Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

Abstract

Two novel supramolecular complexes RuRe ([Ru(dceb)2(bpt)Re(CO)3Cl](PF6)) and RuPt ([Ru(dceb)2(bpt)PtI(H2O)](PF6)2) [dceb = diethyl(2,2′-bipyridine)-4,4′-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO2 reduction and H2 evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy. Inclusion of ethyl-ester moieties led to a reduction in the LUMO energies on the peripheral bipyridine ligand, resulting in localization of the 3MLCT excited state on these peripheral ligands following excitation. RuPt generated hydrogen in solution and when immobilized on NiO in a photoelectrochemical (PEC) cell. RuRe was inactive as a CO2 reduction catalyst in solution, and produced only trace amounts of CO when the photocatalyst was immobilized on NiO in a PEC cell saturated with CO2.

Ruthenium–rhenium and ruthenium–palladium supramolecular photocatalysts for photoelectrocatalytic CO2 and H+ reduction

“…The observed current increase for Re(Hbpt)(CO) 3 Cl matches well with the value observed for the similar Re(Hph)(CO) 3 Cl (3.58) complex, which was previously reported for electrocatalytic CO 2 reduction by Nguyen et al 92 The fairly small difference in catalytic activity between the Re(bpt)(CO) 3 Cl fragment and the supramolecular assembly PC1 is consistent with the limited influence of the Ru fragment on the Re catalytic centre as observed for the analogous supramolecular assembly utilising the bpt bridging ligand but with ester rather than acid groups on the peripheral bpy ligands. 80…”

“…The RuRe compound PC1, performed better than the ester analogue previously reported. 80 In contrast, the RuPd compound PC2 was signicantly less active for hydrogen generation than the ester analogue. 63 The main reason for the change in behaviour is attributed to the difference in lifetime of the excited state.…”

“…Based on previous reports, the reduction takes place in the order dcb > bpy > bpt. 80,88,89 The rst reduction appears to be irreversible while the second and third reduction have some reversible character. A fourth irreversible reduction at −2.8 V vs. Fc, may be assigned to a second reduction taking place on the bpt bridging ligand, as previously reported by Vos et al for the Ru(bpy) 2 (bpt) complex.…”

“…63 In our previous work, the photocatalysts were adsorbed on the NiO surface through ester groups, but only trace amounts of CO were produced and it was noted that carboxylic acids may be preferable for anchoring to metal oxide surfaces. 63,80,81 In this paper, we consider two photoelectrocatalysts, one for CO 2 reduction (PC1), one for H 2 evolution (PC2), with carboxylic acid groups for NiO photocathodes. PC3 was included as a catalystfree compound to benchmark the activity (Fig.…”

“…79 A second advantage is that the absorption coefficient of the bimetallic system is higher than the Ru system alone, which means the NiO film can be quite thin to overcome the slow hole-transport. 80 Co-immobilization of the photosensitizer and catalyst would lead to competition for binding sites at the surface and so the thickness of the NiO would need to be even thicker to immobilize the equivalent number of chromophores as the supramolecular system. The third advantage of the structure is to improve the photophysics.…”