Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal‐Exchanged Zeolites

Thirumalai, Hari; Rimer, Jeffrey D.; Grabow, Lars C.

doi:10.1002/cctc.201901229

Cited by 3 publications

(1 citation statement)

References 101 publications

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“…However, several questions remain regarding the low-temperature NH 3 -SCR mechanism. Although an NH 3 -solvated Cu 2 O 2 dimer is an accepted intermediate in the oxidation half-cycle, ,,,,, the exact structure of the complex relevant to steady-state turnover is challenging to identify by experiment or computation. − Furthermore, the elementary steps that decompose this Cu 2 O 2 species to close the SCR cycle remain unknown. Many intermediates (including NH 3 -solvated Cu–nitrosyl complexes) ,, and pathways (including reactions involving HONO and H 2 O and NH 4 + species) − have been proposed, but reconciling these competing proposals may require interrogation away from “standard” SCR reaction conditions and on varying material compositions , to access regimes in which they can be kinetically discriminated.…”

Section: Discussionmentioning

confidence: 99%

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

et al. 2020

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Conspectus Copper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NO x ) via selective catalytic reduction (SCR) reactions with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NO x under strongly oxidizing conditions and to achieve improved NO x conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants. One critical mechanistic feature is the NH3 solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu–amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu2+/Cu+ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms. In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu+ oxidation with O2 becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the m...

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