Oxy-chlorination as an effective treatment of aged Pd/CeO 2-Al 2O 3 catalysts for Pd redispersion

Abstract

The present work reports on the effects of oxy-chlorine gas treatment (use of Cl 2/O 2/He gas mixture) applied on a 5wt% Pd/20wt% CeO 2-Al 2O 3 catalyst towards Pd redispersion. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), temperature programmed reduction in H 2 (H 2-TPR) and in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) NO chemisorption techniques were employed before and after use of the oxy-chlorine gas treatment to critically evaluate its efficiency. The composition (xvol% Cl 2/18vol% O 2/He), temperature, and time on stream were investigated. The first two parameters were found to largely dictate optimum Pd redispersion, namely the use of a 2vol% Cl 2/18vol% O 2/He gas mixture at 500°C for 1h, where a significant reduction of an initial Pd mean particle size of 17.3-7.5nm was obtained. XPS studies revealed that after oxy-chlorine gas treatment followed by H 2 reduction at 500°C, complete elimination of Cl from the Pd surface was achieved. The oxygen storage capacity (OSC) of the catalyst measured following different oxy-chlorine gas treatments was found to significantly increase. Catalytic activity towards CO oxidation along with in situ DRIFTS NO chemisorption studies proved the large effect of the oxy-chlorine gas treatment on increasing the CO oxidation rate and the extent of NO chemisorption. Alternative treatment in oxygen gas atmosphere at high temperatures (500-850°C) followed by H 2 reduction (300-500°C) applied over the same catalyst failed to cause significant redispersion of Pd as observed with the oxy-chlorine gas treatment. © 2011 Elsevier B.V.