Kinetics Analysis of Palladium/Gold Nanoparticles as Colloidal Hydrodechlorination Catalysts

Abstract

The aqueous-phase hydrodechlorination (HDC) of trichloroethene (TCE) is an important chemical reaction for water pollution control, for which unsupported palladium-on-gold and palladium nanoparticles (Pd/Au and Pd NPs) definitively show the beneficial effects of gold on palladium catalysis. The observed batch reactor kinetics can be erroneously oversimplified when concentration and mass transfer effects are neglected. A comprehensive treatment of NP catalysis is presented here using Pd-based NPs as the catalytic colloid and TCE HDC as the model reaction. Mass transfer effects were quantified for three specific compositions (Pd/Au NPs with 30% and 60% Pd surface coverages, and pure Pd NPs) by analyzing the observed reaction rates as functions of stirring rate and initial catalyst charge. The largest effect on observed reaction rates came from gas−liquid mass transfer. The TCE HDC reaction was modeled as a Langmuir−Hinshelwood mechanism involving competitive chemisorption of dihydrogen and TCE for all three NP compositions. Differences in adsorption affinities of the reactant molecules for the Pd/Au and Pd surfaces are suggested as responsible for the observed difference in TCE reaction order at high TCE concentrations; that is, first-order for Pd/Au NPs and non-first-order for Pd NPs.