Theis, J. and Lambert, C., "The Effects of CO, C2H4, and H2O on the NOx Storage Performance of Low Temperature NOx Adsorbers for Diesel Applications," SAE Int. J. Engines 10(4):2017, doi:10.4271/2017-01-0942.
Model low temperature NOx adsorbers (LTNA) consisting of Pd on a ceria/zirconia washcoat on monoliths were evaluated for low temperature NOx storage under lean conditions to assess their potential for adsorbing the cold-start NOx emissions on a diesel engine during the period before the urea/SCR system becomes operational. A reactor-based transient test was performed with and without C2H4, CO/H2, and H2O to assess the effects of these species on the NOx storage performance. In the absence of C2H4 or CO/H2, H2O severely suppressed the NOx storage of these model LTNAs at temperatures below 100°C, presumably by blocking the storage sites. When C2H4 was included in the feedgas, H2O still suppressed the NOx storage below 100°C. However, the C2H4 significantly increased the NOx storage efficiency above 100°C, attributable to the formation of alkyl nitrites or alkyl nitrates on the catalyst. When the feedgas contained CO/H2, the NOx storage performance was greatly improved below 100°C, even in the presence of H2O. Tests with CO alone and H2 alone indicated that CO was providing the improved NOx storage capability. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis on a Pd/ceria powder indicated that NO and CO react to form NCO at low temperatures, which could account for the improved NOx storage in the presence of CO. The formation of alkyl nitrites or nitrates with NO and C2H4 could not be confirmed or denied because the spectra for these species occur in the same range as that of carbonates which form from the C2H4.