10 research outputs found
Physicochemical Stabilization of Pt against Sintering for a Dehydrogenation Catalyst with High Activity, Selectivity, and Durability
Suppressing irreversible catalyst
deactivation is critical in heterogeneous
catalysis. In particular, deactivation via sintering of active sites
is a significant issue for reactions involving harsh reaction/regeneration
conditions. In this work, we developed a PtGa/Ī³-Al<sub>2</sub>O<sub>3</sub> alkane dehydrogenation catalyst with exceptionally
high activity, selectivity, and long-term stability by markedly suppressing
Pt sintering under harsh conditions (reaction/regeneration at >823
K). To stabilize Pt, physical and chemical stabilization strategies
were synergistically combined. For the former, Pt was introduced during
the synthesis of Ī³-Al<sub>2</sub>O<sub>3</sub> via solāgel
chemistry, which can increase the interfacial contact between Pt and
Ī³-Al<sub>2</sub>O<sub>3</sub> due to the partial entrapment
of Pt in Ī³-Al<sub>2</sub>O<sub>3</sub>. For the latter, atomically
dispersed Ce was doped on Ī³-Al<sub>2</sub>O<sub>3</sub>, which
can stabilize Pt via strong PtāOāCe interactions. Because
of effective Pt stabilization, the catalyst showed remarkably steady
activity and selectivity behaviors over the repeated reaction cycles,
although the catalyst is regenerated via simple oxidation rather than
industrially used oxychlorination. The Pt stabilization strategies
reported in this work can be applied to other metal-catalyzed reactions
that involve severe reaction/regeneration conditions
Synergistic Integration of Ion-Exchange and Catalytic Reduction for Complete Decomposition of Perchlorate in Waste Water
Ion-exchange has
been frequently used for the treatment of perchlorate
(ClO<sub>4</sub><sup>ā</sup>), but disposal or regeneration
of the spent resins has been the major hurdle for field application.
Here we demonstrate a synergistic integration of ion-exchange and
catalytic decomposition by using Pd-supported ion-exchange resin as
an adsorption/catalysis bifunctional material. The ion-exchange capability
of the resin did not change after generation of the Pd clusters via
mild ethanol reduction, and thus showed very high ion-exchange selectivity
and capacity toward ClO<sub>4</sub><sup>ā</sup>. After the
resin was saturated with ClO<sub>4</sub><sup>ā</sup> in an
adsorption mode, it was possible to fully decompose the adsorbed ClO<sub>4</sub><sup>ā</sup> into nontoxic Cl<sup>ā</sup> by
the catalytic function of the Pd catalysts under H<sub>2</sub> atmosphere.
It was demonstrated that prewetting the ion-exchange resin with ethanol
significantly accelerate the decomposition of ClO<sub>4</sub><sup>ā</sup> due to the weaker association of ClO<sub>4</sub><sup>ā</sup> with the ion-exchange sites of the resin, which allows
more facile access of ClO<sub>4</sub><sup>ā</sup> to the catalytically
active Pd-resin interface. In the presence of ethanol, >90% of
the
adsorbed ClO<sub>4</sub><sup>ā</sup> could be decomposed within
24 h at 10 bar H<sub>2</sub> and 373 K. The ClO<sub>4</sub><sup>ā</sup> adsorption-catalytic decomposition cycle could be repeated up to
five times without loss of ClO<sub>4</sub><sup>ā</sup> adsorption
capacity and selectivity
2,6-Di-<i>tert-</i>butylpyridine Sorption Approach to Quantify the External Acidity in Hierarchical Zeolites
This work was aimed to evidence that
substituted pyridine, 2,6-di-<i>tert</i>-butylpyridine,
is a suitable probe for the quantitative
investigation of the external acidity in hierarchically structured
zeolites. The 2,6-di-<i>tert</i>-butylpyridine was too large
to enter the micropores, even in wide pore zeolites, and nearly no
sites in nonmesoporous zeolites were available. Accessibility studies
of acid sites in zeolites TNU-9 and BEA involving quantitative IR
measurements with hindered 2,6-di-<i>tert</i>-butylpyridine
as a probe were performed. The extinction coefficients of the 1615
cm<sup>ā1</sup> diagnostic bands of 2,6-di-<i>tert</i>-butylpyridine interacting with BrĆønsted acid sites were determined.
Lewis acid sites were not detected with the probe. The accessibility
factor (AF) for the 2,6-di-<i>tert</i>-butylpyridine probe
molecule was defined as the number of sites detected by adsorption
of the dTBPy (external sites) divided by the total amount of acid
sites in the studied zeolites as quantified by pyridine sorption.
Upon desilication resulting in the fabrication of the secondary mesopores,
the enhanced accessibility of the protonic sites was observed. In
comparison to the mesoporous zeolites with the secondary system of
mesopores generated by alkaline leaching, considerably higher accessibility
of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated
ITQ-2 zeolite
Rational Design of the Polymeric Amines in Solid Adsorbents for Postcombustion Carbon Dioxide Capture
Substantial
efforts have been made to increase the CO<sub>2</sub> working capacity
of amine adsorbents for an efficient CO<sub>2</sub> capture. However,
the more important metric for assessing adsorbents is the regeneration
heat required for capturing a fixed amount of CO<sub>2</sub>. In this
work, we synthesized polyethyleneimine (PEI)/SiO<sub>2</sub> adsorbents
functionalized with various epoxides. This provided adsorbents with
six different amine structures showing various CO<sub>2</sub>/H<sub>2</sub>O adsorption properties. Our studies revealed that the CO<sub>2</sub> working capacity was not a decisive factor in determining
the regeneration heat required for CO<sub>2</sub> capture. This is
because the benefit of large CO<sub>2</sub> working capacity was canceled
out by the difficulty of CO<sub>2</sub> desorption. Instead, the suppression
of H<sub>2</sub>O co-adsorption was critical for reducing the regeneration
heat because substantial latent heat is required for H<sub>2</sub>O desorption. Consequently, the PEI/SiO<sub>2</sub> functionalized
with 1,2-epoxybutane required a much lower regeneration heat (2.66
GJ tCO<sub>2</sub><sup>ā1</sup>) than the conventional PEI/SiO<sub>2</sub> (4.03 GJ tCO<sub>2</sub><sup>ā1</sup>) because of
suppressed H<sub>2</sub>O co-adsorption as well as moderately high
CO<sub>2</sub> working capacity
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
Optimum Utilization of Biochemical Components in <i>Chlorella</i> sp. KR1 via Subcritical Hydrothermal Liquefaction
Product
distributions in bio-crude, aqueous phase, and solid residue
were rigorously analyzed during the hydrothermal liquefaction (HTL)
of <i>Chlorella</i> sp. KR1 in order to optimize utilization
of energy and chemicals. A non-asphaltene (paraffinic) fraction in
the bio-crude, which can be readily upgraded to high-quality fuels
via a subsequent catalytic process, was mainly produced due to lipid
extraction. Above 170 Ā°C, lipid extraction was almost complete,
and hence, the non-asphaltene content did not increase further with
increasing temperature. Carbohydrates could be extracted, mainly as
polysaccharides, in the aqueous phase at mild temperatures (<200
Ā°C). At high temperatures (>200 Ā°C), they decompose and
react with proteins via the Maillard reaction to form asphaltene (polycyclic
aromatics), which contains large amounts of heteroatoms such as N
and S. Although high-temperature carbohydrate conversion could yield
more bio-crude with high energy values, it dominantly contributed
to formation of the asphaltene fraction, which is difficult to upgrade
catalytically. As high-temperature HTL requires a large energy input,
the recovery and utilization of intact carbohydrates and proteins
at mild temperatures (<200 Ā°C) appears to be more promising.
Energy Return on Investment (EROI) analysis also showed that 170 Ā°C
is the optimum HTL temperature for maximizing the energy production
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
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
Maximizing Biojet Fuel Production from Triglyceride: Importance of the Hydrocracking Catalyst and Separate Deoxygenation/Hydrocracking Steps
Various
parameters in the catalytic hydroconversion of triglycerides
(palm oil) were carefully investigated for maximizing the production
of biojet fuel. The results showed that the deoxygenation of triglyceride
via hydrotreatment should be carried out in a separate reactor prior
to the hydrocracking step (i.e., two-step reaction process). Otherwise,
the CO generated during deoxygenation can poison the metal components
in the metal/acid bifunctional catalysts (Pt/zeolites), which can
cause significant imbalance between the metal and acid functions in
hydrocracking. This leads to fast catalyst deactivation via coke formation,
heavy formation of aromatics, and overcracking of hydrocarbons, resulting
in the reduction of final biojet fuel yield. In the two-step process,
the second hydrocracking step mainly determines the final biojet fuel
yield, and thus, a rational design of the hydrocracking catalysts
that can suppress overcracking is essential. The diffusion characteristics
of the multibranched hydrocarbon (e.g., 2,2,4-trimethylpentane) in
the hydrocracking catalysts could be correlated with the yields of
the jet fuel-range C8āC16 hydrocarbons and the <i>iso</i>/<i>n</i>-paraffin ratios. The result indicates that the
facile diffusion of multibranched isomers out of catalysts before
excessive cracking is important for the suppression of the formation
of light hydrocarbons (ā¤C7). Consequently, Pt supported on
nanocrystalline large-pore BEA zeolite showed the largest biojet fuel
yield with the highest <i>iso</i>-paraffin content. Under
the optimized conditions, 55 wt % of biojet fuel with respect to palm
oil was achieved after final distillation, which satisfied all the
required fuel specifications
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