of the catalysts that will be used in these exhaust aftertreatment systems.