Photoelectrochemical Reduction of 2-t-Butyl-9,10-Anthraquinone at Illuminated P-Type Si: An Approach to the Photochemical Synthesis of Hydrogen Peroxide

The electrochemical behavior of 2-i-butyl-9,10-anthraquinone, BAQ, has been investigated at illuminated (632.8 nm, ~50 mW/cm²) p-type semiconducting Si in CH3CN/0.1M [n-Bu4N]C104 with and without added CH3COOH. In the absence of CH3COOH the BAQ is photoreducible at the photocathode to form BAQ~. Relative to E° (BAQ/BAQ⁷) = -0.95V vs. SCE the photo-voltage Ev, at open circuit is ~0.5V. An efficiency for a regenerative photoelectrochemical cell based on the BAQ/BAQ • is ~4% for the conversion of 632.8 nm light. In the presence of 0.5M CH3COOH the BAQ is photoreducible to 2-t-butyl-9,10-dihydroxyanthracene, H2BAQ, with a measured current efficiency of >90% when the p-type Si photocathode is held at —0.3V vs. SCE in a two-compartment cell. Preparative, controlled-potential photoelectrochemical reduction of BAQ to H2BAQ in CH3CN/0.5M CH₃COOH/0.1M [n-Bu4N]C104 has been demonstrated for >90% conversion of 0.2M BAQ to H2BAQ. Reaction of the photoelectrochemically generated H2BAQ (>0.18M) with O2 from air yields solutions containing >0.18M H2_O2. The BAQ → H2BAQ has only ~2% photoelectrochemical energy conversion efficiency. The low efficiency is apparently a consequence of sluggish electrode kinetics or adsorption problems that lead to a low Ev with respect to the E° (BAQ/H2BAQ) ≈-0.35V us. SCE in the presence of 0.5M CH3COOH.