Structural Characteristics of (NiMgAl)Ox Prepared from a Layered Double Hydroxide Precursor and its Application in Direct Internal Reforming Molten Carbonate Fuel Cells

Structural Characteristics of (NiMgAl)OxPrepared from a Layered DoubleHydroxide Precursor and its Applicationin Direct Internal Reforming MoltenCarbonate Fuel CellsK. Park1, K. Y. Kim1, L. Lu1, T.-H. Lim2, S.-A. Hong2, and H.-I. Lee1*1 School of Chemical and Biological Engineering, Research Center for Energy Conversion and Storage, Seoul National University,

Seoul 151-744, Republic of Korea2 Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

Received February 11, 2006; accepted February 8, 2007

1 Introduction

Molten carbonate fuel cells (MCFCs), operating at 650 °C,are attracting worldwide interest as the next generation elec-trical power supply. Hydrogen, the fuel for the MCFC, is sup-plied by the steam reforming of methane, which has beenconfirmed as the most efficient reaction for obtaining hydro-gen-rich gas [1, 2]. Ni-based catalysts are usually used for thesteam reforming reaction. The MCFC can be divided intoexternal reforming (ER), direct internal reforming (DIR) andindirect internal reforming (IIR) types, based on the position

of the reforming reaction. The DIR-MCFC has the advantagesof simplicity and thermal benefit, through the use of theexothermic heat from the electrochemical reaction at theanode for the endothermic reforming reaction as well asincreased methane conversion, due to the in situ consumptionof hydrogen. In particular, the DIR system has the potentialto produce a uniform fuel cell temperature distribution, dueto the reduction of the cooling load and the freedom to over-

–[*] Corresponding author, [email protected]

Abstract(NiMgAl)Ox materials prepared from layered double hydro-xide (LDH) precursors show a typical mesoporous structurewith an average pore size of about 6 nm, excellent activityfor the methane steam reforming (MSR) reaction at 650 °C,and a strong resistance against Li poisoning, suggesting thepossibility for future application in direct internal reformingmolten carbonate fuel cells (DIR-MCFC). X-ray diffraction(XRD) experiments indicate that the molar ratio of Al/Mgexerts a strong effect on the properties of both the LDH pre-cursors and final catalysts. Temperature-programmedreduction (TPR) analysis reveals that the Al/Mg molar ratioinfluences the properties of Ni in the final catalyst. Thecatalytic performance of the catalysts prepared is greatlyinfluenced by the molar ratio of Al/Mg for MSR. The activ-

ity gradually increases with an increase in the Al/Mg ratio(0.14–1.5). However, with further increases in the amount ofAl added (Al/Mg = 2), the activity decreases. The activity isstrongly related to the BET surface area and Ni dispersion.The Li-poisoning test proves that Mg-rich catalysts lose theiractivity quickly upon exposure to Li, while Al-rich catalystsmaintain virtually all of their original activity. NiMgAl(Al/Mg = 1.5) is found to be an excellent catalyst for theDIR-MCFC, having both the highest activity and strongestresistance against Li poisoning.

Keywords: Al/Mg Molar Ratio, DIR-MCFC, LayeredDouble Hydroxide (LDH), Methane Steam Reforming,Ni Catalyst

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DOI: 10.1002/fuce.200600003

Park et al.: Structural Characteristics of (NiMgAl)Ox Prepared from a Layered Double Hydroxide Precursor…

lap the reforming zone with the heat-producing section ofthe cell [3]. However, in carrying out the reforming reactionin the anode chamber of a DIR-MCFC unit, two major prob-lems exist. One is the deactivation of the catalyst, by the poi-soning of the alkali, originating from the molten carbonateelectrolyte (mixture of Li2CO3 and K2CO3); the other is cokeformation on the catalyst surface, mainly due to the very highmethane steam reforming (MSR) reaction temperature. Exten-sive investigations have been carried out in order to over-come these serious problems [2–6]. It has already been con-cluded that Ni catalyst supported on alumina couldeffectively increase the resistance to electrolyte poisoning andthat Ni on magnesia could suppress carbon deposition. Tothe best of our knowledge, however, a good DIR-MCFCcatalyst with synergetic function of both increasing resistanceto Li poisoning and suppressing coke formation has notbeen discovered yet. Therefore, there is still an urgent needto develop a novel catalyst to meet the strict technicaldemands of the DIR-MCFC. It was expected that well-dis-persed Ni on a homogeneously mixed Mg and Al oxideshould be an ideal candidate, combining the advantages ofboth MgO and Al2O3, based on literature results and our pre-vious studies.

A layered double hydroxide (LDH) material seems to meetthese requirements, having all the cations distributed insidebrucite-like sheets of the layered structure and giving rise tosmall metal crystallites by calcination and reduction [7]. Kungand Ko [8] reviewed several different methods for preparingmixed metal oxides and pointed out that the utilisation of anLDH precursor gave a high surface area and a good porosityto the final mixed oxide. Due to their excellent properties,LDH compounds have recently gained interest as adsorbents,ionic conductors and catalysts as well as acting as catalystsupports [7, 9–11].

In general, the LDH structure can be derived from a bru-cite structure [Mg(OH)2] in which Al3+ substitutes for a partof Mg2+. This substitution leads to a positive layer charge onthe hydroxide layer, compensated for by interlayer anions oranionic complexes. In LDHs, a broad range of compositionsare possible in the form [M1–x

2+Nx3+ (OH)2]x+ [Ax/n

n–]·mH2O,where M2+ and N3+ are the di- and trivalent cations in theoctahedral position within the hydroxide layers, respectively.An– is an exchangeable interlayer anion. Calcination of theLDH at a certain temperature yields homogeneously mixedoxides and the properties of these oxides can be tailored bythe nature of the cations, compensation anions, calcinationconditions, etc. [7].

In this paper, a series of LDH materials, containingNiMgAl components, was synthesised with an Al/Mg molarratio ranging from 0.11 to 2.0. The structural properties of thesynthesised LDHs and their reduced forms were charac-terised by X-ray diffraction (XRD), BET and temperature-pro-grammed reduction (TPR). In addition, these catalysts weretested both for the MSR reaction and electrolyte poisoningunder conditions simulating the operating parameters of theDIR-MCFC.

2 Experimental

2.1 Preparation of the LDH Precursor, Mixed Oxide and Catalyst

In this study, the LDH precursors were prepared by acoprecipitation method, as reported in the literature [7].Briefly, an aqueous solution containing stoichiometricamounts of Ni, Mg and Al nitrates and a solution of NaOH([OH–]/[Metal] = 2.2) were added, dropwise, into deionisedwater under vigorous stirring at room temperature. The twosolutions were quantitatively and simultaneously added tocarefully maintain the pH of the mixture at pH 10. The result-ing mixture was subsequently aged at 70 °C for 6 h with stir-ring. The precipitate was vacuum filtered, washed thor-oughly with hot deionised water until a pH of 7 was attained,and then dried at 110 °C overnight. Then, the dried LDH pre-cursors were decomposed at 450 °C in a nitrogen flow(40 mL min–1) for 1 h. The mixed oxides were reduced byTPR from room temperature to 650 °C at the rate of10 °C min–1 in a mixture of N2 (40 mL min–1) and H2

(30 mL min–1), and then kept at 650 °C for 1 h. The final cata-lysts were obtained and their activities were tested for MSR.The content of nickel in the final catalyst was fixed at25 wt.-%, for all the catalysts mentioned in this paper.The samples were coded as NiAlMg(x), where NiAlMgand (x) were the Ni–Al–Mg mixed oxide obtained and themolar ratio of Al to Mg, respectively.

2.2 Characterisation of the LDH Precursor, Mixed Oxide andCatalyst

Powder XRD was carried out on a diffraction spectrometer(Mac Science, M18XHF22-SRA) using Cu Ka as the radiationsource. The average size of the crystallites in the nickel phasewas estimated from the line broadening of the correspondingXRD peaks using Scherrers formula.

TPR of the mixed oxide was performed by heating thesample from room temperature to 900 °C at the rate of10 °C min–1 with 9% H2 in N2 as the carrier gas at the totalflow rate of 22 mL min–1. A constant testing amount of 50 mgof each sample was used. Before starting TPR, the samplewas pretreated in situ at 450 °C in a flow of pure N2 toremove adsorbed species from the surface. A moisture trapwas employed in the TPR system.

The BET surface area and pore size distribution of the sam-ples were determined by N2 adsorption at –196 °C using aporosimetry analyser (Micromeritics Corp., ASAP 2010).The samples were pretreated at 200 °C in a vacuum for 2 h.The pore size distribution was obtained from a desorptionisotherm using the BJH method.

2.3 Catalytic Activity Test

The catalytic MSR reactions were conducted in a quartztube reactor under atmospheric pressure at 650 °C accordingto our standard MCFC procedure [12–15]. The quartz reactor

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was equipped with a thermocouple inside a thermowell,positioned at the centre of the catalyst bed. A specially de-signed quartz filter was used inside the reactor to support thecatalyst powder. 0.1 g of 45 to 250 lm catalyst powder wasused for each run. CH4 and H2O at a molar ratio of 1:2.5 werefed into the reactor by mass flow controller and microsyringe,respectively. The catalyst was activated, in situ, at 650 °C for1 h in a flow of a H2 (30 mL min–1) and N2 mixture(40 mL min–1), and then applied for the reaction. The gaseouseffluents were passed through a trap at 0 °C to remove waterand then analysed by an online gas chromatograph (Donam,DS6200), equipped with a 0.25 mL autosampler. Efficientseparations were achieved at 150 °C using a 3 M stainlesssteel column packed with SK-4 carbon, with Ar as the carriergas, at the flow rate of 25 mL min–1. All the components weredetected by a TCD detector at 150 °C and quantitative calcu-lations were done using the calibrated area normalisationmethod.

2.4 Electrolyte Poisoning Test

It is well understood that the performance of the DIR-MCFC is seriously affected by the poisoning of the alkali elec-trolyte on a Ni catalyst. Berger et al. [3, 4] investigated themovement of alkali species toward the catalysts in the DIR-MCFC and proposed that, among the alkali species, lithiumcarbonate was the most poisonous to the Ni catalyst. Ourgroup′s previous studies [12, 13] also support this proposal.Therefore, attention was focused on the poisoning effect ofLi2CO3 and the simulation testing was performed by a stan-dard method, described elsewhere [15]. First, lithium carbo-nate powder (Avondale Laboratories, 99%) was added to anactivated fresh catalyst. Then, the Li-containing catalystswere charged into a reactor and pretreated, in situ, at 650 °Cfor 1 h in a gas flow of H2 (30 mL min–1) and N2

(40 mL min–1). The reaction conditions were the same as

those mentioned in Section 2.3. The amount of Li2CO3 usedwas expressed in wt.-%, based on the Li-free catalyst(100 mg). A preliminary test was conducted with commercialcatalyst (HRC) to determine the appropriate amount ofLi2CO3 to be added. The result is shown in Figure 1. Theactivity of HRC gradually decreased by increasing theamount of Li2CO3 added from 10 to 40 wt.-% and dropped toalmost zero with the addition of 40 wt.-%. The Li-poisoningtest was performed on our Ni–Mg–Al catalysts, using differ-ent amounts of Li2CO3. Al- and Mg-rich samples showed dif-ferent resistances to Li poisoning. The difference becamemore obvious with higher additions of Li2CO3. Some resultsfor Li poisoning by 40 wt.-% Li2CO3 are discussed in Sec-tion 3.4.

3 Results and Discussion

3.1 Characterisation of LDH Precursors

The XRD spectra of the precipitates, dried at 110 °C, areillustrated in Figure 2. The XRD results show the presence ofan LDH structure in all the samples. In the Mg-rich sampleswith Al/Mg = 0.11–0.33, the LDH and Mg(OH)2 phases coex-isted and the intensities of typical Mg(OH)2 peaks wereweakened as the Al/Mg ratio increased. Table 1 summarisesthe compositions and their crystallographic parameters,a = 2d (110) and c = 3d (001), calculated for a hexagonal struc-ture. The lattice parameter, c, was related to the electrostaticattraction between the positive brucite-like sheet and negativeinterlayer with the modification of the strength of the OH–Obond [7]. The parameter a was affected by differences in thecationic radii making up the LDH structure. As the content ofAl increased, both lattice parameters, a and c, decreasedbecause of the enhanced strong interaction between the twolayers and also the gradual substitution of Mg2+ (0.66 Å) withAl3+ (0.51 Å).

Fig. 1 Methane conversion of the commercial catalyst (HRC), as a functionof the amount of Li2CO3.

Fig. 2 XRD characterisation of various Ni–Mg–Al precursors before calci-nation.

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3.2 Pore Size Distribution of Ni–Mg–Al Ternary Mixed Oxide

The average pore diameter andpore volume of different Ni–Mg–Al catalysts are shown in Figure 3.When the Al/Mg molar ratioincreased from 0.11 to 0.33, theaverage pore diameter dramati-cally decreased from 20 to 6 nm.However, further increases in themolar ratio had little effect on the

average pore diameter. In contrast, the pore volumes fluctu-ated from 0.361 to 0.558 cm2 g–1, showing a very weak depen-dence on the Al/Mg molar ratio.

Figure 4 shows the N2 adsorption–desorption isothermand pore size distribution of NiMgAl (Mg/Al = 1.0). A fairlysharp and narrow pore size distribution was observed for thissample. Similar N2 isotherm patterns were also detected forother samples with different Mg/Al ratios. The hysteresisloop started at a high relative pressure, showing the charac-teristic of mesoporous materials. Our data indicated that aquite uniform mesoporous Ni–Mg–Al ternary mixed oxidewith a pore size of about 6 nm could be synthesised by thetemplate-free method, starting from an LDH precursor.

3.3 TPR Study on Mixed Oxides

The TPR patterns of Ni–Mg–Al ternary mixed oxides, pre-pared by thermal treatment of their LDH precursors, areshown in Figure 5. Literature studies on NiO–MgO [16] andNiO–MgO–Al2O3 [11, 17] suggest that the TPR peak ataround 400 °C was due to the reduction of NiO on the sur-face, which had not interacted with the support. The peak ataround 700 °C was due to NiO on the mixed oxide of Mg andAl, while the reduction peak above 850 °C was assigned to

Table 1 Composition and characterisation of precursors dried at 110 °C.

Precursor (Al/Mg ratio) Molar composition Identified structure a / Åa) c / Åb)

Ni Mg Al Al/Mg (Mg/Al)

NiMgAl (2.0) 31.6 22.8 45.6 2.0 (0.5) LDH 3.0122 23.0238NiMgAl (1.5) 30.5 27.8 41.7 1.5 (0.67) 3.0174 22.9062NiMgAl (1.0) 28.6 35.7 35.7 1.0 (1) 3.0256 23.0646NiMgAl (0.33) 23.8 57.1 19.1 0.33 (3) LDH + Mg(OH)2 3.0708 23.9790NiMgAl (0.20) 22.2 64.8 13.0 0.20 (5) 3.0820 24.0657NiMgAl (0.14) 21.6 68.6 9.8 0.14 (7) 3.0848 24.6447NiMgAl (0.11) 20.9 71.2 7.9 0.11 (9) 3.0886 24.6447

a) Lattice parameter a = 2d (110).b) Lattice parameter c = 3d (001).

Fig. 3 Average pore diameter and pore volume of various Ni–Mg–Al cat-alysts, as a function of the Al/Mg molar ratio.

Fig. 4 Isotherm and pore size distribution of the NiMgAl(1) catalyst.Fig. 5 Reduction profiles of various Ni–Mg–Al oxides with differentAl/Mg molar ratios, after calcination.

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NiO in the MgO matrices. In Figure 5, the first TPR peak, at atemperature of less than 500 °C, should correspond to thereduction of surface nickel species that do not, or onlyweakly, interact with the support. This peak was the commoncharacteristic between the mixed oxides with Al/Mg = 0.11–0.33, which were obtained by the thermal decomposition oftheir precursors, containing both the LDH and brucite-struc-tured Mg(OH)2, as illustrated in Figure 2. This fact impliesthat the coexistence of different precursor structures signifi-cantly influenced the properties of the mixed oxide materials.Meanwhile, the properties of the final catalysts, after reduc-tion, were also altered.

Besides the low temperature reduction peak, anotherbroad reduction peak from 500 to 800 °C was also observedon those samples. It is well known that NiO and MgO aremutually miscible to form a solid solution at a high tempera-ture, which results in a difficult reduction of nickel oxide inthe solid solution, giving rise to a very broad TPR peak at thehigh temperature range. For the Mg-rich mixed oxide sam-ples with Al/Mg = 0.11–0.33 reported here, a pure MgOphase could be formed because of the existence of a Mg(OH)2

phase in the precursor state, as discussed above. Therefore,NiMg–O solid solution could be formed during the thermaldecomposition step, which consequently required a higherreduction temperature and resulted in a broad TPR peak.

When the Al3+ cation was incorporated into the frameworkof Mg–O, it substituted for the Mg2+ cation and, thus, inhib-ited the crystal growth of the Ni–Mg–O solid solution, result-ing in a relatively easy reduction of a Ni site [11]. From thispoint of view, it can be suggested that the second TPR peak,ranging from 500 to 800 °C, is related to the reduction of Ni2+

interacting with the Al3+-substituted MgO (Mg–Al–O) sup-port. As the Al/Mg ratio increased from 1 to 2, the maximumreduction temperature was slightly shifted to a lower value,indicating the increase in the reducibility of Ni2+ in the mixedoxide. When the Al/Mg molar ratio increased from 0.11 to1.5, the number of Ni reduction peaks in the TPR spectradecreased from 3 to 1. This means that the Ni became morestable and homogeneous in the mixed oxides.

Our catalysts showed good activity for the MSR reaction,when they were reduced at 650 °C. However, according tothe TPR results, 650 °C was insufficient for the reduction ofNi in the mixed oxide. This may suggest that the active cata-lyst is partially reduced Ni in the ternary mixed oxide. How-ever, at the moment, there is not enough data to prove this.The relationship between the degree of reduction and theactivity of the catalyst is still not clear.

3.4 Catalytic Activity of the Methane Steam Reforming Reactionwith/without Li2CO3

The relationship between the average nickel crystallitesize, surface area and MSR activity of various Ni–Mg–Alcatalysts is summarised in Figure 6. It is obvious that all theNi–Mg–Al catalysts showed higher MSR activity than thecommercial catalyst (HRC; methane conversion = 71.6%). It is

known that, generally, the activity of a catalyst is very closelyrelated to the dispersion of the active metal (crystallite size),surface area, etc. As the Al/Mg ratio increased up to 1.5, theBET surface area of the material also increased, while theaverage Ni crystallite size decreased, indicating a high dis-persion of nickel species in the mixed oxide, which conse-quently resulted in increased MSR activity. However, furtherincreases in the amount of Al added caused the nickel size inNiMgAl (2.0) to reversibly increase, resulting in a slightdecrease in the nickel dispersion and a lower MSR activity.The detailed reason is not understood yet. The MSR activitywas found to be in the order: NiMgAl (1.5) > NiMgAl (2.0) ≈NiMgAl (1.0) > NiMgAl (0.33) > NiMgAl (0.20) >NiMgAl (0.14) > NiMgAl (0.11).

Figure 7 shows the relative activities of the catalysts with40 wt.-% Li2CO3 poisoning, compared with the performancesof the catalysts without Li poisoning, Figure 6. The catalysts

Fig. 6 Methane conversion, surface area and Ni crystallite size, as a func-tion of the Al/Mg molar ratio.

Fig. 7 Relative activity (with/without Li2CO3) of the Ni–Mg–Al catalyst, asa function of the Al/Mg molar ratio.

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prepared were damaged by Li poisoning and the Li-poison-ing resistance increased with the Al content. In particular, inthe case of Al-rich catalysts (Al/Mg ≥ 1), the catalysts kepttheir catalytic activity even after Li poisoning and exhibited agood activity for the MSR, as shown in Figure 7. The aboveresults suggest that the activity of the catalyst was signifi-cantly influenced by the alumina content.

Further characterisation was conducted to better under-stand the deactivation mechanism. Figure 8 shows XRD spec-tra of both used and fresh catalysts. There were obvious dif-ferences between the two types. The c-LiAlO2 phase wasclearly detected over used Al-rich catalysts, as shown in spec-trum (b) of NiMgAl (1.5), while it was not evident in usedMg-rich catalyst, as indicated in spectrum (b) of NiMgAl(0.11). This suggests that aluminium oxide absorbs the Li spe-

cies to form lithium aluminate, which protects the activemetallic Ni site from being covered by Li species, and conse-quently maintains the high activity of Al-rich catalysts. Forthe Mg-rich catalyst, the Li species could not be captured,which resulted in the accumulation of Li species around Nisites. As a result, the Ni sites were covered by Li species andthe catalyst lost its activity. The mechanism is illustrated inFigure 9.

Berger et al. [3, 4] performed similar poisoning experi-ments and found satisfactory activity by using a coprecipi-tated Ni/Al2O3 catalyst with a high alumina content. How-ever, the addition of MgO to the Ni/Al2O3 damaged theperformance of the catalyst. These results are in good agree-ment with the results obtained here and can also be explainedby the mechanism discussed above.

It is quite interesting that NiMgAl (1.0) almost maintainedits original activity and showed the same activity as NiM-gAl (2.0). NiMgAl (1.0) contained the same amounts of Mgand Al; therefore, it is reasonable to say that it is necessary tohave Al ≥ Mg so that the Ni–Mg–Al catalyst prepared fromthe LDH precursor has both a high MSR activity and a strongresistance to Li poisoning.

As discussed above, the deactivation caused by Li speciescould be suppressed by adding Al. However, the method andthe amount of Al are very important. Based on the results dis-cussed here, Ni–Mg–Al materials, prepared from the thermaldecomposition of the LDH precursor with an appropriateAl/Mg molar ratio, should be a suitable group of catalysts forthe application in the DIR-MCFC system with a strongLi-resistance capability. The best combination, from thesesamples, was that with Al/Mg = 1.5. The excellent perfor-mance of NiMgAl (1.5) could be ascribed to the synergisticeffect of the homogeneous interdispersion of Al and Mg inthe oxide with this optimum ratio and the formation of verysmall and stable nickel crystallites.

4 Conclusion

Ni–Mg–Al catalysts, prepared by the thermal decomposi-tion of LDH precursors with Al/Mg molar ratios from 0.11 to2.0, were excellent DIR-MCFC catalysts for the MSR reaction.All the Ni–Mg–Al catalysts showed a higher activity than thecommercial catalyst, HRC. The activities were closely relatedto the nickel crystallite size and the surface area of the cata-lyst. In particular, the Al/Mg combination at a ratio of 1.5was the most ideal for controlling the electrostatic interactionbetween the layers, which showed the highest surface area,the smallest crystallite size and the best activity. Al-rich cata-lysts (Al/Mg ≥ 1) showed a strong resistance to Li poisoning,whereas the Mg-rich catalysts had only a very weak resis-tance. This was due to the fact that the Li species wereabsorbed by aluminium oxide to form lithium aluminate,which consequently protected the active metallic Ni sitesfrom being covered. The presence of equal amounts of Mgand Al in the catalyst was the minimum condition required

Fig. 8 Comparison of the XRD spectra of the catalysts before (a) and after(b) reaction (�: c-LiAlO2, �: MgO, �: Ni)

Fig. 9 Mechanism of poisoning by Li2CO3 over Mg-rich (a) and Al-rich (b) catalysts.

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for the Ni–Mg–Al catalyst, obtained from the LDH precursor,to show both good activity and a strong resistance to Li poi-soning.

Acknowledgements

This work was financially supported by the Korea ElectricPower Research Institute, through the Korea Institute ofScience and Technology, and by the MOST/KOSEF (grant no.R11-2002-102-00000-0) ERC Program.

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Structural Characteristics of (NiMgAl)Ox Prepared from a Layered Double Hydroxide Precursor and its Application in Direct Internal Reforming Molten Carbonate Fuel Cells