Comparative study of the anchorage and the catalytic properties of nanoporous TiO2 films modified with ruthenium (II) and rhenium (I) carbonyl complexes

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Rhenium Carbonyl Molecular Catalysts for CO2 Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes

Heterogenization of molecular catalysts on (photo)electrode surfaces is required to design devices performing processes enabling to store renewable energy in chemical bonds. Among the various strategies to immobilize molecular catalysts, direct chemical bonding to conductive surfaces presents some advantages because of the robustness of the linkage. When the catalyst is, as it is often the case, a transition metal complex, the anchoring group has to be connected to the complex through the ligands, and an important question is thus raised on the influence of this function on the redox and on the catalytic properties of the complex. Herein, we analyze the effect of conjugated and non conjugated substituents, structurally close to anchoring functions previously used to immobilize a rhenium carbonyl bipyridyl molecular catalyst for supported CO₂ electroreduction. We show that carboxylic ester groups, mimicking anchoring the catalyst via carboxylate binding to the surface, have a drastic effect on the catalytic activity of the complex toward CO₂ electroreduction. The reasons for such an effect are revealed via a combined spectro-electrochemical analysis showing that the reducing equivalents are mainly accumulated on the electron-withdrawing ester on the bipyridine ligand preventing the formation of the rhenium(0) center and its interaction with CO₂. Alternatively, alkyl-phosphonic ester substituents, not conjugated with the bpy ligand, mimicking anchoring the catalyst via phosphonate binding to the surface, allow preserving the catalytic activity of the complex.

CO2 photoelectroreduction with enhanced ethanol selectivity by high valence rhenium-doped copper oxide composite catalysts

CuO supported catalyst with high valence rhenium doping were specially studied for photoelectrocatalytic reduction of CO₂ to small molecular alcohols, which were synthesized by nitrate thermal decomposition method on anatase TiO₂ nanotube arrays (TiO₂-NTs). Photoelectrochemical measurements indicate that the high valence rhenium doping helps in improving the catalytic activity and selectivity of CuO supported catalysts. For the case of 6 wt% Re-doped CuO/TiO₂-NTs calcined at 723 K, the principal products are methanol and ethanol with yield up to 19.9 μmol and 7.5 μmol after 5 h photoelectrocatalysis at external potential of -0.4 V under simulated solar illumination. In contrast, the products catalyzed by undoped CuO/TiO₂-NTs are only methanol and formaldehyde. These results indicate that the high valence rhenium doping will promote the alcoholization process and benefit the CC coupling, leading to the selective conversion of CO₂ to ethanol. Furthermore, under suitable external potential (-0.5 V) the CO₂ conversion product is almost entirely composed of ethanol.