H2S has been used as a probe molecule both in an “in
situ” poisoning experiment and in intermediate-temperature
heat-treatment steps during and after the preparation of FeNC catalysts
in an attempt to analyze its effect on their ORR activity. The heat
treatments were employed either on the ball-milled precursor of FeNC
or after the Ar-NH3 high temperature heat treatments. ORR
activity of the H2S-treated catalysts was seen to be significantly
lower than the sulfur-free catalysts, whether the sulfur exposure
was during a half-cell testing, or as an intermediate-temperature
exposure to H2S. The incorporation of sulfur species and
interaction of Fe with sulfur were confirmed by characterization using
XPS, EXAFS, TPO, and TPD. This study provides crucial evidence regarding
differences in active sites in FeNC versus nitrogen-containing carbon
nanostructured (CN
x
) catalysts.
While
undesirable in aviation fuel systems, water is both ubiquitous
and tenacious; thus, interactions between water and aviation turbine
fuel occur regularly. From a fuel user perspective, it is important
to know, understand, and be able to predict such fuel–water
interactions, e.g., water solubility, water settling rate, and interfacial
tension, for proper mitigation. We explore these interactions as well
as surface tension of both petroleum-derived and alternative jet fuels
to compare potential differences between product compositions on these
physical interactions. Observations indicate a positive, nonlinear
correlation between water solubility and both aromatic content and
temperature (from 0 to 50 °C). Water settling rates appear to
follow a Stokes’ law model; therefore, bulk chemical composition
indirectly influences settling rates via density and viscosity. Finally,
surface tension appears positively correlated to sample density, while
interfacial tension is correlated to both surface tension and fuel
aromatic content.
To
more effectively utilize jet fuel as a thermal management fluid on
board an aircraft, it is necessary to understand the changing chemical
composition of the fuel under high temperature (350–800 °C)
and pressure (300–1000 psi) pyrolytic conditions. Toward this
aim, we have performed in situ characterization of
neat supercritical fuel surrogates and a fuel (Jet A) primarily comprised
of saturated hydrocarbons using quadrupole mass spectrometry. We directly
probe the pyrolytic fluid via supersonic expansion into a vacuum,
which rapidly cools the reaction mixture. We ionize the resulting
molecular beam using electron impact ionization (10 eV electron kinetic
energy) and identify reactants, intermediates, and products with a
quadrupole mass filter. Although different fuels exhibit distinct
pathways by which they crack into lighter intermediates, the aromatic
product distributions become indistinguishable as the temperature
is increased. We attribute these similarities to the fast cracking
rates at extreme temperatures, which funnel into relatively few small
hydrocarbon intermediates (C2 and C3) from which benzene and larger
aromatics are synthesized. The smaller aromatics and their methyl-substituted
derivatives evolve into larger polycyclic aromatic hydrocarbons at
the highest temperatures, providing insight into the elementary steps
of coke formation. Such a mechanism, which is consistent with that
of soot formation in flames, is sensible in a high-temperature regime,
in which cracking rates increase and residence times lengthen as coke
is deposited on the reactor nozzle.
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