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Keywords:

  • Photocatalysis;
  • Photochemistry;
  • Metal oxides;
  • Artificial photosynthesis;
  • Redox chemistry;
  • Water splitting

Abstract

This essay is based on research leading to the identification of catalysts capable of the selective oxidation of water to molecular oxygen. The real need for such materials relates to the large-scale photochemical dissociation of water into its constituent elements, as opposed, for example, to the electrochemical decomposition of water at macroscopic electrodes. Combining the catalyst with the essential components needed for efficacious photochemistry brings special challenges, as does the ultimate need to scale up the system by a massive amount. Nature has developed a highly successful process for O2 evolution under ambient illumination that makes use of a cubane tetramanganese cluster having a closely associated calcium cation in attendance. This catalyst is surprisingly delicate and it is debatable as to whether we could adapt such a system for use with artificial photosystems. Historically the latter have used colloidal metal oxides, and consideration is given here as to which materials might offer the most promising catalytic performance. Moving towards heterogeneous systems has more practical meaning, but the same materials come to mind. Recent attention to cobalt-based catalysts is highlighted as a possible breakthrough that might lead to interesting electrochemical systems when combined with wind turbines. Molecular catalysts provide interesting opportunities for photochemical O2 evolution but suffer from problems of scale-up. This field has witnessed the most important progress over the past decade or so but still needs urgent attention if advanced materials are to be identified in a timely manner. Finally, consideration is given to the actual status of the field in specific terms of developing an effective artificial photosynthetic apparatus. Moving progressively from using sacrificial redox agents as a simple means to isolate the oxidative photochemical cycle towards full water cleavage will stimulate the development of demonstration models suitable for public display. The reward should be increased investment.