Effect of NO2 and water on the catalytic oxidation of soot

•Oxidation of soot in loose contact with a catalyst is significantly enhanced by NO2.•Loose contact oxidation by NO2 has a volcano-type dependence on bond strength of oxygen on the surface.•Addition of NO2 and loose contact with FeCr2Ox shifts oxidation down by ≥200 °C.•For soot in tight contact with a Co3O4 catalyst hardly any effect of NO2.•Limited effect on oxidation in tight contact consistent with Mars van Krevelen mechanism.

The influence of adding NO2 to 10 vol% O2/N2 on non-catalytic soot oxidation and soot oxidation in intimate or loose contact with a catalyst has been investigated. In non-catalytic soot oxidation the oxidation rate is increased significantly at lower temperatures by NO2. For soot oxidation in tight contact with a Co3O4 catalyst a more reactive NO2-containg atmosphere did not change the oxidation profile significantly during temperature programmed oxidation. This is consistent with the expected Mars van Krevelen mechanism, where the rate limiting step is reaction between carbon and lattice oxygen from the oxide, and where the reaction thus has reached the zero order kinetics regime in the gaseous reactant. In loose contact with a catalyst the presence of NO2 causes a pronounced enhancement of the oxidation rate. The rate constants for loose contact soot oxidation in the presence of NO2 exhibited a volcano-curve dependence on the heat of oxygen chemisorption, and among the tested pure metals and oxides Cr2O3 was the most active catalyst. Further improvements were achieved with a FeaCrbOx binary oxide catalyst.

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