Effect of Pt/γ-Al 2 O 3 Catalyst on Nonthermal Plasma Decomposition of Benzene and Byproducts

Abstract

This article presents the effects of different packing materials in dielectric barrier discharge (DBD) reactor on benzene removal efficiency to clarify the mechanism of combining efficiency between nonthermal plasma and catalyst over organic contaminants degradation. The influence of γ-Al₂O₃ and Pt/γ-Al₂O₃ catalysts on benzene decomposition was examined by passing gaseous benzene through two DBD reactors, each packed with one of the catalysts mentioned above. Concentrations of benzene, CO, CO₂, NO₂, and NO were measured and decomposition efficiency (of benzene), selectivity (of CO and CO₂), and carbon balance were calculated to compare the effects of γ-Al₂O₃ catalyst with that of Pt/γ-Al₂O₃ catalyst in DBD reactors on benzene degradation. Data showed that at the same input energy density, benzene decomposition efficiency increased by about 20%, whereas the amount of CO released decreased by about 50% when using Pt/γ-Al₂O₃ catalyst. From the Arrhenius plot, the activation energy of Pt/γ-Al₂O₃ catalyst was calculated to be 3.01 kJ/mol, which turned out to be less than that of γ-Al₂O₃ catalyst by 57%. Further, the amounts of NO₂ and NO released from the reactor when using Pt/γ-Al₂O₃ catalyst were only about 1/3 and 1/6 of that when using γ-Al₂O₃ catalyst, respectively. Therefore, Pt/γ-Al₂O₃ is a better catalyst because of the higher benzene decomposition efficiency, lower activation energy, and less byproducts generated when it was used in DBD reactor.

Introduction

Nonthermal plasma (NTP) technology is a promising approach to abate volatile organic compounds (VOCs) pollution (Malik and Malik, 1999; Hackam and Akiyama, 2000; Urashima and Chang, 2000; Ogata et al., 2003). However, low energy efficiency and undesirable byproducts (such as CO, NOx, and O₃) have considerably decreased its practicality. Many researchers have attempted to diminish these shortcomings by combining heterogeneous catalyst with NTP technology (Xia et al., 2000; Li et al., 2002; Oda, 2003; Kim et al., 2004, 2005a). The changes in decomposition efficiencies of pollutants in plasma reactors using catalysts such as 13 × zeolite, BaTiO₃, Pd/γ-Al₂O₃ (Xia et al., 2000), and Ag/TiO₂ (Kim et al., 2004) showed that the incorporation of catalysts into NTP technology is a desirable approach to improve NTP performance: The Pd/γ-Al₂O₃ catalyst reduced NOx output and improved methane decomposition efficiency significantly (Xia et al., 2000). The Ag/TiO₂ catalyst suppressed intermediates formation and N₂O release. Previous studies done by our group also showed that photocatalysts could improve benzene decomposition efficiency to a large extent (Song et al., 2007). It has been suggested that catalysts' increasing pollutant molecules' residence time in the plasma zone and lowering reaction energy barrier were the main contributing factors to their effects on pollutant decomposition efficiency (Holzer et al., 2002).

Pt/γ-Al₂O₃ catalyst is widely known as one of the most effective commercial catalysts for CO oxidation. In addition, it is also commonly used to excite hydrogen atoms of aromatic hydrocarbon molecules such as benzene (Margitfalvi et al., 1981; Arribas et al., 2000; Xia et al., 2000; Chen et al., 2005; Kim et al., 2005b). Therefore, there have been considerable researches done on the effects of adding Pt/γ-Al₂O₃ catalysts into NTP reactor on VOCs removal (Ogata et al., 2001, 2003; Kim et al., 2003; Ayrault et al., 2004; Pasquiers, 2004; Demidiouk and Jae, 2005; Kim et al., 2006; Jeon et al., 2007). It has been found that the incorporation of Pt/γ-Al₂O₃ catalyst improved energy efficiency (Ogata et al., 2003), maximized product yield, and minimized byproducts (mainly CO and NOX) formation (Jeon et al., 2007). However, most of the researches conducted have been focusing on the quantity aspect of the improvements mentioned above while failing to investigate the mechanisms by which the catalysts exerted their effects.

In this research, the effects of Pt/γ-Al₂O₃ catalyst on benzene decomposition in a dielectric barrier discharge were compared with that of γ-Al₂O₃. The concentrations of benzene, CO, CO₂, NO₂, and NO were measured after each reaction and the effects of gas temperature on their concentrations were also investigated. In addition, the activation energy and the number of active centers were calculated from the Arrhenius plot. Further, the main causes to the catalyst's effects were also explored.

Experimental

Catalyst preparation

The Pt/γ-Al₂O₃ catalyst was prepared by impregnation of γ-Al₂O₃ with aqueous solution of H₂PtCl₆ (analytically pure; Shanghai Haiqu Chemical Co., Ltd.), followed by evaporation to dryness in a rotary evaporator at 323 K. The resulting sample was then dried and reduced in the nitrogen–hydrogen mixture flow at 373 K for 8 h and calcined at 873 K for 4 h. The final product contained 1 wt% of Pt and the BET surface area of Pt/γ-Al₂O₃ catalyst was measured to be 162 m²/g.

Experimental setup

The structure of the reactor (shown in Fig. 1a) was similar to that in our previous study (Li et al., 2008): a quartz tube (inner diameter = 16 mm, outer diameter = 20 mm, length = 120 mm) was used as the discharge barrier. The inner electrode was a stainless-steel rod (diameter = 10 mm) and the outer electrode was an aluminum mesh. The discharge length was about 40 mm and washed γ-Al₂O₃ beads or prepared Pt/γ-Al₂O₃ catalyst (diameter = 2–3 mm) was used to pack the inside of the reactor.

FIG. 1.

(a) Packed-dielectric barrier discharge reactor. (b) Experimental schematic: 1, benzene bubble bath; 2, packed plasma reactor; 3, oscilloscope; 4, high-voltage AC power supply; 5, optical fiber; 6, spectrometer; 7, personal computer; 8, gas chromatograph; 9, infrared CO gas analyzer; 10, infrared CO₂ gas analyzer; 11, NOx analyzer.

Moreover, the schematic of experiment (shown in Fig. 1b) also resembled our previous study (Li et al., 2008). The experimental setup consisted of a benzene bubble bath, a packed plasma reactor, an oscilloscope, a high-voltage AC power supply (8 kHz), an optical fiber, a spectrometer, a PC, a gas chromatograph, an infrared CO gas analyzer, an infrared CO₂ gas analyzer, and a NOx analyzer. Also, the reactant (gaseous mixture of benzene and air) was preheated by a thermocouple heater, with its temperature monitored by a thermocouple.

Measurement technique

Electrical measurement

We used the V-Q Lissajous method to determine the discharge power of the plasma reactor. The charge Q was calculated from the measured voltage across the capacitor (10,300 pF), through which an aluminum mesh was grounded. The applied high voltage V was measured with a 1,000:1 high-voltage probe (Textronix, P6015A). The signals of V and Q were recorded by a digitizing oscilloscope (Textronix, TDS2022), averaging from 64 scans. Then, the discharge power was calculated by multiplying the area of V-Q parallelogram with the discharge frequency. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} P_{ \rm dis} (W) = f \times \int \limits_0^T V dQ = f \times C \times \int \limits_0^T V du \tag{1} \end{align*} \end{document}

Here, V is the applied high voltage, u is the voltage across the capacitor, and f is the discharge frequency.

The input energy density (IED) was calculated by \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} {\rm IED} \ ({\rm J} / {\rm L}) = \frac {\hbox {\rm Discharge power} \ ({\rm W})} {\hbox {\rm Gas flow rate} \ ({\rm L} / {\rm min})} {\times} 60 \tag {2} \end{align*} \end{document}

Gas analysis

Benzene concentration was monitored by gas chromatography (with thermal conductivity and flame ionization detectors as well as Porapak R column [2m]). The chromatographic temperature program was set to start at 333 K and to then increase to 553 K with a rate of 10 K/min. Temperatures of the injector and transfer line were set to 473 and 523 K, respectively.

The benzene concentration in the discharge gas (a mixture of air and benzene) was about 320 ppm. The discharge gas' flow rate was initialized at 3 L/min and the resulting concentrations of CO, CO₂, NO, and NO₂ were measured by infrared gas analyzers (Beijing BAIF-Maihak Analytical Instrument Co. Ltd., QGS-08C for CO, QGS-08 for CO₂; Thermo Electron Corporation, 42i-HL NO-NO₂-NOx high level analyzer for NO and NO₂).

The carbon balance was given by \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} {\rm Cb} \ (\%) = \frac {[{\rm CO}] {+} [{\rm CO} _2]} {6 ([{\rm benzene}] _ {\rm in} - [{\rm benzene}] _ {\rm out})} {\times} 100 \tag {3} \end{align*} \end{document}

Here, Cb denotes carbon balance.

The selectivity of CO and CO₂ was given by, respectively, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} {\rm S} _ {\rm CO} \ (\%) = \frac {[{\rm CO}]} {6 ([{\rm benzene}]_{\rm in} - [{\rm benzene}] _ {\rm out})} {\times} 100 \tag {4} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} {\rm S} _ {{\rm CO} 2} \ (\%) = \frac {[{\rm CO} _2]} {6 ([{\rm benzene}] _ {\rm in} - [{\rm benzene}] _ {\rm out})} {\times} 100 \tag {5} \end{align*} \end{document}

Here, [benzene]_in denotes the initial concentration of benzene and [benzene]_out denotes the benzene concentration at outlet.

Temperature measurement

Details regarding the method used to determine gas temperature were described in our previous paper (Li et al., 2008). The line (337.1 nm) intensity of the plasma's optical emissions was determined by a monochromator (Tianjin Gangdong Science & Technology Co. Ltd., WGD-8A). The measured emission line profile of the 0-0 band of the second positive system of N₂ (C-B) was compared with that in a mathematical simulation. The N₂ molecules' rotational temperature (Tr) was thus determined, which was equivalent to the gas temperature.

Results and Discussion

Benzene conversion

The correlation between benzene decomposition efficiency and IED is shown in Fig. 2a. It can be seen that benzene decomposition efficiency increases with increasing IED in both reactors regardless of whether preheating is involved in the experimental procedures. Without preheating, benzene decomposition efficiency is approximately 10% higher in the Pt/γ-Al₂O₃–catalyzed reaction (line B) than that in the γ-Al₂O₃–catalyzed reaction (line A). This may be attributed to Pt catalyst's significant lowering of benzene's oxidation energy barrier (Fu et al., 1995, 1999). Also, the catalyst effectively excites the H atoms of benzene, thus making benzene more reactive. Under the effect of preheating, depicted by lines B (Pt/γ-Al₂O₃–catalyzed reaction under nonpreheated condition) and C (Pt/γ-Al₂O₃–catalyzed under preheated condition), there is another 10% increase in benzene decomposition efficiency. This phenomenon is most likely caused by the increased number of activated benzene molecules and the improved catalytic effect of the catalyst under higher temperature (Kim et al., 2006).

FIG. 2.

Correlation between benzene decomposition efficiency and (a) input energy density (IED); (b) Tr. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

To summarize, replacing γ-Al₂O₃ catalyst with Pt/γ-Al₂O₃ along with preheating the reactant mixture would allow for a 20% increase in benzene decomposition efficiency. Further, similar findings were observed in experiments previously done by other researchers using methane and 0.5 wt% Pt/γ-Al₂O₃ as the reactant and catalyst, respectively (Sano et al., 2006).

The aforementioned observations indicate that preheating the discharge gas can improve benzene decomposition efficiency. Therefore, it is important to investigate the effect of gas temperature on benzene decomposition efficiency to shed some light on the kinetics of decomposition. The correlation of benzene decomposition efficiency and gas temperature (Tr) is shown in Fig. 2b. It should be noted that Tr always increases with IED in NTP reactors, although their precise relationship remains unknown. As to the relationship between Tr and benzene decomposition efficiency, it can be concluded from the graph that benzene decomposition efficiency increases almost linearly with increasing Tr. At the same Tr, the γ-Al₂O₃–catalyzed reaction (line A) has the lowest efficiency in all three conditions. Finally, the rate of Pt/γ-Al₂O₃–catalyzed reaction under nonpreheated condition (line B) is a little higher than that preceded by preheating (line C). This indicates that the effect of the discharge on catalytic benzene decomposition is stronger than that of preheating.

The difference between the effects of Pt/γ-Al₂O₃ and γ-Al₂O₃ on benzene decomposition can be explained by the Arrhenius plot (Demidyuk and Whitehead, 2007) in Fig. 3. C₀ and C represent the initial and final concentrations of benzene, respectively. The calculated activation energy and parameter A of the Arrhenius equation are presented in Table 1. From the Arrhenius equation, it can be inferred that the value of parameter A is equivalent to the number of active sites. The value of parameter A is 152.92 s⁻¹ for the γ-Al₂O₃ line, which is lower than those of the Pt/γ-Al₂O₃ lines (183.09 s⁻¹ for the nonpreheated condition and 194.42 s⁻¹ for the preheated condition). Thus, it can be safely concluded that Pt/γ-Al₂O₃ has a much greater number of active sites than γ-Al₂O₃. Further, the activation energies of both Pt/γ-Al₂O₃–catalyzed reactions under preheated and nonpreheated conditions (3.014.73 and 3.06 4.73 kJ/mol, respectively) are considerably lower than that of the γ-Al₂O₃–catalyzed reaction (4.73 kJ/mol). It verifies that Pt catalyst is more beneficial to lower the reaction's activation energy, therefore leading to higher benzene decomposition efficiency. Even though the values of parameter A are higher than those in previous research (Demidyuk and Whitehead, 2007), the values of activation energies are indeed lower than those in the previous research. This is probably due to the difference in the reaction system (such as catalysts, reactants, and the reactor) employed.

FIG. 3.

Arrhenius plot of the influence of catalysts and temperature. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

Table 1.

Parameters of Arrhenius Equation for Benzene Decomposition

Arrhenius Equation: k = A^(−E_a/RT)
Catalysts	A (s⁻¹)	Activation energy (E_ac) (kJ/mol)
γ-Al₂O₃ beads (nonpreheated)	152.93	4.73
Pt/γ-Al₂O₃ beads (nonpreheated)	183.09	3.01
Pt/γ-Al₂O₃ beads (preheated)	194.42	3.06

CO and CO₂ concentrations

CO is one of the major byproducts formed during VOCs decomposition in an NTP system. The relationship between CO concentration and IED is depicted in Fig. 4a. Under nonpreheated condition, in both γ-Al₂O₃–catalyzed and Pt/γ-Al₂O₃–catalyzed reactions, CO concentration increases with increasing IED, although much more rapidly in the former reaction. However, under preheated condition, the CO concentration in the reaction using Pt/γ-Al₂O₃ actually reduces significantly with increasing IED. Further, the maximum CO concentration when using Pt/γ-Al₂O₃ without preheating is only about 200 ppm, as opposed to 400 ppm when using γ-Al₂O₃. Moreover, the former decreases to 150 ppm if the discharge gas has been preheated.

FIG. 4.

Dependence of (a) CO concentration on IED; (b) CO₂ concentration on IED; (c) CO₂ concentration on Tr. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

Figure 4b shows the relationship between CO₂ concentration and IED. It can be seen that CO₂ concentration increases almost linearly with increasing IED in all three reactions. It should be noted that Pt/Al₂O₃ catalyst is significantly more effective in increasing CO₂ concentration. Moreover, the effect of Pt/Al₂O₃ catalyst becomes more significant as the IED increases. For example, when IED is at 439.16 J/L, CO₂ concentrations yielded from Pt/γ-Al₂O₃–catalyzed and γ-Al₂O₃–catalyzed reactions are 606 and 412 ppm, respectively. In contrast, when IED rises to 794.46 J/L, the former and the latter become 1,100 and 791 ppm, respectively. In addition, preheating is also beneficial for improving CO₂ concentration with an effect that also becomes more prominent with increasing IED. For example, at an IED of 439.16 J/L, CO₂ concentration yielded from Pt/γ-Al₂O₃–catalyzed reaction under preheated condition is 882 ppm, as opposed to 606 ppm yielded from the nonpreheated reaction. And when IED increases to 794.46 J/L, it is 1,514 ppm under preheated condition and 1,100 ppm under nonpreheated condition. It can thus be observed that the difference is much more significant in the latter case, when the IED is greater.

It can be inferred from Figs. 4a and 4b that Pt/γ-Al₂O₃ catalyst oxidizes a certain amount of CO molecules into CO₂, leading to lower CO concentration and higher CO₂ concentration than those in γ-Al₂O₃–catalyzed reaction.

The linear relationship between CO₂ concentration and Tr is shown in Fig. 4c. It can be observed that when Pt/γ-Al₂O₃ catalyst is used without preheating, CO₂ concentration increases with increasing Tr at 2.3 times faster than when γ-Al₂O₃ catalyst is used. Moreover, when Pt/γ-Al₂O₃ catalyst is used with preheating, CO₂ concentration increases with increasing Tr twice as fast as that when the same catalyst is used without preheating. These results indicate that both Pt/γ-Al₂O₃ catalyst and preheating improve CO₂ yield.

The above observations show that Pt/γ-Al₂O₃ is the more effective catalyst in promoting CO oxidization when coupled with NTP. This can be attributed to the fact that Pt/γ-Al₂O₃ can promote O₃ (formed during discharging process) decomposition into O more effectively, which consequently accelerates CO oxidation reaction. Further, preheating can also promote the reaction because the increased temperature in reactor lowers the reaction's energy barrier, thus making the reaction proceed more easily.

CO and CO₂ selectivity and the carbon balance

CO selectivity is one of the most important parameters in evaluating the complete oxidization extent. It can be calculated from equation (4). The relationship between CO selectivity and IED and that between CO selectivity and Tr are shown in Fig. 5. It can be seen in Fig. 5a that the relationship between CO selectivity and IED is similar to that between CO concentration and IED in Fig. 4a. Further, it should be noted that under preheated condition, CO selectivity reduces rapidly (from 25.2% to 9%) with increasing IED. The minimum selectivity obtained (9%) is a particularly good result and is seldom reached in previous studies. However, under nonpreheated condition, CO selectivity remains at about 15% after an inducing stage in Pt/γ-Al₂O₃–catalyzed reaction.

FIG. 5.

Dependence of CO selectivity on (a) IED; (b) Tr. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

It can be seen in Fig. 5b that CO selectivity decreases as Tr increases in the preheated reaction using Pt/γ-Al₂O₃ catalyst. This contrasts with the result obtained from the reaction using γ-Al₂O₃, where CO selectivity actually increases linearly with increasing Tr. In addition, even though CO selectivity of the Pt/γ-Al₂O₃–catalyzed reaction is initially higher than that of the γ-Al₂O₃–catalyzed reaction, as the midrange of Tr is reached, it reduces rapidly, ultimately having a very low value at the maximum Tr value.

CO₂ selectivity is expected to improve with reduction in CO selectivity. CO₂ selectivity correlates to IED in a similar fashion to how CO₂ concentration correlates to IED (shown in Figs. 4b and 6a, respectively), whereas the relationship between CO₂ selectivity and Tr (shown in Fig. 6b) is similar to that between CO₂ concentration and Tr (shown in Fig. 4c). Further, under nonpreheated condition, CO₂ selectivity is >70% and is significantly higher in the Pt/γ-Al₂O₃–catalyzed reaction than that in the γ-Al₂O₃–catalyzed reaction. Under preheated condition, however, CO₂ selectivity increases to 95%, which is much higher than those gained in previous studies (Kim et al., 2003, 2006; Ogata et al., 2003).

FIG. 6.

Correlation between CO₂ selectivity and (a) IED; (b) Tr. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

It is widely acknowledged that heavy VOC molecules can decompose into smaller organic molecules such as formaldehyde and methanol as well as inorganic products such as CO₂ and CO in an NTP reactor (Ogata et al., 1999). The carbon balance value, which can be evaluated from equation (3), reflects the extent that VOC molecules can decompose into inorganic products. The correlation between carbon balance and IED as well as that between carbon balance and Tr are shown in Fig. 7. It can be seen in Fig. 7a that the carbon balance increases rapidly in a linear fashion with increasing IED in all three reactions performed. In addition, under nonpreheated condition, the differences between carbon balance values of line A (which depicts the relationship between carbon balance and IED in the reaction using γ-Al₂O₃ catalyst) and those of line B (which depicts the relationship between carbon balance and IED in the reaction using Pt/γ-Al₂O₃ catalyst) are very insignificant. This shows that Pt/γ-Al₂O₃ does not influence the carbon balance extensively at the same IED even though the data presented in previous sections have shown that it has profound effects on the values of benzene decomposition efficiency, CO formation, and CO₂ formation. Nevertheless, line C (which depicts the relationship between carbon balance and IED in the reaction catalyzed by Pt/γ-Al₂O₃ under preheated condition) shows that the carbon balance value rises significantly (from 69.5% to 101.9%) as IED increases from 291.4 to 794.5 J/L. The phenomenon of carbon balance's rising above 100%, which has also taken place during previous studies (Kim et al., 2003), can be attributed to the desorption of benzene molecules. The benzene molecules have been initially absorbed by γ-Al₂O₃. The carbon balance value, however, did not increase with IED as rapidly in the previous studies (Holzer et al., 2002), despite the fact that Pt/γ-Al₂O₃ catalyst was also used in the plasma discharge zone. The discrepancy is due to the lower concentration of benzene used in those studies (Holzer et al., 2002), which were not able to charge carbon balance effectively.

FIG. 7.

Correlation between carbon balance and (a) IED; (b) Tr. A, Results of packing γ-Al₂O₃ without preheating; B, results of packing Pt/γ-Al₂O₃ without preheating; C, results of packing Pt/γ-Al₂O₃ with preheating.

Figure 7b, which depicts the correlation between the value of carbon balance and Tr, shows that the former increases rapidly with increasing Tr in a linear fashion, which coincides with the results reported in previous studies (Ogata et al., 2003; Ayrault et al., 2004), for all three experiments. The difference between Figs. 7a and 7b can be accounted for by the difference between lines A (the relationship between carbon balance and IED and the relationship between carbon balance and Tr in the γ-Al₂O₃–catalyzed reaction under nonpreheated condition) and B (the relationship between carbon balance and IED and the relationship between carbon balance and Tr in the Pt/γ-Al₂O₃–catalyzed reaction under nonpreheated condition). More specifically, throughout both experiments, the carbon balance values are higher in the Pt/γ-Al₂O₃–catalyzed reaction than those in the γ-Al₂O₃–catalyzed reaction at the same Tr. Additionally, in both of the Pt/γ-Al₂O₃–catalyzed reactions (with or without preheating), carbon balance values increase with increasing Tr in a similar fashion. The differences between the values in the two reactions at the same Tr value are not very significant.

NO and NO₂ formation

The highly undesirable byproduct NOx (mainly NO₂ and NO) is one of the most important parameters in evaluating the performance of the NTP reactor used for VOCs decomposition. Thus, NO₂ and NO concentrations have also been monitored and are shown in Fig. 8. It can be seen that NO₂ and NO concentrations increase with increasing IED in all three reactions performed. In addition, NO₂ and NO concentrations increase with increasing IED more rapidly in the γ-Al₂O₃–catalyzed reaction than in the Pt/γ-Al₂O₃–catalyzed reaction. At 600.4 J/L, NO₂ concentration is 109.2 ppm in the γ-Al₂O₃–catalyzed reaction, whereas in the Pt/γ-Al₂O₃–catalyzed reactions, it is only 38.9 ppm under nonpreheated condition and 53 ppm under preheated condition. As IED is increased to 794.4 J/L, NO₂ concentrations become 263, 80, and 102 ppm in the respective reactions mentioned above. In short, NO₂ concentration in the Pt/γ-Al₂O₃–catalyzed reaction is only one-third of that in the γ-Al₂O₃–catalyzed reaction. As for NO, at an IED of 600.4 J/L, its concentration is 11 ppm in the γ-Al₂O₃–catalyzed reaction, whereas in the Pt/γ-Al₂O₃–catalyzed reactions, NO concentrations are 1.2 and 5 ppm, respectively, under nonpreheated and preheated conditions. When IED is increased to 794.4 J/L, NO concentrations for the aforementioned reactions are 108 and 16 and 27 ppm, respectively. Therefore, it can be concluded that using Pt/γ-Al₂O₃ catalyst can reduce NO concentrations to merely 1/6 of that using γ-Al₂O₃ catalyst. This shows the importance of Pt/γ-Al₂O₃ in suppressing NOx formation under NTP condition. It should also be noted that at the same IED, both NO₂ and NO concentrations in the Pt/γ-Al₂O₃–catalyzed reaction under nonpreheated condition are slightly lower than those in the same reaction under preheated condition. This suggests that preheating is disadvantageous for minimizing NOx production.

FIG. 8.

Relationship between NO₂ and NO and IED.

One possible explanation for Pt/Al₂O₃'s suppression of NOx formation is that the catalyst can effectively reduce the energy of excited electrons. High-energy electrons, which can excite N₂ to form NOx, would lose their energy during collisions with the catalyst. Therefore, using the catalyst would decrease the amount of excited N₂, which in turn would decrease the amount of NOx formed. A previous study (Koutsospyros et al., 2003) has also pointed out that modifications to the reacting conditions are also required to suppress NOx formation. These modifications can be done in such a way that the average electron energy is drastically reduced.

In addition, Pt/γ-Al₂O₃ catalyst's suppression of NOx formation is also attributed to its ability to accelerate O₂ excitation and decomposition. This effect would accelerate the rate of benzene oxidation (Fu et al., 1999) while limiting that of N₂ excitation. Other catalysts (such as Pd-Pt/γ-Al₂O₃) have been found to effectively suppress NOx formation via a very similar mechanism (Xia et al., 2000).

Further, Pt/γ-Al₂O₃ catalyst's powerful reduction of NOx is a major contributing factor to its ability to lower NOx formation as well. The experimental data show that part of the NOx formed during the discharges is quickly reduced to N₂ by Pt/γ-Al₂O₃. This is confirmed by the results of previous researches, which have shown that Pt/γ-Al₂O₃ is particularly effective in NOx reduction under NTP (Jeon et al., 2007).

We attribute the discussed phenomena to the synergic effect between the plasma reactor and the Pt/γ-Al₂O₃ catalyst. This leads to high benzene decomposition efficiency and high carbon balance values. In addition, certain characteristics of the Pt/γ-Al₂O₃ catalyst such as a large number of active centers and a very low activation energy (presented in Table 1) also contribute to high benzene decomposition efficiency. Moreover, Pt/γ-Al₂O₃ can activate more benzene molecules and adsorb more O₂ molecules, thus promoting O formation as well as benzene oxidation. The increased number of the active centers on Pt/γ-Al₂O₃ under preheated condition (comparing to that under nonpreheated condition) indicates that raising the gas temperature can improve the synergic effect (between the catalyst and the reactor).

Conclusion

In the attempt of improving the performance of NTP reactor (for VOCs decomposition), the study has discovered that using Pt/γ-Al₂O₃ catalyst in the NTP reactor can increase benzene decomposition efficiency by 20% when compared with that using γ-Al₂O₃, and preheating can also help to improve benzene decomposition efficiency. According to the Arrhenius plot, the activation energy of the Pt/γ-Al₂O₃–catalyzed reaction is 3.01 kJ/mol, whereas that of the γ-Al₂O₃–catalyzed reaction, which shows a 57% decrease, is only 4.73 kJ/mol. The number of available active centers on γ-Al₂O₃ is significantly lower than that on Pt/γ-Al₂O₃. Moreover, in the Pt/γ-Al₂O₃–catalyzed reaction, the amount of CO released is only half of that in the γ-Al₂O₃–catalyzed reaction, whereas CO₂ concentration and selectivity remain higher than those in the γ-Al₂O₃–catalyzed reaction. It has also been found that carbon balance can be improved and NOx formation suppressed by the functioning of Pt/γ-Al₂O₃. These results can be attributed to Pt/γ-Al₂O₃'s powerful catalytic effects such as preventing N₂ excitation, promoting O₂ excitation, and increasing benzene decomposition in the NTP reactor.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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Effect of Pt/γ-Al 2 O 3 Catalyst on Nonthermal Plasma Decomposition of Benzene and Byproducts

Abstract

Abstract

Introduction

Experimental

Catalyst preparation

Experimental setup

Measurement technique

Electrical measurement

Gas analysis

Temperature measurement

Results and Discussion

Benzene conversion

CO and CO2 concentrations

CO and CO2 selectivity and the carbon balance

NO and NO2 formation

Conclusion

Footnotes

Author Disclosure Statement

References

CO and CO₂ concentrations

CO and CO₂ selectivity and the carbon balance

NO and NO₂ formation