CO Coverage Research Articles

Understanding the restructuring and degradation of oxide-derived copper during electrochemical CO2 reduction

Renewable energy produced through electrochemical CO2 reduction is promising for mitigating the energy crisis and environmental issues. Notably, oxide-derived copper (OD-Cu) has attracted considerable attention owing to its unique ability to catalyze CO2 electroreduction to >2e− products, especially C2+ products, which are the key feedstocks for the chemical process. However, Cu under operating conditions usually leads to structure reconstruction and degradation, which hinders understanding the origin of its activity. Understanding the restructuring and degradation of OD-Cu catalyst to form C2+ products is critical for understanding the active sites and, thus for designing effective and durable catalysts for CO2 reduction. This paper describes that Cu defect sites are easily dissolved during electrochemical CO2 reduction, and the presence of defect sites is strongly related to the catalytic performance of C2+ products. Here, the C2+ products Faradaic efficiency (FE) of OD-Cu decreased from 39.4% at 1 h to 26.0% at 5 h at −1.5 V vs. RHE, along with a decrease in low-coordinated defect sites. The low-coordinated defect sites facilitate CO adsorption and help to increase the CO coverage, thus promoting CO–CO coupling. The decrease in the number of defect sites and activity degradation is circumvented by promoting the Cu redeposition process. The C2+ products FE of OD-Cu can be maintained at ca. 47.2% without an obvious decrease in CO2-saturated 0.1 M KHCO3 with 1 mM Cu2+. The results of this study help in understanding the activity of OD-Cu and reveal its deactivation mechanism. This knowledge will aid in designing highly active and stable catalysts for CO2 reduction.

Pressure dependence in aqueous-based electrochemical CO2 reduction

Electrochemical CO2 reduction (CO2R) is an approach to closing the carbon cycle for chemical synthesis. To date, the field has focused on the electrolysis of ambient pressure CO2. However, industrial CO2 is pressurized—in capture, transport and storage—and is often in dissolved form. Here, we find that pressurization to 50 bar steers CO2R pathways toward formate, something seen across widely-employed CO2R catalysts. By developing operando methods compatible with high pressures, including quantitative operando Raman spectroscopy, we link the high formate selectivity to increased CO2 coverage on the cathode surface. The interplay of theory and experiments validates the mechanism, and guides us to functionalize the surface of a Cu cathode with a proton-resistant layer to further the pressure-mediated selectivity effect. This work illustrates the value of industrial CO2 sources as the starting feedstock for sustainable chemical synthesis.

Constrained C2 adsorbate orientation enables CO-to-acetate electroreduction.

The carbon dioxide and carbon monoxide electroreduction reactions, when powered using low-carbon electricity, offer pathways to the decarbonization of chemical manufacture1,2. Copper (Cu) is relied on today for carbon-carbon coupling, in which it produces mixtures of more than ten C2+ chemicals3-6: a long-standing challenge lies in achieving selectivity to a single principal C2+ product7-9. Acetate is one such C2 compound on the path to the large but fossil-derived acetic acid market. Here we pursued dispersing a low concentration of Cu atoms in a host metal to favour the stabilization of ketenes10-chemical intermediates that are bound in monodentate fashion to the electrocatalyst. We synthesize Cu-in-Ag dilute (about 1 atomic per cent of Cu) alloy materials that we find to be highly selective for acetate electrosynthesis from CO at high *CO coverage, implemented at 10 atm pressure. Operando X-ray absorption spectroscopy indicates in situ-generated Cu clusters consisting of <4 atoms as active sites. We report a 12:1 ratio, an order of magnitude increase compared to the best previous reports, in the selectivity for acetate relative to all other products observed from the carbon monoxide electroreduction reaction. Combining catalyst design and reactor engineering, we achieve a CO-to-acetate Faradaic efficiency of 91% and report a Faradaic efficiency of 85% with an 820-h operating time. High selectivity benefits energy efficiency and downstream separation across all carbon-based electrochemical transformations, highlighting the importance of maximizing the Faradaic efficiency towards a single C2+ product11.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models.

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.

Diffusion of O Atoms on a CO-Covered Ru(0001) Surface─A Combined High-Speed Scanning Tunneling Microscopy and Density Functional Theory Study at an Enhanced CO Coverage

The diffusion of adsorbed O atoms on a CO-covered Ru(0001) surface has recently been explained by a “door-opening” mechanism which is driven by fluctuations in the CO layer. Here, we analyze how this mechanism changes at a higher CO coverage than the 0.33 monolayers applied in the previous study and, therefore, lower concentrations of empty sites. High-speed scanning tunneling microscopy was used to track the O atoms. A statistical model based on nearest-neighbor jumps and uniform hopping probabilities into the six equivalent directions of the Ru(0001) lattice describes the data well. Surprisingly, the hopping rates, which involve site exchanges with CO molecules, are higher than the site exchanges at 0.33 monolayers. Density functional theory calculations were performed to explain these observations. The configurations of the CO molecules around the adsorbed O atoms obtained at the higher CO coverage are disordered. Displacements of CO molecules cost less energy than displacements in the ordered (3×3)R30° structure of the previous study. In addition, the activation energies for the jumps of the O atoms are lowered by enhanced O/CO repulsions. The atomic processes are thus faster at the higher coverage, but the general features of the door-opening mechanism remain valid.

Investigation of gas transport resistance in fuel cell catalyst layers via hydrogen limiting current measurements of CO-covered catalyst surfaces

The efficiency of fuel cell operation at high power strongly affects the total cost of ownership of fuel cell vehicles. Gas transport resistance at the cathode strongly suppresses efficiency during high-power operation. The resistance comprises Knudsen diffusion resistance in pores formed among the primary carbon aggregates and local gas transport resistance. The former is independent of the electrochemically active Pt surface area per geometrical electrode area (roughness factor: RF). The latter is proportional to the inverse of RF. Separate identification of these resistances in actual catalyst layers remains challenging because of experimental limitations. Herein, we propose a method that can identify the RF-dependent and RF-independent resistances separately using a single sample. RF can be controlled by partially covering the Pt surface by CO. The gas transport resistance can be ascertained from the hydrogen limiting current. Measuring the hydrogen limiting current at controlled CO coverages allows separate identification of the resistances. The RF-dependent resistance is separable further into ionomer/Pt interfacial resistance and permeation resistance through the ionomer thin film by combining the obtained ionomer-to-carbon weight ratio dependence of each resistance component with structural and electrochemical analyses. The separate resistances show the ionomer/Pt interfacial resistance to be dominant in the catalyst layer.

Quantum mechanical analysis of adsorption for CH4 and CO2 onto graphene oxides

Graphene oxide (GO) surfaces with rich and diverse oxygen-containing functional groups might endow GOs with unique chemical adsorption activity. Exploring the adsorption behavior of gas molecules on GOs with different degrees of oxidation can help design highly efficient adsorbents and catalysts. Herein, the interaction energies between two kinds of gas molecules, CH4 and CO2, and the GO surfaces with varied oxidization degrees were simulated by density functional theory. It is numerically demonstrated that CO2 has a stronger interaction strength than CH4. More specifically, the interaction energies for the gases on GO surfaces do not show monotonous variation with the degree of oxidization. The results suggest that there exists an appropriate range of oxidized degrees on GOs, which can impose higher adsorption interaction. The AIM (atom in molecule) and NOCV (natural orbital for chemical valence) analyses were applied to reveal the interaction mechanisms between the gas molecules and GO surfaces. The adsorption free energy and adsorption coverage of CH4 and CO2 on GO surfaces were calculated. Generally, the gas adsorption on GO is thermodynamically unfavorable under standard conditions. The computational results provide an improved understanding of the adsorption mechanism, which might be valuable for the application of high-performance GO-based adsorbents and catalysts.

New measurement of the diffusion of carbon dioxide on non-porous amorphous solid water

The diffusion of molecules on interstellar grain surfaces is one of the most important driving forces for the molecular complexity in the interstellar medium. Due to the lack of laboratory measurements, astrochemical modeling of grain surface processes usually assumes a constant ratio between the diffusion energy barrier and the desorption energy. This over-simplification inevitably causes large uncertainty in model predictions. We present a new measurement of the diffusion of CO 2 molecules on the surface of non-porous amorphous solid water (np-ASW), an analog of the ice mantle that covers cosmic dust grains. A small coverage of CO 2 was deposited onto an np-ASW surface at 40 K, the subsequent warming of the ice activated the diffusion of CO 2 molecules, and a transition from isolated CO 2 to CO 2 clusters was seen in the infrared spectra. To obtain the diffusion energy barrier and pre-exponential factor simultaneously, a set of isothermal experiments were carried out. The values for the diffusion energy barrier and pre-exponential factor were found to be 1300 ± 110 K and 10 7.6 ± 0.8 s − 1 . A comparison with prior laboratory measurements on diffusion is discussed.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis.

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO2ER) to produce multicarbon (C2+) feedstocks (e.g., C2H4). However, the high energy barriers for CO2 activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in-situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C2H4 selectivity of 62.5% and an industrial-level current density of 160 mA cm-2 at a low potential of -0.72 V were achieved. Our studies revealed that the isolated NiN3 active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO2ER to C2H4.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics‐boosted Ethylene Electrosynthesis

AbstractCu‐based catalysts have been widely applied in electroreduction of carbon dioxide (CO2ER) to produce multicarbon (C2+) feedstocks (e.g., C2H4). However, the high energy barriers for CO2activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N‐enriched carbon support with a sodalite (SOD) topology (Ni‐SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C2H4selectivity of 62.5 % and an industrial‐level current density of 160 mA cm−2at a low potential of −0.72 V were achieved. Our studies revealed that the isolated NiN3active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni‐SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO2ER to C2H4.

Coverage-driven selectivity switch from ethylene to acetate in high-rate CO2/CO electrolysis.

Tuning catalyst microenvironments by electrolytes and organic modifications is effective in improving CO2 electrolysis performance. An alternative way is to use mixed CO/CO2 feeds from incomplete industrial combustion of fossil fuels, but its effect on catalyst microenvironments has been poorly understood. Here we investigate CO/CO2 co-electrolysis over CuO nanosheets in an alkaline membrane electrode assembly electrolyser. With increasing CO pressure in the feed, the major product gradually switches from ethylene to acetate, attributed to the increased CO coverage and local pH. Under optimized conditions, the Faradaic efficiency and partial current density of multicarbon products reach 90.0% and 3.1 A cm-2, corresponding to a carbon selectivity of 100.0% and yield of 75.0%, outperforming thermocatalytic CO hydrogenation. The scale-up demonstration using an electrolyser stack achieves the highest ethylene formation rate of 457.5 ml min-1 at 150 A and acetate formation rate of 2.97 g min-1 at 250 A.

Remote sensing of dissolved CO2 concentrations in meso-eutrophic lakes using Sentinel-3 imagery

Satellite observation can significantly reduce the uncertainties in CO2 emission estimations compared to insufficient field data. However, big challenges remain in developing remote sensing-based models for mapping concentrations of dissolved carbon dioxide (cCO2) in lakes at regional or global scales. We developed a cCO2 estimation model using Sentinel-3-derived lake environmental variables and near-synchronous field cCO2 data from 16 lakes in the middle and lower reaches of the Yangtze and Huai River (ML_YHR) basins in Eastern China (N = 248). Stepwise quadratic polynomial regressions of several combinations of chlorophyll-a (Chl-a), water temperature (Tw), Secchi disk depth (ZSD), and photosynthetic active radiation (PAR)-related variables were tested and validated to select the best approach. The final model showed high performance in calibration and validation (R2 > 0.72, RMSE < 6.35 μmol L−1, MAPE < 30.31%). The model sensitivity analysis, based on Monte Carlo simulations, showed the model's estimated bias as <25% based on uncertainties of all input variables. Spatial and temporal dynamics of dissolved CO2 concentrations in 113 lakes (≥ 10 km2) in the ML_YHR basins were mapped from 2016 to 2021 using the Sentinel-3 data. The result showed that CO2 concentrations were low in the summer and autumn but high in the winter and spring with dramatic variations (e.g., mean coefficient of variation: 52.59%). The annual mean CO2 concentrations of lakes revealed that about 28% of the lakes acted as weak atmospheric CO2 sinks (14.96 ± 1.13 μmol L−1) while the rest were sources (19.22 ± 2.02 μmol L−1), compared with a mean concentration of CO2 atmospheric equilibrium (16.29 μmol L−1). CO2 concentrations decreased with increasing eutrophication and decreasing lake size (p < 0.05). This study advances current knowledge about CO2 emissions from emerging meso-eutrophic lakes and shows how satellite remote sensing can expand the spatiotemporal coverage of lake CO2.

Electrochemical CO2reduction towards formic acid and methanol on transition metal oxide surfaces as a function of CO coverage

Density functional theory is used to study the effect of varying CO coverage on transition metal oxide surfaces on the selectivity and activity of the CO2reduction reaction towards methanol and formic acid formation.

Essential role of CO coverage in CO2 hydrogenation over Pt(111)

The CO2 hydrogenation on Pt(111) proceeds with the increase of CO coverage and this can notably reduce the barrier of CH4 and C2H5OH formation as compared with pristine Pt(111).

A perspective on electrochemical conversion of CO2 to multicarbon chemicals in ionic liquids-based electrolytes

Carbon dioxide electroreduction (CRR) is a promising technology for both intermittent energy storage and emissions mitigation, but it faces challenges such as relatively high overpotential and poor selectivity. Introducing ionic liquids (ILs) into the CRR system has shown impressive activity for CO production, even in electrocatalysts that are primarily active for hydrogen evolution in aqueous electrolytes. However, converting CO2 to high-value C2+ chemicals in IL electrolytes suffers from limitations in *CO coverage, proton accessibility, and specific stabilization effects on *COOH. In this perspective, we emphasize the modification of the steady-state adsorption of *CO and other intermediates and the proton supply to enhance the CO2-to-C2+ conversion. More efforts need to be devoted to electrolyte modulation, involving the functional ILs design, the proton sources, and inorganic additive screening. It is also necessary to develop a descriptor for C2+ selectivity, explore the dynamic evolution of catalyst upon exposure to ILs, and construct novel catalyst/ILs hybrids. Furthermore, developing a molecular understanding of the electrode/ILs interface and the bulk phase of IL-containing electrolytes is essential for designing an efficient electrochemical system for C2+ generation.

Si Doping-Induced Electronic Structure Regulation of Single-Atom Fe Sites for Boosted CO2 Electroreduction at Low Overpotentials.

Transition metal-based single-atom catalysts (TM-SACs) are promising alternatives to Au- and Ag-based electrocatalysts for CO production through CO2 reduction reaction. However, developing TM-SACs with high activity and selectivity at low overpotentials is challenging. Herein, a novel Fe-based SAC with Si doping (Fe-N-C-Si) was prepared, which shows a record-high electrocatalytic performance toward the CO2-to-CO conversion with exceptional current density (>350.0mAcm-2) and~100% Faradaic efficiency (FE) at the overpotential of <400mV, far superior to the reported Fe-based SACs. Further assembling Fe-N-C-Si as the cathode in a rechargeable Zn-CO2 battery delivers an outstanding performance with a maximal power density of 2.44mWcm-2 at an output voltage of 0.30V, as well as high cycling stability and FE (>90%) for CO production. Experimental combined with theoretical analysis unraveled that the nearby Si dopants in the form of Si-C/N bonds modulate the electronic structure of the atomic Fe sites in Fe-N-C-Si to markedly accelerate the key pathway involving *CO intermediate desorption, inhibiting the poisoning of the Fe sites under high CO coverage and thus boosting the CO2RR performance. This work provides an efficient strategy to tune the adsorption/desorption behaviors of intermediates on single-atom sites to improve their electrocatalytic performance.

CO2 coverage on Pd catalysts accelerates oxygen removal in oxy-combustion systems.

Oxy-combustion systems result in enriched CO2 in exhaust gases; however, the utilization of the concentrated CO2 stream from oxy-combustion is limited by remnant O2. CH4 oxidation using Pd catalysts has been found to have high O2-removal efficiency. Here, the effect of excess CO2 in the feed stream on O2 removal with CH4 oxidation is investigated by combining experimental and theoretical approaches. Experimental results reveal complete CH4 oxidation without any side-products, and a monotonic increase in the rate of CO2 generation with an increase in CO2 concentration in the feed stream. Density-functional theory calculations show that high surface coverage of CO2 on Pd leads to a reduction in the activation energy for the initial dissociation of CH4 into CH3 and H, and also the subsequent oxidation reactions. A CO2-rich environment in oxy-combustion systems is therefore beneficial for the reduction of oxygen in exhaust gases.

Gallium Nitride‐based Materials as Promising Catalysts for CO2 Reduction: A DFT Study on the Effect of CO2 Coverage and the Incorporation of Mg Doping or Substitutional In

AbstractCatalytic CO2 conversion to fuels and chemicals is important for mitigating the climate change and reducing the dependence on fossil resources. In order to achieve this goal on a large industrial level, effective catalysts need to be developed. Among them, gallium nitride (GaN) and related Mg‐doped and In‐alloyed systems have been proven as efficient materials for the reduction of highly stable CO2 molecules. This work presents a density functional theory (DFT) investigation, performing periodic boundary condition (PBC) calculations which allow to employ a more extended surface for a detailed analysis of the CO2 coverage, and the effect of Mg doping and In alloying on the CO2 adsorption and its conversion to CO. The results show the great potential of GaN(100) surfaces to simultaneously bind and strongly activate multiple CO2 molecules, which is a crucial aspect for an efficient CO2 conversion process. Moreover, the presence of Mg‐dopant on the top layer is found to be more beneficial for the CO2 adsorption and activation with respect to both the pristine and In‐alloyed system, and this effect is further improved by the inclusion of a second impurity on the top layer. In line with the previous experimental findings, these calculations support the potential of pristine GaN(100) to catalyze the CO2‐to‐CO reduction. The results presented here offer crucial information for the development of more efficient and selective catalysts for the CO2 reduction.

Effects of *CO Coverage on Selective Electrocatalytic Reduction of CO2 to Ethylene over Cu2O with Undercoordinated Cu Sites

Cu2O surface with the coordinatively unsaturated Cu sites reveals advantages in the electroreduction of CO2 toward C2H4 production. Understanding the role of *CO coverage and veritable active sites is of great significance for a good command of the catalytic mechanism. Herein, based on density functional theory, the effects of *CO coverage during the reduction of CO2 to C2H4 on various active sites of Cu2O(111) surface, in terms of the adsorption and structural changes of *CO and key intermediates; the energy profiles of the C–C coupling steps; and the subsequent reaction mechanisms were investigated. Results show that CuCUS on the Cu2O(111) surface is especially reactive toward the *CO adsorption and subsequent reactions, being the preferred site owing to the unsaturated Cu atoms. The *CO coverage obviously tunes the adsorption stability of *COH and *CHO intermediates by affecting the adsorbent–adsorbent interactions. Higher coverage of *CO within 0.13–0.25 promotes the C–C coupling by lowering the energy barrier of *CH2 dimerization, favoring the C2H4 production. Due to the more facile generation of *CHO than *COH, the rate-determining step is speculated to be the C–C coupling with the highest barrier energy occurring in the *CHO pathway. Results provide a fundamental understanding of the CO2 reduction mechanism on Cu-based surfaces, favoring novel catalysts, rational design, and chemical fuel production.

Fast Screening for Copper-Based Bimetallic Electrocatalysts: Efficient Electrocatalytic Reduction of CO2 to C2+ Products on Magnesium-Modified Copper.

Electroreduction of CO2 (CO2 RR) into high value-added chemicals is an attractive route to achieve carbon neutrality. However, the development of an efficient catalyst for CO2 RR is still largely by trial-and-error and is very time-consuming. Herein, we built an electrocatalyst testing platform featuring a home-built automatic flow cell to accelerate the discovery of efficient catalysts. A fast screening of 109 Cu-based bimetallic catalysts in only 55 h identifies Mg combined with Cu as the best electrocatalyst for CO2 to C2+ products. The thus designed Mg-Cu catalyst achieves a Faradaic efficiency (FE) of C2+ products up to 80 % with a current density of 1.0 A cm-2 at -0.77 V versus reversible hydrogen electrode (RHE). Systematic experiments with in situ spectroelectrochemistry analyses show that Mg2+ species stabilize Cu+ sites during CO2 RR and promote the CO2 activation, thus enhancing the *CO coverage to promote C-C coupling.