First-principles and experimental interrogation of fast SCR mechanism in H-SSZ-13 Zeolite

Experiments and density functional theory (DFT) models are combined to develop a unified, quantitative model of the mechanism and kinetics of fast selective catalytic reduction (SCR) of NO/NO2 mixtures over H-SSZ-13 zeolite. Rates, rate orders, and apparent activation energies collected under differential conditions reveal two distinct kinetic regimes. First-principles thermodynamics simulations are used to determine the relative coverages of free Brønsted sites, chemisorbed NH4+ and physisorbed NH3 as a function of reaction conditions. First-principles metadynamics calculations show that all three sites can contribute to the rate-limiting N−N bond forming step in fast SCR. The results are used to parameterize a kinetic model that encompasses the full range of reaction conditions and recovers observed rate orders and apparent activation energies. Observed kinetic regimes are related to changes in most-abundant surface intermediates. 


Experimental and Computational Interrogation of Fast SCR Mechanism and Active Sites on H-Form SSZ-13

S. Li, Y. Zheng, F. Gao, J. Szanyi and W. F. Schneider*. ACS Catal., 2017, 7, 5087-5096

Supporting Information


2016 AIChE Annual Meeting, San Francisco, CA

Metadynamics evaluation of copper ion mobility and its importance in NOx selective catalytic reduction

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temperature rate dependence on copper (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 oxygen (O2)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum 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.

In this project, I apply metadynamics simulation to determine the free energy profile of CuI transport within zeolite cavity and across window, and construct a potential model to rationalize the observation. This finding is a critical component that motivated the proposed mechanism.


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

C. Paolucci, I. Khurana, A. A. Parekh, S. Li, A. J. Shih, H. Li, J. R. Di Iorio, J. D. Albarracin-Caballero, A. Yezerets, J. T. Miller, W. N. Delgass, F. H. Ribeiro, W. F. Schneider* and R. Gounder*. Science, 2017, 357, 898-903 


Fig. 5 Simulated CuI(NH3)2 diffusion up to 11 Å from charge-compensating Al.

On left, the metadynamics-computed free energy at 473 K of CuI(NH3)2 in the 72–T site CHA supercell versus Cu-Al distance. The red line is the energy profile predicted from a point-charge electrostatic model, described in SM section S9. Labeled are reactant state (1) [CuI(NH3)2 in the same cage as Al], transition state (2) [CuI(NH3)2 diffusion through 8-MR], and product state (3) [CuI(NH3)2 in the neighboring cage without Al]. Corresponding representative CuI(NH3)2configurations from the trajectories are shown on the right. Gray, Cu; green, Al; blue, N; and white, H.