Studies of NOx Storage Catalysts - Preparation, Characterization, Sulfur Deactivation and Micro Kinetic Modelling

Doktorsavhandling, 2005

For the protection of the global environment, it is essential to decrease the emissions of CO2 which is a green house gas. Lean-burn and diesel engines offer relatively low CO2 emissions and good fuel economy. However, in oxygen excess, the conversion of NOx is low, since the reducing agents in the exhaust are more favorably oxidized by oxygen than by NOx. The NOx storage and reduction technique solves this problem by trapping NOx for a relatively long period of time (min) in lean conditions, where catalytic conversion of NOx to N2 is unfavorable. The engine is then for a short time (s) tuned to a rich mixture, which causes a release and reduction of the stored NOx. A typical NOx storage catalyst consists of one or more noble metal(s), such as Pt, Rh and Pd, a NOx storage compound, commonly BaO, and a support material like γ-Al2O3. One major problem with NOx storage catalysts is sulfur poisoning, through which the NOx storage capacity and the reduction properties are strongly suppressed. Sulfur comes from sulfur in the fuel and lubricating oils. The main objective of this work was to improve the understanding of the effect of noble metal and storage material dispersion on the catalytic activity for NOx storage and reduction, in the presence and absence of sulfur. The capacity for NOx storage and reduction in Pt/BaO/Al2O3 NOx storage catalysts can be enhanced by enhancing the dispersion of both Pt and BaO. The Pt dispersion can be enhanced by choosing a Pt precursor with chemical properties suitable for a proper interaction with the support. We prepared Pt/BaO/Al2O3 catalysts from different precursors and found that catalysts prepared using platinum nitrate are most active for NOx storage and reduction. In order to understand how sulfur interacts with Pt and NOx storage sites, Pt/BaO/Al2O3 and BaO/Al2O3 samples were treated with SO2, SO2 + O2 or SO2 + H2 prior to performing NOx storage and thermal regeneration cycles. The NOx storage capacity declined for all samples at all sulfur exposure conditions. For the Pt-containing samples, the deactivation was fastest with SO2 + H2 treatment, while all three sulfur exposure conditions had a similar effect on the BaO/Al2O3 samples. Quantitative sulfur analysis showed that more sulfur was trapped in a SO2 + O2-treated Pt/ BaO/Al2O3 sample, compared to a corresponding SO2 + H2-treated sample. No sulfur was found in the BaO/Al2O3 samples, indicating that the sulfur was weakly bound to these samples and desorbed with time. To describe these results with kinetic modelling, we assumed that NOx is stored only on surface storage sites, while sulfur is stored on both surface and bulk sites. The model was able to describe the experimental results fairly well. To attempt to improve the sulfur tolerance of Pt/BaO/Al2O3 catalysts, we studied the effect of metal oxide additives on the NO and SO2 oxidation capacity of metal oxide-promoted Pt/Al2O3 catalysts. We found that the addition of MoO3 to Pt/Al2O3 enhances the oxidation of NO to NO2, while suppressing the oxidation of SO2 to SO3. However, the NOx storage performance of the MoO3 modified Pt/BaO/Al2O3 catalyst declined more rapidly than the performance of the unmodified Pt/BaO/Al2O3 catalyst, due to faster deterioration during the regeneration (rich) periods.