F35. Kinetics of the Fe + NO~2~ reaction at 303 K

K.  [Cappan]{.underline} and C. Vinckier

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 (^5^D~4~) atoms with NO~2~ has been investigated
in a fast flow reactor at a temperature of 303 K

$\text{Fe+NO}_{2}\overrightarrow{\text{k1}}\text{FeO+NO}$ ∆H= -101 kJ
mol^-1^ (1)

For the generation of Fe-atoms in the gasphase , the relatively volatile
metal compound FeCl~3~(s) was first thermally evaporated. The produced
gas phase components Fe~x~Cl~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 H~2~/Ar mixture. Hydrogen atoms rapidly convert a
fraction of the Fe~x~Cl~3x~ oligomers into iron atoms according to the
overall mechanism :

$\text{FeCl}_{3}(s)\overrightarrow{DT}\text{Fe}_{x}\text{Cl}_{\text{3x}}(g)\overrightarrow{H}\text{.}\text{.}\text{.} \rightarrow \text{Fe}(g)\text{+products}$
(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 NO~2~, 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 k~1~ 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 k~1~ has been checked. There was no effect of
the hydrogen concentration as long as \[H~2~\] was below 1.6×10^14^
molecules cm^-3^. A variation of the temperature of the FeCl~3~ pellet ,
which determines the gas phase concentration of Fe~x~Cl~3x~ oligomers,
had no influence on the measured value of k~1~. The reactor pressure was
varried between 6-12 Torr Ar and did not systematically affect the value
of k~1~ which confirms the second order character of reaction 1. The
average value of k~1~ is found to be equal to (6.0 ± 1.1)×10^-11^ cm^3^
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:
$\text{k= }(\text{ 4}\text{.}\text{84}_{\text{-1}\text{.}\text{10}}^{+ 1\text{.}\text{24}}) \times \text{10}^{- \text{10}}\text{exp}( - 2\text{.}\text{60} \pm 0\text{.}\text{56}\text{ kJmol}^{\text{-1}}/\text{RT})$cm^3^
molecule^-1^ s^-1^. This leads to a rate coefficient of (1.7 ±
0.4)×10^-10^ cm^3^ 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 cm^2^ 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

**[Presentation mode]{.underline}**: We prefer to present this paper in
a poster session