to the CO2 discharge bands with rising temperature. As
explained above, this could be caused by a lower CH con-
centration at higher temperatures due to other reaction path-
ways and higher consumption rates or a change in the
percentage of emitting CH radicals. Despite the overall
decrease in emission intensity, the signal obtained with the
nickel-based catalysts always remained stronger than that of
the raw foam and the discharge-only experiment.
reaction schemes (as they were, for example, proposed by
Bradford et al.24), it is evident that there are several steps, which
are found to be similar in the gas phase and in surface reactions.
Since an in situ investigation of the catalytic surface during
plasma interaction was not part of the investigation, the for-
mulation of an exact reaction mechanism for the interaction
between the catalysts and the DBD plasma cannot be done at
this stage. However, a tentative reaction scheme would defi-
nitely include the formation of hydrocarbon species on the
catalyst surface by their direct adsorption from the gas phase
or by adsorption of methane, followed by surface decom-
position. Afterwards, these products would be gasified by
oxygen-containing radicals from the plasma. As the con-
centration of surface oxygen species is enhanced on the catalyst
surface in the presence of the discharge, the generation rate of
carbon monoxide would be increased. Following this argu-
mentation, the result from the CH4 decomposition experiments
that the use of a catalyst in the discharge did not increase the
product yield would be the result of missing ‘‘consumption’’
reactions from hydrocarbon species on the surface.
Conclusion
Although the observed differences in the emission spectra
together with the results from numeric modeling can only be
regarded as a first step towards the understanding of the
plasma–catalyst interaction, we have tried to identify possible
schemes for an interaction between the catalyst and the
plasma. From the experiment with discharge and catalysts it
was evident that a nickel-coated catalyst in the discharge
plasma led to higher product yields in the reforming reaction.
The synergetic effect between the catalyst and the discharge in
the CO2 reforming reaction could, in principle, be caused by
two effects. First, the plasma properties could be influenced by
the presence of a metal layer, which would be measurable as a
change in the vibrational and rotational temperature of the
discharge species, as was presented for example by Chen et al.22
and Suib et al.23 An indication that such plasma-influencing
effects of the nickel deposited on the support is not the main
factor for the observed higher yields is found in the CH4
decomposition with and without catalyst that was also pre-
sented in ref. 6. In these experiments, the yield of the main
product hydrogen was nearly unchanged in the presence of
catalyst. If an influence of the metal on the plasma properties
were to be assumed, it should also be present in other reactions
like CH4 decomposition. Since this was not the case, an
explanation based on the interaction between plasma species
and the catalyst surface is more likely.
The identification of discharge species via optical emission
spectroscopy showed that the relative intensity of the CH
emission in the CO2 reforming experiments was higher when a
catalyst was present in the discharge gap. Furthermore, this
intensity followed the activity trend of the catalysts that was
also found by gas chromatography for the CO yield in the
combined catalyst–discharge experiments (pure gas dis-
charge < raw foam < nickel 4 nickel=calcium), in the sense
that the CH signal was higher for a more active catalyst.
This can be explained by either a lower consumption rate or
a higher production rate for this molecule in the presence of a
catalyst. If the production of CH is regarded as a pure plasma
process, which is reasonable, the focus should be set on the
consumption reactions. If conceivable reactions are taken from
the kinetic modeling, there are two important reactions with
high reaction rates:
Acknowledgements
The authors want to thank Eric Killer for his support of the
experimental investigations.
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