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 act ivity. Hydrated BiC1(3) and FeC1(3) are converted in air into BiOC1 an d FeOC1, which have a moderate soot oxidation activity. MoC1(5), is co nverted into the corresponding metal oxide and also shows a moderate ' loose contact' activity. PbC1(2), CuC1(2) and CuC1 are very active cat alysts; the soot oxidation temperature is lowered by 200-275 K. The ac tivity of metal chlorides is thought to be induced by in situ formatio n of intimate contact between the soot and the metal chloride via 'wet ting' and/or gas phase transport. A correlation between the melting po int and the catalytic activity was found. Furthermore, a catalytic cyc le is proposed involving activation of oxygen on the surface of the (o xy)chloride, followed by transfer of activated oxygen to the soot surf ace. DRIFT analyses showed that this results in the formation of carbo n 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.