Single-Site, Heterogeneous Electrocatalytic Reduction of CO₂ in Water as the Solvent

Creating stable surface-bound molecular catalysts for CO 2 reduction in aqueous solutions for either electrochemical or photoelectrochemical reduction is a continuing challenge. We report here the preparation and characterization of thin oxide/carbon nanotube electrodes on fluorine-doped tin oxide (FTO) electrodes. The electrodes were prepared by atomic layer deposition (ALD) of ∼15 nm of TiO 2 followed by a layer of carbon nanotubes and a thin (2 nm) overlayer of TiO 2 for surface stabilization and binding. The phosphonate-derivatized version of the known aqueous solution catalyst for H 2 /CO reduction, [Ru II (tpy-Ph–CH 2 –PO 3 H 2 )(Mebim-py)(H 2 O)](PF 6 ) 2 (tpy-Ph–CH 2 –PO 3 H 2 = (4-([2, 2′:6′,2″-terpyridin]-4′-yl)benzyl)phosphonic acid; Mebim-py = 3-methyl-1-pyridyl-benzimidazol-2-ylidene), was added to the surface and stabilized by addition of a thin overlayer of TiO 2 by ALD. In the derivatized electrodes, the catalyst maintains its reactivity toward CO 2 reduction in the short-term, giving mixtures of H 2 /CO that vary from 1.5 at −0.96 V to 5.6 at −1.16 V vs NHE in 0.5 M NaHCO 3 with a turnover number of 308 after 15 min of electrolysis.