Oxidation of metallic Cu by supercritical CO(2) and control synthesis of amorphous nano-metal catalysts for CO(2) electroreduction

Rational Construction of Cu Active Sites for CO2 Electrolysis to C2+ Product

Electrocatalytic CO2 reduction reaction (CO2RR) has emerged as a promising approach in advancing towards carbon neutrality and addressing renewable energy intermittency. Copper-based catalysts have received much attention due to their high catalytic activity to convert CO2 into high value-added C2+ products. However, CO2RR exhibits a diversity of reduction products and unavoidable hydrogen precipitation side reactions due to the moderate adsorption strength of *CO on the copper surface and the fact that the electrode potential for CO2 reduction is very close to that for hydrogen precipitation reduction. Here, we summarize recent advances in the structural design and active site construction of copper-based catalysts for CO2RR, and investigate their effects on the improvement of CO2RR performance, with the aim of deepening the understanding of catalyst structure and active sites, thereby facilitating the design of more efficient copper-based catalysts for the sustainable production of value-added chemicals.

Supercritical CO2 Activation Enables an Exceptional Methanol Synthesis Activity Over the Industrial Cu/ZnO/Al2O3 Catalyst

The ternary Cu/ZnO/Al2O3 catalyst is widely used in the industry for renewable methanol synthesis. The tenuous trade-off between the strong metal-support interaction (SMSI)-induced Cu-ZnOx interface and the accessible Cu surface strongly affects the activity of the final catalyst. Successes in the control of oxide migration on adsorbate-induced SMSI catalysts have motivated this to develop a supercritical CO2 activation strategy to synchronously perfect the Cu0-O-Znδ + interface and Cu0-Cu+ surface sites through the manipulation of the adsorbate diffusion kinetics, which involves *OC2H5 and "side-on" fixed CO2 species. This findings illustrate that the adsorbate on ZnOx can facilitate its secondary uniform nucleation and induce a ZnxAl2Oy spinel phase and that CO2 adsorption on metallic Cu0 produces an activated CuxO amorphous shell. Such a structural evolution unlocks a dual-response pathway in methanol synthesis, thus enabling Cu/ZnO/Al2O3 with a twofold increase in catalytic activity. This atomic-level design of active sites and understanding of supercritical CO2-induced structural evolution will guide the future development of high-performance supported metal catalysts.

Cu supraparticles with enhanced mass transfer and abundant C-C coupling sites achieving ampere-level CO2-to-C2+ electrosynthesis

The efficient electrochemical CO2 reduction to C2+ products at high current densities remains a significant challenge. Here we show inherently hydrophobic and hierarchically porous Cu supraparticles comprising sub-10 nm Cu constituent particles for ampere-level CO2-to-C2+ electrosynthesis. These supraparticles feature abundant grain boundaries for high C2+ selectivity, coupled with interconnected mesopores and interparticle macropore cavities to enhance the accessibility of the active sites and mass transfer, breaking the trade-off between activity and mass transfer in Cu-based catalysts. Moreover, the intrinsic hydrophobicity of the supraparticles mitigates the water-flooding issue of catalytic layer in flow cells, improving the stability at high current densities. Consequently, the Cu supraparticles achieve ampere-level CO2 electrolysis up to 3.2 A cm-2 with a C2+ Faradaic efficiency of 74.9% (compared to 1.21 A cm-2 and 55.4% for Cu nanoparticles) and maintain stability at 1 A cm-2 for over 100 h. This work provides profound insights into the effect of the coupling of mass transfer and catalytic reaction under a high current and presents a corresponding solution by superstructure design.

Polymeric ionic liquid promotes acidic electrocatalytic CO2 conversion to multicarbon products with ampere level current on Cu

The acidic electroreduction of CO2 into multicarbon (C2+) products is much attractive for the improved carbon utilization than alkaline or neutral electroreduction. How to improve the efficiency of C2+ products generation by acidic electroreduction of CO2, is important, especially at high current density and in electrolyte with low K+ concentration. Herein, we propose a strategy of capping Cu surface with a polymeric ionic liquid (PIL) adlayer for boosting the acidic electrocatalytic CO2 conversion to C2+ products at high current densities (ampere-level) and low K+ concentration. In the electrolyte with a relatively low K+ concentration (1.0 M), the Faradaic efficiency (FE) for C2+ products reaches 82.2% under a current density 1.0 A·cm-2 in acidic environment (pH=1.8). Particularly, when the current density is as high as 1.5 A·cm-2, the C2+ FE still keeps 75.8%. Experimental and theoretical studies reveal that the presence of PIL adlayer on Cu catalyst can well inhibit H+ diffusion to catalyst surface, enrich more K+ and facilitate C-C coupling reaction.

Ammonia electrosynthesis from nitrate using a stable amorphous/crystalline dual-phase Cu catalyst

Renewable energy-driven electrocatalytic nitrate reduction reaction presents a low-carbon and sustainable route for ammonia synthesis under mild conditions. Yet, the practical application of this process is currently hindered by unsatisfactory electrocatalytic activity and long-term stability. Herein we achieve high-rate ammonia electrosynthesis using a stable amorphous/crystalline dual-phase Cu catalyst. The ammonia partial current density and formation rate reach 3.33 ± 0.005 A cm-2 and 15.5 ± 0.02 mmol h-1 cm-2 at a low cell voltage of 2.6 ± 0.01 V, respectively. Remarkably, the dual-phase Cu catalyst can maintain stable ammonia production with a Faradaic efficiency of around 90% at a high current density of 1.5 A cm-2 for up to 300 h. A scale-up demonstration with an electrode size of 100 cm2 achieves an ammonia formation rate as high as 11.9 ± 0.5 g h-1 at a total current of 160 A. The impressive electrocatalytic performance is ascribed to the presence of stable amorphous Cu domains which promote the adsorption and hydrogenation of nitrogen-containing intermediates, thus improving reaction kinetics for ammonia formation. This work underscores the importance of stabilizing metastable amorphous structures for improving electrocatalytic reactivity and long-term stability.

Novel crystalline/amorphous heterophase Fe-Mn core-shell chains on-site generate hydrogen peroxide in aqueous solution

Hydrogen peroxide (H2O2) is a crucial eco-friendly oxidizer with increasing demand due to its wide range of applications. Activating O2 with catalysts to generate H2O2 on-site offers a promising alternative to traditional production methods. Here, we design unique crystalline/amorphous heterophase Fe-Mn core-shell chains (ZVI-Mn) for efficient on-site generation of H2O2 and manipulation of subsequent H2O2 activation. The yield of H2O2 on-site produced by ZVI-Mn in water within 5 min was 103.7 mg·L-1, which was much greater than that of zero-valent iron (ZVI) and amorphous Mn (A-Mn) (0 and 42.5 mg·L-1). Raman and density functional theory (DFT) calculations confirmed that *OOH is the key species involved in the on-site generation of H2O2. Electrochemical tests confirmed the excellent electron-transferring ability, while electron paramagnetic resonance (EPR) revealed oxygen vacancy defects in the catalysts, which proved to be conducive to improving the catalytic activity of ZVI-Mn. Additionally, by regulating the pH of aqueous solution, ZVI-Mn can simultaneously achieve efficient on-site generation of H2O2 and in-situ removal of enrofloxacin from aqueous solution.

Highly Selective CO2 Electroreduction to Multi-Carbon Alcohols via Amine Modified Copper Nanoparticles at Acidic Conditions

Electroreduction of CO2 into multi-carbon (C2+) products (e.g. C2+ alcohols) offers a promising way for CO2 utilization. Use of strong alkaline electrolytes is favorable to producing C2+ products. However, CO2 can react with hydroxide to form carbonate/bicarbonate, which results in low carbon utilization efficiency and poor stability. Using acidic electrolyte is an efficient way to solve the problems, but it is a challenge to achieve high selectivity of C2+ products. Here we report that the amine modified copper nanoparticles exhibit high selectivity of C2+ products and carbon utilization at acidic condition. The Faradaic efficiency (FE) of C2+ products reach up to 81.8 % at acidic media (pH=2) with a total current density of 410 mA cm-2 over n-butylamine modified Cu. Especially the FE of C2+ alcohols is 52.6 %, which is higher than those reported for CO2 electroreduction at acidic condition. In addition, the single-pass carbon efficiency towards C2+ production reach up to 60 %. Detailed studies demonstrate that the amine molecule on the surface of Cu cannot only enhance the formation, adsorption and coverage of *CO, but also provide a hydrophobic environment, which result in the high selectivity of C2+ alcohols at acidic condition.

Highly Selective Production of C2+ Oxygenates from CO2 in Strongly Acidic Condition by Rough Ag-Cu Electrocatalyst

The acidic electrocatalytic conversion of CO2 to multi-carbon (C2+) oxygenates is of great importance in view of enhancing carbon utilization efficiency and generating products with high energy densities, but suffering from low selectivity and activity. Herein, we synthesized Ag-Cu alloy catalyst with highly rough surface, by which the selectivity to C2+ oxygenates can be greatly improved. In a strongly acidic condition (pH=0.75), the maximum C2+ products Faradaic efficiency (FE) and C2+ oxygenates FE reach 80.4 % and 56.5 % at -1.9 V versus reversible hydrogen electrode, respectively, with a ratio of FEC2+ oxygenates to FEethylene up to 2.36. At this condition, the C2+ oxygenates partial current density is as high as 480 mA cm-2. The in situ spectra, control experiments and theoretical calculations indicate that the high generation of C2+ oxygenates over the catalyst originates from its large surface roughness and Ag alloying.

Pressure-induced generation of heterogeneous electrocatalytic metal hydride surfaces for sustainable hydrogen transfer

Metal hydrides are crucial intermediates in numerous catalytic reactions. Intensive efforts have been dedicated to constructing molecular metal hydrides, where toxic precursors and delicate mediators are usually involved. Herein, we demonstrate a facile pressure-induced methodology to generate a cost-effective heterogeneous electrocatalytic metal hydride surface for sustainable hydrogen transfer. Taking carbon dioxide (CO2) electroreduction as a model system and zinc (Zn), a well-known carbon monoxide (CO)-selective catalyst, as a model catalyst, we showcase a homogeneous-type hydrogen atom transfer process induced by heterogeneous hydride surfaces, enabling direct hydrogenation pathways traditionally considered "prohibited". Specifically, the maximal Faradaic efficiency for formate is enhanced by ~fivefold to 83% under ambient conditions. Experimental and theoretical analyses reveal that unlike the distal hydrogenation route for CO2 to CO over pristine Zn, the Zn hydride surface enables direct hydrogenation at the carbon site of CO2 to form formate. This work provides a promising material platform for sustainable synthesis.

Three-layered nanoplates and amorphous/crystalline interface synergism boost CO2 photoreduction on bismuth oxychloride nanospheres

Structural features like 3D nano-size, ultrathin thickness and amorphous/crystalline interfaces play crucial roles in regulating charge separation and active sites of photocatalysts. However, their co-occurrence in a single catalyst and exploitation in photocatalytic CO2 reduction (PCR) remains challenging. Herein, nano-sized bismuth oxychloride spheres (BiOCl-NS) confining three-layered nanoplates (∼2.2 nm ultrathin) and an amorphous/crystalline interface are exclusively developed via intrinsic engineering for an enhanced sacrificial-reagent-free PCR system. The results uncover a unique synergism wherein the three-layered nanoplates accelerate electron-hole separation, and the amorphous/crystalline interface exposes electron-localized active sites (Bi-Ovac-Bi). Consequently, BiOCl-NS exhibit efficient CO2 adsorption and activation with the lowering of rate-determining-step energy barriers, leading to remarkable CO production (102.72 μmol g-1 h-1) with high selectivity (>99%), stability (>30 h), and apparent quantum efficiency (0.51%), outperforming conventional counterparts. Our work provides a facile structural engineering approach for boosting PCR and offers distinct synergism for advancing diverse materials.

Tailoring Cu-Based Electrocatalysts for Enhanced Electrochemical CO2 Reduction to Alcohols: Structure-Selectivity Relationship

The use of CO2 as a feedstock for the production of carbon-based fuels and value-added chemicals offers a promising route toward carbon neutrality. In this study, two Cu-based electrocatalysts, namely, Cu24/N-C and Cu2/N-C, are successfully prepared by thermal treatment of Cu24 metal-organic polyhedron-loaded zeolitic imidazolate framework-8 (ZIF-8) nanocrystals (Cu24/ZIF-8) and Cu2 dinuclear compound-loaded ZIF-8 nanocrystals (Cu2/ZIF-8), respectively. Extensive structural and compositional analyses were conducted to confirm the formation of Cu nanocluster-loaded N-doped porous carbon supports in both Cu24/N-C and Cu2/N-C and Cu nanoparticles encapsulated by graphitic carbons in Cu2/N-C as well. These two Cu-based electrocatalysts exhibited different behaviors in the electrochemical CO2 reduction reaction (CO2RR). The Cu24/N-C electrocatalyst showed high selectivity for CO production, while Cu2/N-C showed a preference for alcohol generation. The excellent stability of Cu2/N-C over a 30 h continuous electrochemical reduction further highlights its potential for practical applications. The difference in electrocatalytic performance observed in the two catalysts for CO2RR was attributed to distinct catalytic sites associated with Cu nanoclusters and nanoparticles. This research reveals the significance of their structures and compositions for the development of highly selective electrocatalysts for CO2 reduction.

Facet-switching of rate-determining step on copper in CO2-to-ethylene electroreduction

Reduction of carbon dioxide (CO2) by renewable electricity to produce multicarbon chemicals, such as ethylene (C2H4), continues to be a challenge because of insufficient Faradaic efficiency, low production rates, and complex mechanistic pathways. Here, we report that the rate-determining steps (RDS) on common copper (Cu) surfaces diverge in CO2 electroreduction, leading to distinct catalytic performances. Through a combination of experimental and computational studies, we reveal that C─C bond-making is the RDS on Cu(100), whereas the protonation of *CO with adsorbed water becomes rate-limiting on Cu(111) with a higher energy barrier. On an oxide-derived Cu(100)-dominant Cu catalyst, we reach a high C2H4 Faradaic efficiency of 72%, partial current density of 359 mA cm-2, and long-term stability exceeding 100 h at 500 mA cm-2, greatly outperforming its Cu(111)-rich counterpart. We further demonstrate constant C2H4 selectivity of >60% over 70 h in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%.

Revealing the structural evolution of CuAg composites during electrochemical carbon monoxide reduction

Comprehending the catalyst structural evolution during the electrocatalytic process is crucial for establishing robust structure/performance correlations for future catalysts design. Herein, we interrogate the structural evolution of a promising Cu-Ag oxide catalyst precursor during electrochemical carbon monoxide reduction. By using extensive in situ and ex situ characterization techniques, we reveal that the homogenous oxide precursors undergo a transformation to a bimetallic composite consisting of small Ag nanoparticles enveloped by thin layers of amorphous Cu. We believe that the amorphous Cu layer with undercoordinated nature is responsible for the enhanced catalytic performance of the current catalyst composite. By tuning the Cu/Ag ratio in the oxide precursor, we find that increasing the Ag concentration greatly promotes liquid products formation while suppressing the byproduct hydrogen. CO2/CO co-feeding electrolysis and isotopic labelling experiments suggest that high CO concentrations in the feed favor the formation of multi-carbon products. Overall, we anticipate the insights obtained for Cu-Ag bimetallic systems for CO electroreduction in this study may guide future catalyst design with improved performance.

Unveiling the Mechanism of Plasma-Catalyzed Oxidation of Methane to C2+ Oxygenates over Cu/UiO-66-NH2

Nonthermal plasma (NTP) offers the potential for converting CH4 with CO2 into liquid products under mild conditions, but controlling liquid selectivity and manipulating intermediate species remain significant challenges. Here, we demonstrate the effectiveness of the Cu/UiO-66-NH2 catalyst in promising the conversion of CH4 and CO2 into oxygenates within a dielectric barrier discharge NTP reactor under ambient conditions. The 10% Cu/UiO-66-NH2 catalyst achieved an impressive 53.4% overall liquid selectivity, with C2+ oxygenates accounting for ∼60.8% of the total liquid products. In situ plasma-coupled Fourier-transform infrared spectroscopy (FTIR) suggests that Cu facilitates the cleavage of surface adsorbed COOH species (*COOH), generating *CO and enabling its migration to the surface of Cu particles. This surface-bound *CO then undergoes C-C coupling and hydrogenation, leading to ethanol production. Further analysis using CO diffuse reflection FTIR and 1H nuclear magnetic resonance spectroscopy indicates that in situ generated surface *CO is more effective than gas-phase CO (g) in promoting C-C coupling and C2+ liquid formation. This work provides valuable mechanistic insights into C-C coupling and C2+ liquid production during plasma-catalytic CO2 oxidation of CH4 under ambient conditions. These findings hold broader implications for the rational design of more efficient catalysts for this reaction, paving the way for advancements in sustainable fuel and chemical production.

Copper Electrocatalyst Produced by Cu2(OH)2CO3-Mediated In Situ Deposition for Diluted CO2 Reduction to Multicarbon Products

Pure CO2 is commonly used in most of the current studies for electrochemical CO2 reduction which will need a further cost of gas purification and separation. However, the limited works on diluted CO2 reduction are focused on CO or CH4 production other than C2 products. In this work, copper electrocatalysts were prepared by Cu2(OH)2CO3-mediated in situ deposition for diluted CO2 reduction to multicarbon products. Using in situ Raman spectroscopy, constant amounts of CO and OH* were observed on the catalyst surface, which could effectively suppress the high kinetics of hydrogen evolution and promote C-C coupling, especially under the condition of diluted CO2 reduction. The optimized Cu catalyst achieves a C2 Faradaic efficiency as high as 60.72% in the presence of merely 25% CO2, which is almost equivalent to that observed with pure CO2.

Green solvent systems for material syntheses and chemical reactions

It is of great significance to develop environmentally benign, non-volatile and recyclable green solvents for different applications. This feature article overviews the properties of green solvent systems (e.g., ionic liquids, supercritical carbon dioxide, deep eutectic solvents and mixed green solvent systems) and their applications in (1) framework material syntheses, including metal-organic frameworks, covalent organic frameworks and hydrogen-bonded organic frameworks, and (2) CO2 conversion reactions, including photocatalytic and electrocatalytic reduction reactions. Finally, the future perspective for research on green solvent systems is proposed from different aspects.

Selective CO2 -to-Syngas Conversion Enabled by Bimetallic Gold/Zinc Sites in Partially Reduced Gold/Zinc Oxide Arrays

Electrocatalytic CO2 -to-syngas (gaseous mixture of CO and H2 ) is a promising way to curb excessive CO2 emission and the greenhouse gas effect. Herein, we present a bimetallic AuZn@ZnO (AuZn/ZnO) catalyst with high efficiency and durability for the electrocatalytic reduction of CO2 and H2 O, which enables a high Faradaic efficiency of 66.4 % for CO and 26.5 % for H2 and 3 h stability of CO2 -to-syngas at -0.9 V vs. the reversible hydrogen electrode (RHE). The CO/H2 ratios show a wide range from 0.25 to 2.50 over a narrow potential window (-0.7 V to -1.1 V vs. RHE). In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy combined with density functional theory calculations reveals that the bimetallic synergistic effect between Au and Zn sites lowers the activation energy barrier of CO2 molecules and facilitates electronic transfer, further highlighting the potential to control CO/H2 ratios for efficient syngas production using the coexisting Au sites and Zn sites.

Tuning Coordination Structures of Zn Sites Through Symmetry-Breaking Accelerates Electrocatalysis

Manipulating the coordination environment of individual active sites in a precise manner remains an important challenge in electrocatalytic reactions. Herein, inspired by theoretical predictions, a facile procedure to synthesize a series of symmetry-breaking zinc metal-organic framework (Zn-MOF) catalysts with well-defined structures is presented. Benefiting from the optimized coordination microenvironment regulated by symmetry-breaking, Zn-N2 S2 -MOF exhibits the best performance of nitrogen (N2 ) reduction reaction (NRR) with NH3 yield rate of 25.07 ± 1.57 µg h-1  cm-2 and Faradaic efficiency of 44.57 ± 2.79% compared with reported Zn-based NRR catalysts. X-ray absorption near-edge structure shows that the symmetry-breaking distorts the coordination environment and modulates the delocalized electrons around the Zn sites, which favors the formation of unpaired low-valence Znδ+ , thereby facilitating the adsorption/activation of N2 . Theoretical calculations elucidate that low-valence Znδ+ in Zn-N2 S2 -MOF can effectively lower the energy barrier of potential determining step, promoting the kinetics and boosting the NRR activity. This work highlights the relationship between the precise coordination environment of metal sites and the catalytic activity, which offers insightful guidance for rationally designing high-efficiency electrocatalysts.

Br-doped Cu nanoparticle formed by in situ restructuring for highly efficient electrochemical reduction of CO2 to formate

Electrochemical conversion of CO2 into chemical feedstock, such as an energy-dense liquid product (formate), is desirable to address the excessive emission of greenhouse gases and store energy. Cu-based catalysts exhibit great advantages in electrochemical CO2 reduction reaction (eCO2RR) due to their low cost and high abundance, but suffer from low selectivity of formate. In this work, a facile one-pot approach is developed to synthesize CuBr nanoparticle (CuBr NP) that can conduct in situ dynamic restructuring during eCO2RR to generate Br-doped Cu NP. The in situ-formed Br-doped Cu NP can afford up to 91.6% Faradaic efficiency (FE) for formate production with a partial current density of 15.1 mA·cm-2 at -0.94 V vs. reversible hydrogen electrode (RHE) in an H-type cell. Moreover, Br-doped Cu NP can deliver excellent long-term stability for up to 25 h. The first-principles density functional theory (DFT) calculations show that the doped Br can regulate the electronic structure of Cu active sites to optimize the adsorption of the HCOO* intermediate, greatly hindering the formation of CO and H2. This work provides a strategy for electronic modulation of metal active site and suggests new opportunities in high selectivity for electrocatalytic reduction of CO2 to formate over Cu-based catalysts.

Alkaline Ionic Liquid Microphase Promotes Deep Reduction of CO2 on Copper

Electrochemical reduction of CO2 to multicarbon (C2+) products using renewable energy sources is an important route to storing sustainable energy and achieving carbon neutrality. It remains a challenge to achieve high C2+ product faraday efficiency (FE) at ampere-level current densities. Herein, we propose the immobilization of an alkaline ionic liquid on copper for promoting the deep reduction of CO2. By this strategy, a C2+ FE of 81.4% can be achieved under a current density of 0.9 A·cm-2 with a half-cell energy conversion efficiency of 47.4% at -0.76 V vs reversible hydrogen electrode (RHE). Particularly, when the current density is as high as 1.8 A·cm-2, the C2+ FE reaches 71.6% at an applied potential of -1.31 V vs RHE. Mechanistic studies demonstrate that the alkaline ionic liquid plays multiple roles of improving the accumulation of CO2 molecules on the copper surface, promoting the activation of the adsorbed CO2, reducing the energy barrier of CO dimerization, stabilizing intermediates, and facilitating the C2+ product formation.

Local compressive strain regulation of atomically dispersed NiN4 sites for enhancing CO2 electroreduction to CO

Electroreduction of CO2 to valuable chemicals powered by renewable electricity provides a sustainable approach to reduce the environmental issues originating from CO2 emission. However, insufficient current density and production selectivity hinder its further application. In this case, precisely regulating the CO2 reduction reaction (CO2RR) active sites is an excellent strategy to simultaneously reduce the reaction barrier and suppress the hydrogen evolution reaction (HER) pathway. Herein, the strain regulation of atomically dispersed NiN4 active sites is investigated in helical carbon. Ni-N coordination in the curved carbon lattice displays a reduced distance compared to that in a straight lattice, inflicting local compressive strain on NiN4. The resultant catalyst shows the highest CO selectivity of up to 99.4% at -1.4 V (vs. RHE), the FECO is maintained at over 85% over a wide potential range from -0.8 to -1.8 V (vs. RHE), and the maximum partial current density for CO reaches a high of 458 mA cm-2 at -1.8 V (vs. RHE). Theoretical investigations show the superior CO2 electroreduction performance of curved NiN4 stems from its remarkable ability to generate the *COOH intermediate and to suppress the hydrogen combination simultaneously. Our findings offer a novel strategy to rationally regulate the local three-dimensional structure of single-atom sites for efficient electrocatalysis.