Nonthermal Plasma-Enhanced Catalytic Removal of Nitrogen Oxides over V 2 O 5 /TiO 2 and Cr 2 O 3 /TiO 2 Young Sun Mok,* Dong Jun Koh, † Kyong Tae Kim, † and In-Sik Nam ‡ Department of Chemical Engineering, Cheju National University, Ara, Cheju 690-756, South Korea A nonthermal plasma process (dielectric-packed bed reactor) was combined with catalyst to remove nitrogen oxides (NOx). Two different honeycomb catalysts such as V 2 O 5 /TiO 2 and Cr 2 O 3 / TiO 2 were compared with respect to the removal characteristic of NOx. The effect of oxygen content, water vapor, feed gas flow rate, reaction temperature, and initial concentration on the removal of NOx was examined. The plasma discharge was found to largely enhance the removal of NOx on the catalyst. Without plasma discharge, V 2 O 5 /TiO 2 was superior to Cr 2 O 3 /TiO 2 in terms of NOx removal activity. However, the degree of enhancement in NOx removal as a result of plasma discharge was similar for each system. Cr 2 O 3 /TiO 2 catalyst reduced NO 2 to both NO and N 2 while the reduction of NO 2 back to NO was not significant on V 2 O 5 /TiO 2 catalyst. The combined system of the nonthermal plasma with V 2 O 5 /TiO 2 catalyst removed nearly 90% of NOx at 150 °C that is a relatively low temperature, compared to the typical temperature window of NOx reduction catalyst. 1. Introduction A nonthermal plasma technique such as pulsed corona discharge and dielectric barrier discharge has been considered as a prospective candidate for the removal of nitrogen oxides (NOx) emitted from a variety of industrial processes. 1-3 Generally, the content of nitric oxide (NO) in NOx is more than 95% in practical exhaust, and the rest is nitrogen dioxide (NO 2 ). Most of the nonthermal plasma processes that have been studied so far convert NO into NO 2 (or HNO 3 ) first by using the active species generated during the discharge, and then form ammonium nitrate through the reaction with ammonia. 4-7 The same approach is not applicable to the other exhaust gases, namely, engine exhausts and small-scale industrial flue gases. The desired approach in such cases may be to reduce NOx to molecular nitrogen (N 2 ). One important problem regarding this kind of approach is that nonthermal plasma alone cannot reduce NOx to N 2 when oxygen exists in exhaust gas. 3,4 As proved in many laboratories, the principle action of the nonthermal plasma in the presence of oxygen is the oxidation of NO to NO 2 . 3,4,8,9 Accordingly, to attain the purpose of NOx reduction to N 2 , nonther- mal plasma should be combined with another process. Yamamoto et al. 10 made use of the wet scrubbing method using Na 2 SO 3 solution to form N 2 from NO 2 previously produced by the nonthermal plasma. Bro ¨er and Hammer, 11 Yoon et al., 12 and Penetrante et al. 13 combined nonthermal plasma with catalyst to achieve the same purpose. From the practical point of view, the combination of nonthermal plasma with catalyst rather than that with wet scrubbing may be advisable since both of them are dry processes. One major issue with the catalytic removal of NOx may be the high activation temperature. 14,15 According to a published paper in the area of catalytic reduction of NOx, it is reported that the performance of catalyst is elevated by the increase in the ratio of NO 2 to NO. 16 This result implies that the temperature window for NOx removal on catalyst can be lowered when a part of NO is converted to NO 2 . As mentioned above, one easy method to increase the portion of NO 2 in NOx may be nonthermal plasma discharge. Several researchers ex- perimentally verified that installing a nonthermal plasma process in the front of the catalytic reactor lowers the activation temperature of NOx removal catalyst and enhances the removal efficiency. 11-13,17 It can thus be said that nonthermal plasma discharge complements the demerit of the catalytic process and vice versa. In the present work, nonthermal plasma combined with catalyst for the removal of NOx has been studied. A dielectric-packed bed reactor was utilized as the nonthermal plasma reactor, and two different honey- comb catalysts such as V 2 O 5 /TiO 5 and Cr 2 O 3 /TiO 2 were put downstream from the nonthermal plasma reactor. The main objectives of this study are to examine the effects of several crucial variables including discharge power, reaction temperature, water vapor, oxygen con- tent, and space velocity in the catalytic reactor on the removal of NOx. As well, a comparison between the two catalysts in terms of NOx reduction characteristics is also an important objective. 2. Experimental Details Experimental Apparatus. The schematic of the experimental setup composed of a nonthermal plasma reactor and a catalytic reactor is presented in Figure 1. The coaxial plasma reactor makes use of dielectric barrier discharge operated with ac high voltage (60 Hz). A glass tube (inner diameter, 25.8 mm; outer diameter, 30.2 mm) was utilized as the dielectric material and a 3 / 8 in. stainless steel rod was used as the discharging electrode to which ac high voltage was applied. The * To whom correspondence should be addressed. Tel.: 82- 64-754-3682.Fax: 82-64-755-3670.E-mail: smokie@cheju.cheju.ac.kr. † Air Protection Research Team, Research Institute of Industrial Science and Technology, Hyoja, Pohang, Kyungbuk 790-330, South Korea. ‡ Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja, Pohang, Kyungbuk 790-781, South Korea. 2960 Ind. Eng. Chem. Res. 2003, 42, 2960-2967 10.1021/ie0208873 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003 Downloaded via UNIV COLLEGE LONDON on August 2, 2019 at 15:51:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.