14 research outputs found
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Investigation of Electrocatalysts Produced by a Novel Thermal Spray Deposition Method.
Common methods to produce supported catalysts include impregnation, precipitation, and thermal spray techniques. Supported electrocatalysts produced by a novel method for thermal spray deposition were investigated with respect to their structural properties, elemental composition, and electrochemical performance. This was done using electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Various shapes and sizes of catalyst particles were found. The materials exhibit different activity towards oxidation and reduction of Fe. The results show that this preparation method enables the selection of particle coverage as well as size and shape of the catalyst material. Due to the great variability of support and catalyst materials accessible with this technique, this approach is a useful extension to other preparation methods for electrocatalysts
Comparative NEXAFS, NMR and FTIR study of various-sized nanodiamonds â as-prepared and Fluorinated
International audienceVarious 4â50 nm in size diamond nanoparticles prepared by different synthesis methods and their fluorinated derivatives were studied by NEXAFS, solid state NMR and FTIR spectroscopy. C 1s and F 1s NEXAFS spectra of as-prepared and fluorinated nanodiamonds (NDs and F-NDs) were analyzed based on a comparison with the known ones of reference compounds (graphitized carbon nanodiscs, phenol and amino acid molecules, graphite oxide and monofluoride). It has been found that all the studied diamond nanoparticles have crystalline diamond cores and their surfaces are covered with graphite-like carbon clusters. These clusters are partially amorphized and oxidized with the formation of functional groups CâOH, CâO, or OâCâOH and the properties of these surface shells depend on the synthesis method of nanodiamonds. The fluorination of diamond nanoparticles has a purely superficial character; it almost completely cleans the NDs particles from carbon clusters and saturates dangling bonds on the surface of the diamond nanoparticles with F atoms forming covalent Ï(CâF) bonds. NEXAFS data are further supported by NMR and FTIR spectroscopy, leading to similar conclusions concerning the properties of various NDs and the chemical bonding between C and F atoms in F-NDs. A combination of NEXAFS, solid state NMR and FTIR spectroscopy is demonstrated to be very efficient in investigating various NDs and their functionalized derivative
Near-Ambient Pressure XPS and MS Study of CO Oxidation over Model Pd-Au/HOPG Catalysts: The Effect of the Metal Ratio
In this study, the dependence of the catalytic activity of highly oriented pyrolytic graphite (HOPG)-supported bimetallic Pd-Au catalysts towards the CO oxidation based on the Pd/Au atomic ratio was investigated. The activities of two model catalysts differing from each other in the initial Pd/Au atomic ratios appeared as distinctly different in terms of their ignition temperatures. More specifically, the PdAu-2 sample with a lower Pd/Au surface ratio (~0.75) was already active at temperatures less than 150 °C, while the PdAu-1 sample with a higher Pd/Au surface ratio (~1.0) became active only at temperatures above 200 °C. NAP XPS revealed that the exposure of the catalysts to a reaction mixture at RT induces the palladium surface segregation accompanied by an enrichment of the near-surface regions of the two-component Pd-Au alloy nanoparticles with Pd due to adsorption of CO on palladium atoms. The segregation extent depends on the initial Pd/Au surface ratio. The difference in activity between these two catalysts is determined by the presence or higher concentration of specific active Pd sites on the surface of bimetallic particles, i.e., by the ensemble effect. Upon cooling the sample down to room temperature, the reverse redistribution of the atomic composition within near-surface regions occurs, which switches the catalyst back into inactive state. This observation strongly suggests that the optimum active sites emerge under reaction conditions exclusively, involving both high temperature and a reactive atmosphere
Evidence for the Bifunctional Nature of PtâRe Catalysts for Selective Glycerol Hydrogenolysis
Rhenium
substantially promotes the rate of Pt-catalyzed glycerol
hydrogenolysis to propanediols and shifts the product selectivity
from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This
work presents experimental evidence for a tandem dehydrationâhydrogenation
mechanism that occurs over a bifunctional PtâRe catalyst. Infrared
spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis
of propyl acetate were used to identify and quantify BrĂžnsted
acid sites associated with the Re component. Near-ambient-pressure
XPS revealed a range of Re oxidation states on the PtâRe catalysts
after reduction in H<sub>2</sub> at 393 and 493 K, which accounts
for the presence of BrĂžnsted acidity. A mechanism involving acid-catalyzed
dehydration followed by Pt-catalyzed hydrogenation was consistent
with the negative influence of added base, a primary kinetic isotope
effect with deuterated glycerol, an inverse isotope effect with dideuterium
gas, and the observed orders of reaction
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
Bimetallic PdâAu/Highly Oriented Pyrolytic Graphite Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity
The
model Pd and Au mono- and bi-metallic (PdâAu) catalysts
were prepared using vapor deposition of metals (Au and/or Pd) under
ultrahigh vacuum conditions on the defective highly oriented pyrolytic
graphite (HOPG) surface. The model catalysts were investigated using
the X-ray photoelectron spectroscopy and scanning tunneling microscopy
at each stage of the preparation procedure. For the preparation of
bimetallic catalysts, different procedures were used to get different
structures of PdAu particles (Au<sub>shell</sub>âPd<sub>core</sub> or alloyed). All prepared catalysts showed rather narrow particles
size distribution with an average particles size in the range of 4â7
nm. Parahydrogen-enhanced nuclear magnetic resonance spectroscopy
was used as a tool for the investigation of PdâAu/HOPG, Pd/HOPG,
and Au/HOPG model catalysts in propyne hydrogenation. In contrast
to Au sample, Pd, PdAu<sub>alloy</sub>, and Au<sub>shell</sub>âPd<sub>core</sub> samples were shown to have catalytic activity in propyne
conversion, and pairwise hydrogen addition routes were observed. Moreover,
bimetallic samples demonstrated the 2- to 5-fold higher activity in
pairwise hydrogen addition in comparison to the monometallic Pd sample.
It was shown that the structures of bimetallic PdâAu particles
supported on HOPG strongly affected their activities and/or selectivities
in propyne hydrogenation reaction: the catalyst with the Au<sub>shell</sub>âPd<sub>core</sub> structure demonstrated higher pairwise
selectivity than that with the PdAu<sub>alloy</sub> structure. Thus,
the reported approach can be used as an effective tool for the synergistic
effects investigations in hydrogenation reactions over model bimetallic
PdâAu catalysts, where the active component is supported on
a planar support