Elucidation of the activity and pH stability limits of polyoxometalate-intercalated layered double hydroxide nanocomposites towards water oxidation catalysis

The inclusion of water oxidation active polyoxometalates (POMs) inside layered materials is a promising strategy to increase their catalytic efficiency while overcoming their fragility under homogeneous conditions. In this sense, intercalation of POMs in the interlaminar space of layered double hydroxides (LDHs), formed by positively charged brucite-type inorganic layers, is a very interesting strategy that is gaining attention in the field. Despite their huge potential, there is a lack of accurate characterization of the materials, especially after their use as water oxidation catalysts in pH conditions in which the POM counterpart has been demonstrated to be unstable (strong alkali media). For this reason and as a proof-of-concept, we have intercalated the well-known [Co4(H2O)2(PW9O34)2]10- POM (Co4-POM) in the lamellar space of the Mg2Al-LDH layered double hydroxide, to study its catalytical capabilities and stability. Remarkably, the nanocomposites show improved water oxidation efficiencies with excellent stability in close-to-neutral media compared to the water-insoluble cesium salt of Co4-POM or commercial Co3O4. However, thorough post-catalytic characterization of the nanocomposites demonstrates that the polyoxotungstate framework of the POM suffers from hydrolytic instability in strong alkali conditions, leading to the formation of a mixed-valence cobalt(II/III) oxide in the interlayer space of Mg2Al-LDH. This work highlights the importance of accurately assessing the fate of the catalytical POM after the catalytic reaction, especially when employing conditions outside the pH stability window of the POM, which can lead to erroneous conclusions and mistaken catalytic activities.