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

Abstract

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 parametrize 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.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…For example, at reaction temperatures above~350 • C, fast SCR can proceed so readily on H/SSZ-13 that the presence of Cu has little influence on NO x conversions [6]. While recent studies by Li et al [48] of fast SCR mechanisms for H/SSZ-13, using both experimental and theoretical approaches are outside the scope of this review, clearly it is the understanding of lower temperature fast SCR mechanisms on Cu that is of higher significance.…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…Schneider and co‐workers carried a combined experimental and theoretical study on the mechanisms of the selective catalytic reduction (SCR) of NO/NO 2 by an H‐SSZ‐13 zeolite . A detailed kinetic model was constructed based on the results of DFT, metadynamics and aiMD calculations.…”

“…(b) Arrhenius plot of total rate of NOx SCR. Rates were computed at 180 ppm of NO and NO 2 and 360 ppm of NH 3 . Reprinted with permission from ACS Catal ., 2017, 7, 5087.…”

Formation and Reactions of NH4NO3 during Transient and Steady-State NH3-SCR of NOx over H-AFX Zeolites: Spectroscopic and Theoretical Studies

“…Pioneering kinetics and theoretical studies concerning fast or NO2 SCR utilized H-ZSM-5 zeolites, 10,13 and very little research 20 has focused on the effect of NO2 on SCR in conjunction with small-pore H-type zeolites. 20 To the best of our knowledge, there is only limited information regarding the relative reaction rates for standard, fast and NO2 SCR over H-type small-pore zeolites. 27 AFX zeolite is an 8-membered ring (that is, small pore) material having both aft cages (0.55 × 1.35 nm) and smaller gme cages (0.33 × 0.74 nm).…”