Catalytic oxidation of model soot by chlorine based catalysts

Abstract

Different oxidic compounds potentially present in a highly active Cu/K/Mo/Cl soot oxidation catalyst (copper-molybdates, potassium-molybdates, and a mixed copper-potassium-molybdate (K2Cu2(MoO4)3) have been tested individually on their activity in soot oxidation. Although the oxides induce oxidation between 685 K and 720 K after intensive mixing in a ball mill, i.e. in ‘tight contact’ conditions, in ‘loose contact’ the oxides are hardly active and only show a high activity above approximately 790 K. DRIFT and XRD analyses have shown that reaction of KCl with CuMoO4 (two compounds present in the Cu/K/Mo/Cl catalyst), results in the formation of potassium containing molybdates and volatile copper and chlorine containing compounds (such as CuCl and K2CuCl4). These volatile compounds possess a high ‘loose contact’ soot oxidation activity between 600 and 690 K. Several other metal chlorides have been screened on their catalytic activity in the oxidation of model soot (Printex-U) in ‘loose contact’. Only PbCl2, CuCl2, and CuCl are very active. The activity of these 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 metal (oxy)chloride, followed by transfer of activated oxygen to the soot surface. Practical application of metal chlorides for the removal of soot from diesel exhaust is not recommended, because they suffer from instability or high vapor pressures.