Role and reduction of NOx in the catalytic combustion of soot over iron–ceria mixed oxide catalyst

The iron–ceria mixed oxide catalysts with different iron contents were prepared by a citrate acid sol–gel method, and were investigated by low-temperature nitrogen adsorption–desorption, XRD, H2-TPR, Raman spectroscopy, etc. The performances of catalysts were evaluated for the soot combustion with O2 or O2/NO gases in “tight contact” condition. The results show that in the Fe–Ce–O catalyst Fe3+ can be doped into the ceria lattice and Fe–Ce–O solid solution is formed; the presence of Fe in CeO2 can increase the oxygen vacancy concentration and decrease the crystallite sizes of the catalyst; the Fe–Ce–O catalyst (Fe/Ce = 1/9, mol) calcined at 600 °C possesses higher oxygen vacancy concentration on its surface, resulting in the maximal storage ability for nitrite/nitrate species on its surface and high catalytic activities for NO oxidation and the “NO2-assistance” soot combustion. In “tight contact” condition, as the oxidation of NO is inhibited, the “NO2-assistance” soot combustion is partly limited. With the soot particles gradually consumed, the contact between the soot and catalyst becomes loose, resulting in an improvement of the NO oxidation to NO2 and “NO2-assistance” soot combustion. We suppose that soot catalytic combustion by O2 and NO2 is simultaneously existed, in which the oxygen vacancy concentration on the Fe–CeO2 catalysts is a key factor to influence the catalytic performance.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights• FeCe9–O exhibits excellent catalytic activities for soot combustion. • Fe3+ ions can be evenly doped in ceria lattice to increase the oxygen vacancy concentration. • O vacancy amount in FeCe9–O affect pivotally the catalytic (and NO2-assistance) soot combustion. • The NO2-assistance for soot combustion is partly limited in “tight contact” condition. • NOx reduction by soot can occur during the soot combustion over FeCe9–O catalyst.