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Surface and Bulk Reactivity of Iron Oxyhydroxides: A Molecular Perspective
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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 [en]
Iron oxyhydroxide, adsorption, water, carbon dioxide, FTIR, molecular resolution
National Category
Physical Chemistry Inorganic Chemistry Geosciences, Multidisciplinary
Identifiers
URN: urn:nbn:se:umu:diva-70289ISBN: 978-91-7459-669-4 (print)OAI: oai:DiVA.org:umu-70289DiVA: diva2:621003
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
List of papers
1. Variable Hydrogen Bond Strength in Akaganeite
Open this publication in new window or tab >>Variable Hydrogen Bond Strength in Akaganeite
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 3, 2303-2312 p.Article in journal (Refereed) Published
Abstract [en]

Akaganeite (beta-FeOOH) is a chloride-bearing iron oxy-hydroxide with a hollandite-type structure. This high specific surface area mineral has been the object of numerous studies given its high reactivity and involvement in natural and industrial processes. The important ion exchange attributes of this mineral involve similar to 0.4 x 0.4 nm wide channels in which chloride ions are stabilized by hydrogen bonding from bulk OH groups. This work provides further details on the relationship between bulk chloride ion loadings and hydrogen bond strengths. Molecular dynamics calculations were first carried out on chloride-free and bearing lattices to build a conceptual model for possible interactions in the akaganeite bulk. Experimental work was thereafter carried out on synthetic acicular particles (7 x 80 to 11 x 110 nm) reacted to aqueous solutions of HCl, then dried under dry N-2(g). These samples were studied by Fourier transform infrared spectroscopy, temperature-programmed desorption, X-ray photoelectron spectroscopy, and X-ray powder diffraction as well as transmission electron spectroscopy. Results collectively show that Cl/Fe molar ratios increasing from 0.169 up to 0.442 induce important changes in the hydrogen bonding environment of bulk hydroxyls. This can specifically be seen through shifts in bulk O-H stretching frequencies from 3496/3395 to 3470/3350 cm(-1). These changes are associated with a substantial shortening of particle lengths (97 to 45 nm), expansion of crystallographic lattice size (up to 0.9%), and increases in median thermal dehydroxylation temperatures (260 to 305 degrees C). Our work thereby highlights important variations in physicochemical attributes of akaganeite particles reacted with HCl. Such variations should consequently be considered in settings involving submicrometer-sized akaganeite particles.

Place, publisher, year, edition, pages
Washington DC: American Ceramic Society, 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-52653 (URN)10.1021/jp2097108 (DOI)000299584400035 ()
Available from: 2012-03-01 Created: 2012-02-28 Last updated: 2017-12-07Bibliographically approved
2. Competitive ligand exchange on akaganéite surfaces enriches bulk chloride loadings
Open this publication in new window or tab >>Competitive ligand exchange on akaganéite surfaces enriches bulk chloride loadings
2012 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 376, no 1, 331-3 p.Article in journal (Refereed) Published
Abstract [en]

Akaganéite (β-FeOOH) is a nanosized iron oxyhydroxide mineral with a hollandite structure containing chloride ions in 0.4×0.4nm wide channels. Proton and chloride co-sorption into these channels induces variations in bulk O-H stretching vibrations, crystallographic lattice size, and thermal stability, as a result of hydrogen bond formation with chloride ions. In this work, we show that chloride ions bound to akaganéite surfaces can be dislodged into aqueous solutions by competitive adsorption of foreign ions and then transferred alongside co-sorbed protons into the akaganéite bulk. Fourier transform infrared and X-ray photoelectron spectroscopic measurements show that HClO(4), H(2)SO(4), and benzoic acid, and thereby, many other anions of various charge-to-size ratios can all effectively contribute to this phenomenon. This linked surface-bulk reactivity should be accounted for in mixed anion systems containing akaganéite-like materials.

Place, publisher, year, edition, pages
Elsevier, 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-54395 (URN)10.1016/j.jcis.2012.03.006 (DOI)22459024 (PubMedID)
Available from: 2012-04-25 Created: 2012-04-25 Last updated: 2017-12-07Bibliographically approved
3. Surface Hydroxyl Identity and Reactivity in Akaganeite
Open this publication in new window or tab >>Surface Hydroxyl Identity and Reactivity in Akaganeite
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 34, 17036-17045 p.Article in journal (Refereed) Published
Abstract [en]

Hydroxyl groups on surfaces of well-defined akaganeite (beta-FeOOH) particles were identified by Fourier transform infrared spectroscopy. These efforts, assisted by molecular dynamics simulations, enabled the extraction of spectral signatures for these groups of the dominant (100), (001), and (010) crystallographic planes. Band assignments were supported by spectral variations induced by proton and chloride adsorption as well as temperature-programmed desorption. Molecular dynamics simulations were used to determine patterns and free energies of formation of hydrogen bonds. Surface Fe-O distances as well as hydrogen-bond numbers were also used to predict proton affinities. All spectral component concentrations display highly comparable responses to proton loadings with those of other FeOOH minerals previously studied with our coupled experimental-theoretical approach. These similarities underpin common thermodynamic stabilities for hydroxyls of a given Fe nuclearity on different planes of different minerals.

Place, publisher, year, edition, pages
Washington DC: American Chemical Society, 2011
National Category
Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-46679 (URN)10.1021/jp204550k (DOI)
Available from: 2011-09-13 Created: 2011-09-09 Last updated: 2017-12-08Bibliographically approved
4. Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces
Open this publication in new window or tab >>Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces
2012 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 8, 2579-86 p.Article in journal (Refereed) Published
Abstract [en]

Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.

Keyword
Bio-Oss;bone graft;cell culture;ICP-OES;interface;mineralization;xenograft
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-52406 (URN)10.1039/c2cp22715k (DOI)22261841 (PubMedID)
Available from: 2012-02-20 Created: 2012-02-20 Last updated: 2017-12-07Bibliographically approved
5. Identification of fluoride and phosphate binding sites at FeOOH surfaces
Open this publication in new window or tab >>Identification of fluoride and phosphate binding sites at FeOOH surfaces
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 41, 21939-21947 p.Article in journal (Refereed) Published
Abstract [en]

Iron oxyhydroxide minerals occur widely in nature and play important roles in environmental and industrial processes. Owing to their high reactivity, these minerals can act as sinks and/or transformation centers for a variety of inorganic and organic ions. Interfacial reactions are often mediated by surface (hydr)oxo groups. These groups can be singly, doubly, or triply coordinated with respect to underlying Fe atoms. In order to investigate the reactivity of these differently coordinated groups, Fourier transform infrared (FTIR) spectroscopy was used to examine adsorption products formed on iron oxyhydroxide surfaces. The absence of water was required to probe the O-H stretching region after initial reactions in aqueous media. This work was specifically focused on synthetic, submicrometer-sized lepidocrocite and goethite particles reacted with aqueous solutions of sodium fluoride and monosodium phosphate. Langmuir-Freundlich adsorption isotherms were calibrated on adsorption data in aqueous media at various pH values to obtain the maximum sorption densities for these ions under these conditions. FTIR measurements of the resulting solids dried under N-2(g) show that fluoride and phosphate ions preferentially exchange with singly coordinated hydroxyls. Doubly coordinated groups can, however, be exchanged with fluoride ions at relatively high loading densities. Triply coordinated groups remain, in contrast, resilient to exchange. They may, however, stabilize phosphate species by hydrogen bonding. These findings add further constraints to our understanding of adsorption reactions and to the formulation of molecularly adequate thermodynamic models.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-61771 (URN)10.1021/jp3083776 (DOI)000309902100037 ()
Available from: 2012-11-27 Created: 2012-11-26 Last updated: 2017-12-07Bibliographically approved
6. Water vapor interactions with FeOOH particle surfaces
Open this publication in new window or tab >>Water vapor interactions with FeOOH particle surfaces
2013 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 560, 1-9 p.Article in journal (Refereed) Published
Abstract [en]

Interactions between iron (oxyhydr)oxide particle surfaces and water are of fundamental importance to natural and technological processes. In this Letter, we probe the interactions between submicron-sized lepidocrocite (γ-FeOOH) surfaces and gaseous water using Fourier transform infrared spectroscopy. Formation of hydrogen bonds between different lepidocrocite surface OH functional groups and water was specifically monitored in the O–H stretching region. Molecular dynamics simulations of the dominant crystallographic terminations of these particles provided insights into interfacial water structures and hydrogen bonding networks. Theoretical power spectra were moreover used to validate interpretations of experimental spectra. This Letter constrains our understanding of incipient water adsorption reactions leading to thermodynamically stable and reversible thin water films at FeOOH particle surfaces. It also suggests that these water layers are structurally analogous precursors to those occurring at a FeOOH surfaces contacted with liquid water.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-67779 (URN)10.1016/j.cplett.2012.12.048 (DOI)
Available from: 2013-04-03 Created: 2013-04-03 Last updated: 2017-12-06Bibliographically approved
7. Water vapor adsorptions on goethite surfaces
Open this publication in new window or tab >>Water vapor adsorptions on goethite surfaces
(English)Manuscript (preprint) (Other academic)
National Category
Geochemistry Environmental Sciences
Identifiers
urn:nbn:se:umu:diva-70287 (URN)
Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-03-24Bibliographically approved
8. Water Vapor Diffusion into a Nanostructured Iron Oxyhydroxide
Open this publication in new window or tab >>Water Vapor Diffusion into a Nanostructured Iron Oxyhydroxide
2013 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 52, no 12, 7107-7113 p.Article in journal (Refereed) Published
Abstract [en]

Water diffusion through 0.4 nm × 0.4 nm wide tunnels of synthesized akaganéite (β-FeOOH) nanoparticles was studied by a coupled experimental–molecular modeling approach. A sorption isotherm model obtained from quartz crystal microbalance measurements suggests that the akaganéite bulk can accommodate a maximum of 22.4 mg of water/g (44% bulk site occupancy) when exposed to atmospheres of up to 16 Torr water vapor. Fourier transform infrared spectroscopy also showed that water molecules interact with (hydr)oxo groups on both the akaganéite bulk and surface. Diffusion reactions through the akaganéite bulk were confirmed through important changes in the hydrogen-bonding environment of bulk hydroxyl groups. Molecular dynamics simulations showed that water molecules are localized in cavities that are bound by eight hydroxyl groups, forming short-lived (<0.5 ps) hydrogen bonds with one another. Diffusion coefficients of water are three orders of magnitude lower than they are in liquid water (D = 0.0–11.1 × 10–12 m2·s–1), whereas large integral rotational correlation times are 4 to 15 times higher (τr = 8.4–31.8 ps). Moreover, both of these properties are strongly loading-dependent. The simulations of the interface between the water vapor phase and the (010) surface plane of the akaganéite, where tunnel openings are exposed, revealed sluggish rates of incorporation between interfacial water species and their tunnel counterparts. The presence of defects in the synthesized particles are suspected to contribute to different diffusion rates in the laboratory when compared to those observed in pristine crystalline materials, as studied by molecular modeling.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-70285 (URN)10.1021/ic400661d (DOI)000320689200040 ()
Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
9. Carbon dioxide binding at dry FeOOH mineral surfaces: evidence for structure-controlled speciation
Open this publication in new window or tab >>Carbon dioxide binding at dry FeOOH mineral surfaces: evidence for structure-controlled speciation
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
Keyword
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:nbn:se:umu:diva-80753 (URN)10.1021/es4020597 (DOI)000323471700031 ()
Available from: 2013-10-03 Created: 2013-09-25 Last updated: 2017-12-06Bibliographically approved

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