Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13

Synthesis and Structural Analysis of High-Silica ERI Zeolite with Spatially-Biased Al Distribution as a Promising NH3-SCR Catalyst

Erionite (ERI) zeolite has recently attracted considerable attention for its application prospect in the selective catalytic reduction of NOx with NH3 (NH3-SCR), provided that the high-silica (Si/Al > 5.5) analog with improved hydrothermal stability can be facilely synthesized. In this work, ERI zeolites with different Si/Al ratios (4.6, 6.4, and 9.1) are synthesized through an ultrafast route, and in particular, a high-silica ERI zeolite with a Si/Al ratio of 9.1 is obtained by using faujasite (FAU) as a starting material. The solid-state 29Si MAS NMR spectroscopic study in combination with a computational simulation allows for figuring out the atomic configurations of the Al species in the three ERI zeolites. It is revealed that the ERI zeolite with the highest Si/Al ratio (ERI-9.1, where the number indicates the Si/Al ratio) exhibits a biased Al occupancy at T1 site, which is possibly due to the presence of a higher fraction of the residual potassium cations in the can cages. In contrast, the Al siting in ERI-4.6 and ERI-6.4 proves to be relatively random.

Effect of NO2 on N2 O production and NOx emission reduction in NH3 Selective Catalytic Reduction

With the introduction of increasingly strict emission regulations, reducing nitrogen oxide (NOx ) emissions and nitrous oxide (N2 O) production from diesel engines have become the focus of research. At high temperature, the reaction of NO2 in the catalyst generates the intermediate product NH4 NO3 , which first crystallizes below 300 °C. These crystals tend to block the pores and inhibit the reaction. Subsequently, N2 O is produced through the decomposition of NH4 NO3 , leading to additional pollution. Therefore, the concentration of NO2 has a direct impact on both the NOx conversion efficiency and the generation of N2 O, requiring consideration of the optimal proportion of NO2 in SCR. Considering these two factors, it is concluded that the optimal amount of NO2 varies with temperature. To improve the NOx conversion rate of the Cu-SSZ-13 catalyst at low temperatures and reduce N2 O generation, the optimal NO2 ratio of the Cu-SSZ-13 catalyst under various operating conditions is studied using numerical simulations. As the temperature rises, the optimal NO2 /NOx ratio first increases and then decreases. Under the optimal NO2 /NOx ratio, the NOx conversion rate significantly increases, while N2 O generation decreases considerably. The optimal NO2 /NOx ratio also provides suggestions for the optimization of the DOC-DPF-DCR system.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Zeolite Properties, Methods of Synthesis, and Selected Applications

Zeolites, a group of minerals with unique properties, have been known for more than 250 years. However, it was the development of methods for hydrothermal synthesis of zeolites and their large-scale industrial applications (oil processing, agriculture, production of detergents and building materials, water treatment processes, etc.) that made them one of the most important materials of the 20th century, with great practical and research significance. The orderly, homogeneous crystalline and porous structure of zeolites, their susceptibility to various modifications, and their useful physicochemical properties contribute to the continuous expansion of their practical applications in both large-volume processes (ion exchange, adsorption, separation of mixture components, catalysis) and specialized ones (sensors). The following review of the knowledge available in the literature on zeolites aims to present the most important information on the properties, synthesis methods, and selected applications of this group of aluminosilicates. Special attention is given to the use of zeolites in agriculture and environmental protection.

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.

Copper Site Motion Promotes Catalytic NOx Reduction under Zeolite Confinement

Ammonia-mediated selective catalytic reduction (NH3-SCR) is currently the key approach to abate nitrogen oxides (NOx) emitted from heavy-duty lean-burn vehicles. The state-of-art NH3-SCR catalysts, namely, copper ion-exchanged chabazite (Cu-CHA) zeolites, perform rather poorly at low temperatures (below 200 °C) and are thus incapable of eliminating effectively NOx emissions under cold-start conditions. Here, we demonstrate a significant promotion of low-temperature NOx reduction by reinforcing the dynamic motion of zeolite-confined Cu sites during NH3-SCR. Combining complex impedance-based in situ spectroscopy (IS) and extended density-functional tight-binding molecular dynamics simulation, we revealed an environment- and temperature-dependent nature of the dynamic Cu motion within the zeolite lattice. Further coupling in situ IS with infrared spectroscopy allows us to unravel the critical role of monovalent Cu in the overall Cu mobility at a molecular level. Based on these mechanistic understandings, we elicit a boost of NOx reduction below 200 °C by reinforcing the dynamic Cu motion in various Cu-zeolites (Cu-CHA, Cu-ZSM-5, Cu-Beta, etc.) via facile postsynthesis treatments, either in a reductive mixture at low temperatures (below 250 °C) or in a nonoxidative atmosphere at high temperatures (above 450 °C).

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.

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.

Study on the Performance of the Zr-Modified Cu-SSZ-13 Catalyst for Low-Temperature NH3-SCR

Cu-SSZ-13 and Zr-modified Cu-SSZ-13 catalysts with different Zr/Cu mass ratios were prepared by ion-exchange and impregnation methods, respectively. The NH₃-SCR performance tests were performed using the catalyst performance evaluation device to investigate the effects of different Zr/Cu mass ratios on the catalyst ammonia-selective catalytic reduction (NH₃-SCR) performance. X-ray diffraction, ICP-OES, BET, NH₃ temperature-programed desorption (NH₃-TPD), H₂ temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectrometry, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were used to characterize the catalysts. The results show that the prepared Cu-SSZ-13 catalyst had good catalytic activity. Zr introduction was carried out on this basis. The results showed that proper Zr doping improved the catalytic activity at low temperatures and widened the high-temperature stage, with an optimal activity stage at a Zr/Cu mass ratio of 0.2. The NO _x conversion efficiency was close to 100% at 200 °C and over 80% at 450 °C. The active species were well dispersed on the catalyst surface, and the metal modification did not change the crystal structure of the zeolite. The NH₃-TPD results showed that the Zr-modified catalyst had more abundant acid sites, and the H₂-TPR results indicated that the Cu species on the catalyst had excellent reducibility at low temperatures. The interaction between Cu and Zr could regulate the Cu⁺ and Cu²⁺ proportion on the catalyst surface, which facilitated the increase in the Cu⁺ for fast SCR reaction at low temperatures. With abundant acid sites and both SCR reactions following the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism on the catalyst surface at a low temperature of 150 °C, more abundant acid sites and reaction paths created favorable conditions for NH₃-SCR reactions at low temperatures.

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH₃. However, their activity is unexpectedly inhibited in the presence of NO₂ at low temperatures. This is strikingly distinct from the NO₂-accelerated NO_x conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO₂ forces Cu ions to exist mainly as Cu^II species (fw-Cu²⁺ and NH₃-solvated Cu^II with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these Cu^II sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO₂-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the Cu^II sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

Cu-SSZ-13 zeolites are highly active for standard NH₃-SCR, but their activity is unexpectedly inhibited in the presence of NO₂. This work demonstrates that strong oxidation by NO₂ forces Cu ions to exist mainly as Cu^II species with low mobility, which is responsible for this distinctive behavior.

Rigid Body Approximation for the Anharmonic Description of Molecule-Surface Vibrations

We present an anharmonic approach for molecule-surface vibrations that employs rigid body coordinates, based on the Rodrigues rotation formula, to describe curvilinear displacements (rotations and translations) of the molecule along normal modes. These displacements are used to calculate energy data points from which one-dimensional polynomial potentials are fitted using cubic splines. In these potentials, for each of the six rigid body modes separately, one-dimensional Schrödinger equations are solved with harmonic oscillator or Fourier functions (for pure rotations) as basis sets. The anharmonic vibrational energies obtained are used to calculate partition functions and from them enthalpies, entropies, and Gibbs free energies of adsorption. Our numerical implementation has been successfully tested for Morse and cosine potentials with known analytical solutions. The methods have been applied to adsorption of CH₄ on the hydroxyl group of the proton form of the chabazite zeolite (H-CHA), as well as to adsorption of CH₄ and CO on the Mg²⁺ ions of the metal-organic framework (MOF) Mg₂(dobdc). To obtain the best estimates for thermodynamic functions, we include the coupling between molecule-surface vibrations and intrasystem vibrations at the harmonic level. The calculated Gibbs free energies show that the coupling is small for CH₄/H-CHA and CO/MOF (between -0.7 and +0.1 kJ/mol) but substantial for CH₄/MOF (-3.4 kJ/mol). The predicted anharmonic effect on the Gibbs free energy of adsorption for CH₄/H-CHA, CH₄/MOF, and CO/MOF is -4.7, 0.3 ± 0.7, and -2.4 ± 0.6 kJ/mol, respectively, which results in +4.2, +0.9 ± 0.7, and -0.4 ± 0.6 kJ/mol, respectively, for the deviation from experiment. This is well within chemical accuracy limits (±4.2 kJ/mol) for the adsorption of CH₄ and CO in the MOF. The larger deviation for CH₄/H-CHA, at the edge of the chemical accuracy range, is most likely due to contributions from soft zeolite modes which are neglected in our approach.

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.

Titration of Cu(I) Sites in Cu-ZSM-5 by Volumetric CO Adsorption

Cu-exchanged zeolites are widely studied materials because of their importance in industrial energetic and environmental processes. Cu redox speciation lies at the center of many of these processes but is experimentally difficult to investigate in a quantitative manner with regular laboratory equipment. This work presents a novel technique for this purpose that exploits the selective adsorption of CO over accessible Cu(I) sites to quantify them. In particular, isothermal volumetric adsorption measurements are performed at 50 °C on a series of opportunely pre-reduced Cu-ZSM-5 to assess the relative fraction of Cu(I); the setup is fairly simple and only requires a regular volumetric adsorption apparatus to perform the actual measurement. Repeatability tests are carried out on the measurement and activation protocols to assess the precision of the technique, and the relative standard deviation (RSD) obtained is less than 5%. Based on the results obtained for these materials, the same CO adsorption protocol is studied for the sample using infrared spectroscopy, and a good correlation is found between the results of the volumetric measurements and the absorbance of the peak assigned to the Cu(I)-CO adducts. A linear model is built for this correlation, and the molar attenuation coefficient is obtained, allowing for spectrophotometric quantification. The good sensitivity of the spectrophotometric approach and the precision and simplicity of the volumetric approach form a complementary set of tools to quantitatively study Cu redox speciation in these materials at the laboratory scale, allowing for a wide range of Cu compositions to be accurately investigated.

New insights into the NH3-selective catalytic reduction of NO over Cu-ZSM-5 as revealed by operando spectroscopy

To control diesel vehicle NO _x emissions, Cu-exchanged zeolites have been applied in the selective catalytic reduction (SCR) of NO using NH₃ as reductant. However, the harsh hydrothermal environment of tailpipe conditions causes irreversible catalyst deactivation. The aggregation of isolated Cu²⁺ brings about unselective ammonia oxidation along with the main NH₃-SCR reaction. An unusual 'dip' shaped NO conversion curve was observed in the steamed zeolite Cu-ZSM-5, resulting from the undesired NH₃ oxidation that produced NO. Here we gain further insights into the NH₃-SCR reaction and its deactivation by employing operando UV-vis diffuse reflectance spectroscopy (DRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on fresh and steamed zeolite Cu-ZSM-5. We found that tetragonally distorted octahedral Cu²⁺ with associated NH₃ preferentially forms during low temperature NH₃-SCR (<250 °C) in fresh Cu-ZSM-5. The high coordination number of Cu²⁺ ensures the availability for high coverage of nitrate intermediates. Whilst in the steamed Cu-ZSM-5, [Cu _x (OH)_2x-1]⁺ oligomers/clusters in pseudo-tetrahedral symmetry with coordinated NH₃ accumulated during the low-temperature NH₃-SCR reaction. These clusters presented a strong adsorption of surface NH₃ and nitrates/nitric acid at low temperatures and therefore limited the reaction between surface species in the steamed Cu-ZSM-5. Further release of NH₃ with increased reaction temperature favors NH₃ oxidation that causes the drop of NO conversion at ∼275 °C. Moreover, competitive adsorption of NH₃ and nitrates/nitric acid occurs on shared Lewis-acidic adsorption sites. Prompt removal of surface nitrates/nitric acid by NO avoids the surface blockage and tunes the selectivity by alternating nitrate-nitrite equilibrium. The formation of adsorbed NO₂ and HNO _x points to the necessity of an acid adsorbent in practical applications. The structural similarity under the NH₃-SCR reaction and unselective NH₃ oxidation confirmed the entanglement of these two reactions above 250 °C.

Promotion of the selective catalytic reduction of NOx with NH3 over microporous Cu-SSZ-13 by H2O and OH group at low temperatures: a density functional theory study

The successful commercialization of microporous Cu-SSZ-13 catalysts in the selective catalytic reduction (SCR) of NOx with NH3 has attracted extensive attention and debate on the mechanism of their excellent activity...

Revisiting NH3–catalyst interactions in Cu-SSZ-13 SCR catalysts: an in situ spectro-kinetics study

At low surface coverages, NH3 consumption was found to occur during adsorption at 120 °C over both Cu-SSZ-13, and H-SSZ-13; albeit much faster on Cu-SSZ-13.

How changes in Cu-SSZ-13 catalytic oxidation activity via mild hydrothermal aging influence sulfur poisoning extents

Sulfur poisoning of Cu-SSZ-13 is a function of the catalyst's oxidation ability.

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).

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)....

Selective catalytic reduction of NO x with NH3: opportunities and challenges of Cu-based small-pore zeolites

Zeolites, as efficient and stable catalysts, are widely used in the environmental catalysis field. Typically, Cu-SSZ-13 with small-pore structure shows excellent catalytic activity for selective catalytic reduction of NO _x with ammonia (NH₃-SCR) as well as high hydrothermal stability. This review summarizes major advances in Cu-SSZ-13 applied to the NH₃-SCR reaction, including the state of copper species, standard and fast SCR reaction mechanism, hydrothermal deactivation mechanism, poisoning resistance and synthetic methodology. The review gives a valuable summary of new insights into the matching between SCR catalyst design principles and the characteristics of Cu²⁺-exchanged zeolitic catalysts, highlighting the significant opportunity presented by zeolite-based catalysts. Principles for designing zeolites with excellent NH₃-SCR performance and hydrothermal stability are proposed. On the basis of these principles, more hydrothermally stable Cu-AEI and Cu-LTA zeolites are elaborated as well as other alternative zeolites applied to NH₃-SCR. Finally, we call attention to the challenges facing Cu-based small-pore zeolites that still need to be addressed.

A novel CNTs functionalized CeO2/CNTs-GAC catalyst with high NO conversion and SO2 tolerance for low temperature selective catalytic reduction of NO by NH3

Low-temperature selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) for diminishing SO₂ poisoning remains an issue in flue gas denitrification (DeNO_x). Herein, A novel CNTs functionalized low temperature NH₃-SCR catalyst CeO₂/CNTs-GAC was prepared, which showed high NO conversion activity (100% at 150 °C) and SO₂ resistance. The addition of CNTs restrained SO₂ adsorption but improved the selective adsorption of NO, which restricted the deposition of (NH₄)₂SO₄ and/or Ce₂(SO₄)₃, and resulted in high SO₂ resistance. The addition of CNTs facilitated the diffusion and transportation of NH₃ and NO, and the electron transfer on CeO₂/CNTs-GAC, leading to higher content of Ce³⁺ and adsorbed O species on the CeO₂/CNTs-GAC surface and promoted formation of surface-adsorbed oxygen O_A. Therefore, CeO₂/CNTs-GAC provided abundant NO adsorption and activation sites, facilitating "fast SCR" reaction and enhancing the NH₃-SCR reaction. The proposed CeO₂/CNTs-GAC catalyst exhibited higher NH₃-SCR activity, N₂ selectivity, catalytic durability and SO₂ resistance than CeO₂/GAC.

Analysis of NH3 -TPD Profiles for CuSSZ-13 SCR Catalyst of Controlled Al Distribution - Complexity Resolved by First Principles Thermodynamics of NH3 Desorption, IR and EPR Insight into Cu Speciation*

NH₃ temperature-programmed desorption (NH₃ -TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NO_x . Herein, we propose an interpretation of NH₃ -TPD results, which takes into account the temperature-induced dynamics of NH₃ interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH₃ adsorption on single Cu²⁺ , Cu⁺ , [CuOH]⁺ centers, dimeric [Cu-O-Cu]²⁺ , [Cu-O₂ ^2- -Cu]² species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH₃ desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH₃ -TPD experiment. Thus, a rigorous interpretation of the NH₃ -TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers.

The Study of C3H6 Impact on Selective Catalytic Reduction by Ammonia (NH3-SCR) Performance over Cu-SAPO-34 Catalysts

In present work, the catalytic performance of Cu-SAPO-34 catalysts with or without propylene during the NH3-SCR process was conducted, and it was found that the de-NOx activity decreased during low temperature ranges (<350 °C), but obviously improved within the range of high temperatures (>350 °C) in the presence of propylene. The XRD, BET, TG, NH3-TPD, NOx-TPD, in situ DRIFTS and gas-switch experiments were performed to explore the propylene effect on the structure and performance of Cu-SAPO-34 catalysts. The bulk characterization and TG results revealed that neither coke deposition nor the variation of structure and physical properties of catalysts were observed after C3H6 treatment. Generally speaking, at the low temperatures (<350 °C), active Cu2+ species could be occupied by propylene, which inhibited the adsorption and oxidation of NOx species, confining the SCR reaction rate and causing the deactivation of Cu-SAPO-34 catalysts. However, with the increase of reaction temperatures, the occupied Cu2+ sites would be recovered and sequentially participate into the NH3-SCR reaction. Additionally, C3H6-SCR reaction also showed the synergetic contribution to the improvement of NOx conversion at high temperature (>350 °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.

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.

Controlled synthesis of Cu-based SAPO-18/34 intergrowth zeolites for selective catalytic reduction of NOx by ammonia

Cu-based SAPO-18, SAPO-18/34 intergrowth and SAPO-34 zeolites were applied for the selective catalytic reduction of NO_x by ammonia (NH₃-SCR) catalysts. Comprehensive characterization results revealed that the SAPO-18/34 with higher amount and strength of acid sites could facilitate the generation of more isolated copper ions (Cu²⁺ and Cu⁺) and suppress the formation of CuO_x, which might account for the fact that intergrowth crystal structure of Cu-SAPO-18/34 exhibited higher fresh NH₃-SCR activity, more robust hydrothermal durability and better SO₂-resistance ability than that Cu-SAPO-18 and Cu-SAPO-34. In situ DRIFTS results provided the formation of reaction intermediates, such as -NH₂, NH₄⁺, NO₃^-, NO₂^-, etc. Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms were both involved in Cu-based SAPO-18 and SAPO-18/34 intergrowth zeolites, but the L-H mechanism dominated the NH₃-SCR reaction, in addition, Cu-SAPO-34 only followed "L-H" mechanism.

One-pot synthesis of Na+-free Cu-SSZ-13 and its application in the NH3-SCR reaction

Na+-free Cu-SSZ-13 zeolites have been rationally synthesized via a cooperative strategy, which has the advantages of rapid crystallization (9-48 h), high yield (86-94%) and adjustable Cu content. The NH3-SCR catalytic performance and hydrothermal stability of the calcined materials were studied and correlated with their physicochemical properties.

Superior Oxidative Dehydrogenation Performance toward NH3 Determines the Excellent Low-Temperature NH3-SCR Activity of Mn-Based Catalysts

Mn-based oxides exhibit outstanding low-temperature activity for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) compared with other catalysts. However, the underlying principle responsible for the excellent low-temperature activity is not yet clear. Here, the atomic-level mechanism and activity-limiting factor in the NH₃-SCR process over Mn-, Fe-, and Ce-based oxide catalysts are elucidated by a combination of first-principles calculations and experimental measurements. We found that the superior oxidative dehydrogenation performance toward NH₃ of Mn-based catalysts reduces the energy barriers for the activation of NH₃ and the formation of the key intermediate NH₂NO, which is the rate-determining step in NH₃-SCR over these oxide catalysts. The findings of this study advance the understanding of the working principle of Mn-based SCR catalysts and provide a fundamental basis for the development of future generation SCR catalysts with excellent low-temperature activity.

Dynamic Interconversion of Metal Active Site Ensembles in Zeolite Catalysis

Catalysis science is founded on understanding the structure, number, and reactivity of active sites. Kinetic models that consider active sites to be static and noninteracting entities are routinely successful in describing the behavior of heterogeneous catalysts. Yet, active site ensembles often restructure in response to their external environment and even during steady-state catalytic turnover, sometimes requiring non-mean-field kinetic treatments to describe distance-dependent interactions among sites. Such behavior is being recognized more frequently in modern catalysis research, with the advent of experimental methods to quantify turnover rates with increasing precision, an expanding arsenal of operando characterization tools, and computational descriptions of atomic structure and motion at chemical potentials and timescales increasingly relevant to reaction conditions. This review focuses on dynamic changes to metal active site ensembles on zeolite supports, which are silica-based crystalline materials substituted with Al that generate binding sites for isolated and low-nuclearity metal site ensembles. Metal sites can become solvated and mobilized during reaction, facilitating interactions among sites that change their nuclearity and function. Such intersite communication can be regulated by the zeolite support, resulting in non-single-site and potentially non-mean-field kinetic behavior arising from mechanisms of catalytic action that combine elements of those canonically associated with homogeneous and heterogeneous catalysis.We discuss recent literature examples that document dynamic active site behavior in metal-zeolites and outline methodologies to identify and interpret such behavior. We conclude with our outlook on future research directions to develop this evolving branch of catalysis science and harness it for practical applications.

Mobility of Cu Ions in Cu-SSZ-13 Determines the Reactivity of Selective Catalytic Reduction of NOx with NH3

Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) in Cu-SSZ-13 has been proposed to have a unique homogeneous-like mechanism governed by the spatial proximity of mobile Cu ions. Among factors that determine the proximity, the effect of ion density on the SCR reaction is well established; however, it has not been verified how the different mobility of the Cu ion influences the SCR reaction. Herein, we try to reveal the mobility-dependent SCR reaction by controlling the Cu species with different ion mobilities in Cu-SSZ-13. Since the reaction kinetics is governed by the diffusion of Cu ions, the Cu ion mobility determines the reactivity of the Cu-SSZ-13. In terms of this correlation, enhanced ion mobility leads to improved NH₃-SCR activity. These findings help understand the behavior of Cu ions in Cu-SSZ-13 under a catalytic reaction and provide insights to design rational catalysts by tuning the ion mobility.

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.