9 research outputs found
A New Geometric Method Based on Two-Dimensional Transmission Electron Microscopy for Analysis of Interior versus Exterior Pd Loading on Hollow Carbon Nanofibers
Hollow carbon nanofibers (CNFs) are being explored as catalyst supports because of their unique properties. Internal versus external loading of metal nanoparticles impacts catalytic performance; we developed a fast and accurate geometric analysis method based on two-dimensional transmission electron microscopy (2D TEM) images to estimate Pd internal versus external loading percentages. Three different Pd-loaded CNF catalysts were prepared using methods reported in the literature to yield different amounts of Pd inside loading. Results indicate the percentage of inside-loaded Pd increases as expected in the three samples (from 22.7 ± 17.8%, to 47.2 ± 22.8%, to 71.4 ± 19.7%, based on Pd nanoparticle number). We compared percent inside loading values for one segment of a Pd-loaded CNF using our method and three-dimensional scanning transmission electron microscopy (3D STEM), and observed adequate agreement (27.8% vs 32.7%). Our geometric analysis method is proposed as a more straightforward and fast way to evaluate metal nanoparticles on tubular supports
Enhanced Activity and Selectivity of Carbon Nanofiber Supported Pd Catalysts for Nitrite Reduction
Pd-based catalyst treatment represents an emerging technology
that
shows promise to remove nitrate and nitrite from drinking water. In
this work we use vapor-grown carbon nanofiber (CNF) supports in order
to explore the effects of Pd nanoparticle size and interior versus
exterior loading on nitrite reduction activity and selectivity (i.e.,
dinitrogen over ammonia production). Results show that nitrite reduction
activity increases by 3.1-fold and selectivity decreases by 8.0-fold,
with decreasing Pd nanoparticle size from 1.4 to 9.6 nm. Both activity
and selectivity are not significantly influenced by Pd interior versus
exterior CNF loading. Consequently, turnover frequencies (TOFs) among
all CNF catalysts are similar, suggesting nitrite reduction is not
sensitive to Pd location on CNFs nor Pd structure. CNF-based catalysts
compare favorably to conventional Pd catalysts (i.e., Pd on activated
carbon or alumina) with respect to nitrite reduction activity and
selectivity, and they maintain activity over multiple reduction cycles.
Hence, our results suggest new insights that an optimum Pd nanoparticle
size on CNFs balances faster kinetics with lower ammonia production,
that catalysts can be tailored at the nanoscale to improve catalytic
performance for nitrite, and that CNFs hold promise as highly effective
catalyst supports in drinking water treatment
Graphitic Carbon Nitride Supported Ultrafine Pd and Pd–Cu Catalysts: Enhanced Reactivity, Selectivity, and Longevity for Nitrite and Nitrate Hydrogenation
Novel Pd-based catalysts
(i.e., Pd and Pd–Cu) supported
on graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) were prepared
for nitrite and nitrate hydrogenation. The catalysts prepared by ethylene
glycol reduction exhibited ultrafine Pd and Pd–Cu nanoparticles
(∼2 nm), and they showed high reactivity, high selectivity
toward nitrogen gas over byproduct ammonium, and excellent stability
over multiple reaction cycles. The unique nitrogen-abundant surface,
porous structure, and hydrophilic nature of g-C<sub>3</sub>N<sub>4</sub> facilitates metal nanoparticle dispersion, mass transfer of reactants,
and nitrogen coupling for nitrogen gas production to improve catalytic
performance
Structure Sensitivity Study of Waterborne Contaminant Hydrogenation Using Shape- and Size-Controlled Pd Nanoparticles
Catalytic
reduction with Pd has emerged as a promising technology
to remove a suite of contaminants from drinking water, such as oxyanions,
disinfection byproducts, and halogenated pollutants, but low activity
is a major challenge for application. To address this challenge, we
synthesized a set of shape- and size-controlled Pd nanoparticles and
evaluated the activity of three probe contaminants (i.e., nitrite, <i>N</i>-nitrosodimethylamine (NDMA), and diatrizoate) as a function
of facet type (e.g., (100), (110), (111)), ratios of low- to high-coordination
sites, and ratios of surface sites to total Pd (i.e., dispersion).
Reduction results for an initial contaminant concentration of 100
μM show that initial turnover frequency (TOF<sub>0</sub>) for
nitrite increases 4.7-fold with increasing percent of (100) surface
Pd sites (from 0% to 95.3%), whereas the TOF<sub>0</sub> for NDMA
and for diatrizoate increases 4.5- and 3.6-fold, respectively, with
an increasing percent of terrace surface Pd sites (from 79.8% to 95.3%).
Results for an initial nitrite concentration of 2 mM show that TOF<sub>0</sub> is the same for all shape- and size-controlled Pd nanoparticles.
Trends for TOF<sub>0</sub> were supported by results showing that
all catalysts but one were stable in shape and size up to 12 days;
for the exception, iodide liberation in diatrizoate reduction appeared
to be responsible for a shape change of 4 nm octahedral Pd nanoparticles.
Density functional theory (DFT) simulations for the free energy change
of hydrogen (H<sub>2</sub>), nitrite, and nitric oxide (NO) adsorption
and a two-site model based on the Langmuir–Hinshelwood mechanism
suggest that competition of adsorbates for different Pd sites can
explain the TOF<sub>0</sub> results. Our study shows for the first
time that catalytic reduction activity for waterborne contaminant
removal varies with the Pd shape and size, and it suggests that Pd
catalysts can be tailored for optimal performance to treat a variety
of contaminants for drinking water
Enhancement of Nitrite Reduction Kinetics on Electrospun Pd-Carbon Nanomaterial Catalysts for Water Purification
We
report a facile synthesis method for carbon nanofiber (CNF)
supported Pd catalysts via one-pot electrospinning and their application
for nitrite hydrogenation. A mixture of Pd acetylacetonate (PdÂ(acac)<sub>2</sub>), polyacrylonitrile (PAN), and nonfunctionalized multiwalled
carbon nanotubes (MWCNTs) was electrospun and thermally treated to
produce Pd/CNF-MWCNT catalysts. The addition of MWCNTs with a mass
loading of 1.0–2.5 wt % (to PAN) significantly improved nitrite
reduction activity compared to the catalyst without MWCNT addition.
The results of CO chemisorption confirmed that the addition of MWCNTs
increased Pd exposure on CNFs and hence improved catalytic activity
Efficacy of a Rose Bengal-Embedded Antimicrobial Packaging Film in Inactivating <i>Escherichia coli</i> under Visible Light Irradiation
Antimicrobial packaging reduces the
extent of microbial contamination;
however, conventional antimicrobial packaging, which releases antimicrobial
agents into food, may experience rapid agent depletion and can adversely
affect food flavors. In this study, a novel photocatalytic antimicrobial
nanofiber film embedded with Rose Bengal (RB) dye that generates reactive
oxygen species (ROS) in visible light was designed for inactivating
microorganisms. The film’s antimicrobial properties under various
light intensities and exposure times were evaluated, using Escherichia coli as a test microorganism. The results demonstrated
that RB generates singlet oxygen as its principal ROS and has potent
antimicrobial effects when incorporated into a film, achieving a 4.4
± 0.1 log CFU reduction in E. coli after 45
h under a light intensity of 6500 lx. The film’s antimicrobial
efficacy was dependent on light intensity, with significant E. coli inactivation occurring above 2000 lx. Overall, the
RB-incorporated film effectively inactivates E. coli, providing a promising alternative to conventional antimicrobial
packaging methods
Tailored Synthesis of Photoactive TiO<sub>2</sub> Nanofibers and Au/TiO<sub>2</sub> Nanofiber Composites: Structure and Reactivity Optimization for Water Treatment Applications
Titanium
dioxide (TiO<sub>2</sub>) nanofibers with tailored structure
and composition were synthesized by electrospinning to optimize photocatalytic
treatment efficiency. Nanofibers of controlled diameter (30–210
nm), crystal structure (anatase, rutile, mixed phases), and grain
size (20–50 nm) were developed along with composite nanofibers
with either surface-deposited or bulk-integrated Au nanoparticle cocatalysts.
Their reactivity was then examined in batch suspensions toward model
(phenol) and emerging (pharmaceuticals, personal care products) pollutants
across various water qualities. Optimized TiO<sub>2</sub> nanofibers
meet or exceed the performance of traditional nanoparticulate photocatalysts
(e.g., Aeroxide P25) with the greatest reactivity enhancements arising
from (i) decreasing diameter (i.e., increasing surface area), (ii)
mixed phase composition [74/26 (±0.5) % anatase/rutile], and
(iii) small amounts (1.5 wt %) of surface-deposited, more so than
bulk-integrated, Au nanoparticles. Surface Au deposition consistently
enhanced photoactivity by 5- to 10-fold across our micropollutant
suite independent of their solution concentration, behavior that we
attribute to higher photocatalytic efficiency from improved charge
separation. However, the practical value of Au/TiO<sub>2</sub> nanofibers
was limited by their greater degree of inhibition by solution-phase
radical scavengers and higher rate of reactivity loss from surface
fouling in nonidealized matrixes (e.g., partially treated surface
water). Ultimately, unmodified TiO<sub>2</sub> nanofibers appear most
promising for use as reactive filtration materials because their performance
was less influenced by water quality, although future efforts must
increase the strength of TiO<sub>2</sub> nanofiber mats to realize
such applications
Visible-Light-Responsive Graphitic Carbon Nitride: Rational Design and Photocatalytic Applications for Water Treatment
Graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) has recently
emerged as a promising visible-light-responsive polymeric photocatalyst;
however, a molecular-level understanding of material properties and
its application for water purification were underexplored. In this
study, we rationally designed nonmetal doped, supramolecule-based
g-C<sub>3</sub>N<sub>4</sub> with improved surface area and charge
separation. Density functional theory (DFT) simulations indicated
that carbon-doped g-C<sub>3</sub>N<sub>4</sub> showed a thermodynamically
stable structure, promoted charge separation, and had suitable energy
levels of conduction and valence bands for photocatalytic oxidation
compared to phosphorus-doped g-C<sub>3</sub>N<sub>4</sub>. The optimized
carbon-doped, supramolecule-based g-C<sub>3</sub>N<sub>4</sub> showed
a reaction rate enhancement of 2.3–10.5-fold for the degradation
of phenol and persistent organic micropollutants compared to that
of conventional, melamine-based g-C<sub>3</sub>N<sub>4</sub> in a
model buffer system under the irradiation of simulated visible sunlight.
Carbon-doping but not phosphorus-doping improved reactivity for contaminant
degradation in agreement with DFT simulation results. Selective contaminant
degradation was observed on g-C<sub>3</sub>N<sub>4</sub>, likely due
to differences in reactive oxygen species production and/or contaminant-photocatalyst
interfacial interactions on different g-C<sub>3</sub>N<sub>4</sub> samples. Moreover, g-C<sub>3</sub>N<sub>4</sub> is a robust photocatalyst
for contaminant degradation in raw natural water and (partially) treated
water and wastewater. In summary, DFT simulations are a viable tool
to predict photocatalyst properties and oxidation performance for
contaminant removal, and they guide the rational design, fabrication,
and implementation of visible-light-responsive g-C<sub>3</sub>N<sub>4</sub> for efficient, robust, and sustainable water treatment
Lignocellulose Fiber- and Welded Fiber- Supports for Palladium-Based Catalytic Hydrogenation: A Natural Fiber Welding Application for Water Treatment
In our study, lignocellulose yarns
were fabricated via natural
fiber welding (NFW) into a robust, free-standing, sustainable catalyst
for water treatment. First, a series of powder catalysts were created
by loading monometallic palladium (Pd) and bimetallic palladium–copper
(Pd–Cu) nanoparticles onto ball-milled yarn powders via incipient
wetness (IW) followed by a gentle reduction method in hydrogen gas
that preserved the natural fiber while reducing the metal ions to
their zerovalent state. Material characterization revealed Pd preferentially
reduced near the surface whereas Cu distributed more uniformly throughout
the supports. Although no chemical bonding interactions were observed
between the metals and their supports, small (5–10 nm), near-spherical
crystalline nanoparticles were produced, and a Pd–Cu alloy
formed on the surface of the supports. Catalytic performance was evaluated
for each Pd-only and Pd–Cu powder catalyst via nitrite and
nitrate reduction tests, respectively. Next, the optimized Pd–Cu
linen powder catalyst was fiber-welded onto a macroporous linen yarn
scaffold via NFW and its catalyst performance and reusability were
evaluated. This fiber-welded catalyst reduced nitrate as effectively
as the corresponding powder, and remained stable during five consecutive
cycles of nitrate reduction tests. Although catalytic activity declined
after the fiber-welded catalyst was left in air for several months,
its reactivity could easily be regenerated by thermal treatment. Our
research highlights how lignocellulose supported metal-based catalysts
can be used for water purification, demonstrating a novel application
of NFW for water treatment while presenting a sustainable approach
to fabricate functional materials from natural fibers