Plants use photosynthesis to convert carbon dioxide and water into sugars and oxygen. The process starts in a cluster of manganese, calcium and oxygen atoms at the heart of a protein complex called photosystem II, which splits water to form oxygen gas, protons and electrons.
Numerous researchers have attempted to develop synthetic catalysts that mimic this cluster, using light or electricity to convert water into fuels such as hydrogen gas. Unlike plants, however, these artificial catalysts can only split alkaline water, which makes the process less sustainable. Now, researchers at the RIKEN Center for Sustainable Resource Science in Japan have developed a manganese oxide-based catalyst system that can split water efficiently at neutral pH. They report on their work in an open access paper in the journal Nature Communications.
The development of efficient catalysts for the oxidation of water to molecular oxygen has long been the focus of intense research. Such studies are motivated by the desire to understand the water splitting process in natural systems, such as photosystem II (PSII) of oxygenic photosynthesis, and artificial photosynthetic systems designed to produce hydrogen through proton reduction or convert carbon dioxide to fuels. In nature, water oxidation proceeds with extraordinarily high catalytic activity in PSII, in which a Ca-containing tetrameric manganese cluster (CaMn4O5) supported by bridged oxides or hydroxides and carboxylate and histidine side chains from the protein serves as the multi-electron oxidation catalyst. Notably, all species capable of O2 evolution possess a qualitatively identical reaction centres, and no metal element other than Mn has been identified in the catalytic cluster of PSII. Therefore, extensive research efforts have been aimed at developing water-oxidation catalysts composed of the abundant element Mn.
However, a remarkable contradiction still exists on the catalytic performance between naturally occurring and synthetic Mn catalysts, particularly under neutral pH conditions. Although bioinspired water-oxidation catalysts, particularly Mn oxides, function as effective electrocatalysts under alkaline conditions, the activity of most Mn oxides is markedly reduced at neutral pH, resulting in a large electrochemical overpotential (η) ranging from 500 to 700 mV. This high η contrasts that of the PSII tetrameric Mn cluster, which catalyzes water oxidation with an η of only 160 mV. … This property prohibits the successful application of Mn oxides as components of artificial photosynthetic systems.—Yamaguchi et al.
In photosystem II, charged manganese (Mn) ions gradually give up electrons as they tear protons away from water molecules. This causes manganese in the 2+ and 3+ valence states to become oxidized, resulting in Mn4+ ions. Although the less-oxidized Mn3+ ions are quite stable in photosystem II, Nakamura and his colleagues previously found that they are unstable in synthetic manganese oxide catalysts at neutral pH.
To overcome this instability, the researchers sped up the regeneration of Mn3+ ions, which usually occurs when a water–Mn2+ complex loses a proton and an electron in two separate steps. Ryuhei Nakamura and colleagues realized that ring-shaped organic molecules called pyridines could help those steps to happen at the same time—a process likely promoted by amino acids in photosystem II.
They found that the manganese oxide catalyst produced 15 times more oxygen at neutral pH when used in conjunction with a pyridine called 2,4,6-trimethylpyridine.
The team also tested the reaction in deuterated water, which contains a heavier isotope of hydrogen than normal water. The catalyst generated oxygen much more slowly in the presence of 2,4,6-trimethylpyridine, suggesting that removal of a proton from the water–Mn2+ complex is the key step that determines the overall rate of the water-splitting reaction.
As pyridines would not be suitable for large-scale water splitting because they are potential environmental pollutants, the team now hopes to identify safer alternative proton-removing molecules that could be immobilized onto the surface of the manganese oxide catalyst to enhance its activity.
Yamaguchi, A., Inuzuka, R., Takashima, T., Hayashi, T., Hashimoto, K. & Nakamura, R. (2014) “Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH,” Nature Communications 5, 4256 doi: 10.1038/ncomms5256