F35. Kinetics of the Fe + NO₂ reaction at 303 K

Department of Chemistry
K.U.Leuven
Celestijnenlaan 200F
B-3001 Leuven Belgium

E-mail: kevin.cappan@chem.kuleuven.ac.be
E-mail: chris.vinckier@chem.kuleuven.ac.be

The reaction (1) of Fe (⁵D₄) atoms with NO₂ has been investigated in a fast flow reactor at a temperature of 303 K

D H= -101 kJ mol^-1 (1)

For the generation of Fe-atoms in the gasphase , the relatively volatile metal compound FeCl₃(s) was first thermally evaporated. The produced gas phase components Fe_xCl_3x,g (with x = 1 or 2) were then mixed with hydrogen atoms present in the afterglow of a microwave-induced plasma afterglow in a H₂/Ar mixture. Hydrogen atoms rapidly convert a fraction of the Fe_xCl_3x oligomers into iron atoms according to the overall mechanism :

(2)

The iron atoms were detected by atomic absorption spectroscopy at 248.3 nm. The iron absorbance was followed as a function of the reaction time in the presence of an excess of NO₂, so that pseudo first order conditions for iron atom decays were fully established. The observed decays are determined by the pseudo first order rate constant k₁ and by the loss of iron atoms due their diffusion to the reactor wall. The full mathematical formalism has been described in previous papers [ 1-3] .

The influence of a number of microwave-induced plasma afterglow parameters on the value of k₁ has been checked. There was no effect of the hydrogen concentration as long as [ H_2] was below 1.6´ 10¹⁴ molecules cm^-3. A variation of the temperature of the FeCl₃ pellet , which determines the gas phase concentration of Fe_xCl_3x oligomers, had no influence on the measured value of k₁. The reactor pressure was varried between 6-12 Torr Ar and did not systematically affect the value of k₁ which confirms the second order character of reaction 1. The average value of k₁ is found to be equal to (6.0 ± 1.1)´ 10^-11 cm³ molecule^-1 s^-1. Plane and coworkers [ 4] also investigated the same reaction using a pulsed laser photolysis/ LIF technique. They reported an expression in the temperature range from 192-471 K : cm³ molecule^-1 s^-1. This leads to a rate coefficient of (1.7 ± 0.4)´ 10^-10 cm³ molecule^-1 s^-1 at 303 K which is aproximately a factor 2.8 higher than our result.

Since iron is a non-volatile metal atom at our reactor conditions, the loss on the reactorwall is almost entirely diffusion controlled. A value of 251.4 cm² s^-1 Torr has been derived for the diffusion coefficient of iron in Ar at 303 K.

The temperature dependence of the title reaction and the diffusion coefficient will be reported at the meeting.

References

[ 1] Talcot C., Ager III J., Howard C., J.Chem.Phys., 1986, 84, 6161

[ 2] Vinckier C, Christiaens P, J. Phys. Chem., 1992, 96, 8423

[ 3] Vinckier C, Christiaens P, J. Phys. Chem., 1992, 96, 2146

[ 4] Plane JMC., Rollason RJ.,Phys. Chem. Chem. Phys., 1999 , 1 , 1843