Subsurface oxide plays a critical role in CO2 activation by Cu(111) surfaces to form chemisorbed CO2, the first step in reduction of CO2

Subsurface Carbon: A General Feature of Noble Metals

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble-metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low-coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low-coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity.

Surface strategies for catalytic CO2 reduction: from two-dimensional materials to nanoclusters to single atoms

This work constructively reviewed and predicted the surface strategies for catalytic CO₂ reduction with 2D material, nanocluster and single-atom catalysts

Dealing with Cu reduction in X-ray absorption spectroscopy experiments

We prove in the exemplary case of the Cu(ii) amyloid-β peptide complex that, at cryogenic temperatures, the time needed for collecting a good quality spectrum is significantly shorter than the time after which structural damage becomes appreciable.

A mini review of in situ near-ambient pressure XPS studies on non-noble, late transition metal catalysts

The rich surface chemistry of Fe, Co, Ni and Cu during heterogeneous catalytic reactions from the perspective of NAP-XPS studies.

A Surface Reconstruction Route to High Productivity and Selectivity in CO2 Electroreduction toward C2+ Hydrocarbons

Electrochemical carbon dioxide reduction (CO₂ ) is a promising technology to use renewable electricity to convert CO₂ into valuable carbon-based products. For commercial-scale applications, however, the productivity and selectivity toward multi-carbon products must be enhanced. A facile surface reconstruction approach that enables tuning of CO₂ -reduction selectivity toward C₂₊ products on a copper-chloride (CuCl)-derived catalyst is reported here. Using a novel wet-oxidation process, both the oxidation state and morphology of Cu surface are controlled, providing uniformity of the electrode morphology and abundant surface active sites. The Cu surface is partially oxidized to form an initial Cu (I) chloride layer which is subsequently converted to a Cu (I) oxide surface. High C₂₊ selectivity on these catalysts are demonstrated in an H-cell configuration, in which 73% Faradaic efficiency (FE) for C₂₊ products is reached with 56% FE for ethylene (C₂ H₄ ) and overall current density of 17 mA cm^-2 . Thereafter, the method into a flow-cell configuration is translated, which allows operation in a highly alkaline medium for complete suppression of CH₄ production. A record C₂₊ FE of ≈84% and a half-cell power conversion efficiency of 50% at a partial current density of 336 mA cm^-2 using the reconstructed Cu catalyst are reported.

Structural, Magnetic, and Catalytic Evaluation of Spinel Co, Ni, and Co-Ni Ferrite Nanoparticles Fabricated by Low-Temperature Solution Combustion Process

Here, we present the low-temperature (∼600 °C) solution combustion method for the fabrication of CoFe₂O₄, NiFe₂O₄, and Co_0.5Ni_0.5Fe₂O₄ nanoparticles (NPs) of 12-64 nm range in pure cubic spinel structure, by adjusting the oxidant (nitrate ions)/reductant (glycine) ratio in the reaction mixture. Although nitrate ions/glycine (N/G) ratios of 3 and 6 were used for the synthesis, phase-pure NPs could be obtained only for the N/G ratio of 6. For the N/G ratio 3, certain amount of Ni²⁺ cations was reduced to metallic nickel. The NH₃ gas generated during the thermal decomposition of the amino acid (glycine, H₂NCH₂COOH) induced the reduction reaction. X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometry, and X-ray photoelectron spectroscopy techniques were utilized to characterize the synthesized materials. XRD analyses of the samples indicate that the Co_0.5Ni_0.5Fe₂O₄ NPs have lattice parameter larger than that of NiFe₂O₄, but smaller than that of CoFe₂O₄ NPs. Although the saturation magnetization (M _s) of Co_0.5Ni_0.5Fe₂O₄ NPs lies in between the saturation magnetization values of CoFe₂O₄ and NiFe₂O₄ NPs, high coercivity (H _c, 875 Oe) of the NPs indicate their hard ferromagnetic behavior. Catalytic behavior of the fabricated spinel NPs revealed that the samples containing metallic Ni are active catalysts for the degradation of 4-nitrophenol in aqueous medium.

Structure- and Electrolyte-Sensitivity in CO2 Electroreduction

The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO₂. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO₂ to chemicals and fuels with high energy density driven by electricity derived from renewable energy has been recognized as a promising strategy toward sustainable energy. The activation and reduction of CO₂, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO₂ electroreduction reaction (CO₂RR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CO₂RR conditions leads to lower selectivity toward CO₂RR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CO₂RR. The reduction of CO₂ involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CO₂RR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CO₂RR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CO₂RR. In situ and operando characterization methods must be employed to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO₂RR catalysts under working conditions. This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CO₂RR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CO₂RR is discussed.

Adsorption of CO2 on Heterostructures of Bi2O3 Nanocluster-Modified TiO2 and the Role of Reduction in Promoting CO2 Activation

The capture and conversion of CO₂ are of significant importance in enabling the production of sustainable fuels, contributing to alleviating greenhouse gas emissions. While there are a number of key steps required to convert CO₂, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidized TiO₂ or CeO₂ are unable to promote this step. In addressing this difficult problem, a recent experimental work shows the potential for bismuth-containing materials to adsorb and convert CO₂, the origin of which is attributed to the role of the bismuth lone pair. In this paper, we present density functional theory (DFT) simulations of enhanced CO₂ adsorption on heterostructures composed of extended TiO₂ rutile (110) and anatase (101) surfaces modified with Bi₂O₃ nanoclusters, highlighting in particular the role of heterostructure reduction in activating CO₂. These heterostructures show low coordinated Bi sites in the nanoclusters and a valence band edge that is dominated by Bi-O states, typical of the Bi³⁺ lone pair. The reduction of Bi₂O₃-TiO₂ heterostructures can be facile and produces reduced Bi²⁺ and Ti³⁺ species. The interaction of CO₂ with this electron-rich, reduced system can produce CO directly, reoxidizing the heterostructure, or form an activated carboxyl species (CO₂ ^-) through electron transfer from the reduced heterostructure to CO₂. The oxidized Bi₂O₃-TiO₂ heterostructures can adsorb CO₂ in carbonate-like adsorption modes, with moderately strong adsorption energies. The hydrogenation of the nanocluster and migration to adsorbed CO₂ is feasible with H-migration barriers less than 0.7 eV, but this forms a stable COOH intermediate rather than breaking C-O bonds or producing formate. These results highlight that a reducible metal oxide heterostructure composed of a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO₂, potentially overcoming the difficulties associated with the difficult first step in CO₂ conversion.

Steps Control the Dissociation of CO2 on Cu(100)

CO₂ reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO₂ on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO₂ dissociation provides an understanding that can be used in the design of future CO₂ reduction catalysts.

Copper-on-nitride enhances the stable electrosynthesis of multi-carbon products from CO2

Copper-based materials are promising electrocatalysts for CO₂ reduction. Prior studies show that the mixture of copper (I) and copper (0) at the catalyst surface enhances multi-carbon products from CO₂ reduction; however, the stable presence of copper (I) remains the subject of debate. Here we report a copper on copper (I) composite that stabilizes copper (I) during CO₂ reduction through the use of copper nitride as an underlying copper (I) species. We synthesize a copper-on-nitride catalyst that exhibits a Faradaic efficiency of 64 ± 2% for C₂₊ products. We achieve a 40-fold enhancement in the ratio of C₂₊ to the competing CH₄ compared to the case of pure copper. We further show that the copper-on-nitride catalyst performs stable CO₂ reduction over 30 h. Mechanistic studies suggest that the use of copper nitride contributes to reducing the CO dimerization energy barrier-a rate-limiting step in CO₂ reduction to multi-carbon products.

On the origin of the elusive first intermediate of CO2 electroreduction

We resolve the long-standing controversy about the first step of the CO₂ electroreduction to fuels in aqueous electrolytes by providing direct spectroscopic evidence that the first intermediate of the CO₂ conversion to formate on copper is a carboxylate anion *CO₂ ^- coordinated to the surface through one of its C-O bonds. We identify this intermediate and gain insight into its formation, its chemical and electronic properties, as well as its dependence on the electrode potential by taking advantage of a cutting-edge methodology that includes operando surface-enhanced Raman scattering (SERS) empowered by isotope exchange and electrochemical Stark effects, reaction kinetics (Tafel) analysis, and density functional theory (DFT) simulations. The SERS spectra are measured on an operating Cu surface. These results advance the mechanistic understanding of CO₂ electroreduction and its selectivity to carbon monoxide and formate.

Robustness of surface activity electronic structure-based descriptors of transition metals

Efficient yet simple electronic structure-based descriptors of transition metal surfaces are key in material design for many scientific fields in research and technology. Density functional theory-based methods provide the framework to systematically explore the performance and transferability of such descriptors. Using appropriate surface models and the Vosko-Wilk-Nussair (VWN), Perdew-Burke-Ernzerhof (PBE), PBE adapted for solids (PBEsol), revised PBE (RPBE), and Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation functionals, we study the transferability of three descriptors: the d-band centre, the width-corrected d-band centre, and the Hilbert transform highest peak, among the low-index Miller surfaces for the metals of transition elements. We show that the d-band centre and the width-corrected d-band centre descriptors are almost independent of the functional used whereas a dependency is seen in the Hilbert transform highest peak. Moreover, it is seen that the differences between the surface descriptor values and predictions from the bulk ones are affected by the presence of surface states. Interestingly, a direct relation between the surface coordination number and the d-band centre electronic descriptor is found when surface states are absent.

Dopant-induced electron localization drives CO2 reduction to C2 hydrocarbons

The electrochemical reduction of CO₂ to multi-carbon products has attracted much attention because it provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the efficiency of CO₂ conversion to C₂ products remains below that necessary for its implementation at scale. Modifying the local electronic structure of copper with positive valence sites has been predicted to boost conversion to C₂ products. Here, we use boron to tune the ratio of Cu^δ+ to Cu⁰ active sites and improve both stability and C₂-product generation. Simulations show that the ability to tune the average oxidation state of copper enables control over CO adsorption and dimerization, and makes it possible to implement a preference for the electrosynthesis of C₂ products. We report experimentally a C₂ Faradaic efficiency of 79 ± 2% on boron-doped copper catalysts and further show that boron doping leads to catalysts that are stable for in excess of ~40 hours while electrochemically reducing CO₂ to multi-carbon hydrocarbons.

CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface

Carbon dioxide (CO₂) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO₂ to ethylene with 70% faradaic efficiency at a potential of -0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO₂ reduction and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.

Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time

Copper oxides have been of considerable interest as electrocatalysts for CO₂ reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu₂O nanoneedles, Cu₂O nanocrystals, and Cu₂O nanoparticles. We show experimentally that the selective formation of gaseous C₂ products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu₂OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu₂O is present at the surface because Cu₂OR is kinetically and energetically more favorable than CO2R.

Is Subsurface Oxygen Necessary for the Electrochemical Reduction of CO2 on Copper?

It has recently been proposed that subsurface oxygen is crucial for the adsorption and subsequent electroreduction of CO₂ on copper. Using density functional theory, we have studied the stability and diffusion of subsurface oxygen in single crystals of copper exposing (111) and (100) facets. Oxygen is at least 1.5 eV more stable on the surface than beneath it for both crystal orientations; interstitial sites are too small to accommodate oxygen. The rate of atomic oxygen diffusion from one layer below a Cu(111) surface to the surface is 5 × 10³ s^-1. Oxygen can survive longer in deeper layers, but it does not promote CO₂ adsorption there. Diffusion of subsurface oxygen is easier to the less-dense Cu(100) surface, even from lower layers (rate ≈ 1 × 10⁷ s^-1). Once the applied voltage and dispersion forces are properly modeled, we find that subsurface oxygen is unnecessary for CO₂ adsorption on copper.

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations

C1s and O1s core level binding energy shifts have been calculated for various adsorbates on Cu(111) using the ΔSCF method.

Stability of Residual Oxides in Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction Investigated with 18 O Labeling

Oxide-derived (OD) Cu catalysts have high selectivity towards the formation of multi-carbon products (C₂ /C₃ ) for aqueous electrochemical CO₂ reduction (CO₂ R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction, and these residual oxides play a crucial catalytic role. The stability of residual oxides was investigated by synthesizing ¹⁸ O-enriched OD Cu catalysts and testing them for CO₂ R. These catalysts maintain a high selectivity towards C₂ /C₃ products (ca. 60 %) for up to 5 h in 0.1 m KHCO₃ at -1.0 V vs. RHE. However, secondary-ion mass spectrometry measurements show that only a small fraction (<1 %) of the original ¹⁸ O content remains, showing that residual oxides are not present in significant amounts during CO₂ R. Furthermore, we show that OD Cu can reoxidize rapidly, which could compromise the accuracy of ex situ methods for determining the true oxygen content.

Role of the Adsorbed Oxygen Species in the Selective Electrochemical Reduction of CO2 to Alcohols and Carbonyls on Copper Electrodes

The electrochemical reduction of CO₂ into fuels has gained significant attention recently as source of renewable carbon-based fuels. The unique high selectivity of copper in the electrochemical reduction of CO₂ to hydrocarbons has called much interest in discovering its mechanism. In order to provide significant information about the role of oxygen in the electrochemical reduction of CO₂ on Cu electrodes, the conditions of the surface structure and the composition of the Cu single crystal electrodes were controlled over time. This was achieved using pulsed voltammetry, since the pulse sequence can be programmed to guarantee reproducible initial conditions for the reaction at every fraction of time and at a given frequency. In contrast to the selectivity of CO₂ reduction using cyclic voltammetry and chronoamperometric methods, a large selection of oxygenated hydrocarbons was found under alternating voltage conditions. Product selectivity towards the formation of oxygenated hydrocarbon was associated to the coverage of oxygen species, which is surface-structure- and potential-dependent.