The deactivation of NO oxidation by SO 2 was studied with the use of commercial diesel oxidation catalysts (DOC) and Pt/Al 2 O 3 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 SO 2 dosing. For analysis of SO 2 conversion and uptake, SO 2 and SO 3 /H 2 SO 4 were determined separately in the gas phase by absorption and titration. Under typical exhaust gas conditions (1 ppm SO 2 , 250 °C), the catalysts functioned as sulfur traps and stored a large part of the emitted SO 2 . The SO x 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.

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The regional ARA in Visp, in which waste water from the communities and from industry is treated, is set up for the daily biological purification of waste water from 350 000 people. Since the quality of the sewage sludge does not meet the requirements for agricultural use, the sludge is burnt after appropriate preparation, in a Fluidised Bed Oven. With the intention of reducing emissions of nitrogen oxides, the mode of operation of the oven has been optimised. The recycling of off-gases would enable emissions to remain below the permitted levels.