On the influence of the alumina precursor in Fe-K/Al 2 O 3 structured catalysts for the simultaneous removal of soot and NO x : From surface properties to reaction mechanism

摘要

Cordierite monolith-supported and powder Fe–K/Al2O3 catalysts were prepared and thoroughly characterized by bulk (XRD, Raman spectroscopy, XRF, ICP–OES, TPR) and surface (XPS, IR, N2-BET, NOx-TPD, K-TPD) sensitive methods. The catalytic activity was tested in TPO (temperature programmed oxidation) of model soot. The influence of the calcination temperature, i.e. 450 and 650 °C, on the catalysts physicochemical properties and reactivity, type of surface states of K promoter and their role in the soot oxidation mechanism in the presence of NOx was evaluated. In the catalysts calcined at 450 °C potassium was found to be mostly as free KNO3 whereas calcination at 650 °C successfully transformed KNO3 into K2O, which during the soot oxidation yielded K2CO3 species. Such carbonates underwent decomposition in the presence of the oxygen surface groups of the support, leading to the formation of active basic O2− groups. NOx species were found to adsorb strongly on these O2− sites. The increased stability of thus formed NOx-species resulted in lower soot oxidation activity. In the same time, the catalysts calcined at 650 °C evidenced higher ability towards the reduction of NOx, occurring simultaneously with the soot oxidation process. Additionally, the higher calcination temperature led to significantly higher stability of K promoter species as evidenced by potassium thermodesorption experiments. An overall schematic model of the catalyst morphology and relative distribution of the active components (K, Fe) over the Al2O3 support is proposed.