Structure-activity relation of iron oxide catalysts in soot oxidation

In this paper, a systematic series of bare iron oxide catalysts was evaluated towards their activity for the oxidation of soot under oxygen-rich conditions. The catalytic studies were performed by temperature programmed oxidation (TPO) using tight contact mixtures of catalyst and soot. A home-made carbon black originated from propene combustion was taken as model soot. The catalysts were thoroughly characterised by employing powder X-ray diffraction (PXRD), N-2 physisorption, high resolution transmission electron microscopy (HRTEM), temperature programmed reduction by H-2 (HTPR), temperature programmed desorption of NH3 (NH3-TPD), temperature programmed desorption of O-2 (O-2-TPD) and thermogravimetry coupled with difference thermal analysis (TG/DTA). Special notice was put on the NH3-TPD data, which were used for the modelling of the NH3 adsorption and desorption. This model included elementary reactions and implied the specific number of Bronsted and Lewis acid sites. The obtained kinetic parameters were found to be close to that of related oxides such as ZrO2 and H-BEA zeolite. The results of the physical-chemical characterisation as well as NH3 adsorption/desorption kinetics were coupled with the catalytic performance to identify determining properties of the iron oxide catalysts. From these correlations it was derived that both Lewis acid surface sites as well as crystallinity drive the catalytic activity for soot oxidation. The effect of these characteristics is in line with the postulated role of the catalyst implying the transport of oxygen from iron oxide to soot by surface and bulk diffusion. (C) 2013 Elsevier B.V. All rights reserved.