22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Plasmacatalysis: a sustainable and efficient indoor air treatment K. Van Wesenbeeck, B. Hauchecorne and S. Lenaerts Research group of Sustainable Energy and Air Purification, Department of Bioscience Engineering, University of Antwerp, 2020 Antwerpen, Belgium Abstract: The detrimental impact of polluted indoor air requires the development of an innovative air purification technology. Plasma catalysis based on corona discharge offers a sustainable way to remove pollutants from indoor air. This way the disadvantages of the plasma, as for instance the formation of by-products, are tackled. Plasma catalysis achieved by applying a photocatalytic coating on the collector electrode is an innovative and sustainable technology for air purification. Keywords: corona discharge, photocatalysis, titanium dioxide, plasma catalysis 1. Introduction Indoor air quality control is recognised as an important and challenging problem, both nationally and worldwide. Nowadays, air pollution, both indoors and outdoors, is a serious problem for human health as well as for the environment in general. Numerous studies report the occurrence of surprisingly high amounts of pollutants in enclosed environments [1-3]. These studies concluded that the indoor air pollutant concentrations are often two to five times higher than outdoor levels due to a combined effect of insufficient air exchange and high levels of indoor emission sources [4-5]. Although people spend the largest fraction (85%) of their time indoor and despite the fact that according to WHO 4.3 million people per year die from the exposure to household air pollution, poor indoor air quality is still an underestimated problem [6]. It is thus clear that it forms a significant health risk and efforts have to be made to improve indoor air quality. In selecting the most effective air cleaning system, a number of factors must be taken into account. They include long-term performance, minimum energy consumption and minimum amount of unwanted byproduct formation. Furthermore, the capability to work in indoor conditions is also an important parameter [7]. The most significant advantage of using a non-thermal plasma (NTP) in ambient air for air cleaning is the production of highly reactive oxidising radicals such as O◦, HO◦, and O 3 . Such species are produced at room temperature and at a low energy cost compared to any alternative method. Unfortunately NTP used as stand-alone technology in an air-cleaning process produces a high amount of byproducts that are sometimes more toxic than the initial pollutants. To overcome the deficiencies of the NTP plasmacatalysis as a combined technology between catalysis and plasma is proposed. In plasmacatalysis the presence of a catalyst and the synergetic effect of plasma and catalyst enhances the removal efficiency and total oxidation of the components. This synergetic effect results in a higher removal efficiency compared to the P-III-9-31 sum of plasma and catalyst processes, when they are used separately [7]. Consequently, the focus of this work lies on the implementation of an appropriate photocatalytic coating in a corona discharge unit. Selecting an appropriate photocatalyst is an important challenge for enhancing the removal efficiency in a plasmacatalytic process. The presence of the photocatalyst increases the probability of surface reactions between the reactants and the reactive species, which leads to more selective reactions and a higher removal efficiency. Therefore, the catalyst surface textural properties including specific surface area, pore volume, pore size and size distribution, as well as particle size and crystal phase have an important role on the plasmacatalyst performance. Among semiconductor photocatalysts TiO 2 is the most studied one, due to its photo-stability, strong oxidising power, non-toxicity, chemical and biological inertness, stability, as well as its low cost [8]. Balasubramanian [9-10] developed a TiO 2 photocatalytic film on stainless steel using a P25-powdermodified-sol-gel method (PPMSGM). Enhanced photocatalytic activity and adhesion are achieved in comparison to conventional sol-gel procedures by using this methodology. In our previous work [11-13], this coating is optimised with respect to TTIP:P25, DEA:TTIP and H 2 O:TTIP molar ratio while a good adhesion to a metal substrate, a low resistivity and a good photocatalytic activity in the gas phase are achieved. The measurements in this study are performed on the coating with the optimised molar ratios. 2. Material and methods 2.1. The plasma reactor A schematic diagram of the experimental setup is shown in Fig. 1. The configuration of the plasma reactor based on an electrostatic precipitator (ESP) with corona discharge, is a wire-to-cylinder type with an inner electrode (SS 316, 140 mm long) and an outer cylinder electrode (SS 316, 150 mm long). On the discharge 1 electrode, a set of pin pairs (galvanized steel, 1 mm diameter and 15 mm long) is equally distributed. The gap between the pin pairs and the outer electrode is equal to 37 mm. In the centre of the inner electrode an UV-lamp (365 nm) can be placed. A high DC voltage supply (PHYWE systeme GMBH, type 13671.93) is used in the experiments. cylindrical electrode to a flat surface so that a homogeneous coating is obtained. Thereafter, the electrode was vertically hung up in order to let the excess of sol run off the wall. After this step, the cylinder was dried for 24 h at room temperature. Subsequently, the coated substrate was heated in air with a gradient of 3 °C min-1 until a temperature of 100 °C was reached. This temperature was held for 1 h. Afterwards, the temperature was further increased with 3 °C min-1 until 500 °C was reached. The temperature was again kept for 1 h. Finally; the coating was cooled to room temperature by natural convection. The complete cooling process took approximately 12 h. As a result, a deposition of 0.45 mg cm-2 was obtained on the electrode. 3. Fig. 1. Schematic diagram of the experimental set-up. The dashed line represents the bypass. The polluted gas flow (2000 cm3 min-1 , 250 ppmv ethylene) is controlled by four mass flow controllers (MFC, MKS instruments) and consists of ethylene (1% ethylene in N 2 , Air Liquide), O 2 (Air Liquide) and N 2 . The latter can be moisturised by guiding the flow through a gas wash bottle filled with water, as shown in Fig. 1. It is always ensured that the oxygen concentration was 21% in order to mimic the indoor air conditions best. To have a look at the synergetic effect of the plasmacatalytic system, the experiments are performed both with and without coating applied on the collector electrode. Each experiment consists of three different steps, all performed with the high voltage supply turned on. The first step is in absence of an external UV source. In the second and the third step an external UV lamp is placed inside the reactor. In the second step the lamp is turned on, while in the third step the lamp is turned off. 2.2. The TiO 2 photocatalytic film The standard procedure for preparing the P25-basedpowder-modified-sol-gel is similar to the method previously published by our group [12, 13]. For this, commercial titanium isopropoxide (TTIP, 97 %, Sigma), isopropanol (i-PrOH, Sigma-Aldrich), diethanolamine (DEA, Sigma-Aldrich) and Aeroxide TiO 2 P25 (Evonik) were used. The uncoated SS 316 cylinder was pretreated with ethanol (96%, Royal Nedalco) after which it was dried at 105 °C for 24 h prior to coating. Afterwards, 15 mL of the sol was applied on the inner wall by unrolling the 2 Results In our previous work an optimal window of operation for our plasma reactor is determined by varying several characteristics, namely polarity, applied voltage, relative humidity and reactor configuration [13]. To recapitulate, a negative corona generally gives higher conversion efficiencies compared to positive corona. Secondly, it became clear that with a higher applied voltage the conversion efficiency increases. Thus, working with a negative polarity and a voltage higher than 15 kV is preferable. A third conclusion was that the influence of the relative humidity was small. The last parameter that was changed in our previous set of experiments, was the configuration of the plasma reactor and more specifically, the amount of pin pairs that are attached to the discharge electrode. It could be concluded that 10 pin pairs give the highest conversion efficiency. This optimal window of operation is also used in the final stage of the study, where the coating was applied on the collector electrode of the plasma reactor. In this study, we used ethylene to support our previous results [13]. The conversion efficiency of ethylene, the CO 2 formation and the formation of ozone in the reactor were determined before and after applying the coating when using the predetermined window of operation: 21% O 2 , negative corona and 15 kV. Fig. 2 illustrates that the coating does not have an adverse effect on the efficiency of the corona discharge reactor. The risk of implementing a coating on the collector electrode involves that the charged particles are not attracted to the collector electrode anymore since the coating gives a loss in conductivity of the electrode. As can be seen on the graph the efficiencies of ethylene conversion stay almost the same in the 3 steps of the experiment. On the other hand, in Fig. 3 it can be seen that when the external UV lamp is turned on, a higher CO 2 formation is obtained when a coating is applied. Without the presence of a coating the effect of the UV lamp is negligible. Since the ethylene conversion stays more or less the same in these circumstances (Fig. 2), it can be concluded that there is more mineralisation of intermediates of ethylene. Fig. 4 shows that the formation of ozone decreases P-III-9-31 drastically when the UV lamp is turned on while a coating is applied on the collector electrode. At the same time, the ethylene conversion is not affected by the UV-lamp, as shown in Fig. 2. So, the presence of the coating and the UV lamp significantly decreases the formation of ozone and at the same time it promotes the mineralisation of intermediates of ethylene. Fig. 2. A comparison of the conversion efficiency of ethylene (%) by using corona discharge with uncoated and coated collector electrode by a negative polarity and a voltage of 15 kV. 21% O 2 is applied. Fig. 3. A comparison of the CO 2 formation (a.u.) by using corona discharge with uncoated and coated collector electrode by a negative polarity and a voltage of 15 kV. 21% O 2 is applied. Fig. 4. A comparison of the formation of ozone (a.u.) by using corona discharge with uncoated and coated collector electrode by a negative polarity and a voltage of 15 kV. 21% O 2 is applied. P-III-9-31 4. Conclusion The purpose of this research is to combine photocatalysis and corona discharge in order to obtain a plasmacatalytic system as a sustainable and reliable indoor air purification technology. By applying a coating on the collector electrode of the plasma reactor we studied the influence of the coating on the performance of the plasma system when operating in the selected optimal window of operation from previous research. It was thereby clear that the coating does not have a detrimental effect on the plasma. Even more, when a coating is applied on the collector electrode and when a UV lamp is turned on; more CO 2 and less ozone formation is observed. We have illustrated that the implementation of a photocatalytic coating within a corona discharge reactor, also referred to as plasmacatalysis, has high potential as an integrated and sustainable indoor air purification technology. Further research about the implementation of the coating into an ESP is ongoing in order to investigate a possible synergy between the plasma and the photocatalytic activity of the coating. 5. Acknowledgments The authors wish to thank the University of Antwerp for supporting and funding this research. Tom Tytgat and Hilde Vanderstappen are greatly acknowledged for their help during the experiments. 6. References [1] E. Rehfuess, C. Corvalan and M. Neira. Bull. World Health Organ., 84, 508 (2006) ]2] M. Stranger, S.S. Potgieter-Vermaak and R. Van Grieken. Sci. Total Environ., 407, 1182-1192 (2009) [3] J. Van Durme. (Ghent: Ghent University) (2008) [4] B. Kolarik, P. Wargocki, A. Skorek-Osikowska and A. Wisthaler. Build. Environ., 45, 1434-1440 ( ) [5] EPA and the C.P.S. Commission. The Inside Story: A Guide to Indoor Air Quality. (2011) [6] WHO. Fact Sheet n° 292 (2014) [7] M. Bahri and F. Haghighat. CLEAN - Soil, Air, Water, 42, (2014) [8] H. Fujishima, Y. Ueda, K. Tomimatsu and T. Yamamoto. J. Electrostat., 62, 291-308 ( ) [9] G. Balasubramanian, D.D. Dionysiou, M.T. Suidan, Y. Subramanian, I. Baudin and J.M. Laine. J. Mat. Sci., 38, 823-831 (2003) 3 [10] G. 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