Computational modeling predicts the stability of both Pd+ and Pd2+ ion-exchanged into H-CHA

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 9; no. 4; pp. 2161 - 2174
• Main Authors: Jeroen Van der Mynsbrugge, Head-Gordon, Martin, Bell, Alexis T
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 28.01.2021
• Subjects: Candidate species; Cation exchanging; Cold starts; Computer applications; Electronic structure; emission control; Free energy; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Investigations; Ion exchange; Oligomers; Oxidation; passive NOx adsorbers; Phase diagrams; QM/MM; Quantum chemistry; Sensitivity analysis; Speciation; Spectroscopy; Stability; thermodynamics; Vehicle emissions; Zeolites
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d0ta11254b

Passive NOx adsorbers (PNA) using Pd/zeolites have emerged as a promising solution for the reduction of cold-start emissions from vehicle exhaust. However, the nature of the active sites and the mechanisms underlying NOx adsorption in Pd/zeolites remain a subject of ongoing investigation. In this study, we employ quantum chemical simulations to investigate the structure of Pd species in cation-exchange sites at isolated Al and Al pairs in the 6-ring and 8-ring of the CHA framework before the introduction of NOx. Our calculations show that the speciation of Pd in these exchange sites strongly depends on the precise Al arrangement within the framework, as well as the operating conditions. Ionically dispersed Pd is found to be the most favorable species over a wide range of oxidizing and reducing conditions. Small oligomers of PdO and metallic Pd do not appear to be competitive at either isolated Al or Al pairs. Notably, our calculations show that ion exchange sites other than next–next–nearest neighbor Al pairs in the 6-ring will be preferentially occupied by Pd+ instead of Pd2+. The stability of Pd+ in the zeolite environment is an interesting contrast with its rareness in molecular Pd compounds. Nonetheless, a detailed analysis of the electronic structure shows that predicted Pd oxidation states are consistent with chemical intuition for all complexes investigated in this study. We also discuss the potential ambiguity in Pd characterization provided by typical experimental techniques such as XANES, EXAFS and UV-Vis, and highlight the need for additional EPR spectroscopy studies to further elucidate the initial Pd speciation in zeolites for PNA applications.