O 2 ‐Yield Enhancement by CH 3 CN Addition to Photocatalytic Water Oxidation Systems Using [Ru II (bpy) 3 ] 2+ and S 2 O 8 2–

Photocatalytic water oxidation systems employing [Ru II (bpy) 3 ] 2+ as a photosensitizer and S 2 O 8 2− as a sacrificial electron acceptor are widely used to evaluate the activity of various catalysts. However, the stoichiometric amount of O 2 hardly evolves due to the decomposition of [Ru II (bpy) 3 ] 2+ during the reaction. We report herein that the addition of 5–20 vol% of acetonitrile (CH 3 CN) to the reaction solution increases the O 2 yield irrespective of water oxidation catalysts (WOCs). For example, the addition of 20 vol% of CH 3 CN increased the O 2 yield from 38% to 93% in a borate buffer containing [Ru II (bpy) 3 ] 2+ , S 2 O 8 2− and a cobalt porphyrin complex working as a homogeneous WOC. An investigation into the effect of organic‐solvent addition on each reaction step indicated that the oxidative decomposition of [Ru II (bpy) 3 ] 2+ observed in a buffer without organic solvents was suppressed by the sacrificial oxidation of the organic solvents. In the presence of a WOC, the oxidation of MeOH and acetone more readily proceeds than water oxidation, resulting in low O 2 yields. In contrast, the lower oxidation susceptibility of CH 3 CN scarcely interfered with water oxidation, thereby affording high O 2 yields. CH 3 CN addition to the reaction solution is promising for the evaluation of intrinsic catalysis of each WOC.