A UV-Vis micro-spectroscopic study to rationalize the influence of Cl−(aq) on the formation of different Pd macro-distributions on γ-Al2O3 catalyst bodies

Abstract

The influence of the Cl⁻(aq) concentration, solution pH and equilibration time on the PdCl₄²⁻(aq) dynamics and molecular structure after impregnation of γ-Al₂O₃ catalyst bodies has been studied using UV-Vis micro-spectroscopy. To do so, 0.2 wt% Pd catalysts have been prepared from acidic solutions (pH 1 and 5) of the Na₂PdCl₄ precursor salt with different amounts of NaCl. It was found that egg-shell catalysts are obtained when a less acidic pH (pH 5) is combined with [Cl⁻(aq)] < 0.6 M and less than 24 h of equilibration time are implemented, while to achieve egg-white catalysts the solution pH should be 1. Moreover, by increasing the equilibration time up to 96 h, the egg-shell profiles vanish to provide a uniform Pd distribution, while the egg-white distribution becomes egg-yolk. Additionally, Pd complexes appeared with different molecular structures depending on the solution pH, equilibration time and macro-distribution achieved. The protocol developed to create different Pd macro-distributions has been applied to prepare two 1 wt% Pd/γ-Al₂O₃ egg-shell and egg-white catalysts. The Pd dynamics and molecular structure have been followed after impregnation, drying and calcination, demonstrating that the profiles created after impregnation are retained.