Surface Polymerization of Mo(VI) and W(VI) Anions on Hematite Revealed by in Situ Infrared Spectroscopy and DFT+U Theoretical Study

The study of the interactions between dissolved Mo­(VI) or W­(VI) species and the surfaces of metal (hydr)­oxides is relevant for two main areas: the optimization of the preparation of catalytic supports and the understanding of the environmental fate of these elements. For the latter, iron (hydr)­oxides are the most important sinks for pollutants, and recently, their surface reactivity was the focus of many research works. In this work, we develop a joint approach, using in situ infrared spectroscopy and DFT simulations, to characterize the Mo­(VI) and W­(VI) species adsorbed on hematite (α-Fe₂O₃). Surface sorbed polymers of tungstate and molybdate on hematite were identified for low pH and at high concentration of these elements, which is similar to the formation of polyoxometalates in solution phase. However, the surface speciation is different from the adsorption of polymolybdate or polytungstate already formed in solution and should be consistent with the growth of a surface polymer. For low concentrations/high pH conditions, the spectra are consistent with a monodentate surface complexation