10 research outputs found
Adjusting the Chemical Reactivity of Oxygen for Propylene Epoxidation on Silver by Rational Design: The Use of an Oxyanion and Cl
The development of
catalysts for propylene oxide production from
direct epoxidation using propylene and oxygen remains a challenge.
Compared to ethylene epoxidation, where selectivity on silver catalysts
is high, the low selectivity to produce propylene oxide over silver
is partially attributed to the lack of electrophilic oxygen under
propylene epoxidation reaction conditions. Here, we investigate how
to mediate the chemical reactivity of oxygen by theory-inspired experiments
for propylene epoxidation. We show how adding electrophilic-O via
SO4 oxyanions to the surface of silver increases epoxide
selectivity. Moreover, we show how the addition of Cl to the SO4-modified catalyst activates the oxyanion, giving a more than
4-fold increase in selectivity to propylene oxide. Finally, we explore
different systems using DFT and draw a picture on how the next catalyst/co-catalyst
systems should be tuned to design a catalyst with high selectivity
for direct propylene oxidation
Ambient-Pressure Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate
In order to close the pressure gap
in the investigation of catalyst surfaces under real operation conditions
we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis
system to study the pressure dependency of the electronic and catalytic
properties of catalyst surfaces in reactive atmospheres at elevated
temperatures. With this setup we investigated the vanadium L<sub>3</sub>-edge and catalytic performance of polycrystalline vanadyl pyrophosphate
in the selective oxidation of <i>n</i>-butane to maleic
anhydride between 10 and 1000 mbar at 400 °C. As a result, major
gas phase and pressure dependent spectral changes are observed at
energies attributed to V 2p-3d<sub><i>z</i><sup>2</sup></sub> excitations assigned to vanadium atoms square-pyramidally coordinated
to oxygen atoms. This can be interpreted in terms of a shortened vanadyl
bond (VO) and an increased vanadium oxidation state with higher
pressures. Since this is accompanied by an increasing catalytic activity
and selectivity, it indicates that vanadyl oxygen is actively involved
in the selective oxidation of the alkane
When a Metastable Oxide Stabilizes at the Nanoscale: Wurtzite CoO Formation upon Dealloying of PtCo Nanoparticles
Ambient pressure photoelectron and absorption spectroscopies were applied under 0.2 mbar of O<sub>2</sub> and H<sub>2</sub> to establish an unequivocal correlation between the surface oxidation state of extended and nanosized PtCo alloys and the gas-phase environment. Fundamental differences in the electronic structure and reactivity of segregated cobalt oxides were associated with surface stabilization of metastable wurtzite-CoO. In addition, the promotion effect of Pt in the reduction of cobalt oxides was pronounced at the nanosized particles but not at the extended foil
Platinum Nanoparticles on Gallium Nitride Surfaces: Effect of Semiconductor Doping on Nanoparticle Reactivity
Platinum nanoparticles supported on n- and p-type gallium
nitride
(GaN) are investigated as novel hybrid systems for the electronic
control of catalytic activity via electronic interactions with the
semiconductor support. <i>In situ</i> oxidation and reduction
were studied with high pressure photoemission spectroscopy. The experiments
revealed that the underlying wide-band-gap semiconductor has a large
influence on the chemical composition and oxygen affinity of supported
nanoparticles under X-ray irradiation. For as-deposited Pt cuboctahedra
supported on n-type GaN, a higher fraction of oxidized surface atoms
was observed compared to cuboctahedral particles supported on p-type
GaN. Under an oxygen atmosphere, immediate oxidation was recorded
for nanoparticles on n-type GaN, whereas little oxidation was observed
for nanoparticles on p-type GaN. Together, these results indicate
that changes in the Pt chemical state under X-ray irradiation depend
on the type of GaN doping. The strong interaction between the nanoparticles
and the support is consistent with charge transfer of X-ray photogenerated
free carriers at the semiconductor–nanoparticle interface and
suggests that GaN is a promising wide-band-gap support material for
photocatalysis and electronic control of catalysis
Methanol Steam Reforming over Indium-Promoted Pt/Al<sub>2</sub>O<sub>3</sub> Catalyst: Nature of the Active Surface
The surface state of the Pt/In<sub>2</sub>O<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub> catalyst coated
onto a microchannel stainless
steel reactor was investigated under working conditions using synchrotron-based
ambient pressure photoelectron (APPES) and X-ray absorption near-edge
structure (XANES) spectroscopies, combined with online mass spectrometry.
The surface of the fresh catalyst consists of metallic Pt, In<sub>2</sub>O<sub>3</sub>, and Al<sub>2</sub>O<sub>3</sub>. Reduction
under 0.2 mbar of H<sub>2</sub> at 250 °C leads to surface enhancement
of Pt and partial reduction of In<sub>2</sub>O<sub>3</sub>, while
Al<sub>2</sub>O<sub>3</sub> remains unchanged. Reoxidation in O<sub>2</sub> atmosphere stimulates surface segregation of In<sub>2</sub>O<sub>3</sub> over Pt, accompanied by partial oxidation of Pt to
PtO<sub><i>x</i></sub>. Based on these results a dynamic,
gas-phase-dependent surface state is demonstrated. Under methanol
steam reforming conditions, the surface state rapidly adapts under
the reaction stream regardless of the pretreatment. However, correlation
of gas phase with spectroscopic results under working conditions pointed
out the beneficial effect of surface indium to reduce the CO selectivity.
Finally, evidence of a distorted symmetry of Al sites on Pt/In<sub>2</sub>O<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub> catalyst compared
to that of γ-Al<sub>2</sub>O<sub>3</sub> is given. The findings
obtained in the present study are of fundamental significance in understanding
the relation between the surface state and the catalytic performance
of a functional methanol reforming catalyst
Nature of the N–Pd Interaction in Nitrogen-Doped Carbon Nanotube Catalysts
In this work, the geometric and electronic
structure of N species
in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission
(XPS) and absorption spectroscopy (NEXAFS) of the N 1s core excitation.
Substitutional N species in pyridine-like configuration and another
form of N with higher thermal stability are found in NCNTs. The structural
configuration of the high thermally stable N species, in the literature
often referred to as graphitic N, is assessed in this work by a combined
theoretical and experimental study as a 3-fold substitutional N species
in an NCNT basic structural unit (BSU). Furthermore, the nature of
the interaction of those N species with a Pd metal center immobilized
onto NCNTs is of σ-type donation from the filled π-orbital
of the N atom to the empty d-orbital of the Pd atom and a π
back-donation from the filled Pd atomic d-orbital to the π*
antibonding orbital of the N atom. We have found that the interaction
of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical
bond with partial ionic character, consistent with the chemical shift
observed in the Pd 3d core level of divalent Pd. Graphitic N sites
interact with Pd by a covalent bond without any charge redistribution.
In this case, the electronic state of the Pd corresponds to metallic
Pd nanoparticles electronically modified by the interaction with the
support. The catalytic reactivity of these samples in hydrogenation,
CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying
some aspects of the metal carbon support interaction in catalysis
Assessment of the Degradation Mechanisms of Cu Electrodes during the CO<sub>2</sub> Reduction Reaction
Catalyst degradation and product
selectivity changes are two of
the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here,
we combine in situ X-ray spectroscopy, in
situ electron microscopy, and ex situ characterization
techniques to follow the long-term evolution of the catalyst morphology,
electronic structure, surface composition, activity, and product selectivity
of Cu nanosized crystals during the CO2 reduction reaction.
We found no changes in the electronic structure of the electrode under
cathodic potentiostatic control over time, nor was there any build-up
of contaminants. In contrast, the electrode morphology is modified
by prolonged CO2 electroreduction, which transforms the
initially faceted Cu particles into a rough/rounded structure. In
conjunction with these morphological changes, the current increases
and the selectivity changes from value-added hydrocarbons to less
valuable side reaction products, i.e., hydrogen and
CO. Hence, our results suggest that the stabilization of a faceted
Cu morphology is pivotal for ensuring optimal long-term performance
in the selective reduction of CO2 into hydrocarbons and
oxygenated products
Assessment of the Degradation Mechanisms of Cu Electrodes during the CO<sub>2</sub> Reduction Reaction
Catalyst degradation and product
selectivity changes are two of
the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here,
we combine in situ X-ray spectroscopy, in
situ electron microscopy, and ex situ characterization
techniques to follow the long-term evolution of the catalyst morphology,
electronic structure, surface composition, activity, and product selectivity
of Cu nanosized crystals during the CO2 reduction reaction.
We found no changes in the electronic structure of the electrode under
cathodic potentiostatic control over time, nor was there any build-up
of contaminants. In contrast, the electrode morphology is modified
by prolonged CO2 electroreduction, which transforms the
initially faceted Cu particles into a rough/rounded structure. In
conjunction with these morphological changes, the current increases
and the selectivity changes from value-added hydrocarbons to less
valuable side reaction products, i.e., hydrogen and
CO. Hence, our results suggest that the stabilization of a faceted
Cu morphology is pivotal for ensuring optimal long-term performance
in the selective reduction of CO2 into hydrocarbons and
oxygenated products
Assessment of the Degradation Mechanisms of Cu Electrodes during the CO<sub>2</sub> Reduction Reaction
Catalyst degradation and product
selectivity changes are two of
the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here,
we combine in situ X-ray spectroscopy, in
situ electron microscopy, and ex situ characterization
techniques to follow the long-term evolution of the catalyst morphology,
electronic structure, surface composition, activity, and product selectivity
of Cu nanosized crystals during the CO2 reduction reaction.
We found no changes in the electronic structure of the electrode under
cathodic potentiostatic control over time, nor was there any build-up
of contaminants. In contrast, the electrode morphology is modified
by prolonged CO2 electroreduction, which transforms the
initially faceted Cu particles into a rough/rounded structure. In
conjunction with these morphological changes, the current increases
and the selectivity changes from value-added hydrocarbons to less
valuable side reaction products, i.e., hydrogen and
CO. Hence, our results suggest that the stabilization of a faceted
Cu morphology is pivotal for ensuring optimal long-term performance
in the selective reduction of CO2 into hydrocarbons and
oxygenated products
Reactivity of Carbon in Lithium–Oxygen Battery Positive Electrodes
Unfortunately,
the practical applications of Li–O<sub>2</sub> batteries are
impeded by poor rechargeability. Here, for the first time we show
that superoxide radicals generated at the cathode during discharge
react with carbon that contains activated double bonds or aromatics
to form epoxy groups and carbonates, which limits the rechargeability
of Li–O<sub>2</sub> cells. Carbon materials with a low amount
of functional groups and defects demonstrate better stability thus
keeping the carbon will-o’-the-wisp lit for lithium–air
batteries