Formation of sulfur surface species on a commercial NOx-storage reduction catalyst

The influence of SO2 exposure under lean (oxidizing) and rich (reducing) reaction conditions on the storage and oxidation/reduction function of a commercial NOx storage-reduction catalyst was investigated by temperature-programmed uptake experiments and high temperature XRD. Both the storage capacity and the oxidation/reduction function of the catalyst were deactivated by SO2 exposure under lean and rich reaction conditions. The deactivation of the storage component, i.e. the loss of the NOx storage capacity, resulted mainly from the formation of Ba-sulfates accumulating in the bulk phase, which have a high thermal stability (>800°C) and, therefore, cannot be removed under the typical operation conditions of a NSR catalyst. For the oxidation function only a temporarily deactivation during lean reaction conditions was observed. Besides the formation of SO2-4 species on the storage component at the beginning of the SO2 exposure under rich conditions, an adsorption of SO2 on the noble metal component was observed resulting in the formation of sulfur deposits. The oxidation of these sulfur species with a subsequent spillover of SO2-4 species to the storage component during lean conditions could accelerate the deactivation of the storage capacity.