Asymmetric Tubular Oxygen-Permeable Ceramic Membrane Reactor

Jun 14, 2007 - ... Membranes as Oxygen Sources for High-Temperature Reactors. Qiying Jiang , Sedigheh Faraji , David A. Slade , Susan M. Stagg-William...
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9194

J. Phys. Chem. C 2007, 111, 9194-9202

Asymmetric Tubular Oxygen-Permeable Ceramic Membrane Reactor for Partial Oxidation of Methane Xiong Yin,† Liang Hong,*,†,‡ and Zhao-Lin Liu‡ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, BLK E5 02-02, 4 Engineering DriVe 4, Singapore 117576, and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ReceiVed: December 3, 2006; In Final Form: March 15, 2007

We report a successful paradigm of implementing the nickel catalyst for partial oxidation of methane (POM) in an oxygen-permeable ceramic membrane reactor (OPCMR). With using air feed, the OPCM (or the cathode of O2 reaction) was completely gastight but presented a desirable oxygen anion flux, which is required by POM in the anodic side. An almost 100% of CH4 conversion with high selectivity of CO and H2 was achieved in the anode, which was initially a composite of YSZ and NiO. It is also found that a mutual diffusion of Ni2+ and Zr4+ ions happens between the YSZ and NiO phases, which affected performance of POM. Besides experimental work, a new activation pathway was proposed to describe cleavage of the first C-H bond of methane. This activation pathway was simulated using the density function-theory and the computed activation energy for breaking up the first C-H bond falls in the experimental measurement range.

1. Introduction Conversion of natural gas (mainly CH4) into value-added chemicals has nowadays attracted great efforts due to its telling economic implications.1-4 Direct conversion of methane into liquid chemicals still faced the issue of either low yield or low selectivity.5-8 Catalytic transferring methane to syngas, the key precursor leading to liquid products, has a very high production yield (>90%) and selectivity. Steam methane reforming is the most commonly utilized industrial process to transform methane into syngas, but this process is capital expensive and energy consuming due to its high endothermic nature.9-12 On the contrary, the catalytic partial oxidation of methane (POM) with pure oxygen is a mild exothermic reaction,4,13-16 but it requires a large quantity of pure oxygen, which is traditionally generated from the expensive cryogenic air separation process. Integrating air separation and methane reforming in an oxygen-permeable ceramic membrane reactor (OPCMR) will reduce the cost of POM significantly and will make it possible to build housesized syngas plants.1,17 Oxygen-permeable ceramics have been employed as the reactive material to separate oxygen form air with the infinite perm selectivity.18-20 Unfortunately, most of the materials with high O2- conductivity have low mechanical or/and chemical stability at reducing atmosphere (i.e., methane or syngas), while the materials with the adequate stability (e.g., YSZ) have low oxygen ionic conductivity. The asymmetric membrane mode, in which the reactive oxygen electrolyte material forms a very thin dense layer on a thick porous support, is therefore the logical option.21,22 Up to now, a majority of studies have focused on the planar membrane structure.23-28 Compared with the planar disc, the tubular design is obviously advantageous in the industrial applications although it is much more difficult to attain this * To whom correspondence should be addressed. E-mail: chehongl@ nus.edu.sg. Fax: 65-6779-1936. Tel: 65-6516-5029. † National University of Singapore. ‡ Institute of Materials Research and Engineering.

structure.29 The electrochemical vapor deposition30-32 and the multisteps sintering process33 have been developed to fabricate asymmetric tubular OPCMR. In this work, we report a novel design of a asymmetric tubular catalytic reactor consisting of three coaxial annular layers of different ceramic materials and structure, which are porous La0.2Sr0.8MnO3-δ (LSM80)-Ce0.8Gd0.2O2-δ (CGO20)/dense YSZ-Ag composite/porous YSZNi(0) composite. This design resembles solid oxide fuel cell but its electrolyte layer (YSZ-Ag) is not only oxygen conductive but electronic conductive as well; as such the electrons released by O2- at the anode will be able to return to the cathode. Herein, the cathodic reaction is O2 + 4e- f 2O2- and the anodic reaction is CH4 + O2- f CO + 2H2 + 2e-. It is crucial that the electrolyte layer be gastight so that the gaseous reactants and products at both electrodes will not be in contact by each other. On the basis of experimental investigation, we also study the mechanism of syngas formation in the POM. Historically, two mechanisms have been proposed respectively to describe the details of forming syngas on the nickel catalyst.2,34-37 To date, it has been widely considered that the first C-H bond cleavage of methane is the rate-determining step for methane activation. Many theoretical works have been performed38-42 to simulate the first C-H bond cleavage process on Ni(111) surface; however, the calculated energy barriers were much higher than the experimental values measured by Lee et al. (51 kJ/mol)43 and Beebe et al. (53 kJ/mol).44 These theoretical simulations used the same cracking pathway of the first C-H, by which the CH4 molecule adsorbed and dissociated on a single Ni atom, and this is followed by the diffusion of the CH3 and H species toward the adjacent 3-fold sites. In this paper, we propose a new view on the dissociation of the first C-H bond on the Ni(111) surface, wherein the CH4 molecule is adsorbed and dissociated on a 3-fold site, and the generated H shift to an adjacent 3-fold site while the CH3 undergoes rotation and moves more closely to the surface of the underlying nickel cluster, where the CH3 will proceed further dissociation to generate

10.1021/jp0682917 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007

Ceramic Membrane Reactor for Partial Oxidation of CH4

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Figure 1. The flowchart illustrating the technical details for the fabrication of a triple-layer cylindrical membrane reactor.

carbon and hydrogen eventually. On the basis of this dissociative adsorption model, the calculated energy barrier and initial methane-sticking coefficients are very close to the experimental values. 2. Experimental and Computational Methods 2.1. Membrane Reactor Fabrication. The fabrication procedure for making the tubular membrane reactor is shown in Figure 1 in which submicron powders of LSM80 and CGO20 were fabricated in house22 and submicron YSZ [Yttria (8 mol %) fully stabilized zirconia, D50 ) 0.5∼1 µm, specific surface area >15 m2/g] powder was purchased from Stanford Material Corporation (U.S.A.). The NiO (50 wt %)-YSZ mixture was fabricated by suspending a given amount of the YSZ powder in an aqueous solution containing Ni(NO3)2 (1 mol/L), ethylenediaminetetraacetic acid (1 mol/L), and poly(vinyl alcohol) [15 wt % of YSZ], and it was followed by drying and calcination. Details about the preparation of the polymerceramic mixture, green tube extrusion, fine ceramic powder fabrication, and ceramic sintering were reported in our previous work.21,22,45 After precalcinating the NiO-YSZ polymer green tube at 1000 °C for 2 h, the resultant porous NiO-YSZ tube possessed sufficient mechanical strength to withstand developing a coating layer on it. A slurry was formulated by mixing 150 g of the YSZ fine powder in an organic solution comprising Butiva-79 (6.25 g) (polyvinylbutyral resin, Monsanto), Span80 (3 mL) (sorbitan monostearate, a nonionic surfactant, Aldrich), fish oil (6 mL), dibutyl phthalate (3 mL), and toluenemethylethyketone (v/v ) 1:1, 1 L). The slurry was used to form a thin layer of YSZ via slip coating on the exterior side of the NiO-YSZ tube. Subsequently, the coated object was sintered at 1450 °C for 4 h to produce a solid tube with double porous layer YSZ/NiO-YSZ. An electroless silver plating (ESP) solution (1 l, pH ) 11) was formulated by dissolving AgNO3 (0.112 mol), HCHO (0.093 mol), and ammonia (25%) in deionized water. The porous YSZ/NiO-YSZ tube was dipped into the ESP solution for a few seconds to allow the external YSZ pores to be filled with the ESP solution. After wiping away the surface-attached ESP solution, the tube was placed in an oven at 60 °C for 1 h to allow silver metal deposition to take place inside the pore channels of the YSZ layer. The exterior surface layer of the tube then became dense (confirmed by N2 permeation test) after repeating a few rounds of this filling-and-plating manipulations. As the ESP solution is unstable, only the fresh ESP solution

was appropriate to serve this purpose. In the last step, a powdercoating layer was developed on the exterior surface of the tube by using a slurry of LSM80 and CGO20. The slurry contained 15 wt % of LSM80 and CGO20 fine particles (1:1 by wt) and the organic additives that were the same as those used to formulate the YSZ slurry in toluene-methylethylketone. The LSM80/CGO20 powder-coating layer was then subjected to calcination at 1150 °C of which the details have been described in the previous report.21 Finally, the triple-layer membrane reactor was realized according to the microscopic investigation (field emission scanning electron microscopy (FESEM), JEOL JSM-6700F). The thickness of the porous Ni(0)-YSZ tube wall, the dense YSZ-Ag layer, and the porous LSM80/CGO20 layer were about 1 mm, 5∼10 µm, and 0.5∼2 µm, respectively. 2.2. Characterizations of the NiO-YSZ Mixture. Temperature Programmed Reduction (TPR). The NiO and NiO-YSZ powders prepared in house were then calcined at 1450 °C for 4 h, and after that they were placed respectively in a quartz tube connected with a thermal conductor detector (Shimadzu, GC-8A). Before TPR, the sample (100 mg) was pretreated with Ar gas at 700 °C for 1 h. After cooling down to room temperature, the TPR was carried out under the reducing atmosphere of H2 (2.5 sccm) and Ar (47.5 sccm), and the temperature was increased from 25 to 800 °C using the heating rate of 10 °C/min. X-ray powder diffraction (XRD) analysis was carried out on a SHIMADZU XRD-6000 diffraction meter using Cu KR radiation (λ ) 1.54056 Å) with the scanning rate of 2.5 °/min and scanning angles between 20 and 70°. The X-ray photon spectroscopy (XPS) was carried out on an instrument (Kratos Axis His, Manchester, U.K.) equipped with the Al KR X-ray source (1486.6 eV) by using the takeoff angle of 90° with pass energy of 40 eV. 2.3. Coupling Air Separation with POM. The membrane reactor setup has been illustrated in our previous reports.21,22 The reactor was continuously purged with He while it was heated up to the designated reaction temperatures. After that, the inner tube was connected to a feeding drift comprised of in CH4 (10 sccm, FCH ) and He (20 sccm), and the external 4 cathodic side, viz. the LSM80/CGO20 layer, to an air flow (100 sccm). The outlet stream from the interior tube was led into an on-line gas chromatograph (Clarus 500, Perkin-Elmer ARNEL) for composition analysis, and the outlet gas flow rates out out out of CH4 (FCH ), CO (Fout CO), H2 (FH2 ), and CO2 (FCO2) were then 4 determined by multiplying the total outlet gas flow rate with

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Yin et al.

the respective mole fraction. The methane conversion (XCH4), CO selectivity (SCO), CO2 selectivity (SCO2) and H2 selectivity (SH2) were calculated by the formulas as follows

XCH4 ) SCO )

SCO2 )

S H2 )

in FCH 4

-

out FCH 4

in FCH 4

Fout CO in out FCH - FCH 4 4 out FCO 2 in out FCH - FCH 4 4

0.5FHout2 in out FCH - FCH 4 4

× 100%

(1)

× 100%

(2)

× 100%

(3)

× 100%

(4)

The carbon selectivity (SC) was estimated from the measured SCO and SCO2 (SC ) 100% - SCO - SCO2). 2.4. Thermal Decomposition of Methane over NiO, YSZ, and NiO-YSZ Composites. Metal oxide (NiO 1 g, or NiOYSZ (1:1 by wt.) composite 2 g, or YSZ 1 g) was placed in the middle of a quartz tube (i.d. ) 8 mm) and was heated up to 850 °C. After purging the quartz tube with He (20 sccm) for 0.5 h, a diluted methane stream (CH4 10 sccm/He 10 sccm) was introduced into the quartz tube, and the outlet stream was connected to the GC for composition analysis. 2.5. Quantum Calculations. The quantum calculations were performed using density functional theory (DFT),46 Hartwigsen-Goedecker-Hutter pseudopotential,47 the Teter Pade parametrization approximation,48 and a plane-wave basis set with kinetic energy cutoff of 1080 eV, which were implanted in the ABINIT code.49-52 As an initial study, we ignored the relaxations of nickel surface, C-H bond length, and H-C-H angle. The face-centered cubic nickel crystal lattice parameter was fixed at 3.52 Å and the C-H bond length and H-C-H angle in both CH4 and CH3 were fixed at 1.09 Å and 109.5°, respectively. A super cell with the dimension of 6 × 6 × 22.32 Å was used. This super cell containing more than three vacuum layers is desirable for the crystal surface simulation and also gives reasonable calculation results to the H2 and CH4 bond length. The calculated H2 bond length is 0.77 Å (experimental value, 0.74 Å) and the calculated C-H bond length of CH4 is 1.09 Å (experimental value, 1.09 Å). The difference between the calculated bond length and its experimental value is controlled within 5%.

TABLE 1: The XRD Angular Positions (2θ) in Figure 2 2θ of YSZ (degree)

2θ of of NiO (degree)

30.1 30.2 30.4 30.0 30.1

59.6 59.8 59.9 59.6 59.6

compound Pure YSZ NiO (16.7 wt %)/YSZ NiO (33.4 wt %)/YSZ NiO (50 wt %)/YSZ NiO (50 wt %)/YSZ reduction with CH4

34.9 35.0 35.2 34.8 34.9

50.2 50.3 50.5 50.1 50.2

37.2 37.5 37.3

43.2 43.5 43.4

al.53 However, we observed in this study that a further increase in the NiO content to 50 wt % brought about reversely an expansion of YSZ lattice (i.e., exhibiting a set of 2θ values slightly smaller even than that of the native YSZ). We deem that ionic size effect instigates this backward structural adjustment. There might be a coincident diffusion of Zr4+ ions into NiO phase, and its impact became more influential to the YSZ lattice with increasing NiO content. Because departure of Zr4+ ions from the YSZ phase promoted the Y/Zr ratio in its bulk and therefore led to an expansion of lattice because Y3+ is bigger than Zr4+. Hence, these two opposite effects on the lattice size of YSZ, namely the entering of Ni2+ ions and the leaving of Zr4+ ions, caused the YSZ phase experience contraction and then expansion. We also found that the XRD pattern of the YSZ phase resumed to its native form after the NiO phase was reduced to Ni(0) by methane; such change could be attributed to the exclusion of Zr4+ ions from the NiO1-x phase upon the reduction (where 0 < 1 - x , 1). On the other hand, the YSZ phase also affected the lattice parameter of NiO phase in the composite (Table 1). The existence of the Ni3+ in NiO, which was reported to account for the color change of the NiO, happened in its sintering process.54 It is possible that the oxygen-ion conductive YSZ facilitated the oxidation of Ni2+ into Ni3+ via supplying O2- to the NiO phase. When a certain amount of Ni3+ was generated in the NiO lattice (due to calcination at 1450 °C), a reduction in the lattice parameter (i.e., increasing of 2θ values) was the outcome as expected. Besides happening in the NiO phase (33.4 wt % in the composite), the lattice contraction should also occur in the NiO phase of 16.7 wt % in the composite with respect to pristine NiO. Nevertheless, similar to the YSZ phase in the composite, when the content of the NiO phase reached 50 wt % a lattice expansion (i.e., decreasing of 2θ values) of it

3. Results and Discussion 3.1. Bilateral Diffusions of Cations in YSZ-NIO Composite. As described in section 2.1., YSZ-NiO composites with different NiO contents were prepared through the solution impregnation method and co-calcination at 1450 °C for 4 h. The XRD angular positions of the characteristic peaks of the NiO and YSZ phases are listed in Table 1. The 2θ parameters of YSZ shift to higher values (representing contraction of lattice) with increasing the NiO content up to 33.4 wt % (Figure 2). This trend could be explained as the diffusion of Ni2+ ions into the YSZ lattice. It is believed that doping the Zr4+ (0.84 Å) lattice site by smaller Ni2+ (0.78 Å) ions was responsible for the contraction of YSZ lattice, which was primarily a size adjusting effect. The dissolution of nickel ions in the ZrO2 matrix at a low NiO content was previously reported by Dongare et

Figure 2. X-ray diffraction patterns of the three NiO-YSZ composites with different NiO contents. The * indicates the NiO peaks and the ∆ the Ni peaks.

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Figure 3. XPS of Ni 2p3/2 core level of the three NiO-containing specimens.

Figure 4. TPR of the three NiO-YSZ composites with different NiO loadings including pristine NiO, whose intensity is divided by a factor of 9.

was observed. As forementioned, the doping of Zr4+ ion in the NiO phase was furthered in the equimass composite, and the lattice expansion of NiO was triggered by the lodging of bigger Zr4+ ions in the lattice site of Ni2+. The XPS peak of NiO, Ni-2p3/2, was used to probe structural environments of the NiO phase in the composite, and the Ni2p3/2 of native NiO, prepared also by calcinations at 1450 °C, was used as the reference (Figure 3). The Ni-2p3/2 peak consists of two partially overlapping subpeaks with binding energy (BE) of ∼854 and ∼856 eV, respectively. The high BE peak was assigned to Ni3+ species whereas the low BE peak to Ni2+.55,56 For composite NiO 16.7 wt %)-YSZ, it presented a very weak but still discernible Ni-2p3/2 hump peak. It is indicative of a significantly high dispersion of the NiO phase in the YSZ phase. With the increase of NiO loading to 33.4 wt % in the composite, the ratio of high BE peak to low BE peak (viz. Ni3+/Ni2+) becomes clear after peak fitting and is greater than that of the native NiO. This reflects the effect of YSZ on that promoting the oxidative conversion of Ni2+ to Ni3+. 3.2. TPR of NiO-YSZ Composite. Dongare et al.53 have investigated the TPR of pure NiO in the temperature range from 350 to 1000 K and found that the pure NiO presented only one peak at near 340 °C. Mori et al.57 reported also the TPR profiles of pure NiO, pure YSZ, and NiO-YSZ composites in the temperature range from 350 to 1400 K. Identical to their observations, our study verified that the pure NiO showed one TPR reduction peak. We also observed that the NiO-YSZ composite showed a TPR profile consisting of three peaks whose temperatures vary with the loading of NiO in the composite (Figure 4), which is similar to the NiO-YSZ TPR profile peaks reported by Mori et al.57 The reduction initiation temperature (the temperature when the measured sample starts to be reduced) increases with the increase in YSZ content (100 wt % NiO, ∼340 °C; 50 wt % NiO, ∼350 °C; 33.4 wt % NiO, ∼380 °C; 16.7 wt % NiO, ∼410 °C). The pure YSZ has no any reduction peak in the TPR in the temperature range of study ( NiO (16.7 wt %) > NiO (33.4 wt %). On the basis of this sequence, the equimass composite (i.e., NiO of 50 wt %) is deemed to contain the most intimate interfacial boundary between NiO and YSZ amid the three composites. It is likely that in the NiO (50 wt %) case these two oxide phases formed a highly mixed matrix rather than just a dispersion of NiO in YSZ. In light of the relative intensity of peak II in a TPR profile because it stands for the allocation of Ni2+ ions at the interface in a particular composite, NiO (50 wt %) has the highest percentage of interfacial Ni2+ ions, and this is followed by NiO (16.7 wt %) and then NiO (33.4 wt %). The reduction temperatures of peak III also displayed the same order. Such a consistency was because, in principle, a greater interfacial concentration of Ni2+ ions would favor diffusion of the ions deeper into the YSZ bulk phase, and therefore these dissolved Ni2+ ions were more difficult to be reduced relative to those locating nearby to the interfacial boundary. In short, a high concentration of interface-located Ni2+ is the desired feature for POM catalysis, which will be elaborated in the following sections. 3.3. Characterizations of Dual Functional Tubular Membrane Reactor. 3.3.1. Fabrication of Membrane Reactor. As illustrated in Figure 1, a thin layer of YSZ powder was coated on the precalcined NiO-YSZ tube via dip-coating. The resulting tube was cofired (at 1450 °C for 4 h) and a shrinkage rate of around 20% and the overall porosity of about 10% were acquired in the tubular body after sintering. The thickness of the sintered YSZ layer was about 8-10 µm. This thickness suited the design for attaining a dense Ag-YSZ layer through the electroless depositing of Ag in the porous channels of the YSZ layer. Furthermore, the Ag-YSZ layer was topped up by a highly porous LSM80/CGO20 layer using the conventional slurrycoating/-sintering methods. The resultant tube was subjected to reduction [by passing a stream of H2 (10 sccm)/He (20 sccm) through the inner tube at 900 °C for 2 h] to verify that the

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Figure 5. SEM of the cross-sectional view of the triple-layer tubular membrane reactor.

LSM80/CGO20 layer (the cathode of the reactor) can remain intact during the chemical reduction process. The electron micrograph of the cross section of the tube indeed demonstrates this structural feature (Figure 5). The result signifies that the YSZ-Ag layer is completely gastight and therefore successfully prohibits access of H2 to the LSM80/CGO20 layer, otherwise this cathodic layer would shatter forthwith. Moreover, the air permeation test was conducted at 900 °C using He (20 sccm) as the sweeping gas and revealed exclusively an oxygen flux through this triple-layer tubular reactor, confirming that the AgYSZ layer was gastight and the cathodic layer worked as expected. 3.3.2. Partial Oxidizing of Methane by the Permeated Oxygen Stream. The NiO of the inner tube wall started to be reduced to NiO1-x when the CH4/He stream was introduced into the tubular membrane reactor (LSM80-CGO20/Ag-YSZ/NiO-YSZ) at 700 °C. The generation of CO2 and H2O became mild after the initiation period (ca. 30 min) because the majority of Ni2+ ions were reduced to Ni(0), namely forming NiO1-x, and the measurement of the POM was carried out since that moment. The measurement data came from the POM performance in the first 20 min at each temperature point because coke formation on Ni(0) catalyst affects insignificantly the POM activity within this short interval. The methane conversion (XCH4) increased steadily with raising the operation temperature from 700 to 900 °C. The chemical selectivity of H2 (SH) evaluated by eq 4 also remained slowly increasing over the temperature range of investigation. However, the chemical selectivity of CO (SCO) increased drastically from 700 to 800 °C (Figure 6). This is primarily due to the nature of POM, which gives a high SCO at temperature above 800 °C. At 900 °C (20 sccm He and 10 sccm CH4), a very high methane conversion (97%), CO selectivity (94%, and H2 selectivity (96%) were obtained. On the other hand, as the POM relies on using a lattice oxygen anion as the reactant, the oxygen anion flux (through mainly the YSZ phase in both electrolyte and anodic layer) became an important factor. In accordance to the impedance spectrum of YSZ,22 the oxygen conductivity gains a noticeable leap from 750 to 800 °C. 3.4. The Synergy between NiO and YSZ in Catalytic Decomposing CH4. The thermal dissociation of methane on the three designated catalyst powders, NiO (1 g), YSZ (1 g), and NiO-YSZ (2 g, w/w ) 1), respectively, is a study that could help understand the unique role of NiO-YSZ interface in the catalysis of POM (Figure 7 a-c). It may note that this study was performed in the absence of oxygen stream, and thus both

Yin et al.

Figure 6. Temperature-dependent methane conversion, CO selectivity, and H2 selectivity based on a feeding stream consisting of 20 sccm He and 10 sccm CH4.

Figure 7. Thermal decomposition of methane: (a) on pure NiO (1 g), (b) on pure YSZ (1 g), and (c) on NiO (50 wt %)-YSZ composite (2 g).

CO and CO2 were generated from reduction of the NiO enclosed in the reaction system. Yet the reaction parameters could remain approximately a steady state within the examination time period (30 min < t < 80 min), which is likely because of a low flow rate of the dilute CH4 stream and also a fixed diffusion rate of

Ceramic Membrane Reactor for Partial Oxidation of CH4

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Figure 8. The POM selectivity for H2 on the three oxides: (a) pure NiO, (b) pure YSZ, and (c) NiO(50 wt %)-YSZ composite.

the lattice oxygen moving toward the catalytic surface from bulk. On both the NiO and the NiO-YSZ catalysts, the methane conversion (XCH4) was very high (>91%) over the entire process of reaction. However, these two catalysts gave rise to significantly different ratios of SCO2/SCO. The ratio was around 4:1 on the NiO but below 0.15:1 on the NiO-YSZ after 50 min (Figure 7). The result suggests that the NiO may easily release lattice oxygen ion [O]s to the reaction than the NiO in the NiOYSZ composite. YSZ is therefore deemed to exert some sort of constraints on accessibility of [O]s of the adjacent NiO phase. This inference is valid because YSZ showed negligible activity to methane conversion when the reaction became stable. Compared with the NiO and the NiO-YSZ, a very low XCH4 ( 0) ∼ H1 (Figure 11). The rectangular potential barrier has a height of 52 kJ/mol and the width of 1.28 Å. In this way, the S0(E) can be approximately expressed as the one-dimensional

Ceramic Membrane Reactor for Partial Oxidation of CH4

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9201 This work further proposes a methane decomposition route, which could be simulated using a simple Ni4-CH4 cluster model. The calculated activation energy value is very close to the experimental data reported by other authors. Acknowledgment. We thank Dr. J.-Y. Lin and Dr. J.-Z. Luo of ICES Singapore for their help carrying out TPR measurements. This work has received financial support from the National University of Singapore (RP R-279-000-114-112). References and Notes

Figure 13. A comparison of the calculated methane thermal-sticking coefficients with the experimental data.

transmission probability with energy E either below or over the rectangular potential barrier height (eq 8)

S0(E) )

{

[ [

]

(eKIa - e-KIa)2 -1 for E < V0 E E 16 1V0 V0 -1 sin2KIIa 1+ for E > V0 E E 4 -1 V0 V0

1+

(

)

(

KI )

)

x ( ) x ( ) 2mV0

}

E V0

(8a)

2mV0 E -1 p2 V0

(8b)

2

p

KII )

]

(8)

1-

It can be seen from Figure 13, a good agreement between our calculated methane thermal-sticking coefficients with the experimental data measured by Beebe et al.44 has been achieved. To the best of our knowledge, the calculated activation energy and the thermal-sticking coefficients have been to date the values closest to the experimental data. In brief, the methane molecule prefers to engage dissociation via landing to the 3-fold site instead of atop site on the Ni(111) surface. 4. Conclusion An asymmetric tubular oxygen-permeable ceramic membrane POM reactor has been fabricated, which is composed of a cathode layer (