FB100 Plasma Chemical
Processes
Mgr. Ondřej Jašek, Ph.D.
jasek@physics.muni.cz
Outline
• CO2 decompostion
• Fullerenes, Diamond, Carbon nanotubes
• Graphene
• Nanoparticle formation
CO2 dissociation
CO2 → CO + ½ O2, H = 2.9 eV/mol (1)
CO2 → CO + O, H = 5.5 eV/mol (2)
CO + H2O → CO2 + H2, H = −0.4 eV/mol (3)
η = H/ECO (4)
CO2 dissociation is an important process in CO2
lasers and can be stimulated with high efficiency
with vibration excitation. It is model case for similar
“oxides” molecules and their reduction. And its
related to CO2 mitigation in exhaust gases, industrial
synthesis of fuel related applications and possible
future hydrogen production on Mars.
A. Fridman, Plasma Chemistry, Chapter 5, Cambridge University Press, 2008
CO2 dissociation
Thermal plasma systems such as electric arcs or high-pressure RF
discharges provide CO2 dissociation by a high-temperature shift of
thermodynamic equilibrium in the direction of product formation. It
can be seen from Fig. 5–3 that significant CO2 dissociation requires
plasma temperatures of about 2500–3000 K in conditions of
thermodynamic quasi equilibrium. Plasma in this case is only a
heater – a provider of the required high temperature. a) The
products of decomposition (1) generated at high temperature (Fig.
5–3), CO and oxygen, can be protected from reverse reactions only
by quenching, that is, by fast non-adiabatic cooling. If the products
escape from the hot plasma zone too slowly, the thermodynamic
quasi equilibrium is continuously sustained during the temperature
decrease and products can be converted back to CO2. Quenching
(cooling rate) 107–108 K/s. b) Absolute quenching of the CO2
dissociation requires a cooling process sufficient to save all CO
formed in the quasi-equilibrium high-temperature zone. As a result
the maximum energy efficiency value of CO2 dissociation in quasiequilibrium
thermal plasmas is only 43%.
 Higher efficiency can only be reached in non-equilibrium plasma
systems
CO2 dissociation
CO2 dissociation
CO2 Dissociation in Plasma, Stimulated by Vibrational Excitation of Molecules
The major portion of the discharge energy is transferred from plasma electrons to CO2
vibration at electron temperature typical for non-thermal discharges (Te ≈ 1 eV). Excitation
rate keV = 1–3 × 10−8 cm3/s and vibrational - translational losses are kVT ~ 10-10 exp(-72/T0
1/3)
cm3/s.
Non adiabatic CO2 dissociation is favored (5,5 eV against 7 eV) and is stimulated by step by
step vibrational excitation. Oxygen atoms can then participate in further reactions
O + CO∗
2 → CO + O2 , H = 0.3 eV/mol, Ea ≈ 0.5–1 eV/mol
This reaction is faster than the three-body recombination of atomic oxygen (O + O + M → O2 +
M) and permits one to produce a second CO molecule per dissociation event, when the
vibrational temperature is not too low (Tv ≥ 0.1 eV).
No less than 95% of the total non-thermal discharge energy at electron temperature Te = 1–2 eV
can be transferred from plasma electrons to vibrational excitation of CO2 molecules.
The CO2 dissociation through electronic excitation can be a dominant mechanism of dissociation
in non-thermal plasma with high values of reduced electric fields E/p (usually low-pressure
discharges) or when plasma is generated by degradation of very energetic particles. CO2
dissociation in plasma by means of dissociative attachment of electrons has small cross section
10-18 cm2 and energy high penalty in loss of electron, similar results are achieved with other CO2
dissociation mechanisms related to losses of charged particles, like dissociative recombination
or ion–molecular reactions.
CO2 dissociation
Asymmetric and Symmetric CO2 Vibrational Modes
At Te = 1–2 eV, plasma electrons mostly provide excitation of low vibrational levels CO∗
2(1S+).
Then VV relaxation processes lead to population of highly excited vibrational states with nonadiabatic
transition 1S+→ 3B2 and dissociation.
There are three main vibrational modes: asymmetric valence vibration n3 (energy quantum hw3 =
0.30 eV), symmetric valence vibration n1 (energy quantum hw1 = 0.17 eV), and a double degenerate
symmetric deformation vibration n2 (energy quantum hw2 = 0.085 eV).
The VV-exchange process proceeds at low levels of vibrational excitation independently along
symmetric and asymmetric CO2 vibrational modes.
The symmetric and asymmetric CO2 vibrational modes can be characterized by individual vibrational
temperatures Tva and Tvs, as well as by individual vibrational energy distribution functions. A major
contribution to the population of highly vibrationally excited states of CO2 and, hence, to
dissociation is related to the excitation of the asymmetric vibrational modes.
a) The asymmetric vibrational mode of CO2 is 1, which can be predominantly excited by
plasma electrons at electron temperatures Te = 1–3 eV. Discharge energy localization within that
vibrational mode becomes stronger when CO2 is mixed with CO, which is a product of its
dissociation.
b) The VT-relaxation rate from the asymmetric vibrational mode is much slower than that
of symmetric vibrations.
c) VV exchange along the asymmetric mode is several orders of magnitude faster than that
along symmetric modes.
CO2 dissociation
As the level of excitation increases, the vibrations of different types are collisionlessly mixed and
the situation results in in the vibrational quasi continuum of the highly excited states of CO2
molecules.
This situation can be then characterized by one-temperature approximation and two-temperature
approximation. If one mode (usually the asymmetric one) is predominantly excited, then the
asymmetric vibrational temperature significantly exceeds the symmetric one (Tva >> Tvs) and twotemperature
approximation is used. If Tva ≈ Tvs ≡ Tv one-temperature approximation is valid and in
kinetic approximation the situation is called quasi equilibrium of vibrational modes.
Note: The CO2 dissociation rate is limited not by elementary dissociation itself, but via energy
transfer from a low to high vibrational excitation level of the molecule in the VV-relaxation
processes and referred as the fast reaction limit.
From kinetics point of view The one- and two-temperature dissociation rate coefficients are not
much different numerically at typical values of non-thermal discharge parameters.
CO2 dissociation
CO2 dissociation in thermal plasma includes two phases: heating to temperatures necessary to shift
chemical equilibrium in the direction of products and fast cooling (quenching) of the dissociation
products to stabilize them from reverse reactions.
The maximum energy efficiency of quasi-equilibrium plasma-chemical systems corresponds to the ideal
quenching mode, when the CO2 conversion degree achieved in the high-temperature (heating) phase is
saved during the cooling. Ideal quenching means not only saving all main dissociation products (in this
case CO) generated at high temperature, but also conversion during fast cooling of all relevant atoms
and radicals such as C,C2, or C2O into CO. Conversion rate can also be maintained using energy
accumulated in internal degrees of freedom, mainly in molecular vibrations. It can take place when
cooling is faster than VT relaxation, and the VT non-equilibrium (Tv > T0) is achieved during quenching
this process is known as super-ideal quenching.
During slow cooling rate the products can be converted back to initial composition, if cooling is very
fast the products are stable and there are two main competing reactions recombination of the oxygen
atoms, doesn’t change conversion rate, and recombination of the oxygen atoms with the dissociation
products (CO), which decreases the conversion. But reaction rate coefficients for reaction of oxygen
atoms is 100x higher than oxygen and CO and final change is small.
The maximum value of the CO2 dissociation energy efficiency in non-thermal plasma (90%) is
significantly higher than that in thermal plasma (60%) even in the case of super-ideal quenching.
Optimal values of specific energy input are also different: in thermal plasma they are 5–10 times
higher than in non-thermal plasma and are about 3–5 eV/mol. Non equlibrium plasma also doesn’t
require quenching.
CO2 dissociation
The most common types of plasma used for CO2 conversion are dielectric barrier discharges
(DBDs), microwave (MW) plasmas and gliding arc (GA) discharges. The highest energy efficiency
was reported for a MW plasma, i.e., up to 90% but this was under very specific conditions, i.e.,
supersonic gas flow and reduced pressure (100–200 Torr), and a pressure increase to atmospheric
pressure, which would be desirable for industrial applications, yields a dramatic drop in energy
efficiency. Indeed, at normal flow conditions and atmospheric pressure, an energy efficiency up to
40% was reported. A GA plasma also exhibits a rather high energy efficiency, even at atmospheric
pressure, i.e., around 43% for a conversion of 18% in the case of CO2 splitting.
The energy efficiency of a DBD is more limited, i.e., in the order of 2–10%, but as demonstrated
already for other applications, it should be possible to improve this energy efficiency by inserting a
(dielectric) packing into the reactor, i.e., a so-called packed bed DBD reactor.
Adding catalytic functionality should enable further improvement in conversion efficiency and
selectivity of final products.
A. Bogart, T. Kozak, K. van Laer, R. Snoeckx, Plasma-based conversion of CO2: current status and
future challenges, Faraday Discuss., 2015, 183, 217.
A. Ozkan, T. Dufour, T. Silva, N. Britun, R. Snyders, F. Reniers and A. Bogaerts, DBD in burst mode:
solution for more efficient CO2 conversion, Plasma Sources Sci. Technol. 25 (2016) 055005 (9pp).
G. Chena, V. Georgieva, T. Godfroid, R. Snyders ,M.-P. Delplancke-Ogletree, Plasma assisted
catalytic decomposition of CO2, Applied Catalysis B: Environmental 190 (2016) 115–124.
T. Nunnally, K. Gutsol, A. Rabinovich, A. Fridman, A. Gutsol and A. Kemoun, Dissociation of CO2 in a
low current gliding arc plasmatron, J. Phys. D: Appl. Phys. 44 (2011) 274009 (7pp).
G. Chena, V. Georgieva, T. Godfroid, R. Snyders ,M.-P. Delplancke-Ogletree, Plasma assisted catalytic
decomposition of CO2, Applied Catalysis B: Environmental 190 (2016) 115–124.
CO2 dissociation
CO2 dissociation
A. Bogart, T. Kozak, K. van Laer, R. Snoeckx, Plasma-based conversion of CO2: current status and future challenges, Faraday Discuss., 2015, 183, 217.
A. Ozkan, T. Dufour, T. Silva, N. Britun, R. Snyders, F. Reniers and A. Bogaerts, DBD in burst mode: solution for more efficient CO2 conversion, Plasma
Sources Sci. Technol. 25 (2016) 055005 (9pp).