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 b ased on oxides or sulphates of molybdenum, vanadium, and cesium, was studie d 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 ti mes higher than the activity of chemically related solid oxide catalysts an d this activity is observed throughout the complete conversion trajectory o f the soot, The high activity is explained by the better contact between so ot and catalyst, which is probably mainly achieved by wetting of the soot b y the liquid catalyst, and not via the vapour phase. The eutectic catalysts studied do not evaporate at high temperature. Furthermore, MoO3 and KCl-KV O3 were studied, two catalysts reported in literature to be significantly a ctive. It was found that the high activity of MoO3 is a result of vapour ph ase mobility. Catalyst selection procedures were evaluated. Thermobalance e quipment should be used with caution for the selection of soot oxidation ca talysts, because catalytic activities can be overestimated. A low heating r ate, preferably 1 K/min or lower, is recommended for temperature programmed oxidation experiments with these types of catalyst. With higher heating ra tes, promising catalysts can be overlooked. (C) 1999 Elsevier Science B.V. All rights reserved.