Metal Complexes and Manganese Oxides for Heterogeneous Water Oxidation

1 Metal Complexes and Manganese Oxides for Heterogeneous Water Oxidation 

Very valuable conclusions have emerged from the study of water oxidation using molecular manganese complexes. In particular, none of the manganese complexes reported so far can be considered an accurate structural and/or functional mimic of the and it is not always clear what the actual 

Crabtree et al. reported the immobilisation of the dimeric mixed-valence species [(tpy)(H20)Mn (0)2Mn (tpy)(H20)] (5) onto titanium dioxide nanoparticles with different degrees of crystallinity. The complex was directly adsorbed onto the titanium dioxide surface for ciystalline titanium dioxide, the manganese(iii)-manganese(iv) dimer transformed to manganese(iv)-manganese(iv) dimer or tetramer-like structure, whereas it remained stable when anchored to titanium-dioxide nanoparticles with low crystallinity. The o ygen-evolution measurements performed in the presence of CAN showed that the dimeric mixed-valence complex adsorbed on amorphous titanium dioxide support was not catalytically active, whereas the oxidised/dissociated complex formed in the crystalline titanium dioxide nanoparticles showed catalytic activity however, no detailed explanation was provided for the differences in catalytic activity.  

Additionally, several reports of homogeneous water-oxidation catalysts pointed to the possibility that the complexes are not the catalysts and are acting as precursors to the respective metal oxides, which are ultimately the actual water-oxidation catalysts. Metal oxides have been known to catalyse water oxidation, but the most active were often oxides of rare- and precious-metal ions, operating best in highly basic conditions. The past decade has seen major advances in the development of highly active metal-oxide catalysts based on cost-effective, abundant and nontoxic elements that can act under near-neutral pH conditions. These metal oxides can 

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