Probing the electronic and mechanistic roles of the μ4-sulfur atom in a synthetic CuZ model system

Nitrous oxide (N₂O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N₂O fixation by nitrous oxide reductase (N₂OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the Cu_Z catalytic site in N₂OR reported to date. Quantitative N₂ measurements during synthetic N₂O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N₂O reduction emerging from this computational modeling involves cooperative activation of N₂O across a Cu/S cluster edge. Direct interaction of the μ₄-S ligand with the N₂O substrate during coordination and N–O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of Cu_Z and related metal–sulfur clusters. Consistent with hypothetical participation of the μ₄-S unit in two-electron reduction of N₂O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ₄-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological Cu_Z site.

Experimental data and computational modeling indicates an active role for the bridging sulfide ligand in a synthetic Cu_Z model.     

Atomic origins of the strong metal–support interaction in silica supported catalysts

Silica supported metal catalysts are most widely used in the modern chemical industry because of the high stability and tunable reactivity. The strong metal–support interaction (SMSI), which has been widely observed in metal oxide supported catalysts and significantly affects the catalytic behavior, has been speculated to rarely happen in silica supported catalysts since silica is hard to reduce. Here we revealed at the atomic scale the interfacial reaction induced SMSI in silica supported Co and Pt catalysts under reductive conditions at high temperature using aberration-corrected environmental transmission electron microscopy coupled with in situ electron energy loss spectroscopy. In a Co/SiO₂ system, the amorphous SiO₂ migrated onto the Co surface to form a crystallized quartz-SiO₂ overlayer, and simultaneously an interlayer of Si was generated in-between. The metastable crystalline SiO₂ overlayer subsequently underwent an order-to-disorder transition due to the continuous dissociation of SiO₂ and the interfacial alloying of Si with the underlying Co. The SMSI in the Pt–SiO₂ system was found to remarkably boost the catalytic hydrogenation. These findings demonstrate the universality of the SMSI in oxide supported catalysts, which is of general importance for designing catalysts and understanding catalytic mechanisms.

This work tracked at the atomic scale the interfacial reaction induced strong metal–support interaction between SiO₂ and metal catalysts and evolution under reactive conditions by aberration-corrected environmental transmission electron microscopy.     

Two-dimensional heterostructures built from ultrathin CeO2 nanosheet surface-coordinated and confined metal–organic frameworks with enhanced stability and catalytic performance

Two-dimensional (2D) metal–organic framework (MOF) based heterostructures will be greatly advantageous to enhance catalytic performance because they increase the contact surface and charge transfer. Herein, a novel 2D heterostructure named CeO₂@NiFe-MOFs, in which monolayer NiFe-MOFs is coordinated with ceria (CeO₂) to improve catalytic and stability performance, is successfully constructed by the strategy of in situ growth on the surface of ultrathin CeO₂ nanosheets being functionalized with monolayer carboxylic acid groups. The 2D heterostructure possesses a sandwich structure, where monolayer NiFe-MOFs are coordinated to both the top and bottom surface of CeO₂ nanosheets via joining carboxylic acid groups. In particular, CeO₂ with robust coordination plays a significant role in the anchoring of carboxylic acid groups and binding strength of heterostructures. The 2D CeO₂@NiFe-MOF heterostructure with a joint effect of metal–ligand coordination not only presents good structural stability but also significantly enhances the oxygen evolution reaction (OER) efficiencies in comparison to bare NiFe-MOFs, achieving a current density of 20 mA cm⁻² at a low overpotential of 248 mV as well as durability for at least 40 h. Meanwhile, the electronics, optics, band gap energy and local strains of CeO₂ decorated with 2D NiFe-MOFs are different to the properties of bare CeO₂. Our study on the construction of an ultrathin CeO₂ surface-coordinated and confined MOF layer may pave a new way for novel 2D MOF composites/heterostructures or multi-functional 2D CeO₂ materials to be used in energy conversion or other fields.

A synthetic strategy to prepare 2D heterostructures from ultrathin CeO₂ surface-coordinated metal–organic framework was proposed, proving multiple effects of metal-coordinated interactions in 2D heterostructures.     

Fast oxygen ion migration in Cu–In–oxide bulk and its utilization for effective CO2 conversion at lower temperature

Efficient activation of CO₂ at low temperature was achieved by reverse water–gas shift via chemical looping (RWGS-CL) by virtue of fast oxygen ion migration in a Cu–In structured oxide, even at lower temperatures. Results show that a novel Cu–In₂O₃ structured oxide can show a remarkably higher CO₂ splitting rate than ever reported. Various analyses revealed that RWGS-CL on Cu–In₂O₃ is derived from redox between Cu–In₂O₃ and Cu–In alloy. Key factors for high CO₂ splitting rate were fast migration of oxide ions in the alloy and the preferential oxidation of the interface of alloy–In₂O₃ in the bulk of the particles. The findings reported herein can open up new avenues to achieve effective CO₂ conversion at lower temperatures.

Efficient activation of CO₂ at low temperature was achieved by reverse water–gas shift via chemical looping (RWGS-CL) by virtue of fast oxygen ion migration in a Cu–In structured oxide, even at lower temperatures.     

CO2 Hydrogenation over Oxide‐Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity

By simply changing the oxide support, the selectivity of a metal–oxide catalysts can be tuned. For the CO₂ hydrogenation over PtCo bimetallic catalysts supported on different reducible oxides (CeO₂, ZrO₂, and TiO₂), replacing a TiO₂ support by CeO₂ or ZrO₂ selectively strengthens the binding of C,O‐bound and O‐bound species at the PtCo–oxide interface, leading to a different product selectivity. These results reveal mechanistic insights into how the catalytic performance of metal–oxide catalysts can be fine‐tuned.     

Modulation of the superficial electronic structure via metal–support interaction for H2 evolution over Pd catalysts

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle''s surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pd^δ−) with positive Pd atoms distributed on the interface (interfacial Pd^δ+). More superficial Pd^δ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pd^δ− species are responsible for improved catalytic performance for H₂ evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle''s electronic structure precisely and exploiting high-performance catalysts.

A new type of electronic effect, polarized metal-support interaction (pEMSI), in oxide-supported Pd nanoparticles describing the enhanced accumulation of electrons at the superficial surface is responsible for improved catalytic H₂ evolution.     

Metal–ligand–Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation. The [(SCS)Pd]₂ complex featuring a non-innocent indenediide-based ligand was found to be a very efficient and versatile catalyst for the Conia-ene reaction, when associated with Mg(OTf)₂. The reaction operates at low catalytic loadings under mild conditions with HFIP as a co-solvent. It works with a variety of substrates, including those bearing internal alkynes. It displays complete 5-exo vs. 6-endo regio-selectivity. In addition, except for the highly congested ^tBu-substituent, the reaction occurs with high Z vs. E stereo-selectivity, making it synthetically useful and complementary to known catalysts.

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation.     

The precise editing of surface sites on a molecular-like gold catalyst for modulating regioselectivity

It is extremely difficult to precisely edit a surface site on a typical nanoparticle catalyst without changing other parts of the catalyst. This precludes a full understanding of which site primarily determines the catalytic properties. Here, we couple experimental data collection with theoretical analysis to correlate rich structural information relating to atomically precise gold clusters with the catalytic performance for the click reaction of phenylacetylene and benzyl azide. We also identify a specific surface site that is capable of achieving high regioselectivity. We further conduct site-specific editing on a thiolate-protected gold cluster by peeling off two monomeric RS–Au–SR motifs and replacing them with two Ph₂P–CH₂–PPh₂ staples. We demonstrate that the surface Au–Ph₂P–CH₂–PPh₂–Au motifs enable extraordinary regioselectivity for the click reaction of alkyne and azide. The editing strategy for the surface motifs allows us to exploit previously inaccessible individual active sites and elucidate which site can explicitly govern the reaction outcome.

Editing surface motifs on gold cluster catalysts achieves high regioselectivity for the click reactions of azides and alkynes.     

Direct observation of the evolving metal–support interaction of individual cobalt nanoparticles at the titania and silica interface

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties. Spatially resolved soft X-ray absorption spectroscopy (XAS) in combination with Atomic Force Microscopy (AFM) and Scanning Helium Ion-Milling Microscopy (SHIM) has been applied to visualise and characterise the behaviour of individual cobalt nanoparticles (CoNPs) supported on two-dimensional substrates (SiO_xSi(100) (x < 2) and rutile TiO₂(110)) after undergoing reduction–oxidation–reduction (ROR). The behaviour of the Co species is observed to be strongly dependent on the type of support. For SiO_xSi a weaker MSI between Co and the support allows a complete reduction of CoNPs although they migrate and agglomerate. In contrast, a stronger MSI of CoNPs on TiO₂ leads to only a partial reduction under H₂ at 773 K (as observed from Co L₃-edge XAS data) due to enhanced TiO₂ binding of surface-exposed cobalt. SHIM data revealed that the interaction of the CoNPs is so strong on TiO₂, that they are seen to spread at and below the surface and even to migrate up to ∼40 nm away. These results allow us to better understand deactivation phenomena and additionally demonstrate a new understanding concerning the nature of the MSI for Co/TiO₂ and suggest that there is scope for careful control of the post-synthetic thermal treatment for the tuning of this interaction and ultimately the catalytic performance.

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties.     

Pressure-induced phosphorescence enhancement and piezochromism of a carbazole-based cyclic trinuclear Cu(i) complex

Interest in piezochromic luminescence has increased in recent decades, even though it is mostly limited to pure organic compounds and fluorescence. In this work, a Cu₃Pz₃ (Cu3, Pz: pyrazolate) cyclic trinuclear complex (CTC) with two different crystalline polymorphs, namely 1a and 1b, was synthesized. The CTC consists of two functional moieties: carbazole (Cz) chromophore and Cu3 units. In crystals of 1a, discrete Cz–Cu3–Cu3–Cz stacking was found, showing abnormal pressure-induced phosphorescence enhancement (PIPE), which was 12 times stronger at 2.23 GPa compared to under ambient conditions. This novel observation is ascribed to cooperation between heavy-atom effects (i.e., from Cu atoms) and metal–ligand charge-transfer promotion. The infinite π–π stacking of Cz motifs was observed in 1b and it exhibited good piezochromism as the pressure increased. This work demonstrates a new concept in the design of piezochromic materials to achieve PIPE via combining organic chromophores and metal–organic phosphorescence emitters.

One molecule, two response mechanisms: a pair of newly-designed cyclic trinuclear Cu(i) complex crystalline polymorphs are engineered, which show excellent luminescent piezochromism and pressure-induced phosphorescence enhancement, respectively.     

Spatial-confinement induced electroreduction of CO and CO2 to diols on densely-arrayed Cu nanopyramids

The electroreduction of carbon dioxide (CO₂) and carbon monoxide (CO) to liquid alcohol is of significant research interest. This is because of a high mass-energy density, readiness for transportation and established utilization infrastructure. Current success is mainly around monohydric alcohols, such as methanol and ethanol. There exist few reports on converting CO₂ or CO to higher-valued diols such as ethylene glycol (EG; (CH₂OH)₂). The challenge to producing diols lies in the requirement to retain two oxygen atoms in the compound. Here for the first time, we demonstrate that densely-arrayed Cu nanopyramids (Cu-DAN) are able to retain two oxygen atoms for hydroxyl formation. This results in selective electroreduction of CO₂ or CO to diols. Density Functional Theory (DFT) computations highlight that the unique spatial-confinement induced by Cu-DAN is crucial to selectively generating EG through a new reaction pathway. This structure promotes C–C coupling with a decreased reaction barrier. Following C–C coupling the structure facilitates EG production by (1) retaining oxygen and promoting the *COH–CHO pathway, which is a newly identified pathway toward ethylene glycol production; and, (2) suppressing the carbon–oxygen bond breaking in intermediate *CH₂OH–CH₂O and boosting hydrogenation to EG. Our findings will be of immediate interest to researchers in the design of highly active and selective CO₂ and CO electroreduction to diols.

Densely-arrayed Cu nanopyramids have spatial confinement induced by the additional Cu–O bond. This promotes C–C coupling, regulates post-C–C coupling, and retains both oxygen atoms in an alternative pathway toward ethylene glycol formation from CO.     

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O₂-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H₂O₂-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO₂, WO₃, Ta₂O₅, Nb₂O₅, and ZrO₂), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O₂-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.     

Combustion of Carbon Black Catalyzed by Transition Metal-Promoted Y2O3–CeO2–ZrO2 Solid Solutions1

The activity of M-free and M-loaded 10YO_1.5–10CeO₂–80ZrO₂ solid solution (M = Cu, V, or W) towards carbon black combustion was studied using TG/DTA and TPO techniques. It was demonstrated that all studied catalysts lower the temperature of carbon black combustion. The selectivity of the catalytic reaction in CO₂ formation was 100%. It was evidenced that the fast oxidation of carbon at lower temperatures, observed only in the TG/DTA apparatus, was due to heat- and mass-transfer limitations, resulting in a runaway reaction. Using TPR technique, it was shown that, in the temperature range of DTA curve, oxygen on the catalyst surface was rather reactive (and, therefore, it could be easily released by support for the oxidation of carbon), whereas the reactivity of bulk oxygen was negligible. The activity of the metal-loaded 10YO_1.5–10CeO₂–80ZrO₂ (Y-10) samples varied according to the following sequence: Cu/Y-10 > V/Y-10 > W/Y-10. For Cu- and V-containing catalysts, a contribution of a surface redox mechanism in reaction was proposed by comparing EPR spectra of pure catalysts with those of the samples (catalysts mixed with carbon black) after catalysis.     

Pt/oxide nanocatalysts synthesized via the ultrasonic spray pyrolysis process: engineering metal–oxide interfaces for enhanced catalytic activity

We show that Pt nanoparticles synthesized on oxide nanocatalysts exhibit catalytic activity enhancement depending on the type of the oxide support. To synthesize the Pt/oxide nanocatalysts, we employed a versatile synthesis method using Pt nanoparticles (NPs) supported on various metal oxides (i.e., SiO₂, CeO₂, Al₂O₃, and FeAl₂O₄) utilizing ultrasonic spray pyrolysis. Catalytic CO oxidation was carried out on these catalysts, and it was found that the catalytic activity of the Pt NPs varied depending on the supporting oxide. While Pt/CeO₂ exhibited the highest metal dispersion and active surface area, Pt/FeAl₂O₄ exhibited the lowest active surface area. Among the Pt/oxide nanocatalysts, Pt NPs supported on CeO₂ showed the highest catalytic activity. We ascribe the enhancement in turnover frequency of the Pt/CeO₂ nanocatalysts to strong metal–support interactions due to charge transport between the metal catalysts and the oxide support. Such Pt/oxide nanocatalysts synthesized via spray pyrolysis offer potential possibilities for large-scale synthesis of tailored catalytic systems for technologically relevant applications.     

Copper doped ceria porous nanostructures towards a highly efficient bifunctional catalyst for carbon monoxide and nitric oxide elimination

Copper doped ceria porous nanostructures with a tunable BET surface area were prepared using an efficient and general metal–organic-framework-driven, self-template route. The XRD, SEM and TEM results indicate that Cu²⁺ was successfully substituted into the CeO₂ lattice and well dispersed in the CeO₂:Cu²⁺ nanocrystals. The CeO₂:Cu²⁺ nanocrystals exhibit a superior bifunctional catalytic performance for CO oxidation and selective catalytic reduction of NO. Interestingly, CO oxidation reactivity over the CeO₂:Cu²⁺ nanocrystals was found to be dependent on the Cu²⁺ dopants and BET surface area. By tuning the content of Cu²⁺ and BET surface area through choosing different organic ligands, the 100% conversion temperature of CO over CeO₂:Cu²⁺ nanocrystals obtained from thermolysis of CeCu–BPDC nanocrystals can be decreased to 110 °C. The porous nanomaterials show a high CO conversion rate without any loss in activity even after five cycles. Furthermore, the activity of the catalysts for NO reduction increased with the increase of BET surface, which is in accordance with the results of CO oxidation.     

Controlled synthesis of Bi- and tri-nuclear Cu-oxo nanoclusters on metal–organic frameworks and the structure–reactivity correlations

Precisely tuning the nuclearity of supported metal nanoclusters is pivotal for designing more superior catalytic systems, but it remains practically challenging. By utilising the chemical and molecular specificity of UiO-66-NH₂ (a Zr-based metal–organic framework), we report the controlled synthesis of supported bi- and trinuclear Cu-oxo nanoclusters on the Zr₆O₄ nodal centres of UiO-66-NH₂. We revealed the interplay between the surface structures of the active sites, adsorption configurations, catalytic reactivities and associated reaction energetics of structurally related Cu-based ‘single atoms’ and bi- and trinuclear species over our model photocatalytic formic acid reforming reaction. This work will offer practical insight that fills the critical knowledge gap in the design and engineering of new-generation atomic and nanocluster catalysts. The precise control of the structure and surface sensitivities is important as it can effectively lead to more reactive and selective catalytic systems. The supported bi- and trinuclear Cu-oxo nanoclusters exhibit notably different catalytic properties compared with the mononuclear ‘Cu₁’ analogue, which provides critical insight for the engineering of more superior catalytic systems.

The controlled synthesis of novel bi- and trinuclear Cu-oxo nanoclusters supported on UiO-66-NH₂ that show notably different catalytic properties in the photocatalytic formic acid decomposition reaction is reported.     

Engineering plasticization resistant gas separation membranes using metal–organic nanocapsules

Membrane technologies hold great potential for industrial gas separation. Nevertheless, plasticization, a common phenomenon that is responsible for the loss of gas pair selectivity and the decrease of membrane lifespan, is one of the top challenges withholding the deployment of advanced membrane materials in realistic applications. Here, we report a highly generalizable approach, that utilizes PgC₅Cu, a copper metal–organic nanocapsule (MONC) containing 24 open metal sites (OMSs) as a multi-dentate node to coordinatively crosslink polymers. By adding merely 1–3 wt% of PgC₅Cu, a wide range of carbonyl group-containing polymers can be effectively crosslinked. Through rigorous dissolution tests, molecular dynamic simulations, and in situ FT-IR spectroscopy, we qualitatively and quantitatively unveiled the coordinative binding nature at the polymer–MONC interface. As a result, we produced a series of composite membranes showing near complete plasticization resistance to CO₂, C₂H₄, and C₂H₆ under high pressure with no loss of mechanical and gas transport properties.

Ultra-small metal–organic nanocapsules (MONCs) with open metal sites (OMSs) are used as multi-dentate nodes to form coordinative crosslinking networks with polymers.     

Catalysis of water oxidation in acetonitrile by iridium oxide nanoparticles

Water oxidation catalysed by iridium oxide nanoparticles (IrO₂ NPs) in water–acetonitrile mixtures using [Ru^III(bpy)₃]³⁺ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. It was observed that under acidic conditions (HClO₄) and at high water contents (80% (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈25% H₂O (v/v)). The results can be rationalized based on the structure of water in water–acetonitrile mixtures. At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water clustering gives rise to the formation of water-rich micro-domains, and the number of bonded water molecules decreases monotonically. The results presented herein indicate that non-bonded water present in the water micro-domains is considerably more reactive towards oxygen production. Finally, long term electrolysis of water–acetonitrile mixtures containing [Ru^II(bpy)₃]²⁺ and IrO₂ NPs in solution show that the amount of oxygen produced is constant with time demonstrating that the redox mediator is stable under these experimental conditions.     

Reductive amination of cyclohexanol/cyclohexanone to cyclohexylamine using SBA-15 supported copper catalysts

Amination of cyclohexanol was investigated in vapour phase over copper catalysts supported on mesoporous SBA-15. The different products identified during reductive amination of cyclohexanol reaction were cyclohexanone, cyclohexylamine, along with small amounts of N-Cyclohexylidinecyclohexylamine and dicyclohexylamine. Among several catalysts tested for the reductive amination, 5% Cu supported on SBA-15 exhibited better catalytic performance than other catalysts with 36% selectivity towards cylclohexylamine at 80% cyclohexanol conversion. The optimum reaction conditions employed to achieve the best catalyst performance were at 250 °C, 0.1 MPa of H₂/NH₃, TOS-10h. The active Cu sites, acidity of the catalyst, and effect of reaction parameters play a pivotal role in the reductive amination reaction. The prepared catalysts were characterized by XRD, BET, SEM, H₂-TPR and NH₃-TPD. The dispersion of Cu, particle size, and metal surface area (m²/g) calculated from pulse N₂O decomposition method. TPR findings reveal the presence of substantially dispersed copper oxide species at lower loadings which is easily reducible than the bulk copper oxide species found at higher Cu loadings. The acidity measurements by NH₃-TPD analysis suggest that the maximum acidic strength was obtained at 5 wt% copper on porous SBA-15, and decreased with Cu loadings. The catalytic properties are well in agreement with the findings of catalysts characterization.