A simple redox model of low-T NO + CO adsorption onto Pd-CHA as effective passive NOx adsorbers†

Abstract

Pd-exchanged chabazite (Pd-CHA) catalysts show NO adsorption and desorption features which comply well with the requirements for low-T passive NOx adsorber (PNA) applications. An earlier work based on transient adsorption tests investigated the NO storage pathway on Pd-CHA, a still debated topic in the literature. Such research highlighted a Pd-redox mechanism (Pd²⁺ ↔ Pd⁺) underlying the NO storage chemistry over these systems. CO and NO were capable of reducing Pd²⁺ at low temperatures, and the newly formed Pd⁺ acted as the main NO storage site. Increasing temperatures activated a Pd-oxidation process, which reduced the fraction of Pd⁺ sites, and consequently the NO storage, but was inhibited by H₂O. Herein we challenge quantitatively such a scheme relying on transient kinetic analysis. We show that a simple redox kinetic model of NO + CO storage on Pd-CHA, based on the above, reproduces the main features of the species evolution and of the NO storage observed under variable operating conditions over Pd-CHA samples with two Pd-loadings, thus lending support to the proposed Pd-redox chemistry.