(547b) Impact of Dopants in Ba-Based NOx Storage Reduction (NSR) Catalysts On Sulfation, Desulfation and Performance
Lean-burn engines are being introduced into the US automobile market in order to increase fuel efficiency. A major challenge with these engines is the reduction of NOx emissions in the lean exhaust. The NOx storage-reduction (NSR) catalyst is a potential solution to NOx abatement. It is formulated with a NOx storage component, usually alkali or alkaline earth metal oxides such as Ba or K, and precious metals, such as Pt, dispersed over a high surface area γ-alumina washcoat. Unfortunately, fuel-borne sulfur is also trapped on the storage component in the form of sulfates, diminishing the storage capacity of the NSR catalyst. The sulfates must be removed through periodic high temperature excursions, which can have detrimental effects on catalyst surface area due to sintering.
The performance of an NSR catalyst is strongly dependent on the relative stabilities of nitrates and sulfates on the catalyst surface. This effort studies the effects of introducing La, Ca, and K dopants into the BaO lattice. The dopants were chosen with a range of properties to affect the BaO lattice spacing and/or the number of oxygen vacancies. The resulting changes in the storage material, in turn, impact the stability of stored nitrates and sulfates, as measured by NOx conversions and desulfation temperatures.
The Ca-doped material shows a significant increase in performance at 200 and 300°C, while also maintaining high conversion at 400°C. Conversely, both the K- and La-doped NSR catalysts show moderate decreases in performance at 200 and 400°C while matching the Ba-only performance at 300°C. Following the performance measurements, the samples are then sulfated to 5.5 mg S/g-cat and desulfated to 1000°C. The Ba, K+Ba, and La+Ba all show similar desulfation behavior, with moderate differences in desulfation onset and peak release temperatures (see Table 2) indicating that sulfates on the K+Ba and La+Ba are slightly more stable than those on the undoped Ba. The Ca+Ba catalyst, on the other hand, demonstrates a markedly different bimodal desulfation behavior with desulfation onset occurring 50°C before the undoped Ba catalyst. Although the peak release temperature is higher than the Ba-only sample, the significant amount of less stable sulfur released before 550°C is very beneficial to the overall desulfation behavior of an NSR-catalyst.
These results suggest that the electronic manipulation of the storage phase does not have a positive effect on the performance or desulfation characteristics of the NSR catalyst; however, introducing metals with the same valence but different radii can benefit both the NOx performance and desulfation. Additional experimentation and characterization are performed on the fresh and desulfated NSR catalysts to understand the physical impact of the dopants on the BaO and their durability. Specifically, lattice spacing, surface area, pore size distributions, and elemental analysis are evaluated.