Molten salts as promising catalysts for oxidation of diesel soot: importance of experimental conditions in testing procedures

The catalytic activity for soot oxidation of three eutectic salt mixtures based on oxides or sulphates of molybdenum, vanadium, and cesium, was studied and compared with the activity of catalysts reported in literature. It is shown that some of these mixtures have a high activity above their melting point of about 625 K. The activity of these catalysts can be up to five times higher than the activity of chemically related solid oxide catalysts and this activity is observed throughout the complete conversion trajectory of the soot, The high activity is explained by the better contact between soot and catalyst, which is probably mainly achieved by wetting of the soot by the liquid catalyst, and not via the vapour phase. The eutectic catalysts studied do not evaporate at high temperature. Furthermore, MoO3 and KCl-KVO3 were studied, two catalysts reported in literature to be significantly active. It was found that the high activity of MoO3 is a result of vapour phase mobility. Catalyst selection procedures were evaluated. Thermobalance equipment should be used with caution for the selection of soot oxidation catalysts, because catalytic activities can be overestimated. A low heating rate, preferably 1 K/min or lower, is recommended for temperature programmed oxidation experiments with these types of catalyst. With higher heating rates, promising catalysts can be overlooked. (C) 1999 Elsevier Science B.V. All rights reserved.