Mass-spectrometric investigation of reaction of oxygen atoms with methane

Abstract

A mass-spectrometric stirred-reactor technique was used t o study the reaction of atomic oxygen with methane with low concentrations of molecular oxygen. An overall rate constant for oxygen-atom disappearance due t o added methane was obtained for the temperature range 375 to 576 OK. This rate constant was 4 X 1014 exp (-10 000/RT) cm3 m~l e -~s -l and was compared with a previous rate constant for this reaction that had been measured in the presence of a large excess of molecular oxygen. This comparison showed that the presence of a n excess of molecular oxygen produced a considerable increase in oxygen-atom consumption a t low temperatures, but only a small increase a t high temperatures. From product analyses, the stoichiometry of the reaction could be approximately represented by From the reaction stoichiornetry and the rate constant for the oxygen-atom disappearance, the rate constant for methane disappearance was calculated t o be 1 X 1014 exp (-10 000/RT) cm3 mole-' s-'. The presence of excess molecular oxygen has very little effect on this rate constant, contrary to the result found for the disappearance of atomic oxygen.

The kinetics of gas phase reactions studied in a "homogeneous reactor"

“…Such behaviour was, in fact, observed in experiments in which the CH30H concentration covered an order of magnitude or more and plots of the left hand side of eq. [7] against [CH30H] were linear as expected. This leads to the conclusion that at least a significant part of the difficulty in applying the homogeneous reactor equations was a result of quenching of the NO2 chemiluminescence by methanol.…”

“…Although much of the work in the literature which uses this method has employed relatively expensive instrumentation (4)(5)(6)(7)(8)(9), it has also been used successfully with much less costly apparatus (12)(13)(14). We were surprised, then, when our initial efforts to apply the homogeneous reactor method gave results which were in very poor agreement with data reported in the literature.…”

“…In the first method (4)(5)(6)(7)(12)(13)(14)(15), the inlet and exit atom concentrations are measured and are related to the rate constant for the reaction by where [Ale and [AIL are the inlet and exit atom concentrations respectively, v is the volumetric flow rate through the reactor and V is the reactor volume. The assumption is made that diffusional mixing is so fast that the exit atom concentration equals the bulk (homogeneous) atom concentration.…”

“…The principles of kinetic measurements in a "well-stirred or 1 "homogeneous" reactor, first developed by Denbigh (1, 2), 1 have had sporadic application to the study of gas phase reactions 1 of atoms and radicals (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). While considerable success has i been attained at the relatively low pressures of the VLPP method of Benson and co-workers (10, l l ) , the validity of the I homogeneity assumption at the higher pressures used in other work (3)(4)(5)(6)(7)(8)(9)(12)(13)(14)(15) is unclear and has not been examined 1 directly.…”

“…While considerable success has i been attained at the relatively low pressures of the VLPP method of Benson and co-workers (10, l l ) , the validity of the I homogeneity assumption at the higher pressures used in other work (3)(4)(5)(6)(7)(8)(9)(12)(13)(14)(15) is unclear and has not been examined 1 directly. The criteria for efficient mixing have received some attention (4,9,12), but the effect on the use of homogeneous reactors of the homogeneous atom sink represented by the excess reactant has been largely neglected in this higher pressure region.…”