Activation of oxygen on (NH3Cu NH3)+ in NH3-SCR over Cu-CHA

First-Principles Thermodynamic Background of the Comprehensive Reaction Network of NO Oxidation over CuSSZ-13 Catalysts-Influence of Copper Speciation and Interpretation of TPD and TPSR Profiles

A thorough molecular DFT modeling coupled with first-principles thermodynamic (FPT), spectroscopic (EPR/IR), and catalytic investigations into a complex network of reactions involved in the interaction of NO and O2 with a comprehensive variety of active centers present in the CuSSZ-13 zeolites (Cu2+, Cu+, Cu2+-OH-, Cu2+-O2--Cu2+, Cu2+-O2 2--Cu2+, and segregated CuO) were carried out. The molecular structure, energetics, and electronic and magnetic properties of the identified profuse adspecies and intermediates were ascertained. Their thermal stability and reactivity at a wide range of experimental conditions were interpreted by using the constructed thermodynamic ΔG(p,T) diagrams. The course of selective catalytic oxidation of NO (NO-SCO) with 16O2 or 18O2 was examined by a temperature-programmed surface reaction (TPSR) using two types of CuSSZ-13 catalysts of intentionally diverse copper speciation. The results obtained, supported by the corroborative IR and EPR measurements, revealed multiple molecular pathways of the NO and O2 interactions with the single (Cu2+, Cu+, Cu2+-OH-) and dual (Cu2+-O2--Cu2+, Cu2+-O2 2--Cu2+) copper centers of the 6MR and 8MR topologies and with segregated CuO. The complex reaction network and temperature behavior of the critical intermediates (HONO, nitrate, and nitrite), and their evolvement routes into NO2, were rationalized using the calculated FPT thermodynamic profiles. The unraveled reactions were classified into metal (cationic) redox, ligand (anionic) redox, and HONO redox cycles. The Cu2+-OH- species were identified as prime active centers for the formation of NO2 via the HONO pathway. The elusive HONO intermediates allow for chemical communication between the individual redox cycles. Depending on the actual reaction conditions, HONO can act as a reduction agent for Cu2+ with the electroprotic formation of NO2, a source of nitrites upon deprotonation, or as an oxidant of Cu+ with the formation of H2O and NO. For the metal redox pathway, a significant difference in the reactivity between the Cu2+ cations accommodated in the 6MRs and 8MRs was observed, with the Cu2+/6MR being spectators and the Cu2+/8MR active species. Dimeric copper centers with bridging oxo and peroxy moieties can produce a variety of nitrates and nitrites via ligand redox mechanisms. Segregated CuO nanocrystals contribute to NO oxidation only at high temperatures (T > 400 °C), leading to the isotopic scrambling of 18O-labeled oxygen and nitric oxide. The established complex reaction network was successfully used to clarify the temperature dependence of the experimental NO-SCO profiles, also providing a suitable mechanistic background for interpreting the nature of the oxidative half-cycle of the selective catalytic reduction of NO over Cu-SSZ-13 catalysts.

Toward synergetic reduction of pollutant and greenhouse gas emissions from vehicles: a catalysis perspective

It is a great challenge for vehicles to satisfy the increasingly stringent emission regulations for pollutants and greenhouse gases. Throughout the history of the development of vehicle emission control technology, catalysts have always been in the core position of vehicle aftertreatment. Aiming to address the significant demand for synergistic control of pollutants and greenhouse gases from vehicles, this review provides a panoramic view of emission control technologies and key aftertreatment catalysts for vehicles using fossil fuels (gasoline, diesel, and natural gas) and carbon-neutral fuels (hydrogen, ammonia, and green alcohols). Special attention will be given to the research advancements in catalysts, including three-way catalysts (TWCs), NOx selective catalytic reduction (SCR) catalysts, NOx storage-reduction (NSR) catalysts, diesel oxidation catalysts (DOCs), soot oxidation catalysts, ammonia slip catalysts (ASCs), methane oxidation catalysts (MOCs), N2O abatement catalysts (DeN2O), passive NOx adsorbers (PNAs), and cold start catalysts (CSCs). The main challenges for industrial applications of these catalysts, such as insufficient low-temperature activity, product selectivity, hydrothermal stability, and poisoning resistance, will be examined. In addition, the future development of synergistic control of vehicle pollutants and greenhouse gases will be discussed from a catalysis perspective.

A Computational Mechanistic Study on Copper Autoreduction in Cu-CHA Zeolite Catalysts

The activation of Cu-zeolite catalysts is accompanied by an autoreduction reaction, in which a part of Cu(II) species is spontaneously reduced to Cu(I) species. The stoichiometry of autoreduction in which the release of one O2 is accompanied by the reduction of four Cu(II) to Cu(I) has been proposed, but the detailed mechanism of this autoreduction remains unclear. In this work, we used DFT calculations to study the autoreduction mechanism in Cu-CHA zeolites. The two reduction mechanisms of [CuOH]+ to Cu+ in CHA-type zeolite were systematically studied. In Mechanism I, two [CuOH]+ react via dehydration to form [Cu-O-Cu]2+, and the further reaction of two [Cu-O-Cu]2+ to produce O2 is the most critical step, which requires four charge-compensating framework Al in close proximity. In Mechanism II, the production of O2 occurs via [CuO]+ intermediates, and the generation of possible [CuO]+ is the most critical step. The exploration of autoreduction reactions in a variety of Cu-CHA models with different Al sittings shows that the O-O distances between two intermediate precursors, i. e., two [Cu-O-Cu]2+ in Mechanism I, or two [CuO]+ in Mechanism II, are key factors determining the activation barriers of O2 production during autoreduction.

Influence of aluminium distribution on the diffusion mechanisms and pairing of [Cu(NH3)2]+ complexes in Cu-CHA

The performance of Cu-exchanged chabazite (Cu-CHA) for the ammonia-assisted selective catalytic reduction of NOx (NH3-SCR) depends critically on the presence of paired[ Cu ( NH 3 ) 2 ] + complexes. Here, a machine-learning force field augmented with long-range Coulomb interactions is developed to investigate the effect of Al-distribution and Cu-loading on the mobility and pairing of[ Cu ( NH 3 ) 2 ] + complexes. Performing unbiased and constrained molecular dynamics simulations, we obtain unique information inaccessible to first-principle calculations and experiments. The free energy barrier for[ Cu ( NH 3 ) 2 ] + diffusion between CHA-cages depends sensitively on both the local and distant Al-distribution. Importantly, certain Al-distributions and arrangements of neighboring[ Cu ( NH 3 ) 2 ] + andNH 4 + cations make paired[ Cu ( NH 3 ) 2 ] + complexes exothermic with respect to separated configurations. Our results suggest that the NH3-SCR activity can be enhanced by increasing the Cu-loading and Al-content. The dynamic interplay between[ Cu ( NH 3 ) 2 ] + andNH 4 + diffusion is crucial for the[ Cu ( NH 3 ) 2 ] + mobility and stresses the need to explore large systems including long-range Coulomb interactions when studying diffusion of charged species in zeolites.

Unraveling the NH3-SCR-DeNOx Mechanism of Cu-SSZ-13 Variants by Spectroscopic and Transient Techniques

Commercial SSZ-13 zeolite with different n(Si)/n(Al) ratios and from different suppliers were subjected to a post-synthetic treatment in order to create mesopores of up to 15 nm. Furthermore, the materials were modified with copper ions and thoroughly physico-chemically characterized. The modified textural properties varied the nature of copper species, and thus, activity in the selective catalytic reduction of NOx with ammonia (NH3-SCR-DeNOx). Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) studies with hexane as probe liquid revealed improved intracrystalline diffusion for some Cu-containing SSZ-13 materials. The NH3-SCR-DeNOx pathways are verified via in situ DR UV-Vis, in situ FT-IR and EPR, temperature-programmed studies as well as SSITKA studies that provide a mechanistic understanding of the reaction. Kinetic modelling results demonstrate the highest NH3-SCR-DeNOx reaction rates and up to 20 % lower energy barriers with n(Si)/n(Al) ratio of 6.5 for all modified forms (i. e., (NH4)Cu-SSZ-13_6.5 and Cu-SSZ-13_6.5_NaOH/0.1) and cause only negligible parasitic ammonia oxidation. The modelling of the stop-flow experiments further demonstrates that the SCR pathway via the HONO surface intermediate is present but barely contributes to the overall NO conversion compared to the dominant path between adsorbed NH3 and NO from the gas phase.

Kinetic Monte Carlo Simulations of Low-Temperature NH3-SCR over Cu-Exchanged Chabazite

Cu-exchanged chabazite (Cu-CHA) is widely used for ammonia assisted selective catalytic reduction of nitrogen oxides (NH3-SCR). The Cu+ ions are at low temperatures solvated by NH3 forming mobile [Cu(NH3)2]+ complexes. The dynamic behaviour of the complexes is critical as O2 adsorption requires a pair of complexes to form a [Cu2(NH3)4O2]2+ peroxo-species over which NO couples with NH3. Here we introduce a first principles-based kinetic Monte Carlo approach to explore the effect of the Al-distribution on the reaction kinetics of NH3-SCR over Cu-CHA. The method allows us to scrutinize the interplay between the pairing of [Cu(NH3)2]+ complexes and the reaction landscape for the NH3-SCR reaction over the peroxo-complex. The Al-distribution affects the stability of the [Cu(NH3)2]+ pairs as well as the kinetic parameters of the SCR-reaction. The turn-over frequency is determined by the stability of the [Cu(NH3)2]+ pairs and the relative strength of NO and NH3 adsorption once a pair is present. The results establish the hierarchy of effects that influences the performance of Cu-CHA over NH3-SCR and provide a computational basis for further development of the Cu-CHA material.

Thermally stable high-loading single Cu sites on ZSM-5 for selective catalytic oxidation of NH3

Rigorous comparisons between single site- and nanoparticle (NP)-dispersed catalysts featuring the same composition, in terms of activity, selectivity, and reaction mechanism, are limited. This limitation is partly due to the tendency of single metal atoms to sinter into aggregated NPs at high loadings and elevated temperatures, driven by a decrease in metal surface free energy. Here, we have developed a unique two-step method for the synthesis of single Cu sites on ZSM-5 (termed CuS/ZSM-5) with high thermal stability. The atomic-level dispersion of single Cu sites was confirmed through scanning transmission electron microscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance spectroscopy. The CuS/ZSM-5 catalyst was compared to a CuO NP-based catalyst (termed CuN/ZSM-5) in the oxidation of NH3 to N2, with the former exhibiting superior activity and selectivity. Furthermore, operando XAFS and diffuse reflectance infrared Fourier transform spectroscopy studies were conducted to simultaneously assess the fate of the Cu and the surface adsorbates, providing a comprehensive understanding of the mechanism of the two catalysts. The study shows that the facile redox behavior exhibited by single Cu sites correlates with the enhanced activity observed for the CuS/ZSM-5 catalyst.

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NOx with NH3

The formation of dimer-Cu species, which serve as the active sites of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies on the mobility of CuI species in the channels of the Cu-SSZ-13 catalysts. Herein, the key role of framework Brønsted acid sites in the mobility of reactive Cu ions was elucidated via a combination of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance ultraviolet-visible spectroscopy. When the number of framework Al sites decreases, the Brønsted acid sites decrease, leading to a systematic increase in the diffusion barrier for [Cu(NH3)2]+ and less formation of highly reactive dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of Al sites is uneven, the [Cu(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage (e.g., cage with paired Al sites), which effectively accelerates the formation of dimer-Cu species and hence promotes the SCR reaction. These findings unveil the mechanism by which framework Brønsted acid sites influence the intercage diffusion and reactivity of [Cu(NH3)2]+ complexes in Cu-SSZ-13 catalysts and provide new insights for the development of zeolite-based catalysts with excellent SCR activity by regulating the microscopic spatial distribution of framework Brønsted acid sites.

Interpretation of H2-TPR from Cu-CHA Using First-Principles Calculations

Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H2 temperature-programmed reduction (H2-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NOx (NH3-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z2Cu, where Z represents an Al site. Here, we analyze H2-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H2 can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z2CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z2Cu and Z2CuHOOHCu species (360 °C) or paired Z2Cu and Z2CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.

Unexpected Promotion Effect of H2O on the Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-39 Catalysts

Water molecules commonly inhibit the selective catalytic reduction (SCR) of NOx with NH3 on most catalysts, and water resistance is a long-standing challenge for SCR technology. Herein, by combining experimental measurements and density functional theory (DFT) calculations, we found that water molecules do not inhibit and even promote the NOx conversion to some extent over the Cu-SSZ-39 zeolites, a promising SCR catalyst. Water acting as a ligand on active Cu sites and as a reactant in the SCR reaction significantly improves the O2 activation performance and reduces the overall energy barrier of the catalytic cycle. This work unveils the mechanism of the unexpected promotion effect of water on the NH3-SCR reaction over Cu-SSZ-39 and provides fundamental insight into the development of zeolite-based SCR catalysts with excellent activity and water resistance.

Revealing the Formation and Reactivity of Cage-Confined Cu Pairs in Catalytic NOx Reduction over Cu-SSZ-13 Zeolites by In Situ UV-Vis Spectroscopy and Time-Dependent DFT Calculation

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH3-SCR) of toxic NOx pollutants from heavy-duty vehicles, remains a debate and needs to be clarified for further improvement of NH3-SCR performance. In this study, we established experimental protocols to follow the dynamic redox cycling (i.e., CuII ↔ CuI) of Cu sites in Cu-SSZ-13 during low-temperature NH3-SCR catalysis by in situ ultraviolet-visible spectroscopy and in situ infrared spectroscopy. Further integrating the in situ spectroscopic observations with time-dependent density functional theory calculations allows us to identify two cage-confined transient states, namely, the O2-bridged Cu dimers (i.e., μ-η2:η2-peroxodiamino dicopper) and the proximately paired, chemically nonbonded CuI(NH3)2 sites, and to confirm the CuI(NH3)2 pair as a precursor to the O2-bridged Cu dimer. Comparative transient experiments reveal a particularly high reactivity of the CuI(NH3)2 pairs for NO-to-N2 reduction at low temperatures. Our study demonstrates direct experimental evidence for the transient formation and high reactivity of proximately paired CuI sites under zeolite confinement and provides new insights into the monomeric-to-dimeric Cu transformation for completing the Cu redox cycle in low-temperature NH3-SCR catalysis over Cu-SSZ-13.

Influence of Solvent on Selective Catalytic Reduction of Nitrogen Oxides with Ammonia over Cu-CHA Zeolite

The copper-exchanged zeolite Cu-CHA has received considerable attention in recent years, owing to its application in the selective catalytic reduction (SCR) of NO_x species. Here, we study the NH₃-SCR reaction mechanism on Cu-CHA using the hybrid quantum mechanical/molecular mechanical (QM/MM) technique and investigate the effects of solvent on the reactivity of active Cu species. To this end, a comparison is made between water- and ammonia-solvated and bare Cu species. The results show the promoting effect of solvent on the oxidation component of the NH₃-SCR cycle since the formation of important nitrate species is found to be energetically more favorable on the solvated Cu sites than in the absence of solvent molecules. Conversely, both solvent molecules are predicted to inhibit the reduction component of the NH₃-SCR cycle. Diffuse reflectance infrared fourier-transform spectroscopy (DRIFTS) experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) identified spectroscopic signatures of Cu-nitrate and Cu-nitrosamine (H₂NNO), important species which had not been previously observed experimentally. This is further supported by the QM/MM-calculated harmonic vibrational analysis. Additional insights are provided into the reactivity of solvated active sites and the formation of key intermediates including their formation energies and vibrational spectroscopic signatures, allowing the development of a detailed understanding of the reaction mechanism. We demonstrate the role of solvated active sites and their influence on the energetics of important species that must be explicitly considered for an accurate understanding of NH₃-SCR kinetics.

Simplified Kinetic Model for $$\hbox {NH}_3$$-SCR Over Cu-CHA Based on First-Principles Calculations

Selective catalytic reduction with ammonia as reducing agent ($$\hbox {NH}_3$$ NH 3 -SCR) is an efficient technology to control $$\hbox {NO}_\mathrm{x}$$ NO x emission during oxygen excess. Catalysts based on Cu-chabazite (Cu-CHA) have shown good performance for $$\hbox {NH}_3$$ NH 3 -SCR with high activity and selectivity at low temperature and good hydrothermal stability. Here, we explore a first-principles based kinetic model to analyze in detail which reaction steps that control the selectivity for $$\hbox {N}_2$$ N 2 and the light-off temperature. Moreover, a simplified kinetic model is developed by fitting lumped kinetic parameters to the full model. The simplified model describes the reaction with high accuracy using only five reaction steps. The present work provides insight into the governing reaction mechanism and stimulates design of knowledge-based Cu-CHA catalysts for $$\hbox {NH}_3$$ NH 3 -SCR.

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

This contribution aims at analysing the current understanding about the influence of Al distribution, zeolite topology, ligands/reagents and oxidation state on ions mobility in Cu-zeolites, and its relevance toward reactivity of the metal sites. The concept of Cu mobilization has been originally observed in the presence of ammonia, favouring the activation of oxygen by formation of NH3 oxo-bridged complexes in zeolites and opening a new perspective about the chemistry in single-site zeolite-based catalysts, in particular in the context of the NH3-mediated Selective Catalytic Reduction of NO x (NH3-SCR) processes. A different mobility of bare Cu+/Cu2+ ions has been documented too, showing for Cu+ a better mobilization than for Cu2+ also in absence of ligands. These concepts can have important consequences for the formation of Cu-oxo species, active and selective in other relevant reactions, such as the direct conversion of methane to methanol. Here, assessing the structure, the formation pathways and reactivity of Cu-oxo mono- or multimeric moieties still represents a challenging playground for chemical scientists. Translating the knowledge about Cu ions mobility and redox properties acquired in the context of NH3-SCR reaction into the field of direct conversion of methane to methanol can have important implications for a better understanding of transition metal ions redox properties in zeolites and for an improved design of catalysts and catalytic processes.

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNO x and NH3-SCO

The reduction of NO x emissions has become one of the most important subjects in environmental protection. Cu-containing SSZ-13 is currently the state-of-the-art catalyst for the selective catalytic reduction of NO x with ammonia (NH3-SCR-DeNO x ). Although the current-generation catalysts reveal enhanced activity and remarkable hydrothermal stability, still open challenges appear. Thus, this review focuses on the progress of Cu-containing SSZ-13 regarding preparation methods, hydrothermal resistance and poisoning as well as reaction mechanisms in NH3-SCR-DeNO x . Remarkably, the paper reviews also the progress of Cu-containing SSZ-13 in the selective ammonia oxidation into nitrogen and water vapor (NH3-SCO). The dynamics in the NH3-SCR-DeNO x and NH3-SCO fields make this review timely.

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNOx Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy

Cu-exchanged chabazite is the catalyst of choice for NO_x abatement in diesel vehicles aftertreatment systems via ammonia-assisted selective catalytic reduction (NH₃-SCR). Herein, we exploit in situ X-ray absorption spectroscopy powered by wavelet transform analysis and machine learning-assisted fitting to assess the impact of the zeolite composition on NH₃-mobilized Cu-complexes formed during the reduction and oxidation half-cycles in NH₃-SCR at 200 °C. Comparatively analyzing well-characterized Cu-CHA catalysts, we show that the Si/Al ratio of the zeolite host affects the structure of mobile dicopper(II) complexes formed during the oxidation of the [Cu^I(NH₃)₂]⁺ complexes by O₂. Al-rich zeolites promote a planar coordination motif with longer Cu-Cu interatomic distances, while at higher Si/Al values, a bent motif with shorter internuclear separations is also observed. This is paralleled by a more efficient oxidation at a given volumetric Cu density at lower Si/Al, beneficial for the NO_x conversion under NH₃-SCR conditions at 200 °C.

In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR

The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA).

Understanding the superior NH3-SCR activity on CHA zeolite synthesized via template-free interzeolite transformation

Synthesizing CHA-type zeolite via template-free interzeolite transformation has been considered as a green manner, but the resultant zeolite exhibits low activity in selective catalytic reduction of NOx by ammonia (NH3-SCR)....

Inhibition Effect of Phosphorus Poisoning on the Dynamics and Redox of Cu Active Sites in a Cu-SSZ-13 NH3-SCR Catalyst for NOx Reduction

Phosphorus (P) stemming from biodiesel and/or lubricant oil additives is unavoidable in real diesel exhausts and deactivates gradually the Cu-SSZ-13 zeolite catalyst for ammonia-assisted selective catalytic NO_x reduction (NH₃-SCR). Here, the deactivation mechanism of Cu-SSZ-13 by P-poisoning was investigated by ex situ examination of the structural changes and by in situ probing the dynamics and redox of Cu active sites via a combination of impedance spectroscopy, diffuse reflection infrared Fourier transform spectroscopy, and ultraviolet-visible spectroscopy. We unveiled that strong interactions between Cu and P led to not only a loss of Cu active sites for catalytic turnovers but also a restricted dynamic motion of Cu species during low-temperature NH₃-SCR catalysis. Furthermore, the Cu^II ↔ Cu^I redox cycling of Cu sites, especially the Cu^I → Cu^II reoxidation half-cycle, was significantly inhibited, which can be attributed to the restricted Cu motion by P-poisoning disabling the formation of key dimeric Cu intermediates. As a result, the NH₃-SCR activity at low temperatures (200 °C and below) decreased slightly for the mildly poisoned Cu-SSZ-13 and considerably for the severely poisoned Cu-SSZ-13.

Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

The speciation of framework-interacting CuII sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NOx with NH3 is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O2 mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O2 followed by the reaction. XAS and UV-Vis data clearly show the main presence of Z2CuII sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted. EXAFS wavelet transform analysis showed a non-negligible fraction of proximal Z2CuII monomers, possibly stabilized into two 6-membered rings within the same cage. These sites are not able to form Cu-nitrates by interaction with NO/O2. By contrast, framework-anchored Z[CuII(NO3)] complexes with a chelating bidentate structure are formed in samples with a higher Si/Al ratio, by reaction of NO/O2 with Z[CuII(OH)] sites or structurally similar mono- or multi-copper Zx[CuIIxOy] sites. Linear combination fit (LCF) analysis of the XAS data showed good agreement between the fraction of Z[CuII(OH)]/Zx[CuIIxOy] sites formed during activation in O2 and that of Z[CuII(NO3)] complexes formed by reaction with NO/O2, further confirming the chemical inertia of Z2CuII towards these reactants in the absence of solvating NH3 molecules.

Structures and catalytic performances of Me/SAPO-34 (Me = Mn, Ni, Co) catalysts for low-tem perature SCR of NOx by ammonia

Me/SAPO-34 (Me = Mn, Ni, Co) series of catalysts were prepared by a wetness impregnation method and investigated for the selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR). Among them, Mn/SAPO-34 catalyst was found as the most promising candidate based on its superior low-temperature activity. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy images (TEM), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction and desorption (TPR and TPD), and diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) of NH₃/NO_x adsorption. Mn/SAPO-34 is obviously different from Ni/SAPO-34 and Co/SAPO-34 in the active species state and distribution. Surface MnO_x species which play an essential role in NO oxidation and NO₂ adsorption, act as better active sites than nickel and cobalt mostly in the form of the aluminates and silicates.

Highly Efficient NO Abatement over Cu-ZSM-5 with Special Nanosheet Features

Conventional Cu-ZSM-5 and special Cu-ZSM-5 catalysts with diverse morphologies (nanoparticles, nanosheets, hollow spheres) were synthesized and comparatively investigated for their performances in the selective catalytic reduction (SCR) of NO to N₂ with ammonia. Significant differences in SCR behavior were observed, and nanosheet-like Cu-ZSM-5 showed the best SCR performance with the lowest T₅₀ of 130 °C and nearly complete conversion in the temperature range of 200-400 °C. It was found that Cu-ZSM-5 nanosheets [mainly exposed (0 1 0) crystal plane] with abundant mesopores and framework Al species were favorable for the formation of high external surface areas and Al pairs, which influenced the local environment of Cu. This motivated the preferential formation of active copper species and the rapid switch between Cu²⁺ and Cu⁺ species during NH₃-SCR, thus exhibiting the highest NO conversion. In situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that the Cu-ZSM-5 nanosheets were dominated by the Eley-Rideal (E-R) mechanism and the labile nitrite species (NH₄NO₂) were the crucial intermediates during the NH₃-SCR process, while the inert nitrates were more prone to generate on Cu-ZSM-5 nanoparticles and conventional one. The combined density functional theory (DFT) calculations revealed that the decomposition energy barrier of nitrosamide species (NH₂NO) on the (0 1 0) crystal plane of Cu-ZSM-5 was lower than those on (0 0 1) and (1 0 0) crystal planes. This study provides a strategy for the design of NH₃-SCR zeolite catalysts.

On the Redox Mechanism of Low-Temperature NH3 -SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

Cu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications. Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathways in SCR working conditions. Herein we address the redox mechanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques, transient-response methods, operando UV/Vis-NIR spectroscopy with modelling tools based on transient kinetic analysis and density functional theory calculations. We show that the rates of the Reduction Half-Cycle (RHC) of LT-SCR display a quadratic dependence on Cu^II , thus questioning mechanisms based on isolated Cu^II -ions. We propose, instead, a Cu^II -pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates accounts for Cu^II reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade.

Understanding the nature of NH3-coordinated active sites and the complete reaction schemes for NH3-SCR using Cu-SAPO-34 catalysts

Cu-SAPO-34 zeolite catalysts show excellent NH3-SCR performance at low temperature, which is due to the catalytic capacity of copper species. Isolated CuII ions and CuIIOH are active sites, but their nature and role are not fully understood. This paper reports the DFT calculations in combination with ab initio thermodynamics to investigate NH3 and H2O coordination to copper species under typical NH3-SCR reaction conditions. In the reduction part of the NH3-SCR reaction, NH2NO and NH4NO2 intermediates will form on CuII-2NH3/3NH3 and CuIIOH-2NH3 complexes, respectively. The Brønsted acid sites are crucial for the decomposition of these intermediates, rather than copper species. Furthermore, the decomposition of NH2NO is more energetically favorable than NH4NO2 which are formed on the Brønsted acid sites. In the re-oxidation part of the NH3-SCR reaction, O2 dissociation and NO2 formation occur on CuI-2NH3 complexes in the presence of NO, and the regeneration of CuIIOH-2NH3 requires the participation of H2O. The proposed complete mechanisms highlight the importance of ligand coordinated copper species for intermediate formation and O2 activation in NH3-SCR.

To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule

Loewenstein's rule, which states that Al-O-Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.

In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

Ammonia-mediated selective catalytic reduction (NH3-SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NOx removal from exhaust gases, with CuII/CuI redox cycles being the basis of the catalytic reaction. The amount of CuII ions reduced by NO/NH3 can be quantified by the consumption of NO during temperature-programmed reduction experiments (NO-TPR). In this article, we show the capabilities of in situ X-ray absorption near-edge spectroscopy (XANES), coupled with multivariate curve resolution (MCR) and principal component analysis (PCA) methods, in following CuII/CuI speciation during reduction in NO/NH3 after oxidation in NO/O2 at 50 °C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH3 ramp CuII ions are fully reduced to CuI in the 50–290 °C range. The number of species involved in the process, their XANES spectra and their concentration profiles as a function of the temperature were obtained by MCR and PCA. Mixed ligand ammonia solvated complexes [CuII(NH3)3(X)]+ (X = OH−/O− or NO3−) are present at the beginning of the experiment, and are transformed into mobile [CuI(NH3)2]+ complexes: these complexes lose an NH3 ligand and become framework-coordinated above 200 °C. In the process, multiple CuII/CuI reduction events are observed: the first one around 130 °C is identified with the reduction of [CuII(NH3)3(OH/O)]+ moieties, while the second one occurs around 220–240 °C and is associated with the reduction of the ammonia-solvated Cu-NO3− species. The nitrate concentration in the catalysts is found to be dependent on the zeolite Cu loading and on the applied pretreatment conditions. Ammonia solvation increases the number of CuII sites available for the formation of nitrates, as confirmed by infrared spectroscopy.

Comprehensive effect of tuning Cu/SAPO-34 crystals using PEG on the enhanced hydrothermal stability for NH3-SCR

PEG fabricates advantageous hierarchical zeolite crystals, enhancing the high-temperature and low-temperature hydrothermal stability of Cu/SAPO-34.

Selective catalytic reduction of NO over Cu-AFX zeolites: mechanistic insights from in situ/operando spectroscopic and DFT studies

In situ/operando spectroscopic experiments and DFT calculations unravel the redox mechanism of NH₃-SCR over Cu-AFX zeolites.

Recent Understanding of Low-Temperature Copper Dynamics in Cu-Chabazite NH3-SCR Catalysts

Dynamic motion of NH3-solvated Cu sites in Cu-chabazite (Cu-CHA) zeolites, which are the most promising and state-of-the-art catalysts for ammonia-assisted selective reduction of NOx (NH3-SCR) in the aftertreatment of diesel exhausts, represents a unique phenomenon linking heterogeneous and homogeneous catalysis. This review first summarizes recent advances in the theoretical understanding of such low-temperature Cu dynamics. Specifically, evidence of both intra-cage and inter-cage Cu motions, given by ab initio molecular dynamics (AIMD) or metadynamics simulations, will be highlighted. Then, we will show how, among others, synchrotron-based X-ray spectroscopy, vibrational and optical spectroscopy (diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and diffuse reflection ultraviolet-visible spectroscopy (DRUVS)), electron paramagnetic spectroscopy (EPR), and impedance spectroscopy (IS) can be combined and complement each other to follow the evolution of coordinative environment and the local structure of Cu centers during low-temperature NH3-SCR reactions. Furthermore, the essential role of Cu dynamics in the tuning of low-temperature Cu redox, in the preparation of highly dispersed Cu-CHA catalysts by solid-state ion exchange method, and in the direct monitoring of NH3 storage and conversion will be presented. Based on the achieved mechanistic insights, we will discuss briefly the new perspectives in manipulating Cu dynamics to improve low-temperature NH3-SCR efficiency as well as in the understanding of other important reactions, such as selective methane-to-methanol oxidation and ethene dimerization, catalyzed by metal ion-exchanged zeolites.

Cu- and Fe-speciation in a composite zeolite catalyst for selective catalytic reduction of NOx: insights from operando XAS

Cu-SAPO-34 (Cu-CZC) and Fe-mordenite (Fe-MOR) and their mechanical mixture (50 : 50) have been exhaustively investigated by means of operando X-ray absorption spectroscopy under NH₃-SCR conditions.

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.

Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH3-Mediated Selective Catalytic Reduction

The NH3-mediated selective catalytic reduction (NH3-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH3-SCR, oxygen only reacts with CuI ions, which are present as mobile CuI diamine complexes [CuI(NH3)2]+. To determine the structure and reactivity of the species formed by oxidation of these CuI diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu2(NH3)4O2]2+ mobile complex with a side-on μ-η2,η2-peroxo diamino dicopper(II) structure, accounting for 80-90% of the total Cu content. These [Cu2(NH3)4O2]2+ are completely reduced to [CuI(NH3)2]+ at 200 °C in a mixture of NO and NH3. Some N2 is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH3-SCR reaction. The reaction of [Cu2(NH3)4O2]2+ complexes with NH3 leads to a partial reduction of the Cu without any formation of N2. The reaction with NO results in an almost complete reduction to CuI, under the formation of N2. This indicates that the low-temperature NH3-SCR reaction proceeds via a reaction of these complexes with NO.

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.