Photocatalytic and Electrocatalytic Reduction of Carbon Dioxide in Pressurized Systems

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

Interfacial Photoreduction of Supercritical CO2 by an Aqueous Catalyst

Not just a solvent anymore: CO2 reduction under supercritical conditions was achieved in a biphasic water–supercritical CO2 system using an aqueous soluble catalyst (see picture). The introduction of such an interface provides a suitable reaction medium where adsorption, CO2 binding, and protonation of intermediates are intimately linked

Artificial Photosynthesis at Soft Interfaces

The concept of artificial photosynthesis at a polarised liquid membrane is presented. It includes two photosystems, one at each interface for the hydrogen and oxygen evolution respectively. Both reactions involve proton coupled electron transfer reactions, and some ultrafast steps at the photosensitization stage

Electrochemistry in supercritical fluids:a mini review

A brief overview of the literature relating to electrochemical studies and processes undertaken in supercritical fluids is presented. This review mostly concerns carbon dioxide and hydrofluorocarbons, given the accessibility of their supercritical states, and does not consider the emerging body of research in expanded phase electrochemistry

Photoreduction of CO2 using [Ru(bpy)(2)(CO)L](n+) catalysts in biphasic solution/supercritical CO2 systems

The reduction of CO2 in a biphasic liquid-condensed gas system was investigated as a function of the CO2 pressure. Using 1-benzyl-1,4-dihydronicotinamide (BNAH) as sacrificial electron donor dissolved in a dimethylformamide-water mixture and [Ru(bpy)(2)(CO)L](n+) as a catalyst and [Ru(bpy)(3)](2+) as a photosensitizer, the reaction was found to produce a mixture of CO and formate, in total about 250 mu mol after just 2 h. As CO2 pressure increases, CO formation is greatly favored, being four times greater than that of formate in aqueous systems. In contrast, formate production was independent of CO2 pressure, present at about 50 mu mol. Using TEOA as a solvent instead of water created a single-phase supercritical system and greatly favored formate synthesis, but similarly increasing CO2 concentration favored the CO catalytic cycle. Under optimum conditions, a turnover number (TON) of 125 was obtained. Further investigations of the component limits led to an unprecedented TON of over 1000, and an initial turnover frequency (TOF) of 1600 h(-1)

Steady-state macroscale voltammetry in a supercritical carbon dioxide medium

The electrochemical oxidation and reduction of decamethylferrocene is demonstrated in supercritical carbon dioxide at a macro gold disc electrode at 100 bar and 313 K. Fast mass transport effects were exhibited and the corresponding steady-state voltammetry was observed at high scan rates. A highly lipophilic room temperature ionic liquid that readily dissolved in supercritical CO2 with acetonitrile as a co-solvent was used as an electrolyte, allowing for a conducting supercritical single phase. Experimental observations along with simulation results confirmed the hypothesis that a thin layer of liquid-like phase of co-solvent is formed at the electrode surface and is restricted by a more supercritical phase of high natural convection and bulk concentration

Photoreduction of CO₂ Using [Ru(bpy)₂(CO)L]^<i>n+</i> Catalysts in Biphasic Solution/Supercritical CO₂ Systems

The reduction of CO₂ in a biphasic liquid-condensed gas system was investigated as a function of the CO₂ pressure. Using 1-benzyl-1,4-dihydronicotinamide (BNAH) as sacrificial electron donor dissolved in a dimethylformamide–water mixture and [Ru­(bpy)₂(CO)­L]^<i>n</i>+ as a catalyst and [Ru­(bpy)₃]²⁺ as a photosensitizer, the reaction was found to produce a mixture of CO and formate, in total about 250 μmol after just 2 h. As CO₂ pressure increases, CO formation is greatly favored, being four times greater than that of formate in aqueous systems. In contrast, formate production was independent of CO₂ pressure, present at about 50 μmol. Using TEOA as a solvent instead of water created a single-phase supercritical system and greatly favored formate synthesis, but similarly increasing CO₂ concentration favored the CO catalytic cycle. Under optimum conditions, a turnover number (TON) of 125 was obtained. Further investigations of the component limits led to an unprecedented TON of over 1000, and an initial turnover frequency (TOF) of 1600 h^–1

Electrochemical reduction of protic supercritical CO2 on copper electrodes

The electrochemical reduction of carbon dioxide is usually studied in aqueous solutions under ambient conditions. However, the main disadvantages of this method are high hydrogen evolution and low faradaic efficiencies of carbon based products. Supercritical CO2 (scCO2) can be used as a solvent itself to suppresses hydrogen evolution and tune carbon based product yield, however, it received low attention. Therefore, the focus of this study was on the electrochemical reduction of supercritical CO2 (at 40 ˚C and 80 bar). The conductivity of scCO2 was increased through addition of supporting electrolyte and co-solvent (acetonitrile). Besides, the addition of protic solutions with different pH to supercritical CO2 was investigated. 1 M H2SO4, trifluoroethanol, H2O, KOH, and CsHCO3 solutions were used to determine the effect on current density, faradaic efficiency, and selectivity of scCO2 reduction. Reduction of supercritical CO2 to methanol and ethanol were reported for the first time. However, methane and ethylene were not observed. Additionally, corrosion of Cu was noticed