ISSN:
0538-8066
Keywords:
Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The large absorption cross sections for infrared radiation possessed by a few rather stable molecular species, their relatively fast vibration-translation relaxation times, and the availability of large laser fluxes at their characteristic frequencies have been effectively exploited to generate high bath temperatures for gas-phase reactions under strictly homogeneous conditions. Controlled temperatures from 500 to 1500°K have been developed with a modest CO2 laser. Rise times for heating are estimated to be in the millisecond range, with reaction times ≈ 10 seconds. Operating pressures [reagents, radiation absorber (generally SF6), and diluent] range from 10 to 100 torr. Thus laser-powered homogeneous pyrolysis (LPHP) is complementary to the single-pulse shock-tube technique [rise time ∼10-8 sec; reaction time ∼10-3 sec] for obtaining product distributions in gas-phase pyrolyses, and for measuring relative rates in the absence of hot walls. The latter may (and generally do) introduce catalytic effects. Further, LPHP is very simple and ideally suited for exploratory investigations of small amounts of reagent (≈10-6 mole); also, sample cells may be heated externally to augment the vapor pressure of slightly volatile reagents. Conventional analytical procedures (gas chromatography, mass spectrometry) were used to follow the course of pyrolysis.The primary chemical kinetic parameters, the temperature and density of the reactants, have both spatial and temporal distributions. Computer programs were developed for estimating T(r,x;t), given the radial intensity distributon in the incident beam, the absorption coefficient of the SF6 (as a function of density and temperature) for the irradiating line, as well as the mean heat capacity and thermal conductivity of the gas mixture. However, the computed profiles did not prove useful since the effects of convection, which could not be included in the calculations, perturb the initial distribution significantly within a few milliseconds. Indeed, it proved practical to enhance convection so as to homogenize the cell composition. When the incident radiation was chopped, frequencies ranging from 100 Hz to 1 Hz gave the same product distributions. A practical procedure is to use a “chemical thermometer” to establish a mean effective temperature for a given experimental configuration. Examples showing how to measure relative rate constants for molecular conversions, and the use of LPHP for qualitative determinations of product distributions as a function of temperature are presented. Finally, a number of high-temperature conversions are described to illustrate the utility of LPHP for flash thermolysis.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/kin.550070403
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