Photoelectrochemical CO2 Reduction to Acetate on Iron–Copper Oxide Catalysts

Photoactive p-Type Spinel CuGa2O4 Nanocrystals

Herein we report the synthesis and characterization of spinel copper gallate (CuGa₂O₄) nanocrystals (NCs) with an average size of 3.7 nm via a heat-up colloidal reaction. CuGa₂O₄ NCs have a band gap of ∼2.5 eV and marked p-type character, in agreement with ab initio simulations. These novel NCs are demonstrated to be photoactive, generating a clear and reproducible photocurrent under blue light irradiation when deposited as thin films. Crucially, the ability to adjust the Cu/Ga ratio within the NCs, and the effect of this on the optical and electronic properties of the NCs, was also demonstrated. These results position CuGa₂O₄ NCs as a novel material for optoelectronic applications, including hole transport and light harvesting.

Recent Advances in Electrochemical, Photochemical, and Photoelectrochemical Reduction of CO2 to C2+ Products

Environmental problems such as global warming are one of the most prominent global challenges. Researchers are investigating various methods for decreasing CO₂ emissions. The CO₂ reduction reaction via electrochemical, photochemical, and photoelectrochemical processes has been a popular research topic because the energy it requires can be sourced from renewable sources. The CO₂ reduction reaction converts stable CO₂ molecules into useful products such as CO, CH₄ , C₂ H₄ , and C₂ H₅ OH. To obtain economic benefits from these products, it is important to convert them into hydrocarbons above C₂ . Numerous investigations have demonstrated the uniqueness of the CC coupling reaction of Cu-based catalysts for the conversion of CO₂ into useful hydrocarbons above C₂ for electrocatalysis. Herein, the principle of semiconductors for photocatalysis is briefly introduced, followed by a description of the obstacles for C₂₊ production. This review presents an overview of the mechanism of hydrocarbon formation above C₂ , along with advances in the improvement, direction, and comprehension of the CO₂ reduction reaction via electrochemical, photochemical, and photoelectrochemical processes.

Extreme Ultraviolet Reflection-Absorption Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective

Extreme ultraviolet light sources based on high harmonic generation are enabling the development of novel spectroscopic methods to help advance the frontiers of ultrafast science and technology. In this Account, we discuss the development of extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy at near grazing incident reflection geometry and highlight recent applications of this method to study ultrafast electron dynamics at surfaces. Measuring core-to-valence transitions with broadband, femtosecond pulses of XUV light extends the benefits of X-ray absorption spectroscopy to a laboratory tabletop by providing a chemical fingerprint of materials, including the ability to resolve individual elements with sensitivity to oxidation state, spin state, carrier polarity, and coordination geometry. Combining this chemical state sensitivity with femtosecond time resolution provides new insight into the material properties that govern charge carrier dynamics in complex materials. It is well-known that surface dynamics differ significantly from equivalent processes in bulk materials and that charge separation, trapping, transport, and recombination occurring uniquely at surfaces govern the efficiency of numerous technologically relevant processes spanning photocatalysis, photovoltaics, and information storage and processing. Importantly, XUV-RA spectroscopy at near grazing angle is also surface sensitive with a probe depth of ∼3 nm, providing a new window into electronic and structural dynamics at surfaces and interfaces. Here we highlight the unique capabilities and recent applications of XUV-RA spectroscopy to study photoinduced surface dynamics in metal oxide semiconductors, including photocatalytic oxides (Fe₂O₃, Co₃O₄ NiO, and CuFeO₂) as well as photoswitchable magnetic oxide (CoFe₂O₄). We first compare the ultrafast electron self-trapping rates via small polaron formation at the surface and bulk of Fe₂O₃ where we note that the energetics and kinetics of this process differ significantly at the surface. Additionally, we demonstrate the ability to systematically tune this kinetics by molecular functionalization, thereby providing a route to control carrier transport at surfaces. We also measure the spectral signatures of charge transfer excitons with site specific localization of both electrons and holes in a series of transition metal oxide semiconductors (Fe₂O₃, NiO, Co₃O₄). The presence of valence band holes probed at the oxygen L₁-edge confirms a direct relationship between the metal-oxygen bond covalency and water oxidation efficiency. For a mixed metal oxide CuFeO₂ in the layered delafossite structure, XUV-RA reveals that the sub-picosecond hole thermalization from O 2p to Cu 3d states of CuFeO₂ leads to the spatial separation of electrons and holes, resulting in exceptional photocatalytic performance for H₂ evolution and CO₂ reduction of this material. Finally, we provide an example to show the ability of XUV-RA to probe spin state specific dynamics in a photoswitchable ferrimagnet, cobalt ferrite (CoFe₂O₄). This study provides a detailed understating of ultrafast spin switching in a complex magnetic material with site-specific resolution. In summary, the applications of XUV-RA spectroscopy demonstrated here illustrate the current abilities and future promise of this method to extend molecule-level understanding from well-defined photochemical complexes to complex materials so that charge and spin dynamics at surfaces can be tuned with the precision of molecular photochemistry.

Unexpected high selectivity for acetate formation from CO2 reduction with copper based 2D hybrid catalysts at ultralow potentials

Copper-based catalysts are efficient for CO₂ reduction affording commodity chemicals. However, Cu(i) active species are easily reduced to Cu(0) during the CO₂RR, leading to a rapid decay of catalytic performance. Herein, we report a hybrid-catalyst that firmly anchors 2D-Cu metallic dots on F-doped Cu_xO nanoplates (Cu_xOF), synthesized by electrochemical-transformation under the same conditions as the targeted CO₂RR. The as-prepared Cu/Cu_xOF hybrid showed unusual catalytic activity towards the CO₂RR for CH₃COO⁻ generation, with a high FE of 27% at extremely low potentials. The combined experimental and theoretical results show that nanoscale hybridization engenders an effective s,p-d coupling in Cu/Cu_xOF, raising the d-band center of Cu and thus enhancing electroactivity and selectivity for the acetate formation. This work highlights the use of electronic interactions to bias a hybrid catalyst towards a particular pathway, which is critical for tuning the activity and selectivity of copper-based catalysts for the CO₂RR.

A two-dimensional (2D) copper hybrid catalyst (Cu/Cu_xOF) composed of metallic Cu well dispersed on 2D F-doped Cu_xO nanoplates (Cu_xOF) is reported, which shows high catalytic activity toward the CO₂RR for acetate generation.

C2 feedstock-based biomanufacturing of value-added chemicals

Engineering microbes to produce value-added chemicals from C6/C5 sugars sometimes requires long biosynthetic pathways, which causes carbon loss due to involving multiple metabolic branch nodes, leading to a lower product yield. Using C2 feedstocks derived from gaseous, cellulosic, and plastic wastes could establish shorter biosynthetic pathways to produce some target chemicals, for example, acetyl-CoA-derived natural products. Utilizing these waste-derived feedstocks would also contribute to reducing the carbon footprint of the chemical industry. In this review, we highlighted the promising waste-processing technologies that could provide C2 feedstocks that are compatible with microbial fermentation. We also analyzed the recent metabolic engineering works in which the microorganisms/fermentation processes were modified/optimized to utilize acetate, ethanol, or ethylene glycol more efficiently.

CO2 photoelectroreduction with enhanced ethanol selectivity by high valence rhenium-doped copper oxide composite catalysts

CuO supported catalyst with high valence rhenium doping were specially studied for photoelectrocatalytic reduction of CO₂ to small molecular alcohols, which were synthesized by nitrate thermal decomposition method on anatase TiO₂ nanotube arrays (TiO₂-NTs). Photoelectrochemical measurements indicate that the high valence rhenium doping helps in improving the catalytic activity and selectivity of CuO supported catalysts. For the case of 6 wt% Re-doped CuO/TiO₂-NTs calcined at 723 K, the principal products are methanol and ethanol with yield up to 19.9 μmol and 7.5 μmol after 5 h photoelectrocatalysis at external potential of -0.4 V under simulated solar illumination. In contrast, the products catalyzed by undoped CuO/TiO₂-NTs are only methanol and formaldehyde. These results indicate that the high valence rhenium doping will promote the alcoholization process and benefit the CC coupling, leading to the selective conversion of CO₂ to ethanol. Furthermore, under suitable external potential (-0.5 V) the CO₂ conversion product is almost entirely composed of ethanol.

Colloidal Nanocrystals as Precursors and Intermediates in Solid State Reactions for Multinary Oxide Nanomaterials

ConspectusPolyelemental compounds with dimensions in the nanosized regime are desirable in a large variety of applications, yet their synthesis remains a general challenge in chemistry. One of the major bottlenecks to obtaining multinary systems is the complexity of the synthesis itself. As the number of elements to include in one single nano-object increases, different chemical interactions arise during nucleation and growth, thus challenging the formation of the targeted product. Choosing the reaction conditions and identifying the parameters which ensure the desired reaction pathway are of the uttermost importance. When, in addition to composition, the simultaneous control of size and shape is sought after, the development of new synthetic strategies guided by the fundamental understanding of the formation mechanisms becomes crucial.In this Account we discuss the use of colloidal chemistry to target multinary oxide nanomaterials, with focus on light absorbers which can drive chemical reactions. We propose the combination of soft and solid-state chemistries as one successful strategy to target this family of polyelemental compounds with control on composition and morphological features. To start with, we highlight studies where in situ forming nanoparticles act as reaction intermediates, which we found in both oxide (i.e., Bi-V-O) and sulfide (Cu-M-S, with M = V, Cr, Mn) nanocrystals (NCs). Examples of ternary sulfides are mentioned only with the purpose of showing that similar mechanisms can apply to different families of multinary nanomaterials. Using this new knowledge, we demonstrate that reacting pre-synthesized NCs with well-defined composition and size with molecular precursors allows significant control of these same property-dictating features (i.e., composition and grain size) in the resulting ternary and quaternary compounds. For example, nanostructured BiV_1-xSb_xO₄ thin films with tunable composition and nanostructured β-Cu₂V₂O₇ with tunable grain size were accessed from colloidally synthesized Bi_1-xSb_x NCs (0 < x < 1) and size-controlled Cu NCs reacted with a vanadium molecular precursor, respectively. The analysis of reaction aliquots revealed that the formation of these materials occurs via a solid-state reaction between the NC precursors and V-containing amorphous nanoparticles, which form in situ from the molecular precursors. With the aim to achieve better control on the reaction product, we finally propose the use of colloidally synthesized NCs as reactants in solid state reactions. As the first proof of concept, ternary metal oxide NCs, including CuFe₂O₄, CuMn₂O₄, and CuGa₂O₄ with defined size and shape regulated by the NC precursors were obtained. Considering the huge library of single component and binary NCs accessible by colloidal chemistry, the extension of this synthetic concept, which combines soft and solid-state chemistries, to a larger variety of polyelemental nanomaterials is foreseen. Such an approach will contribute to facilitate a more rapid translation of design principles to materials with the desired composition and structural features.

Shaping non-noble metal nanocrystals via colloidal chemistry

Non-noble metal nanocrystals with well-defined shapes have been attracting increasingly more attention in the last decade as potential alternatives to noble metals, by virtue of their earth abundance combined with intriguing physical and chemical properties relevant for both fundamental studies and technological applications. Nevertheless, their synthesis is still primitive when compared to noble metals. In this contribution, we focus on third row transition metals Mn, Fe, Co, Ni and Cu that are recently gaining interest because of their catalytic properties. Along with providing an overview on the state-of-the-art, we discuss current synthetic strategies and challenges. Finally, we propose future directions to advance the synthetic development of shape-controlled non-noble metal nanocrystals in the upcoming years.

Research Progress in Conversion of CO2 to Valuable Fuels

Rapid growth in the world's economy depends on a significant increase in energy consumption. As is known, most of the present energy supply comes from coal, oil, and natural gas. The overreliance on fossil energy brings serious environmental problems in addition to the scarcity of energy. One of the most concerning environmental problems is the large contribution to global warming because of the massive discharge of CO₂ in the burning of fossil fuels. Therefore, many efforts have been made to resolve such issues. Among them, the preparation of valuable fuels or chemicals from greenhouse gas (CO₂) has attracted great attention because it has made a promising step toward simultaneously resolving the environment and energy problems. This article reviews the current progress in CO₂ conversion via different strategies, including thermal catalysis, electrocatalysis, photocatalysis, and photoelectrocatalysis. Inspired by natural photosynthesis, light-capturing agents including macrocycles with conjugated structures similar to chlorophyll have attracted increasing attention. Using such macrocycles as photosensitizers, photocatalysis, photoelectrocatalysis, or coupling with enzymatic reactions were conducted to fulfill the conversion of CO₂ with high efficiency and specificity. Recent progress in enzyme coupled to photocatalysis and enzyme coupled to photoelectrocatalysis were specially reviewed in this review. Additionally, the characteristics, advantages, and disadvantages of different conversion methods were also presented. We wish to provide certain constructive ideas for new investigators and deep insights into the research of CO₂ conversion.

Photoelectrochemical Reduction of CO2 to Syngas by Reduced Ag Catalysts on Si Photocathodes

The photoelectrochemical reduction of CO2 to syngas that is used for many practical applications has been emerging as a promising technique to relieve the increase of CO2 in the atmosphere. Si has been considered to be one of the most promising materials for photoelectrodes, but the integration of electrocatalysts is essential for the photoelectrochemical reduction of CO2 using Si. We report an enhancement of catalytic activity for CO2 reduction reaction by Ag catalysts of tuned morphology, active sites, and electronic structure through reducing anodic treatment. Our proposed photocathode structure, a SiO2 patterned p-Si photocathode with these reduced Ag catalysts, that was fabricated using electron-beam deposition and electrodeposition methods, provides a low onset-potential of −0.16 V vs. the reversible hydrogen electrode (RHE), a large saturated photocurrent density of −9 mA/cm2 at −1.23 V vs. RHE, and faradaic efficiency for CO of 47% at −0.6 V vs. RHE. This photocathode can produce syngas in the ratio from 1:1 to 1:3, which is an appropriate proportion for practical application. This work presents a new approach for designing photocathodes with a balanced catalytic activity and light absorption to improve the photoelectrochemical application for not only CO2 reduction reaction, but also water splitting or N2 reduction reaction.

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using hetero- and homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.

Cathodized copper porphyrin metal-organic framework nanosheets for selective formate and acetate production from CO2 electroreduction

An efficient and selective Cu catalyst for CO2 electroreduction is highly desirable since current catalysts suffer from poor selectivity towards a series of products, such as alkenes, alcohols, and carboxylic acids. Here, we used copper(ii) paddle wheel cluster-based porphyrinic metal-organic framework (MOF) nanosheets for electrocatalytic CO2 reduction and compared them with CuO, Cu2O, Cu, a porphyrin-Cu(ii) complex and a CuO/complex composite. Among them, the cathodized Cu-MOF nanosheets exhibit significant activity for formate production with a faradaic efficiency (FE) of 68.4% at a potential of -1.55 V versus Ag/Ag+. Moreover, the C-C coupling product acetate is generated from the same catalyst together with formate at a wide voltage range of -1.40 V to -1.65 V with the total liquid product FE from 38.8% to 85.2%. High selectivity and activity are closely related to the cathodized restructuring of Cu-MOF nanosheets. With the combination of X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy, we find that Cu(ii) carboxylate nodes possibly change to CuO, Cu2O and Cu4O3, which significantly catalyze CO2 to formate and acetate with synergistic enhancement from the porphyrin-Cu(ii) complex. This intriguing phenomenon provides a new opportunity for the rational design of high-performance Cu catalysts from pre-designed MOFs.

A microfluidic photoelectrochemical cell for solar-driven CO2 conversion into liquid fuels with CuO-based photocathodes

Schematic representation of photoelectrochemical CO₂ reduction set-up.

Countercations and Solvent Influence CO2 Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates

One-electron reduction of Cu₃EL (L^3- = tris(β-diketiminate)cyclophane, and E = S, Se) affords [Cu₃EL]^-, which reacts with CO₂ to yield exclusively C₂O₄^2- (95% yield, TON = 24) and regenerate Cu₃EL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( k_{obs, 298K} = 115(2) s^-1), which is 10⁶ larger than those for reported copper complexes. The k_obs values are dependent on the countercation and solvent (e.g., k_obs is greater for [K(18-crown-6)]⁺ vs (Ph₃P)₂N⁺, and there is a 20-fold decrease in k_obs in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.

Semiconductor-Based Photoelectrochemical Conversion of Carbon Dioxide: Stepping Towards Artificial Photosynthesis

The photoelectrochemical (PEC) carbon dioxide reduction process stands out as a promising avenue for the conversion of solar energy into chemical feedstocks, among various methods available for carbon dioxide mitigation. Semiconductors derived from cheap and abundant elements are interesting candidates for catalysis. Whether employed as intrinsic semiconductors or hybridized with metallic cocatalysts, biocatalysts, and metal molecular complexes, semiconductor photocathodes exhibit good performance and low overpotential during carbon dioxide reduction. Apart from focusing on carbon dioxide reduction materials and chemistry, PEC cells towards standalone devices that use photohybrid electrodes or solar cells have also been a hot topic in recent research. An overview of the state-of-the-art progress in PEC carbon dioxide reduction is presented and a deep understanding of the catalysts of carbon dioxide reduction is also given.

CO2 reduction to acetate in mixtures of ultrasmall (Cu) n ,(Ag) m bimetallic nanoparticles

Monodispersed mixtures of 6-nm Cu and Ag nanoparticles were prepared by electrochemical reduction on electrochemically polymerized poly-Fe(vbpy)₃(PF₆)₂ film electrodes on glassy carbon. Conversion of the complex to poly-Fe(vbpy)₂(CN)₂ followed by surface binding of salts of the cations and electrochemical reduction gave a mixture of chemically distinct clusters on the surface, (Cu) _m ,(Ag) _n |polymer|glassy carbon electrode (GCE), as shown by X-ray photoelectron spectroscopy (XPS) measurements. A (Cu)₂,(Ag)₃|(80-monolayer-poly-Fe(vbpy)₃²⁺|GCE electrode at -1.33 V vs. reversible hydrogen electrode (RHE) in 0.5 M KHCO₃, with 8 ppm added benzotriazole (BTA) at 0 °C, gave acetate with a faradaic efficiency of 21.2%.

Strategies for Efficient Charge Separation and Transfer in Artificial Photosynthesis of Solar Fuels

Converting sunlight to solar fuels by artificial photosynthesis is an innovative science and technology for renewable energy. Light harvesting, photogenerated charge separation and transfer (CST), and catalytic reactions are the three primary steps in the processes involved in the conversion of solar energy to chemical energy (SE-CE). Among the processes, CST is the key "energy pump and delivery" step in determining the overall solar-energy conversion efficiency. Efficient CST is always high priority in designing and assembling artificial photosynthesis systems for solar-fuel production. This Review not only introduces the fundamental strategies for CST but also the combinatory application of these strategies to five types of the most-investigated semiconductor-based artificial photosynthesis systems: particulate, Z-scheme, hybrid, photoelectrochemical, and photovoltaics-assisted systems. We show that artificial photosynthesis systems with high SE-CE efficiency can be rationally designed and constructed through combinatory application of these strategies, setting a promising blueprint for the future of solar fuels.

Beyond Solar Fuels: Renewable Energy-Driven Chemistry

The future feasibility of decarbonized industrial chemical production based on the substitution of fossil feedstocks (FFs) with renewable energy (RE) sources is discussed. Indeed, the use of FFs as an energy source has the greatest impact on the greenhouse gas emissions of chemical production. This future scenario is indicated as "solar-driven" or "RE-driven" chemistry. Its possible implementation requires to go beyond the concept of solar fuels, in particular to address two key aspects: i) the use of RE-driven processes for the production of base raw materials, such as olefins, methanol, and ammonia, and ii) the development of novel RE-driven routes that simultaneously realize process and energy intensification, particularly in the direction of a significant reduction of the number of the process steps.