Exhaust aftertreatment under lean conditions, regenerable SOx traps and NOx reduction catalysts

Licentiatavhandling, 2007

The aim of this thesis was to investigate i) potential materials for regenerable SOx traps and ii) the influence of silver distribution within Ag/Al2O3 catalysts on the selective reduction of NOx by hydrocarbons (HC-SCR). Both areas are highly relevant for present and future vehicle aftertreatment technologies for lean exhausts. In the first part, CeO2, Al2O3 and Al2O3:MgO samples, with or without Pt, were prepared and evaluated by flow-reactor measurements for SOx storage and release performance. To facilitate the understanding of the SOx adsorption mechanism, combined diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy and mass spectrometry were employed. In the second part, 2wt% Ag/Al2O3 samples were prepared and calcined at 550, 850 and 950°C, respectively, in order to gain catalysts with different Ag distribution. By flow-reactor measurements, the oxidation of NO and hydrocarbons was investigated and correlated to NOx reduction performance. Moreover, X-ray photoelectron spectroscopy (XPS) was used to study dispersion and redox-properties of the silver species. The results show that Pt/CeO2 samples are efficient SOx adsorbents under normal lean exhaust temperatures, 200-500°C, but also possible to regenerate at 600°C under lean conditions. The SOx adsorption and regeneration properties are improved by increased Pt content at 250°C. DRIFTS analysis reveal that at 250°C, SOx is stored both as surface and bulk sulfates on CeO2 samples and that the presence of Pt increases the rate of bulk sulfate formation. For the Pt/CeO2 sample, both the SOx storage capacity and the rate of bulk sulfate formation are higher at 400°C compared to 250°C. The results from this study indicate the presence of different SOx storage sites on Pt/CeO2 samples, both regenerable and non-regenerable, but the effect of the latter seems here to be minor. The Ag/Al2O3 samples calcined at 550 and 850°C reveal similar HC-SCR performance, while the sample calcined at 950°C show lower NOx reduction capacity but higher activity for HC oxidation. Contrary to the samples calcined at lower temperatures, the latter sample shows ability for oxidation of NO into NO2 in the absence of hydrocarbons. XPS measurements reveal that the high temperature treatment at 950°C resulted in samples with low silver dispersion and Ag species showing less redox-properties compared to the Ag/Al2O3 sample calcined at 550°C.