The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO ) with ammonia (NH), 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 (O)-mediated Cu→Cu 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.

Theory and experiment reveal relationships between observed UV-visible spectra and ion exchange site types, ion nuclearity, and finite-temperature dynamics in Cu exchanged chabazite (SSZ-13) zeolites.

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Cu in aged zeolites changes structure during SCR.

In this contribution, we synthesized three model Cu-SSZ-13 catalysts with primarily ZCuOH, Z2Cu, and extraframework Cu x O y species and then measured their N2O formation rates during standard selective catalytic reduction (SCR). We first present evidence that the formation of extraframework Cu x O y species after sequential aqueous ion exchange and calcination correlates with the formation of Cu­(OH)2 precipitates during ion exchange. These Cu x O y species are not active for standard SCR, and unchanged apparent activation energies and reaction orders demonstrate that these Cu x O y species do not induce transport limitations to accessible Cu2+ active centers. During standard SCR, N2O formation rates on a per Cu basis were the fastest (and exhibited higher selectivities) on ZCuOH, followed by Z2Cu and then extraframework Cu x O y . Because N2O formation apparent activation energies were indistinguishable from the standard SCR apparent activation energies associated with the reduction-limited step, we posit that N2O is formed during the standard SCR reduction step. Additionally, using sulfur poisons to force the ZCuOH rate-limiting step to the oxidation half-cycle resulted in an unchanged N2O formation apparent activation energy, further supporting our hypothesis. These results suggest that utilizing Cu-SSZ-13 catalysts with higher fractions of Z2Cu active centers in commercial aftertreatment systems can lead to reduced N2O emissions.

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