⁵¹V NMR of homogeneous multicomponent vanadium oxide solutions

There is growing interest in forming vanadium-containing mixed transition metal oxides, particularly for the preparation of oxidation catalysts. It is desirable in many cases to prepare the solid from homogeneous (stable or metastable) metal oxide solutions and gels in order to maintain the molecular-scale mixing that homogeneous solutions can provide. While this is always possible at extreme pH values, it is also desirable to use moderate pH so that the coating solution should be benign to common substrates. We provide an overview of the insight that ⁵¹V NMR can provide into typical aqueous and alkoxide solution routes suggested by the literature for preparing multicomponent vanadium-containing solutions at moderate pH. Details of the preparation protocol can determine whether the solutions remain homogeneous; we examine the starting pH for aqueous solutions and the alcohol used for alkoxide solutions. Solution ⁵¹V NMR shows that the vanadium speciation differs markedly between stable and unstable solutions.