Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH<sub>3</sub>–SCR–NO<sub><i>x</i></sub>) Conditions
Reisel Millán, Pieter Cnudde, Alexander E. J. Hoffman, Christian W. Lopes, Patricia Concepción, Véronique Van Speybroeck, Mercedes Boronat
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.