Deactivation of commercial three-way catalysts: Experimental and theoretical studies

Doktorsavhandling, 2014

This PhD thesis is a summary of my work with thermal deactivated commercial three way catalysts (TWCs). An examination of catalytic properties, before and after accelerated aging was performed, such as light-off temperature, specific surface area (BET), noble metal dispersion and oxygen storage capacity was performed. X-ray photoelectron spectroscopy (XPS) was used to measure changes in the oxidation state of the Pd and Rh present in the washcoat of the catalyst before and after hydrothermal aging. X-ray diffraction (XRD) was used to measure changes in the crystallite size of washcoat components. Element map SEM-EDX analysis was performed in order to characterize the catalytic design and morphology, as well as the poisoning of the catalyst. The experiments were performed using gasoline or E85 similar gas mixture. Gas composition, temperature, space velocity and stoichiometry were kept on levels appropriate to automotive conditions. The effect of hydrothermal aging on commercial TWCs was investigated as well as the effect of various accelerated aging procedures used in the automotive industry, including fuel-cut (FC), secondary air injection (SAI), standard bench cycle (SBC) and standard road cycle (SRC). It is shown that aging mainly caused changes in the state of noble metals that are presented in the washcoat of the examined catalysts. Sintering of the support and of the noble metals could be reported. The effect of an oxidative atmosphere on the deactivation of TWCs’ was confirmed. Growing exotherms were observed through aging and were attributed to the loss of OSC and the oxidation of the accumulated carbon on the catalytic surface. The importance of decreased OSC for the deactivation of TWC has been confirmed. A correlation between crystallite size of CeZrO and catalytic performance is proposed. The literature review resulted in critical points of current insights including substantive findings as well as theoretical and methodological contributions to the deactivation of TWCs.