Photochemical Reduction of Carbon Dioxide to Methanol and Formate in a Homogeneous System with Pyridinium Catalysts

Abstract

Photochemical catalytic CO2 reduction to formate and methanol has been demonstrated in an aqueous homogeneous system at pH 5.0 comprising ruthenium(II) trisphenanthroline as the chromophore, pyridine as the CO2 reduction catalyst, KCl, and ascorbic acid as a sacrificial reductant, using visible light irradiation at 470 ± 20 nm. Isotopic labeling with (13)CO2 yields the six-electron-reduced product (13)CH3OH. After 1 h photolysis, the two-electron-reduced product formate and the six-electron-reduced product methanol are produced with quantum yields of 0.025 and 1.1 × 10(–4), respectively. This represents 76 and 0.15 turnovers per Ru for formate and methanol, respectively, and 152 and 0.9 turnovers per Ru on an electron basis for formate and methanol, respectively. The system is inactive after 6 h irradiation, which appears largely to be due to chromophore degradation. A partial optimization of the methanol yield showed that high pyridine to Ru ratios are needed (100:1) and that the optimum pH is near 5.0. The presence of potassium salts enhances the yield in formate and methanol by 8- and 2-fold, respectively, compared to electrolyte-free solutions; however, other alkali and alkali earth cations have little effect. The addition of small amounts of solid metal catalysts immobilized on carbon had either no effect (M = Pt or Pd) or deleterious effects (M = Ni or Au) on methanol production. Addition of colloidal Pt resulted in no methanol production at all. This is in notable contrast with the pyridine-based electrocatalysis of CO2 to methanol in which metallic or conductive surfaces such as Pt, Pd, or p-type GaP are necessary for methanol formation.

Isotopic Probe Illuminates the Role of the Electrode Surface in Proton Coupled Hydride Transfer Electrochemical Reduction of Pyridinium on Pt(111)

“…8 Evidence for the proposed mechanism included digital simulation of the cyclic voltammetric scan rate dependence on a Pt electrode, an observed first order dependence of current on concentrations of both PyrH + and CO 2 , as well as density functional theory (DFT) calculations. 6,8,9 Reduction of CO 2 to methanol has also been observed photochemically in the presence of PyrH + , indicating that PyrH + reduction may be possible in homogeneous solution using a photocatalyst with suitably negative excited state redox potentials, 10 as had been indicated by quenching measurements. 11 Additionally, a vibrational spectrum indicative of carbamate formation has been observed in the gas phase upon reaction of CO 2 •− with pyridine.…”

Design of a Novel Voltage Controller for Conversion of Carbon Dioxide into Clean Fuels Using the Integration of a Vanadium Redox Battery with Solar Energy

“…In integrated PV and EC, sunlight is first converted to electricity by a photovoltaic cell and CO 2 is then reduced electrochemically [7], which shows outstanding potential for the CO 2 catalytic hydrogenation route [5]. In photoelectrochemical (PEC) routes, photogenerated electrons are used to convert CO 2 into synthetic fuels either directly at certain non-oxide semiconducting photoelectrodes or indirectly using a redox mediator [8]. These routes have been inadequate for performing on a commercial scale.…”

Electrocatalytic Reduction of Carbon Dioxide using Sol-gel Processed Copper Indium Sulfide (CIS) Immobilized on ITO-Coated Glass Electrode

“…tackling two major challenges of our society, i.e., to alleviate looming climate changes and to create storage and transport capabilities for renewable energy [23,24]. Especially the exploration of sunlight as an effective energy source to reduce carbon dioxide into synthetic fuels (solar fuel) [6,[25][26][27] and chemical feed stocks could create a sustainable energy source that does not compromise our environment.…”

“…Though the thermodynamic formal reduction potentials vs normal hydrogen electrode (NHE) for carbon dioxide in to such products (−0.7 to −0.2 V vs NHE at pH 7) [6,27,30] look feasible, these reactions are kinetically very slow, suffer from high overpotential, and compete with hydrogen formation reaction in protic solvents where CO 2 is less soluble [33]. Particularly, the large reorganizational energy required to rearrange the linear CO 2 molecule into the bent radical anion (CO 2 − ) makes a reduction process via a rate limiting initial one-electron transfer highly energy intensive (formal reduction potential −1.9 V vs NHE at pH 7) and difficult to proceed with respect to thermodynamics [19,34,35].…”