SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 2001; 32: 198–201

XPS characterization of zeolite catalyst in plasmacatalytic methane conversion†

Fei He,1∗ Chang-jun Liu,1 Baldur Eliasson2 and Bingzhang Xue2

1 State Key Laboratory of C1 Chemical Technologies, Tianjin University, Tianjin 300072, P. R. China2 ABB Corporate Research Ltd, Baden, CH-5405, Switzerland

Received 23 October 2000; Revised 13 December 2000; Accepted 12 January 2001

Direct plasma methane conversion to higher hydrocarbons using pure methane feed is almost impossiblewithout using zeolite. In the presence of zeolite, such direct conversion leads to a very high yield ofhigher hydrocarbons. The use of zeolite not only inhibits the formation of non-selective carbon blackand plasma-polymerized films but also significantly increases the selectivity of light hydrocarbons duringplasma methane conversion with dielectric barrier discharge (DBD). The DBD plasma-used zeolite hasbeen characterized by x-ray photoelectron spectroscopy (XPS) and suggests the DBD plasmas have led toreactions within the channel of zeolite. The present investigation gives us more supporting results for theproposed mechanism of plasma-promoted catalysis. Copyright 2001 John Wiley & Sons, Ltd.

KEYWORDS: XPS analysis; zeolite; plasma; methane conversion

INTRODUCTION

Plasma methane conversion can lead to a much highermethane conversion compared with conventional catalyticmethane conversion. There are usually two ways for plasmamethane conversion: an indirect plasma methane conversionvia a syngas step and direct plasma methane conversion tomore valuable hydrocarbons. The former method generatessyngas first using plasma partial oxidation or plasma CO2

reforming of methane and then produces more valuablehydrocarbons via syngas. The pilot or industrial test ofsyngas production using plasmas has been reported recently.The latter method produces more valuable hydrocarbonsdirectly from methane using plasmas. More and moreinvestigations on direct plasma methane conversion havebeen conducted. In particular, the combination of catalystand discharge plasmas has recently become a commonoperation for methane conversion.

The major objectives of introducing a heterogeneouscatalyst into cold plasmas (e.g. corona discharge or dielectricbarrier discharge) are to improve the selectivity and toincrease the energy yield of desired products. However,the understanding of the interaction between plasmas andheterogeneous catalyst is still very poor. During a plasmacatalytic methane conversion via corona discharge,1,2 thepresence of an oxide catalyst or zeolite within the discharge

ŁCorrespondence to: F. He, State Key Laboratory of C1 ChemicalTechnologies, Tianjin University, Tianjin 300072, P. R. China.E-mail: hefei30@hotmail.com†Paper presented at APSIAC 2000: Asia–Pacific Surface andInterface Analysis Conference, 23–26 October 2000, Beijing, China.Contract/grant sponsor: Natural Science Foundation of China;Contract/grant number: 29806011.Contract/grant sponsor: Ministry of Education of China.Contract/grant sponsor: ABB Corporate Research Ltd, Switzerland.

plasma has led to a significant change in the characteristicsof optic emissions. This suggests that there must be somechanges in plasma or in catalyst. With more experimentalresults achieved by either authors or other groups, one couldconclude that the mechanism of the interaction betweencatalyst and plasmas includes two aspects: the catalyst-enhanced non-equilibrium of plasma and plasma-promotedor plasma-induced catalysis in some references. The non-equilibrium means, in a cold plasma, that the electrontemperature can reach as high as 104K while the gastemperature remains as low as room temperature. Thisis direct evidence of the mechanism of catalyst (zeolite)-enhanced non-equilibrium of discharge plasmas. Here, wepresent some supporting experimental results on plasma-promoted catalysis for direct higher hydrocarbon formationfrom methane with and without using carbon dioxide.

EXPERIMENTAL

Plasma reactorA dielectric barrier discharge (DBD) plasma reactor has beenapplied for this plasma methane conversion, as shown inFig. 1. The DBD used is initiated when alternating voltagesof sufficiently high amplitudes are applied between twoelectrodes separated by a non-conducting medium (dielectricbarrier) in addition to a gas space. The frequency of the a.c.electric field can vary over a wide range, from line frequencyto several megahertz. Glass, quartz or ceramics can be usedas dielectric materials. The radial width of the dischargespace was 1 mm and its length was 300 mm. A high-voltagegenerator working at ¾30 kHz applies the power. The feedgases, CH4 and CO2, were introduced into the systemflowing through the reactor. A back-pressure valve at the

DOI: 10.1002/sia.1036 Copyright 2001 John Wiley & Sons, Ltd.

XPS of zeolite methane conversion catalyst 199

exit of the reactor was used to adjust the pressure. Two MTI(Microsensor Technology Inc., M200H) dual-module microgas chromatographs containing a Poraplot Q column, anAl2O3 ‘S’ column and a molecular sieve 5 A plot column withthermal conductivity detectors were used to detect gaseousproducts. The liquid sample was analysed at the Labaratoired’Analyse (Shell Raffinerie de Cressier, Switzerland) withgas chromatographs. It was found to contain more than200 branched higher hydrocarbon components in the rangeof C5 –C11. A HP6890 gas chromatograph with a flameionziation detector also was used to analyse the carbonaceousspecies on the plasma-used zeolite.

CatalystThe experimental results obtained with NaX zeolite arediscussed. In each case about 9 g of zeolite powder wasintroduced to the discharge gap in this work. The loading ofzeolite also has been described elsewhere.3,4

Catalyst characterizationX-ray photoelectron spectra were performed with a PHIQuantum 2000 scanning ESCA microprobe equipped witha hemispherical electron energy analyser and an Al K˛monochromator source (1486.8 eV). The pressure in analysischamber during data acquisition was ¾10�7 Pa. The beamdiameter for the analysis area was set at 200 µm. The high-resolution spectra were recorded with a pass energy of23.5 eV. The survey spectra were carried out with a passenergy of 187.85 eV. The contaminative C 1s peak at 284.6 eVwas used to calibrate the energy shift. The accuracy of thebinding energy was estimated to be š0.2 eV. All the dataof spectra were treated using PHI MultiPak Version 6.0software. In order to obtain information within the channelof the zeolite, the depth profiles were performed usingan ion gun with a 3 keV argon ion and a current densityof 16.0 µA cm�2. The sputter rate was 58 A min�1 and thesputter time was 10 s. The depth of sputtering was estimatedto be ¾10 A for the depth profile analysis. In addition, theelectron microscopy studies were performed with a PhilipsXL30 ESEM system.

RESULTS AND DISCUSSION

Direct methane conversion using DBD plasmasover zeoliteThe effective active region for DBD plasma methane conver-sion is within the gap between the dielectric (quartz) and thegrounded electrode (the outer stainless-steel cylinder). Car-bon dioxide has been used here as a co-reactant for methane

Centre lineHigh-voltage electrode Dielectric-barrier material

AC CH4 Product

High-voltage generator

Grounded electrode

Zeolites

Figure 1. The DBD plasma reactor system.

conversion. All the plasma reactions were limited within thecentre area that was covered by carbon black and plasma-polymerized film. Because a metal film electrode that hasbeen connected to the high-voltage generator attaches thecentre area of the inner surface of quartz tube, this centrearea forms the active DBD plasma region for methane con-version. This means that the plasma environment is basic forDBD methane conversion. Out of this active area, there isno reaction observed. Moreover, at the entrance of the DBDplasma region, carbon black was observed. This suggests thatthe energy of the DBD plasma is sufficiently strong to breakdown methane totally into hydrogen and carbon. In additionto carbon black, the radical chain reactions may thereby startfrom CH3, CH2 and CH to generate higher hydrocarbonsup to the plasma-polymerized film. The formation of carbonblack and plasma-polymerized film has a negative effect onthe stability of DBD plasma methane conversion, especiallyfor pure methane feed. The experimental investigation, how-ever, shows that the presence of zeolite within DBD plasmascan inhibit totally the formation of carbon black and plasma-polymerized film, especially for the pure methane feed. TheDBD plasma methane conversion therefore has become sus-tainable. Instead of carbon black and polymers formed onthe dielectric, the oily carbonaceous species were producedon the zeolite.

Table 1 shows a GC analysis of the composition of oilycarbonaceous species from the DBD reactions using puremethane feed over NaX zeolite. Before the GC analysis, theplasma-used zeolite was put into a pure CCl4 solution. Thecarbonaceous species were dissolved into the CCl4 solutionto form a mixture. Then the GC analysis was conductedto analyse the component of this mixture. Table 1 alsoshows that significant amounts of branched hydrocarbonshave been produced, which are characteristics of DBDplasma reactions. The carbon number of these branchedhydrocarbons ranges from C15 to more than C26. Becausethe gas temperature of DBD plasma reactions keeps below180 °C, most of the hydrocarbons with high boiling pointwill be deposited on the zeolite. Figure 2 shows the electronmicroscopy image of fresh and DBD plasma-used zeolites.

Table 1. Composition of carbonaceous species by 9 g DBDplasma-used NaX zeolite

Compositiona CompositionComponent (mol.%) Component (mol.%)

�C15 5.47 C20 2.51C15 14.00 C21 4.31

iC16 C iC17 12.50 iC21 C C22 C iC23 8.99C17 3.32 C23 1.64iC18 1.48 iC24 9.62C18 0.35 C24 5.58iC19 3.76 iC25 3.60C19 0.95 iC26 7.80iC20 1.13 C26 10.77

>C26 2.21

aThe composition here represents the composition of the totalcarbonaceous species detected.

Copyright 2001 John Wiley & Sons, Ltd. Surf. Interface Anal. 2001; 32: 198–201

200 F. He et al.

(a)

(b)

Figure 2. Scanning electron micrographs of (a) fresh and(b) plasma-used zeolite for methane conversion in thepresence of CO2 (magnification, ð10 000).

It is clear that there is a big difference in the surface of thezeolites, which would be from the carbon species depositedon the zeolite.

Characterization of DBD plasma-used zeoliteusing XPSIn the absence of zeolite, electrons and other plasma speciescan move around more independently within the electrodegap. However, when zeolite presents in the gap betweenthe dielectric and the grounded electrode, the generationand reactions of plasma species occurred within the zeolitebed. The plasma reactions occur within the zeolite bed orprobably within the channel of zeolite. In this work, anXPS characterization has been performed to investigate the

composition change with the DBD plasma-used zeolite. Fromthis investigation, indirect information on the interactionbetween plasmas and catalyst can be obtained.

Figure 3 and Table 2 present the comparative XPSanalysis results of fresh and DBD plasma-used zeolites. Thereare two methods applied for the XPS analysis: XPS analysiswithout using sputtering and XPS analysis with sputteringusing an ion gun. Therefore, the results shown in Fig. 3 andTable 2 include the information after sputtering off 10 A fromthe surface that represents the composition of inner pores of

1200

Plasma-used

Fresh

1000

C K

LLC

KLL

Na1

s

O K

LL

O1s

Na

KLL

C1s

Si2

sA

l2s S

i2p

Al2

p

Na1

s

O K

LL

O1s

Na

KLL

C1s

Si2

sA

l2s

Si2

pA

l2p

800 600

Binding Energy (eV)

400 200 0

1200

Plasma-used

Fresh

1000

C K

LLC

KLL

O K

LL

O1s

Na

KLL

C1s

Si2

sS

i2p

Na1

s

O K

LL

O1s

Na

KLL

C1s

Si2

sA

l2s

Si2

pA

l2p

800 600

Binding Energy (eV)

400 200 0

(a)

(b)

Figure 3. The XPS spectra of zeolite for methane conversionin the presence of CO2: (a) without using sputtering; (b) withsputtering.

Table 2. X-ray photoelectron spectroscopy analysis of plasma-used zeolites with and withoutusing sputtering

At.%

O Si Na C Al C/O O/�Al C Si� Si/Al

No Fresh sample 51.3 14.4 12.5 11.4 10.4 0.222 2.07 1.38

sputtering Plasma-used 44.1 7.9 6.5 35.5 6.0 0.805 3.17 1.32

SputteringFresh sample 52.9 12.3 12.2 14.3 8.3 0.270 2.57 1.48

Plasma-used 23.9 2.4 0.4 72.5 0.8 3.033 7.47 3.00

Copyright 2001 John Wiley & Sons, Ltd. Surf. Interface Anal. 2001; 32: 198–201

XPS of zeolite methane conversion catalyst 201

zeolite. For the fresh zeolite, there is no significant differencein the composition of samples from the analyses with andwithout using sputtering. However, a big difference betweenthe composition of DBD plasma-used samples before andafter sputtering has been observed. One would think that alot of carbonaceous species, including contaminated carbon,would be adsorbed on the surface of the plasma-usedsample. These carbonaceous species would be producedin plasma phases or in catalyst sites during plasma methaneconversion. For the XPS analysis without using sputtering,the composition of carbon species would represent theseabove-mentioned carbonaceous species. For the XPS analysisusing sputtering, one would consider that the compositionof carbon species represents information on the channelof the zeolite. Because the composition of carbon speciessignificantly increases, this may suggest that some reactionsoccur in the channel of zeolite to induce the formation ofcarbonaceous species (or higher hydrocarbons). This is directevidence that the inner pore involves reactions for plasmamethane conversion in the presence of carbon dioxide.

In addition, because the plasma-used catalyst has beenexposed to air before ESCA analysis, the zeolite will adsorboxygen from air that will present on the sample surface.Therefore, a low C/O ratio (0.8) has been observed over thesamples without using sputtering. After sputtering, the C/Oratio is 0.27 with fresh sample. The oxygen species detectedmainly belong to the oxygen in the framework of the zeolite(O/(SiCAl) is ¾2.6). However, the C/O ratio becomes ¾3over the plasma-used sample after sputtering, whereas themeasured O/(SiCAl) is 7.47 for plasma-used zeolite. Thisrepresents information on carbon-containing species andoxygen-containing species within the channel of the zeolite.A curve fitting of the O 1s spectrum has been conductedand has been proposed in Fig. 4. The curve fitting clearlyshows three peaks in the spectrum that can be identified asC O (531.09 eV), C–O (532.42 eV) and oxygen species in theframework of zeolite (530.14 eV), respectively. Because thereis no reaction observed between methane and carbon dioxideover the zeolite used here without using DBD plasma, it is theDBD plasmas that have led to reactions within the channel ofzeolite. Thus, plasma-promoted or plasma-induced catalysiswould be suggested.

CONCLUSION

The present investigation gives us more supporting resultsfor the proposed mechanism of plasma-promoted catalysis.

Pos. %Area530.14 10.53531.09 45.22532.42 44.25

535 534 532 533

Binding energy (eV)

531 530 529 528

zeolite

C O

C O

Figure 4. Curve fitting of O 1s spectrum of DBD plasma-usedNaX zeolite (after sputtering).

An optic emission analysis of plasma methane conversionwith and without using catalysts is being conducted. AFaradaic cup measurement also is being performed to analysethe charge nature and charge amount of catalyst passingthrough the plasma-active region. These would provide usmore information on the interaction between catalyst andplasma, which will be discussed in a later paper.

AcknowledgementsSupports from the Natural Science Foundation of China (contract no.29806011), the Ministry of Education of China and ABB CorporateResearch Ltd, Switzerland is much appreciated. Assistance from theAdvanced Surface and Materials Analysis Center (ASMAC) of theChinese University of Hong Kong is also appreciated.

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Copyright 2001 John Wiley & Sons, Ltd. Surf. Interface Anal. 2001; 32: 198–201