Cu/Chabazite catalysts for ‘Lean-Burn’ vehicle emission control

Kinetic Monte Carlo Analysis Reveals Non-mean-field Active Site Dynamics in Cu-Zeolite-Catalyzed NO x Reduction

Copper-exchanged chabazite (Cu-CHA) zeolites are the preferred catalysts for the selective catalytic reduction of NO x with NH3. The low temperature (473 K) SCR mechanism proceeds through a redox cycle between mobile and ammonia-solvated Cu(I) and Cu(II) complexes, as demonstrated by multiple experimental and computational investigations. The oxidation step requires two Cu(I) to migrate into the same cha cage to activate O2 and form a binuclear Cu(II)-di-oxo complex. Prior steady state and transient kinetic experiments find that the apparent rate constants for oxidation (per Cu ion) are sensitive to catalyst composition and follow nonmean-field kinetics. We develop a nonmean-field kinetic model for NO x SCR that incorporates a composition-dependent Cu(I) volumetric footprint centered at anionic [AlO4]- tetrahedral sites on the CHA lattice. We use Bayesian optimization to parameterize a kinetic Monte Carlo model against available experimental composition-dependent SCR rates and in situ Cu(II) fractions. We find that both rates and Cu(II) fractions of a majority of catalyst compositions can be captured by single oxidation and reduction rate constants combined with a composition-dependent Cu(I) cation footprint, highlighting the contributions of both Cu and Al densities to steady-state SCR performance of Cu-CHA. The work illustrates a pathway for extracting robust molecular insights from the kinetics of a dynamic catalytic system.

Effect of Framework Composition and NH3 on the Diffusion of Cu+ in Cu-CHA Catalysts Predicted by Machine-Learning Accelerated Molecular Dynamics

Cu-exchanged zeolites rely on mobile solvated Cu+ cations for their catalytic activity, but the role of the framework composition in transport is not fully understood. Ab initio molecular dynamics simulations can provide quantitative atomistic insight but are too computationally expensive to explore large length and time scales or diverse compositions. We report a machine-learning interatomic potential that accurately reproduces ab initio results and effectively generalizes to allow multinanosecond simulations of large supercells and diverse chemical compositions. Biased and unbiased simulations of [Cu(NH3)2]+ mobility show that aluminum pairing in eight-membered rings accelerates local hopping and demonstrate that increased NH3 concentration enhances long-range diffusion. The probability of finding two [Cu(NH3)2]+ complexes in the same cage, which is key for SCR-NOx reaction, increases with Cu content and Al content but does not correlate with the long-range mobility of Cu+. Supporting experimental evidence was obtained from reactivity tests of Cu-CHA catalysts with a controlled chemical composition.

Entropy and Energy Profiles of Chemical Reactions

The description of chemical processes at the molecular level is often facilitated by the use of reaction coordinates or collective variables (CVs). The CV measures the progress of the reaction and allows the construction of profiles that track how specific properties evolve as the reaction progresses. Whereas CVs are routinely used, especially alongside enhanced sampling techniques, the links among reaction profiles, thermodynamic state functions, and reaction rate constants are not rigorously exploited. Here, we report a unified treatment of such reaction profiles. Tractable expressions are derived for the free-energy, internal-energy, and entropy profiles as functions of only the CV. We demonstrate the ability of this treatment to extract quantitative insight from the entropy and internal-energy profiles of various real-world physicochemical processes, including intramolecular organic reactions, ionic transport in superionic electrolytes, and molecular transport in nanoporous materials.

Vehicle emissions of primary air pollutants from 2009 to 2019 and projection for the 14th Five-Year Plan period in Beijing, China

Over the past decade, the emission standards and fuel standards in Beijing have been upgraded twice, and the vehicle structure has been improved by accelerating the elimination of 2.95 million old vehicles. Through the formulation and implementation of these policies, the emissions of carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NO_x), and fine particulate matter (PM_2.5) in 2019 were 147.9, 25.3, 43.4, and 0.91 kton in Beijing, respectively. The emission factor method was adopted to better understand the emissions characteristics of primary air pollutants from combustion engine vehicles and to improve pollution control. In combination with the air quality improvement goals and the status of social and economic development during the 14th Five-Year Plan period in Beijing, different vehicle pollution control scenarios were established, and emissions reductions were projected. The results show that the emissions of four air pollutants (CO, VOCs, NO_x, and PM_2.5) from vehicles in Beijing decreased by an average of 68% in 2019, compared to their levels in 2009. The contribution of NO_x emissions from diesel vehicles increased from 35% in 2009 to 56% in 2019, which indicated that clean and energy-saving diesel vehicle fleets should be further improved. Electric vehicle adoption could be an important measure to reduce pollutant emissions. With the further upgrading of vehicle structure and the adoption of electric vehicles, it is expected that the total emissions of the four vehicle pollutants can be reduced by 20%-41% by the end of the 14th Five-Year Plan period.

A review on the characterization of metal active sites over Cu-based and Fe-based zeolites for NH3-SCR

Cu-based and Fe-based zeolites are promising catalysts for NH3-SCR due to their high catalytic activity, wide temperature window and good hydrothermal stability, while the detailed investigation of NH3-SCR mechanism should be based on the accurate determination of active metal sites. This review systematically summarizes the qualitative and quantitative determination of metal active sites in Cu-based or Fe-based zeolites for NH3-SCR reactions based on advanced characterization methods such as UV-vis absorption (UV-vis), temperature-programmed reduction with H2 (H2-TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), Infrared spectroscopy (IR), Electron paramagnetic resonance (EPR), Mössbauer spectroscopy and DFT calculations. The application and limitations of different characterization methods are also discussed to provide insights for further study of the NH3-SCR reaction mechanism over metal-based zeolites.

From free-energy profiles to activation free energies

Given a chemical reaction going from reactant (R) to the product (P) on a potential energy surface (PES) and a collective variable (CV) discriminating between R and P, we define the free-energy profile (FEP) as the logarithm of the marginal Boltzmann distribution of the CV. This FEP is not a true free energy. Nevertheless, it is common to treat the FEP as the “free-energy” analog of the minimum potential energy path and to take the activation free energy, [Formula: see text], as the difference between the maximum at the transition state and the minimum at R. We show that this approximation can result in large errors. The FEP depends on the CV and is, therefore, not unique. For the same reaction, different discriminating CVs can yield different [Formula: see text]. We derive an exact expression for the activation free energy that avoids this ambiguity. We find [Formula: see text] to be a combination of the probability of the system being in the reactant state, the probability density on the dividing surface, and the thermal de Broglie wavelength associated with the transition. We apply our formalism to simple analytic models and realistic chemical systems and show that the FEP-based approximation applies only at low temperatures for CVs with a small effective mass. Most chemical reactions occur on complex, high-dimensional PES that cannot be treated analytically and pose the added challenge of choosing a good CV. We study the influence of that choice and find that, while the reaction free energy is largely unaffected, [Formula: see text] is quite sensitive.

Rate Controlling in Low-Temperature Standard NH3-SCR: Implications from Operando EPR Spectroscopy and Reaction Kinetics

A series of seven Cu/SSZ-13 catalysts with Si/Al = 6.7 are used to elucidate key rate-controlling factors during low-temperature standard ammonia-selective catalytic reduction (NH₃-SCR), via a combination of SCR kinetics and operando electron paramagnetic resonance (EPR) spectroscopy. Strong Cu-loading-dependent kinetics, with Cu atomic efficiency increasing nearly by an order of magnitude, is found when per chabazite cage occupancy for Cu ion increases from ∼0.04 to ∼0.3. This is due mainly to the release of intercage Cu transfer constraints that facilitates the redox chemistry, as evidenced from detailed Arrhenius analysis. Operando EPR spectroscopy studies reveal strong connectivity between Cu-ion dynamics and SCR kinetics, based on which it is concluded that under low-temperature steady-state SCR, kinetically most relevant Cu species are those with the highest intercage mobility. Transient binuclear Cu species are mechanistically relevant species, but their splitting and cohabitation are indispensable for low-temperature kinetics.

SO2 Poisoning of Cu-CHA deNO x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH3-assisted selective catalytic reduction of NO (NH3-SCR) for the abatement of NO x emission from diesel vehicles. However, the presence of a small amount of SO2 in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO2 exposure, we have selectively prepared CuI and CuII species with different ligations, which are relevant for the NH3-SCR reaction. The highest reactivity toward SO2 was observed for CuII species coordinated to both NH3 and extraframework oxygen, in particular for [CuII 2(NH3)4O2]2+ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO2 uptake was significantly lower. These results explain why SO2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [CuII 2(NH3)4O2]2+ is a crucial intermediate in the low-temperature NH3-SCR catalytic cycle.

Understanding the direct relationship between various structure-directing agents and low-temperature hydrothermal durability over Cu-SAPO-34 during the NH3-SCR reaction

Hydrolysis of Si–O(H)–Al bonds and the loss of active Cu(OH)+ species jointly contribute towards the deactivation of Cu-SAPO-34 under a moist environment at low temperature (<100 °C).

Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations

The mobility of the copper cations acting as active sites for the selective catalytic reduction of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temperature and feed composition. Herein, the migration of [Cu(NH₃)₂]⁺ complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O₂, H₂O, and NH₃), in the initial and final cavities is investigated using ab initio molecular dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH₃ or NO in the initial cavity, since both reactants form with [Cu(NH₃)₂]⁺ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O₂ modifies strongly the interaction of NO with Cu⁺. At low temperatures, we observe NO detachment from Cu⁺ and increased mobility of the [Cu(NH₃)₂]⁺ complex, while at high temperatures, NO reacts spontaneously with O₂ to form NO₂. The present simulations give evidence for recent experimental observations, namely, an NH₃ inhibition effect on the SCR reaction at low temperatures, and transport limitations of NO and NH₃ at high temperatures. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temperatures, the former involving dimeric Cu(NH₃)₂-O₂-Cu(NH₃)₂ species and the latter occurring by direct NO oxidation to NO₂ in one single cavity.

Emission Variations of Primary Air Pollutants from Highway Vehicles and Implications during the COVID-19 Pandemic in Beijing, China

According to the traffic flow variation from January 2019 to August 2020, emissions of primary air pollutants from highway vehicles were calculated based on the emission factor method, which integrated the actual structure of on-road vehicles. The characteristics of on-highway traffic flow and pollution emissions were compared during various progression stages of coronavirus disease (COVID-19). The results showed that the average daily traffic volume decreased by 38.2% in 2020, with a decrease of 62% during the strict lockdown due to the impact of COVID-19. The daily emissions of primary atmospheric pollutants decreased by 29.2% in 2020 compared to the same period in 2019. As for the structure of on-highway vehicle types, the small and medium-sized passenger vehicles predominated, which accounted for 76.3% of traffic, while trucks and large passenger vehicles accounted for 19.7% and 4.0%, but contributed 58.4% and 33.9% of nitrogen oxide (NO_x) emissions, respectively. According to the simulation results of the ADMS model, the average concentrations of NO_x were reduced by 12.0 µg/m³ compared with the same period in 2019. As for the implication for future pollution control, it is necessary to further optimize the structure of on-highway and the road traffic vehicle types and increase the proportions of new-energy vehicles and vehicles with high emission standards.

High NH3-SCR reaction rate with low dependence on O2 partial pressure over Al-rich Cu-*BEA zeolite

Dependence of NH₃-SCR reaction rate on O₂ partial pressure was investigated at 473 K over Cu ion-exchanged MOR, MFI, CHA and *BEA zeolites with varying “Cu density in micropores”. Among the zeolites, Cu–*BEA zeolite demonstrated promising potential as an effective catalyst for NH₃-SCR over a wide range of O₂ partial pressure.

It was revealed that Al-rich Cu–*BEA zeolite exhibit high reaction rate for NH₃-SCR at 473 K in low P_O2 reaction condition.

Kinetic and spectroscopic insights into the behaviour of Cu active site for NH3-SCR over zeolites with several topologies

Zeolite topology has a great effect on the dependence of NH₃-SCR rates over Cu–zeolites at 473 K on Cu density. It is revealed by the time-resolved UV-vis measurements that zeolites mainly affect the oxidation property of Cu ion by O₂.

Mechanistic studies of NH3-assisted reduction of mononuclear Cu(ii) cation sites in Cu-CHA zeolites

Spectroscopic, titrimetric, and gas-phase product analysis methods reveal a six-electron process for NH3-assisted reduction of mononuclear Cu(ii) sites to Cu(i) in Cu-CHA zeolites of different Cu(ii) site speciation and density.

Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions

The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu⁺ and Cu²⁺ cations from their positions coordinated to the zeolite framework, forming mobile Cu⁺(NH₃)₂ and Cu²⁺(NH₃)₄ complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800-1000 cm^-1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions.

Fe-Exchanged Small-Pore Zeolites as Ammonia Selective Catalytic Reduction (NH3-SCR) Catalysts

Cu-exchanged small-pore zeolites have been extensively studied in the past decade as state-of-the-art selective catalytic reduction (SCR) catalysts for diesel engine exhaust NOx abatement for the transportation industry. During this time, Fe-exchanged small-pore zeolites, e.g., Fe/SSZ-13, Fe/SAPO-34, Fe/SSZ-39 and high-silica Fe/LTA, have also been investigated but much less extensively. In comparison to their Cu-exchanged counterparts, such Fe/zeolite catalysts display inferior low-temperature activities, but improved stability and high-temperature SCR selectivities. Such characteristics entitle these catalysts to be considered as key components of highly efficient emission control systems to improve the overall catalyst performance. In this short review, recent studies on Fe-exchanged small-pore zeolite SCR catalysts are summarized, including (1) the synthesis of small-pore Fe/zeolites; (2) nature of the SCR active Fe species in these catalysts as determined by experimental and theoretical approaches, including Fe species transformation during hydrothermal aging; (3) SCR reactions and structure-function correlations; and (4) a few aspects on industrial applications.

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

ConspectusCopper-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 mobility constraints imposed by the CHA framework on NH₃-solvated Cu ions, which regulate the pore volume accessible to these ions and their ability to pair and complete the catalytic cycle. This highlights the key characteristics of the CHA framework that enable superior performance under low-temperature SCR reaction conditions.This work illustrates the power of precise control over a catalytic material, simultaneous kinetic and spectroscopic interrogation over a wide range of reaction conditions, and computational strategies tailored to capture those reaction conditions to reveal in microscopic detail the mechanistic features of a complex and widely practiced catalysis. In doing so, it highlights the key role of ion mobility in catalysis and thus potentially a more general phenomenon of reactant solvation and active site mobilization in reactions catalyzed by exchanged metal ions in zeolites.

Combining Kinetics and Operando Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NOx Selective Catalytic Reduction with NH3 on Cu-Zeolites

NO_x selective catalytic reduction (SCR) with NH₃ on Cu-zeolites is a commercial emissions control technology for diesel and lean-burn engines. Mitigating low-temperature emissions remains an outstanding challenge, motivating an improved understanding of the reaction mechanism, active site requirements, and rate-determining processes at low temperatures (<523 K). In this Perspective, we discuss how operando spectroscopy provides crucial information about how the structures, coordination environments, and oxidation states of Cu active sites depend on reaction conditions and sample composition; when combined with kinetic measurements, such operando data provide insights into the Cu site and spatial density requirements for reduction and oxidation steps relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated to zeolite oxygen atoms ex situ become coordinated to NH₃ in situ and dynamically interconvert between mononuclear and binuclear NH₃-solvated Cu complexes to catalyze SCR turnovers. We conclude with future research directions that can benefit from combining quantitative kinetic measurements with operando spectroscopy.