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In Situ Study of Ozone and Hybrid Plasma Ag–Al Catalysts for the Oxidation of Toluene: Evidence of the Nature of the Active Sites
Abstract
Silver colloids have been prepared by reducing AgNO3 in aqueous solution and embeded in alumina following a sol–gel procedure in the presence of Pluronic 84 ((EO)19(PO)39(EO)19), as surfactant. Plasma-catalytic experiments aimed at the mineralization of toluene showed that the selectivity to CO2 was significantly increased in the presence of Ag catalysts compared with results obtained using the plasma alone. In-situ studies of the ozone interaction with catalysts provide an insight into the nature of the active sites of supported silver colloids for mineralization reactions. It is noticeable that when ozone is chemisorbed on embedded Ag colloidal catalysts no change in the silver oxidation state or size is found. The population of the chemisorbed species is higher at lower temperatures, where the non-selective decomposition of ozone is smaller. The catalysts exhibit high stability, preserving the structural and textural properties after the catalytic tests, that is indeed very important in the presence of ozone.Graphical abstractHighlights► Silver colloids embedded in alumina following a sol–gel procedure in the presence of Pluronic 84 are stable catalysts in the presence of ozone. ► Plasma-catalytic experiments in the total oxidation of toluene showed a significantly increased selectivity to CO2 in the presence of Ag embedded colloids. ► In situ studies of the ozone interaction with catalysts provide an insight into the nature of the active sites of supported silver colloids for total oxidation reactions.
... Plasma reactions are very complicated processes both in the presence and in the absence of catalyst. Several mechanisms may explain the formation of reactive species when accelerated electrons collide with background gaseous molecules [92, 93]. The nature of the catalyst plays an important role on the type of formed reactive species and reaction mechanisms, as well as the product selectivity [63, 89]. ...
... It is postulated that the highly reactive oxygen species, which are mostly formed on the catalyst surface, are responsible for decomposition of HCHO in the catalyst presence. Magureanu et al. [92] studied the decomposition of toluene in the presence of Ag/Al 2 O 3 catalyst in a post plasma reactor. They found that, during the decomposition of toluene, ozone is chemisorbed on the catalyst surface without any change in the size and oxidation state of the catalysts. ...
... They found that, during the decomposition of toluene, ozone is chemisorbed on the catalyst surface without any change in the size and oxidation state of the catalysts. These interactions result in formation of activated ozone, which in the next step participates in toluene mineralization [92]: ...
... Fig. 3 shows the energy efficiencies as a function of reaction temperature when using different catalysts and different power supplies. In this study, the energy efficiency was 89.5 g*kWh − 1 at 25 • C when using the Au/γ-Al 2 O 3 catalyst, and increased to 125.0 g*kWh − 1 at 250 • C. At a fixed reaction temperature, the energy efficiency using the Au/γ-Al 2 O 3 catalyst was higher than that using CeO 2 /γ-Al 2 O 3 catalyst [37], and also higher than the results given by other researchers [30,37,44,[46][47][48][49][50][51][52][53][54]. It can be seen that the energy efficiency in this study is highest when at comparable toluene conversion and 25 • C (Table S2). ...
... And the peaks at 1702 and 1724 cm − 1 were corresponding to the C = O stretching vibration in aldehydes, which was the characteristic of benzaldehyde [61,62]. As the discharges Co/MCM-41 (AC) [48] TiO 2 /γ-Al 2 O 3 /nickel foam (AC) [52] Ag/TiO 2 (AC) [54] Glidarc (AC) [55] Ag/Al 2 O 3 (AC) [53] CeO 2 /γ-Al 2 O 3 (Pulse) [37] Ag-Mn-O/honeycomb (AC) [44] Ag/Al 2 O 3 (AC) [54] Ag/CeO 2 /Al 2 O 3 (AC) [49] Back-Corona (DC) [47] CeO 2 -MnO x /SBA-15 (AC) [50] MnO 2 -Co 3 O 4 (AC) [51] Mn/γ-Al 2 O 3 (Pulse) [46] Without catalyst (Pulse) [50] Energy efficiency (g*kWh -1 ) continued, all of above peaks became apparent. ...
Plasma catalysis technology has shown a great prospect in the oxidation of volatile organic compounds with a low concentration. However, low energy efficiency and large numbers of nanoparticle by-products are still the bottleneck for its practical application. Herein, a dielectric barrier discharge (DBD) reactor coupled with nano-sized Au (0.1 wt%) supported on γ-Al2O3 (denoted as Au/γ-Al2O3) was used for plasma catalytic oxidation of toluene. It was found that the energy efficiency was 89.5 g*kWh⁻¹ at 25 °C, and improved to 125.0 g*kWh⁻¹ at 250 °C. If a catalyst was not used in the DBD reactor, the by-products including benzaldehyde, phenol and benzoic acid were formed during the toluene oxidation. Those by-products can form nanoparticles which cause another risk to the atmosphere and human beings. With the use of Au/γ-Al2O3 nanocatalyst, the emission of nanoparticle by-products was reduced by 99.99%. Meanwhile, in-situ plasma DRIFTS analysis showed that Au-based catalysts can promote the oxidation of by-products and the gasification of carbonates to CO2.
... However, plasma activation is rather non-selective, so in order to obtain simultaneously high conversion and high selectivity toward total oxidation, the combination of plasma and catalysis appears more promising (Delagrange et al., 2006;Van Durme et al., 2008;Magureanu et al., 2011). Noble metal catalysts (Pd, Pt, Au, Ag) in combination with plasma were tested in many works, due to their high efficiency for VOCs abatement (Harling et al., 2007;Kim et al., 2008;Van Durme et al., 2009). ...
... The discharge was operated in ac mode, with sinusoidal voltage, at 50 Hz frequency. The electrical circuit is described in detail in (Magureanu et al., 2011). The high voltage was applied to the inner electrode, while the outer electrode was grounded. ...
The oxidation of toluene in air was investigated using a dielectric barrier discharge (DBD) combined with a Pd/Al2O3 catalyst. When using only plasma, rather low selectivity toward CO2 was obtained: 32–35%. By filling the DBD reactor with Pd/Al2O3 catalyst the CO2 selectivity was significantly enhanced (80–90%), however, a large amount of toluene was desorbed from the catalyst when the discharge was operated. By filling a quarter of the discharge gap with catalyst and placing the rest of the catalyst downstream of the plasma reactor, an important increase of CO2 selectivity (~75%) and a 15% increase in toluene conversion were achieved as compared to the results with plasma alone. The catalyst exhibited a very good stability in this reaction.
... It exhibits superior properties such as low cost, high stability and reduced nanoparticle emission (Kasprzyk-Hordern et al., 2004;Jiang et al., 2021;Yao et al., 2019). Zhu et al. (2018) used Ag/CeO 2 /Al 2 O 3 catalyst to get a conversion as high as 93% for 2260 mg/m 3 (600 ppmv) toluene degradation under an energy density of 1800 J/L at 25 • C. Magureanu et al. (2013) reported that, for 188 mg/m 3 (50 ppmv) toluene degradation, toluene conversion was 56% with Pd/Al 2 O 3 catalyst at an energy density of 325 J/L, and 60% using Ag/Al 2 O 3 catalyst at an energy density of 350 J/L (Magureanu et al., 2011). Harling et al. (2008) found that toluene conversion of 96% can be achieved at an energy density of 60 J/L over Ag/Al 2 O 3 catalyst for 1883 mg/m 3 (500 ppmv) toluene degradation at a temperature around 257 • C. In recent years, Au-based catalysts have been widely investigated. ...
The emission of toluene into the atmosphere can seriously affect the environmental quality and endanger human health. A dielectric barrier discharge reactor filled with a small amount of Au nanocatalysts was used to decompose toluene in He and O2 gases mixtures at room temperature and atmospheric pressure. Normally, the oxidation of toluene using Au nanocatalysts suffers from low reaction activity and facile catalyst deactivation. Herein, the effects of Au loading, calcination time and calcination temperature were systematically investigated. It was found that 0.1wt%Au/γ-Al2O3 calcined at 300 °C for 5 h can keep an average size around 6 nm with good dispersion on γ-Al2O3 surface and display the best catalytic performance. Moreover, the influences of energy density, gas flow rate, toluene concentration and O2 concentration on toluene degradation using 0.1wt%Au/γ-Al2O3 were evaluated. It showed the best catalytic performance of near 100% conversion for toluene degradation under the reaction conditions of the energy density was 20 J/L, the gas flow rate was 300 mL/min, the concentration of toluene was 376 mg/m³ and the oxygen content was 10%. Combining experimental results and theoretical calculations, the values of reaction constant k were 8.6 × 10⁻⁵, 3.53 × 10⁻⁵ and 3.09 × 10⁻⁵ m⁶/(mol*J), when O2 concentration, power or flow rate changed, respectively. Therefore, O2 concentration has the greatest effect on toluene decomposition compared to other factors in the presence of Au/γ-Al2O3.
... Compared with solid urchin α-MnO 2 , the promoted conversion of ozone over hollow urchin α-MnO 2 can be attributed to the unique hollow structure for enhancing adsorption towards gas and the non-agglomerated morphology for promoting oxygen species concentration as well as low-temperature reducibility. Owing to the highly efficient ozone conversion, the toluene decomposition and energy efficiency over hollow urchin α-MnO 2 are 72-100% and 9.0-13.1 g kWh −1 in the SIE range of 100-250 J L −1 , respectively, which are among the best of state-of-theart works in terms of toluene decomposition (Fig. 8d) [9,19,20,60,[62][63][64][65][66][67][68]. The CO 2 selectivity and carbon balance of NTP and PPC processes are displayed in Fig. 9. ...
... Several factors influence the energy efficiency, including the plasma reactor geometric structure, plasma reactor catalyst, voltage waveform, and operational conditions such as the temperature and gas composition. Fig. 2b and Table 1 summarize the plasma decomposition studies performed on toluene or benzene to show the effect of reaction temperature on their energy efficiencies (Magureanu et al., 2013;Wang et al., 2017aWang et al., , 2017bZhu et al., 2018;Wang et al., 2017b;Harling et al., 2008;Huang et al., 2011aHuang et al., , 2011bFeng et al., 2015;Magureanu et al., 2011;Xu et al., 2016aXu et al., , 2016bXu et al., , 2016cChang et al., 2018). Most energy efficiencies for toluene and benzene decomposition at 25 C were less than 13.1 g/kWh with or without catalysts using dielectric barrier discharge (DBD) with catalysts (cDBD), gliding arc, and corona discharge reactors. ...
Plasma-catalysis technologies (PCTs) have the potential to control the emissions of volatile organic compounds, although their low-energy efficiency is a bottleneck for their practical applications. A plasma-catalyst reactor filled with a CeO2/γ-Al2O3 catalyst was developed to decompose toluene with a high-energy efficiency enhanced by the elevating reaction temperature. When the reaction temperature was raised from 50 °C to 250 °C, toluene conversion dramatically increased from 45.3% to 95.5% and the energy efficiency increased from 53.5 g/kWh to 113.0 g/kWh. Conversely, the toluene conversion using a thermal catalysis technology (TCT) exhibited a maximum of 16.7%. The activation energy of toluene decomposition using PCTs is 14.0 kJ/mol, which is far lower than those of toluene decomposition using TCTs, which implies that toluene decomposition using PCT differs from that using TCT. The experimental results revealed that the Ce3+/Ce4+ ratio decreased and Oads/Olatt ratio increased after the 40-h evaluation experiment, suggesting that CeO2 promoted the formation of the reactive oxygen species that is beneficial for toluene decomposition.
... According to the above data in this study, the CO 2 selectivity was lower because of the incomplete oxidation mixture. The mixture was decomposed during the plasma discharging, whose reactions mainly included two parts, one was the collision between accelerated electrons and VOC molecules directly, and the other was that the active species (such as O 3 , OH) interacted with pollutants in gas phase (Magureanu et al. 2011). The organics in the outlet of the system were analyzed by GC-MS (GC-MS, 6890N, Agilent, USA; 5973N, Agilent, USA), and the results were given in Fig. 8. ...
In this study, the recycling of gas flow was added to oxidize mixture (toluene and xylene) in the post-plasma catalysis (PPC) system, and the MnOx catalysts using impregnation method were used to further oxidize the VOC mixture. The circulation and catalysts were of enhancement for the plasma degradation on both toluene and xylene. The improvement of CO2 selectivity and the reduction of NO, NO2, and O3 were 64.4%, 92.0%, 62.2%, and 51.9%, respectively. The fresh and used catalysts were characterized for the ozone decomposition and mixture degradation in the NTP-REC-CATAL system with the 15 wt% loading amount of catalysts. The results showed that OH groups, lattice oxygen, and manganese sites were potential and significant for the catalytic ability for O3 and mixture conversion. Aldehyde was detected from FT-IR characterization after treating, which indicates that it is the main intermediate NTP-REC-CATAL process. The air plasma was employed to reactive catalytic activity.
... It has been observed that the risks associated with the newly formed organic compounds were sometimes greater than that of the parent compound [9]. The production of solid deposits was also widely reported [10][11][12][13][14][15][16]. It has been reported that for the long-term stable operation of discharges, solid formation should be removed/prevented [17]. ...
A dielectric barrier discharge reactor was used to convert toluene (tar analogue) into methane (>90%). This study showed that wall temperature and plasma power played key roles on the product distribution. At ambient conditions, solid formation was observed inside the reactor at all tested powers (50–85 W). The maximum selectivity to lower hydrocarbon (C1-C6) was 63% at 85 W. However, complete conversion of the toluene to lower hydrocarbons was seen at power (75 W), when the surrounding temperature raised to 200 °C. Significant increase in selectivity also observed at 50 W, where selectivity increased from 35% to 84%, at 200 °C. The selectivity to various lower hydrocarbons was strongly dependent upon power. Methane showed maximum selectivity >90% at 85 W and 200 °C, whereas at 50 W the other hydrocarbons >C1 were >40% along with methane. The selectivity of C2-C6 started to decrease when increasing power due to increased cracking into methane.
... They found that the formation of solid residue increased at lower oxygen concentrations. In some reports, these deposits were described as polymeric substances, or carbonaceous deposits [35,36]. It was also reported that solid particles formed during the cracking of toluene in air, leading to the formation of solid deposits on the surface of the catalyst, thereby decreasing catalytic activity [37]. ...
Non-thermal plasma (NTP) is an attractive method for decomposing biomass gasification tars. In this study, the removal of toluene (as a gasification tar analogue) was investigated in a dielectric barrier discharge (DBD) reactor at ambient and elevated temperatures with hydrogen as the carrier gas. This study demonstrated that higher temperature in the presence of a DBD opens up new (thermal) reaction pathways to increase the selectivity to lower hydrocarbons via DBD promoted ring-opening reactions of toluene in H2 carrier gas. The effect of plasma power (5 – 40 W), concentration (20-82 g/Nm3), temperature (ambient-400 oC) and residence time (1.43-4.23 s) were studied. The maximum removal of toluene was observed at 40 W and 4.23 s. The major products were lower hydrocarbons (C1-C6) and solids. The synergetic effect of power and temperature was investigated to decrease the unwanted solid deposition. It was observed that the selectivity to lower hydrocarbons (LHCs) increased from 20 to 99.97 %, as temperature was increased from ambient to 400 oC, at 40 W and 4.23 s. Methane, C2 (C2H6 + C2H4), and benzene were the major gaseous products, with a maximum selectivity of 97.93% (60 % methane, 9.93 % C2 (C2H6 + C2H4), and 28% benzene). It is important to note that toluene conversion is not a function of temperature, but the selectivity to lower hydrocarbons increases significantly at elevated temperatures under plasma conditions.
... Most VDBD ozone generators are employed for water treatment; surface DBD ozone generators are rarely manufactured [8][9][10][11][12][13]. There are many studies of the use of both surface and VDBDs for ozone generation [14][15][16][17][18][19][20][21][22][23][24][25]. ...
Volume dielectric barrier discharge (VDBD) is considered to be the most effective method for ozone generation. This paper reports a comparison between a simple ‘classic’ VDBD cylindrical ozone generator and a ‘metallic-mesh-filled-air-gap cylindrical dielectric barrier discharge’ model. The obtained results show that although the majority of ozone generators are of the volume discharge type, the novel volume discharge model has provided better results in terms of ozone generation and energy efficiency. A skid was built by using eight proposed generators and was successfully used for wastewater treatment.
... Similar deposits have been described as polymeric substances or carbonaceous deposits. 28 These solid residues can clog the reactor if not managed properly. These deposits can be removed from the surfaces of the DBD reactor by converting them into CO, CO 2 , and lower hydrocarbons. ...
The decomposition of toluene (a model tar compound) in CO2 was investigated at ambient and elevated temperatures in a dielectric barrier discharge (DBD). The effects of reaction parameters, such as the residence time (0.47–4.23 s), plasma power (5–40 W), toluene concentration (20–82 g/Nm3), and temperature (20–400 °C), were investigated. The DBD was shown to be an effective technique for tar removal. The percentage removal of tar increased with increasing the plasma power and residence time (to as high as 99% at the residence time of 4.23 s). The maximum selectivity to the two major gaseous products, CO and H2, was 73.5 and 21.9%, respectively. Solid residue formation was also observed inside the reactor. The synergetic effect of the temperature and plasma power was studied. As temperature increased, the decomposition of toluene decreased slightly from 99 to 88% (from ambient to 400 °C at 40 W) and the selectivity of CO and H2 decreased as a result of the increased rate of recombination of CO and O. The selectivity to lower hydrocarbons increased with the temperature.
... Guo et al. observed a solid deposit product which is yellow in coloration during the treatment of toluene in SDBD, and expressed it as an aromatic polymer [36]. Few researches exist on solid deposit formation when treated VOCs with NTP alone and plasma catalyst hybrid systems, and explained the deposits as polymeric substances or carbonaceous deposits [3,37]. As showed in Fig. 6, our results are quite similar to these studies in both plasma alone and plasma-catalyst combine system. ...
Non-thermal plasma (NTP) an emerging technology to treat volatile organic compounds (VOCs) present in unhygienic point source air streams. In present study, double dielectric barrier discharge (DDBD) reactors were used for the first time to evaluate the removal efficiency of VOCs mixture of different nature at constant experimental conditions (input power 16-65.8 W, VOCs mixture feeding rate 1-6 L/min, 100-101 ppm inlet concentration of individual VOC). Reactor A and B with discharge gap at 6 mm and 3 mm respectively, were used in current study. When treated at an input power of 53.7 W with gas feeding rate of 1 L/min in DDBD reactor A, removal efficiency of the VOCs were: tetrachloroethylene (100%), toluene (100%), trichloroethylene (100%), benzene (100%), ethyl acetate (100%) and carbon disulfide (88.30%); whereas in reactor B, the removal efficiency of all VOCs were 100%. Plasma-catalyst (Pt-Sn/Al2O3, BaTiO3 and HZSM-5) synergistic effect on VOCs removal efficiency was also investigated. Highest removal efficiency i.e 100% was observed for each compound with BaTiO3 and HZSM-5 at an input power 65.8 W. However, integrating NTP with BaTiO3 and HZSM-5 leads to enhanced removal performance of VOCs mixture with high activity, increase in energy efficiency and suppression of unwanted byproducts.
... It was reported that silver-based catalyst shows remarkable activity and stability, even at minus temperatures [36]. Among the most active materials in the reaction of ozone decomposition are silver oxides [37][38][39][40][41]. In ozone-containing environment, they are unstable and decompose rapidly, releasing highly active oxygen, which makes silver a very suitable catalyst in oxidation reactions. ...
Silver modified (5 and 2 wt% loading) mesoporous molecular sieves (H-MCM-41, with Si/Al ratio 20, 40 and 50) and silica were synthesized by incipient wetness impregnation and ion-exchange methods. The obtained catalysts were characterized by different techniques (ICP, XRD, XRF, SEM, FTIR and nitrogen physisorption) and they were tested in heterogeneous catalytic decomposition of ozone and oxidation reactions involving ozone at ambient temperature. All the mesoporous catalysts have very high catalytic activities towards ozone decomposition at room temperature and they do not reveal any deactivation with the time on stream. The activities of the catalysts are enhanced upon increasing the amount of supported silver, decreasing the support acidity and modifying the catalyst with some additional metal having basic properties, such as Ce. The most active catalyst in the reaction of ozone decomposition—5Ag-H-MCM-41-50, shows also high activity at ambient temperature in the oxidation of CO and iso-propanol with ozone.
... Guo et al. observed a solid deposit product which is yellow in coloration during the treatment of toluene in SDBD, and expressed it as an aromatic polymer [36]. Few researches exist on solid deposit formation when treated VOCs with NTP alone and plasma catalyst hybrid systems, and explained the deposits as polymeric substances or carbonaceous deposits [3,37]. As showed in Fig. 6, our results are quite similar to these studies in both plasma alone and plasma-catalyst combine system. ...
Fugitive methane (CH4) from waste treatment facilities (landfill mining), power industries (oil and gas process plants) and coal mining etc. into atmosphere is an increasing environmental concern. In this study, CH4 conversion efficiency in double dielectric barrier discharge (DDBD) has been investigated at various operating parameters including input power, feed gas-mixture flow rate, CH4 initial concentrations, and discharge gap distance between two dielectrics. Increase in input power, decrease in the gas-mixture flow rate and discharge gap distance; results in increases of CH4 conversion efficiency. In plasma alone, maximum CH4 conversion efficiency of 76% was obtained using 3 mm plasma discharge gap distance at flow rates of 2 L/min, input power of 65.8 W and is limited by experimental conditions. In addition, CH4 conversion efficiency in plasma alone and plasma-catalytic is compared by introducing various catalysts includes Pt–Sn/Al2O3, BaTiO3 and HZSM-5 in single plasma discharge zone. Results revealed that plasma combined with Pt–Sn/Al2O3 showed higher CH4 conversion efficiency (84.93%) as compare to plasma alone (56.42%) using 6 mm plasma discharge gap distance at flow rates of 2 L/min, input power of 65.8 W. Moreover, maximum energy efficiency of CH4 conversion (limited by experimental conditions) was 27.24 × 10⁻¹² mol/kJ at 32.6 W observed in plasma-catalyst. Analysis of the exhaust gas showed that DDBD is a promising alternative reactor not only to achieve high CH4 conversion efficiency, but also to overcome the drawbacks of formation of undesirable byproducts. Moreover, deposition of carbon residues on the surface of internal electrode is not observed, which is often occurred in single DBD reactors.
... Volatile organic compounds (VOCs) are major air pollutants and can cause serious environmental pollution at low concentrations [1][2][3][4]. Hydrocarbons (HCs), one of the main components of VOCs, are mainly emitted from industrial processes or mobile sources and the quantity that is released in the atmosphere is continuously increasing. Light alkanes are the largest fraction of hydrocarbons (HCs) in automobile exhaust and their removal is difficult due to the stability of their molecular structure [5,6]. ...
A series of NiCeOx mixed metal oxide catalysts with various Ce/(Ce + Ni) ratios were prepared using hydrothermal methods The NiCeOx catalyst with a 4% Ce/(Ni + Ce) molar ratio (NiCeOx-4) demonstrated excellent catalytic performance for propane oxidation. Furthermore, the preparation method strongly affected the morphology and surface structure of the NiCeOx-4 catalyst as well as its catalytic activity for propane oxidation. The NiCeOx-4 catalyst that was prepared with the hydrothermal method exhibited a better catalytic performance compared with catalysts that were prepared by the co-precipitation method, sol-gel method and physical mixing of pure NiO and CeO2 powders. The results demonstrated that Ni-containing CeO2 (NiCeOx) nanoparticles were located on the surface of the NiCeOx-4 catalyst that was prepared using the hydrothermal method. As a result, the NiCeOx-4 catalyst had strong reducibility, a large number of active oxygen species, and strong ability to break the C-H bond of propane, which led to higher catalytic activity for propane combustion.
... Non thermal plasma is a quick and high efficient technique for abatement of VOCs at ambient pressure [4,5]. Electrons in non-thermal plasma are accelerated, dissociated and ionized without heating the gas, forming chemically active species like ozone, atomic oxygen and hydroxyl radicals which are oxidizing agents of VOCs [6][7][8]. But electrons in non-thermal plasma do not have enough energy for complete mineralization of BTX molecules, leading to the formation of resistant intermediates. ...
Bifunctional nanocatalyst of NiMn over alkali activated montmorillonite was prepared with different Mn contents. The prepared catalysts were examined in hybrid plasma-catalytic system for oxidation of volatile organic compounds. The prepared catalysts were characterized by XRD, FESEM, BET, FTIR and EDX analyses. FESEM pictures showed complete and uniform coverage of montmorillonite with metal oxide particles in nano scale. Increasing Mn content from 0 to 9 wt% decreased the BET surface area from 31 to 18 m² g⁻¹. FTIR spectra illustrated just peaks of montmorillonite and Ni or Mn related peaks were not observed even at high concentrations of Mn. The highest VOC abatement was related to NiMn/montmorillonite with 6 wt% of Mn. This catalyst in hybrid plasma-catalytic oxidation could remove 92% of benzene from polluted air with initial benzene concentration of 1000 ppm in plasma voltage of 20 kV and 9.8 kHz of frequency. While using plasma without catalyst could just remove 74% of benzene in similar conditions. NiMn/montmorillonite nanocatalyst with 6 wt% of Mn was tested at different plasma voltages (10–30 kV) and the highest VOC removal was observed at a voltage of 20 kV. Among 1–9 min residence time, the longest residence time of 9 min had supreme abatement of benzene. Study on mixture of benzene, toluene and xylene in plasma-catalytic system illustrated the highest removal for xylene and the lowest removal for benzene in plasma environment of 20 kV and 9.8 kHz.
... Guo et al. [34] reported that a yellow product was observed after the treatment of toluene vapors in a DBD, and described it as an aromatic polymer. A few reports exist on deposit formation after the treatment of VOCs with non-thermal plasma and catalyst hybrid systems [44][45][46]. They generally described the deposits as polymeric substances, or carbonaceous deposits, some were identified as benzoic acid crystals. ...
... Metal oxide catalysts, promoted with AgO x , are used for total oxidation of mixture of diluted volatile organic compounds (Karuppiah et al. 2012). In situ studies of the ozone interaction with Ag-Al catalysts for mineralization of toluene proved that ozone is chemisorbed on the catalyst surface and at lower temperatures the non-selective decomposition of ozone is occurring at lower rate (Magureanu et al. 2011). In our recent works Nikolov et al. 2010) we found out that the modification of different catalytic supports such as clinoptilolite, silica and perlite with silver increases the catalytic activity in the reaction of ozone decomposition. ...
An alumina-supported silver composite system has been
investigated in the reaction of heterogeneous catalytic ozone
decomposition. Perlite loaded with silver and silver modified
zeolites have also been tested as catalysts for gas phase
conversion of ozone to molecular oxygen. The catalysts have
been activated by calcination and the changes occurring on
the catalytic surface as a result of the reaction have been
characterized in details using XPS. The SEM analysis has
revealed the role of surface morphology in the process of
ozone decomposition on the surface of Ag/α-Al2O3 catalyst.
A catalytic cycle is proposed that supports the hypothesis
about the important role of the peroxide species as intermediates
participating in the process and catalyzing the complete
ozone conversion into molecular oxygen. It has been found
out that the maximum conversion degree, achieved with the
alumina-supported silver system, is up to 90%.
http://www.tandfonline.com/eprint/YfIruDsi4nKfhAAz2M5f/full
Plasma catalysis technology is suitable for the emission control of low concentration volatile organic compounds (VOCs), but the energy cost and complete oxidation are the bottleneck for its application. Herein, MnOx/γ-Al2O3 catalysts incorporated with small amounts of Pt nano particles were designed for plasma catalytic oxidation of o-xylene which is one typical kind of VOCs from paint industry. A dielectric barrier discharge reactor installed with Pt0.1Mn6/γ-Al2O3 catalyst can decompose 90% o-xylene oxidation with 90% CO2 selectivity at an energy density of 211 J/L and a reaction temperature of 150 °C, while only 29% o-xylene conversion and 5% CO2 selectivity were obtained without discharges under the same reaction condition. The energy cost is as high as 12.3 g/kWh which is reached top level at the same VOCs concentration. The excellent performance of [email protected]/γ-Al2O3 catalyst is due to the synergistic effect of Pt nano particles and MnOx, which promoted the activation of O2 and the complete oxidation to CO2 of by-products.
Volatile organic compounds (VOCs) emitted from various industrial processes are extremely harmful pollutants. They are involved in the formation of ozone, photochemical smog, and fine particles (PM2.5) in the atmosphere, which pose considerable threat to human healthy and ecosystem safety. The hybrid plasma-catalytic technology that uses non-thermal plasma (NTP) and catalysts is an efficient method for VOC abatement. This review provides a comprehensive insight into the removal of VOCs with this technology. First, the synergistic effects and mechanisms of NTP and catalysts are discussed. Then, the properties of the catalysts, including types, positions, and other parameters, are explored. Specific examples of VOCs abated using the NTP-catalyst technology are reviewed, along with the main types and the causes of by-products. Several methods, such as the optimization of process parameters and the utilization of end control, are considered efficient for the regulation of product formations. Finally, future perspectives on the applications of this hybrid technology are briefly discussed.
Simultaneous ozonation and coagulation can be realized in one unit in the developed hybrid ozonation-coagulation (HOC) process. To reveal the reaction sequence within the HOC process, the ibuprofen (IBP) removal efficiency of the ozonation only, HOC and HOC-PO43- (inhibition of the reactions between ozone and metal coagulant) processes at pH 5 and different ozone dosages were investigated. The removal efficiency is almost the same for the three processes at a low ozone dosage (4.8 mg/L), and higher removal performance can be achieved by the HOC process with increasing ozone dosage. It can be implied that ozone preferentially reacts with OH- to generate OH which react with IBP in the HOC process, and subsequently reacts with the surface hydroxyl groups of hydrolysed Al species to enhance OH generation. Moreover, based on the kinetics, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses, the synergistic reactions between ozone and the metal coagulants (SOC) started to take effect from ozone dosage of 9.6 mg/L, which further verified that ozone will be involved in the IBP ozonation prior to the SOC reactions. The subsequent SOC reactions also resulted in the increased generation of polymeric Al species and more abundant intermediates in the HOC process.
Mn/MCM-41 and imp-Mn/MCM-41 catalysts were prepared in this study through metal heteroatom substitution and impregnation methods, respectively, and their catalytic oxidation performance of toluene under non-thermal plasma (NTP) in a dielectric barrier discharge reactor was studied. The stability of the catalyst and the parameters of catalytic oxidation conditions (initial concentration of toluene, O2 ratio, and relative humidity of carrier gas) were optimized. Characterization proved that the impregnation method resulted in the existence of manganese in the form of oxides (MnO2, Mn2O3) outside the pores of MCM-41. Meanwhile, the metal heteroatom substitution method implanted manganese into the mesoporous structure and replaced part of it with Si-O-Mn. NTP catalytic oxidation of toluene, ozone emission, and GC–MS by-product analysis confirmed that 60Mn/MCM-41 catalyst has high catalytic activity. Moreover, the catalyst affected the production of by-products. The stability test revealed that the 60Mn/MCM-41 catalyst still respectively reached 84.6% and 61% in the conversion of toluene and the selectivity of CO2 under the SED of 558 J/L after 40 h discharge reaction.
Manganese oxide catalysts have been synthesized from the used batteries via hydrometallurgical method and effect of hydrometallurgical parameters such as the effect of acid type (H2SO4, HNO3, HCl), acid concentration (0.5, 1, 1.5, 2 %v/v) and powder to acid ratio (1/50, 1/60, 1/70, 1/80) were in detail investigated. The physico-chemical properties of as-prepared catalysts were characterized by FT-IR, XRD, FESEM, EDX, BET, TEM, and TPR-H2 analysis. The activity of as-prepared catalysts were investigated towards the oxidation of benzene, toluene, and xylene (BTX) in a plasma-catalytic process. The results show that benzene and toluene conversion were almost constant in the range of 97–98% in case of various acid types, acid concentrations and solid to liquid ratios. However, the xylene conversion were varied in case of different hydrometallurgical factors. The highest xylene conversion was obtained in the presence of MnS0.5–60, which was prepared using H2SO4 with concentration of 0.5%v/v and solid to liquid ratio of 1/60. The effect of the input voltage and BTX flow rate on the BTX conversion was also investigated using MnS0.5–60 catalyst in detail.
Ag-Mn catalysts with excellent water resistance and ozone decomposition activity were successfully synthesized by simple precipitation and impregnation methods. Under a relative humidity of 65% and space velocity of 840,000 h-1, the 6%Ag/α-Mn2O3-I catalyst showed 99% conversion of 40 ppm O3 after 6 h, which was far superior to the performance of the 6%AgMnO
x
-C (49%), 6%Ag/MnCO3-I (32%), and α-Mn2O3 (5%) catalysts. Physicochemical characterization indicated that the chemical state of Ag on the Ag-Mn catalysts determined the O3 decomposition activity of the catalysts. The Ag species on the 6%Ag/α-Mn2O3-I catalyst were mainly metallic silver nanoparticles (Agn0), which exhibited much better ozone decomposition performance than the Ag1.8Mn8O16 and oxidized silver clusters (Agnδ+) existing on the 6%Ag/MnCO3-I and 6%AgMnO
x
-C catalysts. The 6%Ag/α-Mn2O3-I catalyst still had above 85% ozone conversion after 60 h under a relative humidity of 65% and space velocity of 840,000 h-1. The slight deactivation of the catalyst was ascribed to the oxidation of Agn0, and its activity could be completely recovered by treatment at 350 °C under an N2 atmosphere, which indicated that it is a promising catalyst for ozone decomposition. This research provides guidance for the subsequent development of Ag-Mn catalysts for ozone decomposition with high activity.
In this study, the removal performance of a hybrid ozonation-coagulation (HOC) process using AlCl36H2O (Al–HOC) and FeCl36H2O (Fe–HOC) as coagulants for the treatment of wastewater treatment plant (WWTP) effluent and ibuprofen (IBP) was investigated. Compared with the conventional coagulation process and pre-ozonation-coagulation process, much better organic matter removal performance can be achieved for both the Al–HOC and Fe–HOC processes. The Fe–HOC process showed an obviously higher dissolved organic carbon (DOC) removal efficiency than that of the Al–HOC process. Surface hydroxyl groups were determined to be the active sites in generating OH in the HOC process, and the hydrolysed Fe species possessed a higher content of surface hydroxyl groups than the hydrolysed Al species according to fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectra (XPS) analyses. In addition, the hydrolysed Fe species contained a higher portion of tetrahedral sites that were more likely to be stronger Lewis acid sites to react with ozone to generate OH. Furthermore, peroxone reactions in the HOC process were other possible way to enhance the OH generation, and higher H2O2 generation was observed in the Fe–HOC process due to higher O2⁻ generation. Therefore, better removal performance of the Fe–HOC process can be obtained due to the increased OH generation in the Fe–HOC process.
In this paper, the powder of manganese oxide was recovered from spent alkaline batteries and the effect of calcination temperature on the physiochemical properties of resultant powder was studied. Synthesized manganese oxide was impregnated at different percentages on alumina and the resultant material was used as catalyst for oxidation of benzene, toluene, and xylene (BTX) in a hybrid plasma-catalytic process. Manganese oxide/alumina (MnAl) catalysts were analyzed by FTIR, XRD, FESEM, EDX, BET, TEM, and TPR-H2 and the results indicated that high crystalline big cubes of manganese oxide are in good connection with highly dispersed ultra-fine nanoparticles of alumina. Enhancing the Mn content increased the agglomeration of alumina particles probably around the cubes of manganese oxide. Nitrogen adsorption measurements (BET) showed that the specific surface area of these catalysts was high but reduced with increasing Mn percent. TPR-H2 showed that using alumina had a good impact on the redox ability of the catalyst. Oxidation of BTX using synthesized MnAl catalysts was carried out in the presence of plasma. While MnAl catalysts showed similar results in benzene and toluene oxidation percent (97–98%), the oxidation of xylene was difficult and depended on the Mn percent so that the catalyst with 10% wt. of manganese oxide indicated 74% of xylene oxidation while higher or lower Mn contents exhibited lower oxidation percent. In order to evaluate the experimental parameters, the influence of plasma input voltage, catalyst location in the plasma reactor, BTX flow rate, and the catalyst loadings were investigated. Results showed that benzene and toluene were oxidized almost completely regardless of input plasma voltage but the amount of xylene oxidation soared with increasing the voltage. Moreover, the catalyst location in the reactor had no significant influence on the conversion of BTX whereas increasing the flow rate led to a decline in BTX removal efficiency.
Plasma-assisted MnOx catalysis technology has shown promising results in the decomposition of air pollution. In this work, MnOx catalysts with different precursor were prepared by impregnation method to enhance the oxidation ability of plasma for xylene. The performance of the catalyst was characterized by XRD, FT-IR, SEM, XPS and BET. The combined plasma and MnOx promoted the xylene removal efficiency CO2 selectivity and O3 conversion, while reduced NO2 in air discharge concentration. Among three catalysts prepared by precursors, the MnOx(MN) showed outstanding performance with excellent capacity in xylene oxidation (94.1%), CO2 selectivity (80.1%), O3 suppression (76.4%), and NO2 prohibition (78.5%) compared to MnOx(MA) and MnOx(MS). The excellent catalytic activity may be attributed to the fact that the anions containing in the precursors play a significant role on the distribution of MnOx on the support. Also, the Fourier infrared results show that a small amount of precursor anion are present in the MnOx gap, which affects the content of MnO2 (Mn⁴⁺) crystals and OH group on the surface the catalyst.
The role of N2 carrier gas towards the conversion of tar analogue (toluene) was studied in a non-thermal plasma dielectric barrier discharge (DBD) reactor. The parameters investigated were power (5-40 W), residence time (1.41-4.23 s), toluene concentration (20-82 g/Nm³) and wall temperature (ambient-400 oC). Almost complete removal (99 %) of toluene was observed at 40 W and 4.23 s. The main gaseous product was H2 with a maximum selectivity of 40 %. The other gaseous products were lighter hydrocarbons (5.5 %). The selectivity to these LHCs could be increased to 10 % by increasing the temperature to 400 oC. Introducing H2 to the N2 carrier gas at elevated temperatures opened up new reaction routes to enhance the selectivity to lower hydrocarbons (LHCs). The selectivity to methane reached 42 % at 35 % H2 at 400 oC, and the total selectivity to LHCs (lower hydrocarbons) reached 57 %.
Ruthenium (Ru) nanoparticles (~3 nm) with mass loading ranging from 1.5 to 3.2 wt.% are supported on a reducible substrate, cerium dioxide (CeO2, the resultant sample named as Ru/CeO2), for application in the catalytic combustion of propane. Because of the unique electronic configuration of CeO2, a strong metal-support interaction is generated between the Ru nanoparticles and CeO2 to well stabilize Ru nanoparticles for oxidation reactions. In addition, the CeO2 host with high oxygen storage capacity can provide an abundance of active oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the redox steps. As a result of such advantages, a remarkably high performance in the total oxidation of propane at low temperature is achieved on Ru/CeO2. This work exemplifies a promising strategy for developing robust supported catalysts for short-chain VOC removal.
Non-thermal plasma (NTP) catalysis has attracted widespread attention in volatile organic compounds (VOCs). Combining NTP with heterogeneous catalyst has been proved to reduce the formation of unwanted by-products and improve the energy efficiency of the process. The mechanisms of interaction with plasma and catalyst and VOCs oxidation in plasma catalysis are particularly hot research topic. This paper reviewed the mechanism of non-thermal plasma catalysis on volatile organic compounds removal. In the first part, the interaction between plasma and catalyst is disscussed. And the different catalysis systems and discharge types are presented in the second part. In the third part, attention is given to the influence of critical parameters on the removal processes. Finally, based on a large number of literatures, an extended review is presented detailly on the treatment of VOCs with plasma-catalysis system including the oxidation pathways. Moreover, future works of this promising technology are discussed.
Non-thermal plasma catalysis with high efficiency, low by-products selectivity and superior carbon balance is one of the most promising technologies in the control of volatile organic compounds (VOCs). The non-thermal plasma catalysis can be sub-divided into three types, that is, continuous operation in plasma catalyst system (CIPC), sequential operation in plasma catalyst system (SIPC) and post plasma catalyst system (PPC). Previous some reviews lacked of detail discussion of different operation modes of in plasma catalyst system (IPC). Others only discussed the influence of the operation modes of plasma catalysis systems on VOCs abatement, while ignoring post plasma catalysis system (PPC). In this review, the effect of catalyst on plasma properties such as discharge behaviors, electric field, and active species amount and influence of plasma discharge on catalyst status such as dispersion and sintering were summarized. The behavior and mechanism of catalyst adsorption within plasma reactor and plasma oxidation were discussed. Moreover, the synergistic effects of plasma and catalysts in CIPC, SIPC, and PPC systems on VOCs destruction were critically reviewed. Finally, the recent progresses and outlooks of non-thermal plasma catalysis coupled system in VOCs control was proposed. It can be reasonably anticipated that this review is meaningful for deepening our understanding of the fundamental scientific principles regarding the catalytic oxidation of VOCs in non-thermal plasma catalysis system, providing valuable and feasible references for researchers and designers in VOCs efficient reduction and control.
BACKGROUND
To find an appropriate catalyst in the adsorption plasma catalytic process, degradation of toluene over 13X zeolite supported catalysts were studied.
RESULTS
13X zeolite exhibited excellent toluene adsorption capacity but weak catalytic ability. After loading Cu, Co, Ce and Mg active compounds by the impregnation method, adsorption capacity was reduced and catalytic performance was significantly improved. Co/13X exhibited good adsorption capacity and excellent catalytic performance, which carbon balance was 81.6% with 74.4% of COx for toluene adsorption capacity of 0.51 mmol. The effect of cobalt loading and calcination temperature on Co/13X for toluene degradation were studied, and it was found that 5% and 500°C were the optimum loading value and calcination temperature. The characterization results of BET, FTIR, XRD, XPS and SEM were correlated with catalyst performance.
CONCLUSIONS
The active compounds block the pores and reduce the specific surface area of 13X zeolite, but active sites of catalysts can decompose ozone to produce more atomic oxygen species. Co³⁺ could form π‐complexation bonding with toluene and Co3O4 is a P‐type semiconductor, which has large oxygen adsorption capacity and converts oxygen into O⁻ and O²⁻ easily. Co3O4 present in Co/13X is the active point for toluene degradation by adsorption and plasma catalytic activity. © 2017 Society of Chemical Industry
The catalytic decomposition of gaseous ozone (O3) is investigated using anatase TiO2 (A-TiO2) and Aluminum-reduced A-TiO2 (ARA-TiO2) at high concentration and high relative humidity (RH) without light illumination. Compared with the pristine A-TiO2, the ARA-TiO2 sample possesses a unique crystalline core-amorphous shell structure. It is proved to be an excellent solar energy “capture” for solar thermal collectors due to lots of oxygen vacancies. The results indicate that the overall decomposition efficiency of O3 without any light irradiation has been greatly improved from 4.8% on A-TiO2 to 100% on ARA-TiO2 under the RH=100% condition. The ozone conversion over T500/ARA-TiO2 catalyst is still maintained at 95% after a 72 h test under the reaction condition of 18.5 g/m³ ozone initial concentration, and RH=90%. The results can be explained that T500/ARA-TiO2 possesses the largest amorphous contour, the lowest crystallinity, the most surface-active Ti³⁺/Tⁱ⁴⁺couples, and the most oxygen vacancies. This result opens a new door to widen the application of TiO2 in the thermal-catalytic field.
This article reports the use of innovative diagnostics to monitor toluene adsorption and oxidation on CeO2 surface under non-thermal plasma (NTP) exposure. Two plasma-catalytic configurations are explored, namely: post-plasma catalysis (PPC) and in-plasma catalysis (IPC). Since heterogeneous processes are pointed out as key steps of the plasma-catalyst coupling, the catalyst surface has been monitored by two complementary in situ diagnostics: (i) diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and (ii) transmission fourier transform infrared spectroscopy using Sorbent track (ST) device. Dielectric barrier discharges (DBD) are used in both PPC and IPC configurations to induce adsorbed toluene oxidation. Toluene in dry air is first adsorbed on the selected catalytic surface: ceria (CeO2). Subsequently, the plasma is switched on. During the experiment, the ceria surface is monitored by infrared to study toluene adsorption and oxidation mechanisms. The adsorption capacity of toluene on ceria is, respectively, measured in the configurations of PPC and IPC by DRIFTS and ST. The oxidation of toluene by plasma follows a first-order reaction regardless of plasma configuration and injected power and IPC is more effective for the toluene removal than PPC. Intermediates of toluene (benzyl alcohol, benzaldehyde and benzoic acid) are also identified on the surface and their respective temporal evolutions as a function of the plasma exposure are studied.
The reactivitys of toluene degradation were investigated at room temperature under atmospheric pressure by using a non-thermal plasma reactor loaded with SiO2, Al2O3 and NiO/Al2O3. The different reactivities on these catalysts may originate from their dielectric constant, toluene adsorption and ozone decomposition abilities on their surface. In addition, in-situ infrared spectrum technology was used to study adsorption species on catalyst surface during the toluene degradation. The results showed that, within a certain range, the degradation rate of toluene increased along with the energy density, dielectric constant, adsorption and the ozone-decomposing ability. Toluene adsorption species on the catalyst surface played important roles on toluene degradation. The toluene degradation occurred in the vapor phase if SiO2 was loaded in the discharge region. However, when Al2O3 or NiO/Al2O3 was loaded, the oxidation of toluene to benzoic mainly occurred on the catalyst surface, which was the key step of toluene degradation, and the accumulation of benzoic acid on the active sites would decrease the catalyst reactivity.
Plasma-assisted catalysis has been employed for the degradation of benzene by hybrid surface/packed-bed discharge (HSPBD) plasmas over a series of Agx Ce1-x/γ-Al2O3 catalysts in in-plasma catalysis (IPC) and post-plasma catalysis (PPC) configurations. In order to study the influence of catalysts placement on discharge characteristics and the consequent synergetic effect in plasma-catalysis process, the catalysts were introduced inside and downstream the surface discharge region (region I) and packed-bed discharge region (region II), respectively, and the benzene degradation performance was investigated in these systems. The effects of the Ag/Ce molar ratio and water vapor have also been investigated in terms of benzene degradation efficiency and CO2 selectivity. Compared with the plasma-only process, the combination of plasma with Agx Ce1-x/γ-Al2O3 catalyst significantly improved the reaction performance, and the combined degradation efficiency is a synergistic effect rather than simply an additive effect. Besides, the emission of discharge products (O3 and NOx) and hazardous intermediates (formic acid and CO) was markedly suppressed with the introduction of catalyst. The highest benzene degradation efficiency of 96.2% and CO2 selectivity of 77.3% can be achieved with Ag0.9Ce0.1/γ-Al2O3 catalyst at the SIE of 400J/L. This result suggests that the interaction between a certain proportion of Ag and Ce species over the catalyst is capable of activating the surface lattice and generating more surface adsorbed oxygen (Oads), which favors the plasma-catalytic oxidation reaction. PPC processes can decompose O3 and destroy benzene more effectively than IPC processes, especially when the catalyst was introduced downstream the region II. Adding a small amount of water vapor into plasma-catalysis system enhanced the catalyst activity, however, further increased the water vapor caused an obvious negative impact on the catalyst activity.
Volatile organic compounds (VOCs) are common air pollutants existing in various atmospheric environments; as they present both acute and chronic effects on the health of a number of different human systems and organs, methods for their efficient removal are essential. To this end, catalysis is being researched to prevent environmental pollution. While catalytic and photocatalytic oxidation, which can eliminate low concentrations of various kinds of VOCs as well as O3, offer potential to improve indoor air quality, neither is effective in areas with high VOC concentration. To overcome this limitation, many researchers have concentrated on synergetic effects of plasma catalysis, combining the advantages of high-selectivity catalysis and fast ignition and response of a nonthermal plasma. The authors’ research group has demonstrated that our nanosecond (ns)-pulsed-discharge plasma decomposes NOx as well as produces O3 with an energy efficiency higher than that of the reported nonthermal discharges. However, the performance of the ns-pulsed-discharge plasma in VOC decomposition in comparison with or without catalyst has not yet been investigated systematically. Thus, this paper experimentally clarified the combined effects of MnOx catalyst supported by Ni foam on toluene decomposition. The input energy density to our plasma catalysis reactor using a ns-pulsed discharge was 11% of a cited dielectric barrier discharge plasma catalysis when toluene removal ratio reached 100%.
This paper studies the toluene removal by a two-stage dielectric barrier discharge (DBD)-catalyst system with three catalysts: MnOx
/ZSM-5, CoMnOx
/ZSM-5, and CeMnOx
/ZSM-5. V-Q Lissajous method, Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), and X-ray photoelectron (XPS) are used to characterize the DBD and catalysts. The DBD processing partially oxidizes the toluene, and the removal efficiency has a linear relationship with ozone generation. Three DBD-catalyst systems are compared in terms of their toluene removal efficiency, Fourier transform infrared (FTIR) spectra, carbon balance, CO selectivity, CO2 selectivity, and ozone residual. The results show that the DBD-catalyst system with CoMnOx
/ZSM-5 performs better than the other two systems. It has the highest removal efficiency of about 93.7 %, and the corresponding energy yield is 4.22 g/kWh. The carbon balance and CO2 selectivity of CoMnOx
/ZSM-5 is also better than the other two catalysts. The measurements of two important byproducts including aerosols and ozone are also presented.
Cheap natural magnetite (NM) was modified with oxygen plasma owing to its cleaning effect by chemical etching and with argon plasma due to its sputtering effect resulting in more surface roughness. These plasmas were utilized individually or in the order of first O2 and then Ar plasmas, respectively. The performance of the plasma treated magnetites (PTMs) was higher than NM for treatment of Basic Blue 3 (BB3) in catalytic ozonation (O3/PTM). The properties of NM and the most efficient treated magnetite (PTM4) samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) and scanning electron microscopy (SEM) methods. The optimal values were chosen for operational parameters including ozone concentration (0.3 g/L), initial pH (6.7) and PTM4 dosage (600 mg/L). GC–Mass analysis was applied to detect intermediates. Environmentally-friendly treatment of the NM, simple separation of the catalyst, negligible leached iron concentration, successive reusability at milder pH and unaffected efficiency in the presence of inorganic salts are the main advantages of the PTM4.
Metal ions-modified cryptomelane-type manganese oxide octahedral molecular sieves M′-OMS-2 (M′ = Co,Ce,Cu) were synthesized and characterized by XRD, BET, EDS, ICP, TEM, XPS, FTIR, H2-TPR in this work. The obtained materials were supported on Al2O3 pellets and investigated for acetaldehyde degradation in a post plasma-catalysis system. The results showed that the introduction of M′-OMS-2/Al2O3 catalysts improved acetaldehyde removal efficiency and inhibited ozone formation of plasma significantly, while the intermediates such as acetic acid, amine and nitromethane were suppressed. Co-OMS-2/Al2O3 exhibited the best catalytic activity to combine with plasma among all the prepared catalysts. The excellent activity is ascribed to the redox property, oxygen vacancies, oxygen mobility and surface area of catalyst. It can be inferred that, ozone formed in plasma was catalytically dissociated into active oxygen species by metal ions in M′-OMS-2 catalysts and contributed to acetaldehyde oxidation and intermediates removal.
The current paper reports on a newly developed DRIFTS-MS system for the investigation of non-thermal plasma (NTP) assisted heterogeneously catalyzed reactions. Specifically, this methodology has been utilized to investigate the surface changes during the NTP-activated hydrocarbon selective catalytic reduction (HC-SCR) deNOx reaction over a silver-based catalyst at ambient temperature using simulated diesel fuels (toluene and n-octane). The experimental setup and the methods used to investigate the plasma activation operating with helium as the carrier gas in order to examine low-temperature reactions are described. The technique has identified the importance, even at low temperatures, of isocyanate species in the HC-SCR deNOx reaction as well as the critical role of water in the formation of N2.Keywords: low-temperature hydrocarbon-selective catalytic reduction; Ag/Al2O3; non-thermal plasma; DRIFTS-MS; NOx reduction; toluene; octane
Introduction Specific Features Generated by Plasma-Assisted Catalytic Applications Chemical Composition and Texture Methodologies Used for the Preparation of Catalysts for Plasma-Assisted Catalytic Reactions Catalysts Forming Regeneration of the Catalysts Used in Plasma Assisted Reactions Plasma Produced Catalysts and Supports Conclusions References
The adsorption of 4-nitroanisole on silver colloidal nanoparticles was investigated by surface-enhanced Raman spectroscopy (SERS). Actually, the chemical binding with a metal substrate may play a role in changing the electronic structure of this molecule, which can be considered a push–pull chromophore, because an internal charge-transfer occurs between methoxy and nitrogroup. A SERS signal could be detected only in chloride-activated silver colloids, but the spectrum recorded with green-light excitation was not related to adsorbed 4-nitroanisole, but to its azoderivative, formed by photoreduction of the nitrogroup on the surface of the silver substrate. Copyright © 2013 John Wiley & Sons, Ltd.
Atmospheric pressure nonthermal-plasma-activated catalysis for the removal of NOx using hydrocarbon selective catalytic reduction has been studied utilizing toluene and n-octane as the hydrocarbon reductant. When the plasma was combined with a Ag/Al2O3 catalyst, a strong enhancement in activity was observed when compared with conventional thermal activation with high conversions of both NOx and hydrocarbons obtained at temperature ≤250 °C, where the silver catalyst is normally inactive. Importantly, even in the absence of an external heat source, significant activity was obtained. This low temperature activity provides the basis for applying nonthermal plasmas to activate emission control catalysts during cold start conditions, which remains an important issue for mobile and stationary applications.
The effect of water vapor on the performance of a combined non-thermal plasma catalysis (CPC) system with nickel oxide catalysts loaded on different supports was investigated. The catalysts NiO/gamma-Al2O3, NiO/SBA-15 (Santa Barbara Amorphous-15), and NiO/TiO2 were prepared and their activities were tested in the absence and presence of water vapor. Complete destruction of toluene was achieved in the absence of water vapor at ambient temperature and pressure. The activities of catalysts for the toluene conversion in dry air decreased in the following order: NiO/gamma-Al2O3 > NiO/SBA-15 > NiO/TiO2. The presence of water vapor in the feed stream had a significant negative impact on the performance of the CPC systems. This reduction in performance was primarily due to the quenching by water vapor of active species in the plasma and the competitive adsorption of water vapor on the catalyst surfaces. A novel in situ FTIR system was constructed and used to obtain in situ FTIR spectra of the reactive surfaces of the catalysts, revealing that the water molecules that adsorbed on the catalyst surfaces came from both water vapor present in the gas stream and from water vapor formed during the oxidation of toluene. H2O-TPD results indicated that the activation energies of water desorption from the catalysts decreased in the following order: NiO/gamma-Al2O3 > NiO/SBA-15 > NiO/TiO2. The catalyst with lower water vapor desorption activation energy had higher resistance to water vapor. Therefore, the durability towards water vapor poisoning of these catalysts followed the order of: NiO/TiO2 > NiO/SBA-15 > NiO/gamma-Al2O3.
A series of manganese based catalysts have been tested in a combined plasma-catalyst reactor in the reaction of toluene removal from air. In the standard conditions (toluene = 240 ppm, energy density = 172 J/L, 1 g of catalyst, 315 mL/min), the best catalyst (manganese oxide supported on active carbon) is able to transform 55% of the toluene into carbon oxides. According to the study of the reaction mechanism, it appears that the toluene is oxidized both in-plasma by short-lived species generated by plasma and in post-plasma on the catalyst surface by the ozone formed in the plasma, the reaction on the catalyst being more selective in carbon dioxide formation than the reaction in plasma. We have shown that the toluene conversion increases when the toluene concentration in air decreases. A model able to describe the behavior of the plasma reactor and the plasma-catalyst reactor is proposed.
In the present paper a mathematical model of a gas–gas reaction between ozone and benzene in a tubular reactor is considered. Usually, mathematical models of chemical process are governed by a set of ordinary differential equations assuming that the corresponding concentration dynamics depends only on time. On the other hand, the spatial distribution of the mass, energy and concentrations may be observed in the case of a more complex model structure that demands the use of models described by partial differential equations. The example of such complex model describing, the reaction between benzene and ozone in the gas phase, is considered here. The approach suggested in this study is based on the differential neural network (DNN) technique which permits to convert the task of mathematical modeling of a tubular reactor containing an uncertain (not well-defined) dynamics to a non-parametric identification problem. The asymptotic convergence of the obtained identification error to an ellipsoidal zone containing the origin is shown using the Lyapunov-like analysis. The coincidence between the benzene and ozone concentrations variation calculated by the suggested DNN-algorithm and those generated by a kinetic model is shown to be good enough.
The book details biofilter design and operation concepts used by engineers and others; conveys a basic understanding of how biofiltration works by means of contaminant adsorption and biodegradation; and includes otherwise hard-to-find information on the economics of choosing among various biofiltration systems, including details on important designs used in the field.
The combined techniques of in situ Raman microscopy and scanning electron microscopy (SEM) have been used to study the selective oxidation of methanol to formaldehyde and the ethene epoxidation reaction over polycrystalline silver catalysts. The nature of the oxygen species formed on silver was found to depend critically upon the exact morphology of the catalyst studied. Bands at 640, 780 and 960 cm–1 were identified only on silver catalysts containing a significant proportion of defects. These peaks were assigned to subsurface oxygen species situated in the vicinity of surface dislocations, AgIIIO sites formed on silver atoms modified by the presence of subsurface oxygen and O2– species stabilized on subsurface oxygen-modified silver sites, respectively. The selective oxidation of methanol to formaldehyde was determined to occur at defect sites, where reaction of methanol with subsurface oxygen initially produced subsurface OH species (451 cm–1) and adsorbed methoxy species.
Two distinct forms of adsorbed ethene were identified on oxidised silver sites. One of these was created on silver sites modified by the interaction of subsurface oxygen species, and the other on silver crystal planes containing a surface coverage of atomic oxygen species. The selective oxidation of ethene to ethylene oxide was achieved by the reaction between ethene adsorbed on modified silver sites and electrophilic AgIIIO species, whereas the combustion reaction was perceived to take place by the reaction of adsorbed ethene with nucleophilic surface atomic oxygen species. Defects were determined to play a critical role in the epoxidation reaction, as these sites allowed the rapid diffusion of oxygen into subsurface positions, and consequently facilitated the formation of the catalytically active AgIIIO sites.
Reaction of toluene with ozone in acetic anhydride was studied. The dependence of the direction, selectivity, and extent of
oxidation on the reaction temperature, solvent nature, and catalyst composition was examined.
Silver colloids prepared by reducing AgNO3 in aqueous solution with sodium citrate were embedded in alumina following two different preparation procedures resulting in samples containing 3 and 5wt.% silver. Characterization of these materials using TEM, XPS, XAES, CP/MAS NMR, XRD, and adsorption–desorption isotherms of nitrogen showed that embedding the pre-prepared silver colloids into the alumina via the sol–gel procedure preserved the particle size of silver. However, as XAES demonstrates, the catalysts prepared in a sol–gel with a lower amount of water led to embedded colloids with a higher population of Ag+ species. The catalytic behaviors of the resultant catalysts were well correlated with the concentration of these species. Thus, the active silver species of the catalysts containing more Ag+ species selectively converts NO to N2. However, subsequent thermal aging leads to an enhancement of the conversion of NO parallel to slight alteration of the selectivity with the appearance of low amounts of N2O despite an increase of Ag+ species. Accordingly, an optimal surface Ag0/Ag+ ratio is probably needed, independently of the size of silver particles. It was found that this optimal ratio strongly depends on the operating conditions during the synthesis route.
A silica-supported Ag system made by the incipient wetness impregnation method was investigated in the reaction of heterogeneous catalytic decomposition of ozone. It was established that the catalytic ozone decomposition on Ag/SiO2 proceeded in the temperature interval −40°C to 25°C as a first order reaction with activation energy of 65kJ/mol (pre-exponential factor 5.0×1014s−1). Based on the results from the instrumental methods (SEM, XRD, XPS, EPR, TPD) it can be concluded that in presence of ozone the silver is oxidized to a complicated mixture of Ag2O3 and AgO. Due to the high activity and stability of the Ag/SiO2 catalyst, it is promising for neutralization of waste gases containing ozone.
In situ X-ray absorption fine structure spectroscopic studies were carried out to investigate the structural changes in manganese oxides supported on alumina in the catalytic decomposition of ozone at room temperature. In the ozone decomposition with water vapor, Mn atom was oxidized to higher oxidation state with the coordination of water to Mn site, which was caused by the cleavage of Mn-O-Al bond. The used catalyst was regenerated by the heat treatment in an 02 flow at 723 K. (c) 2005 Elsevier B.V. All rights reserved.
The oxidative decomposition of trichloroethylene (TCE) in dry air was investigated in non-thermal plasma at atmospheric pressure and room temperature, both in the absence and in the presence of gold containing mesoporous silica (GMS) catalysts. In the absence of catalyst, TCE removal reached 100% for average powers dissipated in the plasma above 3W, for a TCE concentration of 430ppmv. Carbon monoxide and carbon dioxide were the major reaction products with CO2 selectivity up to 25% and CO selectivity up to 70%. In the presence of gold containing mesoporous catalysts, the concentrations of CO and CO2 increased as compared to those obtained with plasma alone. The GMS catalysts can dissociate ozone produced in plasma to oxygen radicals that decompose TCE. Among these catalysts, the one containing the least amount of Au (0.5% GMS) showed the best catalytic performance. In the presence of ozone generated in the plasma, isolated gold cations might play a critical role for the catalytic behavior.
The gas-phase toluene removal efficiencies by photocatalytic oxidation (TiO2/UV), the combination of ozone and photocatalytic oxidation (TiO2/UV/O3), and the UV/O3 reaction were tested using a quartz tube photoreactor. The experiments were conducted under various ozone concentrations (3.3–15 ppm), toluene concentrations (1–9 ppm), relative humidity (5–80%), and gas flow rates (200–1200 mL/min). The toluene oxidation rates (TORs) of TiO2/UV/O3, and UV/O3 reactions were proportional to the ozone concentrations. The TORs of TiO2/UV, TiO2/UV/O3, and UV/O3 reactions increased with toluene concentration. However, there were negative correlations between the toluene removal efficiencies of these three kinds of reactions and the toluene concentrations. The order of the TORs and the CO2 yield rates of these three reactions were TiO2/UV/O3 > TiO2/UV > UV/O3. The kinetics of TiO2/UV, and TiO2/UV/O3 reactions fit the Langmuir–Hinshelwood rate form. The rate constants (k) and Langmuir adsorption constants (K) are as follows: TiO2/UV: k = 0.0102 ppm m/s, K = 0.146 ppm−1; TiO2/UV/O3: k = 0.0268 ppm m/s, K = 0.0796 ppm−1. The reciprocal of UV/O3 reaction rate showed a positive linear relationship with the reciprocals of humidity and of toluene concentration. Ozone, also an air pollutant, was removed in the TiO2/UV/O3, and UV/O3 reactions. The ozone removal efficiency of TiO2/UV/O3 reaction in the presence and absence of toluene ranged from 61.1 to 99.5% and 38.1 to 95.1%, respectively.
Oxidative removal of toluene in a dielectric barrier discharge reactor combined with manganese catalysts downstream was investigated. Toluene input concentration was varied in the range of 415–2227 ppm. The discharge was operated in pulsed mode, with short pulses of 23–35 kV peak voltage. At 7 W average power, toluene conversion was 60%–70%, independent on the toluene input concentration and on the total gas flow rate in the range of 110–330 SCCM (SCCM denotes cubic centimeter per minute at STP). Toluene total oxidation was favored at high residence time of the gas in the discharge zone and low toluene concentration, when the main reaction product was CO2 with selectivities of 80%–85%. The addition of the catalysts led to a 15%–20% increase in toluene conversion with respect to the values obtained in the plasma, due to oxidation with ozone on the catalyst surface.
The mechanistic cause of the dramatic activity improvement of alumina-supported silver (Ag/Al2O3) by H2 addition for the selective catalytic reduction of NO with propane (C3H8-SCR) was investigated by catalytic and spectroscopic studies. In situ UV−vis, in situ EXAFS, IR, and microcalorimetric experiments show that H2 reduction of Ag+ ions on Ag/Al2O3 at 573 K yields protons on alumina and partially reduced Agnδ+ clusters, which are subsequently aggregated to larger Ag clusters. During H2 + O2 and H2-assisted C3H8-SCR reactions, Ag+ ions and Agnδ+ coexist. Reoxidation with O2 results in the redispersion of the cluster to Ag+ ion, accompanying a reaction of protons. The relationship between cluster size, redox properties, and catalytic activity is examined using Ag/Al2O3 of different Ag loadings. The steady-state NO reduction rate correlates fairly well with the amount of Agnδ+ during the H2-assisted C3H8-SCR reaction. It is shown that Agnδ+ is the active species, whereas monomeric Ag+ ion and metallic Ag particles are inactive. With Ag loading, the Ag+ reduction rate increases and the rate of cluster reoxidation decreases. A balance between the rate of reduction and reoxidation of Ag species is an important factor that controls the size and oxidation state of the Ag species and consequently the catalytic activity of Ag/Al2O3. ESR has provided evidence, for the first time, for the in situ generation of superoxide ions in H2 + O2 and H2-assisted C3H8-SCR reactions. The comprehensive reason for the hydrogen effect in HC-SCR is discussed, focusing on the role of the cluster and protons on the reductive O2 activation to superoxide ion.
Decomposition of ozone was carried out on metal oxide catalysts. The activity of the metal oxide catalysts increased roughly in the order of the increase in their surface area and in the amount of surface oxygen on them. Conductance change of these metal oxides on an introduction of ozone suggested that negatively charged oxygen species were formed on their surface. The Ag catalyst showed the highest activity, and the reactivity of the oxygen species produced on the surface of Ag catalyst toward carbon monoxide was much higher than that of the oxygen species on Co, Ni, Fe, and Mn oxides. Formation of superoxide ions and their precursors on the Ag catalyst was suggested by ESR analysis and by an activity measurement of these oxygen species. The latter, probably oxygen ion (O-), seemed to be an active species for low-temperature oxidation of CO.
The gas phase reaction of ozone with alkenes plays a major role in tropospheric chemistry including urban air quality. The reaction of trans-2,2-dimethyl-3-hexene and 2,4-dimethyl-2-pentene with ozone has been studied at ambient temperature and p = 1 atm of air (RH = 55% ± 10%) with sufficient cyclohexane added to scavenge the hydroxyl radical. Carbonyl products have been identified, and their formation yields are reported. The results are compared to those previously obtained for 35 other alkenes under the same conditions. For these alkenes, the sums of the formation yields of the primary carbonyls are close to the value of 1.0 that is consistent with the following reaction mechanism: O3 + R1R2CCR3R4 → α(R1COR2 + R3R4COO) + (1 − α)(R1R2COO + R3COR4), where R1COR2 and R3COR4 are the primary carbonyls and R1R2COO and R3R4COO are the corresponding biradicals. For 26 nonsymmetrical alkenes, the coefficients α range from 0.28 to 0.82 and indicate preferential formation of the more substituted biradicals and of the biradicals that bear the less bulky substituents. The results are directly relevant to the atmospheric chemistry of alkenes and to their role in oxidant and aerosol formation.
In this paper measurements of the behaviour of single microdischarges between metal and glass electrodes for both negative and positive polarities of the metal (designated and transitions, respectively), as well as the overall discharge behaviour between glass electrodes (GG), will be presented for different nitrogen - oxygen and water vapour - air mixtures. Increasing the oxygen concentration in nitrogen decreases the transferred charge per microdischarge for both polarities ( and ), while the total transferred charge per cycle is increased. With increasing water content in air, more charge is transferred per microdischarge for the polarity , but no significant change in the amount of transferred charge was found for a microdischarge. Less total charge is transferred per cycle with increasing water content. This is also true for double-dielectric barrier discharges (GG). The results suggest that water vapour coats the dielectric, reducing the surface resistance and increasing the effective dielectric capacity. This mechanism is supported by analysis of the voltage versus charge plot (Lissajous figure) for the involved capacitances of a double-barrier discharge. The reduction of the surface resistance of the dielectric and the resulting increase in the effective dielectric capacitance, are shown by photographs of spatially isolated microdischarges in a metal - pin dielectric arrangement.
TiO2-supported metal oxides such as CoOx, CuOx, NiOx and FeOx have been used for catalytic wet oxidation of trichloroethylene (TCE) in a continuous flow type fixed-bed reactor system, and the most promising catalyst for this wet catalysis has been characterized using XPS and XRD techniques. All the supported catalysts gave relatively low conversions for the wet oxidation at 36 °C, except for 5 wt% CoOx/TiO2 which exhibited a steady-state conversion of 45% via a transient activity behavior up to 1 h on stream. XPS measurements yielded that a Co 2p3/2 main peak at 779.8 eV appeared with the 5 wt% CoOx/TiO2 catalyst after the continuous wet TCE oxidation at 36 °C for ca. 6 h (spent catalyst) and this binding energy value was equal to that of Co3O4 among reference Co compounds used here, while the catalyst calcined at 570 °C (fresh catalyst) possessed a main peak at 781.3 eV, very similar to that for CoTiOx species such as CoTiO3 and Co2TiO4. Only characteristic reflections for Co3O4 were indicated upon XRD measurements even with the fresh catalyst sample. The simplest model, based on these XPS and XRD results, for nanosized Co3O4 particles existing with the fresh catalyst could reasonably explain the transient activity behavior observed upon the wet TCE oxidation.
Mesoporous molecular sieve Na-MCM-41 with Si/Al ratio 20 and 50 and Na-beta zeolite with Si/Al ratio 11 were synthesized and characterized using X-ray powder diffraction, scanning electron microscope and nitrogen adsorption. The Ag (5 and 2 wt%) modifications of H-MCM-41-20, H-MCM-41-50 and H-beta-11 catalysts were carried out using impregnation method. The catalysts were tested in the decomposition of ozone at ambient temperature. The 5 wt% Ag-H-MCM-41 catalyst showed a very high decomposition of ozone (~98%). The 5 wt% Ag-H-MCM-41-50 catalyst exhibited higher decomposition rate than 2 wt% Ag-H-MCM-41-50. The Ag modified H-MCM-41 catalyst with higher Si/Al ratio showed higher reaction rate than the catalyst with lower Si/Al ratio. The H-MCM-41 catalyst without Ag exhibited the lowest decomposition of ozone indicating an important role of Ag in the reaction.
A comparison has been made of plasma-catalysis with thermal-catalysis and plasma alone for the removal of low concentrations
of propane and propene from synthetic air using a one-stage, catalyst-in discharge configuration. In all cases, plasma-catalysis
produces better hydrocarbon destructions (~40%) than thermal catalysis at low temperatures. At higher temperatures, little
difference is observed between plasma-catalytic and thermal-catalytic operation. Plasma operation by itself had a similar
effectiveness to plasma-catalysis at low temperatures but was significantly lower (up to 50%) as the temperature was raised.
By examining the form of the temperature dependence for the plasma-catalytic destruction processes, it is possible to phenomenologically
distinguish two contributions to the destruction; one that is specifically plasma-induced and another (at higher temperatures)
in which both plasma and thermal activation have similar mechanisms.
The introduction of ferroelectric and catalytically active materials into the discharge zone of NTP reactors is a promising
way to improve their performance for the removal of hazardous substances, especially those appearing in low concentrations.
In this study, several coaxial barrier-discharge plasma reactors varying in size and barrier material (glass, Al2 O3, and TiO2) were used. The oxidation of methyl tert-butyl ether (MTBE), toluene and acetone was studied in a gas-phase plasma and in various packed-bed reactors (filled with
ferroelectric and catalytically active materials). In the ferroelectric packed-bed reactors, better energy efficiency and
CO2 selectivity were found for the oxidation of the model substances. Studies on the oxidation of a toluene/acetone mixture in
air showed an enhanced oxidation of the less reactive acetone related to toluene in the ferroelectric packed-bed reactors.
It can be concluded that the change of the electrical discharge behaviour was caused by a larger number of non-selective and
highly reactive plasma species formed within the ferroelectric bed. When combining ferroelectric (BaTiO3) and catalytically active materials (LaCoO3), only a layered implementation led to synergistic effects utilising both highly energetic species formed in the ferroelectric
packed-bed and the potential for total oxidation provided by the catalytically active material in the second part of the packed
bed.
Methods for hybridization of silent discharge plasma and catalysts in different forms are presented, and their synergy in benzene decomposition is discussed. TiO2 deposition on the inside wall of the coaxial type of the silent discharge plasma reactor promotes benzene decomposition in air and increases CO2 yield. TiO2–silica gel granules housed inside of the punched internal electrodes also facilitate the oxidative decomposition of benzene. Comparison of the TiO2 surface before and after the reactions by FTIR suggests that the positive effect of TiO2 can be ascribed to the active oxygen species generated on its surface. Replacement of TiO2–silica gel by MnO2 also promotes the oxidative decomposition of benzene in silent discharge plasma. Ozone, which is generated from gaseous oxygen, is decomposed by MnO2, but not by TiO2. Catalytic effects of TiO2 and MnO2 can be ascribed to formation of active oxygen species on their surfaces and that of the triplet oxygen atom from ozone on the MnO2 surface. It has been shown that both of TiO2 and MnO2 can sustain their catalytic activities in silent discharge plasma.
The role of ozone was studied for two different configurations combining non-thermal plasma (NTP) and heterogeneous catalysis, namely the use of a gas phase plasma with subsequent exposure of the effluent to a catalyst in a packed-bed reactor (post-plasma treatment) and the placement of the catalyst directly in the discharge zone (in-plasma catalysis). Non-porous and porous alumina and silica were deployed as model catalysts. The oxidation of immobilised hydrocarbons, toluene as a volatile organic compound and CO as an inorganic pollutant were studied in both operational modes.While conversion and selectivity of hydrocarbon oxidation in the case of catalytic post-plasma treatment can be fully explained by the catalytic decomposition of O3 on γ-Al2O3, the conversion processes for in-plasma catalysis are more complex and significant oxidation was also measured for the other three materials (α-Al2O3, quartz and silica gel). It became obvious that additional synergetic effects can be utilised in the case of in-plasma catalysis due to short-lived species formed in the NTP.The capability of porous alumina for ozone decomposition was found to be correlated with its activity for oxidation of carbon-containing agents. It could be clearly shown that the reaction product CO2 poisons the catalytic sites at the γ-Al2O3 surface. The catalytic activity for O3 decomposition can be partially re-established by NTP treatment. However, for practical purposes the additional reaction pathways provided by in-plasma catalytic processes are essential for satisfactory conversion and selectivity.
A novel catalytic reactor with dielectric barrier discharge (DBD) at atmospheric pressure was developed for the abatement of volatile organic compounds (VOCs). The novelty of DBD reactor is the metallic catalyst serving also as the inner electrode. The catalytic electrode was prepared from sintered metal fibers (SMF) in the form of a cylindrical tube. Oxides of Mn and Co were deposited on SMF by impregnation. Decomposition of toluene taken as the model VOC compound (<1000 ppm in air) was investigated. The catalyst composition, toluene concentration, applied voltage and frequency were systematically varied to evaluate the performance of the DBD reactor. At 100 ppm of toluene, the conversion ∼100% was achieved in the DBD reactor using a specific input energy (SIE) ∼ 235 J/l independently of the chemical composition of the SMF catalytic electrode, but the selectivity to CO2 was observed to be a function of the catalyst composition. The MnOx/SMF catalytic electrode showed the best performance towards total oxidation. At a SIE of 295 J/l, the selectivity to CO2 was 80% with 100% conversion of toluene. No carbon solid residues were deposited on the electrode.
Ozonation of toluene over NaX, NaY and MCM-41 adsorbents was studied targeting for indoor air purification. The combined use of ozone and the various micro- or meso-porous adsorbents aimed to take advantage of the strong oxidizing capability of ozone. At the same time the residual ozone would be minimized due to the enhanced catalytic reaction in the porous structure. To lower the residual ozone level is a crucial issue as ozone is itself an indoor pollutant. The Lewis acid sites in the adsorbents were believed to decompose ozone into atomic oxygen, and the subsequent reactions would then convert the adsorbed toluene into CO2 and H2O. In the dry conditions, the MCM-41 required the smallest amount of material to achieve the 90% reduction target, followed by NaY and NaX. In the more humid environment (50% RH), extra amounts of MCM-41 and NaX adsorbents were required to reach the target as compared with the dry conditions. Desorption experiments were also conducted to study the amounts of various major species held in the adsorbents during the catalytic process. A material balance analysis of the major species in both the effluents and the adsorbents showed that within our experimental conditions, about 20–40% of the removed toluene was carried out via catalytic ozonation while adsorption covered the rest. Trace amount of intermediate species such as aldehydes and organic acids were identified in the desorbed gas indicating that they were withheld by the adsorbents during the air purification process and those in the effluent were below detection levels.
The kinetics of the heterogeneous reactions of O(3) with 17 polycyclic aromatic hydrocarbons (PAHs) present on laboratory generated kerosene soot surface was studied at T=255-330 K in a low pressure flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAHs consumption in reaction with O(3) was monitored using off-line HPLC measurements of their concentrations in soot samples as a function of time of exposure to O(3). Concentration of ozone in the gas phase was analyzed by mass spectrometry. The first-order rate constants measured for individual PAHs ranged from 0.004 to 0.008 s(-1) and were found to be independent of the ozone concentration ([O(3)]=(0.5-92) x 10(12) mol cm (-3)) and temperature (255-330 K). Results show that reaction with ozone can be an important degradation pathway of the particulate PAHs in the atmosphere.
The decomposition of trichloroethylene ITCE) by non-thermal plasma was investigated in a dielectric barrier discharge (DBD) reactor with a copper rod inner electrode and compared with a plasma-catalytic reactor. The particularity of the plasma-catalytic reactor is the inner electrode made of sintered metal fibers (SMF) coated by transition metal oxides. In order to optimize the geometry of the plasma reactor, the efficiency of TCE removal was compared for different discharge gap lengths in the range of 1-5 mm. Shorter gap lengths (1-3 mm) appear to be more advantageous with respect to TCE conversion. In this case TCE conversion varies between 67% and 100% for input energy densities in the range of 80-480 J/l, while for the 5 turn discharge gap the conversion was lower (53-97%) for similar values of the input energy. As a result of TICE oxidation carbon monoxide and carbon dioxide were detected in the effluent gas. Their selectivity was rather low, in the range 14-24% for CO2 and 11-23% for CO, and was not influenced by the gap length. Several other chlorinated organic compounds were detected as reaction products. When using MnOx/SMF catalysts as the inner electrode of the DBD reactor, the TCE conversion was significantly enhanced, reaching similar to 95% at 150 J/l input energy. The selectivity to CO2 showed a major increase as compared to the case without catalysts, reaching 58% for input energies above 550 J/l. (C) 2007 Elsevier B.V. All rights reserved.
This paper provides a comprehensive review regarding the application of plasma catalysis, the integration of nonthermal plasma and catalysis, on VOC removal. This novel technique combinesthe advantages of fast ignition/response from nonthermal plasma and high selectivity from catalysis. It has been successfully demonstrated that plasma catalysis could serve as an effective solution to the major bottlenecks encountered by nonthermal plasma, i.e., the reduction of energy consumption and unwanted/hazardous byproducts. Instead of working independently, the combination could induce extra performance enhancement mechanisms either in a single-stage or a two-stage configuration, in which the catalyst is located inside and downstream from the nonthermal plasma reactor, respectively. These mechanisms are believed to be responsible for the higher energy efficiency and better CO2 selectivity achieved with plasma catalysis. A comprehensive discussion on the performance enhancement mechanisms is provided in this review paper. Moreover, the current status of the applications of two different plasma catalysis systems on VOC abatement are also given and compared. The catalyst plays an important role in both configurations. Especially for the single-stage type, depositing an inappropriate active component on catalytic support would decrease the VOC removal efficiency instead. To date, no definite conclusion on catalyst selection forthe single-stage plasma catalysis is available. However, MnO2 seems to be the best catalyst for two-stage configuration because it could effectively decompose ozone and generate active species toward VOC destruction. On the other hand, although the single-stage plasma catalysis has been proved to be superior to the two-stage configuration, it does not mean that the former is always the best choice. Considering the typical VOC concentrations from different sources and the characteristics of different plasma catalysis systems, the single-stage and two-stage configurations are suggested to be more suitable for industrial and indoor air applications, respectively.
Carbon nanotubes are expected to play an important role in sensing, pollution treatment and separation techniques. This study examines the adsorption behaviors of volatile organic compounds (VOCs), n-hexane, benzene, trichloroethylene and acetone on two multiwall carbon nanotubes (MWCNTs), CNT1 and CNT2. Among these VOCs, acetone exhibits the highest adsorption capacity. The highest adsorption enthalpies and desorption energies of acetone were also observed. The strong chemical interactions between acetone and both MWCNTs may be the result from chemisorption on the topological defects. The adsorption heats of trichloroethylene, benzene, and n-hexane are indicative of physisorption on the surfaces of both MWCNTs. CNT2 presents a higher adsorption capacity than CNT1 due to the existence of an exterior amorphous carbon layer on CNT2. The amorphous carbon enhances the adsorption capacity of organic chemicals on carbon nanotubes. The morphological and structure order of carbon nanotubes are the primary affects on the adsorption process of organic chemicals.
Gaseous pollution control technologies for acid gases (NO<sub>x
</sub>, SO<sub>x</sub>, etc.), volatile organic compounds (VOC),
greenhouse gases, ozone layer depleting substance (ODS), etc., have been
commercialized based on catalysis, incineration and adsorption methods.
However, non-thermal plasma techniques based on electron beams and
corona discharges become significant due to advantages such as lower
cost, higher removal efficiency, smaller space volume, etc. In order to
commercialize this new technology, the following needs faster
investigation: pollution gas removal rates, energy efficiency of
removal, pressure drop of reactors, usable byproduct production rates,
identification of major fundamental processes, and optimization of
reactor and power supply for an integrated system. In this work, recent
development of gaseous pollution control technology based on discharge
plasmas is reviewed critically and the principle of processes and
reactor technologies are outlined
Plasma chemical behavior of trichloroethylene (TCE) was
investigated with a packed-bed ferroelectric pellet reactor and a pulsed
corona reactor. Volatile byproducts were identified by gas
chromatography and mass spectrometry (GC-MS), and it was shown that
reactor type, TCE concentration, flow rate, background gas, and moisture
affected TCE decomposition efficiency and product distribution.
Byproduct distributions in nitrogen and the negative effect of oxygen
and moisture on TCE decomposition efficiency show that TCE decomposition
proceeds via initial elimination of chlorine and hydrogen atoms, the
addition of which to TCE accelerates its decomposition. Active oxygen
species like OH radical is less likely involved in the initial step of
TCE decomposition in plasma. Triplet oxygen molecules
(<sup>3</sup>O<sub>2</sub>) scavenge intermediate carbon radicals
derived from TCE decomposition to give much lower yields of organic
byproducts
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