The term amorphous metal oxide is becoming widely used in the catalysis community. The term is generally used when there are no apparent peaks in an X-ray diffraction pattern. However, the absence of such features in X-ray diffraction can mean that the material is either truly amorphous or that it is better described as nanocrystalline. By coprecipitating a sodium birnessite-like phase with and without phosphate (1.5%), we are able to engineer two very similar but distinct materials - one that is nanocrystalline and the other that is amorphous. The two closely related phases were characterized with both Mn K-edge X-ray absorption spectroscopy and high-resolution transmission electron microscopy. These structural results were then correlated with catalytic and electrocatalytic activities for water oxidation catalysis. In this case, the amorphous phosphate-doped material was less catalytically active than the nanocrystalline material.
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