6 research outputs found
Evaluating Differences in the Active-Site Electronics of Supported Au Nanoparticle Catalysts Using Hammett and DFT Studies
Supported metal catalysts, which are composed of metal nanoparticles dispersed on metal oxides or other high-surface-area materials, are ubiquitous in industrially catalysed reactions. Identifying and characterizing the catalytic active sites on these materials still remains a substantial challenge, even though it is required to guide rational design of practical heterogeneous catalysts. Metal-support interactions have an enormous impact on the chemistry of the catalytic active site and can determine the optimum support for a reaction; however, few direct probes of these interactions are available. Here we show how benzyl alcohol oxidation Hammett studies can be used to characterize differences in the catalytic activity of Au nanoparticles hosted on various metal-oxide supports. We combine reactivity analysis with density functional theory calculations to demonstrate that the slope of experimental Hammett plots is affected by electron donation from the underlying oxide support to the Au particles
Modulation of Spatiotemporal Particle Patterning in Evaporating Droplets: Applications to Diagnostics and Materials Science
Spatiotemporal
particle patterning in evaporating droplets lacks a common design
framework. Here, we demonstrate autonomous control of particle distribution
in evaporating droplets through the imposition of a salt-induced self-generated
electric field as a generalized patterning strategy. Through modeling,
a new dimensionless number, termed âcapillary-phoresisâ
(CP) number, arises, which determines the relative contributions of
electrokinetic and convective transport to pattern formation, enabling
one to accurately predict the mode of particle assembly by controlling
the spontaneous electric field and surface potentials. Modulation
of the CP number allows the particles to be focused in a specific
region in space or distributed evenly. Moreover, starting with a mixture
of two different particle types, their relative placement in the ensuing
pattern can be controlled, allowing coassemblies of multiple, distinct
particle populations. By this approach, hypermethylated DNA, prevalent
in cancerous cells, can be qualitatively distinguished from normal
DNA of comparable molecular weights. In other examples, we show uniform
dispersion of several particle types (polymeric colloids, multiwalled
carbon nanotubes, and molecular dyes) on different substrates (metallic
Cu, metal oxide, and flexible polymer), as dictated by the CP number.
Depending on the particle, the highly uniform distribution leads to
surfaces with a lower sheet resistance, as well as superior dye-printed
displays
Using Thiol Adsorption on Supported Au Nanoparticle Catalysts To Evaluate Au Dispersion and the Number of Active Sites for Benzyl Alcohol Oxidation
Two techniques to study the surface
chemistry of supported gold
nanoparticles were developed. First, phenylethyl mercaptan (PEM) adsorption
from hexane solution was followed with UVâvis spectroscopy
to evaluate the total amount of surface Au available. Two catalysts,
Au/Al<sub>2</sub>O<sub>3</sub> and Au/TiO<sub>2</sub>, were found
to have Au:S surface stoichiometries of âŒ2:1, whereas a Au/SiO<sub>2</sub> catalyst had a Au:S surface stoichiometry of âŒ1:1.
The room temperature equilibrium binding constants for PEM adsorption
on the Au/Al<sub>2</sub>O<sub>3</sub> and Au/TiO<sub>2</sub> catalysts
were similar (âŒ3 Ă 10<sup>5</sup> M<sup>â1</sup>; Î<i>G</i> â â31 kJ/mol); the PEMâAu/SiO<sub>2</sub> binding constant was somewhat larger (âŒ2 Ă 10<sup>6</sup> M<sup>â1</sup>; Î<i>G</i> â
â36 kJ/mol). XPS data for all of the catalysts showed no observable
changes in the Au oxidation state upon adsorption of the thiol. Implications
of these experiments regarding self-assembled monolayers and thiol-stabilized
Au nanoparticles are discussed. Second, kinetic titrations (i.e.,
controlled thiol-poisoning experiments) were developed as a method
for evaluating the number of active sites for selective 4-methoxybenzyl
alcohol oxidation. These experiments suggested only a fraction of
the surface Au (âŒ10â15% of the total Au) was active
for the reaction. When thiol was added with the 4-methoxybenzyl alcohol
substrate, more thiol was required to poison the catalyst, suggesting
that the thiol and substrate compete for initial adsorption sites,
possibly at the metalâsupport interface. These two methods
were combined to evaluate the magnitude of the support effect on selective
4-methoxybenzyl alcohol oxidation. Correcting the catalytic activity
of the catalysts to the number of sites determined by thiol titration
provided clear evidence that the support has a strong influence on
the catalytic activity of Au in benzyl alcohol oxidation
PEGylated Carbon Nanocapsule: A Universal Reactor and Carrier for In Vivo Delivery of Hydrophobic and Hydrophilic Nanoparticles
We have developed PEGylated mesoporous
carbon nanocapsule as a universal nanoreactor and carrier for the
delivery of highly crystalline hydrophobic/hydrophilic nanoparticles
(NPs) which shows superior biocompatibility, dispersion in body fluids,
good biodistribution and NPs independent cellular uptake mechanism.
The hydrophobic/hydrophilic NPs without surface modification were
synthesized in situ inside the cavities of mesoporous carbon capsules
(200â850 nm). Stable and inert nature of carbon capsules in
a wide range of reaction conditions like high temperature and harsh
solvents, make it suitable for being used as nano/microreactors for
the syntheses of a variety of NPs for bioimaging applications, such
as NaYF<sub>4</sub>:Eu<sup>3+</sup>(5%), LaVO<sub>4</sub>:Eu<sup>3+</sup>(10%), GdVO<sub>4</sub>:Eu<sup>3+</sup>(10%), Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>(5%), GdF<sub>3</sub>:Tb<sup>3+</sup>(10%),
Mo, Pt, Pd, Au, and Ag. Multiple types of NPs (Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>(5%) (hydrophobic) and GdF<sub>3</sub>:Tb<sup>3+</sup>(10%) (hydrophilic)) were coloaded inside the carbon capsules
to create a multimodal agent for magneto-fluorescence imaging. Our
in vivo study clearly suggests that carbon capsules have biodistribution
in many organs including liver, heart, spleen, lungs, blood pool,
and muscles