Sequential catalysis enables enhanced C-C coupling towards multi-carbon alkenes and alcohols in carbon dioxide reduction: a study on bifunctional Cu/Au electrocatalysts

Electrochemical reduction of carbon dioxide (CO2) to multi-carbon products such as ethylene, ethanol and n-propanol offers a promising path for utilization of excessive CO2 and energy storage. Oxide-derived Cu electrodes are among the best electrocatalysts for the selective formation of ethylene and ethanol. However, a large fraction of the faradaic current still goes to hydrogen evolution, even at optimal conditions (electrolyte, potential, etc.). Here we employ the concept of sequential catalysis using judiciously designed CuAu bimetallic catalysts through galvanic exchange between Au3+ and Cu2O nanowires. By controlling the concentration of the Au3+ precursor and the exchange time, Au nanoparticles were evenly dispersed onto the Cu2O nanowires. The optimized oxide-derived CuAu catalyst showed remarkable improvement towards the formation of ethylene, ethanol and n-propanol, in terms of faradaic efficiency and current density. Our analysis of the electrochemical formation of carbon monoxide, ethylene and hydrogen suggests that the presence of Au, an electrocatalyst for CO2-to-CO conversion, helps enhance *CO-coverage on Cu, thus promoting the production of multi-carbon products and suppressing hydrogen formation on the CuAu catalyst. We propose promising strategies for designing electrochemical systems, which would enable the selective and scalable reduction of CO2 to ethylene and ethanol.

Photocatalytic and Electrocatalytic Reduction of Carbon Dioxide in Pressurized Systems

Patrick Voyame

The depletion of carbon-based fossil fuels and the rise in atmospheric carbon dioxide concentration will force an inevitable change in the future global energy landscape. CO2 reduction presents the advantages of decreasing its atmospheric concentration and storing energy in chemical form in CO2 reduction products. With a predicted conversion to renewable energy such as solar or wind energy, energy storage will become a key process in the near future for buffering the fluctuating energy production. The objective of the present work was to study the efficiency towards CO2 reduction of different molecular catalysts. In particular, conducting experiments in high-pressure and supercritical conditions and observing the effect of CO2 pressure or concentration on the efficiency and selectivity of the reaction. As supercritical CO2 (scCO2) has poor solubilisation capabilities, experiments were conducted in biphasic systems or with addition of an organic co-solvent. To drive the reduction reaction of CO2, a catalyst is needed to overcome the kinetic limitations of the reaction, but an energy input is also necessary. Three different forms for this energy input were used in this work. In a first time a sacrificial product, decamethylferrocene (DMFc), was used to transfer electrons to CO2 in biphasic water/scCO2 systems. Complete oxidation of the DMFc was observed in presence of anion capable of transporting protons from water to the DMFc present in the supercritical phase. A photosensitization cycle was used to supply a water soluble catalyst, NI(II)Cyclam, in electron at the required potential to drive the reduction of CO2 into carbon monoxide in water/scCO2 system. The creation of the interface in the system appeared highly favourable to the efficiency of the catalyst. A second catalyst, a ruthenium polypyridyl carbonyl complex, was used for the photocatalytic reduction of CO2. Pressure had an important impact on the production of one of the two reduction product, carbon monoxide, while the production of formate was unaffected by CO2 pressure. As limitations in productivity in photocatalytic experiments were coming principally from the photosensitizer cycle or the sacrificial electron donor, photosensitizer was replaced by an electrode to provide the catalyst in electrons. Voltammetry in CO2-expanded liquids was described and determination of important parameters such as catalyst concentration and diffusion coefficient as a function of pressure was performed. Electrocatalytic reduction of CO2 by ruthenium and rhenium polypyridyl carbonyl complexes was studied in CO2-expanded liquids. The catalytic mechanisms were observed to be highly influenced by CO2 concentration.

EPFL2016

Carbon dioxide hydrogenation to methanol at low pressure and temperature

The measurements obtained were used to investigates the methanol synthesis reaction from pure carbon dioxide and hydrogen over binary (CuO/ZrO2) and ternary supported catalysts (CuO/ZnO/Al2O3). A comparison of different catalysts is presented. The influence of the most important reaction parameters, i.e. temperature, pressure, space velocity and feed gas composition is examined at moderate temperatures (≤ 300 °C) and pressures (≤ 20 bar). The influence of the partial pressures of hydrogen, carbon dioxide, carbon monoxide, water and methanol are studied in detail with the most efficient catalyst. According to the results, water has a decisive effect on the catalyst activity and performance. The measured results were used to derive a possible Langmuir-Hinshelwood kinetic model of the methanol synthesis reaction. The catalyst surface is analyzed by in situ diffuse reflectance Fourier transform infrared spectroscopy. The identification and evolution of intermediates on CuO/ZnO/Al2O3 catalysts confirm that the reaction proceeds by prior formation of the carbonate on the copper, followed by hydrogenation of the carbonate to the formate and thereafter to methoxy and methanol. For CuO/ZrO2 catalysts results demonstrate that the methanol formation occurs via π – bound formaldehyde and methoxy. Responses of sine shape variation applied to the reactants are also examined. The conversion of CO2 with hydrogen to methanol is investigated in dielectric-barrier discharges with and without catalysts at low temperatures (≤ 100 °C) and pressures (≤ 10 bar). The combination of discharges and catalysts lower the activation energy of the reaction resulting in a decrease in the catalyst optimum temperature. The presence of the catalyst in the discharge increases the methanol yield and selectivity by more than a factor of 10. Electrochemical reduction of carbon dioxide is briefly investigated by using TiO2/Ni complex and TiO2/Ru complex thin film electrodes. A considerable decrease in overvoltage is achieved together with respectable current densities.

EPFL1998

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

A rapidly growing population and industrialization have caused the exploitation of natural resources in keeping with fossil fuels shortage. Due to the combustion of fossil fuels, the concentration of CO2 in the earth's atmosphere, which is a potent greenhouse gas, has been increasing dramatically and contributes to the current climate change. Meanwhile, CO2 can be considered as an abundant and inexpensive carbon feedstock, especially if carbon sequestration is required for future fossil fuel-based power stations. Electrochemical reduction of CO2 to form useful chemicals or fuels is a potentially efficient method of CO2 utilization and recycling. The advantage of the electrochemical CO2 reduction is that natural renewable sources, like solar power, wind power, hydropower or nuclear power can supply electric power for CO2 conversion. Various products, such as carbon monoxide, oxalic acid, formic acid, alcohols and hydrocarbons have been reduced from CO2 First in chapter 1, the general remarks about electrochemical reduction of CO2, especially on metal electrodes are introduced. It also includes the introduction of photoelectochemical CO2 reduction and literature review study about electrocatalytic CO2 reduction with organic molecules as mediators and catalysts. In chapter 2, our interest in Mo-based electrocatalysts led us to study the activity of MoO2 for CO2 reduction. MoO2 microparticles indeed act as an active catalyst for the electrochemical reduction of CO2 in organic solvents such as acetonitrile (MeCN) and dimethylformamide (DMF). In the context of this study, it was found that the CO2 reduction activity is much higher at -20 oC than at RT, and it can be promoted by a small amount of water. The selectivity of CO2 reduction depends on the temperature, overpotential and water concentration. Chapter 3 describes the electrochemical reduction of CO2, improved and modified by using imidazolium ionic liquids (ILs) as an electrolyte in organic solvent with MoO2/Pb electrode. The important role of ILs in changing the pathway of the reduction was observed. Particularly, the use of imidazolium ILs instead of tetraalkylammonium salt as an electrolyte altered the product from oxalate to CO. Besides, the overpotential of the CO2 reduction was shifted to the less negative potentials compared to previous work, and the current density as well increased. At a low water concentration, the catalytic activity was promoted and the total Faradaic efficiency (FE) was up to 100% with more than 60% of CO formation. Furthermore, imidazolium ILs together with our MoO2/Pb electrode lowered the overpotential of CO2 reduction by about 40 mV. In addition to the high selectivity for CO formation, the imidazolium ILs can perform a role as co-catalyst for lowering the CO2 reduction potential. Finally, chapter 4 shows the possibility of photoelectrochemical CO2 reduction to extend our research with various catalysts. In this work, we could derive a photoelectrochemical method to deposit Sn/SnOx catalyst on the surface protected cuprous oxide (Cu2O) photocathodes. The deposition of Sn/SnOx catalyst was optimized on the photocathode and it exhibited reliable CO2 reduction activity in a mild aqueous condition. Sn/SnOx-Cu2O photocathode showed the onset potential of the photocurrent at +0.1 V vs. RHE, which is very promising result. At a constant potential electrolysis of -0.1 V vs. RHE, a FE of 50% for formate formation was obtained and the total FE was around 86%.

EPFL2015