Effect of Polarity on Ozone Production of DC Corona Discharge with and Without Photocatalyst

Effect of polarity on ozone production of DC corona discharge with and without photocatalyst

S. Pekárek

Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic

Abstract: Experimental results dealing with polarity effects of the needle to mesh DC corona discharge in air with or without TiO 2 photocatalyst on ozone production are presented. It was found that discharge without TiO 2 with the needle positive produces higher ozone concentra- tions than the discharge with the needle negative. Addition of TiO 2 increases ozone production and in case of the needle negative about six times increases ozone production yield.

Keywords: Ozone, DC corona discharge, polarity, photocatalyst.

1. Introduction charge in a streamer regime. Present efforts in the research of ozone generation by Discharge ozone production also depends on presence electrical discharges are focused on obtaining higher of catalysts or ferroelectrics in the discharge region. In ozone concentrations, higher ozone generating efficien- [10] the ozone generation characteristics of a cies via better understanding of ozone formation and de- point-to-mesh DC corona discharge with ferroelectric struction plasma-chemistry, adjusting electrical parame- pellet barrier on the mesh have been experimentally in- ters of the discharge and involving new principles and vestigated. This configuration utilizes a wider plasma ideas to the process of ozone formation [1]. reacting area between the top surfaces of every pellet bar- In case of DC corona discharge the ozone production rier and the mesh electrode. It was found that the mean depends among others also on the polarity of the coronat- corona current and ozone concentration significantly in- ing electrode. Results presented in the literature dealing crease when a negative DC voltage was applied. Introduc- with the polarity effects on discharge ozone production tion of photocatalyst such as TiO 2 in the discharge region are a little bit ambiguous. Thus in [2, 3] were studied po- can increase production of reactive species. Titanium di- larity effects of the DC corona with coaxial electrode ar- oxide TiO 2 is an n-type semiconductor with the width of rangement on discharge ozone production. The discharge the forbidden gap 3.2 eV, which corresponds to the wave- took place in air stream flowing transverse to the corona length of radiation 388 nm. The TiO 2 can be activated by discharge wire. It was found that ozone production rate in ultraviolet radiation of the discharge, which for the dis- the negative corona is an order of magnitude higher than charge in air at atmospheric pressure comes mainly from in the positive corona. This result was explained by the the de-excitation of nitrogen molecules. In [11] it was effect of discharge polarity on the number of energetic found that introduction of TiO 2 in the discharge region electrons in the corona plasma. Assessment of ozone gen- and its subsequent photoexcitation contributes to the for- - eration in dc coronas was performed in [4]. Presented mation of superoxide anion O 2 , which can increase total results show that the ozone generation rate from the nega- catalytic activity. In [12] it was experimentally demon- tive corona is almost seven to ten times that from the strated that ozone concentration and corresponding energy positive corona. Corona ozone production in N 2-O2 mix- yield achieved by packed-bed reactors with glass beads ture at volume ratio 4:1 for the wire-duct and point-plane and Al 2O3 pellets are significantly higher than that reactors stressed by the negative and positive pulses was achieved by dielectric barrier discharge only. Ozone gen- treated in [5]. It was found that concentration of ozone eration by hollow needle to mesh negative corona generated by positive pulses is higher than that generated discharge enhanced by the flow of air through the needle by negative pulses. This result was explained by the in- without and with TiO 2 on the mesh was treated in [13]. It crease of the volume reaction zone in case when the reac- was found that addition of TiO 2 on the mesh shifts the tor was stressed by positive pulse voltages. Similar results transition from the glow into the streamer regime of the were obtained for the discharge in air with wire-cylinder discharge into higher currents, increases discharge ozone or wire-plane electrode configuration in [6,7]. The effect production as well as the ozone production yield. It was of polarity on DC corona discharge ozone production is also found that ozone production depends on the mass of sometimes associated with different corona regimes [8]. the TiO 2 and its location in the discharge chamber. Thus about seven times increase in ozone production in In this study we focused on the effect of polarity on negative corona discharge [9] in comparison with positive electrical parameters and ozone production of DC corona corona was found for the glow regime. Results presented discharge at atmospheric pressure enhanced by the flow in [5] for the positive corona were obtained for the dis- of air through the needle electrode with and without TiO 2. 2. Experimental set up characteristics of the breakdown and the formation of the The experimental set up is shown in Fig. 1. The elec- discharge were sensitive to the value of the ballast resistor, trode arrangement consisted from a hollow stainless steel we used for the needle negative ballast resistor R = 3.92 needle and a stainless steel mesh. M and for the needle positive R = 17.86 M . The mesh was situated perpendicularly to the needle. Results concerning ozone production are presented as The electrodes were placed in a circular glass discharge a function of energy density, which is defined as a ratio of chamber of the inner diameter 32.1 mm. The needle had the power delivered to the discharge divided by the air- an inner and outer diameter 0.7 mm and 1.2 mm respec- flow through the needle. The energetic efficiency of the tively. The tip of the needle was sharpened at the angle discharge ozone production is described by the ozone 15 o. The mesh had rhombus cells dimensions 0.60×0.50 production yield α, which is defined in the following way: mm and thickness 0.15 mm. For experiments we used hollow needle to mesh elec- 21 4. ×(Ozone conc .)× Airflow × 6 ×10 −3 trodes or the same electrodes with the globules of Aero- α = [g/kWh], U × I lyst 7706 TiO 2 photocatalyst on the mesh. The anatase content of this photocatalyst is minimum 70 %, the den- where ozone concentration is substituted in ppm, airflow sity is ~3.8 g/cm 3 and the surface area is 40-50 m 2/g. The in slm, discharge voltage U and current I in kV and mA cylindrical globules had the diameter ~3 mm and the respectively. height ~4 mm. Air from a cylinder was supplied into the needle. Mass 3.1. Electrical parameters of the discharge flow controller MFC adjusted the airflow through the Discharge ozone production could not be separated needle. At the output of the discharge tube, was placed the from electrical parameters of the discharge. Fig. 2 shows sensor of temperature T and the sensor of relative humid- the discharge voltage-current characteristics (V-A) for ity RH. A fan cooled the discharge tube. both polarities of the coronating needle electrode and for For study of polarity effects on discharge ozone pro- the discharge without and with TiO 2 photocatalyst on the duction we used two regulated DC HV power supplies. mesh. It is seen that for the needle biased negatively for The first one with negative output terminal provided particular current addition of TiO 2 photocatalyst on the voltage up to 30 kV. The second one with positive output mesh electrode substantially decreases discharge voltage. terminal provided voltage up to 25 kV. Thus for the current 0.25 mA addition of TiO 2 photocata- Ozone concentration was measured by the absorption of lyst on the mesh decreases the discharge voltage from the 254 nm U.V. spectral line with API 450 ozone monitor. 20.2 to 12.7 kV. On the other hand for the needle biased positively for MFC lower currents addition of TiO 2 photocatalyst on the mesh electrode slightly increases discharge voltage and for higher currents addition of this photocatalyst on the mesh Needle R V does not influence discharge voltage. Thus for the current Catalyst 0.25 mA addition of the same mass of TiO 2 photocatalyst Discharge chamber as for the needle negative on the mesh increases discharge mA Mesh voltage from 14.8 kV to 15.3 kV. T RH

API 450 22,5

20,0 Mesh without TiO2, negative Output Mesh+TiO2, 17,5 positive Fig. 1. Experimental set up. 15,0

12,5 Discharge [kV] voltage 3. Experimental results and discussion Mesh without TiO2, positive Experiments were performed with the needle biased 10,0 Mesh+TiO2, negative either negatively or positively, with the airflow through 7,5 the needle electrode 1.5 slm, the distance between the 5,0 needle and the mesh 12 mm and relative humidity of air 0,0 0,1 0,2 0,3 0,4 0,5 0,6 3.7 %. Temperature of air at the output from the discharge Discharge current [mA] chamber varied from 21 to 25 ºC. Mass of the TiO 2 photocatalyst globules arranged in one layer on the mesh Fig. 2. Effect of polarity and TiO photocatalyst on was 2.070 g. 2 voltage-current characteristics of the discharge. As far as the stability of the discharge as well as the The differences in V-A characteristics for the needle needle is biased negatively for particular energy density 3 biased negatively and positively are caused by the differ- (e.g. 202 kJ/m ) addition of TiO 2 photocatalyst on the ent discharge mechanisms as well as the presence of the mesh more than five times increases concentration of photocatalyst on the mesh. ozone produced by the discharge (from 160 to 825 ppm). If the needle is negative, then avalanche multiplica- In case of the needle biased positively maximum ozone tion takes place. The secondary process is the emission of concentration is reached for lower energy density (148 electrons from the cathode, mostly induced by ion impact. kJ/m 3), however increase of ozone concentration due to The photoionization in the bulk plasma is also possible. If the addition of TiO 2 is not so significant. the needle is positive the remote cathode does not partici- Ozone production yield versus energy density for pate in electron multiplication because of the weak elec- both polarities of the needle, airflow rate through the nee- tric field in its vicinity. The reproduction of electrons is dle 1.5 slm and for the discharge without and with TiO 2 ensured by secondary photo-processes in the gas around on the mesh is shown in Figure 4. As it is seen from this the needle tip. figure maximum ozone production yield is obtained for the needle negative with TiO 2 photocatalyst on the mesh. In this case addition of TiO on the mesh about six times 3.2 Discharge ozone production 2 increases ozone production yield in comparison with At our first experiment we placed TiO photocatalyst 2 ozone production yield for the needle positive. on the mesh in the discharge chamber. It was found that when the discharge was switched off there was no ozone 45 in the stream of air leaving the discharge. 40

Another experimental fact that has been observed 35 when TiO 2 was placed on the mesh was that there was a 30 Negative, mesh+TiO2 time lag between the moment when the discharge was 25 switched on and the time when a stable ozone production 20 Positive, mesh 15 was reached. When the cycle of switching off and without TiO2 10 switching on the discharge was repeated several times the Positive, 5 ozone production was increased until it reached saturation Ozoneproduction [g/kWh] yield mesh+TiO2 0 state. Ozone concentrations presented in the following Negative, mesh without TiO2 -5 figure correspond to this saturation state. 0 50 100 150 200 250 300 The concentration of ozone produced by the dis- Energy density [kJ/m 3] charge versus energy density for both polarities of the Fig. 4. Ozone production yield versus energy density for needle, airflow rate through the needle 1.5 slm and for the both polarities of the needle. discharge without and with TiO globules on the mesh is 2 shown in Figure 3. Our results dealing with the ozone production for the case of the hollow needle to the mesh discharge enhanced 1000 by the flow of air through the needle without TiO 2 photo- catalyst show that when the needle is positive the dis- 800 Negative, mesh+TiO2 charge generates higher concentrations of ozone than

600 when the discharge is negative. This result is in agreement with the results presented in [5,9]. 400 According to [3] the most significant differences in

Positive, mesh Positive, mesh+TiO2 positive and negative corona plasma are the size of the 200 without TiO2 plasma region, the distribution of the number density of Concentrationof [ppm] ozone electrons and the effect of the gas temperature on that 0 Negative, mesh without TiO2 distribution. In our case due to the supply of air through 0 50 100 150 200 250 300 the needle electrode into the discharge region we affect Energy density [kJ/m 3] motion of negative ions (e.g. oxygen), we shift transi- Fig. 3. Concentration of ozone produced by the discharge tion of the discharge from the glow to the streamer regime versus energy density for both polarities of the needle. to higher current and consequently we influence the size of the plasma region, the distribution of the number First conclusion, which can be taken from this figure, density of electrons and the gas temperature. All these quantities influence discharge ozone production is that for the discharge without TiO 2 photocatalyst maximum ozone concentration is higher for the discharge For the discharge with TiO 2 photocatalyst on the with the needle positive (563 ppm) than that for the dis- mesh the discharge is more efficient source of ozone than charge with the needle negative (160 ppm). When the the discharge without TiO 2 photocatalyst. This conclusion is valid for both polarities of the Though our experiments were performed with hol- needle electrode. It is possible to explain this result by the low-needle to mesh electrode configuration, it is reason- fact that apart of two classical processes leading to the able to expect that qualitatively similar results concerning ozone generation for the discharge without photocatalyst the TiO 2 effect on ozone production could be obtained there appears additional third process associated with the also with other discharge electrode systems e.g. wire to presence of photocatalyst on the mesh. cylinder. The first two processes involve: (a). dissociation of oxygen and nitrogen molecules by energetic electrons Acknowledgements produced by the discharge with the subsequent ozone This work was supported by the Grant Agency of the generation according to O + O 2 + M → O 3 + M. (b) Czech Republic under contract 202/09/0176. The author dissociation of oxygen molecule by UV radiation pro- would also like to thank Dr. K. Hensel for valuable com- duced by the discharge of wavelengths shorter than 242 ments dealing with the effect of polarity on DC corona nm with the subsequent ozone generation. discharge ozone production. The third process involves activation of TiO 2 by UV radiation emitted by the discharge and its contribution to References ozone production. TiO 2 is a n-type semiconductor with [1] U. Kogelschatz, Plasma Physics and Controlled Fu- valence band separated from the conduction band by a sion, 46, Suppl. 12B, B63 (2004). forbidden energy gap of 3.2 eV. Incidence of UV radiation [2] J. Chen and J. H. Davidson, Plasma Chem. and of a wavelength shorter than 388 nm promotes electrons Plasma Processes, 23, 501 (2003). from the valence band to the largely vacant conductance [3] J. Chen and J. H. Davidson, Plasma Chem. and band. Simultaneously, a positive hole with strong oxidiz- Plasma Processes , 23, 83 (2003). ing capability is formed: [4] A. Yehia, M. Abdel Salam and A. Mizuno, J. Phys. D: Appl. Phys. 33 , 831 (2000). + - TiO 2 + hf → h + e [5] M. Abdel Salam, A. Mizuno and K. Shimizu, J. Phys. The negative electron from titanium dioxide and D: Appl. Phys. 30 , 864 (1997). positive-hole can react with the molecules in the vicinity [6] W. J. M. Samaranayake, Y. Miyahara, T. Namihira, S. of the catalyst. In air the electron reacts with an oxygen Katsuki, T. Sakugawa, R. Hackam, H. Akiyama, - molecule to form the superoxide anion O 2 : IEEE Trans. on Diel. & El. Ins. 7, 254 (2000). [7] B. Held, Proc. of the 11 th Int. Conf. on Gas Discharges - - e + O 2 → O 2 and their Applications, Tokyo, 2, 514 (1995). This cycle continues as long as the ultraviolet radia- [8] H.H. Kim, Plasma Process. Polym. 1, 91 (2004). tion is available. The superoxide anions contribute to the [9] D.K. Brandvold, P. Martinez, D. Dogruel, Atmos. En- ozone generation processes. viron. 23 , 1881 (1989). [10] J.D. Moon, S.T. Geum, G.T. Lee, D.K. Park, J. of the 4. Conclusions Korean Physical Society, 38, 680 (2001). Hollow needle to mesh negative or positive DC co- [11] S. Chavadey, W. Kiatubolpaiboon, P. Rangsunvigit, T. rona discharge at atmospheric pressure without and with a Streethawong, J. Mol. Catal A: Chem. 263, 128 TiO 2 photocatalyst was investigated from the standpoint (2007). of its ozone production. The discharge was enhanced by [12] H. L. Chen, H. M. Lee, M. B. Chang, Ozone Sc. & the flow of air through the needle electrode. Eng. E 28, 111 (2006). For the discharge without TiO 2 the discharge with the [13] S. Pekárek, Eur. Phys. J. D, 50 , 171 (2008). needle positive produces higher ozone concentrations than the discharge with the needle negative. Maximum ozone concentration for the needle positive (569 ppm) is ob- tained for lower energy density (147 kJ/m 3) than maxi- mum ozone concentration (163 ppm) for the needle nega- tive (201 kJ/m 3). We showed that addition TiO 2 on the mesh electrode strongly influences V-A characteristics of the discharge. For particular energy density addition of TiO 2 on the mesh electrode increases discharge ozone production for both polarities of the coronating electrode. This effect is stronger for the needle negative. We also found that addi- tion TiO 2 on the mesh for the needle negative about six times increases ozone production yield.