7 research outputs found
Comprehensive Understanding of the Effects of Carbon Nanostructures on Redox Catalytic Properties and Stability in Oxidative Dehydrogenation
The
intrinsic redox catalytic properties of metal-free carbons
have been widely investigated due to their fundamental interest as
well as potential practical applications. Although a large variety
of nanostructured carbons are now available, the effects of carbon
nanostructures on redox properties have not been comprehensively understood.
In this work, the redox catalytic properties and thermochemical stabilities
of 16 different types of carbons, including activated carbon, carbon
nanotubes, onion-like carbons, and microporous/mesoporous templated
carbons were systematically investigated using <i>n</i>-butane
oxidative dehydrogenation as a model reaction. The results demonstrate
that the overall catalytic activity increases with increasing content
of Cī»O active sites. However, with increasing Cī»O content,
the activity per site (i.e., turnover frequency) gradually decreases,
while the alkene selectivity increases due to the decreased reducibility
of each Cī»O site. Since more Cī»O sites are present in
a thermochemically less stable amorphous framework, the carbons generally
exhibit a trade-off relationship between catalytic activity and stability.
However, a graphitic carbon with ācoin-stackingā carbon
layers showed exceptionally high activity and stability simultaneously.
This is attributed to its unique carbon structure that simultaneously
provides high graphitic order and abundant carbon edge sites where
Cī»O active sites are grafted
Characterization of various shaped 5 GHz TFBARs based on 3D full-wave modeling
In this paper, three dimensional finite element method is used for the analysis of thin film bulk acoustic wave resonator (TFBAR) at 5GHz. The TFBARisplaced on thin membrane after removal of substrate materialfor the suppression of loading effects and threedifferent geometries (rectangular, polygonal and circular)are implemented and modeled. The size of fabricated TFBAR are from100x100um2 to 200x200 um2 and full-wave modeled results are compared with the measurement.It is found that the modeled and measured results agree within 1% in terms of series and parallel resonant frequencies. Furthermore, the different shapes of TFBAR revealed slightly different bandwidth characteristics, whichis defined on frequency spacing between the series and parallel resonant frequencies.The another goal of this workis to study the variation of the size of resonator on how affects the performance of TFBAR. As the size of resonator incease, the electrical impedance of TFBAR decrease at theresonant frequencies and out of resonant frequencies.Thesephenomenacontributeto improve the effects of theattenuations of TFBAR filter in stop-band to some degree
Significant Roles of Carbon Pore and Surface Structure in AuPd/C Catalyst for Achieving High Chemoselectivity in Direct Hydrogen Peroxide Synthesis
Direct synthesis
of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) from hydrogen (H<sub>2</sub>) and oxygen (O<sub>2</sub>) has been
widely investigated as an attractive way for small-scale/on-site H<sub>2</sub>O<sub>2</sub> production. Among various catalysts, carbon-supported
AuPd catalysts have been reported to exhibit the most promising H<sub>2</sub>O<sub>2</sub> productivity and selectivity. In this work,
to better understand the catalytic role of the surface properties
and porous structures of the carbon supports, we systematically investigated
AuPd catalysts supported on various nanostructured carbons including
activated carbon, carbon nanotube, carbon black, and ordered mesoporous
carbons. The results showed that a high density of oxygen functional
groups on the carbon surface was essential for synthesizing highly
dispersed bimetallic catalysts with effective AuPd alloying, which
is a prerequisite for achieving high H<sub>2</sub>O<sub>2</sub> selectivity.
Regarding porous structure, a solely mesoporous carbon support was
superior to microporous ones. Microporous carbons such as activated
carbon suffered from diffusion limitation, leading to significantly
slower H<sub>2</sub> conversion than mesoporous catalysts. Furthermore,
H<sub>2</sub>O<sub>2</sub> produced from AuPd catalyst in the micropores
was more prone to subsequent disproportionation/hydrogenation into
H<sub>2</sub>O due to retarded diffusion of the H<sub>2</sub>O<sub>2</sub> out of the microporous structure, which led to decreased
H<sub>2</sub>O<sub>2</sub> selectivity. The present study showed that
solely mesoporous carbons with high surface oxygen content are most
desirable as a support for AuPd catalyst in order to achieve high
H<sub>2</sub>O<sub>2</sub> productivity and selectivity
Cross-Linked āPoisonousā Polymer: Thermochemically Stable Catalyst Support for Tuning Chemoselectivity
Designed catalyst poisons can be
deliberately added in various
reactions for tuning chemoselectivity. In general, the poisons are
ātransientā selectivity modifiers that are readily leached
out during reactions and thus should be continuously fed to maintain
the selectivity. In this work, we supported Pd catalysts on a thermochemically
stable cross-linked polymer containing diphenyl sulfide linkages,
which can simultaneously act as a catalyst support and a āpermanentā
selectivity modifier. The entire surfaces of the Pd clusters were
ligated (or poisoned) by sulfide groups of the polymer support. The
sulfide groups capping the Pd surface behaved like a āmolecular
gateā that enabled exceptionally discriminative adsorption
of alkynes over alkenes. H<sub>2</sub>/D<sub>2</sub> isotope exchange
revealed that the capped Pd surface alone is inactive for H<sub>2</sub> (or D<sub>2</sub>) dissociation, but in the presence of coflowing
acetylene (alkyne), it becomes active for H<sub>2</sub> dissociation
as well as acetylene hydrogenation. The results indicated that acetylene
adsorbs on the Pd surface and enables cooperative adsorption of H<sub>2</sub>. In contrast, ethylene (alkene) did not facilitate H<sub>2</sub>āD<sub>2</sub> exchange, and hydrogenation of ethylene
was not observed. The results indicated that alkynes can induce decapping
of the sulfide groups from the Pd surface, while alkenes with weaker
adsorption strength cannot. The discriminative adsorption of alkynes
over alkenes led to highly chemoselective hydrogenation of various
alkynes to alkenes with minimal overhydrogenation and the conversion
of side functional groups. The catalytic functions can be retained
over a long reaction period due to the high thermochemical stability
of the polymer
Hydrogen Peroxide Synthesis via Enhanced Two-Electron Oxygen Reduction Pathway on Carbon-Coated Pt Surface
Continuous
on-site electrochemical production of hydrogen peroxide
(H<sub>2</sub>O<sub>2</sub>) can provide an attractive alternative
to the present anthraquinone-based H<sub>2</sub>O<sub>2</sub> production
technology. A major challenge in the electrocatalyst design for H<sub>2</sub>O<sub>2</sub> production is that O<sub>2</sub> adsorption
on the Pt surface thermodynamically favors āside-onā
configuration over āend-onā configuration, which leads
to a dissociation of OāO bond via dominant 4-electron pathway.
This prefers H<sub>2</sub>O production rather than H<sub>2</sub>O<sub>2</sub> production during the electrochemical oxygen reduction reaction
(ORR). In the present work, we demonstrate that controlled coating
of Pt catalysts with amorphous carbon layers can induce selective
end-on adsorption of O<sub>2</sub> on the Pt surface by eliminating
accessible Pt ensemble sites, which allows significantly enhanced
H<sub>2</sub>O<sub>2</sub> production selectivity in the ORR. Experimental
results and theoretical modeling reveal that 4-electron pathway is
strongly suppressed in the course of ORR due to a thermodynamically
unfavored end-on adsorption of O<sub>2</sub> (the first electron transfer
step) with 0.54 V overpotential. As a result, the carbon-coated Pt
catalysts show an onset potential of ā¼0.7 V for ORR and remarkably
enhanced H<sub>2</sub>O<sub>2</sub> selectivity up to 41%. Notably,
the produced H<sub>2</sub>O<sub>2</sub> cannot access the Pt surface
due to the steric hindrance of the coated carbon layers, and thus
no significant H<sub>2</sub>O<sub>2</sub> decomposition via disproportionation/reduction
reactions is observed. Furthermore, the catalyst shows superior stability
without considerable performance degradation because the amorphous
carbon layers protect Pt catalysts against the leaching and ripening
in acidic operating conditions