Catalytic oxidation of model soot by metal chlorides

Several metal chlorides were screened for their catalytic activity in the oxidation of model soot (Printex-U) in 'loose contact' by means of TGA/DSC. HgC1(2), CaC2, BaC1(2), CoC1(2), and NiC1(2) show little activity. Hydrated BiC1(3) and FeC1(3) are converted in air into BiOC1 and FeOC1, which have a moderate soot oxidation activity. MoC1(5), is converted into the corresponding metal oxide and also shows a moderate 'loose contact' activity. PbC1(2), CuC1(2) and CuC1 are very active catalysts; the soot oxidation temperature is lowered by 200-275 K. The activity of metal chlorides is thought to be induced by in situ formation of intimate contact between the soot and the metal chloride via 'wetting' and/or gas phase transport. A correlation between the melting point and the catalytic activity was found. Furthermore, a catalytic cycle is proposed involving activation of oxygen on the surface of the (oxy)chloride, followed by transfer of activated oxygen to the soot surface. DRIFT analyses showed that this results in the formation of carbon surface oxygen complexes. Decomposition of those complexes yields CO and CO2. Practical application of metal chlorides for the removal of soot from diesel exhaust is not recommended, because they suffer from instability or high vapour pressures.