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Partial oxidation of methane with Cu–Zn–Al catalyst in a dielectric barrier discharge Antonius Indarto a,b , Dae Ryook Yang b , Jelliarko Palgunadi c , Jae-Wook Choi a , Hwaung Lee a , Hyung Keun Song a,∗ a

Plasma-Catalyst Chemical Process Lab., Korea Institute of Science and Technology, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Republic of Korea c Hydrogen Energy Research Center, Korea Institute of Science and Technology, Republic of Korea

Received 20 November 2006; received in revised form 23 December 2006; accepted 24 December 2006

Abstract A series of methanol synthesis catalyst containing Cu–Zn–Al (CZA) were prepared by co-precipitation method and applied for partial oxidation of methane into methanol using dielectric barrier discharge (DBD). The methanol synthesis process was occurred at ambient temperature and atmospheric pressure. In our experiment, CZA showed a high catalytic activity to increase the production of methanol. The methanol selectivity of CZA-assisted plasma process was twice higher than that of non-catalytic plasma process. The addition of other metals on CZA catalyst also produced a significant effect on the methanol production and it was found that yttrium could the best addition metal compared to Pt, Fe, and Ni. Instead of methanol, the reaction products of plasma reactions were dominated by H2 , CO, CO2 , C2 and water. The optimum methanol selectivity reached 27% when 3% yttrium metal was doped over CZA. © 2007 Published by Elsevier B.V. Keywords: Methane oxidation; Methanol synthesis; Dielectric barrier discharge; Cu–Zn–Al; Heterogeneous catalyst

1. Introduction The catalytic conversion of methane into methanol is one of the major challenges for chemists. Methane, as the major part of natural gas, is the cheapest and promising source for direct conversion chemical process to produce methanol. Nowadays, the need for methanol increases due to its importance as an intermediate material for industries. Well-established methanol production process is occurred via synthesis gas (CO, CO2 , and H2 ) using Cu–Zn–Al2 O3 (CZA) catalyst. However, this process route can be changed drastically when the effective method to oxidize methane to methanol is found. Catalytic oxidation of methane at low temperatures is economically interesting, but, very difficult to achieve as a result of the high stability of C–H bonds.

∗

Corresponding author at: Plasma-Catalyst Chemical Process Lab., Korea Institute of Science and Technology, Republic of Korea. Tel.: +82 2 958 5241. E-mail address: [email protected] (H.K. Song).

The investigations of methane oxidation to methanol in non-thermal plasmas have been made worldwide by corona discharges, spark discharges, gliding arc, or dielectric barrier discharges (DBD) at atmospheric pressure and ambient temperature. Some researchers have tried to add auxiliary gases in the process, such as hydrogen [1], air [1–3], oxygen [2–5], noble gas [3,6]. Some others used the different types of plasma discharges in order to produce different types of products [7,8]. However, the production of methanol was still low and not economically meets the market requirement. In the present research, comprehensive study on the performance of plasma process combined with Cu–Zn–Al (CZA)-based catalyst for methane partial oxidation has been undertaken. Dielectric barrier discharge was chosen as the plasma media due to mild characteristics which probably offers better process for methanol synthesis. In methanol synthesis via synthesis gas, CZA has been claimed as the active component for methanol production [9,10]. Although the catalytic reaction mechanism is still not clear, the existence of CZA in the process was proved to be better material catalyst than other metals. The synergy of Cu–Zn–Al was able to absorb the molecules

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and conducted a series of surface reactions on the catalyst surface [11–13]. It is well known that for the hydrogenation of CO to methanol on Cu–Zn catalysts, Cu acts as the active center for the CO adsorption and activation. Although there are many studies concerning the role of Zn, the ideas that have been proposed are widely diversified, ranging from hydrogen activation center, structural-electronic promoter, to bifunctional Cu–Zn interfacial active center [11]. The similar mechanisms are hopefully occurred in the plasma environment and due to abundant amounts of radicals and ions and the production of methanol can be higher than that of conventional thermal process. In accordance with the above discussion, this study investigates the influence of CZA on direct partial oxidation of methane to produce methanol. CZA and metal-supported CZA catalysts were prepared by co-precipitation method and the efficiencies were measured by calculating the production of methanol. 2. Experimental set-up Fig. 1 shows a schematic diagram of the experimental setup. Methane and oxygen were introduced to the reactor at room temperature and atmospheric pressure. The products were analyzed by a gas chromatography (GC). Details of each parts of the system are described in the next sections.

voltage and current of 10 kV and 100 mA, respectively. A digital power meter (Metex, model M-3860M) was inserted to the electricity line of the ac power supply in order to measure the total supplied power to the reactor. 2.3. Materials All experiments were carried out by introducing methane (CH4 , purity >99.99%) and oxygen (O2 , purity >99.9%) at fixed methane to oxygen ratio of 4:1 (volume basis). The input gases were controlled by calibrated mass flow controllers (Milipore, model FC-280SAV). Analysis of the products was done by a gas chromatography (YoungLin, model M600D, Column: 10 ft Hayesep D 80/100 + 10 ft Carbowax 10% 80/100) with a thermal conductivity detector (TCD) for measuring H2 and CH4 and a flame ionized detector (FID) for measuring CH4 , methanol, and higher hydrocarbons. The performance evaluation of the process was calculated on the basis of products selectivity and methane conversion, formulated as selectivity of H2 (%) =

moles of H2 produced × 100 2 × moles of CH4 converted

(1)

selectivity of Cx Hy (%) 2.1. Reactor

=

The reactor is a cylindrical pyrex tube (i.d. of 7.5 mm) with two parallel-straight wires (0.2 mm diameter, stainless steel) as the inner metal electrode and silver film coated at the outer side of tube as the outer electrode. The effective volume and length of the reactor were 8.8 mL and 200 mm, respectively. In order to maintain the similarity of the reactor configuration, e.g. electrodes gap distance, the reactor capacitance was checked first by an RCL meter (Fluke PM6304) before and after experiments. 2.2. Power supply

x × moles of Cx Hy produced × 100 moles of CH4 converted

(2)

selectivity of CH3 OH (%) =

moles of CH3 OH produced × 100 moles of CH4 converted

(3)

selectivity of COx (%) =

moles of COx produced × 100 moles of CH4 converted

(4)

conversion of CH4 (%)

Plasma was generated by an alternating current (ac) power supply (Auto electric, model A1831) which has a maximum

=

moles of CH4 converted × 100 moles of initial CH4

(5)

Fig. 1. Experimental set-up.

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2.4. Preparation and characterization of catalyst The precursors of methanol synthesis catalysts, denoted as CZA, with the % mole ratios of Cu:Zn:Al = 6:2.5:1.5 were prepared by simultaneous co-precipitation method from their nitrates solution at a pH about 7.0. A buffer solution with a pH 8.50 containing a mixture of NaHCO3 and Na2 CO3 was used as the precipitation agents throughout the works. In a typical catalyst preparation, metal nitrates of Cu(NO3 )2 ·3H2 O, Zn(NO3 )2 ·6H2 O, and Al(NO3 )·9H2 O were dissolved in H2 O to get 250 mL of metal nitrate solution with total molar concentration of 0.3 M. In a 1000 mL beaker glass initially filled with 200 mL of distilled water, a solution of metal nitrates and a solution of precipitation agent were dropt simultaneously for 1.5 h under vigorous agitation at room temperature while the pH of the slurry was maintained in a range from 6.90 to 7.10. Following the completion of co-precipitation process, the slurry was stirred further, which is so-called aging, for 2 h at room temperature. The formed precipitate was filtered and washed with distilled water under stirring for 12 h. This procedure was repeated six times which were found to be sufficient for the slurry reached pH about 7.0. The cake was dried at room temperature for 24 h and then oven dried at 110 C for 24 h. The catalyst precursor was calcined under air condition at 350 ◦ C for 2 h. In order to compare the performance of prepared catalyst with commercial CZA catalysts, the CZA produced by ICI (ICI Katalco 51-8; noted as CZAi) and BASF (BASF K3-110; noted as CZAb) was ordered. Moreover, using the same method, some other metals, e.g. Pt, Ni, and Fe, were doped to CZA by incipient wetness to increase the activity of pure CZA.

2.4.1. XRD patterns Crystal structures were identified by powder X-ray diffraction spectroscopy on a Shimadzu 6000 X-ray diffractometer system (Cu K␣ radiation, 40 kV, 30 mA). Fig. 2a presents the XRD pattern changes of the prepared CZA at various conditions. The CZA structure was greatly changed before and after calcinations. After oven drying, the catalyst shows clearly a hydorotalcite-like (HTlc) and malachite phases. This result is consistent with the previous report which suggested that HTlc phase coexists with a malachite phase when the atomic ration of Cu/(Cu + Zn) is in the range from 0.5 to 0.9. CuO also appears due to the high Cu/Zn ratio. The presence of CuO along with HTlc occurs due to the Jahn–Taller effect. The existence of those phases on the fresh uncalcined CZA catalyst will guaranty its activity of the after calcinations treatment. Those forms were also appeared at all series CZA catalyst with addition of Pt, Ni, Fe, and Y as shown in (b). The catalyst structure was greatly changed after calcinations treatment which mostly produced CuO and ZnO as the most dominant phases. However, in (a), the structure of catalyst before and after 6 h plasma process was not so much different. The results also showed the stable production of methanol during that operation period. It concludes that the deactivation of catalyst was not occurred during 6 h operation due to plasma reactions.

Fig. 2. The XRD patterns of the catalyst: (a) the XRD phases change of CZA at various catalyst treatments; (b) the uncalcined XRD pattern of CZA as function of various types of metal loading.

2.4.2. SEM, EDS, and Cu surface area Dispersed copper, incorporated with ZnO structure, is believed as the active side on the catalyst [11]. In this experiment, the copper surface areas were determined by N2 O titration using pulse flow experiment following to the previous method [11]. In a typical experiment about 50 mg of sample was loaded and reduced using 5 vol.% H2 in Ar at 250 ◦ C for 1 h and purged the reactor in a He stream at 250 ◦ C for 30 min and cooled down to 90 ◦ C at which the N2 O titrations were carried out. A surface copper density of 1.47 × 1019 atoms/m2 assuming Cu:O = 2:1 was used in this investigation. Surface analysis was done using Hitachi FE-SEM S-400 microscope at an accelerating voltage 0.5–30 kV. Fig. 3 shows the surface images of the CZA catalyst. Fig. 3 also present that there is no significant different of the catalyst structure before plasma reaction (b) and after plasma reaction (c). The elemental compositions of the samples were determined by an energy dispersive spectrometer (EDS) analysis using the same instrument. The typical result of EDS analysis of the catalyst is shown in (d).

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Fig. 3. (a–c) SEM surface picture of CZA: (a and b) after calcinations; (c) after 6 h plasma reaction; (d) EDS elemental analysis.

3. Results and discussion 3.1. Plasma stability and products distribution The direct methane oxidation using dielectric barrier discharge (DBD) was conducted at room temperature and atmospheric pressure. The total flow rate and the volumetric ratio of methane to oxygen were maintained constant at 30 ml/min and 4:1, respectively. The supplied power to the reactor was also fixed at 50 W. Our previous study showed that in this region, the selectivity of methanol, the most profitable products, reached the highest value for non-catalytic partial oxidation of methane by dielectric barrier discharge [14]. The products were analyzed at three different sampling times to ensure the reproducibility data and each point of the experimental variables was repeated twice. Fig. 4 shows the comparison of the catalyst performance between CZA made by co-precipitation method with the commercialized one. By addition of CZA catalyst, the selectivity of methanol was sharply increased, almost two times higher compared to non-catalytic plasma reaction. In non-catalyst reaction, the most products were dominated by CO, CO2 , and H2 . The selectivity of methanol was less than 11%. From the result of non-catalytic process, it can be said that the non-catalytic

plasma will convert methane and O2 into synthesis gas (CO, CO2 , and H2 ). C2 H6 was found as the single major product of higher hydrocarbon (Table 1). Based on our previous kinetics calculation [14], the methanol from methane oxygen plasma reaction can be possible produced

Fig. 4. Methanol increment selectivity and its stability as comparison between prepared CZA catalyst and commercialized CZA catalysts.

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Table 1 Products distribution as a function of various CZA catalyst Catalyst

Blank CZA CZAi CZAb

Conversion CH4 (%) 28.1 25.3 31.0 29.8

Selectivity (%) H2

CO

CO2

C2

C3

C4

17.0 10.9 11.2 14.6

45.5 36.8 35.7 34.8

21.5 12.0 10.6 12.3

9.3 5.6 6.7 7.5

1.3 0.0 0.0 0.0

1.7 0.0 0.0 0.0

from two reaction mechanisms: CH3 + OH → CH3 OH

(6)

CH2 + H2 O → CH3 OH

(7)

Insertion reaction of single atom oxygen to methane was rarely occurred because methane is easily dissociated to CH3 and H or C and H2 in the bulk plasma. CH4 → CH3 + H

(8)

CH4 → C + 2H2

(9)

Between above two reactions, the second reaction of methanol synthesis was higher chance to be occurred. By employing CZA catalyst, which is well-known catalyst for converting synthesis gas to methanol, the selectivity of methanol was higher. We can conclude here that the plasma-catalytic process of methane and oxygen is a two-stage process which consists of methane and oxygen conversion to synthesis gas then with catalyst, it will be converted to methanol. Fig. 4 shows that the value difference among the catalysts was relatively small means that the performance of CZA catalyst made by co-precipitation method was satisfying enough to be used for further experiments as the selectivity to produce methanol was similar to the commercialized catalysts. The stability of the catalyst used in long-time process was also good represented by small fluctuations of methanol production. The deactivation of catalyst, e.g. by carbon deposition or coke formation, was not occurred. Our previous kinetic model results that at the ratio of CH4 :O2 = 4:1, the produced solid carbon from CH4 fragmentation will react with oxygen radical to form CO [14]. C + O → CO

(10)

Fig. 3c also proves that there is no significant amount of carbon attached on the surface of the catalyst after plasma reactions. The catalyst should turn into dark or black color when the carbon or coke deposit on the surface of the catalyst. However, our physical observation found the similar catalyst color between before and after 6 h operation. 3.2. Effect of catalyst loading The effects of catalyst loading on the methanol selectivity were shown in Fig. 5 and Table 2. The experimental results represented that the methanol selectivity was increased with the increase of catalyst loading. The methanol selectivity was increase from 24% to 27.4% when the catalyst loading increased

Fig. 5. Methanol selectivity as a function of catalyst loading.

from 0.5 to 1.5 g. However, the catalyst loading was also affecting the conversion of methane. The methane conversion was increased from 28% to 33% when the catalyst loading was changed from 0.5 to 1.5 g. Increasing amount of catalyst in the reactor will increase the methanol selectivity means that more catalytic reactions of methanol production will be occurred on the surface of the catalyst. Although the selectivity increased, the efficiency of the catalyst, defined as a ratio of methanol production per weight of catalyst loading, was decreasing. The selectivity of other products is listed in Table 2. The main products were dominated by CO, CO2 , and H2 . The selectivity of C2 H6 was between 3% and 9% or around 80% of the total C2 hydrocarbons. Similar with previous results, the increment of methanol production was followed by reducing concentration of CO, CO2 and H2 in the products stream. Compared to noncatalytic process, the CO and CO2 decreased around 20–30% while H2 around 30%. 3.3. Effect of power increment In order to increase the methane conversion and probably the yield of methanol, the supplied power to the reactor was increased from 80 to 120 W. The effects of applied power to the products are shown in Fig. 6 and Table 3. The conversion rate Table 2 Products distribution as a function of catalyst loading in the reactor Metal loading (g)

Conversion CH4 (%)

Blank 0.5 1 1.5

28.1 25.3 30.6 33.9

Selectivity (%) H2

CO

CO2

C2

C3

C4

17.0 10.9 13.6 12.0

45.5 36.8 34.8 32.9

21.5 12.0 9.7 6.6

9.3 5.6 3.6 6.9

1.3 0.0 0.0 0.0

1.7 0.0 0.0 0.0

Table 3 Products distribution as a function of supplied power Power (W)

50 80 110 140

Conversion CH4 (%) 28.5 38.0 39.5 45.0

Selectivity (%) H2

CO

CO2

C2

C3

C4

10.9 14.7 18.9 20.5

36.8 39.5 40.0 42.7

12.0 13.3 15.2 18.3

5.6 4.8 6.2 4.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

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Conversion CH4 (%)

Selectivity (%) H2

CO

CO2

C2

C3

C4

Blank CZA 1% Pt/CZA 3% Ni/CZA 3% Fe/CZA 3% Y/CZA

28.1 25.3 24.6 26.0 30.8 25.3

17.0 10.9 15.4 18.9 13.7 10.1

45.5 36.8 35.6 35.0 34.4 33.8

21.5 12.0 9.5 15.7 12.1 9.0

9.3 5.6 3.5 7.5 9.5 5.6

1.3 0.0 0.0 2.2 0.0 0.0

1.7 0.0 0.0 0.0 0.0 0.0

Fig. 6. Effect of power supply on the conversion of methane and methanol selectivity.

of methane increased from 27% at 50 W to 45% at 140 W. It can be deduced trivially that by supplying more energy to the reactor, the conversion of reactant will increased. As conversion of methane and supplied power increased, the methanol selectivity of non-catalytic process tends to decrease. In other words, fragmentation reactions of molecules were preferable than recombination or higher molecule synthesis reactions [15–17]. This idea was supported by the result presented in Table 3 which the concentration of CO, CO2 , and H2 also increased. In non-catalytic plasma reaction, this condition resulted lower production of methanol. The existence of CZA catalyst will help maintaining the production of methanol from synthesis gas produced by fragmentation reactions of methane and oxygen. As shown in figure, although the selectivity of methanol decreases as power increases, the decrement rate of plasma with CZA catalyst is lower than that of non-catalytic plasma process. 3.4. Effect of metal addition on CZA In order to increase the activity of the catalyst, other transition metals, e.g. Pt, Ni, Fe, and Y, have been to the CZA catalyst. The percentage of additional metal loading was only 3%, except for Pt which is only 1%. Meanwhile, the total catalyst weight loaded in the reactor was only 0.5 g. As shown in Fig. 5, except Ni, the addition of metal catalysts on CZA gives a positive effect by increasing the selectivity of methanol around 15–20% compared to the methanol selectivity of CZA catalyst. Among those, yttrium produced the highest value of methanol selectivity that able to convert methane to methanol around 31%. The addition of Y, also Pt and Fe, gave a significant effect on the direction of oxidation reaction. From Table 4, the addition of Pt decreases the selectivity of CO2 and CO (Fig. 7). The probability of methane conversion to methanol would be higher as there is no significant different found on the remaining products, e.g. C2 and higher hydrocarbons. Instead of above reaction mechanism of methanol production, another

Fig. 7. Effect of various types of metal loading of CZA catalyst on the methanol selectivity.

mechanism of methanol by CH3 and OH reaction could be occurred. Pt and Fe are well-known metal catalysts which were able to activate CH4 [18,19] and adsorb OH [20]. Methanol can be formed by surface reaction between methyl methane with OH: CHx → CHx(surf)

(11)

CHx(surf) + OH(surf) + (3 − x)H(surf) → CH3 OH(surf)

(12)

For the case of yttrium addition, the above mechanism is not proved yet. Our calculation of the CO and CO2 and the ratio of CO2 to CO in the product stream for yttrium/CZA catalyst are the lowest value compared to other catalysts. The existence of yttrium inhibits the second-stage oxidation of carbon from CO to CO2 . However, this will create a chance for CO to be converted into methanol. The detail phenomena of partial oxidation of methane with yttrium-stabilized zirconia were observed thoroughly in our previous research [21]. Different phenomena could be found in case of Ni–CZA catalyst. The selectivity of methanol production decreased compared to pure CZA catalyst. In this case, we can think about different response of Ni on the reaction. Ni catalyst is also able to decompose methanol by oxidation reaction [22,23]. 4. Conclusions The well-known methanol catalyst – copper–zinc–alumna – was tested to produce methanol by direct methane conversion in a dielectric barrier discharge. The CZAs catalysts displayed higher activity and better performance to produce methanol than

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non-catalytic plasma process. The existence of CZA increased the selectivity and yields of methanol. Yttrium, also Pt and Fe, was found to be the best addition for the conventional CZA system in the plasma process for methanol synthesis. Acknowledgement The authors thank the Korea Institute of Science and Technology for supports. References [1] D.J. Liu, R. Mallinson, L. Lobban, Nonoxidative methane conversion to acetylene over zeolite in a low temperature plasma, J. Catal. 179 (1998) 326–334. [2] L.M. Zhou, B. Xue, U. Kogelschatz, B. Eliasson, Partial oxidation of methane to methanol with oxygen or air in a nonequilibrium discharge plasma, Plasma Chem. Plasma Process 18 (1998) 375–393. [3] A. Indarto, J.W. Choi, H. Lee, H.K. Song, Effect of additive gases on methane conversion using gliding arc discharge, Energy 31 (2006) 2986–2995. [4] J.M. Cormier, I. Rusu, Syngas production via methane steam reforming with oxygen: plasma reactors versus chemical reactors, J. Phys. D: Appl. Phys. 34 (2001) 2798–2803. [5] T. Nozaki, A. Hattori, K. Okazaki, Partial oxidation of methane using a microscale non-equilibrium plasma reactor, Catal. Today 98 (2004) 607–616. [6] K. Thanyachotpaiboon, S. Chavadej, T.A. Caldwell, R.G. Mallinson, Conversion of methane to higher hydrocarbons in ac nonequilibrium plasmas, AIChE J. 44 (2004) 2252. [7] H.K. Song, H. Lee, J.W. Choi, B.K. Na, Effect of electrical pulse forms on the CO2 reforming of CH4 using atmospheric barrier discharge, Plasma Chem. Plasma Process 24 (2004) 57–72. [8] X.S. Li, A.M. Zhu, K.J. Wang, Y. Xu, Z.M. Song, Methane conversion to C2 hydrocarbons and hydrogen in atmospheric non-thermal plasmas generated by different electric discharge techniques, Catal. Today 98 (2004) 617.

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