Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction

William  Schneider, University of Notre Dame

Christopher Paolucci,¹ Ishant Khurana,² Atish A. Parekh,¹ Sichi Li,¹ Arthur J. Shih,² Hui Li,¹ John R. Di Iorio,² Jonatan D. Albarracin-Caballero,² Aleksey Yezerets,³ Jeffrey T. Miller,² W. Nicholas Delgass,² Fabio H. Ribeiro,² William F. Schneider,¹ Rajamani Gounder²

¹Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA

²Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

³Cummins Inc., 1900 McKinley Ave., MC 50183, Columbus, IN 47201, USA

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of NO_x with NH₃, but the low-temperature rate dependence on Cu volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an O₂-mediated Cu^I → Cu^II redox step integral to SCR.  Electrostatic tethering to framework Al centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst and suggests strategies for improving the low-temperature performance of SCR catalysts.

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