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Water film-mediated mineralogical transformations and photocatalytic reactions
Umeå University, Faculty of Science and Technology, Department of Chemistry. (J.-F. Boily)ORCID iD: 0000-0002-0118-8207
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mineral particles capture water vapor in the atmosphere in the form of water films that are only few monolayers thick. Water films form nanoscale hydration environments that mediate a wide range of important reactions in nature and technology. This thesis explored two important phenomena that commonly occur within the confines of water films: mineralogical transformations (Topic 1) and photocatalytic decomposition of organics (Topic 2). These transformations were chiefly identified by vibrational spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and (Transmission and Scanning) electron microscopy. Interpretations of reaction mechanisms were partially supported by chemometrics, kinetic and thermodynamic modeling, as well as molecular simulations.

Mineralogical transformations (Topic 1) resolved in this thesis involved the hydroxylation (Papers I, II) and carbonation (Paper III) of periclase (MgO), and the oxidation of rhodochrosite (MnCO3) (Paper IV). Two types of MgO nanocubes with contrasting physical properties were used to resolve nucleation- and diffusion-limited hydroxylation reactions to brucite and carbonation reactions to amorphous magnesium carbonate (AMC). While nucleation-limited reactions completely transformed (8 nm) small and aggregated MgO nanocubes to brucite, the reactions became diffusion-limited in larger (32 nm) monodispersed MgO nanocubes because of brucite surface nanocoatings (Paper I). Additionally, brucite nanosheets grew under (GPa-level) crystallization pressures because of the important volumetric expansion of the reaction, which took place in a complex network of microporosity between the small and within the larger MgO nanocubes. Brucite stacking mechanisms, explored in Paper II, focused on the early stages of MgO-water interaction in water films of different thicknesses. These were suggested to involve the stacking and (epitaxial-like) growth of precursor Mg(OH)2 nanosheets in water films. Carbonation reactions explored in Paper III completely hampered hydroxylation reactions studied in Papers I and II, and produced AMC nanocoatings grown over an unreacted MgO core. Finally, oxidation-driven reactions involving rhodochrosite in Paper IV produced MnO2, Mn3O4, and MnOOH nanocoatings with growth rates being scaled with water loadings.

Photocatalytic decomposition reactions of organics (Topic 2) were focused on the case of oxalate bound to TiO2 nanoparticles (Paper V). Photodecomposition rates scaled with humidity in oxygenated water films, and were explained by the combination of hole transfer (HT), ligand-to-metal charge transfer (LMCT), and the formation of hydroxyl radicals and reactive oxygen species. Decreasing rates in oxygen-free water films were, on the other hand, explained by water-driven charge localization, which eventually limited radical production and charge transfers via HT and LMCT. The reactions involved limited HT and LMCT processes which also competed with a charge recombination process across all humidity ranges.

This thesis provides new insight into two key types of transformations mediated by water films on minerals. This knowledge can be used to understand the reactivity of mineral (nano)particles exposed to variations in atmospheric humidity and oxygen content, which are both highly relevant to a wide range of settings in nature and technology. It can also advance new ideas in the study of mineral growth, especially within the confines of nanometer-thick water films.

Place, publisher, year, edition, pages
Umeå: Umeå University , 2023. , p. 70
Keywords [en]
mineral, water films, carbon dioxide, MgO, MnCO3, TiO2, hydroxylation, carbonation, oxidation, transformation, photocatalysis
National Category
Materials Chemistry Inorganic Chemistry Physical Chemistry Geochemistry
Research subject
Inorganic Chemistry; nanomaterials; Physical Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-213817ISBN: 9789180701501 (print)ISBN: 9789180701518 (electronic)OAI: oai:DiVA.org:umu-213817DiVA, id: diva2:1793679
Public defence
2023-09-29, Lilla hörsalen, KB.E3.01, KBC building, Linnaeus väg 10, Umeå, 09:00 (English)
Opponent
Supervisors
Part of project
Chemistry within the confines of mineral-bound thin water films, Swedish Research CouncilDirect Mineralization of Atmospheric CO2 by Enhanced Weathering, Swedish Research Council FormasRust in Ice: The Geochemistry of Iron in Freezing Water, Swedish Research Council
Funder
Swedish Research Council, 2016-03808Swedish Research Council, 2020-05853Swedish Research Council Formas, 2022-01246Available from: 2023-09-08 Created: 2023-09-01 Last updated: 2023-09-04Bibliographically approved
List of papers
1. MgO nanocube hydroxylation by nanometric water films
Open this publication in new window or tab >>MgO nanocube hydroxylation by nanometric water films
2023 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 15, no 24, p. 10286-10294Article in journal (Refereed) Published
Abstract [en]

Hydrophilic nanosized minerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry, we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)2) nanosheets. We show that three monolayer-thick water films first triggered the nucleation-limited growth of brucite, and that water film loadings continuously increased as newly-formed brucite nanosheets captured air moisture. Small (8 nm-wide) nanocubes were completely converted to brucite under this regime while growth on larger (32 nm-wide) nanocubes transitioned to a diffusion-limited regime when (∼0.9 nm-thick) brucite nanocoatings began hampering the flux of reactive species. We also show that intra- and inter-particle microporosity hosted a hydration network that sustained GPa-level crystallization pressures, compressing interlayer brucite spacing during growth. This was prevalent in aggregated 8 nm wide nanocubes, which formed a maze-like network of slit-shaped pores. By resolving the impact of nanocube size and microporosity on reaction yields and crystallization pressures, this work provides new insight into the study of mineralogical transformations induced by nanometric water films. Our findings can be applied to structurally related minerals important to nature and technology, as well as to advance ideas on crystal growth under nanoconfinement.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2023
National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-209178 (URN)10.1039/d2nr07140a (DOI)000988100900001 ()37194306 (PubMedID)2-s2.0-85160450592 (Scopus ID)
Funder
Swedish Research Council, 2020-05853Swedish Research Council Formas, 2022-01246
Available from: 2023-06-20 Created: 2023-06-20 Last updated: 2023-09-04Bibliographically approved
2. Water film-driven brucite nanosheet growth and stacking
Open this publication in new window or tab >>Water film-driven brucite nanosheet growth and stacking
2023 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 39, no 31, p. 11090-11098Article in journal (Refereed) Published
Abstract [en]

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH)2] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH)2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH)2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH)2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023
National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-212986 (URN)10.1021/acs.langmuir.3c01411 (DOI)001035007600001 ()37486722 (PubMedID)2-s2.0-85167468412 (Scopus ID)
Funder
Swedish Research Council, 2020-05853Swedish Research Council, 2022-06725Swedish Research Council Formas, 2022-01246National Supercomputer Centre (NSC), Sweden
Available from: 2023-08-21 Created: 2023-08-21 Last updated: 2023-09-04Bibliographically approved
3. CO2 mineralization by MgO nanocubes in nanometric water films
Open this publication in new window or tab >>CO2 mineralization by MgO nanocubes in nanometric water films
2023 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 15, no 38, p. 45055-45063Article in journal (Other academic) Published
Abstract [en]

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer to 4 monolayer-thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC reaction for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023
Keywords
air moisture, CO2, mineralization, magnesium oxide, magnesium carbonate, water films, nanomaterials
National Category
Materials Chemistry Inorganic Chemistry Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-213927 (URN)10.1021/acsami.3c10590 (DOI)001067290600001 ()37707796 (PubMedID)2-s2.0-85174704540 (Scopus ID)
Funder
Swedish Research Council, 2020-04853Swedish Research Council, 2016-03808Swedish Research Council Formas, 2022-01246
Note

Originally included in thesis in manuscript form. 

Available from: 2023-08-31 Created: 2023-08-31 Last updated: 2023-10-30Bibliographically approved
4. Water film-driven Mn (oxy)(hydr)oxide nanocoating growth on rhodochrosite
Open this publication in new window or tab >>Water film-driven Mn (oxy)(hydr)oxide nanocoating growth on rhodochrosite
2022 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 329, p. 87-105Article in journal (Refereed) Published
Abstract [en]

Minerals exposed to moist air stabilize thin water films that drive a score of chemical reactions of great importance to water-unsaturated terrestrial environments. In this study, we identified Mn (oxy)(hydr)oxide nanocoatings formed by the dissolution, oxidation and precipitation of Mn in oxygenated water films grown on rhodochrosite (MnCO3) microparticles. Nanocoatings that could be identified by vibrational spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and (scanning and transmission) electron microscopy formed in water films containing the equivalent of at least 7 monolayers (∼84 H2O/nm2). These films were formed by exposing microparticles to moist air with at least 50% relative humidity (RH). Films of neutral pH reacted up to 14% of the MnII located in the topmost ∼5 nm region of the microparticles in atmospheres of up to 90% RH for 7 d. These reactions produced MnOOH, birnessite (MnO2) and hausmannite (Mn3O4) nanoparticles of low crystallinity, while exposure to atmospheric air for 1 yr. converted only 2% of MnII in this region to MnOOH. In contrast, reactions in alkaline water films converted up to ∼75% of the MnII but only after 16 d of reaction. These films produced MnOOH and MnO2 of low crystallinity, as well as crystalline hausmannite. Kinetic modeling of the time-resolved growth of the Mn[sbnd]O stretching vibrational bands of these nanocoatings revealed two concurrent reaction processes. A 1rst-order process was assigned to nucleation events terminating only after a few hours, and a 0-order process was assigned to the sustained growth of nanocoatings from these nuclei over longer reaction time. By identifying nanocoatings formed by water film-driven reactions on rhodochrosite, our study adds new insight into mineralogical transformations relevant to anoxic–oxic boundaries in water-unsaturated environments.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Oxidation, Precipitation, Rhodochrosite, Water films
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-203210 (URN)10.1016/j.gca.2022.05.019 (DOI)000818523300006 ()2-s2.0-85132406337 (Scopus ID)
Funder
Swedish Research Council, 2020-04853Swedish Research Council, 2016-03808Umeå University
Available from: 2023-01-18 Created: 2023-01-18 Last updated: 2023-09-04Bibliographically approved
5. Water film-mediated photocatalytic oxidation of oxalate on TiO2
Open this publication in new window or tab >>Water film-mediated photocatalytic oxidation of oxalate on TiO2
2024 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 432, article id 115425Article in journal (Refereed) Published
Abstract [en]

Water films on minerals under humid environment can be photocatalytic hotspots when exposed to sunlight or artificial sources of ultraviolet (and visible) radiation. In this study, we resolved the water film-mediated photocatalysis of oxalate adsorbed on TiO2 using in situ infrared spectroscopy. We found that 0.5 to 4 monolayer- (ML) thick water films enhanced the photodecomposition rates of oxalate under 21 kPa O2. We explained this through the combined actions of direct hole transfer, ligand-to-metal-charge transfer, as well as the production of hydroxyl radicals and reactive oxygen species. Rates were, however, substantially slower in the absence of O2 because charge recombination, together with water film-mediated charge localization, disrupted hole transfer and hydroxyl radical production. Our work adds insight into the impact of humidity on controlling important photocatalytic processes in nature (drying soils, atmospheric aerosols), and technology (water and air treatment).

Place, publisher, year, edition, pages
Academic Press, 2024
Keywords
photocatalysis, TiO2, water films, oxalate, reactive oxygen species, hydroxyl radicals.
National Category
Inorganic Chemistry Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-213926 (URN)10.1016/j.jcat.2024.115425 (DOI)2-s2.0-85188433491 (Scopus ID)
Funder
Swedish Research Council, 2020-04853Swedish Research Council, 2016-03808Swedish Research Council Formas, 2022-01246
Note

Originally included in thesis in manuscript form. 

Available from: 2023-08-31 Created: 2023-08-31 Last updated: 2024-04-15Bibliographically approved

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