Dye-Sensitized Photoelectrosynthesis Cells for Benzyl Alcohol Oxidation Using a Zinc Porphyrin Sensitizer and TEMPO Catalyst

Abstract

Exploring a catalytic reaction other than water oxidation at the photoanode of a photoelectrochemical cell is probably a key feature to more efficiently generate the electrons needed to produce solar fuels. In this framework, we describe herein the fabrication of a TiO2-based dye-sensitized photo-electrosynthesis cell (DSPEC) using a zinc porphyrin (ZnP) sensitizer and a 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) organo-catalyst that quite efficiently catalyzes light-driven oxidation of methoxybenzyl alcohol into aldehyde. Two dyads ZnP–TEMPO, differing by the anchoring group (carboxylic acid and hydroxamic acid) on ZnP, were prepared and their electrochemical, absorption, and emission properties were recorded and quantum chemical modeling was realized. The photovoltaic performances in dye-sensitized solar cells were first examined in order to optimize the dyeing conditions and compare the relative efficiencies of the compounds. The dyads substituted with TEMPO outperform the reference zinc porphyrin lacking TEMPO with a much higher Jsc and Voc. The photocatalytic properties after immobilization on TiO2 nanocrystalline films toward para-methoxy benzyl alcohol oxidation were explored in borate buffer and in an acetonitrile electrolyte. In borate buffer, the optimal pH was 8 and using dyad ZnP–TEMPO anchored with hydroxamic acid, para-methoxy benzaldehyde was selectively produced with an average photocurrent density of 200 μA/cm2, a Faradaic efficiency of 82%, a turnover number (TON) of 26, and a turnover frequency (TOF) of 47 h–1. In acetonitrile, in the presence of 0.1 M N-methyl-imidazole, the same dyad gives an average photocurrent density of about 100 μA/cm2, a Faradaic efficiency of 76%, a TON of 13, and a TOF of 24 h–1. The stability of the anchor is crucial in the acetonitrile electrolyte, where the dyad is quite soluble since only the dyad functionalized with hydroxamic acid is compatible with these organic solvent conditions. Overall, this study paves the way to the development of more efficient and probably more stable TiO2-based DSPECs for alcohol oxidation that could advantageously complement those devoted to water oxidation.