(674a) Mechanism of Oxygen Activation during Selective Catalytic Reduction of NO with NH3 in Cu-SSZ-13 Zeolites

Paolucci, C., University of Notre Dame
Parekh, A. A., Purdue University
Li, H., University of Notre Dame
Schneider, W. F., University of Notre Dame
Khurana, I., Purdue University
Gounder, R., Purdue University
Di Iorio, J. R., Purdue University
Delgass, W. N., Purdue University
Ribeiro, F. H., Purdue University
Miller, J., Purdue University
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, 11828â??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