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 C 2 H 4 , CO/H 2 , and H 2 O to assess the effects of these species on the NO x storage performance. In the absence of C 2 H 4 or CO/H 2 , H 2 O severely suppressed the NO x storage of these model LTNAs at temperatures below 100°C, presumably by blocking the storage sites. When C 2 H 4 was included in the feedgas, H 2 O still suppressed the NO x storage below 100°C. However, the C 2 H 4 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/H 2 , the NO x storage performance was greatly improved below 100°C, even in the presence of H 2 O. Tests with CO alone and H 2 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 C 2 H 4 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 C 2 H 4 .Cross Reference: