Ind. Eng. Chem. Res. 2009, 48, 9847–9857
9847
Adsorption and Desorption of SOx on Diesel Oxidation Catalysts Oliver Kro¨cher,*,† Markus Widmer,† Martin Elsener,† and Dieter Rothe‡ Paul Scherrer Institute, Villigen PSI, Switzerland, and MAN Nutzfahrzeuge AG, Nu¨rnberg, Germany
The deactivation of NO oxidation by SO2 was studied with the use of commercial diesel oxidation catalysts (DOC) and Pt/Al2O3 as reference material, coated on cordierite monolith. Despite their slightly different elemental compositions, the NO conversion rates of the fresh commercial catalysts were very similar. The maximum NO conversion was 38% at 350 °C, and above this temperature conversion started to be limited by the thermodynamics of the reaction. The rates of NO conversion strongly decreased with the start of SO2 dosing. For analysis of SO2 conversion and uptake, SO2 and SO3/H2SO4 were determined separately in the gas phase by absorption and titration. Under typical exhaust gas conditions (1 ppm SO2, 250 °C), the catalysts functioned as sulfur traps and stored a large part of the emitted SO2. The SOx storage was divided into two phases: a fast saturation of the catalyst surface with sulfuric acid, which hampered NO conversion, and a slow, long-lasting sulfation of the washcoat. The storage capacities of the oxidation catalysts reached their maxima at 250 °C due to the temperature dependency of sulfur adsorption and desorption. Adsorbed sulfuric acid desorbed between 350 and 400 °C, whereas more stable compounds, such as aluminum sulfate, were decomposed at higher temperatures. Deactivated catalysts could be completely regenerated within a few minutes at temperatures above 350 °C. However, repeated or lengthier thermal treatments resulted in a reduced sulfur storage capacity and irreversible activity losses for NO oxidation due to a reduction of the active surface by sintering. 1. Introduction NO2 is an important molecule for exhaust gas catalysis, as it facilitates a series of advantageous reactions. In the field of NOx reduction, NO2 is stored as nitrate in NOx storage and reduction (NSR) catalysts, is responsible for the fast SCR reaction in the selective catalytic reduction with urea (urea SCR), and allows for the regeneration of diesel particulate filters (DPF) by oxidizing soot at moderate temperatures. However, in raw diesel exhaust gas NO and NO2 are not in thermodynamic equilibrium: the share of NO2 is only about 5-10%. Therefore, in exhaust gas systems a diesel oxidation catalyst (DOC) is usually applied that is positioned in front of the NSR catalyst, the SCR catalyst, or the DPF, respectively, and that oxidizes part of the NO to NO2. In mobile applications there is a pronounced need for an NO oxidation catalyst with a low light-off temperature in order to provide the NO2 needed for these processes. It has to be mentioned that the oxidation catalysts not only have the function to oxidize NO to NO2, but also to oxidize CO and HC to harmless CO2 and water. It is known that the functions of some exhaust gas catalysts, such as NOx storage and reduction (NSR) catalysts and diesel oxidation catalysts (DOC), are affected by SO2 formed from sulfur in fuel during engine combustion processes.1,2 The introduction of low-sulfur fuels (
