22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Novel reaction path in discharge induced reduction of oxidized metal catalyst
D.H. Lee1, T. Kim2, S. Jo2, S.H. Pyun2, K.-T. Kim2 and Y.-H. Song2
1
2
KIMM Industrial Plasma Laboratory, Daejeon, South Korea
Chosun University Department of Aerospace Engineering, Gwangju, South Korea
Abstract: Reduction of metal oxide catalyst under discharge condition is compared to that
of typical thermal reduction process. Discharge produces reactive primary radicals that are
crucial in forming excited intermediate species by Eely-Rideal type reaction path. This
kind of excited species easily desorbs without further reaction with surface hydrogen and
this process is essential in avoiding rate determining step (water desorption) in thermal
reduction. Emission spectroscopy reveals the evidence of this novel reaction path.
Resultantly, plasma reduction process can much lower the active temperature window and
rate of the reduction process compared to typical thermal reduction process.
Keywords: discharge, reduction, metal oxide, catalyst, reaction path
1. Introduction
Plasma catalysis has been drawing attention for the
reason of possible synergistic effect of low temperature
activation by plasma and high selectivity of catalyst.
However, most of the previous works are focused on the
oxidation process such as plasma catalysis for the removal
of VOC and fluoro-compound [1-6]. Oxidation by
plasma catalysis is mainly a process that utilize discharge
generated oxygen species or ozone. But the processes
using ozone are limited in their use because of the
temperature condition of ozone decomposition.
This work introduces reduction process of metal oxide.
Oxidation is one of the reasons for catalyst deactivation
and typically the oxygen is removed by hydrogen stream
in high temperature condition. Lowering the temperature
of reduction and reducing time required for the reduction
is essential in protecting a catalyst from sintering or
permanent deactivation. This work probes reaction
mechanism of accelerated reduction by plasma.
2. Experiment
Cu/ZnO/γ-Al 2 O 3 catalyst was prepared for the
experiment by wet impregnation process. ZnO was used
as a stabilizer for Cu (Cu:Zn = 3:7) after sintering at
350 °C for 3 hrs. Cu was fully oxidized to CuO at 300 °C
for 2 hrs. Catalysts are coated on 1 mm alumina bead.
Using the prepared catalyst, pack-bed reactor
configuration as shown in Fig. 1 was constructed for the
experiment. AC high voltage transformer delivers peak
voltage of 7 kV with frequency of 1 kHz. Spectral
measurement was done with the monochromator with
resolution of 0.045 nm and grating of 1200 lines/nm. The
temperature rise inside the pack bed reactor was measured
by inserting thermocouple into the catalyst soon after the
discharge was finished and showed about 3 degrees and
the temperature rise by discharge itself did not affect the
overall reaction.
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Fig. 1. Schematic of overall experimental apparatus
including optical emission spectroscopy.
3. Results
The effect of plasma reduction was evaluated based on
the comparison with that of thermal reduction. Firstly,
physical aspects of results from both the methods were
compared. Possible structural modification such as lattice
structure or pore configuration were evaluated. XRD
results after reduction showed the same pattern of Cu
peak that means both the method produces the same Cu
phase. BET results for both the catalysts reduced
thermally and under discharge showed almost the same
value for surface area and pore volume (Table 1). Pore
size and distribution also showed the same physical
configuration in the two conditions.
All of the results mean plasma reduction does not
induce any structural and morphological change of
catalyst during the reduction process.
1
Table 1. BET results of oxidized and reduced (by thermal
and plasma) state catalyst.
Surface area
(m2∙g−1)
Pore volume
(cm3∙g−1)
Pore diameter
(nm)
CuO
Cu
(Thermal)
Cu
(Plasma)
122.5475
138.2244
135.7695
0.30303
0.356134
0.354411
9.89102
10.30597
10.44157
The rate of hydrogen consumption and time required
for the full reduction in thermal and plasma reduction are
compared in Fig. 2. Reduction under discharge showed
much lowered temperature window of reduction and
much faster conversion of hydrogen. Lower limit of
reduction is lowered about 50 degree and time for total
reduction is reduced about half of that in thermal
reduction.
Fig. 2. Comparison of hydrogen conversion rate and time
for full reduction in thermal and plasma reduction.
Discharge in oxidized state and reduced state showed
much different behaviour. In reduced state, the catalyst
particles are not interconnected and do not show bulk
electrical conductivity. However, once discharge is
hosted, dispersed metal particles (catalyst) on the alumina
bead rapidly re-distribute the accumulated surface charge
around a streamer preventing generation of strong
streamers. Meanwhile, oxidized state of catalyst forms
dielectric layer and the discharge on oxidized state of
catalyst becomes similar to typical dielectric barrier
discharge [7, 8]. The catalyst contributes accumulation of
surface charge and following generation of strong
streamers. As a result, discharge fluorescence in reduced
and oxidized states showed much different appearance as
shown in Fig. 3.
Strong fluorescence around the catalyst bead implies
active generation of collision generated species. In this
case of reduction process, the discharge around the bead
was revealed to be effective in generating hydrogen
radical that is crucial in subsequent reduction process [9].
Optical emission spectroscopy (OES) results clearly
2
showed the existence of hydrogen radicals (Fig. 4)
[10, 11].
Fig. 3. Comparison of fluorescence in oxidized state (left)
and reduced state (right) in the single bead discharge
experiment.
Fig. 4. Hα and Hβ line in OES data showing existence of
gas phase H radicals.
What is interest in OES results was the existence of OH
band (OH [AX]). In comparison of OES data for
oxidized state and reduced state, only in the result of
oxidized state showed the clear existence of OH radical as
shown in Fig. 4 [12, 13].
Only the source of ‘O’ in this reaction system is lattice
O in oxidized catalyst. The result implies that OH is
produced by lattice O and H radical. In typical adsorption
process, adsorbed species lose their excessive energy in
the course of adsorption and rapidly governed by surface
equilibrium condition. Under the equilibrium condition,
adsorbed H does not have enough energy to desorb but
reacts with lattice O that also does not have enough
energy for desorption and follows successive surface
reaction to form adsorbed state of H 2 O. Because of this,
the result that gas phase OH was detected by OES (Fig. 5)
implies that surface OH is not formed by typical
adsorption process (Langmuir-Hinshelwood reaction) but
by Eley-Rideal type reaction. In the case of E-R type
reaction, especially where energetic hydrogen radical is
involved, reaction product can temporarily have energy
enough to desorb from the surface. In this case, formation
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Intensity (a.u.)
of water should be in the form of gas phase reaction
between the OH and H radical.
OH [A X]
CuO
Cu
290
300
310
Time (ns)
320
330
Fig. 5. OES data showing OH in gas phase or desorbed
OH in the case of oxidized state.
Based on the above information, novel reaction path for
the reduction under discharge can be proposed. Typical
thermal reduction process follows the process (1)-(7)
below. Here, Eqn. (4) and (5) are active in high
temperature condition of above 300 °C with sufficiently
supplied hydrogen [14, 15]:
H 2(g)  H 2(ad)
H 2(ad)  H (ad) + H (ad)
CuO + H (ad) Cu + OH (ad)
Cu + CuO  Cu 2 O
Cu 2 O + H (ad)  2Cu + OH (ad)
OH (ad) + H (ad)  H 2 O (ad)
H 2 O (ad)  H 2 O (g)
(7)
(1)
(2)
(3)
(4)
(5)
(6)
What is different under the discharge environment is
1) primary radicals before molecular adsorption,
2) Formation of intermediate OH, 3) desorption of surface
OH, and 4) gas phase reaction of HO and H with the
condition 1),
H 2(g) + e  H (g) + H (g) + e
(8)
H 2(g) + e  H2∗ (g) + e
R
(9)
Actually, the H radicals participate Eley-Rideal type
reaction with surface O as in Eqn. (10) and (11)
(condition 2)
CuO + H (g)  Cu + OH (ad)
(10)
Cu 2 O + H (g)  2Cu + OH (ad)
(11)
Desorption and gas phase reaction of OH can be
expressed as in Eqn. (12) and (13)
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(12)
OH (ad)  OH (g)
(13)
OH (g) + H (g)  H 2 O (g)
Generation of hydrogen radical is not governed by
thermal equilibrium but depends on the energy of the
electron. And because the desorption of surface water by
reaction in Eqn. (7) is rate determining step in typical
thermal reduction, removal of reaction (7) by gas phase
water formation can clearly accelerate the overall
reduction process as shown in Fig. 2 [16, 17].
4. Conclusion
Reduction process for oxidized metal catalyst was
investigated. Reduction was done both thermally and
under discharge. A reduction process under discharge
showed much accelerated reduction rate and lowered
temperature of active reduction. This work introduced the
mechanism to explain the accelerated reduction by
presence of discharge.
Discharge generates novel
reaction path that includes 1) production of hydrogen
radical before adsorption of hydrogen molecule, 2)
increased Eley-Rideal type reaction for the generation of
surface OH, 3) desorption of intermediate HO, and 4)
subsequent gas phase reaction of OH and H to produce
H 2 O. The role of discharge is essential in accelerating
and activating the process. The results prove that a
plasma catalysis can be a powerful tool that can overcome
the equilibrium condition or limitation of thermally driven
reaction.
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