Nancy Washton | Pacific Northwest National Laboratory (original) (raw)

Papers by Nancy Washton

Research paper thumbnail of Dissolution rate dependence on reactive surface sites

Research paper thumbnail of Solid-State NMR Studies of Reactive Hydroxyls on the Surfaces of Natural Samples: New Insights Into Dissolution Mechanisms of Oxide Surfaces

Research paper thumbnail of Multi-Step Crystallization and Chemical Evolution of Sodium Yttrium Fluoride

Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are co... more Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are commonly observed both in the laboratory and in nature. While this pathway may require chemical reactions, subsequent nucleation and growth are often considered as separate, discrete events from the reaction itself. Recent work has also shown a distinct intermediate step involving the formation of an amorphous aggregate. Here we report a novel four-step mechanism in the aqueous synthesis of sodium yttrium fluoride involving 1) the segregation of aqueous ions into a dense liquid phase, 2) the formation of an amorphous aggregate, 3) nucleation of a cubic YF3 phase, and 4) subsequent solid-state diffusion of sodium and fluoride ions to form a final NaYF4 phase. The final step involves a continuous, gradual change of the solid phase’s chemical stoichiometry from YF3 toward NaYF4. Unlike previously studied nucleation and growth mechanisms, the stoichiometry of the final solid phase evolves thro...

Research paper thumbnail of Synthesis and evaluation of Cu-SAPO-34 catalysts for ammonia selective catalytic reduction. 1. Aqueous solution ion exchange

ACS Catalysis, 2013

ABSTRACT SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs)... more ABSTRACT SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs). Cu-SAPO-34 catalysts are prepared via aqueous solution ion exchange (IE). Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties are examined using standard ammonia selective catalytic reduction (NH3–SCR) and ammonia oxidation reactions. During solution IE, different SAPO-34 samples undergo different extent of structural damage via irreversible hydrolysis. Si content within the samples (i.e., Al–O–Si bond density) and framework stress are key factors that affect irreversible hydrolysis. Even using very dilute Cu acetate solutions, it is not possible to generate Cu-SAPO-34 samples with only isolated Cu2+ ions. Small amounts of CuOx species always coexist with isolated Cu2+ ions. Highly active and selective Cu-SAPO-34 catalysts for NH3-SCR are readily generated using this synthesis protocol, even for SAPO-34 samples that degrade substantially during solution IE. High-temperature aging is found to improve the catalytic performance. This is likely due to reduction of intracrystalline mass-transfer limitations via formation of additional porosity in the highly defective SAPO-34 particles formed after IE.

Research paper thumbnail of Surface characterization of nanomaterials and nanoparticles: Important needs and challenging opportunities

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013

Research paper thumbnail of Effects of Si/Al ratio on Cu/SSZ-13 NH3-SCR catalysts: Implications for the active Cu species and the roles of Brønsted acidity

Journal of Catalysis, 2015

Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporatio... more Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporation via solution ion exchange. The implications of varying Si/Al ratios on the nature of the multiple Cu species that can be present in the SSZ-13 zeolite are a major focus of this work, as highlighted by the results of a variety of catalyst characterization and reaction kinetics measurements. Specifically, catalysts were characterized with surface area/pore volume measurements, temperature programmed reduction by H 2 (H 2-TPR), NH 3 temperature programmed desorption (NH 3-TPD), and DRIFTS and solid-state nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties were examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH 3-SCR) reactions on selected catalysts under differential conditions. Besides indicating the possibility of multiple active Cu species for these reactions, the measurements are also used to untangle some of the complexities caused by the interplay between redox of Cu ion centers and Brønsted acidity. All three reactions appear to follow a redox reaction mechanism, yet the roles of Brønsted acidity are quite different. For NO oxidation, increasing Si/Al ratio lowers Cu redox barriers, thus enhancing reaction rates. Brønsted acidity appears to play essentially no role for this reaction. For standard NH 3-SCR, residual Brønsted acidity plays a significant beneficial role at both low-and high-temperature regimes. For NH 3 oxidation, no clear trend is observed suggesting both Cu ion center redox and Brønsted acidity play important and perhaps competing roles.

Research paper thumbnail of Protein-mineral interactions: molecular dynamics simulations capture importance of variations in mineral surface composition and structure

Langmuir : the ACS journal of surfaces and colloids, Jan 31, 2016

Molecular dynamics simulations, conventional and metadynamics, were performed to determine the in... more Molecular dynamics simulations, conventional and metadynamics, were performed to determine the interaction of model protein Gb1 over kaolinite (001), Na+-montmorillonite (001), Ca2+-montmorillonite (001), goethite (100), and Na+-birnessite (001) mineral surfaces. Gb1, a small (56 residue) protein with a well-characterized solution-state nuclear magnetic resonance (NMR) structure and having α-helix, four-fold β-sheet, and hydrophobic core features, is used as a model protein to study protein soil mineral interactions and gain insights on structural changes and potential degradation of protein. From our simulations, we observe little change to the hydrated Gb1 structure over the kaolinite, montmorillonite, and goethite surfaces relative to its solvated structure without these mineral surfaces present. Over the Na+-birnessite basal surface, however, the Gb1 structure is highly disturbed as a result of interaction with this birnessite surface. Unraveling of the Gb1 β-sheet at specific t...

Research paper thumbnail of EMSL and Institute for Integrated Catalysis (IIC) Catalysis Workshop

DOE Scientific and Technical Information. DOE Scientific and Technical Information. ...

Research paper thumbnail of Measurement of the Reactive Surface Area of Clay Minerals Using Solid-State NMR Studies of a Probe Molecule

Journal of Physical Chemistry C, 2010

Understanding the surface reactivity of clay minerals is necessary for accurate prediction of nat... more Understanding the surface reactivity of clay minerals is necessary for accurate prediction of natural weathering rates due to the ubiquity of clays in the environment as weathering products of primary minerals. However, the reactivity of the heterogeneous surfaces of a clay can be difficult to characterize as clay mineral edge sites often react at different rates or via different mechanisms than sites on the basal planes. Ultimately, a method is needed to probe quantitatively the reactive surface sites in order to predict clay mineral dissolution rates. In this study, solid-state NMR spectroscopy has been utilized to investigate surface hydroxyl species and their relation to clay surface reactivity. The surfaces of two kaolinite samples (KGa-1b and KGa-2) and two montmorillonite samples (STx-1b and SWy-2) were reacted with the probe molecule (3,3,3trifluoropropyl)dimethylchlorosilane (TFS), which binds selectively to reactive non-hydrogen bonded Q 3 Si hydroxyl sites. Quantification of 19 F spins in the TFS-treated samples using 19 F magic angle spinning NMR peak intensities provides a sensitive measure of the number of reactive hydroxyl sites on a mass normalized (per gram) basis. The reactive surface site densities of KGa-1b and KGa-2 were found to be proportional to published atomic force microscopy edge site fractions. An example from KGa-1b dissolution after 10 days at pH 2.9 and 21°C revealed no significant change in Brunauer-Emmett-Teller specific surface area, but a 25% decrease in reactive surface site density. We posit this site density determined by solid-state NMR is proportional to the reactive surface area of each clay mineral and its use in future dissolution studies is warranted to investigate how changes in reactive surface area can be tied to decreases in rates of silicon and aluminum release into solution.

Research paper thumbnail of Molecular-level control of oxide dissolution: Insights from NMR and computational chemistry

Research paper thumbnail of Study of a Family of 40 Hydroxylated β-Cristobalite Surfaces Using Empirical Potential Energy Functions

Journal of Physical Chemistry C, 2007

We present a study of a family of 40 unique hydroxylated -cristobalite surfaces generated by clea... more We present a study of a family of 40 unique hydroxylated -cristobalite surfaces generated by cleaving the -cristobalite unit cell along crystallographic planes to include a combination of several low Miller index surfaces. The surface silicon atoms are quantified as percentages of Q 2 and Q 3 centers based on their polymeric state. We find that Q 3 centers are, on average, three times more abundant than Q 2 centers. To study the surface properties, we use two different empirical potential energy functions: the multibody potential proposed by Fueston and Garofalini (J. Phys. Chem. 1990, 94, 5351) and the newly developed CHARMM potential by Lopes et al. (J. Phys. Chem. B 2006, 110, 2782. Our results for the surface water interactions are in good agreement with previous ab initio theoretical studies by Yang et al. (Phys. ReV. B 2006, 73, 146102) for the (100) surface. We find that the most commonly studied family of {100} surfaces is unique and is the only surface with 100% abundance of Q 2 centers, whereas there are nine examples of surfaces with 100% Q 3 centers. The predominantly pure Q 3 surfaces show no hydrogen bonding with the neighboring surface hydroxyl groups and weakly adsorb water overlayers. This is markedly different from the {100} pure Q 2 surface that shows strong hydrogen bonding within the surface groups and with water. As compared to all the surfaces studied in this work, we find that the {100} surfaces are not true representations of the overall -cristobalite surfaces and their properties. We find that the two main factors that characterize the physical properties of silica surfaces are the polymeric state of the silicon atom and surface topography. Two types of pure Q 3 crystallographic planes have been identified and are labeled as Q 3A and Q 3B based on the differences in their topological features. Using the {111} and {011} surfaces as examples, we show that the Q 3A surface adsorbs H 2 O that forms a stable monolayer, but the Q 3B surface fails to form a stable H 2 O overlayer. Other crystallographic planes with different ratios of Q 2 to Q 3 centers are contrasted by the differences in the hydrogenbonding network and their ability to form ordered H 2 O overlayers.

Research paper thumbnail of Solid-State NMR Studies of Reactive Hydroxyls on the Surfaces of Natural Samples: New Insights Into Dissolution Mechanisms of Oxide Surfaces

Surface hydroxyl groups are an important controlling factor for the interfacial reactivity of oxi... more Surface hydroxyl groups are an important controlling factor for the interfacial reactivity of oxide surfaces. However, quantification of the hydroxyl content at oxide surfaces is challenging when presented with low surface area or complex (and changing) morphologies, such as those commonly present in environmentally relevant samples. We have recently developed a 19F magic angle spinning nuclear magnetic resonance (NMR) approach to the quantification of surface hydroxyl groups by covalently attaching a 19F containing probe molecule to hydroxyl moieties on the oxide surface. Two suites of rhyolitic glasses were investigated to determine the appropriateness of utilizing our surface modification technique to accurately identify reactive surface species involved in chemical reactions (e.g. dissolution, precipitation). The first suite consisted of two sets of seven rhyolitic and dacitic glasses of Icelandic origin: both pristine and acid treated glasses were studied. For both pre and post-dissolution samples a clear correlation exists between the mass normalized dissolution rate and the number of hydroxyl groups per gram as measured by our NMR-detected surface modification technique, with the exception of one post-dissolution sample (SLN). The SLN sample showed evidence of advanced weathering as compared to the six other samples within the same suite. To determine whether physical and/or chemical weathering time impacted the number of measured hydroxyl groups, a second suite of rhyolitic glasses were obtained from a chronosequence (Kozushima, Japan). The four samples differ with respect to age (1.1, 1.8, 26, 52 ka) as well as the presence of weathering products (halloysite). Prior to surface modification, the weathering products were removed via ultra-sonication. The number of measured hydroxyl groups increases with increasing age, whether normalized to mass or BET generated surface area. These data are in agreement with the overall change in hydroxyl number per gram measured on the post-dissolution Icelandic suite, which also showed an increase in the number of hydroxyls after dissolution, normalized either to mass or BET surface area. Mechanistic models for the relevance of these measurements to the understanding of complex aluminosilicate surfaces in the environment will also be presented.

Research paper thumbnail of Toward the Integration of Liquid and Solid-State NMR Across the Undergraduate Curriculum

Cheminform, 2007

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Research paper thumbnail of Probing the molecular-level control of aluminosilicate dissolution: A sensitive solid-state NMR proxy for reactive surface area

Geochimica Et Cosmochimica Acta, 2008

For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalen... more For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalent attachment of the fluorine-containing (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS) probe molecule were measured by quantitative 19F nuclear magnetic resonance (NMR) spectroscopy. The first set of samples consists of six rhyolitic and dacitic glasses originating from volcanic activity in Iceland and one rhyolitic glass from the Bishop Tuff, CA. Due to differences in the reactive species present on the surfaces of these glasses, variations in the rate of acid-mediated dissolution (pH 4) for samples in this suite cannot be explained by variations in geometric or BET-measured surface area. In contrast, the rates scale directly with the surface density of TFS-reactive sites as measured by solid-state NMR. These data are consistent with the inference that the TFS-reactive M-OH species on the glass surface, which are known to be non-hydrogen-bonded Q 3 groups, represent loci accessible to and affected by proton-mediated dissolution. The second suite of samples, originating from a chronosequence in Kozushima, Japan, is comprised of four rhyolites that have been weathered for 1.1, 1.8, 26, and 52 ka. The number of TFS-reactive sites per gram increases with duration of weathering in the laboratory for the "Icelandic" samples and with duration of field weathering for both "Icelandic" and Japanese samples. One hypothesis is consistent with these and published modeling, laboratory, and field observations: over short timescales, dissolution is controlled by fast-dissolving sites, but over long timescales, dissolution is controlled by slower-dissolving sites, the surface density of which is proportional to the number of TFS-reactive Q 3 sites. These latter sites are not part of a hydrogen-bonded network on the surface of the glasses, and measurement of their surface site density allows predictions of trends in reactive surface area. The TFS treatment method, which is easily monitored by quantitative 19F solid-state NMR, therefore provides a chemically specific and quantifiable proxy to understand the nature of how sites on dissolving silicates control dissolution. Furthermore, 27Al NMR techniques are shown here to be useful in identifying clays on the glass surfaces, and these methods are therefore effective for quantifying concentrations of weathering impurities. Our interpretations offer a testable hypothesis for the mechanism of proton-promoted dissolution for low-iron aluminosilicate minerals and glasses and suggest that future investigations of reactive surfaces with high-sensitivity NMR techniques are warranted.

Research paper thumbnail of Dissolution rate dependence on reactive surface sites

Research paper thumbnail of NMR and IR Study of Fluorobenzene and Hexafluorobenzene Adsorbed on Alumina

Research paper thumbnail of Probing the molecular-level control of aluminosilicate dissolution: A sensitive solid-state NMR proxy for reactive surface area

Geochimica Et Cosmochimica Acta, 2008

For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalen... more For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalent attachment of the fluorinecontaining (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS) probe molecule were measured by quantitative 19 F nuclear magnetic resonance (NMR) spectroscopy. The first set of samples consists of six rhyolitic and dacitic glasses originating from volcanic activity in Iceland and one rhyolitic glass from the Bishop Tuff, CA. Due to differences in the reactive species present on the surfaces of these glasses, variations in the rate of acid-mediated dissolution (pH 4) for samples in this suite cannot be explained by variations in geometric or BET-measured surface area. In contrast, the rates scale directly with the surface density of TFS-reactive sites as measured by solid-state NMR. These data are consistent with the inference that the TFS-reactive M-OH species on the glass surface, which are known to be non-hydrogen-bonded Q 3 groups, represent loci accessible to and affected by proton-mediated dissolution. The second suite of samples, originating from a chronosequence in Kozushima, Japan, is comprised of four rhyolites that have been weathered for 1.1, 1.8, 26, and 52 ka. The number of TFS-reactive sites per gram increases with duration of weathering in the laboratory for the ''Icelandic'' samples and with duration of field weathering for both ''Icelandic'' and Japanese samples. One hypothesis is consistent with these and published modeling, laboratory, and field observations: over short timescales, dissolution is controlled by fast-dissolving sites, but over long timescales, dissolution is controlled by slower-dissolving sites, the surface density of which is proportional to the number of TFS-reactive Q 3 sites. These latter sites are not part of a hydrogen-bonded network on the surface of the glasses, and measurement of their surface site density allows predictions of trends in reactive surface area. The TFS treatment method, which is easily monitored by quantitative 19 F solidstate NMR, therefore provides a chemically specific and quantifiable proxy to understand the nature of how sites on dissolving silicates control dissolution. Furthermore, 27 Al NMR techniques are shown here to be useful in identifying clays on the glass surfaces, and these methods are therefore effective for quantifying concentrations of weathering impurities. Our interpretations offer a testable hypothesis for the mechanism of proton-promoted dissolution for low-iron aluminosilicate minerals and glasses and suggest that future investigations of reactive surfaces with high-sensitivity NMR techniques are warranted.

Research paper thumbnail of Effects of Systematic Methyl Substitution of Metal (III) Tris( n Methyl8-Quinolinolato) Chelates on Material Properties for Optimum Electroluminescence Device Performance

Journal of The American Chemical Society, 2001

... Linda S. Sapochak,* † Asanga Padmaperuma, † Nancy Washton, † Flocerfida Endrino, †Gregory T. ... more ... Linda S. Sapochak,* † Asanga Padmaperuma, † Nancy Washton, † Flocerfida Endrino, †Gregory T. Schmett, † Jeffrey Marshall, † Daniel Fogarty, † Paul E. Burrows, ‡ § and Stephen R. Forrest ‡. Contribution from the Department ...

Research paper thumbnail of Dissolution rate dependence on reactive surface sites

Research paper thumbnail of Solid-State NMR Studies of Reactive Hydroxyls on the Surfaces of Natural Samples: New Insights Into Dissolution Mechanisms of Oxide Surfaces

Research paper thumbnail of Multi-Step Crystallization and Chemical Evolution of Sodium Yttrium Fluoride

Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are co... more Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are commonly observed both in the laboratory and in nature. While this pathway may require chemical reactions, subsequent nucleation and growth are often considered as separate, discrete events from the reaction itself. Recent work has also shown a distinct intermediate step involving the formation of an amorphous aggregate. Here we report a novel four-step mechanism in the aqueous synthesis of sodium yttrium fluoride involving 1) the segregation of aqueous ions into a dense liquid phase, 2) the formation of an amorphous aggregate, 3) nucleation of a cubic YF3 phase, and 4) subsequent solid-state diffusion of sodium and fluoride ions to form a final NaYF4 phase. The final step involves a continuous, gradual change of the solid phase’s chemical stoichiometry from YF3 toward NaYF4. Unlike previously studied nucleation and growth mechanisms, the stoichiometry of the final solid phase evolves thro...

Research paper thumbnail of Synthesis and evaluation of Cu-SAPO-34 catalysts for ammonia selective catalytic reduction. 1. Aqueous solution ion exchange

ACS Catalysis, 2013

ABSTRACT SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs)... more ABSTRACT SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs). Cu-SAPO-34 catalysts are prepared via aqueous solution ion exchange (IE). Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties are examined using standard ammonia selective catalytic reduction (NH3–SCR) and ammonia oxidation reactions. During solution IE, different SAPO-34 samples undergo different extent of structural damage via irreversible hydrolysis. Si content within the samples (i.e., Al–O–Si bond density) and framework stress are key factors that affect irreversible hydrolysis. Even using very dilute Cu acetate solutions, it is not possible to generate Cu-SAPO-34 samples with only isolated Cu2+ ions. Small amounts of CuOx species always coexist with isolated Cu2+ ions. Highly active and selective Cu-SAPO-34 catalysts for NH3-SCR are readily generated using this synthesis protocol, even for SAPO-34 samples that degrade substantially during solution IE. High-temperature aging is found to improve the catalytic performance. This is likely due to reduction of intracrystalline mass-transfer limitations via formation of additional porosity in the highly defective SAPO-34 particles formed after IE.

Research paper thumbnail of Surface characterization of nanomaterials and nanoparticles: Important needs and challenging opportunities

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013

Research paper thumbnail of Effects of Si/Al ratio on Cu/SSZ-13 NH3-SCR catalysts: Implications for the active Cu species and the roles of Brønsted acidity

Journal of Catalysis, 2015

Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporatio... more Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporation via solution ion exchange. The implications of varying Si/Al ratios on the nature of the multiple Cu species that can be present in the SSZ-13 zeolite are a major focus of this work, as highlighted by the results of a variety of catalyst characterization and reaction kinetics measurements. Specifically, catalysts were characterized with surface area/pore volume measurements, temperature programmed reduction by H 2 (H 2-TPR), NH 3 temperature programmed desorption (NH 3-TPD), and DRIFTS and solid-state nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties were examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH 3-SCR) reactions on selected catalysts under differential conditions. Besides indicating the possibility of multiple active Cu species for these reactions, the measurements are also used to untangle some of the complexities caused by the interplay between redox of Cu ion centers and Brønsted acidity. All three reactions appear to follow a redox reaction mechanism, yet the roles of Brønsted acidity are quite different. For NO oxidation, increasing Si/Al ratio lowers Cu redox barriers, thus enhancing reaction rates. Brønsted acidity appears to play essentially no role for this reaction. For standard NH 3-SCR, residual Brønsted acidity plays a significant beneficial role at both low-and high-temperature regimes. For NH 3 oxidation, no clear trend is observed suggesting both Cu ion center redox and Brønsted acidity play important and perhaps competing roles.

Research paper thumbnail of Protein-mineral interactions: molecular dynamics simulations capture importance of variations in mineral surface composition and structure

Langmuir : the ACS journal of surfaces and colloids, Jan 31, 2016

Molecular dynamics simulations, conventional and metadynamics, were performed to determine the in... more Molecular dynamics simulations, conventional and metadynamics, were performed to determine the interaction of model protein Gb1 over kaolinite (001), Na+-montmorillonite (001), Ca2+-montmorillonite (001), goethite (100), and Na+-birnessite (001) mineral surfaces. Gb1, a small (56 residue) protein with a well-characterized solution-state nuclear magnetic resonance (NMR) structure and having α-helix, four-fold β-sheet, and hydrophobic core features, is used as a model protein to study protein soil mineral interactions and gain insights on structural changes and potential degradation of protein. From our simulations, we observe little change to the hydrated Gb1 structure over the kaolinite, montmorillonite, and goethite surfaces relative to its solvated structure without these mineral surfaces present. Over the Na+-birnessite basal surface, however, the Gb1 structure is highly disturbed as a result of interaction with this birnessite surface. Unraveling of the Gb1 β-sheet at specific t...

Research paper thumbnail of EMSL and Institute for Integrated Catalysis (IIC) Catalysis Workshop

DOE Scientific and Technical Information. DOE Scientific and Technical Information. ...

Research paper thumbnail of Measurement of the Reactive Surface Area of Clay Minerals Using Solid-State NMR Studies of a Probe Molecule

Journal of Physical Chemistry C, 2010

Understanding the surface reactivity of clay minerals is necessary for accurate prediction of nat... more Understanding the surface reactivity of clay minerals is necessary for accurate prediction of natural weathering rates due to the ubiquity of clays in the environment as weathering products of primary minerals. However, the reactivity of the heterogeneous surfaces of a clay can be difficult to characterize as clay mineral edge sites often react at different rates or via different mechanisms than sites on the basal planes. Ultimately, a method is needed to probe quantitatively the reactive surface sites in order to predict clay mineral dissolution rates. In this study, solid-state NMR spectroscopy has been utilized to investigate surface hydroxyl species and their relation to clay surface reactivity. The surfaces of two kaolinite samples (KGa-1b and KGa-2) and two montmorillonite samples (STx-1b and SWy-2) were reacted with the probe molecule (3,3,3trifluoropropyl)dimethylchlorosilane (TFS), which binds selectively to reactive non-hydrogen bonded Q 3 Si hydroxyl sites. Quantification of 19 F spins in the TFS-treated samples using 19 F magic angle spinning NMR peak intensities provides a sensitive measure of the number of reactive hydroxyl sites on a mass normalized (per gram) basis. The reactive surface site densities of KGa-1b and KGa-2 were found to be proportional to published atomic force microscopy edge site fractions. An example from KGa-1b dissolution after 10 days at pH 2.9 and 21°C revealed no significant change in Brunauer-Emmett-Teller specific surface area, but a 25% decrease in reactive surface site density. We posit this site density determined by solid-state NMR is proportional to the reactive surface area of each clay mineral and its use in future dissolution studies is warranted to investigate how changes in reactive surface area can be tied to decreases in rates of silicon and aluminum release into solution.

Research paper thumbnail of Molecular-level control of oxide dissolution: Insights from NMR and computational chemistry

Research paper thumbnail of Study of a Family of 40 Hydroxylated β-Cristobalite Surfaces Using Empirical Potential Energy Functions

Journal of Physical Chemistry C, 2007

We present a study of a family of 40 unique hydroxylated -cristobalite surfaces generated by clea... more We present a study of a family of 40 unique hydroxylated -cristobalite surfaces generated by cleaving the -cristobalite unit cell along crystallographic planes to include a combination of several low Miller index surfaces. The surface silicon atoms are quantified as percentages of Q 2 and Q 3 centers based on their polymeric state. We find that Q 3 centers are, on average, three times more abundant than Q 2 centers. To study the surface properties, we use two different empirical potential energy functions: the multibody potential proposed by Fueston and Garofalini (J. Phys. Chem. 1990, 94, 5351) and the newly developed CHARMM potential by Lopes et al. (J. Phys. Chem. B 2006, 110, 2782. Our results for the surface water interactions are in good agreement with previous ab initio theoretical studies by Yang et al. (Phys. ReV. B 2006, 73, 146102) for the (100) surface. We find that the most commonly studied family of {100} surfaces is unique and is the only surface with 100% abundance of Q 2 centers, whereas there are nine examples of surfaces with 100% Q 3 centers. The predominantly pure Q 3 surfaces show no hydrogen bonding with the neighboring surface hydroxyl groups and weakly adsorb water overlayers. This is markedly different from the {100} pure Q 2 surface that shows strong hydrogen bonding within the surface groups and with water. As compared to all the surfaces studied in this work, we find that the {100} surfaces are not true representations of the overall -cristobalite surfaces and their properties. We find that the two main factors that characterize the physical properties of silica surfaces are the polymeric state of the silicon atom and surface topography. Two types of pure Q 3 crystallographic planes have been identified and are labeled as Q 3A and Q 3B based on the differences in their topological features. Using the {111} and {011} surfaces as examples, we show that the Q 3A surface adsorbs H 2 O that forms a stable monolayer, but the Q 3B surface fails to form a stable H 2 O overlayer. Other crystallographic planes with different ratios of Q 2 to Q 3 centers are contrasted by the differences in the hydrogenbonding network and their ability to form ordered H 2 O overlayers.

Research paper thumbnail of Solid-State NMR Studies of Reactive Hydroxyls on the Surfaces of Natural Samples: New Insights Into Dissolution Mechanisms of Oxide Surfaces

Surface hydroxyl groups are an important controlling factor for the interfacial reactivity of oxi... more Surface hydroxyl groups are an important controlling factor for the interfacial reactivity of oxide surfaces. However, quantification of the hydroxyl content at oxide surfaces is challenging when presented with low surface area or complex (and changing) morphologies, such as those commonly present in environmentally relevant samples. We have recently developed a 19F magic angle spinning nuclear magnetic resonance (NMR) approach to the quantification of surface hydroxyl groups by covalently attaching a 19F containing probe molecule to hydroxyl moieties on the oxide surface. Two suites of rhyolitic glasses were investigated to determine the appropriateness of utilizing our surface modification technique to accurately identify reactive surface species involved in chemical reactions (e.g. dissolution, precipitation). The first suite consisted of two sets of seven rhyolitic and dacitic glasses of Icelandic origin: both pristine and acid treated glasses were studied. For both pre and post-dissolution samples a clear correlation exists between the mass normalized dissolution rate and the number of hydroxyl groups per gram as measured by our NMR-detected surface modification technique, with the exception of one post-dissolution sample (SLN). The SLN sample showed evidence of advanced weathering as compared to the six other samples within the same suite. To determine whether physical and/or chemical weathering time impacted the number of measured hydroxyl groups, a second suite of rhyolitic glasses were obtained from a chronosequence (Kozushima, Japan). The four samples differ with respect to age (1.1, 1.8, 26, 52 ka) as well as the presence of weathering products (halloysite). Prior to surface modification, the weathering products were removed via ultra-sonication. The number of measured hydroxyl groups increases with increasing age, whether normalized to mass or BET generated surface area. These data are in agreement with the overall change in hydroxyl number per gram measured on the post-dissolution Icelandic suite, which also showed an increase in the number of hydroxyls after dissolution, normalized either to mass or BET surface area. Mechanistic models for the relevance of these measurements to the understanding of complex aluminosilicate surfaces in the environment will also be presented.

Research paper thumbnail of Toward the Integration of Liquid and Solid-State NMR Across the Undergraduate Curriculum

Cheminform, 2007

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Research paper thumbnail of Probing the molecular-level control of aluminosilicate dissolution: A sensitive solid-state NMR proxy for reactive surface area

Geochimica Et Cosmochimica Acta, 2008

For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalen... more For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalent attachment of the fluorine-containing (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS) probe molecule were measured by quantitative 19F nuclear magnetic resonance (NMR) spectroscopy. The first set of samples consists of six rhyolitic and dacitic glasses originating from volcanic activity in Iceland and one rhyolitic glass from the Bishop Tuff, CA. Due to differences in the reactive species present on the surfaces of these glasses, variations in the rate of acid-mediated dissolution (pH 4) for samples in this suite cannot be explained by variations in geometric or BET-measured surface area. In contrast, the rates scale directly with the surface density of TFS-reactive sites as measured by solid-state NMR. These data are consistent with the inference that the TFS-reactive M-OH species on the glass surface, which are known to be non-hydrogen-bonded Q 3 groups, represent loci accessible to and affected by proton-mediated dissolution. The second suite of samples, originating from a chronosequence in Kozushima, Japan, is comprised of four rhyolites that have been weathered for 1.1, 1.8, 26, and 52 ka. The number of TFS-reactive sites per gram increases with duration of weathering in the laboratory for the "Icelandic" samples and with duration of field weathering for both "Icelandic" and Japanese samples. One hypothesis is consistent with these and published modeling, laboratory, and field observations: over short timescales, dissolution is controlled by fast-dissolving sites, but over long timescales, dissolution is controlled by slower-dissolving sites, the surface density of which is proportional to the number of TFS-reactive Q 3 sites. These latter sites are not part of a hydrogen-bonded network on the surface of the glasses, and measurement of their surface site density allows predictions of trends in reactive surface area. The TFS treatment method, which is easily monitored by quantitative 19F solid-state NMR, therefore provides a chemically specific and quantifiable proxy to understand the nature of how sites on dissolving silicates control dissolution. Furthermore, 27Al NMR techniques are shown here to be useful in identifying clays on the glass surfaces, and these methods are therefore effective for quantifying concentrations of weathering impurities. Our interpretations offer a testable hypothesis for the mechanism of proton-promoted dissolution for low-iron aluminosilicate minerals and glasses and suggest that future investigations of reactive surfaces with high-sensitivity NMR techniques are warranted.

Research paper thumbnail of Dissolution rate dependence on reactive surface sites

Research paper thumbnail of NMR and IR Study of Fluorobenzene and Hexafluorobenzene Adsorbed on Alumina

Research paper thumbnail of Probing the molecular-level control of aluminosilicate dissolution: A sensitive solid-state NMR proxy for reactive surface area

Geochimica Et Cosmochimica Acta, 2008

For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalen... more For two suites of volcanic aluminosilicate glasses, the accessible and reactive sites for covalent attachment of the fluorinecontaining (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS) probe molecule were measured by quantitative 19 F nuclear magnetic resonance (NMR) spectroscopy. The first set of samples consists of six rhyolitic and dacitic glasses originating from volcanic activity in Iceland and one rhyolitic glass from the Bishop Tuff, CA. Due to differences in the reactive species present on the surfaces of these glasses, variations in the rate of acid-mediated dissolution (pH 4) for samples in this suite cannot be explained by variations in geometric or BET-measured surface area. In contrast, the rates scale directly with the surface density of TFS-reactive sites as measured by solid-state NMR. These data are consistent with the inference that the TFS-reactive M-OH species on the glass surface, which are known to be non-hydrogen-bonded Q 3 groups, represent loci accessible to and affected by proton-mediated dissolution. The second suite of samples, originating from a chronosequence in Kozushima, Japan, is comprised of four rhyolites that have been weathered for 1.1, 1.8, 26, and 52 ka. The number of TFS-reactive sites per gram increases with duration of weathering in the laboratory for the ''Icelandic'' samples and with duration of field weathering for both ''Icelandic'' and Japanese samples. One hypothesis is consistent with these and published modeling, laboratory, and field observations: over short timescales, dissolution is controlled by fast-dissolving sites, but over long timescales, dissolution is controlled by slower-dissolving sites, the surface density of which is proportional to the number of TFS-reactive Q 3 sites. These latter sites are not part of a hydrogen-bonded network on the surface of the glasses, and measurement of their surface site density allows predictions of trends in reactive surface area. The TFS treatment method, which is easily monitored by quantitative 19 F solidstate NMR, therefore provides a chemically specific and quantifiable proxy to understand the nature of how sites on dissolving silicates control dissolution. Furthermore, 27 Al NMR techniques are shown here to be useful in identifying clays on the glass surfaces, and these methods are therefore effective for quantifying concentrations of weathering impurities. Our interpretations offer a testable hypothesis for the mechanism of proton-promoted dissolution for low-iron aluminosilicate minerals and glasses and suggest that future investigations of reactive surfaces with high-sensitivity NMR techniques are warranted.

Research paper thumbnail of Effects of Systematic Methyl Substitution of Metal (III) Tris( n Methyl8-Quinolinolato) Chelates on Material Properties for Optimum Electroluminescence Device Performance

Journal of The American Chemical Society, 2001

... Linda S. Sapochak,* † Asanga Padmaperuma, † Nancy Washton, † Flocerfida Endrino, †Gregory T. ... more ... Linda S. Sapochak,* † Asanga Padmaperuma, † Nancy Washton, † Flocerfida Endrino, †Gregory T. Schmett, † Jeffrey Marshall, † Daniel Fogarty, † Paul E. Burrows, ‡ § and Stephen R. Forrest ‡. Contribution from the Department ...