The Effects of CO, C ₂ H ₄ , and H ₂ O on the NO _x Storage Performance of Low Temperature NO _x Adsorbers for Diesel Applications

Model low temperature NO_x adsorbers (LTNA) consisting of Pd on a ceria/zirconia washcoat on monoliths were evaluated for low temperature NO_x storage under lean conditions to assess their potential for adsorbing the cold-start NO_x 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 NO_x storage performance. In the absence of C₂H₄ or CO/H₂, H₂O severely suppressed the NO_x 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 NO_x storage below 100°C. However, the C₂H₄ significantly increased the NO_x 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 NO_x 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 NO_x 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 NO_x 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.