The effect of Fe on the catalytic behavior of model Pd-Rh/CeO2-Al2O3 three-way catalyst

摘要

The present work attempts to address the issue whether iron (Fe) which is accumulated on the surface of "three-way" catalysts (TWCs) used in gasoline-driven cars is a true chemical poison of their catalytic activity.This important issue from a scientific and technological point of view is addressed via catalytic activity,temperature-programmed surface reaction (TPSR),and X-ray photoelectron spectroscopy (XPS) measurements over a model TWC (1 wt% Pd-Rh/20 wt% CeO2-Al2O3).It was found that deposition of Fe up to the level of 0.4 wt% (an average concentration found in aged commercial TWCs) on the model TWC does not deteriorate its activity towards CO and C3H6 oxidation,and reduction of NO by H2.Instead it was found that iron improves significantly the T50 parameter in the activity versus temperature profile.Small Fe clusters in contact with the noble metal (Pd and Rh) particles due to the lower work function of Fe compared to Pd and Rh act likely as a source of electron flow towards the noble metals (as evidenced by XPS measurements),thus altering their surface work function and adsorption energetics of reaction intermediates.The latter have increased significantly the activity of the model TWC towards oxidation of CO and propylene,and to a lesser extent the activity towards the reduction of NO by H2.The presence of Fe on the surface of the model TWC provided and/or created also new active catalytic sites for the reactions investigated.According to previous work from this laboratory,iron up to the level of 0.4 wt% was shown not to deteriorate the oxygen storage capacity (OSC) of the same model TWC used in the present work.Thus,it could be concluded that Fe when deposited on a commercial TWC at least up to the level of 0.4 wt% acts likely as a promoter than a poison of its catalytic activity.