Oxy-chlorination as an effective treatment of aged Pd/CeO2-Al2O3 catalysts for Pd redispersion

The present work reports on the effects of oxy-chlorine gas treatment (use of Cl-2/O-2/He gas mixture) applied on a 5 wt% Pd/20 wt% CeO2-Al2O3 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 (x vol% Cl-2/18 vol% 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 2 vol% Cl-2/18 vol% O-2/He gas mixture at 500 degrees C for 1 h, where a significant reduction of an initial Pd mean particle size of 17.3-7.5 nm was obtained. XPS studies revealed that after oxy-chlorine gas treatment followed by H-2 reduction at 500 degrees 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 degrees C) followed by H-2 reduction (300-500 degrees C) applied over the same catalyst failed to cause significant redispersion of Pd as observed with the oxy-chlorine gas treatment. (C) 2011 Elsevier B.V. All rights reserved.