Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

Cleaving DNA with DNA: Cooperative Tuning of Structure and Reactivity Driven by Copper Ions

A copper-dependent self-cleaving DNA (DNAzyme or deoyxyribozyme) previously isolated by in vitro selection has been analyzed by a combination of Molecular Dynamics (MD) simulations and advanced Electron Paramagnetic Resonance (Electron Spin Resonance) EPR/ESR spectroscopy, providing insights on the structural and mechanistic features of the cleavage reaction. The modeled 46-nucleotide deoxyribozyme in MD simulations forms duplex and triplex sub-structures that flank a highly conserved catalytic core. The DNA self-cleaving construct can also form a bimolecular complex that has a distinct substrate and enzyme domains. The highly dynamic structure combined with an oxidative site-specific cleavage of the substrate are two key-aspects to elucidate. By combining EPR/ESR spectroscopy with selectively isotopically labeled nucleotides it has been possible to overcome the major drawback related to the "metal-soup" scenario, also known as "super-stoichiometric" ratios of cofactors versus substrate, conventionally required for the DNA cleavage reaction within those nucleic acids-based enzymes. The focus on the endogenous paramagnetic center (Cu2+) here described paves the way for analysis on mixtures where several different cofactors are involved. Furthermore, the insertion of cleavage reaction within more complex architectures is now a realistic perspective towards the applicability of EPR/ESR spectroscopic studies.

New Features of Laboratory-Generated EPFRs from 1,2-Dichlorobenzene (DCB) and 2-Monochlorophenol (MCP)

The present research is primarily focused on investigating the characteristics of environmentally persistent free radicals (EPFRs) generated from commonly recognized aromatic precursors, namely, 1,2-dichlorobenzene (DCB) and 2-monochlorophenol (MCP), within controlled laboratory conditions at a temperature of 230 °C, termed as DCB230 and MCP230 EPFRs, respectively. An intriguing observation has emerged during the creation of EPFRs from MCP and DCB utilizing a catalyst 5% CuO/SiO2, which was prepared through various methods. A previously proposed mechanism, advanced by Dellinger and colleagues (a conventional model), postulated a positive correlation between the degree of hydroxylation on the catalyst's surface (higher hydroxylated, HH and less hydroxylated, LH) and the anticipated EPFR yields. In the present study, this correlation was specifically confirmed for the DCB precursor. Particularly, it was observed that increasing the degree of hydroxylation at the catalyst's surface resulted in a greater yield of EPFRs for DCB230. The unexpected finding was the indifferent behavior of MCP230 EPFRs to the surface morphology of the catalyst, i.e., no matter whether copper oxide nanoparticles are distributed densely, sparsely, or completely agglomerated. The yields of MCP230 EPFRs remained consistent regardless of the catalyst type or preparation protocol. Although current experimental results confirm the early model for the generation of DCB EPFRs (i.e., the higher the hydroxylation is, the higher the yield of EPFRs), it is of utmost importance to closely explore the heterogeneous alternative mechanism(s) responsible for generating MCP230 EPFRs, which may run parallel to the conventional model. In this study, detailed spectral analysis was conducted using the EPR technique to examine the nature of DCB230 EPFRs and the aging phenomenon of DCB230 EPFRs while they exist as surface-bound o-semiquinone radicals (o-SQ) on copper sites. Various aspects concerning bound radicals were explored, including the hydrogen-bonding tendencies of o-semiquinone (o-SQ) radicals, the potential reversibility of hydroxylation processes occurring on the catalyst's surface, and the analysis of selected EPR spectra using EasySpin MATLAB. Furthermore, alternative routes for EPFR generation were thoroughly discussed and compared with the conventional model.

OSDA-Free Seeded Cu-Containing ZSM-5 Applied for NH3-SCR-DeNOx

A series of ZSM-5 zeolite materials were synthesized from organic structure-directing agent (OSDA)-free seeded systems, including nanosized silicalite-1 (12 wt % water suspension or in powder form) or nanosized ZSM-5 (powder form of ZSM-5 prepared at 100 or 170 °C). The physicochemical characterization revealed aggregated species in the samples based on silicalite-1. Contrarily, the catalysts based on ZSM-5 seeds revealed isolated copper species, and thus, higher NO conversion during the selective catalytic reduction of NOx with NH3 (NH3-SCR-DeNOx) was observed. Furthermore, a comparison of the Cu-containing ZSM-5 catalysts, conventionally prepared in the presence of OSDAs and prepared with an environmentally more benign approach (without OSDAs), revealed their comparable activity in NH3-SCR-DeNOx.

Prediction of Cu Zeolite NH3-SCR Activity from Variable Temperature 1H NMR Spectroscopy

Selective catalytic reduction (SCR) of NOx by ammonia is one of the dominant pollution abatement technologies for near-zero NOx emission diesel engines. A crucial step in the reduction of NOx to N2 with Cu zeolite NH3-SCR catalysts is the generation of a multi-electron donating active site, implying the permanent or transient dimerization of Cu ions. Cu atom mobility has been implicated by computational chemistry as a key factor in this process. This report demonstrates how variable temperature 1H NMR reveals the Cu induced generation of sharp 1H resonances associated with a low concentration of sites on the zeolite. The onset temperature of the appearance of these signals was found to strongly correlate with the NH3-SCR activity and was observed for a range of catalysts covering multiple frameworks (CHA, AEI, AFX, ERI, ERI-CHA, ERI-OFF, *BEA), with different Si/Al ratios and different Cu contents. The results point towards universal applicability of variable temperature NMR to predict the activity of a Cu-zeolite SCR catalyst. The unique relationship of a spectroscopic feature with catalytic behavior for zeolites with different structures and chemical compositions is exceptional in heterogeneous catalysis.

Photocatalytic Oxidative Coupling of Ethane to n-Butane Using CO2 as a Soft Oxidant over NiTi-Layered Double Hydroxide

Selective conversion of ethane (C2 H6 ) to high-value-added chemicals is a very important chemical process, yet it remains challenging owing to the difficulty of ethane activation. Here, a NiTi-layered double hydroxide (NiTi-LDH) photocatalyst is reported for oxidative coupling of ethane to n-butane (n-C4 H10 ) by using CO2 as an oxidant. Remarkably, the as-prepared NiTi-LDH exhibits a high selectivity for n-C4 H10 (92.35%) with a production rate of 62.06 µmol g-1 h-1 when the feed gas (CO2 /C2 H6 ) ratio is 2:8. The X-ray absorption fine structure (XAFS) and photoelectron characterizations demonstrate that NiTi-LDH possesses rich vacancies and high electron-hole separation efficiency, which can promote the coupling of C2 H6 to n-C4 H10 . More importantly, density functional theory (DFT) calculations reveal that ethane is first activated on the oxygen vacancies of the catalyst surface, and the C─C coupling pathway is more favorable than the C─H cleavage to C2 H4 or CH4 , resulting in the high production rate and selectivity for n-C4 H10 .

Methane Oxidation over Cu2+ /[CuOH]+ Pairs and Site-Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy

Cu-exchanged mordenite (MOR) is a promising material for partial CH4 oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH4 oxidation involving paired [CuOH]+ and bare Cu2+ species has been identified. The reduction of bare Cu2+ ions facilitated by adjacent [CuOH]+ demonstrates that the frequently reported assumption of redox-inert Cu2+ centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu2+ active sites highlighting their difference in the CH4 oxidation potential.

Quantitative In Situ Monitoring of Cu-Atom Release by Cu2O Nanocatalysts under Photocatalytic CO2 Reduction Conditions: New Insights into the Photocorrosion Mechanism

Cu₂O is among the most promising photocatalysts for CO₂ reduction, however its photocorrosion remains a standalone challenge. Herein, we present an in situ study of the release of Cu ions from Cu₂O nanocatalysts under photocatalytic conditions in the presence of HCO₃ as a catalytic substrate in H₂O. The Cu-oxide nanomaterials were produced by Flame Spray Pyrolysis (FSP) technology. Using Electron Paramagnetic Resonance (EPR) spectroscopy in tandem with analytical Anodic Stripping Voltammetry (ASV), we monitored in situ the Cu²⁺ atom release from the Cu₂O nanoparticles in comparison with CuO nanoparticles under photocatalytic conditions. Our quantitative, kinetic data show that light has detrimental effect on the photocorrosion of Cu₂O and ensuing Cu²⁺ ion release in the H₂O solution, up to 15.7% of its mass. EPR reveals that HCO₃ acts as a ligand of the Cu²⁺ ions, promoting the liberation of {HCO₃-Cu} complexes in solution from Cu₂O, up to 27% of its mass. HCO₃ alone exerted a marginal effect. XRD data show that under prolonged irradiation, part of Cu²⁺ ions can reprecipitate on the Cu₂O surface, creating a passivating CuO layer that stabilizes the Cu₂O from further photocorrosion. Including isopropanol as a hole scavenger has a drastic effect on the photocorrosion of Cu₂O nanoparticles and suppresses the release of Cu²⁺ ions to the solution. Methodwise, the present data exemplify that EPR and ASV can be useful tools to help quantitatively understand the solid-solution interface photocorrosion phenomena for Cu₂O.

Interplay between copper redox and transfer and support acidity and topology in low temperature NH3-SCR

Low-temperature standard NH₃-SCR over copper-exchanged zeolite catalysts occurs on NH₃-solvated Cu-ion active sites in a quasi-homogeneous manner. As key kinetically relevant reaction steps, the reaction intermediate Cu^II(NH₃)₄ ion hydrolyzes to Cu^II(OH)(NH₃)₃ ion to gain redox activity. The Cu^II(OH)(NH₃)₃ ion also transfers between neighboring zeolite cages to form highly reactive reaction intermediates. Via operando electron paramagnetic resonance spectroscopy and SCR kinetic measurements and density functional theory calculations, we demonstrate here that such kinetically relevant steps become energetically more difficult with lower support Brønsted acid strength and density. Consequently, Cu/LTA displays lower Cu atomic efficiency than Cu/CHA and Cu/AEI, which can also be rationalized by considering differences in their support topology. By carrying out hydrothermal aging to eliminate support Brønsted acid sites, both Cu^II(NH₃)₄ ion hydrolysis and Cu^II(OH)(NH₃)₃ ion migration are hindered, leading to a marked decrease in Cu atomic efficiency for all catalysts.

A review on identification, quantification, and transformation of active species in SCR by EPR spectroscopy

Electron paramagnetic resonance (EPR) is the only technique that provides direct detection of free radicals and samples that contain unpaired electrons. Thus, EPR had an important potential application in the field of selective catalytic reduction of nitrogen oxide (SCR). For the first time, this work reviewed recent developments of EPR in charactering SCR. First, qualitative analysis focused on recognizing Cu, Fe, V, Ti, Mn, and free-radical (oxygen vacancy and superoxide radical) species. Second, quantification of the active species was obtained by a double-integral and calibration method. Third, the active species evolved because of different thermal treatments and redox-thermal processes under reductants (NH₃ and NO). The coordination information of the active species in catalysts and their effects on SCR performances were concluded from mechanism viewpoints. Finally, potential perspectives were put forward for EPR developments in characterizing the SCR processes in the future. After all, EPR characterization will help to have a deep understanding of structure-activity relationship in one catalyst.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

The Structure of Monomeric Hydroxo-CuII Species in Cu-CHA. A Quantitative Assessment

Using EPR and HYSCORE spectroscopies in conjunction with ab initio calculations, we assess the structure of framework-bound monomeric hydroxo-Cu^II in copper-loaded chabazite (CHA). The species is an interfacial distorted square-planar [Cu^IIOH(O-8MRs)₃] complex located at eight-membered-ring windows, displaying three coordinating bonds with zeolite lattice oxygens and the hydroxo ligand hydrogen-bonded to the cage. The complex has a distinctive EPR signature with g = [2.072 2.072 2.290], ^CuA= [30 30 410] MHz, and ^HA = [−13.0 −4.5 +11.5] MHz, distinctively different from other Cu^II species in CHA.

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.

Direct Observation of Ammonia Storage in UiO-66 Incorporating Cu(II) Binding Sites

The presence of active sites in metal-organic framework (MOF) materials can control and affect their performance significantly in adsorption and catalysis. However, revealing the interactions between the substrate and active sites in MOFs at atomic precision remains a challenging task. Here, we report the direct observation of binding of NH3 in a series of UiO-66 materials containing atomically dispersed defects and open Cu(I) and Cu(II) sites. While all MOFs in this series exhibit similar surface areas (1111-1135 m2 g-1), decoration of the -OH site in UiO-66-defect with Cu(II) results in a 43% enhancement of the isothermal uptake of NH3 at 273 K and 1.0 bar from 11.8 in UiO-66-defect to 16.9 mmol g-1 in UiO-66-CuII. A 100% enhancement of dynamic adsorption of NH3 at a concentration level of 630 ppm from 2.07 mmol g-1 in UiO-66-defect to 4.15 mmol g-1 in UiO-66-CuII at 298 K is observed. In situ neutron powder diffraction, inelastic neutron scattering, and electron paramagnetic resonance, solid-state nuclear magnetic resonance, and infrared spectroscopies, coupled with modeling reveal that the enhanced NH3 uptake in UiO-66-CuII originates from a {Cu(II)···NH3} interaction, with a reversible change in geometry at Cu(II) from near-linear to trigonal coordination. This work represents the first example of structural elucidation of NH3 binding in MOFs containing open metal sites and will inform the design of new efficient MOF sorbents by targeted control of active sites for NH3 capture and storage.

Purification of Hydrogen from CO with Cu/ZSM-5 Adsorbents

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and FTIR, H₂-TPR, CO-TPD, etc. The AE samples possess mainly isolated Cu²⁺ cations not adsorbing CO. Reduction generates Cu⁺ sites demonstrating different affinity to CO, with the strongest centres desorbing CO at about 350 °C. The SE samples have about twice higher Cu/Al ratios, as one H⁺ is exchanged with one Cu⁺ cation. Although some of the introduced Cu⁺ sites are oxidized to Cu²⁺ upon contact with air, they easily recover their original oxidation state after thermal treatment in vacuum or under inert gas stream. In addition, these Cu⁺ centres regenerate at relatively low temperatures. It is important that water does not block the CO adsorption sites because of the formation of Cu⁺(CO)(H₂O)_x complexes. Dynamic adsorption studies show that Cu/ZSM-5 selectively adsorbs CO in the presence of hydrogen. The results indicate that the SE samples are very perspective materials for purification of H₂ from CO.

17O-EPR determination of the structure and dynamics of copper single-metal sites in zeolites

The bonding of copper ions to lattice oxygens dictates the activity and selectivity of copper exchanged zeolites. By ¹⁷O isotopic labelling of the zeolite framework, in conjunction with advanced EPR methodologies and DFT modelling, we determine the local structure of single site Cu^II species, we quantify the covalency of the metal-framework bond and we assess how this scenario is modified by the presence of solvating H₂¹⁶O or H₂¹⁷O molecules. This enables to follow the migration of Cu^II species as a function of hydration conditions, providing evidence for a reversible transfer pathway within the zeolite cage as a function of the water pressure. The results presented in this paper establish ¹⁷O EPR as a versatile tool for characterizing metal-oxide interactions in open-shell systems.

Hybrid Cu-Fe/ZSM-5 Catalyst Prepared by Liquid Ion-Exchange for NOx Removal by NH3-SCR Process

A series of Cu/ZSM-5, Fe/ZSM-5, and Cu-Fe/ZSM-5 catalysts (Si/Al in ZSM-5 = 25) were prepared by different metal loadings using the liquid ion-exchange method. Several characterization methods were conducted to explore the effects of metals on the physical and chemical properties of catalysts. Meanwhile, the electron paramagnetic resonance method is also used to assess the copper and/or iron elements’ coordination and valence state at intersections or in channels of ZSM-5. The metal-loading effects of all catalysts on the catalytic activities were studied for the removal of NOx in a fixed-bed flow reactor using selective catalytic reduction with ammonia (NH3-SCR). The results showed that the iron’s addition could markedly broaden the operation temperature range of the Cu/ZSM-5 catalyst for NH3-SCR between 200 and 550°C due to the presence of more isolated Cu2+ ions as well as additional oligomeric Fe3+ active sites and FexOy oligomeric species. This paper gives a facile and straightforward way to synthesize the practical-promising catalyst applied in NH3-SCR technology to control NOx emissions.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

Copper-exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time-resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono-μ-oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono-μ-oxo species, while the activation energy is two times lower.

Computational Modeling Predicts the Stability of Both Pd+ and Pd2+ Ion-Exchanged into H-CHA

Passive NOx adsorbers (PNA) using Pd/zeolites have emerged as a promising solution for the reduction of cold-start emissions from vehicle exhaust. However, the nature of the active sites and the mechanisms underlying NOx adsorption in Pd/zeolites remain a subject of ongoing investigation. In this study, we employ quantum chemical simulations to investigate the structure of Pd species in cation-exchange sites at isolated Al and Al pairs in the 6-ring and 8-ring of the CHA framework, before the introduction of NOx. Our calculations show that the speciation of Pd in these exchange sites strongly depends on the precise Al arrangement within the framework, as well as the operating conditions. Ionically dispersed Pd is found to be the most favorable species over a wide range of oxidizing and reducing conditions. Small oligomers of PdO and metallic Pd do not appear to be competitive at either isolated Al or Al pairs. Notably, our calculations show that ion exchange sites other than next-next-nearest neighbor Al pairs in the 6-ring will be preferentially occupied by Pd+ instead of Pd2+. The stability of Pd+ in the zeolite environment is an interesting contrast with its rareness in molecular Pd compounds. Nonetheless, a detailed analysis of the electronic structure shows that predicted Pd oxidation states are consistent with chemical intuition for all complexes investigated in this study. We also discuss the potential ambiguity in Pd characterization provided by typical experimental techniques such as XANES, EXAFS and UV-VIS, and highlight the need for additional EPR spectroscopy studies to further elucidate the initial Pd speciation in zeolites for PNA applications.

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.

Framework coordination of single-ion Cu2+ sites in hydrated 17O-ZSM-5 zeolite

The interfacial coordination chemistry of water solvated single Cu²⁺ sites in ZSM-5 is assessed through pulsed EPR spectroscopy and selective ¹⁷O isotopic labelling.

Environmental benign synthesis of Nano-SSZ-13 via FAU trans-crystallization: Enhanced NH3-SCR performance on Cu-SSZ-13 with nano-size effect

Small pore zeolites with chabazite structure have been commercialized for selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with ammonium (NH₃) from diesel exhaust. However, conventional zeolite synthesis processes detrimental effects on the environment due to the consumption of large amount of water, organic templates. Thus, this study proposed a green synthesis process with addition of minimal amount of water, structure directing agent and shortened steps to prepare nano-sized SSZ-13 (0.12 μm) using trans-crystallization strategy and exhibited enhanced performance for NO_x removal after copper ion-exchange. The operation temperature window (NO_x conversion >90 %) as well as the SO₂ and H₂O resistance over the green-route prepared nano-sized SSZ-13 (178-480 °C) outperformed the conventional SSZ-13 (29.8 μm, 211-438 °C) mainly due to the much shorter diffusion path. This clearly implied that the mass transportation was key for NH₃-SCR of NO_x on such small pore zeolite catalysts, which was further confirmed via an in-depth mass transportation calculation process. These results demonstrate that the Cu-nano-sized SSZ-13 prepared by the environmental benign route has great potential to act as a new generation of deNO_x catalyst for diesel exhaust and provided a guideline for researchers to develop new methods to synthesize nano-catalysts for air pollution control.

Adsorption and activation of molecular oxygen over atomic copper(I/II) site on ceria

Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O₂) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O₂]^3- site selectively adsorbs molecular O₂, forming a rarely reported electrophilic η²-O₂ species at 298 K. Assisted by neighbouring Ce(III) cations, η²-O₂ is finally reduced to two O^2-, that create two Cu-O-Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s^-1 at 373 K and 0.01 bar P_CO. The unique electronic structure of [Cu(I)O₂]^3- site suggests its potential in selective oxidation.

Spectroscopic identification and catalytic relevance of NH4+ intermediates in selective NOx reduction over Cu-SSZ-13 zeolites

Reduction of harmful nitrogen oxides (NO_x) from diesel engine exhausts is one of the key challenges in environmental protection, and can be achieved by NH₃-assisted selective catalytic reduction (NH₃-SCR) using copper-exchanged chabazite zeolites (i.e. Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) as catalysts. Understanding the redox chemistry of Cu-CHA in NH₃-SCR catalysis is crucial for further improving the NO_x reduction efficiency. Here, a series of Cu-SSZ-13 catalysts with different Cu ion exchange levels were prepared, thoroughly characterized by different techniques such as X-ray diffraction, diffuse reflectance ultraviolet-visible spectroscopy and temperature-programmed desorption using NH₃ as a probe molecule, etc., and tested in NH₃-SCR reactions under steady-state conditions. In situ studies by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), supplemented with density-functional theory calculations, provided solid evidence for the formation of ammonium ion (NH₄⁺) intermediates resulting from the reduction of Cu²⁺ to Cu⁺ by co-adsorbed NH₃ and NO molecules on Cu-SSZ-13. Catalytic relevance of the NH₄⁺ intermediates, as demonstrated by an increase of NO conversion over Cu-SSZ-13 pre-treated in NH₃/NO atmosphere, can be attributed to the formation of closely coupled Cu⁺/NH₄⁺ pairs promoting the Cu⁺ re-oxidation and, consequently, the overall NH₃-SCR process. This study thus paves a new route for improving the NH₃-SCR efficiency over Cu-CHA zeolite catalyst.

Nanosized Cu-SSZ-13 and Its Application in NH3-SCR

Nanosized SSZ-13 was synthesized hydrothermally by applying N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as a structure-directing agent. In the next step, the quantity of TMAdaOH in the initial synthesis mixture of SSZ-13 was reduced by half. Furthermore, we varied the sodium hydroxide concentration. After ion-exchange with copper ions (Cu2+ and Cu+), the Cu-SSZ-13 catalysts were characterized to explore their framework composition (XRD, solid-state NMR, ICP-OES), texture (N2-sorption, SEM) and acid/redox properties (FT-IR, TPR-H2, DR UV-Vis, EPR). Finally, the materials were tested in the selective catalytic reduction of NOx with ammonia (NH3-SCR). The main difference between the Cu-SSZ-13 catalysts was the number of Cu2+ in the double six-membered ring (6MRs). Such copper species contribute to a high NH3-SCR activity. Nevertheless, all materials show comparable activity in NH3-SCR up to 350 °C. Above 350 °C, NO conversion decreased for Cu-SSZ-13(2–4) due to side reaction of NH3 oxidation.

The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH3-SCR

Systematic catalytic tests and in situ/operando spectroscopy uncovered structure–performance relationships determining the seagull profile of the NO_x conversion for Cu-SSZ-13 catalysts.

Site selective adsorption and relocation of SOx in deactivation of Cu–CHA catalysts for NH3-SCR

SO₂ selectively interacts with Z-CuOH in Cu–CHA catalysts for NH₃-SCR and relocates to Z₂-Cu during heating at 550 °C.

Consequences of exchange-site heterogeneity and dynamics on the UV-visible spectrum of Cu-exchanged SSZ-13

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.

The speciation and structure of Cu ions and complexes in chabazite (SSZ-13) zeolites, which are relevant catalysts for nitrogen oxide reduction and partial methane oxidation, depend on material composition and reaction environment. Ultraviolet-visible (UV-Vis) spectra of Cu-SSZ-13 zeolites synthesized to contain specific Cu site motifs, together with ab initio molecular dynamics and time-dependent density functional theory calculations, were used to test the ability to relate specific spectroscopic signatures to specific site motifs. Geometrically distinct arrangements of two framework Al atoms in six-membered rings are found to exchange Cu²⁺ ions that become spectroscopically indistinguishable after accounting for the finite-temperature fluctuations of the Cu coordination environment. Nominally homogeneous single Al exchange sites are found to exchange a heterogeneous mixture of [CuOH]⁺ monomers, O- and OH-bridged Cu dimers, and larger polynuclear complexes. The UV-Vis spectra of the latter are sensitive to framework Al proximity, to precise ligand environment, and to finite-temperature structural fluctuations, precluding the precise assignment of spectroscopic features to specific Cu structures. In all Cu-SSZ-13 samples, these dimers and larger complexes are reduced by CO to Cu⁺ sites at 523 K, leaving behind isolated [CuOH]⁺ sites with a characteristic spectroscopic identity. The various mononuclear and polynuclear Cu²⁺ species are distinguishable by their different responses to reducing environments, with implications for their relevance to catalytic redox reactions.

Cu-CHA – a model system for applied selective redox catalysis

We review the structural chemistry and reactivity of copper-exchanged molecular sieves with chabazite (CHA) topology, as an industrially applied catalyst in ammonia mediated reduction of harmful nitrogen oxides (NH₃-SCR) and as a general model system for red-ox active materials (also the recent results in the direct conversion of methane to methanol are considered).