Copper Catalyst Research Articles

Metal-catalyzed divergent synthesis from ylides with 3-arylbenzo[d][1,2,3]triazin-4(3H)-ones.

The present work reveals a new metal-catalyzed synthetic reaction involving 1,2,3-benzotriazinones with carbonyl sulfoxonium ylide and iodonium ylide, resulting in divergent products. Within this catalytic system, 3-phenylbenzo[d][1,2,3]triazin-4(3H)-one derivatives undergo C-H alkylation processes facilitated by a Cp*Rh(III) catalyst when combined with a carbonyl sulfoxonium ylide. On the other hand, when iodonium ylide substrates are used, they undergo an alkenylation reaction facilitated by a Cp*Ir(III) catalyst. In addition, hydrazone products are produced by synthesizing iodonium ylide substrates with the use of a copper catalyst. These transformations demonstrate mild reaction conditions, a wide range of substrates, and excellent compatibility with various functional groups. The strategy and tactics utilized have been effectively implemented on a significant scale.

In situ XPS study of methanol oxidation over a copper catalyst derived from layered double hydroxides

Copper nanoparticles supported on alumina have been synthesized from CuAl-layered double hydroxide and have been studied in methanol oxidation at different molar ratios of the components of the reaction feed by in situ XPS and mass-spectrometry.

Exactly regulated copper catalysts exploiting isolated photoelectrochemical reduction of cuprous oxides and random mesh-structured TiO2 for enhanced photoelectrochemical CO2 conversion

This work presents a novel photocorrosion paradigm to guide CO2RR photocathode design. The isolated photoelectrochemical reduction corrosion (IPRC) method developed through this artificial control provides an efficient route to ensure long-term stability, energy saving, and high performance.

Surface activation via selective dealloying of Cu-based metallic glasses for efficient catalytic behavior.

Surface activation is considered to regulate the electronic structures of materials for enhancing catalytic capability. Herein, we report a controllable strategy for constructing three-dimensional micro-nanoporous copper catalysts with high reactivity and activity for the degradation reaction of organic pollutants. Various micro-nanoporous structures and in situ formation processes by chemical selective dealloying of Cu-based metallic glasses are evaluated due to the surface modification. The porous catalysts exhibit superior catalytic performance, attributing to the catalytic mechanisms related to the superior surface activity of nanoscale copper composites and the strong oxidizing capability of activated radicals. These findings will provide a promising synthesis approach for three-dimensional micro-nanoporous catalysts for many chemical reactions.

Multiple neighboring active sites of an atomically precise copper nanocluster catalyst for efficient bond-forming reactions.

Atomically precise copper nanoclusters (NCs) are an emerging class of nanomaterials for catalysis. Their versatile core-shell architecture opens the possibility of tailoring their catalytically active sites. Here, we introduce a core-shell copper nanocluster (CuNC), [Cu29(StBu)13Cl5(PPh3)4H10]tBuSO3 (StBu: tert-butylthiol; PPh3: triphenylphosphine), Cu29NC, with multiple accessible active sites on its shell. We show that this nanocluster is a versatile catalyst for C-heteroatom bond formation (C-O, C-N, and C-S) with several advantages over previous Cu systems. When supported, the cluster can also be reused as a heterogeneous catalyst without losing its efficiency, making it a hybrid homogeneous and heterogeneous catalyst. We elucidated the atomic-level mechanism of the catalysis using density functional theory (DFT) calculations based on the single crystal structure. We found that the cooperative action of multiple neighboring active sites is essential for the catalyst's efficiency. The calculations also revealed that oxidative addition is the rate-limiting step that is facilitated by the neighboring active sites of the Cu29NC, which highlights a unique advantage of nanoclusters over traditional copper catalysts. Our results demonstrate the potential of nanoclusters for enabling the rational atomically precise design and investigation of multi-site catalysts.

Controllable synthesis of hollow mesoporous organosilica nanoparticles with pyridine-2,6-bis-imidazolium frameworks for CO2 conversion.

A series of hard-template-derived hollow mesoporous organosilica nanoparticles (HMONs) with pyridine-2,6-bis-imidazolium frameworks have been described for the first time. As a part of the investigation, to evaluate the effects of the hard template nature, the Si/CTAB and organosilica/TEOS molar ratios, and the stepwise addition of precursors, four reaction conditions denoted as methods A-D were designed. In the presence of polystyrene latex as a hard template, the HMONs that we wished to synthesize were not yielded with a Si/CTAB molar ratio of 3 (method A), but we could synthesize the desired HMONs with a Si/CTAB molar ratio of 9 and an organosilica : TEOS ratio of 1 : 99 (method B). The ratio of organosilica to TEOS could be improved up to 2.5 : 97.5 if the precursor additions are made in a stepwise manner rather than by simultaneous additions (method C). Using sSiO2 as a hard template, a yolk-shell morphology was observed by adopting a Si/CTAB molar ratio of 3 (method D). The HMONs were modified by iodide ions and their activity was explored toward the coupling of CO2 with epoxides. Among the catalysts, I-HMON-L-C-2.5 exhibited excellent results under mild reaction conditions. Well-oriented pore sizes and short channel length facilitated easy mass transfer from one side and the integration of the interior hollow regions of the catalyst particles from the other side improved the CO2 retention time around pores where the imidazolium organocatalysts were located, which made I-HMON-L-C-2.5 an effective catalyst for title CO2 utilization. The catalyst was reused at least six times without exhibiting any changes in its activity. HMONs can also be used as solid CNC ligands for the preparation of copper catalysts for the click reaction between phenyl acetylene and benzyl azide.

Catalytic asymmetric intramolecular propargylation of cyclopropanols to access the cuparane core.

The catalytic asymmetric propargylation of enol(ate) intermediates is a well-established method for the synthesis of α-propargyl-substituted carbonyl compounds. However, the propargylation of homo-enol(ate) or its equivalents for the synthesis of β-propargyl-substituted carbonyl compounds remains underdeveloped. A catalytic enantioselective decarboxylative intramolecular propargylation of cyclopropanols has been developed using a PyBox-complexed copper catalyst. This reaction offers an effective approach to assemble a cyclopentanone skeleton bearing an all-carbon quaternary stereogenic center and an adjacent quaternary gem-dimethyl carbon center, which is the core scaffold of the naturally occurring cuparenoids. Key to the success of this protocol is the use of a new structurally optimized PyBox ligand. This study represents the first example of catalytic asymmetric intramolecular propargylation of cyclopropanols.

Straightforward α-allylation of carbonyl compounds with alkenes via α-carbonyl radical intermediates

We report that the direct oxidative cross-coupling of carbonyl compounds with simple alkenes is realized to synthesize α-allylated carbonyl compounds by identifying the bisphosphine ligand BINAP that enables the copper catalyst to integrate two different catalytic cycles in a one-pot system.

Comparison of metal-carbenoid reactions of β-2-five-membered heteroaryl substituted α,β-unsaturated ketones.

In this study, β-2-heteroaryl substituted (N-methyl 2-pyrrolyl, 2-thiophenyl, 2-furyl) α,β-unsaturated ketones were reacted with two α-diazo carbonyl compounds that had different characteristics (dimethyl diazo malonate and 1-diazo-1-phenyl-propane-2-one) in the presence of both copper and rhodium catalysts. In the case of reactions with N-methyl 2-pyrrolyl α,β-unsaturated ketones, the major product was the insertion derivative. However, in the reactions of 2-thiophenyl and 2-furyl α,β-unsaturated ketones with dimethyl diazomalonate (acceptor-acceptor disubstituted), only dihydrofuran products were formed over carbonyl ylides. When 2-thiophenyl and 2-furyl α,β-unsaturated ketones were reacted with 1-diazo-1-phenyl-propane-2-one (donor-acceptor disubstituted), 1-phenylpropane-1,2-dione was obtained under our reaction conditions.

Copper-Catalyzed Intramolecular Aldehyde-Ketone Nucleophilic Additions for the Synthesis of Chromans Bearing a Tertiary Alcohol Motif.

The synthesis of chroman-3-ol derivatives via intramolecular nucleophilic additions has been established. Aldehydes can be used as alkyl carbanion equivalents via reductive polarity reversal which is facilitated by a copper catalyst and N-heterocyclic carbene ligand under mild conditions. The key to success is the difference in reaction activity between aldehydes and ketones. Finally, this methodology also can be used to construct other cyclic structures containing tertiary alcohols including tetraline, cyclohexane, indan, and 9,10-dihydrophenanthrene.

Microwave-induced one-pot synthesis of 3-imidazolyl indole clubbed 1,2,3-triazole hybrids as antiproliferative agents and density functional theory study.

Herein, we outline a highly efficient PEG-4000-mediated one-pot three-component reaction for the synthesis of 3-imidazolyl indole clubbed 1,2,3-triazole derivatives (5a-r) at up to 96% yield as antiproliferative agents. This three-component protocol offers the advantages of an environmentally benign reaction, excellent yield, quick response time, and operational simplicity triggered by the copper catalyst under microwave irradiation. All the synthesized compounds were tested for antiproliferative activity against six human solid tumor cell lines, that is, A549 and SW1573 (nonsmall cell lung), HBL100 and T-47D (breast), HeLa (cervix), and WiDr (colon). Among them, six compounds, 5g-j, 5m, and 5p, demonstrated effective antiproliferative action with GI50 values under 10 μM. Furthermore, density functional theory (DFT) calculations were performed for all the synthesized molecules through geometry optimizations, frontier molecular orbital approach, and molecular electrostatic potential (MESP). The theoretical DFT calculation was performed using the DFT/B3LYP/6-31+G (d,p) basis set. Moreover, the biological reactivity of all the representative synthesized molecules was compared with the theoretically calculated quantum chemical descriptors and MESP 3D plots. We also investigated the drug-likeness characteristic and absorption, distribution, metabolism, excretion, and toxicity (ADMET) prediction. In general, our approach enables environmentally friendly access to 3-imidazolyl indole clubbed 1,2,3-triazole derivatives as prospective antiproliferative agents.

Effect of Cu-modified HZSM-5 zeolites on catalytic pyrolysis of lignin to producing aromatic hydrocarbons

Lignin catalytic pyrolysis presents a promising and sustainable approach to producing aromatic molecules. Herein, we investigated the effect of modified zeolite with copper (Cu/HZSM-5) catalyst on efficiently pyrolyzed kraft lignin into aromatic hydrocarbons (AHs) production, especially mononuclear aromatic hydrocarbons (MAHs). Time-of-flight mass spectrometry (TOF-MS) was employed to compare the volatile product profiles of lignin during catalytic pyrolysis. The highest yield of MAHs reached 70.59 mg/g based on dry lignin at 550 °C, accounting for 70.23% among lignin pyrolysis products, compared to the pristine HZSM-5 zeolite catalytic system with MAHs yield of only 54.31 mg/g and of 40.34% in products. At 550 °C and 60 s residence time, the 5% Cu/HZSM-5 catalyst demonstrated the best performance compared to all other catalysts in terms of selectively producing MAHs. The results are ascribed to the enhancement of decarboxylation and aromatization processes by the doping of copper. This work demonstrates a cost-effective pathway that potentially converts kraft lignin into aromatic hydrocarbon production at a phenomenally high selectivity, providing a concept for industrial application toward biomass utilization and high value-added production.

Direct Electroreduction of Carbonate to Formate

A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO2 reduction reaction (eCO2RR) can be attributed to the formation of carbonates (CO3 2–), which is generally considered to be an electrochemically-inert species. Herein, we employ in-situ electrochemical Raman spectroscopy, liquid chromatography, nuclear magnetic resonance spectroscopy, 13C isotope-labelling and density functional theory (DFT) simulations to understand the role of carbonate species formed on copper catalysts during eCO2RR, and its ability to reduce to formate. Direct carbonate-to-formate conversion was confirmed by the the direct reduction of CuCO3, and by pulse reduction of N2-saturated K2CO3 electrolyte on Cu,. DFT simulations elucidated the nature of the active sites generating the Cu-CO3 2– species and revealed the mechanism for the pulse-enabled carbonate reduction to formate. Combining both experimental and computational results, we proposed that a dynamic carbonate intermediate was constantly formed on Cu during eCO2RR, that further evolves to formate between –0.4 to –1.2 VRHE on Cu. Control systems using Pb, Ag and Au electrodes were also studied.

Durability of CO and CO2 Electrolyzers with Copper Electrocatalysts, Gas Diffusion Electrodes, and Anion Exchange Membranes

Recent works have shown that the electrochemical reduction of CO2 or CO to C2 products may be carried out at high rates and selectivities using electrolyzers with copper catalysts, gas diffusion electrodes (GDEs), and anion exchange membranes (AEMs). The current densities and Faradaic efficiencies reported for CO2 reduction are similar to those used in the commercial production of chlorine via the Chlor-alkali process; however, other performance metrics related to the energetic efficiency, carbon efficiency, single-pass conversion, crossover, and durability or operating lifetime are also important considerations.Work in our labs indicates that operating lifetimes are typically related to flooding, precipitation, or catalyst deactivation. As shown in Figure 1a, flooding in CO2 electrolyzers is typically observed with a sharp increase in hydrogen production and increased cathode gas flows. Figure 1b shows evidence of carbonate precipitation including variations in cell potential and lower cathode flow.In this talk, we consider the relationships between AEM properties (conductivity, water transport, counter ion transport), binder properties (viz. hydrophobicity), and catalysts on operating lifetimes of CO2 electrolyzers. Figure 1

Highly Porous Copper Catalysts for Electrochemical Ammonia Production from Nitrate-Containing Waste Water

Nitrate from agricultural runoff and industrial waste streams is one of the major impurities in water. As high level of nitrate in drinking water may cause methemoglobinemia in infants and gastrointestinal cancer in adults, previous efforts on electrochemical wastewater treatment have aimed to solve this problem by converting nitrates to environmentally benign nitrogen gas. In the present study, we adjust this approach and propose an opportunity to transform nitrates to ammonia, which can serve as a key chemical used in fertilizer and fuel. Additionally, the electrochemical reduction of nitrate to ammonia can also offer an appealing alternative to the energy- and resource-intensive Haber-Bosch process. In this work, we explore the structure-activity relationship of copper catalysts using Cu foil, Cu particle, and porous Cu in terms of their performances in electrochemical nitrate reduction toward ammonia. Under the optimal conditions, the synthesized porous Cu exhibits a nitrate conversion of 84% with an ammonia selectivity of 85% and a Faraday efficiency of 58% at an applied potential of -0.75 V. Further investigations on surface structural effects reveal that the electrochemical surface area and the fraction of (111) crystal facet on porous Cu are significantly larger than other Cu materials, impacting their catalytic activity for electrochemical ammonia production. Figure 1

Causes and Effects of Unintended Cation Crossover in CO2 Reduction Cells

Electrochemically converting carbon dioxide into useful products is a promising approach to combat climate change by reducing our reliance on fossil fuels and promoting a sustainable carbon cycle.The operation of membrane electrode assemblies within gas diffusion cells facilitates the efficient reduction of CO2 at rates relevant to industrial applications. However, their long-term stability is often limited by formation of solid precipitates in the cathode pores. This is a consequence of a combination of 1) local alkalization due to the electrochemical reaction, 2) generation of (bi)carbonate by chemical reaction of CO2 with the alkaline electrolyte, and 3) the presence of alkali metal cations. In catholyte-free, zero-gap cells using anion exchange membranes, the presence of electrolyte cations at the cathode is the result of unintended crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviours of copper catalysts in catholyte-free CO2 electrolyzers.Our findings highlight the substantial impact of cation effects, including unintended crossover, even in catholyte-free cells, on reaction pathways. This aspect should be considered in the future development of catalysts and devices. As an outlook a more in-depth knowledge with the help of operando measurements could help to understand and manage cation crossover for optimizing the performance, selectivity, and durability of these electrochemical systems.

Surface Hydroxyl Groups Enhance Ethylene Selectivity in Acidic CO2 Reduction Reaction

Acidic conditions can enable high single-pass conversion in electrochemical CO2 reduction (CO2R) reaction. However, the kinetically facile hydrogen evolution reaction (HER) prevails in acidic conditions and limits the energy efficiency for CO2R towards multi-carbon (C2+) products. By performing electrodeposition under CO2R conditions, we synthesized a copper catalyst (EC-Cu) that promotes selective CO2R to C2+ products in acidic conditions. We combine in situ spectroscopy, DFT calculations, and catalyst performance to show that co-adsorbed CO and OH on the catalyst surface promote C-C coupling, which we correlate with evidence of increased CO residence time. With optimization, the EC-Cu catalyst achieved a 60% faradaic efficiency for ethylene and 90% for C2+. Furthermore, when implemented in a slim flow cell, the EC-Cu catalyst can attain a 20% energy efficiency toward ethylene production.

In Situ STM Investigations of Cu(100) and Bimetallic Ag/Cu(100) Model Catalysts for CO2 Electroreduction in Bicarbonate Solution

The electrochemical conversion of carbon dioxide (CO2RR) to valuable chemicals and fuels plays a crucial role in the sustainable energy economy as it represents a promising potential for the renewable energy storage as well as closing the carbon loop.[1,2] Thus, the design and development of a highly efficient and selective electrocatalyst for CO2RR has become a topic of great importance in recent years. Among elemental metals, copper exhibits unique catalytic behaviour to reduce CO2 towards multi-carbon products with reasonable Faradaic efficiency due to its moderate CO-binding energy.[3,4] However, the selectivity and activity of pure copper is still far away for a practical application. The integration of a secondary metal species provides a promising strategy to improve the catalytical performance of the copper catalyst. Combination with silver is particularly interesting for such bimetallic catalysts, since Ag is a highly efficient catalyst for CO2RR to CO, probably the most important intermediate in the CO2RR, and has been shown to promote the formation of multi-carbon products.[5]Here, we present in situ scanning tunnelling microscopy studies of well-defined Cu and AgCu electrodes in CO2-satured 0.1 M KHCO3, which is one of the most common electrolytes for CO2 electroreduction. For Cu(100) in the double layer potential range, coexistence of two ordered (bi)carbonate adlayer phases was observed (Fig. 1a,b, published in [6]). These adlayer phases have a highly complex structure, exhibit dynamic fluctuations, and undergo a potential dependent order-disorder transition at about 0 V vs. the reversible hydrogen electrode. Detailed density functional calculations reveal that the observed layer is composed of carbonate and water that coadsorb on the electrode surface, underlining the key role of water in the stabilization of the (bi)carbonate adlayers.[6]Upon decreasing the potential to the onset of CO2RR, the formation of Cu nanoclusters is observed on the Cu(100) surfaces by in situ STM. This surface restructuring can be assigned to the influence of CO intermediates formed in the CO2RR. Upon subsequent increase of the potential back into the double layer range, the clusters disperse, resulting in a highly disordered interface structure that contains Cu adatoms, (bi)carbonates, and further molecular adsorbates. This irreversible change of the molecular-scale electrode surface is supported by spectroscopic results.AgCu bimetallic model catalysts were prepared by electrodeposition of submonolayer Ag coverage on Cu(100) in 0.1 M H2SO4. Silver forms islands on Cu(100) with a hexagonal quasi-Ag(111) atomic lattice. After the preparation and characterization of the Ag-decorated Cu(100), the electrolyte was exchanged under potential control to 0.1 M KHCO3. The electrolyte exchange results in the distinct restructuring on the surface in the double layer potential range, in which the islands develop a strongly anisotropic shape. In addition, appearance of new islands and a reduced surface mobility are observed. Furthermore, in contrast to the Ag-free Cu(100), high-resolution STM images show a disordered molecular adlayer.Our results demonstrate the pronounced role of adsorbed (bi)carbonate at the catalyst/electrolyte interface and its impact on the surface morphology and dynamics already at potentials in the double layer regime. These adsorbate-induced structural changes need to be considered for a fundamental understanding and rational design of electrocatalysts for CO2RR.The Authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via SPP 2080, project no. 327886311.

Unintended Cation Crossover in CO2 Conversion MEA Cells: Causes and Effects

Membrane electrode assemblies in gas diffusion cells enable CO2 reduction at industrially relevant rates, yet their long-term operational stability is often limited by the formation of solid precipitates (e.g. K2CO3) in the cathode pores. This is a consequence of a combination of 1) local alkalization due to the electrochemical reaction, 2) generation of (bi)carbonate by chemical reaction of CO2 with the alkaline electrolyte, and 3) the presence of alkali metal cations. In catholyte-free, zero-gap cells using anion exchange membranes, the presence of electrolyte cations at the cathode is the result of unintended crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte ionic strength (using aqueous KOH or KHCO3) correlated with drastic changes in the observed product selectivity – most notably, at low ionic strength, Cu catalysts produced predominantly CO, in contrast to the mixture of C2+ products typically observed on Cu. In this talk, we examine the factors influencing ion crossover and the resulting effects on catalyst structure and activity, under conditions of both CO2 and CO reduction. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy were used to study how the catalyst is affected by operation conditions. Our results show that even in catholyte-free cells, cation effects (including unintended ones) can significantly influence reaction pathways, and this must be considered in future development of catalysts and devices.El-Nagar, G. A., Haun, F., Gupta, S., Stojkovikj, S. & Mayer, M. T. Unintended cation crossover influences CO2 reduction activity in Cu-based zero-gap electrolysers. Nature Communications14, 2062 (2023) doi: doi.org/10.1038/s41467-023-37520-x Figure 1

Highly dispersed copper catalysts for CO preferential oxidation in hydrogen-rich atmosphere

CeO2 and ZnO with finely rod shape were used as carriers to support copper. High-resolution transmission electron microscopy demonstrated that copper species were existed mostly in bilayer clusters containing underlying Cu+ and upper Cu0 on CeO2, but in big particles of 5–10 nm consisting of only Cu0 on ZnO. The amounts of underlying Cu+ in bilayers and oxygen vacancies (Ov) at copper-ceria interface reached maximum when copper loading was about 10 wt%. In CO preferential oxidation reaction, the 10-Cu/CeO2 catalyst showed the highest activity suggesting that CO adsorbed on Cu+ in bilayer clusters reacted rapidly with O2 absorbed on adjacent Ov. The near synergistic effect not only reduced the CO reaction temperature but also enhanced the CO conversion. The Cu/ZnO catalyst had weak CO adsorption and long distance of Cu0 and Ov, thus it showed much low activity. By filling the 10-Cu/CeO2 catalysts in a two-stage reactor, 1 % CO in the methanol steam reforming gas could be removed to 2 ppm after two successive CO oxidation processes, which provided a new solution for H2 purification.