YSZ Catalyst in Dielectric Barrier Discharge: Catalyst Activation by Plasma

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JOURNAL OF DIRECT'

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JOURNAL OF RARE EARTHS 24 (2006) 5 13 - 5 18

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Partial Oxidation of Methane with Sol-Gel Fe/Hf/YSZ Catalyst in Dielectric Barrier Discharge : Catalyst Activation by Plasma Antonius Indarto'**, Jae-Wook Choi' , Hwaung Lee', Hyung Keun Song' * , Jelliarko Palgunadi3 (1. Clean Technology Research Center, Korea Institute of Science and Technology, Korea; 2 . Department of Chemical and Biological Engineering , Korea University , Korea ; 3 . Hydrogen Energy Research Center, Korea Institute of Science and Technology, Korea ) Received 11 May 2006; revised 30 June 2006

Abstract: A 1% Fe-30% Hf over yttria-stabilized zirconia catalyst in cornbination with novel plasma-assisted activation techniques for a direct partial oxidation of methane to methanol was tested using dielectric barrier discharge plasma at ambient temperature and atmospheric pressure. However, instead of methanol, the reaction products were dominated by H2, CO, COZ , C2, and H20. A catalytically activated plasma process increased the production of methanol compared with a noncatalytic plasma process. The maximum selectivity of methanol production was achieved using a catalyst that was treated at higher applied power. Key words : methane oxidation ; dielectric barrier discharge ; catalyst ; plasma activation ; rare earths Article ID: 1002 - 0721(2006)05 - 0513 - 05 Document code: A CLC number:

The catalytic conversion of methane to methanol is one of the major challenges faced by chemists. Methane, as the major constituent of natural gas, is the cheapest source of hydrocarbons, and the demand for methanol is expected to increase in the near future. Currently, methanol is produced using synthesis gas (CO, C 0 2 , and H2) . However, this process can be drastically changed if an effective method to oxidize methane to methanol is found. Catalytic homogeneous oxidation of methane at low temperatures is economically interesting but very difficult to achieve because of the high stability of C - H bonds. Much work was reported in the past decade on catalytically oxidative coupling of methane (OCM) , a promising process for the direct conversion of natural gas into Due to the high degree of oxidation reactions in the gaseous phase as well as on the surface of the catalyst,

the maximum yield of methanol obtained thus far p i n g OCR! on any catalyst is less than 25%, whereas methanol selectivity is higher than 5 0 % , which is far below the requirement for making OCM economically attractive ( > 30% 40%)[41. The oxidation of methane to methanol in nonthermal plasmas has been investigated worldwide using corona discharges, spark discharges, gliding arc, and dielectric barrier discharge (DBD) at atmospheric pressure and ambient temperature. As a part of the investigation process, some researchers tried to add auxiliary gases, such as hydrogenr5], airr5-'] , oxygen'6-91, noble gas'7' l o ] . Others used different types of plasma discharges that possibly led to the distribution of different products"' - 12' . This research is a comprehensive study of the performance of the plasma process and 1%Fe-30% Hf

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* Corresponding author ( E-mail : indarto - antonius @yahoo. corn ) Foundation item: Project supported by the National Research Laboratory Program of the Korea Ministry of Science and Technology Biography: Antonius lndarto (1980 - ), Male, Researcher; Interested field: Plasma and simulation process Copyright @ZOOS, by Editorial Committee of Journal of the Chinese Rare Earths Society, Published by Elsevier B. V . All rights reserved.

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JOURNAL OF RARE EARTHS, Vol. 24, No.5 , Oct 2006

over yttria-stabilized zirconia (YSZ) catalyst for partial oxidation of methane. DBD was chosen as the plasma medium due to the mild temperature condition that probably improves methanol synthesis. Among all the metal catalysts, well-dispersed Fe was believed to be the most active species that has the ability to activate methane molecules on the catalyst ~ u r f a c e " ~ - ' The ~~. reaction that occurs on the catalyst surface is the key for the conversion of methane to methanol. YSZ was used as the catalyst support. In methanol synthesis, YSZ is claimed to be an active component. YSZ showed better performance as the catalyst support for methane reforming compared with A1203and Si0,'151. The existence of Hf in the catalyst structure was used to maintain the conductivity of catalyst due to molecule structure. Conductivity is one of the main factors that can affect the oxidation rate of a reaction.

1 Experimental Fig. 1 shows the experimental setup. Methane and oxygen were introduced into the reactor at room temperature and atmospheric pressure. The products were analyzed by gas Chromatography. Details of each part of the system are described in the following sections.

1.1 Reactor The reactor is a cylindrical Pyrex tube (ID of 7.5 mm) with two parallel straight wires ( 0 . 2 mm diameter, stainless steel) as the inner metal electrode and silver film coated on the outer side of the tube as the outer electrode. A high-frequency alternating current (AC) power supply was connected to the electrodes. The effective volume and length of the reactor were 8.8 ml and 200 mm, respectively. To maintain the similarity of the reactor configuration, e. g. , electrodes gap distance, the reactor capacitance was checked by an RCL meter (Fluke PM6304) before and after the experiments. The reactor capacitance was kept

constant in the range of 8 . 5 gap condition.

1.2 Power supply Plasma was generated by AC power supply (Auto electric, model A1831), which has a maximum voltage and frequency of 10 kV and 20 kHz , respectively. A digital power meter (Metex, model M-3860M) was inserted into the electric line of power supply to measure the total power supplied to the reactor. The typical waveform of the voltage and the current used during the experiments is shown in Fig. 2.

1.3 Materials All experiments were carried out by introducing methane (CH4, purity > 99.99% ) and oxygen ( 02, purity > 99.9% ) with fixed methaneoxygen ratio of 4: 1 (volume basis). The input gases were controlled by calibrated mass flow conbllers (Milipore, model FC-280SAV). Analysis of the products was carried out using gas chromatography ( YoungLin , model M600D ) with a thermal conductivity detector (TCD , Column: Hayesep D 80/100) for measuring Hz and CH4, and a flameionized detector (FID) for measuring CH4 and higher hydrocarbons. The evaluation of system performance was done on the basis of product selectivity and methane conversion that are formulated as: moles of Hz produced x 100% Of H2 = 2 x moles of CH4 converted Selectivity of C,H, =

100%

I Blow heater Catalyst

Heating tape

Fig. 1 Schematic diagram of experimental set up

X

moles of CH4 converted

(2)

moles of CH30H produced Selectivity of CH@H = moles of cH, converted

I

Plasma

x x moles of C,H, produced

100%

AC power supply

Reactor

- 9.5 pF in an air-filled

X

(3)

Antonius Indarto et a1 . Partial Onidation Dielectric Bam'er Discharge : Catalyst Activation by Plasma

Selectivity of CO, =

moles of CO, produced x 100% moles of CH, converted

Conversion of CH4 =

moles of CH, conversion x 100% moles of initial CH4

(5) The catalyst was prepared by sol-gel technique using nitrate solution of metal precursors dissolved in ethanol and oxalic acid, which act as the precipitating agents. The nitrate solution consisted of 8% mol fraction of Yz03and 1% mol fraction of Fe in the solid solution of 30% rnol fraction of Hf02 and 61% mol fraction of ZrO,. The gel was formed by quick mixing of nitrate and oxalic acid solution. To ensure equal species distribution on the gel, the stirring was vigorous during the phase transformation from liquid to gel. The gel thus produced was heated to evaporate the solvent and dried at atmospheric air. Calcination was done by increasing the oven temperature at the rate of 2 . 5 T a m i n - ' from room temperature to 650 C ' and kept constant for 8 h . Fig. 3 shows the XRD spectrum of the catalyst, which confirms the stable cubic structure of YSZ. It also shows that the addition of 30% Hf (Fig.3(b)) does not change the crystal structure of "t

f LUU

515

the YSZ catalyst. Using XRD analysis, the content of Fe , which was only 1% by mass weight, was relatively difficult to detect.

1.4 Catalyst treatment Novel catalyst activation approaches by plasma were used as the variables for the investigation. It began with the inert gas-plasma treatment. In our experiment, first, helium was used at applied power of 75 W for 2 h . The flow of helium was controlled at 10 ml*min-'. The next step was plasma-assisted oxidation (PAO) , which was carried out by flowing a gas mixture of oxygen-helium at the ratio of 6 : 4 for 2 h . The total gas flow rate and supplied power were 10 ml-min-' and 50 W, respectively. The third step was plasma-assisted reduction (PAR) , which was done by flowing a gas mixture of hydrogen-helium in the ratio of 6 : 4. Plasma reduction was done at the supplied power of 50 W and total gas flow rate of 10 ml-min-' for 2 h . Fig. 4 shows the waveform of applied plasma during the activation process. To check the effects of each step, the treatments using the catalyst was divided into four cases as follows: ( 1) no-treatment, ( 2 ) helium plasma treatment, ( 3 ) helium + PA0 treatment, and ( 4 ) helium + PA0 + PAR treatment. To check the stability and durability of the catalyst, continuous reaction was canied out for 80 min and the first data point was obtained at the 20th minute.

<

._2 z 3

2

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-25

-15

-5

5

-2

1-200 25

15

2) lie : 02=4 : 6, so w (3) H e : H,-4 : 6, SOW

'Time cycld !-Is -6

Fig.2 Voltage and current profile

-2

-1

0

I

2

1

2

Timc/lOTmin

I 10

20

30

40

50

60

70

80

00

2 "/(")

Fig.3 XRD spectra of catalyst after calcination ( 1 ) Pure YSZ; (2) 30% Hf/YSZ

(3) Hc : H2=4 : 6, 5 0 W -0.08

-2

-1

0 Timeil0-' inin

Fig.4 Profile of voltage ( a ) and current waveform ( b ) of plasma-assisted catalyst activation

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JOURNALOFRAREEARTHS, Vo1.24, No.5, Oct. 2006

electron and other energetic species. Electron, which plays an important role in cold plasma process, for example, DBD[~], will initiate radical reactions : CH4+ e+CH4 (6) CH4*+CH3* + H - + e ( 7) A recombinant reaction of higher hydrocarbons is brought about by the coupling reactions of methyl radical (CH3 ) . However, the presence of hydrogen in the reactor will result in an opposite effect as the hydrogen radical (Ha) has a tendency to become a reducing or a decomposing agent for higher hydrocarbons. CH3 + CH3 *-CzH6 (8) CH3 + CH3 *+CzH4 + H2 (9) CH3. + CzH3*+C3Hsa ( 10) C,H, +H*+C,H,-,* +H2 (11) Instate of H , redecomposition of higher hydrocarbon can be caused by electrons. Electron collision with molecules is faster than molecule collision with ion or radical“81.

Results and Discussion

2.1 Non-catalytic process The direct methane oxidation process using DBD was conducted at ambient temperature and atmospheric pressure. The total gas flow rate and the ratio of methane to oxygen were kept constant at 30 ml*min-’ and 4 : 1 (vol/vol) respectively, and the power supplied to the plasma reactor was varied by 50, 60,and 80 W. Our previous research shows that different power supplies produce different pathways of reactions for methane ~ o n v e r s i o n [ ~.” ~ ” ~ ~ Table 1 shows the effect of power variations on the methane conversion. The conversion gradually increased and reached a maximum of 25.7 % at the supplied power of 80 W. It can be easily concluded that increasing the power will increase the energy that is required to break the chemical bond of methane. In contrast to the thermal process, in the direct methane oxidation process using DBD, the energy supplied in the plasma reactor is used to activate the molecules to a higher energy level or to produce nonoeutral species, such as electrons and ions. These species, especially electrons, have enough capacity to break the chemical bond of methane. As shown in Table 1 , the products were dominated by H , CO , and CO, The selectivity of hydrogen is in the range of 10% 15%. However, the selectivities of CO and COz, which were classified as undesirable products, were around 48% and 22% , respectively. Higher hydrocarbons (C,) were also detected, and ethane was the most predominant undesirable product. The highest selectivity of H, was produced at the highest supplied power. It is believed that the fiagmentation of methane into smaller molecules, for example, H, and C , is more favorable than recombinant reactions to produce higher hydrocarbons, and this will lead to the redecomposition reactions of higher hydrocarbons produced during plasma reactions by

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2.2 Catalyst-assisted plasma process To increase the selectivity of methanol as the desired product, 1% Fe-30% Hf over YSZ catalyst was added to the process and located at the end of the plasma zone. This location was chosen to prevent the decomposition of catalytic products by plasma. The catalyst loaded in the reactor was only 0.5 g . A novel catalyst activation approaches by plasma were used as the variable of investigation. Table 2 shows the effects of the catalyst-assisted plasma treatment on methane oxidation. The power supply was kept constant at 80 W and methane to oxygen ratio was 4:1. It shows that the plasma treatment activates the catalyst. Using DBD, the gas bulk temperature is equal to the room temperature, which is much lower than the temperature required to activate the catalyst. However, the abundance of energetic-unstable species, such as electron, ion, and radical species, can be the source for catalyst activation in the catalyst-assisted plasma treatment process.

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Table 1 Methane conversion and products distribution (data were obtained at the ratio of CH,:O2= 4: 1 and flow rate of 30 ml*mh-’) Yields/%

Sd&ivity/%

Supplied

CHq

power/W

conversion/%

H~

50

20.17 f 3.07

11.28 f 0 . 9

49.73 f 4.12

22.09 f 3.04

19.67 f 2.29

7.33 f 0.76

5 . 4 4 f 0.35

60

23.43 f 2.95

11.05 f 4.18

48.42 4.18

*

22.39 2.29

*

19.13 f 2.29

7.60 f 2.29

5.17 f 0.1

80

25.76 f 3.56

14.19 i 2.38

48.44 f 3.07

21.47 f 2.63

15.33 i2.04

5.14 i1.22

4.49 f 0.57

co

CP

c02

c3

CH30H

Anfonius Inahrto et a1 . Partial Oxidation Dielectric Barrier Discharge : Catalyst Activation by P l a s m

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Table 2 CH,:O, = 4:1, Effects of catalysts on methane conversion and products distribution (Data were obtained at ratio of flow rate of 30 ml-min-' and suDDlied Dower of 80 W) Selectivity/%

Yields/%

Supplied treatment

CHq conversion1%

Hz

co

co2

Blank

25.76k3.56 30.80k4.17 24.25+4.34 34.91k3.33 30.84 k 2.36 35.34 f 2.56

14.19k2.38 28.46k2.18 17.03k2.13 20.09k2.29 23.72k2.29 20.13k1.98

48.44k3.07 39.01 k3.079 45.54k4.57 45.02k1.73 54.41 f 2.25

21.4722.63 30.48*2.74 20.78k2.67 11.84k1.37 32.05 k 1.78

~ _ 15.33k2.04 14.90k1.54 15.32 k 2.14 14.47*1.58 9.45 f 2.29

46.77k1.13

11.98k1.29

15.57k1.96

YSZ No-treatment He He + PA0

He + PA0 + FAR

Without any treatment, the catalyst does not show any significant activity in terms of the products distribution differences compared with the blank or pure plasma process. The production of methanol was around 4 . 5 % . To study the effect of pure YSZ on the plasma process, commercialized YSZ made by TOSOH , was placed in the reactor. It showed that the yield of methanol increased by factor of 1.4. Another interesting finding in the experiments was the oxidation of CO into CO,. The concentration of COz drastically increased from 21% to 30% and was simultaneously followed by a decrease in the concentration of CO in the products. YSZ has oxygen vacancy sites that adsorb 0 - onto the surface of the catalyst, based on this kinetic scheme"91 : O2 + e-0, (12) 0,- + 2e-O;(13) 0 2 2 - + 2 0 - (dissociation) (14) 0- + Vo,s+ e O,,,,(incorporation) (15) The Kroger-Vink notation is used for lattice defects : denotes an oxygen vacancy on the surface of the catalyst; O,,,, , an oxygen ion in the lattice form; and e , an electron. Using this proposed formulation, YSZ is classified as a material for selective oxidation in the thermal process. This role was not changed in the plasma reaction caused by the dense population of 0 ions and electrons. However, only the surface-lattice oxygen will be accessible for the catalytic reaction. The reaction between methanelC0 and surfacelattice oxygen can be the reason for the increasing methanol and C 0 2 concentrations. By treating the catalyst using helium, the methanol yield increased from 5% to 10%. It can be proposed that the Fe sites that activate methane on the catalyst surface can be also activated by helium plasma exposure. Panov et al . described methane activation on so-called a-oxygen sites that are formed on Fe sites"31. Knops-Gerrits and Goddard showed the evident activation of Fe when the Fe metal concentration was > 0. 5%[201. Based on the methane and oxygen +

cz

c3

_ 5.14t1.22 0.00 * 0 3.26k1.23 0.00*0

o.oo*o 0.00 k 0

CH30H

4.49k0.57 6.64 f 0.31 4.89k0.44 10.14k0.8 7.42 f 0.22 9.63 k0.25

the production of H, will activation decrease as some methane molecules will be adsorbed and reacted with attached 0, to form methanol. This also occurred in case of CO and CO, . However, a different result was found when the catalyst was oxidized by 0,.The conversion of CO and especially that of CO, became higher, which reduced the formation of methanol. Oxidation of the catalyst transforms the metal into metal-oxide catalyst, or weak bonding of metal and oxygen occurs on the surface of the catalyst. Metal-phase transformation decreases the activity to adsorb methane ; the most possible mechanism that results in this reduced activity is attributed to the formation of CO and CO,. This condition can be overcome by conducting the reduction process on the catalyst. Table 2 shows that the production of methanol of the PAR case was 2 % higher than that without-PAR treatment case. Based on the amount of methanol produced, the PAR and PA0 are not urgently required to boost the production of methanol. Helium plasma (or probably other inert gases) treatment was particularly adequate to activate the catalyst.

3 Conclusion The partial oxidation of methane using a DBD in the presence of 1 % Fe-30% Hf over YSZ was discussed. The maximum methane conversion of the noncatalytic process is 2 5 . 7 % at a flow rate of 30 ml-min-' and a supplied power of 80 W. The products are dominated by Hz, CO, CO,, and C,. The plasma activation techniques show that the catalyst can be activated, and the yield is different using different plasma treatment methods. Activated by inert gas (helium), the catalyst shows the highest production of methanol. However, PA0 transforms the reactants to complete oxidation products by producing more CO and CO,. Acknowledgments: The authors are thankful to the Korea Institute of Science and Technology for its support.

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Progress of Separation and Determination Methods for Rare Earth Elements Hu Bin * , Yin Jun ( College of 'Chemistry and Molecular Sciences , Wuhan University, Wuhan 430072, China 1 Abstract: This review coverS the developments in rare earth elements analysis over the period from 2003 to

and ion-imprinted polymers, and some new technologies, such as cloud point extraction, membrane extraction and capillary electrophoresis were successful used in rare ea& elements analysis. The determination methods of NAA , ICP-OES and ICP-MS were also discussed in detail.

2005. Some novel separation and determination methods and their applications in rare earth elements analysis were presented and discussed in detail. Some new materials, such as ionic liquids, nanometer materials Key words : rare earth elements ; separation and determination; review

(See J. Chin. RE.

Soc. (in Chin.), 2006, 2 4 ( 5 ) : 513 for f d text)