Poisoning of Silica-Supported Pd (But Not Pt!) by Residual Chloride for Hydrogen Chemisorption
Haiying Zhou, Alaba Ojo, Ismail Paykar, Santosh K Balijepalli, John R Monnier, John R RegalbutoAbstract
In this work, we compared the effect of residual chloride on chemisorption characterization of silica supported Pd and Pt. Pt/OX50 and Pd/OX50 samples were synthesized by strong electrostatic adsorption using chloride containing and chloride free precursors. Pd and Pt particle sizes were characterized by powder X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and chemisorption. The XRD and STEM confirmed less than 2 nm Pd and Pt particle sizes with a narrow size distribution. The Pd/OX50 synthesized by Pd(NH3)4Cl2 showed much lower chemisorption uptake than expected, which can be attributed to chloride blockage of Pd surface sites. In contrast, Pt/OX50 chemisorption was not poisoned by chloride as the chemisorption size estimate agreed with XRD and STEM. X-ray photoelectron spectroscopy (XPS) revealed the presence of chloride on the Pd sample but not on Pt before and after reduction. It appears that residual chloride has a strong affinity for Pd and not for Pt. A simple way to avoid chloride poisoning on Pd catalysts is to employ entirely chloride-free precursors during synthesis.