Linking transition metal molybdate formation to polyoxometalate and chloride complex speciation: An in situ X-ray total scattering study

A profound mechanistic understanding of material formation is imperative for developing tailored syntheses. However, insights into solution-based oxide syntheses remain limited to selected systems. Hydrothermal MMoO4 (M = Fe, Ni, Co) formation is governed by the initial pH, implying the involvement of pH-dependent polyoxomolybdate cluster chemistry. Nevertheless, the exact role of clusters in forming molybdates remains poorly understood. Here, we use in situ X-ray total scattering to follow the hydrothermal syntheses of MMoO4 and related hydrates at various pH values, using Co-based reaction mixtures as our primary model system. Focussing on the structural investigation of non-crystalline species, we find no heteropolyoxometalates under the probed conditions. This contrasts our results from reported MWO4 formation pathways and thereby reiterates the element-specific nature of nucleation chemistry. Moreover, we observe pH- and temperature-dependent formation of bimetallic phases that cannot be explained by polyoxomolybdate chemistry alone. Our results therefore point towards a product-directing role of the 3d metal complex speciation under hydrothermal conditions, emphasising the mechanistic complexity of ternary oxide formation.