10 research outputs found
Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO<sub>3</sub> Nanocrystals
To
gain an in-depth understanding of the surface properties relevant
for catalysis using ternary oxides, we report the acid–base
pair reactivity of shape-controlled SrTiO<sub>3</sub> (STO) nanocrystals
for the dehydrogenation of ethanol. Cubes, truncated cubes, dodecahedra,
and etched cubes of STO with varying ratios of (001) and (110) crystal
facets were synthesized using a hydrothermal method. Low-energy ion
scattering (LEIS) analysis revealed that the (001) surface on cubes
of STO is enriched with SrO due to surface reconstruction, resulting
in a high ratio of strong base sites. Chemical treatment with dilute
nitric acid to form etched cubes of STO resulted in a surface enriched
with Ti cations and strong acidity. Furthermore, the strength and
distribution of surface acidic sites increase with the ratio of (110)
facet from cubes to truncated cubes to dodecahedra for STO. Kinetic,
isotopic, and spectroscopy methods show that the dehydrogenation of
ethanol proceeds through the facile dissociation of the alcohol group,
followed by the cleavage of the C<sub>α</sub>–H bond,
which is the rate-determining step. Co-feeding of various probe molecules
during catalysis, such as NH<sub>3</sub>, 2,6-di-<i>tert</i>-butylpyridine, CO<sub>2</sub>, and SO<sub>2</sub>, reveals that
a pair of Lewis acid site and basic surface oxygen atom is involved
in the dehydrogenation reaction. The surface density of acid–base
site pairs was measured using acetic acid as a probe molecule, allowing
initial acetaldehyde formation turnover rates to be obtained. Comparison
among various catalysts reveals no simple correlation between ethanol
turnover rate and the percentage of either surface facet ((001) or
(110)) of the STO nanocrystals. Instead, the reaction rate is found
to increase with the strength of acid sites but reversely with the
strength of base sites. The acid–base property is directly
related to the surface composition as a result from different surface
reconstruction behaviors of the shaped STO nanocrystals. The finding
in this work underscores the importance of characterizing the top
surface compositions and sites properties when assessing the catalytic
performance of shape-controlled complex oxides such as perovskites
Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia
The role of Lewis
and Brønsted sites in the dehydration of
glycerol on niobium oxide and Na<sup>+</sup>-exchanged niobium oxide
is investigated using FTIR spectroscopy supported by DFT calculations.
Glycerol is impregnated on the catalysts at room temperature using
an <i>ex-situ</i> method. Under high vacuum conditions,
glycerol forms a stable multidentate alkoxy species through its primary
hydroxyl groups with the Lewis sites. When coordinated this way, the
primary C–O bonds are polarized, favoring dehydration in this
position to form hydroxyacetone. In contrast, dehydration of the secondary
alcohol group is kinetically favored over Brønsted acid sites
in the absence of steric constraints. The primary product of this
reaction, 1,3-propenediol, is further dehydrated to acrolein. When
more than a monolayer of glycerol is impregnated on niobia, monoaromatic
compounds are also formed on the surface upon heating
Important Roles of Enthalpic and Entropic Contributions to CO<sub>2</sub> Capture from Simulated Flue Gas and Ambient Air Using Mesoporous Silica Grafted Amines
The measurement of
isosteric heats of adsorption of silica supported
amine materials in the low pressure range (0–0.1 bar) is critical
for understanding the interactions between CO<sub>2</sub> and amine
sites at low coverage and hence to the development of efficient amine
adsorbents for CO<sub>2</sub> capture from flue gas and ambient air.
Heats of adsorption for an array of silica-supported amine materials
are experimentally measured at low coverage using a Calvet calorimeter
equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized
silica materials, higher amine densities resulted in higher isosteric
heats of adsorption, clearly showing that the density/proximity of
amine sites can influence the amine efficiency of adsorbents. In a
series of materials with fixed amine loading but different amine types,
strongly basic primary and secondary amine materials are shown to
have essentially identical heats of adsorption near 90 kJ/mol. However,
the adsorption uptakes vary substantially as a function of CO<sub>2</sub> partial pressure for different primary and secondary amines,
demonstrating that entropic contributions to adsorption may play a
key role in adsorption at secondary amine sites, making adsorption
at these sites less efficient at the low coverages that are important
to the direct capture of CO<sub>2</sub> from ambient air. Thus, while
primary amines are confirmed to be the most effective amine types
for CO<sub>2</sub> capture from ambient air, this is not due to enhanced
enthalpic contributions associated with primary amines over secondary
amines, but may be due to unfavorable entropic factors associated
with organization of the second alkyl chain on the secondary amine
during CO<sub>2</sub> adsorption. Given this hypothesis, favorable
entropic factors may be the main reason primary amine based adsorbents
are more effective under air capture conditions
Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes
The
kinetics and mechanism of methanol dehydration, redox, and
oxidative coupling were investigated at 300 °C under dilute oxygen
concentration over anatase TiO<sub>2</sub> nanoplates and truncated-bipyramidal
nanocrystals in order to understand the surface structure effect of
TiO<sub>2</sub>. The two TiO<sub>2</sub> nanoshapes displayed both
(001) and (101) facets, with a higher fraction of the (001) facet
exposed on the nanoplates, while truncated-bipyramidal nanocrystals
were dominated by the (101) facet. A kinetic study using in situ titration
with ammonia shows that the active sites for methanol dehydration
are acidic and nonequivalent in comparison to redox and oxidative
coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy
is the dominant surface species for all reactions, while the observed
methanol dimer is found to be a spectator species through isotopic
methanol exchange, supporting the dissociative mechanism for methanol
dehydration via surface methoxy over TiO<sub>2</sub> surfaces. Density
functional theory calculations show that the formation of dimethyl
ether involves the C–H bond dissociation of an adsorbed methoxy,
followed by coupling with another surface methoxy on the 5-fold-coordinated
Ti cations on the (101) surface, similar to the mechanism reported
on the (001) surface. Kinetic isotope effects are observed for dimethyl
ether, formaldehyde, and methyl formate in the presence of deuterated
methanol (CD<sub>3</sub>OH and CD<sub>3</sub>OD), confirming that
the cleavage of the C–H bond is the rate-limiting step for
the formation of these products. A comparison between estimated kinetic
parameters for methanol dehydration over various TiO<sub>2</sub> nanocrystals
suggests that (001) has a higher dehydration reactivity in comparison
to (101), but the surface density of active sites could be limited
by the presence of residual fluorine atoms originating from the synthesis.
The (001) surface of TiO<sub>2</sub> is also more active than the
(101) surface in redox and oxidative coupling of methanol, which is
due to the reactive surface oxygen on (001) in comparison to the (101)
surface
Acid–Base Reactivity of Perovskite Catalysts Probed via Conversion of 2‑Propanol over Titanates and Zirconates
Although perovskite
catalysts are well-known for their excellent
redox property, their acid–base reactivity remains largely
unknown. To explore the potential of perovskites in acid–base
catalysis, we made a comprehensive investigation in this work on the
acid–base properties and reactivity of a series of selected
perovskites, SrTiO<sub>3</sub>, BaTiO<sub>3</sub>, SrZrO<sub>3</sub>, and BaZrO<sub>3</sub>, via a combination of various approaches
including adsorption microcalorimetry, in situ FTIR spectroscopy,
steady state kinetic measurements, and density functional theory (DFT)
modeling. The perovskite surfaces are shown to be dominated with intermediate
and strong basic sites with the presence of some weak Lewis acid sites,
due to the preferred exposure of SrO/BaO on the perovskite surfaces
as evidenced by low energy ion scattering (LEIS) measurements. Using
the conversion of 2-propanol as a probe reaction, we found that the
reaction is more selective to dehydrogenation over dehydration due
to the dominant surface basicity of the perovskites. Furthermore,
the adsorption energy of 2-propanol (Δ<i>H</i><sub><i>ads,</i>2<i>–propanol</i></sub>) is
found to be related to both a bulk property (tolerance factor) and
the synergy between surface acid and base sites. The results from
in situ FTIR and DFT calculations suggest that both dehydration and
dehydrogenation reactions occur mainly through the E<sub>1cB</sub> pathway, which involves the deprotonation of the alcohol group to
form a common alkoxy intermediate on the perovskite surfaces. The
results obtained in this work pave a path for further exploration
and understanding of acid–base catalysis over perovskite catalysts
Stability of Pt/γ-Al<sub>2</sub>O<sub>3</sub> Catalysts in Lignin and Lignin Model Compound Solutions under Liquid Phase Reforming Reaction Conditions
The stability of a 1 wt % Pt/γ-Al<sub>2</sub>O<sub>3</sub> catalyst was tested in an ethanol/water mixture
at 225 °C and
autogenic pressure, conditions at which it is possible to dissolve
and depolymerize various kinds of lignin, and structural changes to
the catalysts were studied by means of X-ray diffraction (XRD), <sup>27</sup>Al MAS NMR, N<sub>2</sub> physisorption, transmission electron
microscopy (TEM), H<sub>2</sub> chemisorption, elemental analysis,
thermogravimetric analysis-mass spectrometry (TGA-MS), and IR. In
the absence of reactants the alumina support is found to transform
into boehmite within 4 h, leading to a reduction in support surface
area, sintering of the supported Pt nanoparticles, and a reduction
of active metal surface area. Addition of aromatic oxygenates to mimic
the compounds typically obtained by lignin depolymerization leads
to a slower transformation of the support oxide. These compounds,
however, were not able to slow down the decrease in dispersion of
the Pt nanoparticles. Vanillin and guaiacol stabilize the aluminum
oxide more than phenol, anisole, and benzaldehyde because of the larger
number of oxygen functionalities that can interact with the alumina.
Interestingly, catalyst samples treated in the presence of lignin
showed almost no formation of boehmite, no reduction in support or
active metal surface area, and no Pt nanoparticle sintering. Furthermore,
in the absence of lignin-derived aromatic oxygenates, ethanol forms
a coke-like layer on the catalyst, while oxygenates prevent this by
adsorption on the support by coordination via the oxygen functionalities
Direct Neutron Spectroscopy Observation of Cerium Hydride Species on a Cerium Oxide Catalyst
Ceria has recently
shown intriguing hydrogenation reactivity in
catalyzing alkyne selectively to alkenes. However, the mechanism of
the hydrogenation reaction, especially the activation of H<sub>2</sub>, remains experimentally elusive. In this work, we report the first
direct spectroscopy evidence for the presence of both surface and
bulk Ce–H species upon H<sub>2</sub> dissociation over ceria
via <i>in situ</i> inelastic neutron scattering spectroscopy.
Combined with <i>in situ</i> ambient-pressure X-ray photoelectron
spectroscopy, IR, and Raman spectroscopic studies, the results together
point to a heterolytic dissociation mechanism of H<sub>2</sub> over
ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric
ceria surface or heterolytic products (Ce–H and OH) with the
presence of induced oxygen vacancies in ceria. The finding of this
work has significant implications for understanding catalysis by ceria
in both hydrogenation and redox reactions where hydrogen is involved
Quantitative Analysis of the Morphology of {101} and {001} Faceted Anatase TiO<sub>2</sub> Nanocrystals and Its Implication on Photocatalytic Activity
The atomistic structure and morphology
(shape and size) of nanomaterials
have strong influences on their physical and chemical properties.
However, many characterization techniques focus exclusively on one
length-scale regime or another when developing quantitative morphology/structural
models. In this article, we demonstrate that powder X-ray diffraction
and neutron pair distribution function (PDF) can be used to obtain
accurate average morphology and atomistic structure of {001} and {101}
faceted anatase TiO<sub>2</sub> nanocrystals based on differential
evolution refinements using Debye scattering equation calculations.
It is also demonstrated that the morphology polydispersity of TiO<sub>2</sub> nanocrystals can be effectively obtained from the diffraction
data via a numerical refinement routine. The morphology refinement
results are in good agreement with those from transmission electron
microscopy and the modeling of small angle neutron scattering data.
This method is successfully used to quantify the facet-specified photocatalytic
hydrogen evolution activity of anatase TiO<sub>2</sub> nanocrystals
with different {001} to {101} ratios. It is found that the sample
with an intermediate amount of both {001} and {101} facets shows the
best photocatalytic hydrogen evolution reaction (HER) activity. It
is expected that the simultaneous structure and morphology refinement
technique can be generally used to study the relationship between
morphology and functionality of nanomaterials
Hydrolysis of Cellobiose over Selective and Stable Sulfonated Activated Carbon Catalysts
Activated
carbon is functionalized by different treatments with
sulfuric acid and hot liquid water and used as catalyst for the hydrolysis
of cellobiose in a continuously operated fixed bed reactor. Characterization
results reveal that the chemically treated materials are more disordered
with a lower degree of graphitization, while adsorption isotherms
demonstrate that van der Waals forces dominate the interaction between
carbohydrates and the surface of catalysts. All catalysts are stable
during the hydrolysis of cellobiose under flow conditions. Carbon
catalysts with a limited fraction of sulfonic acid groups exhibit
moderate cellobiose conversion but a higher and sustained glucose
selectivity. The high selectivity is attributed to a higher fraction
of weak acid sites, where degradation of glucose only occurs to a
limited extent due to less accessibility and competitive adsorption
with cellobiose. Furthermore, the strong sulfonic acid groups are
more accessible for degradation reactions to occur. In contrast, the
catalyst with a higher fraction of sulfonic acid groups shows increased
cellobiose conversion but decreased glucose selectivity because glucose
monomers can be converted to degradation products at these sites
Toward the Design of a Hierarchical Perovskite Support: Ultra-Sintering-Resistant Gold Nanocatalysts for CO Oxidation
An
ultrastable Au nanocatalyst based on a heterostructured perovskite
support with high surface area and uniform LaFeO<sub>3</sub> nanocoatings
was successfully synthesized and tested for CO oxidation. Strikingly,
small Au nanoparticles (4–6 nm) are obtained after calcination
in air at 700 °C and under reaction conditions. The designed
Au catalyst not only possessed extreme sintering resistance but also
showed high catalytic activity and stability because of the strong
interfacial interaction between Au and the heterostructured perovskite
support