Mechanism of Oxygen Activation during Selective Catalytic Reduction of NO with NH3 in Cu-SSZ-13 Zeolites

Cu-SSZ-13 zeolites are used commercially in diesel engine exhaust after-treatment for abatement of NOx pollutants via selective catalytic reduction (SCR) with NH₃. Yet, the mechanistic details of the redox reactions they catalyze, and specifically of the Cu(I)->Cu(II) oxidation half-reaction, are not well understood. A detailed understanding of the SCR reaction mechanism and Cu active site requirements would provide insight into their catalytic performance and specific guidance on how to synthesize materials with improved low temperature (<473 K) reactivity.

The standard SCR reaction uses O₂ as the oxidant (4NH₃ + 4NO + O₂ -> 6H₂O + 4N₂) and involves a Cu(I)/Cu(II) redox cycle, with Cu(II) reduction mediated by NO and NH₃ [1], and Cu(I) oxidation mediated by NO and O₂. In contrast, the fast SCR reaction (4NH₃ + 2NO + 2NO₂ -> 6H₂O + 4N₂) uses NO₂ as the oxidant. Here, we show through a coordinated effort involving theory and experiment, that the fast and standard SCR reactions are not linked via common reactive intermediates but instead proceed with different active site requirements and by different elementary reaction steps.

Low temperature (437 K) standard SCR reaction kinetics depend strongly on the spatial density of Cu ions, varied by changing the Cu/Al and Si/Al ratio of Cu-SSZ-13 zeolites. Facilitated by NH₃ solvation [2], mobile Cu(I) complexes can dimerize with other Cu(I) complexes within diffusion distances to activate O₂, as demonstrated through X-ray absorption spectroscopy and density functional theory calculations. Monte Carlo simulations are used to define average Cu-Cu distances, ab initio metadynamics simulations to quantify Cu mobility, and climbing-image nudged elastic band calculations to calculate Cu diffusion energies between chabazite cages and activation of O₂ by Cu complexes. These theoretical results are validated by kinetic and spectroscopic experiments.

In contrast with O₂-assisted oxidation reactions, NO₂ oxidizes single Cu(I) complexes with similar kinetics among samples of varying Cu spatial density. These findings demonstrate that low temperature standard SCR is strongly dependent on Cu spatial density and requires NH₃ solvation to mobilize Cu(I) sites in order to activate O₂, while fast SCR uses NO₂ to oxidize single Cu sites and complete the catalytic redox cycle at one Cu site for both reduction and oxidation half-reactions.

(1) Paolucci, C.; Verma, A. A.; Bates, S. A.; Kispersky, V. F.; Miller, J. T.; Gounder, R.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F. Angew. Chem., Int. Ed. 2014, 53, 11828−11833.

(2) C. Paolucci, A. A. Parekh, I. Khurana, J. R. Di Iorio, H. Li, J. D. Albarracin Caballero, A. Shih, T. Anggara, W. N. Delgass, J. T. Miller, F. H. Ribeiro, R. Gounder and W. F. Schneider Journal of the American Chemical Society, (2016) DOI: 10.1021/jacs.6b02651