7 research outputs found
Asymmetric Hydrogenation of α‑Amino Ester Probed by FTIR Spectroscopy
Asymmetric hydrogenation
reaction of dehydro-α-amino acid
(i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst
has been studied by an array of in situ infrared spectroscopic methods,
including transmission, diffuse reflectance (DR), and attenuated total
reflectance (ATR). Transmission FTIR spectra probed the hydrogenation
reaction process, revealed OH–O and NH–N hydrogen bonding
interactions between the adsorbed CD and during the reaction. DR and
ATR spectra of the hydrogenation reaction under different conditions,
which are consistent with but slightly different from the transmission
spectra, evidenced the successful hydrogenation of the compound. The
incorporation of DR and microfluidics flow-through design allowed
us to investigate the adsorption of CD on the Pd surface efficiently.
The results revealed that the N-bonded CD on Pd surface in a tilted
configuration had increased abundance on the Pd surface with high
coverage. These valuable insights provided an image of the reaction
pathway to the prochiral structure (precursor state)
Asymmetric Hydrogenation of α‑Amino Ester Probed by FTIR Spectroscopy
Asymmetric hydrogenation
reaction of dehydro-α-amino acid
(i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst
has been studied by an array of in situ infrared spectroscopic methods,
including transmission, diffuse reflectance (DR), and attenuated total
reflectance (ATR). Transmission FTIR spectra probed the hydrogenation
reaction process, revealed OH–O and NH–N hydrogen bonding
interactions between the adsorbed CD and during the reaction. DR and
ATR spectra of the hydrogenation reaction under different conditions,
which are consistent with but slightly different from the transmission
spectra, evidenced the successful hydrogenation of the compound. The
incorporation of DR and microfluidics flow-through design allowed
us to investigate the adsorption of CD on the Pd surface efficiently.
The results revealed that the N-bonded CD on Pd surface in a tilted
configuration had increased abundance on the Pd surface with high
coverage. These valuable insights provided an image of the reaction
pathway to the prochiral structure (precursor state)
<i>In Situ</i> Infrared Study of the Effect of Amine Density on the Nature of Adsorbed CO<sub>2</sub> on Amine-Functionalized Solid Sorbents
<i>In situ</i> Fourier
transform infrared spectroscopy
was used to determine the nature of adsorbed CO<sub>2</sub> on class
I (amine-impregnated) and class II (amine-grafted) sorbents with different
amine densities. Adsorbed CO<sub>2</sub> on amine sorbents exists
in the form of carbamate–ammonium ion pairs, carbamate–ammonium
zwitterions, and carbamic acid. The adsorbed CO<sub>2</sub> on high-amine
density sorbents showed that the formation of ammonium ions correlates
with the suppression of CH stretching intensities. An HCl probing
technique was used to resolve the characteristic infrared bands of
ammonium ions, clarifying that the band observed around 1498 cm<sup>–1</sup> is a combination of the deformation vibration of
ammonium ion (NH<sub>3</sub><sup>+</sup>) at 1508 and 1469 cm<sup>–1</sup> and the deformation vibration of NH in carbamate
(NHCOO<sup>–</sup>) at 1480 cm<sup>–1</sup>. Carbamate
and carbamic acid on sorbents with low amine density desorbed at a
rate faster than those on sorbents with high amine density after switching
the flow from CO<sub>2</sub> to Ar at 55 °C. Evaluation of the
desorption temperature profiles showed that the temperature required
to achieve the maximal desorption of CO<sub>2</sub> (<i>T</i><sub>max. des</sub>) increases with amine density. The adsorbed
CO<sub>2</sub> on sorbents with high amine density is stabilized via
hydrogen bonding interactions with adjacent amine sites. These sorbents
require higher temperature to desorb CO<sub>2</sub> than those with
low amine density
Nature of Active Sites and Surface Intermediates during SCR of NO with NH<sub>3</sub> by Supported V<sub>2</sub>O<sub>5</sub>–WO<sub>3</sub>/TiO<sub>2</sub> Catalysts
Time-resolved
in situ IR was performed during selective catalytic
reduction of NO with NH<sub>3</sub> on supported V<sub>2</sub>O<sub>5</sub>–WO<sub>3</sub>/TiO<sub>2</sub> catalysts to examine
the distribution and reactivity of surface ammonia species on Lewis
and Brønsted acid sites. While both species were found to participate
in the SCR reaction, their relative population depends on the coverage
of the surface vanadia and tungsta sites, temperature, and moisture.
Although the more abundant surface NH<sub>4</sub><sup>+</sup><sub>,ads</sub> intermediates dominate the overall SCR reaction, especially
for hydrothermally aged catalysts, the minority surface NH<sub>3,ads</sub> intermediates exhibit a higher specific SCR activity (TOF). The
current study serves to resolve the long-standing controversy about
the active sites for SCR of NO with NH<sub>3</sub> by supported V<sub>2</sub>O<sub>5</sub>–WO<sub>3</sub>/TiO<sub>2</sub> catalysts
Surface Structure Dependence of SO<sub>2</sub> Interaction with Ceria Nanocrystals with Well-Defined Surface Facets
The
effects of the surface structure of ceria (CeO<sub>2</sub>)
on the nature, strength, and amount of species resulting from SO<sub>2</sub> adsorption were studied using in situ IR and Raman spectroscopies
coupled with mass spectrometry, along with first-principles calculations
based on density functional theory (DFT). CeO<sub>2</sub> nanocrystals
with different morphologies, namely, rods (representing a defective
structure), cubes (100 facet), and octahedra (111 facet), were used
to represent different CeO<sub>2</sub> surface structures. IR and
Raman spectroscopic studies showed that the structure and binding
strength of adsorbed species from SO<sub>2</sub> depend on the shape
of the CeO<sub>2</sub> nanocrystals. SO<sub>2</sub> adsorbs mainly
as surface sulfites and sulfates at room temperature on CeO<sub>2</sub> rods, cubes, and octahedra that were either oxidatively or reductively
pretreated. The formation of sulfites is more evident on CeO<sub>2</sub> octahedra, whereas surface sulfates are more prominent on CeO<sub>2</sub> rods and cubes. This is explained by the increasing reducibility
of the surface oxygen in the order octahedra < cubes < rods.
Bulk sulfites are also formed during SO<sub>2</sub> adsorption on
reduced CeO<sub>2</sub> rods. The formation of surface sulfites and
sulfates on CeO<sub>2</sub> cubes is in good agreement with our DFT
results of SO<sub>2</sub> interactions with the CeO<sub>2</sub>(100)
surface. CeO<sub>2</sub> rods desorb SO<sub>2</sub> at higher temperatures
than cubes and octahedra nanocrystals, but bulk sulfates are formed
on CeO<sub>2</sub> rods and cubes after high-temperature desorption
whereas only some surface sulfates/sulfites are left on octahedra.
This difference is rationalized by the fact that CeO<sub>2</sub> rods
have the highest surface basicity and largest amount of defects among
the three nanocrystals, so they bind and react with SO<sub>2</sub> strongly and are the most degraded after SO<sub>2</sub> adsorption
cycles. The fundamental understanding obtained in this work on the
effects of the surface structure and defects on the interaction of
SO<sub>2</sub> with CeO<sub>2</sub> provides insights for the design
of more sulfur-resistant CeO<sub>2</sub>-based catalysts
Reaction Pathways and Kinetics for Selective Catalytic Reduction (SCR) of Acidic NO<sub><i>x</i></sub> Emissions from Power Plants with NH<sub>3</sub>
Selective catalytic reduction (SCR)
of NO<sub><i>x</i></sub> with NH<sub>3</sub> by supported
vanadium oxide catalysts
is an important technology for reducing acidic NO<sub><i>x</i></sub> emissions from stationary sources and mobile diesel vehicles.
However, rational design of improved catalysts is still hampered by
a lack of consensus about reaction pathways and kinetics of this critical
technology. The SCR fundamentals were resolved by applying multiple
time-resolved in situ spectroscopies (ultraviolet–visible light
(UV-vis), Raman and temperature-programmed surface reaction (TPSR))
and isotopically labeled molecules (<sup>18</sup>O<sub>2</sub>, H<sub>2</sub><sup>18</sup>O, <sup>15</sup>N<sup>18</sup>O, ND<sub>3</sub>). This series of experiments directly revealed that the SCR reaction
occurs at surface V<sup>5+</sup>O<sub>4</sub> sites that are maintained
in the oxidized state by O<sub>2</sub> and the rate-determining step
involves the reduction of V<sup>5+</sup>O<sub>4</sub> sites by NO
and NH<sub>3</sub>, specifically the breaking of N–H bonds
during the course of formation or decomposition of the NO–NH<sub>3</sub> intermediate
Synergistic Effects of Water and SO<sub>2</sub> on Degradation of MIL-125 in the Presence of Acid Gases
The
behavior of metal–organic frameworks (MOFs) in the presence
of acid gases may be decisive in their suitability for industrial
applications. In this study, MIL-125 and MIL-125-NH<sub>2</sub> were
investigated with SO<sub>2</sub> exposure in dry, humid, and aqueous
environments. MIL-125 was found to be unstable in both humid and aqueous
acidic environments, while MIL-125-NH<sub>2</sub> was stable under
these exposure conditions, showing no change in textural properties
or visual degradation, as observed through SEM. Both materials were
stable in the presence of water and dry SO<sub>2</sub>, suggesting
that the reaction of these molecules to form an acidic species is
likely a key factor in the degradation of MIL-125. In situ IR experiments
confirmed the presence of sulfite species, supporting the hypothesis
that the presence of an acidic sulfur species likely leads to the
degradation of the MIL-125 structure. Computational investigation
of several potential reaction mechanisms in MIL-125 indicated reactions
involving the bisulfite ion are favored over reactions with water
or SO<sub>2</sub>. DFT simulations support the observation that MIL-125-NH<sub>2</sub> is stable in humid conditions, as all reactions are less
favorable with the functionalized framework compared to the unfunctionalized
framework. This combined experimental and computational study advances
the fundamental understanding of MOF degradation mechanisms during
acid gas exposure