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Tsukasa Nakasone, [Ya Xu](https://orcid.org/0000-0001-9067-5244), Ryuji Tamura

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[Metallic Honeycomb Catalysts for Methane Steam Reforming: Effect of the Bimetallic Surface Coating on Catalytic Properties](https://mdr.nims.go.jp/datasets/b78df18c-c307-420f-b4f7-e8c2a4f39db7)

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Metallic Honeycomb Catalysts for Methane Steam Reforming: Effect of the Bimetallic Surface Coating on Catalytic PropertiesMetallic Honeycomb Catalysts for Methane Steam Reforming: Effect of theBimetallic Surface Coating on Catalytic PropertiesTsukasa Nakasone1,2,+1, Ya Xu2,+2 and Ryuji Tamura11Department of Materials Science and Technology, Tokyo University of Science, Tokyo 125-8585, Japan2Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, Tsukuba 305-0003, JapanMetallic honeycomb catalysts are promising candidates for fuel cell and small-scale onsite hydrogen production applications. In this study,high-cell-density Ni honeycomb catalysts coated with a series of bimetallic surface layers were synthesized. Their catalytic performance for CH4steam reforming was investigated under a low steam-to-carbon ratio of 1.36 and a gas hourly space velocity of 6400 h¹1 in the temperature rangeof 400–700°C. The catalysts coated with Ni–Mg and Ni–Zr showed excellent catalytic performance, reaching a high CH4 conversion and COselectivity close to the equilibrium values within the test temperature range. The enhanced catalytic performance of the Ni–Mg and Ni–Zrcoatings was attributed to the formation of oxide-supported fine Ni particles. In contrast, the catalysts coated with Ni–Fe and Ni–Sn exhibited anextremely low activity, which was lower than that of the catalyst coated with only Ni. The low activity of the Ni–Fe and Ni–Sn coatings issupposed to be due to the formation of aggregated Ni3Fe, Ni3Sn, and Ni3Sn2 phases. [doi:10.2320/matertrans.MT-MH2022005](Received April 3, 2023; Accepted April 24, 2023; Published May 19, 2023)Keywords: metallic honeycomb catalyst, nickel-based bimetallic coating, methane steam reforming, hydrogen production1. IntroductionIt is essential to develop efficient, low-cost, and small-scalehydrogen production systems for fuel cell and small-scaleonsite hydrogen production applications.1–3) High-perform-ance and low-cost catalysts are required for hydrogenproduction systems. Metallic honeycomb catalysts are morepromising than conventional pellet-type catalysts becausethey have several advantages, such as a low pressure dropper catalyst volume, compactness, and high heat and masstransfer abilities.4–9)In recent years, metallic honeycomb catalysts have beenstudied for hydrogen production reactions. Fukuhara et al.fabricated a structured Ni/Al2O3 catalyst by combining a sol-gel method to form an Al2O3 layer and electroless platingto deposit a Ni component on a stainless-steel honeycombfin. The prepared catalyst exhibited a high activity in CH4steam reforming (MSR).10,11) We also developed a metallichoneycomb catalyst using pure Ni thin foil with a high celldensity of 900 cpsi (number of cells per square inch) forMSR.12) The prepared catalyst exhibited a high activity andan excellent carbon deposition resistance for MSR at 800°Cfor over 8000 h at a low steam-to-carbon ratio (S/C = 1.34)and gas hourly space velocity (GHSV = 335 h¹1). Theactivity of this honeycomb catalyst was further improvedby increasing the cell density of the honeycomb to 2300 cpsiand performing steam pretreatment prior to the reaction.13,14)Furthermore, to enhance the catalytic performance of the Nihoneycomb catalyst for MSR at low temperatures (<800°C),a Ni–Re bimetallic layer was synthesized directly on thesurfaces of Ni honeycomb channels without the use of anoxide support, which maintains the high thermal conductivityof the honeycomb.15) The activity of the honeycomb catalystbelow 700°C was effectively improved by the formation ofNi–Re bimetallic surface layer, which indicates that thecatalytic activity of metallic honeycombs can be improvedby controlling the composition and microstructure of thesurface coating layer. In addition, intermetallic compoundsare known to exhibit different catalytic properties from thoseof component pure metals;16,17) for example, Ni3Al andNi3(Si,Ti) in the form of foils18,19) and Ni3Fe and Ni3Sn inthe form of nanoparticles20,21) both showed high activity formethanol decomposition. The aim of this study was tosynthesize various Ni–X (X = Zr, Mg, Sn, or Fe) bimetallicor intermetallic surface layers on Ni metallic honeycombsubstrates and to examine their catalytic activity for MSR todevelop low-cost, high-performance metallic honeycombcatalysts.2. Experimental Procedure2.1 Catalyst preparationThe Ni honeycomb substrate with a cell density of 2300cpsi was fabricated using flat and corrugated thin Ni foilswith a thickness of 30 µm. The flat Ni foils were fabricated byforging, hot-rolling, and cold-rolling commercially availablepure Ni ingots. The corrugated files were shaped in anapproximate sinusoidal waveform (height of 0.4mm andspacing of 1mm). The honeycomb substrate was cylindrical,with a diameter and height of 8 and 10mm, respectively(Fig. 1), and it weighed approximately 0.6 g. The fabricatedhoneycomb was degreased with acetone, activated withdiluted nitric acid (10 vol% HNO3 aqueous solution), andcoated with a surface layer using a method similar to theconventional sequential impregnation method for preparingsupported catalysts with two sequential solutions, A and B.Solution A was a Ni(NO3)2·6H2O (FUJIFILM Wako PureChem. Co.) aqueous solution that consisted of 5mass% Nirelative to the weight of the honeycomb substrate. Solution Bwas an aqueous solution consisting of one of the following:ZrO(NO3)2·2H2O, Mg(NO3)2·6H2O, Fe(NO3)2·9H2O, orSnCl2·2H2O (FUJIFILM Wako Pure Chem. Co.), each ofwhich contained a third (molar ratio) of the amount of Ni+1Graduate Student, Tokyo University of Science. Present address: NiihamaNickel Refinery, Sumitomo Metal Mining Co. Ltd., Niihama 792-8555,Japan+2Corresponding author, E-mail: XU.Ya@nims.go.jpMaterials Transactions, Vol. 64, No. 10 (2023) pp. 2410 to 2416Special Issue on Metallurgy for Advanced Catalytic Materials©2023 The Japan Institute of Metals and Materialshttps://doi.org/10.2320/matertrans.MT-MH2022005in solution A as a second element, thereby obtaining surfacelayer compositions (by molar ratio) of Ni3Mg, Ni3Zr, Ni3Fe,or Ni3Sn, respectively. The surface layer was formed byimpregnating the honeycomb with solution A and drying at120°C for 12 h, followed by impregnation with solution B,drying in the same manner, and calcination at 500°C for 4 hin air. Hereafter, the prepared samples are denoted as Ni–Zr,Ni–Mg, Ni–Fe, and Ni–Sn, and the sample without a secondelement (impregnated with solution A only) is denoted asNi–0.2.2 Catalytic reaction testMSR tests were conducted on the fabricated Ni honey-comb samples using a fixed-bed reactor at ambient pressure.Prior to the catalytic reaction test, a hydrogen reductiontreatment was performed at 430°C for 1 h in a flow gasmixture of H2 and N2 at flow rates of 30 and 5mLmin¹1,respectively. All reported gas volumes in this studycorrespond to the values at a standard temperature andpressure (STP, 0°C, 1 atm). After the hydrogen reduction, H2was purged by an N2 flow gas. Catalytic reaction tests wereconducted at gas flow rates of 10, 13.6, and 30mLmin¹1 forCH4, H2O (steam), and N2, respectively, and with an S/C of1.36 and GHSV of 6400 h¹1. Two types of catalytic reactiontests were conducted, namely a stepwise heating test from400–700°C that was increased in 100°C increments, whichwere each maintained for 5 h, and an isothermal test at700°C for 20 h. Data was collected using two on-line gaschromatographs (GC; GL Science, GC323). One wasequipped with a column packed with zeolite (MolecularSieve 13X) to detect H2, N2, CH4, and CO, whereas the otherwas equipped with a column packed with a porous polymersorbent (Porapak Q) to detect CO2 and H2O. The flow rate ofeach component during the reaction was determined based onthe flow rate of the N2 gas and the composition of the outputgases analyzed by the GCs. The CH4 conversion (XCH4),hydrogen yield (STY-H2), CO selectivity (SCO), and CO2selectivity (SCO2) were calculated using eqs. (1)–(4):XCH4¼ FCH4=F0CH4� 100 ð1ÞSTY �H2 ¼ FH2=A ð2ÞSCO ¼ FCO=ðFCO þ FCO2Þ � 100 ð3ÞSCO2¼ FCO2=ðFCO þ FCO2Þ � 100 ð4Þwhere F0CH4 is the flow rate of the supplied CH4 (mol s¹1),and FCH4, FH2, FCO, and FCO2 are the outlet flow rates(mol s¹1) of CH4, H2, CO, and CO2, respectively. Thehydrogen yield (eq. (2)) is defined as the H2 flow rate perunit time (molm¹2 s¹1) and is normalized by the geometricsurface area of the honeycomb substrate A (m2).2.3 CharacterizationX-ray diffraction (XRD) measurements were performed toidentify the phase of the coating layer after the calcinationand hydrogen reduction treatment (Rigaku; MiniFlex 600,Cu K¡ line, 40 kV, 15mA). A temperature-programmedreduction (TPR) measurement was performed to determinethe reduction temperature at a heating rate of 10°Cmin¹1from 25 to 1000°C in a stream (50mLmin¹1) of H2 dilutedwith Ar (4 vol% H2).The surface morphologies of the samples were observedusing scanning electron microscopy (SEM; JEOL; JSM-7000F, acceleration voltage: 20 kV). The Brunauer–Emmett–Teller (BET) specific surface area of each sample wasmeasured via N2 adsorption using a specific-surface-area-measuring device (Micromeritics, ASAP2020).Transmission electron microscopy (TEM) and scanningtransmission electron microscopy (STEM) were performedusing a field-emission transmission electron microscopeequipped with an energy-dispersive spectrometry (EDS)analysis system (JEOL, JEM-2100F) at an accelerationvoltage of 200 kV. EDS elemental mapping was performedin the STEM mode.3. Results and Discussions3.1 Catalyst propertiesFigure 2 shows the CH4 conversion of the honeycombcatalysts coated with Ni–Mg, Ni–Zr, Ni–Fe, Ni–Sn, and Ni–0during the stepwise heating test. For comparison, the results8 mm10 mmFig. 1 The fabricated Ni honeycomb structure with a cell density of 2300cpsi.0 5 10 15 20020406080100 Ni-Mg Ni-Zr Ni-Fe Ni-Sn Ni-0 Uncoated Ni honeycomb ---  EquilibriumCH4 conversion (%)Time on stream (h)400℃ 500℃ 600℃ 700℃Fig. 2 The CH4 steam reforming catalytic performance of the Ni honey-comb catalysts coated with various bimetallic surface layers during thestepwise heating test. For comparison, the results of the uncoated Nihoneycomb substrate are also shown.Metallic Honeycomb Catalysts for Methane Steam Reforming 2411of the Ni honeycomb substrate without coating are alsoshown in Fig. 2. The catalysts coated with Ni–Mg and Ni–Zrshowed much higher CH4 conversions that were very closeto the equilibrium values at all test temperatures, whereasthose coated with Ni–Fe and Ni–Sn showed much lower CH4conversions at all test temperatures. The CH4 conversion ofthe sample coated with only Ni (Ni–0) was slightly higherthan those of Ni–Fe and Ni–Sn but much lower than thoseof Ni–Mg and Ni–Zr. In addition, in contrast to the relativelystable CH4 conversion values of the Ni–Mg, Ni–Zr, Ni–Fe,and Ni–Sn samples during the keeping period (5 h) at eachtemperature, the CH4 conversion of the Ni–0 samplegradually decreased with an increase in the keeping time attemperatures below 600°C (Fig. 2). This is likely due tothe aggregation of Ni particles easily occurred in the Ni–0sample. The Ni honeycomb substrate without coating showedno activity below 500°C; at 600°C, however, it exhibited arelatively high initial CH4 conversion, but this activitydecreased quickly with the increasing holding time.Furthermore, at 700°C, it showed a slightly higher CH4conversion than the Ni-coated sample (Ni–0). These resultsindicate that the bare Ni honeycomb can exhibit relativelyhigh activity at high temperatures, which is agreement withour previous results.13)Figure 3 shows the CH4 conversion of the honeycombcatalysts as a function of time on the stream during theisothermal tests at 700°C. The Ni–Mg and Ni–Zr samplesexhibited an initial CH4 conversion of approximately 90%,which was close to the equilibrium conversion of 92%. Theconversion value did not significantly decrease during thewhole period (20 h) at 700°C. In contrast, the Ni–Fe, Ni–Sn,and Ni–0 samples exhibited much lower initial CH4conversions. Ni–Fe initially showed a CH4 conversion of25%, which gradually decreased over time and stabilized at7% after 6 h. Ni–Sn retained its initial CH4 conversion of7% throughout the test. The Ni–0 sample showed an initialCH4 conversion of 13%, which slightly increased over time,reaching approximately 20% after 20 h. On the other hand,the Ni honeycomb substrate without coating showed arestively stable CH4 conversion around 30% throughout thetest at 700°C, which agreed with our previous report, and washigher than those of Ni–0, Ni–Fe, and Ni–Sn samples.The CO selectivity in the isothermal test is shown as afunction of time on the stream in Fig. 4. The Ni–Mg and Ni–Zr samples exhibited high and stable CO selectivity above80%, which was close to the equilibrium value of 87% duringthe test. This indicated that the Ni–Mg and Ni–Zr sampleshad a high selectivity for MSR. In contrast, the Ni–Fe, Ni–Sn, and Ni–0 samples exhibited much lower CO selectivitythan Ni–Mg and Ni–Zr. The Ni–Fe sample showed an initialCO selectivity of 0%, which rapidly increased to approx-imately 50%, whereas the Ni–Sn sample showed exhibited aninitial CO selectivity of approximately 60% before rapidlydecreasing to 0% after 4 h. The Ni–0 sample exhibited a COselectivity of approximately 45%, which remained nearlyunchanged during the test. By contrast, the uncoated Nihoneycomb substrate showed a CO selectivity of approx-imately 55%, which was slightly higher than that of the Ni–0sample. These results indicate that the Ni–Mg and Ni–Zrcatalysts have a high activity, stability, and selectivity forMSR. In contrast, the Ni–Fe and Ni–Sn samples exhibited alow activity and selectivity for MSR.3.2 Characterization3.2.1 X-ray diffraction and temperature-programmedreduction resultsFigure 5 shows the XRD profiles of the Ni–Mg, Ni–Zr,Ni–Fe, Ni–Sn, and Ni–0 surface layers in their calcinationstates (Fig. 5(a)) and after hydrogen reduction at 430°C for1 h (Fig. 5(b)). For the calcined samples, all the diffractionpeaks were identified as oxide phases: (MgNiO2 + NiO) inthe Ni–Mg sample, (ZrO2 + NiO) in the Ni–Zr sample,(Fe3O4 + NiO) in the Ni–Fe sample, (SnO2 + NiO) in theNi–Sn sample, and NiO in the Ni–0 sample. After hydrogenreduction, peaks from the metallic Ni phase were identified,whereas peaks from MgNiO2 and ZrO2 were retained in theNi–Mg and Ni–Zr samples, indicating that only NiO wasreduced to metallic Ni. MgNiO2 and ZrO2 were not reducedby the reduction treatment. In contrast, peaks from the Ni3Fe0 5 10 15 20020406080100CH4 Conversion (%)Time on Stream (h) Ni-Mg Ni-Zr Ni-Fe Ni-Sn Ni-0 Uncoated Ni honeycomb EquilibriumFig. 3 The CH4 conversion of the Ni honeycomb catalysts coated withvarious bimetallic surface layers during the isothermal test at 700°C. Forcomparison, the results of the uncoated Ni honeycomb substrate are alsoshown.0 5 10 15 20020406080100CO Selectivity (%)Time on Stream (h) Ni-Mg Ni-Zr Ni-Fe Ni-Sn Ni-0 Uncoated Ni honeycomb EquilibriumFig. 4 The CO selectivity of the Ni honeycomb catalysts coated withvarious bimetallic surface layers and the uncoated Ni honeycombsubstrate during the isothermal test at 700°C.T. Nakasone, Y. Xu and R. Tamura2412phase were identified in the Ni–Fe sample, and no peaksfrom the Fe and Ni oxides were observed, suggesting thatboth NiO and Fe3O4 were reduced to metallic Ni and Feand further combined to form Ni3Fe during the hydrogenreduction treatment. Peaks from the Ni3Sn and Ni3Sn2phases were identified in the Ni–Sn sample, together withsome weak peaks from SnO2. This suggests that NiO andpart of SnO2 were reduced to metallic Ni and Sn and thatthey were combined further to form the Ni3Sn and Ni3Sn2phases. In addition, only metallic Ni phase was identifiedin the Ni–0 sample, indicating that NiO was completelyreduced to metallic Ni by the hydrogen reduction treatmentat 430°C.Figure 6 shows the TPR profiles of the samples in theircalcined states. A reduction peak at approximately 410°Cwas detected in Ni–0 sample, which is supposed tocorrespond to the reduction of NiO.22) A main reduction atapproximately 510°C and two broad peaks at approximately420°C and 460°C were observed in the Ni–Sn sample. Thepeak at approximately 510°C is supposed to correspond tothe reduction of SnO223,24) and the broad peaks correspondto reduced NiO and SnO2.22–24) A reduction peak atapproximately 385°C was detected in the Ni–Fe sample,which is supposed to correspond to the reduction of Fe3O4and NiO. Noting that the temperature was slightly lower thanthat of the Ni–0 sample, the combination of Ni and Fe oxidesmay be reduced more easily than that of Ni oxide. A mainreduction at 410–450°C was observed in both Ni–Mg andNi–Zr, which is supposed to correspond to the reduction ofNiO. These TPR results are consistent with the XRD results,suggesting that NiO, Fe3O4, and a part of SnO2 were reducedby the reduction treatment at 430°C.3.2.2 Brunauer­Emmett­Teller specific surface areaTable 1 shows the BET specific surface areas of thesamples after they underwent reduction treatment, a stepwiseheating test, and an isothermal test at 700°C. For the samplesafter the hydrogen reduction, the BET specific surface areasof Ni–Mg and Ni–Zr were 2.24 and 1.12m2 g¹1, respectively,which were significantly larger than those of Ni–Fe, Ni–Sn,and Ni–0 (0.57, 0.32, and 0.38m2 g¹1, respectively). Afterthe stepwise heating and isothermal tests, the BET specificsurface areas of all samples significantly decreased. However,the BET specific surface areas of the Ni–Mg and Ni–Zrsamples remained larger than those of the other samples.Ni-MgNi-ZrNi-FeNi-SnNi(a)Intensity (a.u.)2 theta (deg.)20 30 40 50 60 70 8020 30 40 50 60 70 80Intensity (a.u.)2 theta (deg.)Ni-MgNi-ZrNi-FeNi-SnNi-0(b)＊＊＊ ＊ ＊＊MgNiO2＊＊＊ ＊＊MgNiO2＊∆∆ ∆∆ Ni∆ ∆ ∆○○ ○ ○○ ZrO2∆∆∆○○ ○○ ZrO2●●●●● NiO○●●●◊◊◊ Fe3O4●●●●●□●●●□ □□ SnO2×××× Ni3Fe+++ + +↓↓↓↓+ Ni3Sn↓ Ni3Sn2Fig. 5 X-ray diffraction profiles of the Ni–Mg, Ni–Zr, Ni–Fe, Ni–Sn, andNi–0 surface layers after (a) calcination at 500°C for 4 h in air and (b) H2reduction at 430°C for 1 h in a mixture of H2 and N2 at flow rates of 30and 5mLmin¹1, respectively.0 100 200 300 400 500 600 700 800 900 1000Intensity (a.u.)Temperature (℃) Ni-Mg Ni-Zr Ni-Fe Ni-Sn Ni-0Fig. 6 Temperature-programmed reduction profiles of the Ni–Mg, Ni–Zr,Ni–Fe, Ni–Sn, and Ni–0 surface layers after calcination at 500°C for4 h. The measurements were performed from room temperature to 1000°Cat a heating rate of 10°Cmin¹1 and an H2/Ar flow (4 vol% H2) of50mLmin¹1.Table 1 Brunauer–Emmett–Teller specific surface areas (m2 g¹1) of honey-comb catalysts coated with various bimetallic surface layers.Metallic Honeycomb Catalysts for Methane Steam Reforming 24133.2.3 Scanning and transmission electron microscopyanalysesThe surface microstructures of the samples after hydrogenreduction, stepwise testing, and isothermal testing wereobserved using SEM (Fig. 7). In the reduced samples, a largenumber of extremely fine particles with sizes less than a fewdozen nanometers were observed in the Ni–Mg sample. Incontrast, fine particles with sizes ranging from several dozensto several hundreds of nanometers were observed in theother samples, that is, Ni–Zr, Ni–Fe, Ni–Sn, and Ni–0. In thecase of Ni–Zr, fine particles were observed to be supportedon some lumps. Significant particle agglomeration wasobserved in Ni–Fe, Ni–Sn, and Ni–0 after the stepwise andisothermal tests. In contrast, the agglomeration of particleswas relatively minor in Ni–Zr and Ni–Mg after both thestepwise and isothermal tests.Figure 8 shows the TEM image (Fig. 8(a)) and STEM-EDS element mapping (Fig. 8(b)) of the Ni–Mg surface layerafter hydrogen reduction. Fine Ni particles with a size ofa few dozen nanometers or less were confirmed to bedistributed homogeneously with Mg and O. This is likelydue to the Ni and NiMgO2 phases mixing, as identified by theXRD measurements (Fig. 5).Figure 9 shows the TEM image (Fig. 9(a)) and STEM-EDS element mapping (Fig. 9(b)) of the Ni–Zr surface layerafter hydrogen reduction. Fine Ni particles that had diametersof 100–200 nm were distributed in a clump consisting ofZr and O. This clump was assumed to be the ZrO2 phase,considering the XRD measurement results (Fig. 5).3.3 Effect of surface coating on catalytic performanceThese results revealed that the surface coating compositionsignificantly affected the MSR catalytic performance of themetallic honeycomb catalyst. The Ni–Mg and Ni–Zr coatingssignificantly improved the catalytic activity and selectivityfor MSR, whereas the Ni–Fe and Ni–Sn coatings decreasedthe MSR selectivity with the Ni–0 coating (Figs. 2–4). Thehigh catalytic activity of Ni–Zr and Ni–Mg is attributed tothe microstructure generated during the hydrogen reductiontreatment, that is, fine Ni particles supported on the metaloxides NiMgO2 or ZrO2, which showed less aggregation ofFig. 7 Scanning electron microscopy secondary electron images of the surface of the Ni honeycomb catalysts coated with Ni–Mg, Ni–Zr,Ni–Fe, Ni–Sn, and Ni–0 after hydrogen reduction at 430°C for 1 h (upper), a stepwise heating test from 400–700°C (middle), and anisothermal test at 700°C for 20 h (lower).NiMg O(a)(b)Fig. 8 (a) A transmission electron microscopy (TEM) image and (b)annular dark-field (ADF)-STEM and elemental mapping images of theNi–Mg surface layer after hydrogen reduction.T. Nakasone, Y. Xu and R. Tamura2414fine Ni particles during the reaction (Figs. 7–9) and retaineda relatively large BET specific surface area (Table 1). TheNiMgO2 and ZrO2 formed during calcination were notreduced during hydrogen reduction. They were supposed tosuppress the aggregation of fine Ni particles during the MSRreaction. In contrast, oxides of Ni, Fe, and Sn formed duringcalcination were reduced to metallic Ni, Fe, and Sn, resultingthe formation of Ni3Fe, Ni3Sn, and Ni3Sn2 intermetalliccompounds that underwent significant aggregation duringthe hydrogen reduction treatment in the Ni–Fe and Ni–Sncoatings (Fig. 7). This caused a decrease in the BET specificsurface area and activity. A similar aggregation of Niparticles occurred during the hydrogen reduction treatmentof the Ni–0 coating, as shown in Fig. 7. Because Ni–0, Ni–Fe, and Ni–Sn showed similar BET specific surface areas, theactivities of Ni–Fe and Ni–Sn were even lower than thatof Ni–0, suggesting that the formation of Ni3Fe and Ni3Sn(Ni3Sn2) did not play a role in enhancing the MSR activity ofthe Ni honeycomb catalyst.The differences in the Ni–Mg, Ni–Zr, Ni–Fe, and Ni–Snmicrostructures after hydrogen reduction can be understoodby considering the Gibbs free energy of the oxide formation(¦G). Figure 10 shows an Ellingham diagram, which showsthe ¦G of MgO, ZrO2, Fe3O4, SnO2, NiO, and H2O as afunction of temperature. It is clear that the ¦G of theformation of MgO and ZrO2 is much lower than that of theformation of Fe3O4, SnO2, NiO, suggesting that MgO andZrO2 can be formed easily. By comparing the ¦G values ofhydrogen and other elements, it is possible to predict whetherthe oxide of the elements can be reduced by hydrogen, thatis, if the absolute value of ¦G is smaller than that of theline indicated by hydrogen, the oxide can be reduced byhydrogen, and if the absolute value is larger, the oxide cannotbe reduced by hydrogen. Thus, the Ellingham diagramindicates clearly that NiO, Fe3O4, and SnO2 can be reducedby hydrogen at 430°C, which was the hydrogen reductiontemperature used in this study, whereas MgO and ZrO2cannot. The Ellingham diagram provides guidelines forselecting a second element to form an active Ni/oxidecoating on metallic honeycomb catalysts.4. ConclusionA bimetallic coating of Ni–Mg, Ni–Zr, Ni–Fe, and Ni–Snon a high-cell-density Ni honeycomb catalyst was preparedby sequential impregnation, followed by calcination andreduction. Their catalytic performance for MSR wasexamined in the temperature range of 400–700°C. Thecatalytic performance was significantly improved by thecoatings of Ni–Mg and Ni–Zr, whereas it was diminished bythe coatings of Ni–Fe and Ni–Sn compared to that of thesample coated with only Ni. The enhanced catalyticperformance of the Ni–Mg and Ni–Zr coatings was attributedto the formation of oxide-supported fine Ni particles, that is,Ni/NiMgO2 and Ni/ZrO2, which suppressed the aggregationof fine Ni particles during the reaction and lead to animproved catalytic performance. In contrast, no such oxide-supported fine Ni particles, but aggregated intermetalliccompounds, Ni3Fe and Ni3Sn (Ni3Sn2), were formed in theNi–Fe and Ni–Sn coatings after hydrogen reduction, resultingin a low catalytic activity.ONiZr(a)(b)Fig. 9 (a) A TEM image and (b) ADF-STEM and elemental mappingimages of the Ni–Zr surface layer after hydrogen reduction.200 300 400 500 600 700 800-1200-1100-1000-900-800-700-600-500-400-300ΔG (kJ/mol)Temperature (℃)2Ni + O2 = 2NiO2H2 + O2 = 2H2OSn + O2 = SnO23/2Fe + O2 = 1/2Fe3O4Zr + O2 = ZrO22Mg + O2 = 2MgOFig. 10 An Ellingham diagram that shows the Gibbs free energy (¦G) ofthe MgO, ZrO2, Fe3O4, SnO2, NiO, and H2O formation as a function ofthe temperature.Metallic Honeycomb Catalysts for Methane Steam Reforming 2415AcknowledgmentsThe authors are grateful to Ms. Isaka and Ms. Nishimiyaat the Electron Microscopy Analysis Station of NIMS fortheir help in preparing the cross-sectional samples usingFIB equipment and TEM analysis. 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