OER and HER Co-Catalysts

1 Mimicking the Oxygen Evolving Center Water Oxidation Catalysts 

Controlled potential electrolysis of 32 inl0% water in CF3CH2OH gave 21 turnovers (O2 per 32) whereas the activity jumps to 33,500 turnovers for the catalyst immobilized on an ITO electrode in aqueous solution. Unfortunately, no attempts to use chemical oxidants in a completely homogeneous system for 32 are reported. 

Of the water oxidation catalysts mentioned, all use either a powerful chemical oxidant (e.g., CeIV, OC1, Oxone, Ihi OOI I) or an electrode to drive the reaction. Efforts to couple the oxidation to a photoprocess have not yielded an active photocatalyst but nonetheless are beginning to yield some promising results as shown previously in Fig. 9 for the Ru-Mm dyad.254 256 

An early review by Koelle on transition metal catalyzed proton reduction nicely developed the various chemical steps involved in hydrogen evolution including metal hydride formation, hydride acidity (basicity) and protonation and requisite redox potentials.284 The complexes review here have little structural relevance to the hy-drogenase active sites but many show promising catalytic activity. More recently 

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Much less work has appeared on AlGaAs alloys. A bipolar electrode configura tion of Alo.isGao.ssAs (Eg = 1.6 eV) and Si was used in conjunction with OER and HER co catalysts, RuCh and Pt, respectively, to drive water photosplitting at 18.3% conversion efficiency.209 Clearly, among all the photoelectrode materials discussed up till now, the Group III-V compounds, namely, InP and the alloyed materials, have yielded the most impressive results. 

Fig. 6.3 Fundamental principle of semiconductor-based photocatalytic water splitting for hydrogen generation. CB conduction band VB valence band Eg bandgap OER oxygen evolution reaction HER hydrogen evolution reaction WRC water reduction co-catalysts WOC water oxidation co-catalysts SBR surface back reaction AEo and AE/, kinetic overpotentials for OER and HER, respectively...

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