50 Au-catalysts in the Purification of TRS Emissions Satu Ojala 1 *, Jyri-Pekka Mikkola 2 and Riitta L. Keiski 1 1 University of Oulu, Dept.Process and Environmental Engineering, FI-90014 University of Oulu, P.O.Box 4300 2 University of Umeå, Sweden 1 Introduction Total Reduced Sulfur (TRS) compounds are a group of typical emissions of the pulp and paper mills. These compounds include very malodorous compounds, such as H 2 S, methyl mercaptan (MM) and dimethyl disulphide (DMDS) and they cause annoyance to the surrounding areas even if they were very diluted and cause no direct harm to the nature or the human health at these concentrations. The TRS emissions may be concentrated or diluted. The concentrated emissions can be incinerated in a lime kiln and the SO 2 formed may be further used in sodium bisulfte (NaHSO 3 ) production. Concen- trated emissions may also be incinerated in a recovery boiler or in a separate incinerator. The oxidation of concentrated sulphur-containing volatile organic compounds (S-VOCs) increases the energy-efciency of the pulping process unit where they are treated. The problem arises when diluted TRS emissions are considered. They are emitted from several sites in a process and very often they cannot be easily control- led by process optimization. Due to very low odour threshold limits, the requirements for the emission abatement efciency are very high. Furthermore, the explosion limits of these gases should be taken into account, which means that for example mixing of concentrated and diluted emissions and simultaneous treatment cause substantial explosion hazard. Therefore, these two emission types should be treated separately. 1 The abatement of diluted TRS emissions can be carried out efciently and economically by means of catalytic oxidation. However, application of a catalytic process to this kind of feld of operation is not straightforward due to difcult process conditions for the catalyst and the construction materials of the abatement unit. Currently, the greatest problem in industrial scale applications of catalytic oxidation of TRS compounds is that traditionally used noble metal catalysts, such as platinum, are too active in oxidation and cause formation of SO 3 . SO 3 is reported to be formed from SO 2 over Pt at temperatures of 250–600°C depending on the support material (Koutsopoulos et al. 2006). SO 3 is quite unstable and reacts quickly with water vapour present in a gas stream to fnally end up in the formation of sulphuric acid. Corrosion problems can be avoided in a certain level by keeping the inner surfaces of the oxidation system above the dew point temperature of sulphuric acid. Preferably, this could be done by tailoring the catalysts and avoiding the formation of SO 3 . In TRS emission abatement, sulphur-containing compounds and oxidation products may cause problems also with long-term durability of the catalysts. Oxidation products can form sulphates with the supporting metal oxides or very stable surface metal sulphites with the active metal. Sulphur can also be deposited on the catalyst via decomposition of H 2 S and it will then inhibit the chemisorption of small molecules such as H 2 , CO, NO, C 2 H 3 etc. However, sulphur poisoning is reversible and it is dependent on the temperature. In fact, among the transition and noble metals, Au exhibits the lowest reactivity towards sulphur and may therefore ofer a good solution for the catalytic oxidation of TRS emissions. *Correspoding author, E-mail: satu.ojala@oulu.f