The effects of sulfur dioxide, water and temperature on the deactivation of model diesel oxidation catalysts

With the aim to simulate ageing of catalysts on a laboratory scale and to develop methods for accelerated deactivation, model diesel oxidation catalysts were exposed to synthetic diesel exhaust gases in a flow reactor, and the effects of SO2, H2O and temperature on catalyst performance were investigated. Samples based on Pt, -Al2O3 and ZSM-5, and a commercial reference catalyst formed the basis of the model series. Characterisation was performed pre- and post-deactivation with respect to C3H6 adsorption capacity and TPD, BET surface area, CO oxidation, platinum dispersion using CO chemisorption, and quantification of sulfur species using XPS. Ageing conditions were identified for which significant loss in catalyst performance occurred. Hydrothermal treatment was critical to the C3H6 adsorption capacity of the zeolite. Exposure to SO2 affected a number of characteristics; elevated CO light-off temperature and a decrease in Pt dispersion, primarily due to blocking of Pt sites, and a decrease in BET surface area. XPS analysis showed that low temperature was necessary for sulfur adsorption to occur and that the adsorption process was facilitated by H2O. It was also indicated that adsorbed sulfur species were present on the washcoat even at elevated temperatures. Long-term operation of a catalytic converter with diesel exhausts at relatively low temperature induces chemical deactivation to some extent due to accumulation of sulfur compounds. The results presented here show that a lab model can be used to simulate relevant parts of such behaviour