International Journal of Environment Science and Technology, Vol. 8, No. 3, 2011, pp.621-630
Volatile organic compounds decomposition using nonthermal plasma coupled with a combination of catalysts
1 *T. Zhu; 1 Y. D. Wan; 2 J. Li; 1X. W. He; 1D. Y. Xu; 1X. Q. Shu; 2W. J. Liang; 2Y. Q. Jin
1School of Chemical and Environmental Engineering, China University of Mining and Technology,
*Corresponding Author Email: firstname.lastname@example.org Tel./Fax: +861 0623 31360
Received 21 June 2010; revised 17 March 2011; accepted 25 April 2011
Code Number: st11057
A series of experiments were performed for toluene decomposition from a gaseous influent at normal temperature and atmospheric pressure by nonthermal plasma coupled with a combination of catalysts technology. Nonthermal plasma was generated by dielectric barrier discharge. ã-Al2O3 was used to be a sorbent and a catalyst carrier. Nanocatalysts were MnO2/ã-Al2O3 coupled with modified ferroelectric of nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3. ã-Al2O3 played an important role in prolonging reaction time of nonthermal plasma with volatile organic compounds molecules. MnO2/ã-Al2O3 has an advantage for ozone removal, while nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 is a kind of good ferroelectric material for improving energy efficiency. Thus these packed materials were incorporated together to strengthen nonthermal plasma power for volatile organic compounds decomposition. The results showed the synergistic technology resulted in greater enhancement of toluene removal and energy efficiencies and a better inhibition for ozone formation in the gas exhaust. Based on the data analysis of the Fourier transforms infrared spectrum, the reaction process of toluene decomposition and the mechanism of synergistic effect are discussed. The results showed in a complex oxidation mechanism of toluene via several pathways, producing either ringretaining or ringopening products. The final products were carbon dioxide and water.
Keywords: Energy efficiency; Ferroelectric; Synergistic effect; Toluene
Volatile organic compounds (VOCs) are triggering serious environmental problems such as stratospheric ozone depletion and photochemical smog, etc. Control of VOCs in the atmosphere is a major environmental problem now and attracts more and more researchers' attentions (Futamura et al., 1997; Muhamad et al., 2000; Gal et al., 2003; Magureanu et al., 2005; Malik et al., 2005; Kim, 2006; Magureanu et al., 2007; Juang et al., 2009a; b; Zhu et al., 2009a). As one of the typical VOCs, toluene effluents came from many industries, including paints, paint thinners, fingernail polish, lacquers, adhesives, rubber and some print and leather tanning processes. Several strategies have been identified in order to reduce toluene presence in civil and industrial emissions due to its noxiousness. As an emerging technology for environmental protection, Nonthermal plasma (NTP) has been subjected to extensive researches over the past 20 years (Mizuno et al., 1986;
Masuda et al., 1988; Chang et al., 1991; Chang et al., 1996; Guo et al., 2006). The main advantages of NTP technology compared to the conventional technologies include moderate operation conditions (normal temperature and atmospheric pressure), moderate capital cost, compact system, easy operations and short residence times, etc. (Nunez et al., 1993; Tonkyn et al., 1996; Ogata et al., 1999; Urashima et al., 2000 ; Park et al., 2003; Babel and Opiso, 2007; Subrahmanyam et al., 2007).
However, the major bottleneck of developing NTP with catalysis technology is the reduction of energy consumption. If this requirement is not satisfied, the nonthermal plasma process may lose its potential for commercial applications (Yamamoto et al., 1996; Tonkyn et al., 2003; Young et al., 2004). Many researcher found that for VOCs control, ferroelectric could improve energy efficiency significantly (Zhu et al., 2011), but ozone concentration increased due to ferroelectric presence (Yamamoto et al., 1992). Ogata et al. (2003) investigated the effects of alumina and metal ions in plasma discharge using NTP reactors packed with a mixture of BaTiO3 and porous Al2O3 pellets. The results indicated that the oxidative decomposition of benzene was enhanced by concentrating benzene on the Al2O3 pellets. The selected catalyst of MnO2 was well known for high potentials to decompose ozone (Van Durmea et al., 2007; Refaat, 2009). Futamura et al. (2004); Lee et al. (2011) and Refaat, (2011) tested catalytic effects of TiO2 and MnO2 with NTP. The results showed that the ozone generated from gaseous oxygen is decomposed by MnO2, but not by TiO2.
A series of experiments were performed for toluene decomposition from a gaseous influent at normal temperature and atmospheric pressure from 2005 to 2010 in Beijing. In this paper, the prepared nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 catalyst was used in the plasma reactor. Doped some ions (Sr and Zr) into the powder particles and crystal boundary in the experiment. The metal ions such as strontium, zinc and zirconium entered into crystal lattices of BaTiO3 equably and the Curie temperature (Tc) fell. As a result, the permittivity of nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 was up to 104 which were 12 times higher than that of pure BaTiO3, while dielectric loss reduced to 1/6 in normal temperature. This study found that this nano-material could reduce the energy consumption and increase energy efficiency significantly.
The oxidative decomposition of benzene was enhanced by concentrating toluene on the Al2O3 pellets. The selected catalyst of MnO2 was well known for high potential to decompose ozone. In the experiment, the prepared MnO2/ã-Al2O3 was used as catalyst to reduce the byproducts and toluene concentrations (Zhu et al., 2009b) also justified about 10 wt %. The objective of this study was to use a combination of catalysts (MnO2/ã-Al2O3 coupled with modified ferroelectric of nano- Ba0.8 Sr0.2Zr0.1Ti0.9 O3) in the NTP process for toluene decomposition in order to enhance toluene decomposition efficiency and increase energy efficiency and reduce byproducts for commercial applications.
MATERIALS AND METHODS
The reaction system was a tube-wire packed bed reaction system at normal temperature and atmospheric pressure. The schematic diagram of the NTP system used in this investigation is shown in Fig. 1. Dry air (78.5 % N2, 21.5 % O2) was used as a balance gas for toluene decomposition. Air supplied from an air compressor was divided into two air flows. Each flow rate was controlled with a mass flow meter. One air flow was introduced into a toluene liquid bottle (3) which contained liquid toluene. The air with a mass of saturated vapor of toluene was mixed with the other air flow in a blender (5) and the gaseous toluene was diluted to a prescribed concentration of 800~1000 mg/m3. A wire-tube Dielectric barrier discharge (DBD) reactor packed with catalyst was used to study the reaction as shown in Fig. 2.
An alternating current (AC) of 150 Hz was supplied to the NTP reactor in the radial direction, and the voltage extension changed from 0 kV to 50 kV. The experimental parameters of the process of discharge were detected by an oscillograph (model TDS2014, manufactured by American Tektronix Co.). The primary power values were measured with the voltage-charge (V-Q) Lissajous method in the plasma reactor.
Toluene decomposition was studied with a combination of catalysts including MnO2/ã-Al2O3 and nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 catalysts (volume ratio of 1:1). The manganese oxide catalysts (5 wt %, 10 wt %, 15 wt %) were prepared by impregnation of pellet type³-alumina with the granules diameter of 5~7 mm and BET surface area of 228 m2/g detected by Micromeritics (model NOVA 1000, manufactured by American Quantachrome Co.). Nanometer - sized Ba0.8Sr0.2 Zr0.1Ti0.9O3 powders were prepared with inorganic salts, such as TiCl4 and Ba(OH)2, as the raw materials by a water-thermal method at normal pressure. Particulate diameters of Ba0.8Sr0.2Zr0.1Ti 0.9O3 was 59 nm which was detected by XRD (model D8 ADVANCE, manufactured by Germany Bruker Co.) and BET surface area was 8.8 m2/g. The relative permittivity of nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 was about 104 (detected by inductance capacitance resistance (LCR) automatism test instrument 4210). The toluene concentration was determined using a gas chromatography (model HP6890N, manufactured by Agilent Co.) with a Flame ionization detector (FID) and a capillary column of HP-5 (internal diameter of 0.32 mm, length 30 m). The byproducts such as aldehyde, alcohols, amide, hydroxybenzene and polymerization products, etc, were identified by Gas chromatography mass spectrometry (GC-MS) (manufactured by American Thermo Finnegan Co.) and FT-IR (model Vertex 70, manufactured by Germany). Ozone concentration was measured by a chemical titration method of iodine (Zhu et al., 2009b).
As evaluation criterion, the toluene removal efficiency, reactor energy density and energy efficiency in the gas phase were calculated as follows.
Toluene removal efficiency (ç):
RESULTS AND DISCUSSION
Effect of combined catalysts on toluene removal efficiency
As the MnO2/ã-Al2O3 catalyst has the best effect for ozone decomposition but not for toluene decomposition (Delagrange et al., 2006) and nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3, a type of developmental material on base of pure BaTiO3 (typical ferroelectric), enhances energy efficiency because of its higher relative permittivity of 104 (Shi et al., 2008), a combination of nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 with MnO2/ã-Al2O3 as a combined catalyst was tested in this study. The effect of various catalysts such as multiple catalyst, nano- Ba0.8Sr0.2Zr0.1Ti 0.9O3, MnO2/ã-Al2O 3 and no padding on removal efficiency is shown in Fig. 3 (toluene concentration: 800-1000 mg/m3; gas flow rate: 2 L/min; AC frequency: 150 Hz). The removal efficiency increased significantly with the catalysts than that without. The removal efficiency increased in the order of: combined catalyst > nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 > MnO2/ã-Al2O3 > no padding. The best removal efficiency of 98.7 % was achieved in the NTP process. It indicated that the combination of catalysts exhibited a synergistic effect for toluene decomposition.
Effect of combined catalysts on ozone formation
Fig. 4 shows the influence of various catalysts on ozone formation with the order of: combined catalyst > MnO2/ã-Al2O3 > no padding > nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 at RED of 0.5 kJ/L. This result suggested that MnO2/ã-Al2O3 in the combination of catalysts should have a main effect on ozone decomposition.
Effect of the combination of catalysts on energy efficiency
Fig. 5 shows the influence of various catalysts on energy efficiency with the order of: combined catalyst > nano -Ba0.8Sr0.2Zr0.1Ti 0.9O3 > MnO2/ã-Al2O3 > no padding at the same Specific discharge energy density (SED).These results indicated that the nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 in the combination of catalysts should play an important role for improving energy efficiency.
As a result, the combination of catalysts shows the best removal efficiency of toluene, the best decomposition effect of ozone and the best energy efficiency for toluene removal.
Byproducts and decomposition pathways of toluene
Non-thermal plasma has high potential in air cleaning technology, but in some cases unwanted byproducts are formed which could be more harmful than the original VOCs. Fig. 6 shows the FT-IR (Fourier transforms infrared spectrum) of the byproducts of toluene decomposition and Fig. 7 shows the FT-IR spectrum of the byproducts on the surface of the combination of catalysts.
As shown in Fig. 7a, the -NH- and -NH2 peak appeared at 3350/cm while the peak of 2730/cm N=C-N was absent. The peak -NH- with benzene ring appeared at 3450/cm, OH at 3400/cm, -CH3/-CH2 at 2900/cm, benzene derivative (hydroxybenzene, polymerization products, etc) at 1700~1100/cm, and CO2 and CO separately at the range of 2300~2100/cm and 700~500 cm-1 respectively. Thus, the byproducts on the surface of the combination of catalysts involved aldehyde, alcohols, amide, and benzene derivative. However, when the combination of catalysts were packed into the NTP reactor, the byproducts on the surface of the packed materials in the NTP reactor reduced greatly as shown in Fig. 7b. Except of amine, CO2 and CO, no other byproducts were detected on the surface of catalysts. It illuminated that the synergic effect of the NTP with the combination of catalysts could control byproducts effectively. In Fig. 6, the products of toluene decomposition included CO2, CO and H2O. At the same time, there are a mass of ozone (strong peak at 1000/cm), and several amide and benzene derivatives. Compared spectrum `a' with `b' in Fig. 6, the benzene derivatives and ozone concentration reduce while the amounts of CO2 and H2O increase with the increase of the electric field strength. A large number of high-energy electrons, ions and free radicals were produced in the NTP reaction process. Firstly, the high-energy electrons could take part in reaction with oxygen in air as follow (Kim et al., 2008):
The oxygen free radical groups react with oxygen and other molecules to form ozone:
O + O2 + M → O• 3+ M → O3 + M (6)
Toluene bond energy between the carbon of benzene ring and the carbon of the substituent radical is 3.6 eV, which is lower than that of carbon-carbon bond or hydrocarbon bond. As a hydrogen atom in a benzene ring is replaced by a methyl radical to form toluene, the newly formed bond is less stable and the most vulnerable. Of course, the other bonds are also likely to be destroyed by high energy electrons. Formulas 11 to 16 are the possible reaction equations of the process of toluene removal (Kohno et al., 1998).
According to the FT-IR spectrums (Fig. 7), the author speculated the reaction pathways for toluene decomposition with the NTP and the combination of catalysts (Fig. 8). The oxygen and hydroxyl free radicals of should be the inducement during the process of toluene oxidation. The oxidation process of toluene may involve many reactions and these reactions cooperate and interact with each other for toluene decomposition. Firstly, a series of chain reactions take place between OH radicals and toluene molecules due to the higher oxidation ability of OH radicals than that of oxygen radicals:
Then, the idiographic reactions occur because of oxygen free radicals during the subsequent oxidation reaction as follows:
The binding bonds inside the benzene ring break down after the bonds outside the benzene ring break as follows:
At last, the byproducts were oxidized to CO2 and H2O with increasing RED and the help of catalysis. At last, the byproducts were oxidized to CO2 and H2O with increasing RED and the help of catalysis.
Perry et al. (1977) reported that aromatic compounds react with OH radicals by two pathways: hydrogen atom abstraction and OH addition to the aromatic ring. Reaction control pathways IXII were illustrated in
Fig. 8. The results showed in a complex oxidation mechanism of toluene via several pathways, producing either ring-retaining or ring-opening products. The final products were CO2 and H2O.
The synergistic effect of the combination of catalysts with the NTP reactor is presented in Fig. 9.
The catalyst carrier of γ-Al2O3 possesses sorbent characteristic, thus it could improve toluene concentration on the catalyst surface and increase the reaction time. MnO2 is known as a metal oxide catalyst and has been reported to possess a potential activity in redox reactions. MnO2 surface has been found to expose metal (Mnn+), oxide (O2- ) and defect sites of various oxidation states, present degrees of coordination instauration, and exhibit acid and base properties
Furthermore, the dd electrons exchange interactions between intimately coupled manganese ions of different oxidation states [Mnn+OMn(n+1)+] furnish the electron-mobile environment necessary for the surface redox activity (Zhu et al., 2009b):
O3 +Mnn+ → O2 2- +Mn(n+2)+ +O2 (28)
O3 +O2- + Mn(n+2)+ → O2 2- +Mn(n+2)+ +O2 (29)
O2 2- +Mn(n+2)+ → Mnn+ +O2 (30)
These factors would be helpful for toluene decomposition. Radhakrishnan (2001) reported that ozone decomposed to O2- and O22- in the surface of MnO2. Naydenov et al. (1993) believed that O- existed in the surface of MnO2 according to the oxidation of benzene in the surface of MnO2. As a modified ferroelectric, nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3 has a higher dielectric constant than BaTiO3 and is polarized at lower electric field strength. More high energy electrons and active radicals are generated to accelerate the reaction between NTP and toluene molecules.
Series of experiments were performed for removal of toluene gaseous influent at normal temperature and atmospheric pressure. In this study, the prepared combined catalyst was used to improve the NTP process and to take the catalytic advantages of both MnO2/ã-Al2O3 and nano-Ba0.8Sr0.2Zr0.1 Ti0.9O3. From the view of materials application, the authors adopted NTP coupled with the combination of catalysts technology to decompose VOCs in this study. The catalyst
materials could be prepared easily and inexpensing, and at the same time, this combined technology resolved the key bottlenecks effectively. Therefore, the combination of catalysts technology could advance to the NTP technology and improve applications in the industry in the future.
This work was supported by the Youth Research Funding of China University of Mining and Technology (Beijing) and the Fundamental Research Funds for the Central Universities (2009QH03), Public Welfare Project of China Environmental Protection Department (201009052-02) and National 863 key project (2009 AA 063201).
© IRSEN, CEERS, IAU
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