Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations
Density functional calculations are performed on M{sub 3}(OH){sub 7}(H{sub 2}O){sub 6}{sup 2+} and M{sub 3}O(OH){sub 6}(H{sub 2}O){sub 6}{sup +} clusters for M {double_bond} Al, Cr(III), and Fe(III), allowing determination of the relative acidities of the {mu}{sub 3}-hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, {double_bond}Fe{sub 3}OH and {double_bond}Al{sub 3}OH groups have nearly the same intrinsic acidity, while {double_bond}Cr{sub 3}OH groups are significantly more acidic. The gas-phase acidity of the Fe{sub 3}OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. Acidities of aquo functional groups were also computed for Al and Cr. The {double_bond}AlOH{sub 2} site is more acidic than the {double_bond}Al{sub 3}OH site, whereas the {double_bond}Cr{sub 3}OH site is more acidic than the {double_bond}CrOH{sub 2} site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH.
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 20076119
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 64, Issue 10; Other Information: PBD: May 2000; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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