Elucidation of the Activity and pH Stability Limits of Polyoxometalate-Intercalated Layered Double Hydroxide Nanocomposites toward Water Oxidation Catalysis

Abstract

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 under 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, to study its catalytic capabilities and stability. Remarkably, the nanocomposites show improved water oxidation efficiencies with excellent stability in close-to-neutral media compared with the water-insoluble cesium salt of Co₄-POM or commercial Co₃O₄. However, thorough postcatalytic 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 catalytic POM after the catalytic reaction, especially when conditions are employed outside the pH stability window of the POM, which can lead to erroneous conclusions and mistaken catalytic activities.