I. INTRODUCTION

Plasma chemistry and catalyst chemistry both have long history in both research and industry, and they have proven themselves as effective tools in chemical industry [1-4]. Actually, plasma and catalyst has somewhat opposite characteristics. Catalyst is selective in its reactions, but plasma is unselective [5, 6]. Catalyst is thermally controlled or temperature can be the only controlling parameter, but plasma is electrically controlled. Electric field determines the aspects of discharge. In catalytic reaction, thermal activation is the only way to initiate the reaction, but in the case of plasma, there are diverse paths for the initiation. High electric field, focused thermal energy, or high frequency excitation can initiate the discharge, and thus the reaction. Because the characteristics above are obviously opposite, combination of the two in a reaction system seems at a glance simple adding up of independent processes. However, researchers expected a synergistic effect of the hybridization and have investigated it for decades. Most of all what we should answer before investigating the hybridization of plasma and catalyst is whether the hybridization has a positive or negative effect on the chemical reaction. Catalyst deactivation upon poisoning by plasma and charge dissipation by the catalyst can be the possible negative effects of hybridization. Even when the effect of hybridization is positive, we should have means to utilize and maximize the synergy effect. All the aspects above require basic understanding on what happens when plasma and catalyst meet in one reaction. This paper addresses the interactions in plasma-catalyst hybridization, which are classified into two topics: the effect of catalyst on the discharge phenomena and the effect of discharge on the catalytic reaction.

With the preliminary results on this topic, perspectives on the applications of this new technology are presented.

II. INTERACTIVE ASPECTS

A. Effect of Catalyst on Discharge The effect of the catalyst on discharge phenomena

should be understood in view of electrical aspects, discharge of the gas system, and geometric factors.

Regarding the electrical aspects, we should focus on the dielectric properties of the catalyst and distribution of the electric field. In the case of dielectric barrier discharge (DBD), dielectricity of the discharge space determines the electric field distribution. Because there are many different types of catalysts, such as precious metals, transition metals, metal oxides, perovskite, and so on, the material properties of the catalysts themselves can alter the discharge characteristics. Onset voltage can be varied according to the dielectric strength of the catalyst.

Fig. 1 shows the Lissajous plots of a catalyst at different temperatures. As the dielectric strength of the catalyst depends on temperature, appropriate discharge voltage conditions should be considered before designing the reaction system

The shape of the catalyst is also a very important parameter affecting the discharge characteristics. Fig. 2 compares the discharge currents of the DBD reactors empty between the dielectric barriers, filled with spherical alumina beads, or filled with catalyst-coated alumina beads.

The figure clearly shows that the time scale of current flow and current peak configuration depends on the discharge volume conditions. The difference is mainly due to different electric fields. The catalyst or the alumina beads can induce distortion of electric field causing high local electric field, where a strong streamer can be produced.

Interactive Aspects of Plasma-catalyst Hybrid Reaction

D. H. Lee, S. Jo, and Y. -H. Song

Plasma Engineering Laboratory, Korea Institute of Machinery & Materials, Korea

Abstract—In this paper, interactive aspects of plasma-catalyst hybrid reaction are addressed. Plasma and catalyst

have somewhat opposite characteristics. However, the two reaction systems can be combined synergistically to bring new aspects to chemical reactions, such as low temperature catalyst activation. In the course of hybridizing plasma and catalyst in one reaction system, caution is required. Using both plasma and catalyst does not simply sum up their functions, but their interactions affect each reaction. Plasma alters the catalyst in view of reaction kinetics, material properties, and thermal environment of a reaction. Also, the catalyst alters the discharge process by changing the discharge electric fields and thus, resulting in different current flow. The selection of the catalyst and the design of the discharge system should be based on the understanding of these interactive aspects of hybridization. Plasma-catalyst hybridization has potential to produce a synergistic effect that can open new era in the reaction control and chemical industry.

Keywords—Plasma, catalyst, interaction, hybrid reaction, plasma catalysis

Corresponding author: Dae Hoon Lee e-mail address: dhlee@kimm.re.kr Presented at the Joint Symposium on Plasma and Electrostatic Technologies for Environmental Applications, in May 2013

Lee et al. 115

The aspects of the effect of ferroelectric catalyst on the discharge can be analyzed as electrically equivalent circuit model [7]. Detailed discussion on the distortion of

electric field and the resulting change in the electron density can be found in literature [8].

Different discharge conditions inevitably induce different chemistry within the reactor. The results in Fig. 3 [9] on the effect of catalyst (on alumina support) on activation show a clear difference in CH4 conversions.

The catalyst induces different discharge conditions where the electron densities and energies are different. Methane can be decomposed according to the following equations: [10]

CH4 + e → CH3 + H : 9 eV (1) CH4 + e → CH2 + 2H : 10 eV (2) CH4 + e → CH + 3H : 11 eV (3) CH4 + e → C + 4H : 12 eV (4)

From the equations above, higher CH4 conversions

could be explained by higher density of high energy electron that is produced in the case of alumina bead-filled conditions. This suggests the idea on what kind of catalyst (or support) works positively on the discharge.

Here we should remember that the original function of the catalyst is initiating a chemical reaction. It means that during the discharge, catalyst can change the gas

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Fig. 1. Lissajous plots of the same plasma-catalyst system at different temperatures (300 and 500°C).

Fig. 2. Comparison of current flow characteristics in different

reaction systems, and the effect of catalyst packing on discharging volume.

Fig. 3. Comparison of CH4 conversions at different packing

conditions

116 International Journal of Plasma Environmental Science & Technology, Vol.7, No.2, JULY 2013

reactant of the discharging gas system. Catalyst not only affects the discharge phenomena but also catalyzes selective chemical reactions. From detailed analysis of the reaction products presented in Fig. 3, although the reaction proceeds at room temperature, catalyst not only affects the conversion of CH4 but also the successive reaction steps. Fig. 4 shows C3 species in the reaction above. Pt catalyst enhances the selectivity of C3H8.

Collision of a CH4 molecule with high-energy electron most likely produces a CH3 radical. CH3 is the most abundant primary radical in the plasma activation of CH4 according to Equation (1), and the CH3 radicals soon dimerize to C2H6, because the reaction is initiated at room temperature where it has lower probability for thermal dehydrogenation [11]. Here, the probability of reaction in Equation (5) increased in the presence of catalyst. Results on the selectivity are presented in Fig. 4.

C2H5 + CH3 → C3H8 (5)

The geometry of the catalyst is also an important

factor in the discharge. In the case of packed-bed type plasma catalyst system, the spatial dimension of the catalyst (or support) is an important parameter. Small size of the catalyst (or support) can induce high local electric fields and result in stronger discharge. This can be verified by the results of discharge characteristics of different reaction systems, where alumina beads of different diameters were placed in the discharge volume. Fig. 5 shows the results. The smaller the diameter of the alumina bead, the stronger the discharge. This means that adjusting the catalyst (or support) size can be used as a design parameter for controlling the energy of produced electrons in the reaction. The optimal diameter can vary according to the characteristics of catalyst and reaction. Detail discussion on this issue can be found in the literature [12, 13].

B. Effect of Plasma on the Catalyst So far, we discussed the effect of catalyst on the discharge phenomena and next, the effect of plasma on the catalyst will be addressed. The effect of plasma on the catalyst should be investigated in view of reaction kinetics, material structure, and thermal aspects. First, change in the reaction kinetics is the most important effect of plasma on the catalytic reaction. Plasma generates radical based reactions. As is well-known, almost all catalytic reactions are based on the adsorption of molecules on catalytic surface and the subsequent Hinshelwood-type reactions [14]. However, once plasma is combined with the reaction, gas molecules that experience collision with high-energy electrons or ions (also with excited gas molecules) produce radicals before being adsorbed on the catalytic surface. This means that different types of reactions are possible: 1) reaction initiated by the radicals adsorbed on the catalytic surface, and 2) Eley-Rideal-type reaction between the plasma-produced radicals and adsorbed species on the catalytic surface. These new reaction pathways can produce positive enhancement of the reaction. Fig. 6 shows a difference in the hydrogen consumption of a reduction process in the absence and presence of plasma. Hydrogen is introduced in the oxidized Cu/ZnO/-Al2O3 catalyst, which can be reduced, and the reduction rate is controlled by temperature. Profile of the hydrogen consumption reflects the degree of activation in the reduction process. As is clearly shown in Fig. 6, plasma widens the effective temperature range and decreases the reaction temperature. Also, the rate of hydrogen consumption increased significantly by the plasma (data not shown). It means that hydrogen is decomposed into atomic hydrogen before it is adsorbed on the catalytic surface, accelerating the rapid reaction and lowering the energy barrier of H and O. This is critical evidence on the effective acceleration of specific reactions by plasma through radical-induced reaction kinetics.

Fig. 4. C3 species produced during the CH4 activation in the presence of alumina beads only and catalyst-coated beads.

Blank symbol for catalyst coated beads and filled symbol for alumina beads only.

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Fig. 5. Dependence of discharge on the bead size inside the

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Lee et al. 117

In addition to the simple reaction of hydrogen oxidation above, radical-induced reaction can accelerate more complex reactions by bypassing the rate-determining steps. In the case of methanol conversion, there are three rate-determining steps, shown in Fig. 7, [15] where plasma can cut the C-H, C-OH, or O-H bonds directly, and the subsequent catalytic adsorption of these

radicals can omit the rate-determining steps I and III in Fig. 7. The advantage of radical-induced reactions is the acceleration through higher probability of initiation, but it has also negative aspects related to adsorption conditions. In most plasma-catalyst reactions, especially at rather high temperatures, where the catalyst itself has sufficient catalytic activity, the combination of plasma and catalyst has negative effects on the overall reaction rate. As shown in Fig. 8 [14], the conversion of methanol shows a clear decrease especially when the temperature and electric power are high. This is mainly due to the nature of the plasma-catalyst combined reaction. Because radical-induced reaction produces multiple primary radicals from one methanol molecule, these radicals and methanol molecules compete in the occupation of the active sites of catalytic surface. This mechanism actually reduces the number of active sites and results in a decrease in conversion. This is because both plasma (high power) and catalyst (high temperature) have relatively high levels of activation. The result implies that the synergistic effect can be maximized under the conditions where the functioning of plasma or catalyst, or both are somewhat deficient. One should bear in mind that under the conditions where both plasma and catalyst work sufficiently, the generation of synergistic effect for a new mechanism needs to be

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reduction of Cu/ZnO/-Al2O3 catalyst by hydrogen. Blue line represents the conditions in the presence of plasma, red line in the

absence of it.

Fig. 7. Kinetic steps of methanol decomposition, where rate-determining steps have been defined.

118 International Journal of Plasma Environmental Science & Technology, Vol.7, No.2, JULY 2013

considered to avoid the negative effect. Another important issue that should be addressed in the plasma-catalyst hybridization is the heating effect. Generation of discharge inevitably induces heating of the electrode and the dielectric layer [16], and the concern is whether the heating affects the catalytic reaction. Because the catalytic reaction is controlled by temperature, the increase in temperature should be estimated. When testing DBD packed with alumina beads, temperature elevation was within 3oC when specific energy density (SED) was 1.6 kJ/L. The reaction was steam reforming of methanol that is endothermic with heat of reaction 49.3 kJ/mol. For this reason, increase of bulk temperature is limited and local temperature by discharge is estimated to be within 3oC. Temperature increase of 3oC is not critical for the catalyst under the tested conditions, and the contribution of dielectric

heating for overall conversion of methanol is negligible compared to that of direct interaction of plasma and catalyst. In the case of non-thermal plasma, joule heating is also negligible once strong streamer and arc are prevented in the course of discharge. Finally, the adsorption issues should be addressed. Many studies have been conducted on the effects of plasma on the adsorption on metal surfaces [17-19]. However, direct and concrete evidence on the adsorption mechanism is not available, and the adsorption mechanism in the hybridization is not yet fully understood. Future work on this issue is required to optimize the hybrid reaction.

III. DISCUSSION AND CONCLUSION

Plasma catalyst hybridization is an emerging

technology that can help overcoming the physical limitations of existing catalysts based chemical processes. However, the synergistic effect is not yet fully understood. Most important question is whether the effect of hybridization is negative or positive. The interaction can have both positive and negative effects, and some instances for such cases are presented in this paper. The aspects of the interaction should be investigated separately on the viewpoint of the effect of plasma on the catalyst and the effect of catalyst on the plasma. In fact, both are strongly coupled and it is hard to isolate the independent aspects of interaction. In the reactive system design, following items should be considered:

1) Geometric effect of the catalyst. Geometry,

including the porosity, the length scale, as well as smoothness of the surface, is one of important parameters that affects the electric field re-distribution resulting in different current flow.

2) Material properties of the catalyst. Dielectricity and the electrical conductivity affect the onset of discharge and electric field within the reactive system.

3) Reaction temperature. Temperature controls (1) electrical properties of the catalyst (or support) material, and (2) the catalytic effect on the gas system.

4) Radical-based reactions. Kinetic pathways of the reaction can be varied according to the relative amount of primary radicals produced by plasma, and the possibilities of accelerating the reaction by passing some of the rate-determining steps should be considered.

5) Possible increase in temperature. In our preliminary tests, the expected temperature increase was not very high, but in some conditions, depending on the material properties of the catalyst, temperature may possibly increase to the degree that can affect the overall reaction.

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Lee et al. 119

6) The effects of plasma conditions on the adsorption. The degree of adsorption is expected to depend on the degree of excitation by plasma. Proper design of the excitation level should be conducted before designing the reaction, because most of the excitation that affects the adsorption has rather low energy level of around and below 1 eV.

The interaction of plasma and catalyst has diverse aspects controlled by several reaction parameters. It is important to get a picture on the interaction mechanisms for designing the reaction system. Proper design of the plasma-catalyst interactive system will enable overcoming the issues of the yield of catalyst reaction and the cost of plasma reaction.

ACKNOWLEDGMENT

This work is supported by MKE (Ministry of Knowledge Economy) and ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea, Grant number B551179-11-03-00.

REFERENCES [1] J. J. Carberry, Chemical and Catalytic Reaction Engineering,

General Publishing Company, Toronto, 2001. [2] M. P. Dudukovic, "Trends in catalytic reaction engineering,"

Catalysis Today, vol. 48, pp. 5-15, 1999. [3] A. Fridman and L. A. Kennedy, Plasma Physics and Engineering,

CRC Press, 2001. [4] A. Fridman, Plasma Chemistry, Cambridge University Press,

New York, 2008. [5] U. Roland, F. Holzer, and E. D. Kopinke, "Combination of non-

thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds Part 2. Ozone decomposition and deactivation of -Al2O3," Applied Catalysis B-Environmental, vol. 58, pp. 217-226, 2005.

[6] H. L. Long, S. Y. Shang, X. M. Tao, Y. P. Yin, and X. Y. Dai, "CO2 reforming of CH4 by combination of cold plasma jet and Ni/-Al2O3 catalyst," International Journal of Hydrogen Energy, vol. 33, pp. 5510-5515, 2008.

[7] A. Mizuno, Y. Yamazaki, S. Obama, E. Suzuki, and K. Okazaki, "Effect of voltage waveform on partial discharge in ferroelectric pellet layer for gas cleaning," IEEE Transactions on Industry Applications, vol. 29, pp. 262-267, 1993.

[8] W. S. Kang, J. M. Park, Y. Kim, and S. H. Hong, "Numerical study on influences of barrier arrangements on dielectric barrier discharge characteristics," IEEE Transactions on Plasma Science, vol. 31, pp. 504-510, 2003.

[9] S. Jo, T. Kim, D. H. Lee, W. S. Kang, and Y.-H. Song, "Effect of the electric conductivity of a catalyst on methane activation in a dielectric barrier discharge reactor," Plasma Chemistry and Plasma Processing, submitted.

[10] T. Nozaki, A. Hattori, and K. Okazaki, "Partial oxidation of methane using a microscale non-equilibrium plasma reactor," Catalysis Today, vol. 98, pp. 607-616, 2004.

[11] D. H. Lee, K. T. Kim, Y. H. Song, W. S. Kang, and S. Jo, "Mapping Plasma Chemistry in Hydrocarbon Fuel Processing Processes," Plasma Chemistry and Plasma Processing, vol. 33, pp. 249-269, 2013.

[12] H. H. Kim, J. H. Kim, and A. Ogata, "Microscopic observation of discharge plasma on the surface of zeolites supported metal nanoparticles," Journal of Physics D-Applied Physics, vol. 42, 135210, 2009.

[13] M. Chen, K. Takashima, and A. Mizuno, "Effect of pellet-diameter on discharge characteristics and performance of a

packed bed reactor," International Journal of Plasma Environmental Science and Technology, vol. 7, pp. 89-93, 2013

[14] T. Kim, D. H. Lee, S. Jo, and Y. -H. Song, "Synergetic Mechanism of Methanol-Steam Reforming Reaction in a Catalytic Reactor with Electric Discharges," Applied Energy, submitted

[15] D. H. Lee and T. Kim, "Plasma-catalyst hybrid methanol-steam reforming for hydrogen production," International Journal of Hydrogen Energy, vol. 38, pp. 6039-6043, 2013.

[16] A. P. Papadakis, "Numerical Analysis of the Heating Effects of an Atmospheric Air-Dielectric Barrier Discharge," IEEE Transactions on Plasma Science, vol. 40, pp. 811-820, 2012.

[17] J. T. Yardley and C. Bradley Moore, "Laser-excited vibrational fluorescence and energy transfer in methane," Journal of Chemical Physics, vol. 45, pp. 1066-1067, 1966.

[18] T. V. Choudhary and D. W. Goodman, "Methane activation on Ni and Ru model catalysts," Journal of Molecular Catalysis a-Chemical, vol. 163, pp. 9-18, 2000.

[19] A. Bukoski and I. Harrison, "Assessing a microcanonical theory of gas-surface reactivity: Applicability to thermal equilibrium, nonequilibrium, and eigenstate-resolved dissociation of methane on Ni(100)," Journal of Chemical Physics, vol. 118, pp. 9762-9768, 2003.

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