Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3–SCR–NOx) Conditions

Abstract

The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800–1000 cm–1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions. ; This work has been supported by the European Union through ERC-AdG-2014-671093 (SynCatMatch), Spanish Government through ''Severo Ochoa" (SEV-2016-0683, MINECO), and MAT2017-82288-C2-1-P (AEI/FEDER, UE). Red Española de Supercomputación (RES) and Servei d'Informàtica de la Universitat de València (SIUV) are acknowledged for computational resources and technical support. R.M. thanks ITQ for his contract. V.V.S. and P.C. acknowledge funding from the European Union's Horizon 2020 research and innovation program (consolidator ERC grant agreement No. 647755 - DYNPOR (2015–2020)). V.V.S. acknowledges the Research Board of the Ghent University (BOF). The computational resources and services used were provided by Ghent University (Stevin Supercomputer Infrastructure), the VSC (Flemish Supercomputer Center), funded by the Research Foundation - Flanders (FWO). ; Peer reviewed