A deep look into the diverse surface speciation of the mono-molybdate/lepidocrocite system by ATR-IR and polarized ATR-IR spectroscopy
2022
Botella, Romain | Lefèvre, Grégory
The present study is focused on the adsorption of monomeric molybdate anions on the surface of lepidocrocite investigated by several infrared spectroscopy methods. To study the surface speciation of molybdates on lepidocrocite, the behavior of the anion at the interface upon increasing anion concentration (fixed pH) or decreasing pH (fixed concentration) was analyzed in situ. The obtained results expand the knowledge for surface speciation by revealing a second molybdate complex, which is more stable, arising from the interaction with defects on the (010) facets of lepidocrocite. A curve fitting procedure enabled us to confirm what is observed from the pristine spectra, and outer-sphere complexes were probed as well. The evolution of the different bands upon increasing adsorbate concentration and decreasing pH, as obtained by curve fitting, shows a complex evolution for the different surface species with protonation, with the formation of strongly bonded complexes at low pH. The 2D synchronous and asynchronous spectra support the interpretation of the data. Furthermore, insights into the orientation of the complexes with respect to the lepidocrocite surface are obtained by using linearly polarized radiation. The results confirm most of the conclusions ascertained on the basis of the unpolarized spectra. The combination of the different methodologies yields a consistent picture for the complexity of this system and paves the way for subsequent utilization of this approach to study other systems.
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