Enhanced Photocatalytic Alcohol Oxidation at the Interface of RuC-Coated TiO₂ Nanorod Arrays

Abstract

Visible-light-driven organic oxidations carried out under mild conditions offer a sustainable approach to performing chemical transformations important to the chemical industry. This work reports an efficient photocatalytic benzyl alcohol oxidation process using one-dimensional (1D) TiO2 nanorod (NR)-based photoanodes with surface-adsorbed ruthenium polypyridyl photocatalysts at room temperature. The photocatalyst bis­(2,2′-bipyridine)­(4,4′-dicarboxy-2,2′-bipyridine)­Ru­(II) (RuC) was covalently anchored onto TiO2 nanorod arrays grown on fluorine-doped tin oxide (FTO) electrode surfaces (FTO|t-TiO2|RuC, t = the thickness of TiO2 NR). Under aerobic conditions, the photophysical and photocatalytic properties of FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes were investigated in a solution containing a hydrogen atom transfer mediator (4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl, ACT) as cocatalyst. Dye-sensitized photoelectrochemical cells (DSPECs) using the FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes and ACT-containing electrolyte were investigated for carrying out photocatalytic oxidation of a lignin model compound containing a benzylic alcohol functional group. The best-performing anode surface, FTO|1-TiO2|RuC (shortest NR length), oxidized the 2° alcohol of the lignin model compound to the Cα-ketone form with a > 99% yield over a 4 h photocatalytic experiment with a Faradaic efficiency of 88%. The length of TiO2 NR arrays (TiO2 NRAs) on the FTO substrate influenced the photocatalytic performance with longer NRAs underperforming compared to the shorter arrays. The influence of the NR length is hypothesized to affect the homogeneity of the RuC coating and accessibility of the ACT mediator to the RuC-coated TiO2 surface. The efficient photocatalytic alcohol oxidation with visible light at room temperature as demonstrated in this study is important to the development of sustainable approaches for lignin depolymerization and biomass conversion.

Photoelectrochemical approaches for the conversion of lignin at room temperature

“…19 This photoelectrocatalytic oxidation process leads to enhanced selectivity and effective chemical transformations of aliphatic and/or benzylic alcohols under ambient conditions. 13,14,18,19 Our recent work on developing DSPEC processes incorporating ARMs for achieving benzyl alcohol oxidation and lignin degradation will be discussed later. The photoelectrocatalytic mechanisms and the key mechanistic pathways of chemical transformation for solar or visible light driven lignin degradation will also be considered.…”

“…Combining the concepts of mediated electrocatalysis, especially in the context of C-C or C-O bond cleavage of lignin, with that of dye-sensitized photoelectrochemical cells has given rise to aminoxyl mediated DSPECs for biomass conversion or alcohol oxidation in non-aqueous media. 14,18,19 Work related to the development of this specific type of DSPEC will be discussed below, and a detailed review of the recent work related to lignin conversion and mechanistic pathways will be given in the later sections.…”

“…Our recent publications have detailed the use of both mesoporous and nanorod-based TiO 2 photoanodes modified with RuC (TiO 2 -RuC) and shown high photocurrents and excellent conversion efficiencies with ACT or NHPI for generating oxidized LMC products. 14,18,19 To prove electron injection from photoexcited RuC* to TiO 2 and hole transfer from RuC to ACT, the electrochemical and photoelectrochemical measurements were conducted with a TiO 2 -RuC photoanode, ACT, and a phenolic LMC. As displayed in Fig.…”

“…With this motivation, Sherman, Yoo, and Leem research groups have developed photoelectrocatalytic processes for oxidative chemical transformations in primary aliphatic and/or benzyl alcohols by combining a DSPEC and aminoxyl radical mediators (ARMs). [13][14][15][16][17][18][19] Lignin is a three-dimensional amorphous and interlinked biomacromolecule, representing the largest aromatic component in lignocellulosic biomass, and is comprised of three primary monomeric subunits: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Interunit linkages mainly consist of the aryl ether (b-O-4 and a-O-4), phenylcoumaran (b-5), and resinol (b-b) groups, as shown in the representative structures of native lignin in Fig.…”

Solar thermal-activated photocatalysis for hydrogen production and aqueous triethanolamine polymerization

“…Coupling the photocatalytic H 2 production and holes-mediated transformation organic synthesis in particulate photocatalysis system represents a new trend, where the oxidation of alcohols, dehydrocoupling of thiols to disuldes, and oxidative crosscoupling have been tentatively investigated. [37][38][39] We also realized synchronous photocatalytic H 2 evolution and anaerobic oxidation of alcohols to ketones in acetonitrile (MeCN) by Pt/g-C 3 N 4 -mediated hole oxidation. 40 However, the progress of photocatalytic holes-mediated organic transformation and polymerization synthesis in water still faces challenges.…”