Formation of a nanoparticulate birnessite-like phase in purported molecular water oxidation catalyst systems

The fate of [Mn^III/IV₂(μ-O)₂(terpy) ₂(H₂O)₂]³⁺ (1) under conditions typically applied to test its ability to catalyze water oxidation was studied by X-ray absorption spectroscopy and UV/Vis spectrophotometry by using [Mn ^III/IV₂(μ-O)₂(bipy)₄] ³⁺ (2) and Mn²⁺ as controls (terpy=2,2:6,2"- terpyridine, bipy=2,2-bipyridine). The sample matrix, pH and choice of oxidizing agent were found to have a significant effect on the species formed under catalytic conditions. At low range pH values (4-6), homogeneous catalysis testing in oxone implied that 1 remains intact, whereas in clay intercalate there is strong evidence that 1 breaks down to a birnessite-like phase. In homogeneous solutions at higher pH, the results are consistent with the same birnessite-like structure identified in the clay intercalate. The use of the molecular complexes, as a source of manganese instead of simple Mn^II salts, was found to have the effect of slowing down oxide formation and particle aggregation in solution. The original analytical results that implied the systems are molecular are discussed in the context of these new observations.