Stability of Ba,K/CeO2 catalyst during diesel soot combustion: Effect of temperature, water, and sulfur dioxide

In this work we study the stability of Ba,K/CeO2 catalysts, which have been shown to be very active for soot combustion. The effects of high-temperature treatments and the presence of water or sulfur dioxide on the catalytic properties for soot oxidation are studied. Fresh and deactivated catalysts are characterized by XPS, FTIR, XRD, and high-frequency CO2 pulses, and the activity is measured by TPO. Barium has only a minor effect on the activity for soot combustion, whereas potassium has a pronounced effect in decreasing the temperature needed to burn soot. In the case of Ba,K/CeO2, optimum activity as a function of potassium content of around 7 wt% is found. The optimum, which is very smooth, is due to the synergistic effect between K and CeO2. This catalyst is thermally stable up to 830 °C and does not deactivate even after 30 h at 800 °C. At higher temperatures, a decrease in the K/Ce surface ratio and formation of the BaCeO3 perovskite, indicated by the XPS analyses, are the causes of activity loss. In these cases, there is a decrease in the level of interaction with CO2. The presence of water at 400 °C does not lead to any significant modification of catalytic activity. However, the presence of water at 800 °C leads to both a drastic decrease in activity and a change in surface composition as indicated by XPS, with a high degree of hydroxylation and probably a spreading of BaO on the ceria surface. The presence of SO2 (1000 ppm in air) at 400 °C deactivates the catalyst for soot oxidation even after rather short times (32 h). Under these conditions, FTIR and XPS analyses show that barium, potassium, and cerium sulfates are formed. Consequently, the activity for soot oxidation is lost, and there is no interaction between the catalyst and the CO2. This type of catalyst has good thermal stability and very good tolerance to water at low temperatures (e.g., 400 °C). However, high concentrations of SO2 lead to rapid deactivation. This should not be a major drawback, because in the near future a much lower level of sulfur in diesel fuel is expected.