Electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O

Synthesis and Structure-Property Relationship of meso-Substituted Porphyrin- and Benzoporphyrin-Thiophene Conjugates toward Electrochemical Reduction of Carbon Dioxide

A novel series of ZnII-trans-A2B2 porphyrins and benzoporphyrins bearing phenyl and thiophene-based meso-substituents was successfully synthesized and characterized by spectroscopic and electrochemical techniques. Systematic comparison among the compounds in this series, together with the corresponding A4 analogs previously studied by our group, led to the understanding of the effects of π-conjugated system extension of a porphyrin core through β-fused rings, replacement of the phenyl with the thiophene-based meso-groups, and introduction of additional thiophene rings on thienyl substituents on photophysical and electrochemical properties. Oxidative electropolymerization through bithiophenyl units of both A4 and trans-A2B2 analogs was achieved, resulting in porphyrin- and benzoporphyrin-oligothiophene conjugated polymers, which were characterized by cyclic voltammetry and absorption spectrophotometry. Preliminary studies on catalytic performance toward electrochemical reduction of carbon dioxide (CO2) was described herein to demonstrate the potential of the selected compounds for serving as homogeneous and heterogeneous electrocatalysts for the conversion of CO2 to carbon monoxide (CO).

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.

Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy

The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO2 emissions. Outpacing the natural carbon cycle, atmospheric CO2 levels increased by 50 % since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chemical recycling of CO2 to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO2 , CO and their derivatives as potential low-temperature alternatives to the commercial processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.

Nanostructured copper molybdates as promising bifunctional electrocatalysts for overall water splitting and CO2 reduction

Overall water splitting and CO₂ reduction are two very important reactions from the environmental viewpoint. The former produces hydrogen as a clean fuel and the latter decreases the amount of CO₂ emissions and thus reduces greenhouse effects. Here, we prepare two types of copper molybdate, CuMoO₄ and Cu₃Mo₂O₉, and electrochemically investigate them for water splitting and CO₂ reduction. Our findings show that Cu₃Mo₂O₉ is a better electrocatalyst for full water splitting compared to CuMoO₄. It provides overpotentials, which are smaller than the overpotentials of CuMoO₄ by around 0.14 V at a current density of 1 mA cm⁻² and 0.10 V at −0.4 mA cm⁻², for water oxidation and hydrogen evolution reactions, respectively. However, CuMoO₄ adsorbs CO₂ and the reduced intermediates/products more strongly than Cu₃Mo₂O₉. Such different behaviors of these electrocatalysts can be attributed to their different unit cells.

Comparing overall water splitting on the surface two types of copper molybdate.

Descriptor-Based Design Principle for Two-Dimensional Single-Atom Catalysts: Carbon Dioxide Electroreduction

Single-atom catalysis has recently emerged as a promising approach for catalyzing the carbon dioxide reduction reaction (CO₂RR). In this study, we present a principle for designing active single-atom catalysts (SACs) for CO₂RR. We systematically examine totally 24 transition metals supported by a graphitic carbon nitride (g-CN) monolayer and find that their catalytic activities are highly correlated with the adsorption free energies of two intermediate species (OH and OCH). We then identify two important intrinsic descriptors, namely, the number of electrons in the outmost d-shell and the enthalpy of vaporization of the transition metal. Test calculations on transition metals supported by a C₂N monolayer indicate that both descriptors are quite universal for SACs of CO₂RR. Based on these results, we show that Ni@g-CN, Cu@g-CN, and Co@C₂N are promising SACs for CO₂RR. This study offers an effective principle for designing highly active SACs for CO₂RR on the basis of intrinsic properties of transition metals.

How photocorrosion can trick you: a detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production

The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li_0.03Cu_0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of Li_xCu_1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu₂O grows as distinct crystallites on the surface of Li_xCu_1-xO instead of forming a dense layer. Additionally, there is no obvious Cu₂O gradient inside the films, as Cu₂O seems to form on all Li_xCu_1-xO nanocrystals exposed to water. The application of a thin Ti_0.8Nb_0.2O_x coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of Li_xCu_1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes.

Power to methanol technologies via CO2 recovery: CO2 hydrogenation and electrocatalytic routes

Various strategies are proposed to date in order to convert CO2 to large diversity of useful chemicals. The following review discusses two important approaches that produce methanol from CO2. These two includes CO2 hydrogenation and electrocatalytic routes. These processes could recycle CO2, permitting a carbon neutral, closed loop of fuel combustion and CO2 reduction to prevent a rising concentration of this greenhouse gas in the atmosphere. Besides, intermittent electricity generation can be stored in an energy-dense, portable form in chemical bonds. The present review reports more recent findings and drawbacks of these two processes. The present review study revealed that the hydrogenation process could become readily operational in comparison to electrocatalytic process. The electrocatalytic approach still has serious technical issues in terms of kinetically sluggish multi-electron transfer process during CO2 reduction reaction that requires excessive over-potential, relatively poor selectivity, poor durability in the long term, and the absence of the optimized standard experimental and commercial systems.

CO2 Reduction of Hybrid Cu2 O-Cu/Gas Diffusion Layer Electrodes and their Integration in a Cu-based Photoelectrocatalytic Cell

Cu₂ O/gas diffusion layer (GDL) electrodes prepared by electrodeposition were studied for the electrocatalytic reduction of CO₂ . The designed electrode was also tested in solar-light-induced CO₂ conversion in combination with a CuO/NtTiO₂ photoanode using a compact photoelectrocatalytic (PEC) cell. Both PEC cell electrodes were prepared using non-critical raw materials and low cost, easily scalable procedures. In the PEC experiments, a total carbon faradaic selectivity of about 90 % to formate and about 75 % to acetate was obtained after 24 h of operations without application of potential/current or using sacrificial agents. In electrocatalytic tests of CO₂ reduction at -1.5 V, the same electrode yielded high total faradaic selectivity (>95 %) but formed selectively formate (about 80 % selectivity) rather than acetate. The in situ transformation of the Cu₂ O/GDL electrode leads to the formation of a hybrid Cu₂ O-Cu/GDL system. Cyclic voltammetry data indicate that the potential and the presence of CO₂ (not only of HCO₃ ^- species) are both important elements in this transformation. Data also indicate that the surface concentration of CO₂ (or of its products of transformation) on the electrode is an important factor to determine performance in the conversion of CO₂ .

Solvents and Supporting Electrolytes in the Electrocatalytic Reduction of CO2

Different electrolytes applied in the aqueous electrocatalytic CO₂ reduction reaction (CO₂RR) considerably influence the catalyst performance. Their concentration, species, buffer capacity, and pH value influence the local reaction conditions and impact the product distribution of the electrocatalyst. Relevant properties of prospective solvents include their basicity, CO₂ solubility, conductivity, and toxicity, which affect the CO₂RR and the applicability of the solvents. The complexity of an electrochemical system impedes the direct correlation between a single parameter and cell performance indicators such as the Faradaic efficiency; thus the effects of different electrolytes are often not fully comprehended. For an industrial application, a deeper understanding of the effects described in this review can help with the prediction of performance, as well as the development of scalable electrolyzers. In this review, the application of supporting electrolytes and different solvents in the CO₂RR reported in the literature are summarized and discussed.

Synthesis and Evaluation of Copper-Supported Titanium Oxide Nanotubes as Electrocatalyst for the Electrochemical Reduction of Carbon Oxide to Organics

Carbon dioxide (CO2) is considered as the prime reason for the global warming effect and one of the useful ways to transform it into an array of valuable products is through electrochemical reduction of CO2 (ERC). This process requires an efficient electrocatalyst with high faradaic efficiency at low overpotential and enhanced reaction rate. Herein, we report an innovative way of reducing CO2 using copper-metal supported on titanium oxide nanotubes (TNT) electrocatalysts. The TNT support material was synthesized using alkaline hydrothermal process with Degussa (P-25) as a starting material. Copper nanoparticles were anchored on the TNT by homogeneous deposition-precipitation method (HDP) with urea as precipitating agent. The prepared catalysts were tested in a home-made H-cell with 0.5 M NaHCO3 aqueous solution in order to examine their activity for ERC and the optimum copper loading. Continuous gas-phase ERC was carried out in a solid polymer electrolyte (SPE) reactor. The 10% Cu/TNT catalysts were employed in the gas diffusion layer (GDL) on the cathode side with Pt-Ru/C on the anode side. Faradaic efficiencies for the three major products namely methanol, methane, and CO were found to be 4%, 3%, and 10%, respectively at −2.5 V with an overall current density of 120 mA/cm2. The addition of TNT significantly increased the catalytic activity of electrocatalyst for ERC. It is mainly attributed to their better stability towards oxidation, increased CO2 adsorption capacity and stabilization of the reaction intermediate, layered titanates, and larger surface area (400 m2/g) as compared with other support materials. Considering the low cost of TNT, it is anticipated that TNT support electrocatalyst for ECR will gain popularity.

Electrochemical Carbon Dioxide Reduction in Methanol at Cu and Cu2O-Deposited Carbon Black Electrodes

The electrochemical reduction of carbon dioxide in methanol was investigated with Cu and Cu2O-supported carbon black (Vulcan XC-72) nanoparticle electrodes. Herein, Cu or a Cu2O-deposited carbon black catalyst has been synthesized by the reduction method for a Cu ion, and the drop-casting method was applied for the fabrication of a modified carbon black electrode. A catalyst ink solution was fabricated by dispersing the catalyst particles, and the catalyst ink was added onto the carbon plate. The pH of suspension was effective for controlling the Cu species for the metallic copper and the Cu2O species deposited on the carbon black. Without the deposition of Cu, only CO and methyl formate were produced in the electrochemical CO2 reduction, and the production of hydrocarbons could be scarcely observed. In contrast, hydrocarbons were formed by using Cu or Cu2O-deposited carbon black electrodes. The maximum Faraday efficiency of hydrocarbons was 40.3% (26.9% of methane and 13.4% of ethylene) at −1.9 V on the Cu2O-deposited carbon black catalyst. On the contrary, hydrogen evolution could be depressed to 34.7% under the condition.

From Enzymes to Functional Materials-Towards Activation of Small Molecules

The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences.

Cu-CDots nanocorals as electrocatalyst for highly efficient CO2 reduction to formate

Electrochemical reduction of CO₂ is a key component of many prospective artificial technologies for renewable carbon-containing fuels, but it still suffers from the high overpotentials required to drive the process, low selectivity for diversiform products and the high cost of the catalyst. Here, we report that Cu-CDots nanocorals is a highly efficient, low-cost and stable electrocatalyst for CO₂ reduction in aqueous solution. The major product of CO₂ reduction on the Cu-CDots nanocorals is HCOOH with an inconceivable low overpotential of 0.13 V and a high Faraday efficiency of 79% at a moderate potential of -0.7 V vs. RHE. In the present system, CDots can increase the adsorption capacity of CO₂ molecules and H⁺, which play important roles in CO₂ reduction. The high selectivity of HCOOH for CO₂ reduction may be ascribed to that CDots can greatly diminish the HCOOH desorption energy and improve the catalytic selectivity for HCOOH. Furthermore, Cu-CDots nanocorals exhibit a long-term stability during 5 h-electrolysis.

Electrode Build-Up of Reducible Metal Composites toward Achievable Electrochemical Conversion of Carbon Dioxide

At the beginning of the 21st century, our world is faced with a global-warming problem due to the continuous increase in carbon dioxide emission, and thus, the development of novel experimental techniques is needed. The electrochemical conversion of carbon dioxide into high-value organic compounds could be of vital importance to solve this issue. The biggest challenge has always been to develop an electrocatalyst that is chemically active and structurally stable. Herein, previous studies, recent approaches, and current points of view on the electrode structure of metal oxide composites for the advanced electrochemical conversion of carbon dioxide are reviewed.

Concurrent electrochemical CO2 reduction to HCOOH and methylene blue removal on metal electrodes

Experimental setup for CO₂ reduction and MB removal.

Electrochemical reduction of CO2 to HCOOH using zinc and cobalt oxide as electrocatalysts

HCOOH was produced electrochemically from CO₂ in the presence of Zn and Co₃O₄.

Insights into an autonomously formed oxygen-evacuated Cu2O electrode for the selective production of C2H4 from CO2

In situ transformation of bulk Cu₂O to a Cu₂O-derived bulk structure with oxygen-vacant sites, with the oxide layer still on the surface, resulted in highly active, stable, and selective production of ethylene from carbon dioxide.

A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels

This paper reviews recent progress made in identifying electrocatalysts for carbon dioxide (CO2) reduction to produce low-carbon fuels, including CO, HCOOH/HCOO(-), CH2O, CH4, H2C2O4/HC2O4(-), C2H4, CH3OH, CH3CH2OH and others. The electrocatalysts are classified into several categories, including metals, metal alloys, metal oxides, metal complexes, polymers/clusters, enzymes and organic molecules. The catalyts' activity, product selectivity, Faradaic efficiency, catalytic stability and reduction mechanisms during CO2 electroreduction have received detailed treatment. In particular, we review the effects of electrode potential, solution-electrolyte type and composition, temperature, pressure, and other conditions on these catalyst properties. The challenges in achieving highly active and stable CO2 reduction electrocatalysts are analyzed, and several research directions for practical applications are proposed, with the aim of mitigating performance degradation, overcoming additional challenges, and facilitating research and development in this area.

Recycling of carbon dioxide to methanol and derived products - closing the loop

Starting with coal, followed by petroleum oil and natural gas, the utilization of fossil fuels has allowed the fast and unprecedented development of human society. However, the burning of these resources in ever increasing pace is accompanied by large amounts of anthropogenic CO2 emissions, which are outpacing the natural carbon cycle, causing adverse global environmental changes, the full extent of which is still unclear. Even through fossil fuels are still abundant, they are nevertheless limited and will, in time, be depleted. Chemical recycling of CO2 to renewable fuels and materials, primarily methanol, offers a powerful alternative to tackle both issues, that is, global climate change and fossil fuel depletion. The energy needed for the reduction of CO2 can come from any renewable energy source such as solar and wind. Methanol, the simplest C1 liquid product that can be easily obtained from any carbon source, including biomass and CO2, has been proposed as a key component of such an anthropogenic carbon cycle in the framework of a "Methanol Economy". Methanol itself is an excellent fuel for internal combustion engines, fuel cells, stoves, etc. It's dehydration product, dimethyl ether, is a diesel fuel and liquefied petroleum gas (LPG) substitute. Furthermore, methanol can be transformed to ethylene, propylene and most of the petrochemical products currently obtained from fossil fuels. The conversion of CO2 to methanol is discussed in detail in this review.

FIRST-PRINCIPLES STUDY OF OXYGEN-VACANCY Cu2O (111) SURFACE

Geometric and electronic properties and vacancy formation energies for two kinds of oxygen-vacancy Cu 2 O (111) surfaces have been investigated by first-principles calculations. Results show that the relaxation happens mainly on the top three trilayers of surfaces. Two vacancies trap electrons of -0.11e and -0.27e, respectively. The effects of oxygen vacancies on the electronic structures are found rather localized. The electronic structures suggest that the oxygen vacancies enhance the electron donating ability of the surfaces to some extent. The energies of 1.75 and 1.43 eV for the formation of oxygen vacancies are rather low, which indicates the partially reduced surfaces are stable and easy to produce.