Role of chain length of the capping agents of iron oxide based fuel borne catalysts in the enhancement of soot combustion activity

Abstract

A series of fuel borne catalysts (FBC) based on an iron oxide core stabilized by various long chain carboxylic acids were synthesized, characterized and tested in model soot combustion. As the capping agents methacrylic (C4), undecylenic (C11), oleic (C18) and erucic (C22) acids were used. The FBC precursors and the resultant catalysts were examined by Raman, FTIR, EPR, UV–vis and TEM/SAED techniques, whereas the catalytic reactivity in combustion of a model soot (Printex U) was evaluated by TG/DTA. For all the investigated FBC derived catalysts, it was found that the core of the catalyst consists of Fe2O3 of a crystallite size in range of 5–8nm, which is beneficial for the soot combustion activity. While a strong positive effect on the catalyst performance was observed for all the alkyl chain lengths, the best results were obtained for a hybrid FBC catalyst with two types (C11 and C18) of capping ligands. The effect of the chain length of the capping agent on the soot combustion process was discussed in terms of promotion of soot ignition by the more reactive C11 and combustion sustentation by heat transfer from the long chain C18 to soot particles, as implied by the observed overlapping of the combustion temperature windows of those moieties. For such hybrid catalysts, the ignition temperature of the soot combustion was lowered to almost 150°C, and the total soot combustion accomplished below ∼470°C. The high activity of the hybrid FBC catalysts makes them a promising alternative/backup for the particulate filter technology in light oil burners and related applications.