Electrocatalytic CO₂ Reduction with a Ruthenium Catalyst in Solution and on Nanocrystalline TiO₂

Abstract

A RuII complex [Ru(PO3Et2‐ph‐tpy)(6‐mbpy)(NCCH3)]2+ [PO3Et2‐ph‐tpy=diethyl(4‐[(2,2′:6′,2′′‐terpyridin)‐4′‐yl]phenyl)phosphonate; 6‐mbpy=6‐methyl‐2,2′‐bipyridine] is explored as a molecular catalyst for electrocatalytic CO2 reduction in both a homogeneous solution and, as a phosphonated derivative, on nanocrystalline‐TiO2 surfaces. In CH3CN, the complex acts as a selective electrocatalyst for reduction of CO2 to CO at a low overpotential of 340 mV but with a limited turnover number (TON). An enhancement in reactivity was observed by immobilizing the phosphonated derivative of the catalyst on a nanocrystalline‐TiO2 electrode surface, with the catalyst surface protected by a thin overlayer of NiO. The surface‐functionalized electrode was characterized by X‐ray photoelectron and diffuse reflectance spectroscopies (XPS and DRS). Electrocatalytic reduction of CO2 to CO occurred at −1.65 V versus Fc+/0 with a TON of 237 per catalyst site during 4 h of electrocatalysis. Post‐catalysis XPS measurements reveal that the molecular structure of the catalyst is retained on TiO2 after the long‐term electrocatalysis.

Effect of PDI ligand binding pattern on the electrocatalytic activity of two Ru(II) complexes for CO2 reduction

“…TOF was determined using equation from catalytic CVs: where F is Faraday's constant (96485 C/mol), v is the scan rate employed (0.1 V/s), n p is the number of electrons involved in the non‐catalytic redox reaction ( n p = 1), R is the gas constant (8.314 J/K/mol), T is the temperature (293.15 K), and n cat is the number of electrons involved for the catalytic reaction ( n cat = 2 for the reduction of CO 2 to CO), i p and i cat are determined as the peak currents under Ar and CO 2 , respectively. As for complex 1 , the calculated TOF is 4.4 s −1 ( i cat / i p = 4.7) in MeCN solution at a potential of −1.85 V, which is higher than or comparable to that of many reported ruthenium molecular water oxidation catalysts, and 0.4 s −1 ( i cat / i p = 1.5) in THF solution at a potential of −1.94 V and a scan rate of 100 mV/s. As for complex 2 , the calculated TOFs are 0.3 s −1 ( i cat / i p = 1.2) in MeCN solution at a potential of −2.43 V and 0.2 s −1 ( i cat / i p = 0.9) in THF solution at a potential of −2.21 V at 100 mV/s.…”

“…Molecular catalysts based on ruthenium transition metal complexes for electrochemical reduction of CO 2 have attracted considerable research attention due to their unique structural and electronic features, as well as, homogeneous complexes retaining the advantages of elucidating catalytic processes for optimizing the catalysts . A great many remarkable achievements have been reported by Meyer, Kubiak, Tanaka, Fujita, Miller, Huang, and others …”

Photoelectrochemical properties of dyads composed of porphyrin/ruthenium catalyst grafted on metal oxide semiconductors

“…Furthermore, several tpy-Ru complexes have proven to be active catalysts towards CO2 reduction, [28][29][30][31][32] therefore NiO photocathodes, coated with the HOOC-(Zn)DMP-tpy-Ru dyad, were used as working electrodes for the fabrication of DSPECs. The reaction medium was a mixture of acetonitrile/water in a 9/1 ratio with Bu4NPF6 as supporting electrolyte.…”

“…Finally, Meyer and co-workers, along with other groups, have demonstrated that tpy-Ru derivatives can work as catalysts for either alcohol oxidation [23][24][25][26][27] and/or for CO2 reduction. [28][29][30][31][32] Consequently, the dyads containing the tpy-Ru unit could be implemented in DSPECs as photocatalysts for either alcohol oxidation on TiO2 or CO2 reduction on NiO. Alcohol oxidation seems to be more appealing than water oxidation into oxygen, because the latter requires four holes to operate instead of only two that are needed for alcohol oxidation and necessitates a higher potential (1.23 V vs. NHE for water versus -0.143 V vs. NHE for benzyl alcohol oxidation).…”

Photocatalytic CO2 Reduction and Thermal CO Oxidation to CO2 over Cu/Ni-loaded TiO2 Photo and Thermal Catalysts

“…Titanium dioxide (TiO 2 ) has been widely used as a model catalyst material in various areas such as photo and thermal catalytic applications for energy and environment [1][2][3][4][5][6]. To satisfy increasing demand, considerable effort has been devoted to the designing of new catalysts with higher catalytic performance [1,[7][8][9].…”