4 research outputs found
Evidence for the Bifunctional Nature of Pt–Re 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 dehydration–hydrogenation
mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared
spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis
of propyl acetate were used to identify and quantify Brønsted
acid sites associated with the Re component. Near-ambient-pressure
XPS revealed a range of Re oxidation states on the Pt–Re catalysts
after reduction in H<sub>2</sub> at 393 and 493 K, which accounts
for the presence of Brønsted 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 Pd–Au/Highly Oriented Pyrolytic Graphite Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity
The
model Pd and Au mono- and bi-metallic (Pd–Au) 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>–Pd<sub>core</sub> or alloyed). All prepared catalysts showed rather narrow particles
size distribution with an average particles size in the range of 4–7
nm. Parahydrogen-enhanced nuclear magnetic resonance spectroscopy
was used as a tool for the investigation of Pd–Au/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>–Pd<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 Pd–Au particles
supported on HOPG strongly affected their activities and/or selectivities
in propyne hydrogenation reaction: the catalyst with the Au<sub>shell</sub>–Pd<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
Pd–Au catalysts, where the active component is supported on
a planar support
The Selective Species in Ethylene Epoxidation on Silver
Silver’s
unique ability to selectively oxidize ethylene
to ethylene oxide under an oxygen atmosphere has long been known.
Today it is the foundation of ethylene oxide manufacturing. Yet, the
mechanism of selective epoxide production is unknown. Here we use
a combination of ultrahigh vacuum and in situ experimental methods
along with theory to show that the only species that has been shown
to produce ethylene oxide, the so-called <i>electrophilic oxygen</i> appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed
SO<sub>4</sub>. This adsorbate is part of a 2D Ag/SO<sub>4</sub> phase,
where the nonstoichiometric surface variant, with a formally SÂ(V+)
species, facilitates selective transfer of an oxygen atom to ethylene.
Our results demonstrate the significant and surprising impact of a
trace impurity on a well-studied heterogeneously catalyzed reaction