Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide.

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3'-DHBPY)(CO)3Cl] (3,3'-DHBPY = 3,3'-dihydroxy-2,2'-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO2 to CO. The combined techniques of cyclic voltammetry and IR/UV-vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4'-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O-H···O-. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4'-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3'-DHBPY-H+)(CO)3Cl]- under ambient conditions occurs at a cathodic potential close to that of the Re/4,4'-DHBPY-H+ derivative. However, for the stabilized 3,3'-DHBPY-H+ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e--reductive cleavage of the second O-H bond, triggering dissociation of the Cl- ligand from [Re(3,3'-DHBPY-2H+)(CO)3Cl]2-. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3'-DHBPY-2H+)(CO)3]3-, the equivalent of classical 2e--reduced [Re(BPY)(CO)3]-. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm-1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl- dissociation from [Re(3,3'-DHBPY-2H+)(CO)3Cl]2- was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3'-DHBPY-2H+)(CO)3(PrCN)]3-. Useful reference data were obtained with substituted parent [Re(3,3'-DHBPY)(CO)3(PrCN)]+ that also smoothly deprotonates by the initial reduction to [Re(3,3'-DHBPY-H+)(CO)3(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3'-DHBPY-2H+)(CO)3(PrCN)]3- via a counterintuitive ETC step generating the 1e- radical of the parent complex, viz., [Re(3,3'-DHBPY)(CO)3(PrCN)]. The same alternative reduction path is also followed by [Re(3,3'-DHBPY-H+)(CO)3Cl]- at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3'-DHBPY)(CO)3Cl]•- further reducible to [Re(3,3'-DHBPY-2H+)(CO)3]3- as the CO2 catalyst.

In situ study of the low overpotential "dimer pathway" for electrocatalytic carbon dioxide reduction by manganese carbonyl complexes.

The electrocatalytic reduction of CO2 using [fac-Mn(bpy)(CO)3Br] (bpy = 2,2'-bipyridine) and its derivatives has been the subject of numerous recent studies. However the mechanisms of catalysis are still debated. Here we carry out in situ vibrational sum-frequency generation (VSFG) spectroelectrochemistry to examine how this catalyst behaves at an electrode surface. In particular, a low overpotential pathway involving a dimeric manganese has been reported in several studies using substituted bipyridine ligands. Here, we find that the "dimer pathway" can also occur with the unsubstuituted bipyridine complexes. Specifically we can observe spectroscopic evidence of the interaction between [Mn2(bpy)2(CO)6] with CO2 in the presence of a suitable acid. Detailed VSFG studies of [Mn2(bpy)2(CO)6], including of the potential dependence of the surface ν(CO) mode, allow us to construct a model of how it accumulates and behaves at the electrode surface under potentiostatic control.

Directing the mechanism of CO2 reduction by a Mn catalyst through surface immobilization.

Immobilization of a Mn polypyridyl CO2 reduction electrocatalyst on nanocrystalline TiO2 electrodes yields an active heterogeneous system and also significantly triggers a change in voltammetric and catalytic behaviour, relative to in solution. A combination of spectroelectrochemical techniques are presented here to elucidate the mechanism of the immobilized catalyst in situ.

The Role of Electrode-Catalyst Interactions in Enabling Efficient CO2 Reduction with Mo(bpy)(CO)4 As Revealed by Vibrational Sum-Frequency Generation Spectroscopy.

Group 6 metal carbonyl complexes ([M(bpy)(CO)4], M = Cr, Mo, W) are potentially promising CO2 reduction electrocatalysts. However, catalytic activity onsets at prohibitively negative potentials and is highly dependent on the nature of the working electrode. Here we report in situ vibrational SFG (VSFG) measurements of the electrocatalyst [Mo(bpy)(CO)4] at platinum and gold electrodes. The greatly improved onset potential for electrocatalytic CO2 reduction at gold electrodes is due to the formation of the catalytically active species [Mo(bpy)(CO)3]2- via a second pathway at more positive potentials, likely avoiding the need for the generation of [Mo(bpy)(CO)4]2-. VSFG studies demonstrate that the strength of the interaction between initially generated [Mo(bpy)(CO)4]•- and the electrode is critical in enabling the formation of the active catalyst via the low energy pathway. By careful control of electrode material, solvent and electrolyte salt, it should therefore be possible to attain levels of activity with group 6 complexes equivalent to their much more widely studied group 7 analogues.

Improving the efficiency of electrochemical CO2 reduction using immobilized manganese complexes.

Immobilization of [Mn(bpy)(CO)3Br], (1) and [Mn(bpy((t)Bu)2)(CO)3Br] (2, where (bpy((t)Bu)2) = 4,4'-di-tert-butyl-2,2'-bipyridine) in Nafion/multi-walled carbon nanotubes (MWCNT) on glassy carbon yielded highly active electrodes for the reduction of CO2 to CO in aqueous solutions at pH 7. Films incorporating have significantly improved selectivity towards CO2, with CO : H2 ∼ 1 at -1.4 V vs. SCE, exceeding that for the previously reported /MWCNT/Nafion electrode. Furthermore, we report the synthesis and subsequent electrochemical characterization of two new substituted Mn(i) bipyridine complexes, [Mn(bpy(COOH)2)(CO)3Br] (3) and [Mn(bpy(OH)2)(CO)3Br] (4) (where (bpy(COOH)2) = 4,4'-di-carboxy-2,2'-bipyridine and (bpy(OH)2) = 4,4'-di-hydroxy-2,2'-bipyridine). Both 3 and 4 were found to have some activity towards CO2 in acetonitrile solutions; however once immobilized in Nafion membranes CO2 reduction was found to not occur at significant levels.

A functionalised nickel cyclam catalyst for CO2 reduction: electrocatalysis, semiconductor surface immobilisation and light-driven electron transfer.

The immobilisation of electrocatalysts for CO2 reduction onto light harvesting semiconductors is proposed to be an important step towards developing more efficient CO2 reduction photoelectrodes. Here, we report a low cost nickel cyclam complex covalently anchored to a metal oxide surface. Using transient spectroscopy we validate the role of surface immobilisation on enhancing the rate of photoelectron transfer. Furthermore [Ni(1,4,8,11-tetraazacyclotetradecane-6-carboxylic acid)](2+) (2) is shown to be a very active electrocatalyst in solution.

Hybrid photocathode based on a Ni molecular catalyst and Sb2Se3 for solar H2 production.

We report a H2 evolving hybrid photocathode based on Sb2Se3 and a precious metal free molecular catalyst. Through the use of a high surface area TiO2 scaffold, we successfully increased the Ni molecular catalyst loading from 7.08 ± 0.43 to 45.76 ± 0.81 nmol cm-2, achieving photocurrents of 1.3 mA cm-2 at 0 V vs. RHE, which is 81-fold higher than the device without the TiO2 mesoporous layer.

Tuning the local chemical environment of ZnSe quantum dots with dithiols towards photocatalytic CO2 reduction.

Sunlight-driven CO2 reduction to renewable fuels is a promising strategy towards a closed carbon cycle in a circular economy. For that purpose, colloidal quantum dots (QDs) have emerged as a versatile light absorber platform that offers many possibilities for surface modification strategies. Considerable attention has been focused on tailoring the local chemical environment of the catalytic site for CO2 reduction with chemical functionalities ranging from amino acids to amines, imidazolium, pyridines, and others. Here we show that dithiols, a class of organic compounds previously unexplored in the context of CO2 reduction, can enhance photocatalytic CO2 reduction on ZnSe QDs. A short dithiol (1,2-ethanedithiol) activates the QD surface for CO2 reduction accompanied by a suppression of the competing H2 evolution reaction. In contrast, in the presence of an immobilized Ni(cyclam) co-catalyst, a longer dithiol (1,6-hexanedithiol) accelerates CO2 reduction. 1H-NMR spectroscopy studies of the dithiol-QD surface interactions reveal a strong affinity of the dithiols for the QD surface accompanied by a solvation sphere governed by hydrophobic interactions. Control experiments with a series of dithiol analogues (monothiol, mercaptoalcohol) render the hydrophobic chemical environment unlikely as the sole contribution of the enhancement of CO2 reduction. Density functional theory (DFT) calculations provide a framework to rationalize the observed dithiol length dependent activity through the analysis of the non-covalent interactions between the dangling thiol moiety and the CO2 reduction intermediates at the catalytic site. This work therefore introduces dithiol capping ligands as a straightforward means to enhance CO2 reduction catalysis on both bare and co-catalyst modified QDs by engineering the particle's chemical environment.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst.

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H2, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR3) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H2 production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

A stable covalent organic framework for photocatalytic carbon dioxide reduction.

Photocatalytic conversion of CO2 into fuels is an important challenge for clean energy research and has attracted considerable interest. Here we show that tethering molecular catalysts-a rhenium complex, [Re(bpy)(CO)3Cl]-together in the form of a crystalline covalent organic framework (COF) affords a heterogeneous photocatalyst with a strong visible light absorption, a high CO2 binding affinity, and ultimately an improved catalytic performance over its homogeneous Re counterpart. The COF incorporates bipyridine sites, allowing for ligation of the Re complex, into a fully π-conjugated backbone that is chemically robust and promotes light-harvesting. A maximum rate of 1040 µmol g-1 h-1 for CO production with 81% selectivity was measured. CO production rates were further increased up to 1400 µmol g-1 h-1, with an improved selectivity of 86%, when a photosensitizer was added. Addition of platinum resulted in production of syngas, hence, the co-formation of H2 and CO, the chemical composition of which could be adjusted by varying the ratio of COF to platinum. An amorphous analog of the COF showed significantly lower CO production rates, suggesting that crystallinity of the COF is beneficial to its photocatalytic performance in CO2 reduction.

ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water.

A precious metal and Cd-free photocatalyst system for efficient CO2 reduction in water is reported. The hybrid assembly consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-functionalised Ni(cyclam) catalyst, NiCycP. This precious metal-free photocatalyst system shows a high activity for aqueous CO2 reduction to CO (Ni-based TONCO > 120), whereas an anchor-free catalyst, Ni(cyclam)Cl2, produced three times less CO. Additional ZnSe surface modification with 2-(dimethylamino)ethanethiol (MEDA) partially suppresses H2 generation and enhances the CO production allowing for a Ni-based TONCO of > 280 and more than 33% selectivity for CO2 reduction over H2 evolution, after 20 h visible light irradiation (λ > 400 nm, AM 1.5G, 1 sun). The external quantum efficiency of 3.4 ± 0.3% at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe. The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO2 reduction. With this work we introduce ZnSe QDs as an inexpensive and efficient visible light-absorber for solar fuel generation.

A highly active nickel electrocatalyst shows excellent selectivity for CO2 reduction in acidic media.

The development of selective electrocatalysts for CO2 reduction in water offers a sustainable route to carbon based fuels and feedstocks. However, molecular catalysts are typically studied in non-aqueous solvents, in part to avoid competitive H2 evolution. [Ni(cyclam)]2+ (1) is one of the few known electrocatalysts that operate in water and 30 years after its report its activity remains a rarely surpassed benchmark. Here we report that [Ni(cyclam-CO2H)]2+ (cyclam-CO2H = 1,4,8,11-tetraazacyclotetradecane-6-carboxylic acid (2)) shows greatly enhanced activity versus1 for CO production. At pHs < pKa of the pendant carboxylic acid a large increase in catalytic activity occurs. Remarkably, despite the high proton concentration (pH 2), 2 maintains selectivity for CO2 reduction and is believed to be unique in operating selectively in such acidic aqueous solutions.

Photochemical CO2 reduction in water using a co-immobilised nickel catalyst and a visible light sensitiser.

A dye-sensitised CO2 reduction photocatalyst that operates in water is reported. Transient spectroscopy demonstrates that the facile co-immobilisation of a Ru dye and a Ni CO2 reduction electrocatalyst enables efficient on-particle electron transfer leading to photocatalytic activity that greatly exceeds the equivalent solution based system.

Electrocatalytic CO2 reduction with a membrane supported manganese catalyst in aqueous solution.

[Mn(bpy)(CO)3Br] cast in a Nafion membrane is an active heterogeneous electrocatalyst with good selectivity for CO2 reduction to CO in neutral aqueous electrolyte. Addition of multi-walled carbon nanotubes (MWCNT) leads to a ∼10 fold current enhancement and stable CO : H2 yields (1 : 2) at -1.4 V vs. Ag/AgCl at pH 7.