19 research outputs found
Delamination of Layered Zeolite Precursors under Mild Conditions: Synthesis of UCB-1 via Fluoride/Chloride Anion-Promoted Exfoliation
New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray
diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve
exfoliation. UCB-1 consists of a high degree of structural integrity via ^(29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of
hysteresis in the N_2 adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization
Delamination of Layered Zeolite Precursors under Mild Conditions: Synthesis of UCB-1 via Fluoride/Chloride Anion-Promoted Exfoliation
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The Design, Synthesis, and Characterization of Open Sites on Metal Clusters
Coordinatively unsaturated corner and edge atoms have been hypothesized to have the highest activity of sites responsible for many catalytic reactions on a metal surface. Recent studies have validated this hypothesis in varied reaction systems. However, quantification of different types of coordinatively unsaturated sites, and elucidation of their individual catalytic rates has remained a largely unresolved challenge when understanding catalysis on metal surfaces. Yet such structure-function knowledge would be invaluable to the design of more active and selective metal-surface catalysts in the future. I investigated the catalytic contributions of undercoordinated sites such as corner and edge atoms are investigated in a model reaction system using organic ligands bound to the gold nanoparticle surface. The catalyst consisted of 4 nm gold nanoparticles on a metal oxide support, using resazurin to resorufin as a model reaction system. My results demonstrate that in this system, corner atom sites are the most undercoordinated sites, and are over an order of magnitude more active when compared to undercoordinated edge atom sites, while terrace sites remain catalytically inactive for the reduction reaction of resazurin to resorufin.Catalytic activity has been also demonstrated for calixarene-bound gold nanoparticles using the reduction of 4-nitrophenol. With the 4-nitrophenol reduction reaction, a comparative study was undertaken to compare calixarene phosphine and calixarene thiol bound 4 nm gold particles. The results of the study suggested that a leached site was responsible for catalysis and not sites on the original gold nanoparticles. Future experiments with calixarene bound gold clusters could investigate ligand effects in reactions where the active site is not a leached or aggregated gold species, possibly in oxidation reactions, where electron-rich gold is hypothesized to be a good catalyst.The results that emphasize the enhanced catalytic activity of undercoordinated sites led me to synthesize small gold clusters consisting of a high fraction of coordinatively unsaturated open sites. This was enabled through an approach that utilized bulky calix[4]arene ligands that are bound to a gold core. Since the size of the calix[4]arene ligand is commensurate with the size of the gold cluster core, the calix[4]arene ligand does not pack closely together on the gold cluster surface. This in turn results in areas of accessible gold atom sites between ligands. Additionally, these calix[4]arene ligands prevent cluster aggregation and electronically tune the gold core in a manner conceptually similar to enzymes affecting reactivity through organic side-chains acting as ligands. I quantified the number of open sites that result from this packing problem on the gold cluster surface, using fluorescence probe chemisorption experiments. The results of these chemisorption measurements support the mechanical model of accessibility whereby accessibility is not dependent on the identity of the functional group, whether it be calixarene phosphines or N-heterocyclic carbenes, bound to the gold surface, but rather to the relative radii of curvature of bound ligands and the gold cluster core. Additional materials characterization was completed with transmission electron microscopy in both bright-field imaging of zeolites, in MCM-22 and delaminated ITQ-2 and UCB-1 materials, and in dark field imaging of glucan coatings on oxide particles. These materials could prove to be interesting materials as to use as supports for the calixarene-bound metal clusters described above or for other metal clusters
Surfactant- and Ligand-Free Synthesis of Platinum Nanoparticles in Aqueous Solution for Catalytic Applications
The synthesis of surfactant-free and organic ligand-free metallic nanoparticles in solution remains challenging due to the nanoparticles’ tendency to aggregate. Surfactant- and ligand-free nanoparticles are particularly desirable in catalytic applications as surfactants, and ligands can block access to the nanoparticles’ surfaces. In this contribution, platinum nanoparticles are synthesized in aqueous solution without surfactants or bound organic ligands. Pt is reduced by sodium borohydride, and the borohydride has a dual role of reducing agent and weakly interacting stabilizer. The 5.3 nm Pt nanoparticles are characterized using UV-visible spectroscopy and transmission electron microscopy. The Pt nanoparticles are then applied as catalysts in two different reactions: the redox reaction of hexacyanoferrate(III) and thiosulfate ions, and H2O2 decomposition. Catalytic activity is observed for both reactions, and the Pt nanoparticles show up to an order of magnitude greater activity over the most active catalysts reported in the literature for hexacyanoferrate(III)/thiosulfate redox reactions. It is hypothesized that this enhanced catalytic activity is due to the increased electron density that the surrounding borohydride ions give to the Pt nanoparticle surface, as well as the absence of surfactants or organic ligands blocking surface sites
Surfactant- and Ligand-Free Synthesis of Platinum Nanoparticles in Aqueous Solution for Catalytic Applications
The synthesis of surfactant-free and organic ligand-free metallic nanoparticles in solution remains challenging due to the nanoparticles’ tendency to aggregate. Surfactant- and ligand-free nanoparticles are particularly desirable in catalytic applications as surfactants, and ligands can block access to the nanoparticles’ surfaces. In this contribution, platinum nanoparticles are synthesized in aqueous solution without surfactants or bound organic ligands. Pt is reduced by sodium borohydride, and the borohydride has a dual role of reducing agent and weakly interacting stabilizer. The 5.3 nm Pt nanoparticles are characterized using UV-visible spectroscopy and transmission electron microscopy. The Pt nanoparticles are then applied as catalysts in two different reactions: the redox reaction of hexacyanoferrate(III) and thiosulfate ions, and H2O2 decomposition. Catalytic activity is observed for both reactions, and the Pt nanoparticles show up to an order of magnitude greater activity over the most active catalysts reported in the literature for hexacyanoferrate(III)/thiosulfate redox reactions. It is hypothesized that this enhanced catalytic activity is due to the increased electron density that the surrounding borohydride ions give to the Pt nanoparticle surface, as well as the absence of surfactants or organic ligands blocking surface sites
Mesostructure of Mesoporous Silica/Anodic Alumina Hierarchical Membranes Tuned with Ethanol
Chaperonin-Inspired pH Protection by Mesoporous Silica SBA-15 on Myoglobin and Lysozyme
While
enzymes are valuable tools in many fields of biotechnology,
they are fragile and must be protected against denaturing conditions
such as unfavorable solution pH. Within living organisms, chaperonins
help enzymes fold into their native shape and protect them from damage.
Inspired by this natural solution, mesoporous silica SBA-15 with different
pore diameters is synthesized as a support material for immobilizing
and protecting enzymes. In separate experiments, the model enzymes
myoglobin and lysozyme are physically adsorbed to SBA-15 and exposed
to a range of buffered pH conditions. The immobilized enzymes’
biocatalytic activities are quantified and compared to the activities
of nonimmobilized enzymes in the same solution conditions. It has
been observed that myoglobin immobilized on SBA-15 is protected from
acidic denaturation from pH 3.6 to 5.1, exhibiting relative activity
of up to 350%. Immobilized lysozyme is protected from unfavorable
conditions from pH 6.6 to 7.6, with relative activity of up to 200%.
These results indicate that the protective effects conferred to enzymes
immobilized by physical adsorption to SBA-15 are driven by the enzymes’
electrostatic attraction to the material’s surface. The pore
diameter of SBA-15 affects the quality of protection given to immobilized
enzymes, but the contribution of this effect at different pH values
remains unclear
Mesostructure of Mesoporous Silica/Anodic Alumina Hierarchical Membranes Tuned with Ethanol
Hierarchically structured
membranes composed of mesoporous silica
embedded inside the channels of anodic alumina (MS-AAM) were synthesized
using the aspiration method. Ethanol is shown to have a significant
effect on the type and organization of the mesoporous silica phase.
Detailed textural analysis revealed that the pore size distribution
of the mesoporous silica narrows and the degree of ordering increases
with decreasing ethanol concentration used in the synthesis mixture.
The silica mesopores were synthesized with pores as small as 6 nm
in diameter, with the channel direction oriented in lamellar, circular,
and columnar directions depending on the ethanol content. This study
reveals ethanol concentration as a key factor behind the synthesis
of an ordered mesoporous silica–anodic alumina membrane that
can increase its functionality for membrane-based applications
The Contribution of Declines in Blood Lead Levels to Reductions in Blood Pressure Levels: Longitudinal Evidence in the Strong Heart Family Study
Background Chronic lead exposure is associated with both subclinical and clinical cardiovascular disease. We evaluated whether declines in blood lead were associated with changes in systolic and diastolic blood pressure in adult American Indian participants from the SHFS (Strong Heart Family Study). Methods and Results Lead in whole blood was measured in 285 SHFS participants in 1997 to 1999 and 2006 to 2009. Blood pressure and measures of cardiac geometry and function were obtained in 2001 to 2003 and 2006 to 2009. We used generalized estimating equations to evaluate the association of declines in blood lead with changes in blood pressure; cardiac function and geometry measures were considered secondary. Mean blood lead was 2.04 μg/dL at baseline. After ≈10 years, mean decline in blood lead was 0.67 μg/dL. In fully adjusted models, the mean difference in systolic blood pressure comparing the highest to lowest tertile of decline (>0.91 versus <0.27 μg/dL) in blood lead was −7.08 mm Hg (95% CI, −13.16 to −1.00). A significant nonlinear association between declines in blood lead and declines in systolic blood pressure was detected, with significant linear associations where blood lead decline was 0.1 μg/dL or higher. Declines in blood lead were nonsignificantly associated with declines in diastolic blood pressure and significantly associated with declines in interventricular septum thickness. Conclusions Declines in blood lead levels in American Indian adults, even when small (0.1–1.0 μg/dL), were associated with reductions in systolic blood pressure. These findings suggest the need to further study the cardiovascular impacts of reducing lead exposures and the importance of lead exposure prevention