Chemistry Reference
In-Depth Information
concentrations [221-226], see also Section 8.1.2. There is of course a wealth of
literature dealing with the NO
x
and VOC problem and so only three examples dealing
with new aspects are presented here.
One of the key issues when studying plasma processing for gas treatment is to
make sure that no undesirable by-product results from the process. Among them, NO
x
are readily produced in air plasmas. The production of undesirable NO and NO
2
and
the removal of acetylene, an example of a Volatile Organic Compound (VOC), has
been studied in a pulsed low-pressure dc discharge in air. The influence of changing
pulse duration, of the pulse repetition rate and of a photocatalyst is reported.
Both NO and NO
2
could be measured ex situ simultaneously using TDLAS
spectroscopy downstream of the plasma region. In contrast to what was expected, the
use of short pulses did not lead to an effective curtailment of the NO
x
production. It
was found that the NO
x
formation depended only on the average power injected into
the plasma independent of the pulse duration and repetition rate. In order to explain
this feature a simplified analytical calculation has been developed which considers
about 30 kinetic reactions involved in NO
x
formation. The calculations lead to a fair
agreement with the experimental results [227].
The disadvantage of downstream experiments, being separated from the plasma
region and being naturally limited in time resolution, has also been overcome in DC
discharges. TDLAS has been applied in situ in a pulsed low-pressure DC discharge
of dry air. Under these experimental conditions a time resolution of about 1 ms could
be achieved, which was an important step for analyzing plasma chemical phenomena
in single discharge pulses. It was found that the NO concentration is approximately
proportional to the product of the pulse current and the pulse duration. The role of
vibrationally excited nitrogen molecules in NO formation was discussed. Numerical
computation of a simplified kinetic model for NO formation, taking into account the
N
2
(A) excited metastable state, showed good agreement, see Figure 6.13 [228].
A higher time resolution, up to nanoseconds, using pulsed QCL, has opened up
a new approach for studying kinetic phenomena in molecular plasmas in real time
and in situ. The scan through an infrared spectrum is commonly achieved by two
different methods. In the inter pulse mode a bias DC ramp is applied to a series
of short laser pulses of a few tens of nanoseconds [229,230]. Another option is
the intra pulse mode, i.e., scanning in single, longer pulses and acquiring an entire
spectrum [231]. Since this scan is performed in tens up to a few hundred nanoseconds
a time resolution below 100 ns is feasible for quantitative in-situ measurements of
molecular concentrations in plasmas. This fits very well for making measurements
of rapidly changing chemical processes. The time decay of NO in single discharge
pulses has been studied based on this new approach for fast in-situ plasma diagnostics
Figure 6.14. At the center of interest was the kinetics of the destruction of NO in
a pulsed DC discharge. It transpires that the QCLAS measurements, accompanied
by simplified model calculations, serve as a powerful noninvasive temperature probe
with a remarkable time resolution up to the sub-microsecond timescale giving insight
into the gas heating dynamics [232].
Lastly, the effect of combining plasmas and photocatalysts for VOC removal
was investigated in a pulsed low-pressure DC discharge. The photocatalyst was TiO
2
while the VOC was acetylene (1000 ppm) diluted in dry air. The temporal evolution of
