The structure and reactivity of 0-70mol% Al/Fe iron oxyhydroxides (ferrihydrite in the absence and presence of Al) toward gaseous CO2 were investigated with X-ray photoelectron spectroscopy (XPS), atomic absorption (AA), scanning transmission electron microscopy with electron dispersive X-ray spectroscopy (STEM/EDS), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform Infrared spectroscopy (ATR-FTIR) combined with density functional theory (DFT) calculations. Results showed that Al/Fe oxyhydroxide particles containing more than 20 mol% Al consisted at least in part of Fe-oxyhydroxide with incorporated Al and a discrete AlOOH phase. Results from ATR-FTIR experiments and DFT calculations suggested that the bicarbonate complex formed by passing CO2 over the particles was accommodated on at least three distinct binding sites. At the lowest Al concentrations bicarbonate was bound to individual sites with primarily Fe or Al character. At the highest concentrations of Al (>20 mol%) bicarbonate bound to discrete AlOOH phases became apparent. Results also suggested that the amount of CO2 adsorption for a given particle mass increased as the Al concentration was increased from 0 to 30%. This increase was likely due in large part to differences in the morphology of the particle aggregates that formed in the dry state, which would be expected to affect the amount of surface that was available to adsorb CO2.
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