1. Introduction
During collection of food wastes in container, significant amount of odors or volatile organic compounds can be generated. Among various odor compounds from food wastes, acetaldehyde is the most difficult to decompose. Therefore, suitable methods to reduce acetaldehyde vapor should be developed. Until now, many methods to reduce volatile organic compounds including acetaldehyde vapor have been suggested [1-3]. For example, adsorption can be applied to capture acetaldehyde but periodic replacement of used adsorbent may be needed. Also, catalytic combustion can be applied to decompose acetaldehyde but this process requires high temperature above 300 ℃[4]. Because food wastes can be stored in a container more than two days, and collection of food wastes is collected at ambient temperature, a suitable odor removal process operated under high humidity and ambient temperature should be considered.
Recently many researchers propose a hybrid process comprised of catalyst and non-thermal plasma. The catalyst is introduced either in the discharge zone or downstream from the discharge zone (post plas-ma-catalysis, PPC)[5-7]. The PPC system can form reactive oxygen species using O3 emitted from the NTP system, which can enhance oxidation of VOCs. Therefore, the choice of the desirable catalyst is a crucial factor in the efficient decomposition of O3. Among the reported catalysts, manganese loaded catalyst is one of the best catalysts for ozone oxidation of volatile organic compounds. For example, recently, Zheng et al. have used a vertically-oriented graphenes supported manganese catalysts to remove toluene decomposition in PPC system[8]. In addition, Wang et al. have compared Mn, Ag and Ce modified HZSM-5 catalysts for plasma-catalytic oxidation of adsorbed toluene and found that Ag-Mn/HZSM-5 catalyst showed better catalytic ability in both complete oxidation of the adsorbed toluene and suppression of the byproducts[9]. Zhang et al. reported the catalytic performance of series of 10% MnOx/HZSM-5 catalysts with different SiO2/Al2O3 ratios toward toluene decomposition[10]. These papers were performed in dry conditions, comparing the activity of the catalyst for VOCs oxidation and did not consider the effect on humidity levels. Therefore, it is needed to find a suitable catalyst which shows good activity for VOCs oxidation under high humidity.
As a catalyst support, zeolite has a wide range of applications in industry and in the laboratory with large specific surface area, long catalyst life and low cost[11-13]. Furthermore, zeolites are an important class of aluminosilicates and thus existing of wide range of SiO2/Al2O3 ratios. High-silica zeolites are known to be hydrophobic, while low SiO2/Al2O3 ratios is essentially hydrophilic due to the presence of hydroxyl groups on its surface. The hydrophilic zeolites can cause toxicity by occupying active sites on the surface, where the water molecules can cover the catalyst surface. The high SiO2/Al2O3 molar ratio of zeolites provide more hydrophobicity of catalyst surface, and thus prevent adsorption of water on the catalyst. These zeolites are expected to show high activity for acetaldehyde degradation in the humid condition. In previous our study[14], we performed acetaldehyde removal using hybrid system comprised of plasma and Mn/Y catalysts. Among different SiO2/Al2O3 ratio Y supports, Mn/Y (SiO2/Al2O3 = 80) having highest SiO2/Al2O3 ratio showed highest acetaldehyde removal activity. However, other representative microporous zeolites has not been applied to the removal of acetaldehyde using a hybrid system of plasma and catalysts under high humidity.
In this study, the effect of different zeolite structure (Beta and ZSM-5) and SiO2/Al2O3 ratio on the removal of acetaldehyde under high humidity was investigated.
2. Experimental
2.1. Catalyst preparation
Commercial ZSM-5 zeolites with different SiO2/Al2O3 molar ratios (SiO2/Al2O3 = 30 and 280) and Beta zeolites with different SiO2/Al2O3 molar ratios (SiO2/Al2O3 = 25 and 300) were supplied by the Zeolyst International. The detailed preparation method was described in our previous study[15].
2.2. Catalyst characterization
Brunauer-Emmett-Teller (BET) measurements were performed to determine the specific surface area of each catalyst. Temperature programmed reduction (TPR) was performed using BEL-CAT under a gas mixture (10 vol% H2 in Ar) and the temperature of the catalysts was increased up to 600 ℃ at a heating rate of 10 ℃ min-1.
2.3. Experimental setup and activity tests
Figure 1 shows a schematic diagram of catalytic non-thermal plasma system for degradation of acetaldehyde degradation. It included an acetaldehyde generator, plasma reactor, water vapor section and catalyst bed. The detailed experimental procedure was described in literature[15,16].
3. Results and Discussion
3.1. Catalyst characterization
The BET surface area of parent and Mn-loaded zeolites are listed in Table 1. The order of surface area for the zeolites regardless of the SiO2/Al2O3 molar ratios are as follows: BETA (SiO2/Al2O3 = 25) > BETA (SiO2/Al2O3 = 300) > ZSM-5 (SiO2/Al2O3 = 30) > ZSM-5 (SiO2/Al2O3 = 280). The decrease in surface area was observed in the 5 wt.% Mn-loaded zeolites. This can be related to partial blocking of some zeolite pores with manganese oxide formed on the surface of zeolite.
3.2. Catalyst activity
The effects of the acetaldehyde degradation efficiency were investigated under humid air conditions after a 30 min plasma treatment with Mn-loaded Beta zeolites and Mn-loaded ZSM-5 zeolites, which have different SiO2/Al2O3 molar ratios (Figure 2). Mn/Beta(300) and Mn/ZSM-5(280) removed approximately 87% and 66% of acetaldehyde at 30 min, whereas Mn/Beta(25) and Mn/ZSM-5(30) removed 55% and 50% of the acetaldehyde after the same period of time, respectively. In addition, Mn/Beta(300) and Mn/ZSM-5(280) removed the ozone completely after 30 min plasma treatment. For all zeolite, the samples exposed to more hydrophobic zeolite showed higher degradation efficiency than those exposed to lower SiO2/Al2O3 ratios (Figure 2). Accordingly, the acetaldehyde removal efficiency increased with increasing hydrophobicity, which is considered a major factor for the efficient decomposition of organic material under humid conditions. Also, Mn/Beta(300) showed higher acetaldehyde removal activity than Mn/Y(80) which showed good acetaldehyde removal activity in our previous study[14].
The catalytic properties according to the type of support can be explained based on the reduction characteristics analyzed by H2-TPR. Figure 3 shows the reduction behaviors of Mn/Y(80), Mn/Beta(300), and Mn/ZSM-5(280) catalyst. Mn/Y(80) showed broad reduction peaks centered at 335.1 ℃ and 498.4 ℃. Because of the strong interaction of Y with the large pores and MnOx, they could not be reduced easily and broad peaks were formed in the high temperature region[14]. Mn/Beta(300) showed a single reduction peak at 342.7 ℃. Two reduction peaks were observed at 370.3 ℃ and 523.6 ℃ in Mn/ZSM-5(280). A low-temperature peak below 400 ℃ was attributed to the reduction of MnO2 and Mn2O3 to Mn3O4[17]. These low-temperature hydrogen consumption peaks revealed highly dispersed and easily reducible MnOx species[18]. On the other hand, the peak at approximately 500 ℃ in Mn/Y(80) and Mn/ZSM-5 catalysts indicates that Mn3O4 was reduced to MnO. Kim et al. examined the removal of VOCs over manganese oxide catalysts and reported that the catalytic activity was related to the oxygen mobility. The reduction temperature shifting to a higher temperature reduces the lattice oxygen mobility on the catalyst. A higher oxygen mobility means higher catalyst activity[19]. This suggests that Mn/Beta(300), which had a single reduction peak at low temperatures compared to Mn/Y(80) and Mn/ZSM-5(280), showed high activity in the removal of acetaldehyde.
A long term stability test was carried out with 5 wt.% Mn/Beta (SiO2/Al2O3 = 300), which showed high activity for the acetaldehyde removal efficiency (Figure 4). The plasma system was repeatedly turned on for 20 min and turned off for 10 min during the reaction. This is because the amount of ozone generated increased as the plasma apparatus was operated. Figure 8 shows the removal efficiency of acetaldehyde and ozone for 110 h. The acetaldehyde removal efficiency was 100% and 67% at the highest and lowest, respectively, showing an average efficiency of 83% for 110 h. The acetaldehyde degradation efficiency was not constant because acetaldehyde removal was affected by the ozone generated in the plasma. The average concentration of ozone was 60 ppm in the plasma system in on mode and 35 ppm in the system in off mode. The ozone generated was removed completely by the catalytic reaction.
4. Conclusion
The acetaldehyde removal by the hybrid plasma-catalyst system was performed. The catalysts were 5 wt.% Mn loaded zeolites (Beta and ZSM-5) with different SiO2/Al2O3 ratios. The acetaldehyde degradation efficiency was higher with higher SiO2/Al2O3 ratio under high humidity conditions owing to higher hydrophobicity of the zeolite surface. When the relative humidity was 60%, the acetaldehyde removal efficiency was the highest at 87% in the reaction over the 5 wt.% Mn/Beta (SiO2/Al2O3 = 300). This may be due to its highest hydrophobicity and reducibility. Also, a long term stability test showed that removal activity of 5 wt.% Mn/Beta (SiO2/Al2O3 = 300) was relatively maintained well.
