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Methanol Synthesis over Cu and Cu-oxide-containing ZnO/Al2O3 using Dielectric Barrier Discharge Antonius Indarto, Jae W. Choi, Hwaung Lee, and Hyung K. Song, Member, IEEE

Abstract—Initially developed for more than 20 years ago, the copper over zinc and aluminum oxide (ZnO/Al2O3) catalyst has been used for low pressure methanol synthesis. Recently, the Cu/ZnO/Al2O3 showed to be active in a dielectric barrier discharge (low-temperature plasma). In this present study, the investigation on the copper as the active site of the catalyst was discussed on the basis of experimental result and its characterization analysis. The catalyst was attempted to aid the reaction performance of partial oxidation of methane in order to produce methanol. All CZA-based catalysts were successfully increased the methanol selectivity and the Cu-oxide performed better than metallic copper catalyst. Index Terms—Catalytic plasma, methanol, dielectric barrier discharge

I. INTRODUCTION

T

HE use of Cu/ZnO/Al2O3 (CZA) based catalyst for methanol synthesis has been known for long period. In the beginning of 19th century, ICI developed the catalyst which able to convert synthesis gas (mixture of CO, CO2, and H2) to methanol with yields c.a. 99% [1]. Even though the process of methanol is already well-established, still, there are many plenty rooms for the process development, e.g. methanol from direct methane conversion, or the application of new process system, e.g. plasma-aided reaction. The combination of plasma with catalyst has been intensively investigated to enable the production methanol from direct methane conversion. Recent result of our investigation indicates that the CZA showed to be a very promising catalyst for this type of chemical process using dielectric barrier discharge (DBD) [2]. Moreover, it proposed that the Cu and Zn synergy was also occurred in the low-temperature plasma environment. In the thermal process, Manuscript received October 26, 2007. This work was supported by the Global R&D Program of the Korea Foundation for International Cooperation of Science and Technology (KICOS). A. Indarto was with Korea Institute of Science and Technology, Seoul Korea. He is now with the Department of Organic Chemistry, Università di Torino, Turin, 10145 Italy (corresponding author, phone: 349-351-3904; e-mail: [email protected]). J. W. Choi, H. Lee, and H. K. Song are with the Environmental Process Division, Korea Institute of Science and Technology, Seoul, Korea.

Cu2+ or Cu+ have been suspected as the main active center while ZnO could help to disperse the copper metal distribution on the surface of the catalyst [3]. In this research, we would like to focus the study on the effect of the two different Cu forms, i.e. metallic copper (Cu0) and Cu-oxide forms (Cu+ and Cu2+), employed for the methanol synthesis from methane and oxygen in the low-temperature plasma environment.

II. EXPERIMENTAL SETUP A. Plasma Reactor and Instrumental Devices The research was conducted using a dielectric barrier discharge. The plasma reactor was a quartz cylindrical tube with inside diameter of 2 cm and the active length of the plasma zone was 20 cm. The catalyst was packed at the end of the plasma zone in order to avoid the destruction of catalytic reaction products by plasma. In all experiments, we used only 0.5 gram of catalyst and the supplied powers to the reactor were between 60 and 80 W. The ratio of methane to oxygen was maintained at 5:1 by volume basis. To increase the temperature of the catalyst-zone as well as to activate the catalyst, an additional heater was installed. By using the heater, the catalyst-pack temperature reached 80 to 120oC when the plasma was turned off. The product line was connected to the gas chromatograph (GC), covered by heating bend to avoid the liquid products’ condensation. The GC was able to detect eight different hydrocarbons and the column material used for analysis was chosen to be resistance in the present of water and acid compounds. B. Catalyst Preparation and Characterization The catalyst was prepared following the co-precipitation method [2]. To obtain the Cu-oxide form, the catalyst was calcined in mild-temperature below 200oC for 2 hr in atmospheric open-air condition (A-type catalyst). Then, some portions of calcined catalyst were reduced by flowing 2 ml/min of H2 and He mixture (1:2) in DBD-plasma (B-type catalyst). This method was used to avoid the change of catalyst structure and morphology due to high-temperature effect, e.g. sintering, when the catalyst was reduced using thermal method. DBD

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700

CuO Cu2O

Intensity / a.u.

600

ZnO Al2O3

Ga/A-type

500 400

Cr/A-type

300 200

A-type

100 0

0

10

20

30

40

50

60

2theta / degree Cu(111)

700

70 Cu

Intensity / a.u.

600 Cu(200)

500

80

90

ZnO Al2O 3

Cr/B-type

400 300 200

Ga/B-type

100 0 0

10

20

30

40

50

2theta / degree

60

70

80

90

feature of the surface morphology shown by SEM is rather enough to present the similarity of those two types of catalyst.

III. RESULTS AND DISCUSSION Fig. 3 shows the performance of the catalyst in the partial oxidation of methane reaction to produce methanol. Compared to the non-catalytic plasma reaction, the addition of CZA-based catalysts gave almost double increment of methanol selectivity (Fig. 3). Our previous investigation resulted that non-catalytic plasma reaction of partial methane oxidation produced synthesis gas (mixture of CO, CO2, and H2) and water as the most dominant products [2]. This means that non-catalytic plasma process acted similar to steam methane reforming (SMR) which known as the most common method of producing syngas in the industrial synthesis of methanol. Then, in the present of CZA catalyst, syngas could be converted further into methanol. The durability of A and B -type catalyst shows very good performance. For almost 3 hr operation, the production of methanol was stable. We did not find any significant morphology changes of the catalyst after it was used for the plasma experiment. Methanol selectivity / %

treatment will allow us to maintain the catalyst morphology because the treatment occurred at temperature of as low as room temperature, c.a. 25oC. The final catalysts have the surface area in the range between 92 and 108 m2/g while the pore volume was around 0.25 – 0.35 cm3/g. The XRD patterns of the different catalysts are shown in Fig. 1 which indicates the two different forms of Cu in the catalyst. In the calcined-catalyst (A-type), the Cu presented in two oxide forms (CuO and Cu2O). In contrast, the Cu-oxide was drastically transformed into metallic copper (Cu0) after plasma reduction shown by dominant peaks of 2θ = 43o, 50o. We failed to obtain the peak for Ga and Cr as the concentration of those metals was relatively small. The purpose of Ga and Cr addition is to enhance the catalyst performance as those metals were reported to give a positive effect on the methanol production [4].

Weight loss / %

(a)

(b)

98 96 94

CuO/ZnO/Al2 O3

92 90

Cu/ZnO/Al2 O3 0

100

200

300

400

500

600

700

20 15 A-type

10

800

Temp. / oC

Fig. 2. (a) The TGA analysis and (b) the SEM images of the A-type (above) and B-type catalyst (below). The bar is equal to 600 nm.

Cr/A-type

+ catalyst

5

Ga/A-type

0 90

140

190

240

290

340

390

Sampling time / min Methanol selectivity / %

Fig. 1. The X-Ray Diffraction (XRD) spectra: (above) The Cu-oxide catalyst (A-type) and (below) the Cu 0 catalyst (B-type)

100

25

40

The catalyst characterization was done by thermal gravimetric analysis (shown in Fig. 2a). The analysis was conducted by flowing the mixture of He and H2 (9:1) to measure the weight changes while the temperature elevated. The investigation of the catalyst morphology by SEM analysis showed similar surface texture of both catalysts; formed a small cubic-like structure. We did not analyze thoroughly the crystallinity structure of the catalyst; however the similar

2

25 20 15 10

B-type Cr/B-type

+ catalyst

5

Ga/B-type

0 40

90

140

190

240

290

340

390

Sampling time / min

Fig. 3. Methanol selectivity of catalytic plasma process using A-type catalyst (above) and B-type catalyst (below).

The interesting result was found as the A-type catalyst produced more methanol than the B-type catalyst at the same methane conversion. The conversion of methane was mostly affected by supplied power to the reactor and the presence of catalyst did not give a significant influence [2]. The selectivity of methanol in A-type catalyst was in the region between 19 and 23% whereas B-type catalyst was only 15 to 18%. It shows that the difference of Cu forms in the catalyst gives a significant effect on the reaction of methanol synthesis. This result also clearly indicated that the Cu ion forms (Cu+ or Cu2+) could be

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < more active species than metallic copper (Cu0). Although this idea is a controversy and still debatable, Günter et al. showed evidence that Cu-oxide forms gave slightly higher methanol production by Cu-phase cycle method at 523K [5]. This seems to have occurred in the low thermal plasma process, such as DBD in this experiment. Recently, Shishido and co-workers proposed that Cu+ could be the active site for water-shift gas reaction [6]. It was reported that the migration of ZnO on top of Cu was followed by the formation of a (partly) oxidized Cu in a Cu+/ZnO surface with oxygen vacancies worked as the active sites for methanol synthesis. An old work by Lee et al. found that Cu+ and Cu2+ could exist in the CuO lattice with Cu+ concentration was higher than Cu2+ [7].

IV. CONCLUSIONS The present study demonstrated that CZA-based catalyst was also active for methanol synthesis in the plasma environment. In order to distinguish the active form of copper, the Cu-oxide and metallic Cu in the catalyst were used as the experimental variable. The result showed that Cu-oxide resulted higher methanol selectivity than that of metallic copper (Cu0), means that Cu+ and Cu2+ could be the active forms of the catalyst in the low-temperature plasma process.

ACKNOWLEDGMENT A. Indarto thanks the International R&D Academy of the Korea Institute of Science and Technology for the study supports. REFERENCES [1] [2]

[3]

[4] [5]

[6]

[7]

J. M. Thomas, W. J. Thomas, in Principles and Practice of Heterogeneous Catalysis, VCH Publishers, Weinheim, 1996, pp. 515–521. A. Indarto, D. R. Yang, J. Palgunadi, J. W. Choi, H. Lee, H. K. Song, “Partial oxidation of methane with Cu-Zn-Al catalyst in a dielectric barrier discharge,” Chem. Eng. Process., doi:10.1016/j.cep.2006.12.015, 2007. H. Y. Chen, S. P. Lau, L. Chen, J. Lin, C. H. A. Huan, K. L. Tan, J. S. Pan, “Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis,” Appl. Surf. Sci., vol. 152, pp. 193-199, 1999. T. Matsuhisa, in Catalysis, Athenaemum Press Ltd, Gateshead, 1996, Vol. 12, Chap. 1, pp. 6. M. M. Günter, T. Ressler, R. E. Jentoft, B. Bems, “Redox behavior of copper oxide/zinc oxide catalysts in the steam reforming of methanol studied in situ X-ray diffraction and absorption spectroscopy,” J. Catal., vol. 203, pp. 133-149, 2001; M. M. Günter, T. Ressler, B. Bems, C. Büscher, T. Genger, O. Hinrichsen, M. Muhler, R. Schlögl, “Implication of the microstructure of binary Cu/ZnO Catalysts for their catalytic activity in methanol synthesis,” Catal. Lett., vol. 71, pp. 37-44, 2001. T. Shishido, M. Yamamoto, D. Li, Y. Tian, H. Morioka, M. Honda, T. Sano, K. Takehira, “Water-gas shift reaction over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation,” Appl. Catal. A: Gen., vol. 303, pp. 62-71, 2006; T. Shishido, M. Yamamoto, I. Atake, D. Li,Y. Tian, H. Morioka, M. Honda, T. Sano, K. Takehira, “Cu/Zn-based catalysts improved by adding magnesium for water-gas shift reaction,” J. Mol. Catal. A: Chem., vol. 253, pp. 270-278, 2006. H. G. Lee, C. S. Han, M. S. Cho, K. S. Rhee, H. Chon, “Temperature-programmed reduction of copper oxide supported on γ-Al2O3 and SiO2,” J. Korea. Chem. Soc., vol. 30, pp. 415, 1986.

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Antonius Indarto was born in Malang, Indonesia, in 1980. He received the B.S. degree in chemical engineering in 2002 from Institut Teknologi Bandung, and M.S. degree in environmental process from the Korea Institute of Science and Technology, Seoul, Korea in 2006. He also received another M.S. degree from the Asian Institute of Technology, Thailand in 2005. His research focuses on the methanol production by non thermal plasma-chemistry process as well as developing high quality catalysts for plasma processes.