5 research outputs found
Pressure-Dependent IāAtom Yield in the Reaction of CH<sub>2</sub>I with O<sub>2</sub> Shows a Remarkable Apparent Third-Body Efficiency for O<sub>2</sub>
The formation of I atom and Criegee intermediate (CH<sub>2</sub>OO) in the reaction of CH<sub>2</sub>I with O<sub>2</sub> has
potential
relevance for aerosol and organic acid production in the marine boundary
layer. We report measurements of the absolute yield of I atom as a
function of pressure for N<sub>2</sub>, He, and O<sub>2</sub> buffer
at 298 K. Although the overall rate coefficient is pressure-independent,
the I-atom yield, correlated with CH<sub>2</sub>OO, decreases with
total pressure, presumably because of increased stabilization of CH<sub>2</sub>IOO. The extrapolated yield of the I + Criegee channel under
tropospheric conditions is small but nonzero, ā¼0.04. The zero-pressure
limiting I-atom yield is unity, within experimental error, implying
negligible branching to IO + CH<sub>2</sub>O. The apparent collision
efficiency of O<sub>2</sub> in stabilizing CH<sub>2</sub>IOO is a
remarkable factor of 13 larger than that of N<sub>2</sub>, which suggests
unusually strong interaction or possible reaction between the chemically
activated CH<sub>2</sub>IOO<sup>#</sup> and O<sub>2</sub>
Additional file 1: of Comparison of the complications of traditional 12 cores transrectal prostate biopsy with image fusion guided transperineal prostate biopsy
Multiparametric MRI Examination and Analysis. (DOCX 14 kb
Effect of Carbon Supports on Pd Catalyst for Hydrogenation Debenzylation of Hexabenzylhexaazaisowurtzitane (HBIW)
<p>A series of carbon materialāsupported Pd catalysts was prepared and used for hydrogenation debenzylation of hexabenzylhexaazaisowurtzitane (HBIW). The structures and morphologies of carbon supports were characterized by field emission scanning electron microscopy (FESEM), physical adsorption, and Raman spectroscopy. X-ray diffraction (XRD), H<sub>2</sub>-TPR, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize Pd-supported carbon. The species Pd, with the states, PdĀ° and PdOx, were presented to the catalysts and can be reduced to PdĀ° as the active centers during the reaction. Among layer, tube, and ordered/disordered pore structures, disordered mesoporous carbon exhibited a high content of Pd on the surface and can be used as an efficient support in terms of high catalytic activity and good recycled stability for hydrogenation debenzylation of HBIW.</p
Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions
The
present paper describes further development of the multiscale informatics
approach to kinetic model formulation of Burke et al. (Burke, M. P.;
Klippenstein, S. J.; Harding, L. B. <i>Proc. Combust. Inst.</i> <b>2013</b>, <i>34</i>, 547ā555) that directly
incorporates elementary kinetic theories as a means to provide reliable,
physics-based extrapolation of kinetic models to unexplored conditions.
Here, we extend and generalize the multiscale informatics strategy
to treat systems of considerable complexityīøinvolving multiwell
reactions, potentially missing reactions, nonstatistical product branching
ratios, and non-Boltzmann (i.e., nonthermal) reactant distributions.
The methodology is demonstrated here for a subsystem of low-temperature
propane oxidation, as a representative system for low-temperature
fuel oxidation. A multiscale model is assembled and informed by a
wide variety of targets that include <i>ab initio</i> calculations
of molecular properties, rate constant measurements of isolated reactions,
and complex systems measurements. Active model parameters are chosen
to accommodate both āparametricā and āstructuralā
uncertainties. Theoretical parameters (e.g., barrier heights) are
included as active model parameters to account for parametric uncertainties
in the theoretical treatment; experimental parameters (e.g., initial
temperatures) are included to account for parametric uncertainties
in the physical models of the experiments. RMG software is used to
assess potential structural uncertainties due to missing reactions.
Additionally, branching ratios among product channels are included
as active model parameters to account for structural uncertainties
related to difficulties in modeling sequences of multiple chemically
activated steps. The approach is demonstrated here for interpreting
time-resolved measurements of OH, HO<sub>2</sub>, <i>n</i>-propyl, <i>i</i>-propyl, propene, oxetane, and methyloxirane
from photolysis-initiated low-temperature oxidation of propane at
pressures from 4 to 60 Torr and temperatures from 300 to 700 K. In
particular, the multiscale informed model provides a consistent quantitative
explanation of both <i>ab initio</i> calculations and time-resolved
species measurements. The present results show that interpretations
of OH measurements are significantly more complicated than previously
thoughtīøin addition to barrier heights for key transition states
considered previously, OH profiles also depend on additional theoretical
parameters for R + O<sub>2</sub> reactions, secondary reactions, QOOH
+ O<sub>2</sub> reactions, and treatment of non-Boltzmann reaction
sequences. Extraction of physically rigorous information from those
measurements may require more sophisticated treatment of all of those
model aspects, as well as additional experimental data under more
conditions, to discriminate among possible interpretations and ensure
model reliability
Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH<sub>3</sub>)<sub>2</sub>COO
The
Criegee intermediate acetone oxide, (CH<sub>3</sub>)<sub>2</sub>COO,
is formed by laser photolysis of 2,2-diiodopropane in the presence
of O<sub>2</sub> and characterized by synchrotron photoionization
mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy.
The rate coefficient of the reaction of the Criegee intermediate with
SO<sub>2</sub> was measured using photoionization mass spectrometry
and pseudo-first-order methods to be (7.3 Ā± 0.5) Ć 10<sup>ā11</sup> cm<sup>3</sup> s<sup>ā1</sup> at 298 K and
4 Torr and (1.5 Ā± 0.5) Ć 10<sup>ā10</sup> cm<sup>3</sup> s<sup>ā1</sup> at 298 K and 10 Torr (He buffer). These
values are similar to directly measured rate coefficients of <i>anti</i>-CH<sub>3</sub>CHOO with SO<sub>2</sub>, and in good
agreement with recent UV absorption measurements. The measurement
of this reaction at 293 K and slightly higher pressures (between 10
and 100 Torr) in N<sub>2</sub> from cavity ring-down decay of the
ultraviolet absorption of (CH<sub>3</sub>)<sub>2</sub>COO yielded
even larger rate coefficients, in the range (1.84 Ā± 0.12) Ć
10<sup>ā10</sup> to (2.29 Ā± 0.08) Ć 10<sup>ā10</sup> cm<sup>3</sup> s<sup>ā1</sup>. Photoionization mass spectrometry
measurements with deuterated acetone oxide at 4 Torr show an inverse
deuterium kinetic isotope effect, <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = (0.53 Ā± 0.06), for reactions with SO<sub>2</sub>, which may be consistent with recent suggestions that the
formation of an association complex affects the rate coefficient.
The reaction of (CD<sub>3</sub>)<sub>2</sub>COO with NO<sub>2</sub> has a rate coefficient at 298 K and 4 Torr of (2.1 Ā± 0.5) Ć
10<sup>ā12</sup> cm<sup>3</sup> s<sup>ā1</sup> (measured
with photoionization mass spectrometry), again similar to rate for
the reaction of <i>anti</i>-CH<sub>3</sub>CHOO with NO<sub>2</sub>. Cavity ring-down measurements of the acetone oxide removal
without added reagents display a combination of first- and second-order
decay kinetics, which can be deconvolved to derive values for both
the self-reaction of (CH<sub>3</sub>)<sub>2</sub>COO and its unimolecular
thermal decay. The inferred unimolecular decay rate coefficient at
293 K, (305 Ā± 70) s<sup>ā1</sup>, is similar to determinations
from ozonolysis. The present measurements confirm the large rate coefficient
for reaction of (CH<sub>3</sub>)<sub>2</sub>COO with SO<sub>2</sub> and the small rate coefficient for its reaction with water. Product
measurements of the reactions of (CH<sub>3</sub>)<sub>2</sub>COO with
NO<sub>2</sub> and with SO<sub>2</sub> suggest that these reactions
may facilitate isomerization to 2-hydroperoxypropene, possibly by
subsequent reactions of association products