International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 6, Num. 1, 2009, pp. 141-148

International Journal of Enviornmental Science and Technology, Vol. 6, No. 1, Winter, 2009, pp. 141-148

Gaseous phase benzene decomposition by non-thermal plasma coupled with nano titania catalyst

¹ T. Zhu; ¹ *J. Li; ¹ Y. Q. Jin; ² Y. H . Liang; ² G. D. Ma

¹ College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
² Department of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
*Corresponding Author Email: lj@bjut.edu.cn Tel: +86 1067 39 2080; Fax: +86 1067 39 2080

Received 21 July 2008; revised 9 August 2008; accepted 24 November 2008

Code Number: st09014

ABSTRACT

Synergistic effect of atmospheric non-thermal plasma generated by dielectric barrier discharge and nano titania photocatalyst for benzene decomposition was tested. The paper indicated the effect of photocatalyst on removal efficiency of benzene, the compare of photocatalyst characteristic in different high temperatures by heat treatment, analysis of by-products. The results showed that the effect of degradation was visible by added photocatalyst in the plasma reactor. When concentration of benzene was 600 mg/m³ and electric field strength was 10 kV/cm, the removal efficiency of benzene was increased up to 81 % without photocatalyst. At the same condition, the removal efficiency was increased to 15 % higher with photocatalyst. Nano titania crystal was anatase crystal in 450 ^oC heat treatment which is best for benzene removal. The plasma reactor packed with photocatalyst shows a better selectivity of carbon dioxide than that without photocatalyst. By-products are mostly carbon dioxide, water and a small quantity of carbon monoxide.

Key words: Dielectic barrier discharge, removal efficiency, anatase, carbon dioxide

Introduction

As an emerging technology for environmental protection, there have been extensive researches on using non-thermal plasma (NTP) over the past 20 years (Tonkyn et al.,1996; Futamura et al., 1997; Muhamad and Jiang, 2000; Zhu et al., 2008a). The major advantages of NTP technology include the moderate operation conditions (normal temperature and atmospheric pressure), moderate capital cost, compact system, easy operations and short residence times, etc. compare to the conventional technologies (Magureanu et al., 2005; Zhu et al., 2008b). In the field of air pollution control, the NTP technology has been tested for the abatement of various types of hazardous air pollutants such as volatile organic compounds (VOCs) (Ogata et al., 1999; Urashima and Chang, 2000; Futamura et al., 2004; Magureanu et al., 2007; Subrahmanyam et al., 2007), SO₂ (Mizuno, et al.,1986; Changet al.,1991), NOx (Ohkubo et al.,1994; Tonkyn et al.,2003; Young et al., 2004), CFCs (Li and Ma, 2000; Gal et al., 2003; Park et al., 2003; Ricketts et al., 2004) odors (Masuda et al., 1993; Mizuno et al.,1993; Chang and Tseng, 1996; Zhu et al., 2007), mercury (Masuda,1988), etc. In order to further improve the energy efficiency of the VOCs removal decomposition process by the plasma, the cooperation with catalyst has been tested by some researchers (Einaga et al., 2001; Guo et al., 2006; Kim, 2006; Kim et al., 2008). These studies showed that the combination of discharge plasma with catalyst is a very effective method in VOCs removal. In this paper, non-thermal plasma coupled with nanotitania (TiO₂) photocatalyst for benzene removal to further reduce the energy consumption and harmful by-products in plasma process. Discharge plasma as a driving force of photocatalyst furnished a mess of UV light. Hole-electron pairs are produced by supplying energy larger than the band gap energy of TiO₂ (3.2 eV for anatase type). High energy particles such as electrons, excited molecules and radicals may transfer their energy to TiO₂ by bombardment when TiO₂ is placed in a non-thermal plasma.

Various chemical reactions are induced on the excited TiO₂ surface through the following reactions:

T iO₂ + Plasma → h⁺ + e⁻ (1)

h⁺ + H₂ O → OH + H⁺ (2)

e^- +O₂ →O₂ ^- (3)

e^- + H₂ O₂ → OH + OH^- (4)

O₂ ^- + H⁺ → HO₂ (5)

Radicals + Reactant → Products (6)

This paper illuminates the experimental results of the synergistic effect of NTP generated by dielectric barrier discharge (DBD) and nano-TiO₂ catalyst on benzene decomposition. The results showed that the effect of degradation was visible by added photocatalyst in the plasma reactor. The removal efficiency of benzene was increased to 98 % with benzene concentration of 600 mg/m³ and electric field strength of 10 kV/cm. At the same time, a better selectivity of carbon dioxide of 60 % was given.

The reaction activity of anatase catalyst is higher than that of rutileat catalyst. Above all that means this technology of NTP coupled with nano-TiO₂ (anatase) would have a great potential for application in the future.

Materials and Methods

Experimental system

The reaction system was a tube-wire packed-bed reaction system at atmospheric pressure. The schematic diagram of NTP system is shown in Fig. 1. Dry air (78 % N₂, 21% O₂) was used as a balance gas for benzene decomposition. Air supplied from air compressor was divided into two air flows. Each flow rate was controlled with a mass flow meter.

One air flow was introduced into 3 benzene liquid bottle which contained liquid benzene. The air with a mass of saturated vapor of benzene was mixed with the other air flow in 4 blenders and gaseous phase benzene was diluted to a prescribed concentration. A wire-tube DBD reactor with catalyst in situ was used in the reaction as shown in Fig. 2.

A packed reactor with coaxial electrodes was designed as illustrated in Fig. 2.

The packed materials (i.d. 5 mm, length 10 mm, rasching ring) coated with nano-TiO₂ photocatalyst were packed into the reactor. The reactor were made of ceramic tube (i.d. 50 mm, reaction length 500 mm), axes electrode (i.d. 1.5 mm, stainless steel wire), outer electrode (80 circles stainless steel wire). The characteristic of packed materials includes volume density 217g/cm³, hole rate 12.7 % and bibulous 5.9 %.

Nano-TiO₂ films were prepared by the Sol-Gel method (Liang, et al., 2006) in the experiment. Flow chart on preparing nano-TiO₂ thin film by Sol-Gel method referred to Fig.3.

The nanometer TiO₂ thin film was inspected and analyzed by scan electric mirror (SEM, Made in Japan, S-2700).

The results of SEM micrograph showed that average particulate diameters of TiO₂ were less than 100 nm. SEM micrograph of the samples referred to Fig. 4.

The plasma reactor employed an alternating current (AC) power supply of 60 Hz (designed by the authors). The AC voltage was applied to the reactor in the radial direction and the AC voltage extension lied from 0 kV to 30 kV. The benzene concentration was determined on a gas chromatography (manufactured by American Thermo Finnigan Co.TRACE-GC ULTRA) with a flame ionization detector (FID) and a capillary column of DB-1. Separately, another GC (SC—1001) equipped with an FID detector and an methane converter was used to analyze concentration of CO₂ and CO. Reaction gas samples were taken by a syringe from the sampling ports of the reactor.The by-products were identified by GC-MS with a 30 m long wide bore capillary column (DB-1). The experimental condition was in atmospheric pressure (760 mmHg) and temperature (20 ^oC).

Results and Discussion

The Compare of photocatalyzed characteristic by different heat treatment

The packed materials with photocatalyst were to calcine at 450 ^oC, 600 ^oC and 700 ^oC in the muffle. Then, they were packed into the plasma reactor. The samples of packed materials were detected by X-Ray and testified that nano-TiO₂ was anatase at 450 ^oC and nano-TiO₂ was mixture of anatase and rutileat at 600 ^oC and nano-TiO₂ was rutile mostly at 700 ^oC. The experimental results are shown in Fig. 5.

The decomposition efficiency of benzene is best with anatase photocatalyst in the plasma reactor. Next, the decomposition efficiency is better with the mixture photocatalyst of anatase and rutileat and the last to rutileat photocatalyst. The above all are shown in Figs. 5 a and b which show the decomposition efficiency of benzene reduction, gradually when the packed materials with photocatalyst by heat treatment from 450 ^oC to 700 ^oC. On one hand, when sinter temperature is raised, the surface areas of catalyst reduce and the surface adsorption capacity decrease. On the other hand, nano-TiO₂ catalyst will transform from anatase to rutileat. In Fig. 5 a, the test shows the reaction activity of anatase catalyst is higher than that of rutileat catalyst. There are four reasons:

• Because of structural difference, two type of catalyst have different quality densities and different structure of energy gap of electron. The quality density of anatase of 3.894 g/cm³ is less than that of rutileat of 4.250 g/cm³. The energy of energy gap of anatase which is 3.2 eV is higher than that of rutileat which is 3.1 eV. The higher energy of energy gap leads to the higher reaction activity for catalyst.
• The surfaces of anatase possess symmetrical structure with the molecular structure of benzene. Thus, it can adsorb benzene effectively.
• The hydroxyl of surface of rutileat is not more than that of anatase. Since the hydroxyl of surface is helpful for benzene removal, anatase is better than rutileat on benzene degradation.
• The surface area of rutileat catalyst declines sharply because a large number of particles converge under high temperature. The adsorption capacity of rutileat of TiO₂ is not proper for O₂. Thus, catalyst activity is low and nano-TiO₂ photocatalyst of anatase crystal was employed for next experiment.

The Effect of photocatalyst on removal efficiency of benzene

The relationship between benzene degradation and electrostatic field strength with or without catalyst is shown in Fig. 6 (benzene concentration: 600-1500 mg/m³).

The removal efficiency of benzene is 98 % in the reactor with catalyst while the removal efficiency of benzene is 78 % in the reactor without catalyst with concentration of 600 mg/m³, electric field strength of 10 kV/cm and gas flow rate of 14 mm/s as shown in Fig. 6 a. The removal efficiency with catalyst is 19 % higher than without catalyst with concentration of 1500 mg/m³, electric field strength of 12 kV/cm and gas flow rate of 14 mm/s as shown in Fig. 6 b. Obviously, the results indicate that catalyst enhanced the removal efficiency.

By-products analysis

Complete oxidation of benzene to CO₂ is the final goal of the experiment and the catalytic effect on the product distribution had been investigated. Photo-oxidation activity should be expressed as selectivity to CO₂ because other by-products except CO₂ were emitted in plasma process.

Here, the selectivity of CO₂ was calculated as follow:

Where, the [Benzene]₀ and [Benzene] indicate inlet and outlet concentrations of benzene, respectively. Though GC-MS, the main products in the plasma reactor were CO₂, H₂O and a small quantity of CO. Ozone was the only byproduct and no other by-products could be detected in the tail gas. In addition, certain brown-yellow products that were observed in the plasma reactor regardless of with or without catalyst appeared in Fig. 9.

The composition of the brown-yellow products was indistinct and maybe it was aromatic polymer detected by GC-MS. From Fig. 7, it is found the selectivity of CO₂ ranges from 24 % to 48% in the plasma reactor without catalyst while the selectivity of CO₂ ranges from 28 % to 60 % in the reactor with catalyst. The selectivity of CO₂ is enhanced due to the benzene oxidation near or on the catalyst surface. For that, it could be thought that intermediates and secondary products are more oxidized to CO₂ on catalyst surface. With benzene concentration increasing, the total output of C increases as shown in Fig. 8.

Therefore, the hybrid system comprising a non-thermal plasma reactor and nano-TiO₂ catalyst, not only in the gas phase, but also on the catalyst surface resulted in the higher energy efficiency and enhanced performance for the oxidative removal of benzene with lower medium reactivity and higher CO₂ selectivity in non-thermal plasma.

Conclusion

Four important results were obtained experimentally.

• Photocatalyst enhanced the benzene decomposition efficiency obviously. The benzene removal by the non-thermal plasma without photocatalyst was not as good as that by the non-thermal plasma with it. When electrostatic field strength arrived at 10 kV/cm in the plasma reactor, benzene decomposition level of higher than 98% was obtained. Although, the initial benzene concentration was low, i.e. 600 mg/m³ in the dry air. At the same time, benzene decomposition efficiency increased with increasing electrostatic field strengths.
• Nanometer TiO₂ took on anatase by heat treatment of 450 °C and behaved most effective on benzene degradation.
• It is facile and promising to simultaneously hybridize plasma with TiO₂ based on the data presented in this paper. The plasma reactor packed with catalyst shows a better selectivity of CO₂ than that without catalyst. Detected by GC-MS, the main products in the plasma reactor are CO₂, H₂O and a small quantity of CO. There are no other products detected in the tail gas.

Acknowledgements

This work was supported by Research Fund for the Doctoral Program of Higher Education of China and Funding Project for Academic Human Resources Development in Institutions of higher learning under the jurisdiction of Beijing municipality.

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