A highly effective catalyst of Co-CeO2 for the oxidation of diesel soot: The excellent NO oxidation activity and NOx storage capacity

Abstract

In the catalytic combustion of diesel soot, the role of NO in the exhaust emission and the simultaneous reduction of NO by soot are two fascinating problems. Herein, the Co-CeO2 catalyst prepared by the citrate acid sol-gel method was developed to catalyze the NOx-assisted soot oxidation. The effect of the Co amount on the physicochemical and catalytic properties of the Co-CeO2 catalysts was investigated in detail. When the molar ratio of Co/Ce was 50/50, the Co50Ce50 mixed oxide catalyst exhibited the lowest soot oxidation temperature (Tm of 335°C) in the feed gas of NO+O2, and the presence of water can improve obviously the catalytic combustion of soot with Tm of 310°C. The presence of Co can improve the reduction of surface oxygen of ceria, and Ce can promote the reduction of Co3O4 to CoO, resulting in the enhancement of the catalytic activity of Co-CeO2 for NO oxidation and its capacity for NO2 storage as the surface nitrates. The soot oxidation on the Co-CeO2 catalyst can be promoted by “NO2-assistance”: the nitrate species are preferentially formed on the Co active sites and stored on the catalyst surface, and then NO2 produced by NO oxidation and the decomposition of nitrate stored on the surface at lower temperature, reacts with soot. That is, the stored NOx species initiates the beginning of soot oxidation, and the NO oxidation ability of the catalyst is responsible for the extensive soot oxidation by NO+O2, in which NO is reduced by carbon in soot combustion. The combination of high NO oxidation activity and high NO2 storage capacity makes the Co50Ce50 oxide catalyst as an efficient catalyst for the NOx-assisted soot oxidation.