Unveiling the Role and Stabilization Mechanism of Cu+ into Defective Ce-MOF Clusters during CO Oxidation.

Copper single-site catalysts supported on Zr-based metal-organic frameworks (MOFs) are well-known systems in which the nature of the active sites has been deeply investigated. Conversely, the redox chemistry of the Ce-counterparts is more limited, because of the often-unclear Cu2+/Cu+ and Ce4+/Ce3+ pairs behavior. Herein, we studied a novel Cu2+ single-site catalyst supported on a defective Ce-MOF, Cu/UiO-67(Ce), as a catalyst for the CO oxidation reaction. Based on a combination of in situ DRIFT and operando XAS spectroscopies, we established that Cu+ sites generated during catalysis play a pivotal role. Moreover, the oxygen vacancies associated with Ce3+ sites and presented in the defective Cu/UiO-67(Ce) material are able to activate the O2 molecules, closing the catalytic cycle. The results presented in this work open a new route for the design of active and stable single-site catalysts supported on defective Ce-MOFs.

Modulation of [CuOH/O]+ Properties in [2,2'-Bipyridine]2 Homoleptic Complexes through Substitution at the 6,6' Position by Methyl Groups.

In this paper, data from a DFT-based computational study on the reactivity of [Cu(2,2'-S-bpy)2]+PF6- (S indicating substitution by methyl groups at the 6 and/or 6' position and ranging from 0 to 100% through 50%) homoleptic complexes based toward tButOOH were presented. Computational results, supported by cyclic voltammetry analysis, prove the feasibility of finely tuning the chemical properties of the complexes and their reactivity by means of insertion of methyl moieties in selected positions within the bipyridine scaffold.

Deciphering Local Structural Complexity in Zn/Ga-ZrO2 CO2 Hydrogenation Catalysts.

In the last few decades, massive effort has been expended in heterogeneous catalysis to develop new materials presenting high conversion, selectivity, and stability even under high-temperature and high-pressure conditions. In this context, CO2 hydrogenation is an interesting reaction where the catalyst local structure is strongly related to the development of an active and stable material under hydrothermal conditions at T/P > 300 °C/30 bar. In order to clarify the relationship between catalyst local ordering and its activity/stability, we herein report a combined laboratory and synchrotron investigation of aliovalent element (Ce/Zn/Ga)-containing ZrO2 matrixes. The results reveal the influence of similar average structures with different short-range orderings on the catalyst properties. Moreover, a further step toward the comprehension of the oxygen vacancy formation mechanism in Ce- and Ga-ZrO2 catalysts is reported. Finally, the reported results illustrate a robust method to guide local structure determination and ultimately help to avoid overuse of the "solid solution" definition.

Enabling a bioinspired N,N,N-copper coordination motif through spatial control in UiO-67: synthesis and reactivity.

Metal-organic frameworks (MOFs) featuring zirconium-based clusters are widely used for the development of functionalized materials due to their exceptional stability. In this study, we report the synthesis of a novel N,N,N-ligand compatible with a biphenyl dicarboxylic acid-based MOF. However, the resulting copper(I) complex exhibited unexpected coordination behaviour, lacking the intended trifold coordination motif. Herein, we demonstrate the successful immobilization of a bioinspired ligand within the MOF, which preserved its crystalline and porous nature while generating a well-defined copper site. Comprehensive spectroscopic analyses, including X-ray absorption, UV/Vis, and infrared spectroscopy, were conducted to investigate the copper site and its thermal behaviour. The immobilized ligand exhibited the desired tridentate coordination to copper, providing access to a coordination motif otherwise unattainable. Notably, water molecules were also found to coordinate to copper. Upon heating, the copper centre within the MOF exhibited reversible dehydration, suggesting facile creation of open coordination sites. Furthermore, the copper site displayed reduction at elevated temperatures and subsequent susceptibility to oxidation by molecular oxygen. Lastly, both the molecular complexes and the MOF were evaluated as catalysts for the oxidation of cyclohexane using hydrogen peroxide. This work highlights the successful immobilization of a bioinspired ligand in a zirconium-based MOF, shedding light on the structural features, thermal behaviour, and catalytic potential of the resulting copper sites.

Transitioning from Methanol to Olefins (MTO) toward a Tandem CO2 Hydrogenation Process: On the Role and Fate of Heteroatoms (Mg, Si) in MAPO-18 Zeotypes.

The tandem CO2 hydrogenation to hydrocarbons over mixed metal oxide/zeolite catalysts (OXZEO) is an efficient way of producing value-added hydrocarbons (platform chemicals and fuels) directly from CO2via methanol intermediate in a single reactor. In this contribution, two MAPO-18 zeotypes (M = Mg, Si) were tested and their performance was compared under methanol-to-olefins (MTO) conditions (350 °C, PCH3OH = 0.04 bar, 6.5 gCH3OH h-1 g-1), methanol/CO/H2 cofeed conditions (350 °C, PCH3OH/PCO/PH2 = 1:7.3:21.7 bar, 2.5 gCH3OH h-1 g-1), and tandem CO2 hydrogenation-to-olefin conditions (350 °C, PCO2/PH2 = 7.5:22.5 bar, 1.4-12.0 gMAPO-18 h molCO2-1). In the latter case, the zeotypes were mixed with a fixed amount of ZnO:ZrO2 catalyst, well-known for the conversion of CO2/H2 to methanol. Focus was set on the methanol conversion activity, product selectivity, and performance stability with time-on-stream. In situ and ex situ Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), sorption experiments, and ab initio molecular dynamics (AIMD) calculations were performed to correlate material performance with material characteristics. The catalytic tests demonstrated the better performance of MgAPO-18 versus SAPO-18 at MTO conditions, the much superior performance of MgAPO-18 under methanol/CO/H2 cofeeds, and yet the increasingly similar performance of the two materials under tandem conditions upon increasing the zeotype-to-oxide ratio in the tandem catalyst bed. In situ FT-IR measurements coupled with AIMD calculations revealed differences in the MTO initiation mechanism between the two materials. SAPO-18 promoted initial CO2 formation, indicative of a formaldehyde-based decarboxylation mechanism, while CO and ketene were the main constituents of the initiation pool in MgAPO-18, suggesting a decarbonylation mechanism. Under tandem CO2 hydrogenation conditions, the presence of high water concentrations and low methanol partial pressure in the reaction medium led to lower, and increasingly similar, methanol turnover frequencies for the zeotypes. Despite both MAPO-18 zeotypes showing signs of activity loss upon storage due to the interaction of the sites with ambient humidity, they presented a remarkable stability after reaching steady state under tandem reaction conditions and after steaming and regeneration cycles at high temperatures. Water adsorption experiments at room temperature confirmed this observation. The faster activity loss observed in the Mg version is assigned to its harder Mg2+-ion character and the higher concentration of CHA defects in the AEI structure, identified by solid-state NMR and XRD. The low stability of a MgAPO-34 zeotype (CHA structure) upon storage corroborated the relationship between CHA defects and instability.

A Critical Assessment on Calculating Vibrational Spectra in Nanostructured Materials.

Vibrational spectroscopy is an omnipresent spectroscopic technique to characterize functional nanostructured materials such as zeolites, metal-organic frameworks (MOFs), and metal-halide perovskites (MHPs). The resulting experimental spectra are usually complex, with both low-frequency framework modes and high-frequency functional group vibrations. Therefore, theoretically calculated spectra are often an essential element to elucidate the vibrational fingerprint. In principle, there are two possible approaches to calculate vibrational spectra: (i) a static approach that approximates the potential energy surface (PES) as a set of independent harmonic oscillators and (ii) a dynamic approach that explicitly samples the PES around equilibrium by integrating Newton's equations of motions. The dynamic approach considers anharmonic and temperature effects and provides a more genuine representation of materials at true operating conditions; however, such simulations come at a substantially increased computational cost. This is certainly true when forces and energy evaluations are performed at the quantum mechanical level. Molecular dynamics (MD) techniques have become more established within the field of computational chemistry. Yet, for the prediction of infrared (IR) and Raman spectra of nanostructured materials, their usage has been less explored and remain restricted to some isolated successes. Therefore, it is currently not a priori clear which methodology should be used to accurately predict vibrational spectra for a given system. A comprehensive comparative study between various theoretical methods and experimental spectra for a broad set of nanostructured materials is so far lacking. To fill this gap, we herein present a concise overview on which methodology is suited to accurately predict vibrational spectra for a broad range of nanostructured materials and formulate a series of theoretical guidelines to this purpose. To this end, four different case studies are considered, each treating a particular material aspect, namely breathing in flexible MOFs, characterization of defects in the rigid MOF UiO-66, anharmonic vibrations in the metal-halide perovskite CsPbBr3, and guest adsorption on the pores of the zeolite H-SSZ-13. For all four materials, in their guest- and defect-free state and at sufficiently low temperatures, both the static and dynamic approach yield qualitatively similar spectra in agreement with experimental results. When the temperature is increased, the harmonic approximation starts to fail for CsPbBr3 due to the presence of anharmonic phonon modes. Also, the spectroscopic fingerprints of defects and guest species are insufficiently well predicted by a simple harmonic model. Both phenomena flatten the potential energy surface (PES), which facilitates the transitions between metastable states, necessitating dynamic sampling. On the basis of the four case studies treated in this Review, we can propose the following theoretical guidelines to simulate accurate vibrational spectra of functional solid-state materials: (i) For nanostructured crystalline framework materials at low temperature, insights into the lattice dynamics can be obtained using a static approach relying on a few points on the PES and an independent set of harmonic oscillators. (ii) When the material is evaluated at higher temperatures or when additional complexity enters the system, e.g., strong anharmonicity, defects, or guest species, the harmonic regime breaks down and dynamic sampling is required for a correct prediction of the phonon spectrum. These guidelines and their illustrations for prototype material classes can help experimental and theoretical researchers to enhance the knowledge obtained from a lattice dynamics study.

Elucidating the reaction mechanism of SO2 with Cu-CHA catalysts for NH3-SCR by X-ray absorption spectroscopy.

The application of Cu-CHA catalysts for the selective catalytic reduction of NOx by ammonia (NH3-SCR) in exhaust systems of diesel vehicles requires the use of fuel with low sulfur content, because the Cu-CHA catalysts are poisoned by higher concentrations of SO2. Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO2 is crucial for elucidating the SO2 poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO2 reacts with the [Cu2II(NH3)4O2]2+ complex that is formed in the pores of Cu-CHA upon activation of O2 in the NH3-SCR cycle. In order to determine the products of this reaction, we use X-ray absorption spectroscopy (XAS) at the Cu K-edge and S K-edge, and X-ray emission spectroscopy (XES) for Cu-CHA catalysts with 0.8 wt% Cu and 3.2 wt% Cu loadings. We find that the mechanism for SO2 uptake is similar for catalysts with low and high Cu content. We show that the SO2 uptake proceeds via an oxidation of SO2 by the [Cu2II(NH3)4O2]2+ complex, resulting in the formation of different CuI species, which do not react with SO2, and a sulfated CuII complex that is accumulated in the pores of the zeolite. The increase of the SO2 uptake upon addition of oxygen to the SO2-containing feed, evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO2, is explained by the re-oxidation of the CuI species into the [Cu2II(NH3)4O2]2+ complexes, which makes them available for reaction with SO2.

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis.

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Synergic Effect of Isolated Ce3+ and Ptδ+ Species in UiO-66(Ce) for Heterogeneous Catalysis.

In this work, we have synthesized through an efficient electrostatic deposition a Pt single-atom catalyst (SAC) supported on a Ce-MOF. The basic solution employed in the impregnation process favors the deprotonation of the hydroxyl groups allocated on the clusters that can easily interact with the cationic Pt species. The resulting material, denoted as Pt/UiO-66(Ce), shows an increment of Ce3+ content, as demonstrated by UV-vis and Ce L3-edge XANES spectroscopy. These Ce3+ species and their corresponding oxygen vacancies are able to accommodate very disperse Pt single sites. Moreover, Pt L3-edge XANES and CO-FTIR spectroscopy confirm the cationic nature of the supported Ptδ+ (2+ < δ < 4+). For comparison purpose, we have synthesized and characterized a well-known Pt single-site catalyst supported on nanocrystalline ceria, denoted as Pt/nCeO2. Since the simultaneous presence of Ce3+ and Ptδ+ on the MOF clusters were able to activate the oxygen molecules and the CO molecule, respectively, we tested Pt/UiO-66(Ce) for the CO oxidation reaction. Interestingly, this catalyst showed ∼six-fold increment in activity in comparison with the traditional Pt/nCeO2 material. Finally, the characterization after catalysis reveals that the Pt nature is preserved and that the activity is maintained during 14 h at 100 °C without any evidence of deactivation.

Cu-loaded zeolites enable the selective activation of ethane to ethylene at low temperatures and pressure.

Cu-zeolites are found to activate the C-H bond of ethane already at 150 °C in a cyclic protocol and form ethylene with a high selectivity. Both the zeolite topology and Cu content are found to impact the ethylene yield. Ethylene adsorption studies with FT-IR, demonstrate that oligomerization of ethylene occurs over protonic zeolites, while this reaction does not occur over Cu-zeolites. We postulate that this observation is the origin of the high ethylene selectivity. Based on the experimental results, we propose that the reaction proceeds via the formation of an ethoxy intermediate.

CeO2 Frustrated Lewis Pairs Improving CO2 and CH3OH Conversion to Monomethylcarbonate.

Frustrated Lewis pairs (FLPs), discovered in the last few decades for homogeneous catalysts and in the last few years also for heterogeneous catalysts, are stimulating the scientific community's interest for their potential in small-molecule activation. Nevertheless, how an FLP activates stable molecules such as CO2 is still undefined. Through a careful spectroscopic study, we here report the formation of FLPs over a highly defective CeO2 sample prepared by microwave-assisted synthesis. Carbon dioxide activation over FLP is shown to occur through a bidentate carbonate bridging the FLP and implying a Ce3+-to-CO2 charge transfer, thus enhancing its activation. Carbon dioxide reaction with methanol to form monomethylcarbonate is here employed to demonstrate active roles of FLP and, eventually, to propose a reaction mechanism clarifying the role of Ce3+ and oxygen vacancies.

Surface species in direct liquid phase synthesis of dimethyl carbonate from methanol and CO2: an MCR-ALS augmented ATR-IR study.

The reaction mechanism of dimethyl carbonate (DMC) production over ZrO2 from CO2 and CH3OH is well-known, but the level of understanding has not improved in the last decade. Most commonly, the reaction mechanism has been explored in the gas phase, whilst DMC production occurs in the liquid phase. To overcome this contradiction, we exploited in situ ATR-IR spectroscopy to study DMC formation over ZrO2 in the liquid phase. A multiple curve resolution-alternate least square (MCR-ALS) approach was applied to spectra collected during the CO2/CH3OH interaction with the catalyst surface, leading to the identification of five pure components with their respective concentration profiles. CO2 and CH3OH activation to carbonates and methoxide species was found to strongly depend on the reaction temperature. Low temperature prevents methanol dissociation leaving a catalyst covered with stable carbonates, whilst higher temperature decreases the stability of the carbonates and enhances the formation of methoxides. A reaction path involving the methoxide/carbonate interaction at the surface was observed at low temperature (≤50 °C). We propose that a different reaction path, independent of carbonate formation and involving the direct CO2/methoxide interplay, occurs at 70 °C.

From Lab to Technical CO2 Hydrogenation Catalysts: Understanding PdZn Decomposition.

The valorization of CO2 to produce high-value chemicals, such as methanol and hydrocarbons, represents key technology in the future net-zero society. Herein, we report further investigation of a PdZn/ZrO2 + SAPO-34 catalyst for conversion of CO2 and H2 into propane, already presented in a previous work. The focus of this contribution is on the scale up of this catalyst. In particular, we explored the effect of mixing (1:1 mass ratio) and shaping the two catalyst functions into tablets and extrudates using an alumina binder. Their catalytic performance was correlated with structural and spectroscopic characteristics using methods such as FT-IR and X-ray absorption spectroscopy. The two scaled-up bifunctional catalysts demonstrated worse performance than a 1:1 mass physical mixture of the two individual components. Indeed, we demonstrated that the preparation negatively affects the element distribution. The physical mixture is featured by the presence of a PdZn alloy, as demonstrated by our previous work on this sample and high hydrocarbon selectivity among products. For both tablets and extrudates, the characterization showed Zn migration to produce Zn aluminates from the alumina binder phase upon reduction. Moreover, the extrudates showed a remarkable higher amount of Zn aluminates before the activation rather than the tablets. Comparing tablets and extrudates with the physical mixture, no PdZn alloy was observed after activation and only the extrudates showed the presence of metallic Pd. Due to the Zn migration, SAPO-34 poisoning and subsequent deactivation of the catalyst could not be excluded. These findings corroborated the catalytic results: Zn aluminate formation and Pd0 separation could be responsible for the decrease of the catalytic activity of the extrudates, featured by high methane selectivity and unconverted methanol, while tablets displayed reduced methanol conversion to hydrocarbons mainly attributed to the partial deactivation of the SAPO-34.

MOF-Derived CeO2 and CeZrOx Solid Solutions: Exploring Ce Reduction through FTIR and NEXAFS Spectroscopy.

The development of Ce-based materials is directly dependent on the catalyst surface defects, which is caused by the calcination steps required to increase structural stability. At the same time, the evaluation of cerium's redox properties under reaction conditions is of increasing relevant importance. The synthesis of Ce-UiO-66 and CeZr-UiO-66 and their subsequent calcination are presented here as a simple and inexpensive approach for achieving homogeneous and stable CeO2 and CeZrOx nanocrystals. The resulting materials constitute an ideal case study to thoroughly understand cerium redox properties. The Ce3+/Ce4+ redox properties are investigated by H2-TPR experiments exploited by in situ FT-IR and Ce M5-edge AP-NEXAFS spectroscopy. In the latter case, Ce3+ formation is quantified using the MCR-ALS protocol. FT-IR is then presented as a high potential/easily accessible technique for extracting valuable information about the cerium oxidation state under operating conditions. The dependence of the OH stretching vibration frequency on temperature and Ce reduction is described, providing a novel tool for qualitative monitoring of surface oxygen vacancy formation. Based on the reported results, the molecular absorption coefficient of the Ce3+ characteristic IR transition is tentatively evaluated, thus providing a basis for future Ce3+ quantification through FT-IR spectroscopy. Finally, the FT-IR limitations for Ce3+ quantification are discussed.

Zn Redistribution and Volatility in ZnZrOx Catalysts for CO2 Hydrogenation.

ZnO-ZrO2 mixed oxide (ZnZrOx) catalysts are widely studied as selective catalysts for CO2 hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn2+ species are mobile between the solid solution phase with ZrO2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO2-to-hydrocarbon process optimization.

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.

A multi-technique approach to unveil the redox behaviour and potentiality of homoleptic CuI complexes based on substituted bipyridine ligands in oxygenation reactions.

The effect of differently substituted 2,2'-bipyridine ligands (i.e. 6,6'-dimethyl-2,2'-bipyridine, 5,5'-dimethyl-2,2'-bipyridine, 6,6'-dimethoxy-2,2'-bipyridine and 2,2'-bipyridine) on the reversible oxidation of the resulting CuI homoleptic complexes is investigated by means of a multi-technique approach (electronic and vibrational spectroscopies, DFT, electrochemistry). Among the four tested complexes, [CuI(6,6'-dimethyl-2,2'-bipyridine)2] (PF6) shows a peculiar behavior when oxidized with an organic peroxide (i.e. tert-butyl hydroperoxide, tBuOOH). The simultaneous use of UV-Vis-NIR and Raman spectroscopy methods and cyclovoltammetry, supported by DFT based calculations, allowed identifying (i) the change in the oxidation state of the copper ion and (ii) some peculiar modification in the local structure of the metal leading to the formation of a [CuIIOH]+ species. The latter, being able to oxidize a model molecule (i.e. cyclohexene) and showing the restoration of the original CuI complex and the formation of cyclohexanone, confirms the potential of these simple homoleptic CuI complexes as model catalysts for partial oxygenation reactions.

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 NOx 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 [CuI(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 NOx conversion under NH3-SCR conditions at 200 °C.

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

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