Optimization of Photocatalyst Excited- and Ground-State Reduction Potentials for Dye-Sensitized HBr Splitting

Abstract

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO/TiO core/shell mesoporous thin films through 2,2'-bipyridine-4,4'-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4' positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φ) and regeneration quantum yields (Φ) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φ (1.0) and minimal Φ (0.037) to minimal Φ (0.09) and unity Φ (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ (0.99), while a significant Φ was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.

Halide Photoredox Chemistry

“…9,10,236 HX splitting has recently found some successes through molecular approaches such as homogeneous catalysts (see sections 2 and 3) or in molecular photoelectrodes such as in a dye-sensitized photoelectrosynthesis cell (DSPEC), Figure 8. 237 In a DSPEC, 238,239 mesoporous thin films of wide band gap metal oxide semiconductor nanoparticles are sensitized to visible light with molecular photocatalysts. Upon light absorption and formation of an excited-state photocatalyst, subsequent electron injection into the metal oxide conduction band yields an injected electron and an oxidized photocatalyst.…”

“…In a DSPEC, , mesoporous thin films of wide band gap metal oxide semiconductor nanoparticles are sensitized to visible light with molecular photocatalysts. Upon light absorption and formation of an excited-state photocatalyst, subsequent electron injection into the metal oxide conduction band yields an injected electron and an oxidized photocatalyst.…”

Dual-Sensitizer Photoanode for Bromide Oxidation

“…As was recently described, there is a delicate interplay between ground-and excited-state reduction potentials that makes it difficult to tune both for optimal behavior while still maintaining light absorption throughout the visible region. 37 A dual-sensitizer photoanode approach with a molecular catalyst has been reported for water oxidation, 34 but the conversion efficiencies were low even when losses associated with transmitted light were taken into account. 34 Herein we report a mechanistic study designed to identify loss mechanisms in the dual-sensitizer approach.…”

Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting

“…Photoanodes for bromide oxidation include TiO 2 or SnO 2 /TiO 2 core-shell nanostructures functionalized with ruthenium polypyridyl based photosensitizers [11], as well as WO 3 , SnO 2 , and Sb-doped SnO 2 decorated with aggregated dyes belonging to the family of perylene diimides, reaching up to 1 mA/cm 2 photocurrent [9].…”