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Carbon dioxide binding at dry FeOOH mineral surfaces: evidence for structure-controlled speciation
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2013 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 47, no 16, 9241-9248 p.Article in journal (Refereed) Published
Abstract [en]

Interactions between CO2(g) and mineral surfaces are important to atmospheric and terrestrial settings. This study provides detailed evidence on how differences in mineral surface structure impact carbonate speciation resulting from CO2(g) adsorption reactions. It was achieved by resolving the identity of adsorption sites and geometries of (bi)carbonate species at surfaces of nanosized goethite (alpha-FeOOH) and lepidocrocite (gamma-FeOOH) particles. Fourier transform infrared spectroscopy was used to obtain this information on particles contacted with atmospheres of CO2(g). Vibrational modes of surface hydroxo groups covering these particles were first monitored. These showed that only one type of the surface groups that are singly coordinated to Fe atoms (-OH) are involved in the formation of (bi)carbonate species. Those of higher coordination numbers (mu-OH, mu(3)-OH) do not participate. Adsorption geometries were then resolved by investigating the C-O stretching region, assisted by density functional theoretical calculations. These efforts provided indications leaning toward a predominance of mononuclear species, -O-CO2Hx=[0,1]. In contrast, monodentate binuclear species of (-O)(2)center dot COHx=[0,1], are expected to form at particle terminations and surface defects. Finally, calculations suggested that bicarbonate is the dominant species on goethite, while a mixture of bicarbonate and carbonate species is present on lepidocrocite, a result stemming from different hydrogen bonding patterns at these mineral surfaces.

Place, publisher, year, edition, pages
Washington: American Chemical Society (ACS), 2013. Vol. 47, no 16, 9241-9248 p.
Keyword [en]
quantum chemical calculations, metal-oxide catalysts, goethite alpha-FeOOH, atmospheric CO2, vibrational frequencies, complex structures, hydroxyl-groups, adsorption, hematite, climate
National Category
Chemical Sciences Chemical Process Engineering Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-80753DOI: 10.1021/es4020597ISI: 000323471700031OAI: oai:DiVA.org:umu-80753DiVA: diva2:653324
Available from: 2013-10-03 Created: 2013-09-25 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Surface and Bulk Reactivity of Iron Oxyhydroxides: A Molecular Perspective
Open this publication in new window or tab >>Surface and Bulk Reactivity of Iron Oxyhydroxides: A Molecular Perspective
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron oxyhydroxide (FeOOH) mineral plays an important role in a variety of atmospheric, terrestrial and technological settings. Molecular resolution of reactions involving these minerals is thereby required to develop a fundamental understanding of their contributions in processes taking place in the atmosphere, Earth’s upper crust as well as the hydrosphere. This study resolves interactions involving four different types of synthetic FeOOH particles with distinct and well-defined surfaces, namely lath- and rod-shaped lepidocrocite (γ), goethite (α) and akaganéite (β). The surface and bulk reactivities of these particles are controlled by their distinct structures. When exposed to ambient atmospheric or aqueous conditions their surfaces are populated with different types of (hydr)oxo functional groups acting as reaction centers. These sites consist of hydroxyl groups that can be singly- (≡FeOH, -OH), doubly- (≡Fe2OH, μ-OH), or triply-coordinated (≡Fe3OH, μ3-OH) with underlying Fe atoms. Moreover, these sites exhibit different types, densities, distributions, as well as hydrogen bonding patterns on different crystal planes for each mineral. Knowledge of the types and distributions of hydroxyl groups on minerals with different surface structures is fundamental for building a molecular-scale understanding of processes taking place at FeOOH particle surfaces.

In this thesis, Fourier transform infrared (FTIR) spectroscopy was used to resolve the interactions between (hydr)oxo groups of FeOOH particles with (in)organic acids, salts, water vapor as well as carbon dioxide. The focus on such compounds was justified by their importance in natural environments. This thesis is based on 9 articles and manuscripts that can be found in the appendices.

FTIR spectroscopic signatures of hydroxyl groups in the bulk of well crystallized FeOOH minerals were characterized for structural differences and thermal stabilities. Those of akaganéite were particularly resolved for the variable bond strength of bulk hydroxyls induced by the incorporation of HCl through nanostructured channels at the terminations of the particles. FTIR bands of hydroxyl groups at all particle surfaces were monitored for responses to thermal gradients and proton loadings, providing experimental validation to previous theoretical accounts on surface site reactivity. This site reactivity was resolved further in the fluoride (F-) and phosphate (PO43-) ions adsorption study to follow the site selectivity for ligand-exchange reactions. These efforts showed that singly-coordinated groups are the primary adsorption centers for ligands, doubly-coordinated groups can only be exchanged at substantially higher ligand loadings, while triply coordinated groups are largely resilient to any ligand-exchange reaction.

These findings helped consolidate fundamental knowledge that can be used in investigating sorption processes involving atmospherically and geochemically important gases. The latter parts of this thesis were therefore focused on water vapor and carbon dioxide interactions with these FeOOH particles. These efforts showed how surface structure and speciation affect sorption loadings and configurations, as well as how water diffused into and through the akaganéite bulk. Hydrogen bonding is one of the most important forms of interactions between gas phase and minerals. It plays a crucial role in the formation of thin water films and in stabilizing surface (bi)carbonate species. The affinity of surface hydroxyl groups for water and carbon dioxide is strongly dependent on their abilities to form hydrogen bonds. These are controlled by coordination number and site accessibility/steric constraints. In agreement with the aforementioned ligand-exchange studies, surfaces dominated by singly coordinated groups have stronger ability to accumulate water layers than the ones terminated by groups of larger coordination number. Collectively, these efforts consolidate further the concept for structure-controlled reactivities in iron oxyhydroxides, and pave the way for new studies along such lines.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2013. 83 p.
Keyword
Iron oxyhydroxide, adsorption, water, carbon dioxide, FTIR, molecular resolution
National Category
Physical Chemistry Inorganic Chemistry Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:umu:diva-70289 (URN)978-91-7459-669-4 (ISBN)
Public defence
2013-06-05, KBC-huset, KB3B1, Umeå Universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-03-22Bibliographically approved

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Song, XiaoweiBoily, Jean-Francois

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