Polyoxometalates on Cationic Silica Nanoparticles. Physicochemical Properties of an Electrostatically Bound Multi-Iron Catalyst

Reaction of a solution of the multi-iron polyoxometalate (POM) K9[(FeIII(OH2)2)3(A-(alpha)PW9O34)2] (K91) with a colloidal suspension of cationic silica nanoparticles ((Si\AlO2)Cl) results in the production of a new heterogeneous oxidation catalyst (K81\(Si\AlO2)). Dynamic light scattering data, coupled with elemental analysis and streaming potential measurements suggests that there are 58 molecules of POM electrostatically bound to the surface of each silica particle on average. Transmission electron microscopy confirms the presence of the POM on the surface of the cationic silica and shows the diameter of the (Si\AlO2)Cl and of the K81\ (Si\AlO2) nanoparticles to be ~12 and ~17 nm, respectively. Significantly, cryo-high-resolution scanning electron microscopy (cryo-HRSEM) shows that the POM retards the natural gelation process that colloidal silica is known to undergo upon aging. EPR and catalytic data collectively suggest that the exchange of the cationic silica for one of the nine K+ cations associated with each POM is responsible for subtle structural changes in the POM which result in its activation as a catalyst.