Title: Nature-Driven Photochemistry for Catalytic Solar Hydrogen Production: A Photosystem I–Transition Metal Catalyst Hybrid
Authors: Lisa M. Utschig, Sunshine C. Silver, Karen L. Mulfort, and David M. Tiede
Journal: Journal of the American Chemical Society
Affiliation: * Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Hydrogen (H2) production from water and sunlight is a long-held goal in the scientific community because it promises to alleviate environmental stress while still allowing for universal access to cheap energy storage. Enzymes known as hydrogenases are Nature’s preferred method for production of H2, and research chemists have made careers from building molecular models of the transition-metal active site where H2 generation takes place. Other proteins, known as Photosystem I and Photsystem II, are highly efficient at capturing photons and converting the photon energy into electrical energy. Other chemists have taken a different route and have attempted to build molecular catalysts that have no precedent in nature. Both methods rely on the same strategy: the effective shuttling of electrons to protons in solution. This research group (which, it should be noted, contains a member named “Sunshine”), based at Argonne National Lab, is taking a new approach to the development of H2 generation catalysts and combining the ability of photosystem I to generate electrons from sunlight with the ability of cobalt complexes to generate H2 from protons.
Photosystem I is a large protein containing strategically placed chlorophyll subunits containing loosely bound electrons that can be excited by a photon. This excited electron is transferred through other subunits until it eventually ends up at an Iron-Sulfur (Fe-S) cluster. In nature, this electron reduces NADP+, which can be thought of as “biological H2” because it stores two protons and two electrons in one molecule.
Instead of NADP+, the researchers add a cobaloxime complex to solution (seen below). Cobaloximes have been known for the last 25 years to catalyze H2 production and are a great example of an earth-abundant transition-metal complex that is inexpensive and simple to make.
When the researchers combined photosystem I and cobaloximes and shined light on the resulting solution, bubbles were observed immediately. The H2 production slows down and eventually stops, however, suggesting that some sort of decomposition reaction is occurring. The presence of H2 was confirmed with the use of gas chromatography. The researchers were not able to determine the mode of attachment of the cobaloxime to photosystem I, nor were they able to determine the decomposition products of the cobaloxime. The authors propose that this method of attaching molecular catalysts to light-harvesting proteins may be used to drive a wide range of redox reactions; it will be exciting to see what new discoveries will be made based on this work.