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Title: Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations

Abstract

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.

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab., Richland, WA (US)
Sponsoring Org.:
USDOE
OSTI Identifier:
20076119
DOE Contract Number:  
AC06-76RL01830
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 64; Journal Issue: 10; Other Information: PBD: May 2000; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ALUMINIUM HYDROXIDES; CHROMIUM HYDROXIDES; IRON HYDROXIDES; PH VALUE; ELECTRONIC STRUCTURE; HYDRATES; CLAYS; SOILS

Citation Formats

Rustad, J.R., Dixon, D.A., and Felmy, A.R. Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations. United States: N. p., 2000. Web. doi:10.1016/S0016-7037(99)00384-1.
Rustad, J.R., Dixon, D.A., & Felmy, A.R. Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations. United States. doi:10.1016/S0016-7037(99)00384-1.
Rustad, J.R., Dixon, D.A., and Felmy, A.R. Mon . "Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations". United States. doi:10.1016/S0016-7037(99)00384-1.
@article{osti_20076119,
title = {Intrinsic acidity of aluminum, chromium(III) and iron(III) {mu}{sub 3}-hydroxo functional groups from ab initio electronic structure calculations},
author = {Rustad, J.R. and Dixon, D.A. and Felmy, A.R.},
abstractNote = {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.},
doi = {10.1016/S0016-7037(99)00384-1},
journal = {Geochimica et Cosmochimica Acta},
issn = {0016-7037},
number = 10,
volume = 64,
place = {United States},
year = {2000},
month = {5}
}