CERAMICSINTERNATIONAL

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http://dx.doi.org/0272-8842/& 20

nCorrespondinE-mail addre

(2014) 14197–14206

Ceramics International 40 www.elsevier.com/locate/ceramint

Synthesis of Ni-alkaline earth metals particles encapsulated by porous SiO2

(NiMO@SiO2) and their catalytic performances on ethanol steam reforming

Sora Kang, Byeong Sub Kwak, Misook Kangn

Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea

Received 15 January 2014; received in revised form 30 May 2014; accepted 2 June 2014Available online 13 June 2014

Abstract

This study examined hydrogen generation by ethanol steam reforming over 30 wt% Ni-alkaline earth metal (Ni1M0.33O2.66@SiO2, M¼Mg,Ca, Sr, and Ba) oxide catalysts encapsulated by 70 wt% porous SiO2 to depress the sintering that can occur between the nickel particles duringthe ethanol steam reforming reaction. Transmission electron microscopy of a mixture of NiMO (alkaline earth metal/Ni molar ratio¼1/3) andSiO2 particles revealed a core@shell shape with cubic NiMO particles, 100–200 nm in size, encapsulated by spherical, porous SiO2 nanoparticles.In contrast, whisker-shaped silica encapsulated the Ni1Sr0.3O2.66@SiO2 and Ni1Ba0.3O2.66@SiO2 samples. The shape of the hydrogen-reducedsamples changed to a nanowire-like shape. The catalytic performance over the hydrogen-reduced samples varied according to the alkaline earthmetal oxide loading. The Ni1Mg0.3O2.66@SiO2 catalyst exhibited significantly higher reforming reactivity than the other catalysts, NiO@SiO2,Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 because of the synergy between the nickel metal and magnesium oxide ions. H2 production wasmaximized to 88% over Ni1Mg0.3O2.66@SiO2 under the following conditions: reaction temperature of 700 1C, CH3CH2OH:H2O of 1:3, andGHSV (gas hourly space velocity) of 4500 h�1. The nickel in the catalyst had absolute catalytic activity but its activity was improved by thepresence of MgO. MgO might provide oxygen to the nickel species, resulting in an increase in the rate of the CO–water gas shift reaction.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Core@shell shape; Alkaline earth metals; NiMO@SiO2; Hydrogen production; Ethanol steam reforming reaction

1. Introduction

The conversion of hydrocarbons and oxygenated hydrocarbonsto hydrogen has attracted considerable attention. The steamreforming of hydrocarbons [1–3] or light alcohols [4–6] has beenstudied extensively because of the good hydrogen productionyield, and that hydrogen can also be extracted from steam.Ethanol has been used for steam reforming according to thereaction, C2H5OHþ3H2O-2CO2þ6H2 (ΔH0

298=174 KJ/mol),because it is relatively harmless to humans and can be transportedand stored easily. Among the many reforming catalysts, nickel-based catalysts supported on Al2O3 [7,8], SiO2 [9,10], andvarious zeolites [11,12] have been examined in most studies ofethanol steam reforming. On the other hand, aggregation of Niparticles [13] or the Ni and Al2O3 supports as a result of rapid

10.1016/j.ceramint.2014.06.00814 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ82 53 810 2363; fax: þ82 53 815 5412.ss: mskang@ynu.ac.kr (M. Kang).

and strong sintering is a serious problem [14]. As a new trend,other metal oxides, such as ZnO [15], CeO2 [16], ZrO2 [17],La2O3 [18], etc., have been added as a promoter or assistant. Thepromoters also play important roles in the steam reforming ofethanol. Some studies reported the assistant ability of alkali earthmetals for the major catalytic species, particularly Mg [19] andCa [20]. Shi et al. [19] examined hydrogen production from thesteam reforming of ethanol over a Ni/MgO–CeO2 catalyst at lowtemperatures. They reported well-dispersed NiO, bulk-like NiOphases and NiO interacting with the support co-exists in Ni/CeO2

and Ni/MgO–CeO2. As other example, Gong et al. [21] reportedethanol steam reforming over Ni/NixMg1LxO, and concluded thatNixMg1�xO produced higher conversion of ethanol and hydrogenyield than the bare support. On the other hand, there have beenfew studies on the role of alkaline earth metals in improving thecatalysis.Therefore, in the present work, a NiMO core composed of

Ni as the active metal and alkaline earth metal oxides, MgO,

S. Kang et al. / Ceramics International 40 (2014) 14197–1420614198

CaO, BaO, and SrO, as promoters were assessed. Silica wasused as a porous shell to avoid sintering from the strongaggregation among nickel particles. The effects of alkalineearth metals on the properties of NiMO@SiO2 for the steamreforming of ethanol were evaluated. The physicochemicalproperties of the catalysts were examined by X-ray diffraction(XRD), transmission electron microscopy (TEM), H2O- andCO-TPD (temperature-programmed desorption), H2-TPR(temperature-programmed reduction), TPO (temperature-pro-grammed oxidation), and X-ray photon spectroscopy (XPS).

2. Experimental

2.1. Preparation of the NiO@SiO2 and Ni1M0.3O2.66@SiO2

catalysts

Core@shell Ni1M0.33O2.66@SiO2 (NiMgO@SiO2, NiCaO@SiO2, NiSrO@SiO2, and NiBaO@SiO2) were synthesizedusing sequential sol–gel and impregnation methods. In thefirst step, the NiO or NiMO cores were prepared as follows:nickel nitrate–Ni(NO)3 � 5H2O and alkaline earth metals chlor-ide–MCl2 � 5H2O (all the reagents used had purities 499.99%,Junsei Chemical, Japan) were used as the Ni and earth metalprecursors (Mg, Ca, Sr and Ba) to prepare the sol mixture,respectively. One and three moles of the earth metal precursorand nickel nitrate were dissolved sequentially in ethanol, andstirred to homogeneity for 2 h. The final solution wasevaporated at 70 1C for 6 h to remove the ethanol solvent.This step was used to generate a colloidal solution, where theNi and alkaline earth metal ions underwent hydrolysis duringevaporation. The oxygen atom in the solvent binds to eachmetal, and NiO or NiMO (maybe Ni1.0M0.33O2.66) is formed inthis step. The resulting precipitates were obtained, washed withdistilled water, and dried at 70 1C for 24 h. The samples werethen treated thermally at 500 1C for 3 h in air to allowcrystallization. In the second step for core@shell particlesynthesis, 150 mL of ethanol was used as the solvent. Thecore and porous silicate (Wako gel C-100, 150–425 μm) weremixed with the solvent at a 3:7 weight% ratio. The mixturewas stirred to homogeneity for 2 h. The final colloidal solutionwas evaporated at 70 1C for 6 h to remove the ethanol solvent.Similar to the first step, the obtained powder samples weretreated thermally at 500 1C for 3 h in air to remove anyimpurities between particles. Finally, five types of core@shell-structured materials were synthesized: NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, andNi1Ba0.3O2.66@SiO2.

2.2. Characterization of the NiO@SiO2 andNi1M0.3O2.66@SiO2

The catalysts (NiO@SiO2 and four types of Ni1.0M0.3

O2.66@SiO2) were examined by powder XRD (model MPDfrom PANalytical) using nickel-filtered CuKα radiation(30.0 kV, 30.0 mA) over the range, 10–901 2θ. The shapesof the NiO@SiO2 and Ni1M0.3O2.66@SiO2 catalysts particleswere observed by high-resolution TEM (HR-TEM, H-7600,

Hitachi, Japan) operated at 120 kV. The Brunauer, Emmett andTeller (BET) specific surface areas of the NiO@SiO2 andNi1M0.3O2.66@SiO2 catalysts were measured from the N2

adsorption/desorption isotherms at 77 K using a Belsorp IIinstrument. A tube was filled with 0.2 g of the samples under aN2 atmosphere and then out-gassed for 2 h at 150 1C prior tothe measurements. After the pre-treatment, the samples wereexposed to liquid nitrogen gas for 2 h. Finally, the sampleswere heated to 650 1C at a programmed heating rate of10 1C min�1. In the α-plot method, the adsorption volume(Vads(p/p0)) normalized to Vads for the reference material isregarded as α(p/p0), which is used as a new x-axis to plot theadsorption isotherms for the samples of interest. The discreteadsorption data for the reference were interpolated numericallyto generate a continuous α-axis. The atomic compositions onthe catalyst surface were obtained from the energy dispersiveX-ray analysis (EDX). The spatial distribution of discreetchemical phases was evaluated using a cold-cathode EDXsystem (HORIBA/EX-250, Japan) with 30 mm2 ultra-drywindows. The samples were placed on double-sided carbonadhesive tabs and coated with carbon (Edwards 306 highvacuum carbon evaporation) before EDX analysis. The ele-mental distribution on the surface of tablets was investigatedusing EDX while surface analysis to characterize the morphol-ogy of tablets was evaluated using SEM magnification. Theaccelerating voltage of the incident electron beam was set at8 kV. This value was selected in order to minimize beamdamage.The adsorption ability of the NiO@SiO2 and Ni1M0.3O2.66

@SiO2 catalysts for CO gases were measured using the H2O-and CO-TPD (temperature programmed desorption) experi-ments in the same manner using a BELCAT (Bel Japan Inc.).Each catalyst (0.05 g) was charged in the quartz reactor of theTPD apparatus. The catalysts were pretreated at 300 1C for 1 hunder flowing He (30 mL min�1) to remove the physisorbedwater and impurities. H2O (saturated (NH4)2SO4 aqueoussolution for 24 h) and CO (5 vol% CO/He) gases were injectedinto the reactor over a 1 h period at a rate of 50 mL min�1 for100 and 50 1C, respectively. The physisorbed H2O and COgases were removed by evacuating the catalyst samples at50 1C for 30 min. The furnace temperature was increased from50 to 700 1C at a rate of 10 1C min�1 under a He flow. Thedesorbed H2O and CO gases were detected using a TCDdetector. H2-TPR for the NiO@SiO2 and Ni1M0.3O2.66@SiO2

catalysts was carried out using the same equipment used in theTPD experiment. Approximately 0.05 g of the catalysts waspre-treated with flowing argon (30 mL min�1) at 300 1C for1 h and then cooled to 50 1C. The analysis was carried outby increasing the catalyst temperature from 200 to 900 1C at5 1C min�1 under H2 (5 vol%)/Ar with a flow rate of50 mL min�1.

2.3. Apparatus and conditions of the ethanol steam reformingreaction

Fig. 1 shows the reactor used for ethanol steam reforming.The catalytic activities were measured over the temperature

S. Kang et al. / Ceramics International 40 (2014) 14197–14206 14199

range, 300–800 1C, at 1 h reaction time intervals at a steam-to-ethanol ratio of 1:3 (mol%) with a GHSV (gas hourly spacevelocity) of 4500 h�1. The NiO@SiO2 or Ni1M0.3O2.66@SiO2

catalysts (0.5 g) was pelletized to a 20–24 mesh and packedwith a small amount of quartz wool to prevent the catalystfrom moving in the fixed-bed quartz reactor, which had beenmounted vertically inside the furnace. All catalysts werereduced in H2/argon (1:10 ratio) at 700 1C for 2 h prior toethanol steam reforming. In this study, the amount of steamwas adjusted by regulating the temperature according to thelaw of partial pressures [22,23]. The vaporized temperatures ofethanol and water were fixed to 30.0 vol%/carrier gases. Theflow rates of ethanol and steam were kept constant at 12.5 and37.5 mL min�1, respectively. Argon gas was used to carry thevaporized mixture to the reactor. The reaction products duringESR were measured by on-line gas chromatography (DonamDS6200, Donam Company, Korea) equipped with a thermalconductivity detector (TCD) and flame ionizing detector (FID).H2, CO, CO2, CH3CHO, C2H5OH, and CH3COOH weredetected using a TCD, whereas a FID was used to detectCH4, C2H4, C2H6, and the other products. The hydrogen

Reaction conditions

• Space velocity : 4500 h-1

• -Catalyst : 0.5g, 20 24 mesh

• Flow rate :-CH3CH2OH 12.5 mL/min-H2O 37.5 mL/min

Fig. 1. Apparatus of the reactor for ethanol steam-reforming.

Inte

nsity

(a. u

.)

2theta/CuKa

20 40 60 80 100

NiOSrSiO2

BaSiO3

20

Fig. 2. XRD patterns of the NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@S(B) after the H2 pre-treatment.

yield (H2%), ethanol conversion (EtOH%), and selectivity ofcarbon-containing products (SC%) are respectively definedasH2%=(moles of hydrogen produced� 100)/(6�moles of

ethanol feed),EtOH%=(moles of ethanol converted� 100)/(moles of

ethanol feed),SC% of product A=(N�moles of A produced 100)/

(2�moles of ethanol converted), where N=the number ofcarbon atoms in product A.

3. Results and discussion

3.1. Characteristics of the five catalysts, NiO@SiO2,Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2,Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2

Fig. 2A and B shows XRD patterns of NiO@SiO2,Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@-SiO2, and Ni1Ba0.3O2.66@SiO2 catalysts before and after thehydrogen pre-treatment. The main XRD peaks at 37.441 (111),43.471 (200), 63.201 (220), 75.371 (311), 79.871 (222), and95.581 2θ (400) were assigned to the cubic crystalline NiO[24] and were observed in all samples in Fig. 2A. CrystallineSiO2 [25] was also observed in all samples. No peaks assignedto the alkaline earth metal oxides were observed overNiO@SiO2, Ni1Mg0.3O2.66@SiO2, and Ni1Ca0.3O2.66@SiO2

catalysts. On the other hand, peaks corresponding to SrSiO3

[26] or BaSiO3 [27] were observed in the case of theNi1Ca0.3O2.66@SiO2 and Ni1Ba0.3O2.66@SiO2 samples. Thenickel oxide (Ni2þ ) had transformed to nickel metal (Ni0) inthe reduced samples after the hydrogen pre-treatment exceptfor Ni1Mg0.3O2.66@SiO2, as shown in Fig. 2B. The diffractionlines of the metallic cubic Ni phases (Fm3m space group) [28]were observed at 44.371 (111), 51.591 (200), 76.081 (220),92.091 (311), and 98.081 2θ (222) in the reduced catalysts.

NiBaSio3

SrSiO3

NiO

40 60 80 100

2theta/CuKa

a) NiO@SiO2

b) Ni1Mg0.3O2.66@SiO2

c) Ni1Ca0.3O2.66@SiO2

d) Ni1Sr0.3O2.66@SiO2

e) Ni1Ba0.3O2.66@SiO2

iO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalysts (A) before and

S. Kang et al. / Ceramics International 40 (2014) 14197–1420614200

On the other hand, NiO peaks in reduced Ni1Mg0.3O2.66@SiO2

were also observed together Ni metal species at 51.591 (200)and 76.081 (220) planes despite hydrogen reduction occurringat 700 1C for 2 h. On the other hand, the silica gradientsof approximately 201 in the reduced all samples were notchanged, and the intensities of the peaks assigned to SrSiO3

and BaSiO3 in reduced Ni1Ca0.3O2.66@SiO2 and Ni1Ba0.3O2.66@SiO2 were similar to those in the un-reduced samples.Generally, the crystallite size decreases with increasing line-broadening. The Scherrer's equation, t¼0.9λ/β cos θ, was usedto estimate the crystallite size, where λ is the wavelength of theincident X-rays, β is the full width at half maximum height inradians, and θ is the diffraction angle. The estimated crystallitesizes of Ni metal for a peak at 44.371 (111) in the reduced

Table 1The atomic compositions on the surface of catalysts obtained from the energydispersive X-ray analysis (EDX).

Catalysts Atomic compositions on the surface of catalysts

Si Ni Metal O

NiO@SiO2 14.15 28.84 – 57.01Ni1Mg0.3O2.66@SiO2 9.49 18.99 3.39 68.12Ni1Ca0.3O2.66@SiO2 12.64 22.02 4.45 60.93Ni1Sr0.3O2.66@SiO2 14.33 12.82 5.28 57.01Ni1Ba0.3O2.66@SiO2 11.58 15.45 14.95 58.02

NiO@SiO2 Ni1Mg0.3O2.66@SiO2 Ni1Ca0.3O2

100 nm

NiO@SiO2 Ni1Mg0.3O2.66@SiO2 Ni1Ca0.3O2

100 nm

100 nm

100 nm

Fig. 3. TEM images of five un-reduced (A) and reduced samples (B), NiO@Ni1Ba0.3O2.66@SiO2.

NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 samples were29.66, 15.05, 26.91, 22.41, and 14.97 nm, respectively. Herethe size was for NiO in Ni1Mg0.3O2.66@SiO2 using the 43.471(200) peak. This means that the incorporated alkaline earthmetals depressed the growth of Ni particles.Table 1 lists the atomic compositions on the catalyst surface

obtained from the energy dispersive X-ray analysis (EDX). Thesevalues are relative values at the exposed portions on the surface.The loaded amounts were different in all samples. When metalswith a larger size and stronger basicity were loaded, the loadingamounts appeared to be higher and the actual metal/Ni atomicratios in Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalysts were increasedin the order of 0.18o0.20o0.41o0.97, respectively. Conse-quently, the Ni amounts exposed on the each catalytic surfacewere different, which will affect the activity of the catalyst, i.e.the activity will be reduced in the same order. This is believed tobe the unique properties of the alkaline earth metals: the higheralkaline metal ions tightly wrapped the acidic Ni ions.The morphology shapes of the five un-reduced and reduced

samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@-SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2, werecharacterized by TEM, as shown in Fig. 3A and B. All scales(bars) of the images are 100 nm. Rectangular-shaped NiOparticles, approximately 100–200 nm in size, were observed inthe NiO@SiO2 particles, and very fine SiO2 particles were

.66@SiO2 Ni1Ba0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2

.66@SiO2 Ni1Ba0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2

100 nm 100 nm 100 nm

100 nm 100 nm 100 nm

SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and

0

300

600

900

0 0.5 1

V a/c

m3 (

STP)

g-1

p/p0

ADS DES

0

100

200

300

0 0.5 1

V a/c

m3 (

STP)

g-1

p/p0

ADS DES

0

100

200

300

0 0.5 1

V a/c

m3 (

STP)

g-1

p/p0

ADS DES

0

100

200

300

0 0.5 1

V a/c

m3 (

STP)

g-1

p/p0

ADS DES

0

100

200

300

0 0.5 1

V a/c

m3 (

STP)

g-1

p/p0

ADS DES

NiO@SiO2 Ni1Mg0.3O2.66@SiO2Ni1Ca0.3O2.66@SiO2

Ni1Ba0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2

Fig. 4. Adsorption–desorption isotherm curves of N2 at 77 K for the reduced samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2,and Ni1Ba0.3O2.66@SiO2.

Table 2Surface areas and mean pore-diameter of the reduced samples, NiO@SiO2,Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, andNi1Ba0.3O2.66@SiO2.

Materials BET surfacearea [m2 g�1]

Total pore volume(p/p0¼0.994) [cm3g�1]

Average porediameter [nm]

NiO@SiO2 655.1 1.05 6.38Ni1Mg0.3O2.66@SiO2 189.9 0.38 8.06Ni1Ca0.3O2.66@SiO2 148.8 0.36 9.76Ni1Sr0.3O2.66@SiO2 168.1 0.36 8.54Ni1Ba0.3O2.66@SiO2 162.6 0.32 7.99

S. Kang et al. / Ceramics International 40 (2014) 14197–14206 14201

noted on the surface. Interestingly, the large NiO cores weresurrounded completely by the nanosized porous SiO2 shell,indicating a core@shell structure. The shapes were similar onNiO@SiO2, Ni1Mg0.3O2.66@SiO2, and Ni1Ca0.3O2.66@SiO2.The core sizes decreased after adding the alkaline earth metals.In contrast, SrSiO3 or BaSiO3 particles were observed as nano-whisker shapes in the Ni1Sr0.3O2.66@SiO2 and Ni1Ba0.3O2.66@SiO2 samples, and they transformed to a nanowireshape after hydrogen reduction, as shown in Fig. 3B. Other-wise, the pores that formed by porous SiO2 shell particles inthe reduced NiO@SiO2, Ni1Mg0.3O2.66@SiO2, andNi1Ca0.3O2.66@SiO2 were smaller and became dense.

Fig. 4 shows the N2 adsorption–desorption isotherm curvesat 77 K for the reduced samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2. All the isotherms belonged to the IV typeaccording to the IUPAC classification [29]. The hysteresisslopes were observed at intermediate and high relativepressures in all samples, indicating the presence of largemesopores. Table 2 lists the change in the mean pore diameter(Dp) between 6.38 and 9.76 nm. The pores in theNi1M0.3O2.66@SiO2 samples were larger due to alkali earthmetal-incorporation than those in NiO@SiO2. It is well knownthat the agglomeration between pure nickel particles at hightemperatures is very strong and thus there is no pore. Howeverwhen alkali earth metals are incorporated between the nickelparticles resulted that the pores are formed. It is attributed to arelationship between the radius of alkali earth metal ions andNi ion. They have various ionic radii as like Mg2þ¼66 pm,Ca2þ¼99 pm, Sr2þ¼113 pm, and Ba2þ ¼134 pm verseNi2þ¼72 pm. If the ions with a different radius are

aggregated to form the particles resulted that the pores (maybe like bulk aperture) are generated between the different sizedparticles. The BET surface area was highest (655.1 m2g�1) forthe NiO@SiO2 sample, and decreased in the order ofNi1Mg0.3O2.66@SiO24Ni1Sr0.3O2.66@SiO24Ni1Ba0.3O2.66@SiO24Ni1Ca0.3O2.66@SiO2 in the range of 146.8–189.9m2g�1. In general, the specific surface area is strongly relatedto the particle sizes. However the comparisons for particle sizesin this study were difficult because there is only an ambiguouscorrelation. It can be rather attributed to their pore sizes andvolume from the results in the hysteresis slopes. When thealkali-earth metal species are impregnated on the poroussupports using a general wet-impregnation method, the alkali-earth metal gradients can permeate into the porous silica supportresulted that the pores of porous supports are blocked by those.Thus, in general, the surface area decreased largely according tothe impregnated amounts. However there is not necessaryalways. Sometimes the smaller metals are irregularly permitted

a) NiO@SiO2b) Ni1Mg0.3O2.66@SiO2c) Ni1Ca0.3O2.66@SiO2d) Ni1Sr0.3O2.66@SiO2e) Ni1Ba0.3O2.66@SiO2

At 456°C : NiO→Ni

200 400 600 800

-500000

0

500000

1000000

1500000

2000000

2500000

Temperature (°C)

Inte

nsity

(a.u

.)

Fig. 5. H2-TPR profiles of the five fresh samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2.

S. Kang et al. / Ceramics International 40 (2014) 14197–1420614202

into the pores and thus the surface areas are reduced irregularly.The total pore volumes in the alkali earth metal-incorporatedsamples also decreased compared to the non-incorporatedsample, NiO@SiO2.

The changes corresponding to reduction were observed in theH2-TPR profiles of the five fresh samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, andNi1Ba0.3O2.66@SiO2, as shown in Fig. 5. Generally, H2-TPRindicates that the peak area corresponds to the hydrogen uptakeand that the peak location depends on how easy the catalystspecies are reduced. One curve for the maximum reduction ofNiO in the NiO@SiO2 sample was considered to be Ni at456 1C. On the other hand, the reduction curves were separatedinto two curves corresponding to the different interactions ofNiO species and were shifted to higher temperatures in thealkaline earth metal incorporated Ni1M0.3O2.66@SiO2: the firstcurves probably correspond to isolated NiO-Ni reduction, andthe smaller second curves appear to rely on NiO strongly-incorporated with alkaline earth metals-Ni reduction [30]. Thereduction temperatures for NiO-Ni increased in the orderof NiO@SiO24Ni1Sr0.3O2.66@SiO24Ni1Ba0.3O2.66@SiO24Ni1Ca0.3O2.66@SiO24Ni1Mg0.3O2.66@SiO2 in the range of300–900 1C. This means that the incorporated alkaline earthmetals interact strongly with NiO, and affect NiO reduction. Inparticular, less of the NiO in the Ni1Mg0.3O2.66@SiO2 samplewas reduced to metallic Ni at temperatures below 700 1Ccompared to the other samples, meaning that this probablyaffects the catalytic active temperature during ethanol steamreforming. Except of Ni1Mg0.3O2.66@SiO2, the metal ingredi-ents in all other samples were reduced at temperature less than700 1C. On the basis of these results, we have decided anappropriate reduction temperature to 700 1C to reduce the metalingredients of catalysts. Therefore all catalysts were pre-reducedat 700 1C before ESR reaction.

Fig. 6A and B shows the CO- and H2O-TPD profiles. Todetermine the CO and H2O adsorption ability of the catalyst duringethanol steam reforming, the CO- and H2O-TPD profiles of thefive samples reduced after the hydrogen pre-treatment, NiO@SiO2,

Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2,and Ni1Ba0.3O2.66@SiO2, were obtained. Generally, the majorityof adsorbed CO desorbs as CO2, but there is also the moleculardesorption of CO at temperatures as low as 50–100 1C, and COdesorption begins immediately upon the increase in temperature[31]. In this study, the CO adsorption or CO2 adsorption profiles,as shown in Fig. 6A, were characterized at three temperatureranges, approximately 100 1C, 300–500 1C, and 500–600 1C.A sharp curve at approximately 100 1C, which correspondsto physically adsorbed CO, was observed only over theNi1Ca0.3O2.66@SiO2 sample, and there was no curve forchemically-bounded CO adsorption. Generally, CO-adsorption onmetallic species is often observed at higher temperatures than onthe oxidized species [32]. A broad curve for CO molecules thatadsorbed on the surface of the metallic Ni particles of NiO@SiO2

was observed at 300–600 1C. In particular, more CO adsorptionwas observed over the Ni1Mg0.3O2.66@SiO2 sample at the sametemperature range as NiO@SiO2. On the other hand, CO-adsorption over the Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2

samples was observed at higher temperatures, albeit in smalleramounts. This means that CO adsorption for transforming to CO2

during the water gas shift reactions occur easily over the reducedNi species over NiO@SiO2 or Ni1Mg0.3O2.66@SiO2 samplescompared to those over Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2,and Ni1Ba0.3O2.66@SiO2 samples. Otherwise, as shown in Fig. 6B,the H2O molecules adsorbed on the surfaces of the catalysts wouldbe desorbed below 100 1C in all samples, but there were nochemically hydrogen-bonded water molecules with the catalysts.On the other hand, the maximum curves for the desorbed H2Omolecules were shifted to higher temperatures in the orderof NiO@SiO2¼Ni1Mg0.3O2.66@SiO24Ni1Ca0.3O2.66@SiO24Ni1Sr0.3O2.66@SiO24Ni1Ba0.3O2.66@SiO2, and the amountsadsorbed also showed a similar trend. The higher desorptiontemperature means that water molecules interact strongly with thesurfaces of the catalyst. This suggests that the catalysts themselvesare oxidized by the adsorbed water rather than by a reaction thattransfers the CO molecules adsorbed on the surface Ni species intoCO2 by water molecules (CO–water gas shift reaction) during theethanol steam reforming reaction. Consequently, the CO–water gasshift during reaction might be enhanced further on the surface ofthe NiO@SiO2 or Ni1Mg0.3O2.66@SiO2 samples compared toNi1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@-SiO2 catalysts.

3.2. Ethanol steam-reforming reaction over the five catalysts,NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2,Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2

Fig. 7a and b shows the catalytic activities for ethanolconversion and hydrogen selectivity over the reduced fivecatalysts, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@-SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalystsaccording to the temperature-on-steam at GHSV 4500 h�1.Two pathways from ethanol to hydrogen and carbon dioxideexist in ethanol steam reforming [33]. One is that an ethanolmolecule is transformed to reaction intermediates, such asacetaldehyde and/or acetone, by dehydrogenation and is

200 400 600 800

Temperature (°C)

a) NiO@SiO2b) Ni1Mg0.3O2.66@SiO2c) Ni1Ca0.3O2.66@SiO2d) Ni1Sr0.3O2.66@SiO2e) Ni1Ba0.3O2.66@SiO2

40 60 80 100 120 140 160 180 200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Temperature (oC)

a) NiO@SiO2

b) Ni1Mg0.3O2.66@SiO2

c) Ni1Ca0.3O2.66@SiO2

e) Ni1Ba0.3O2.66@SiO2

d) Ni1Sr0.3O2.66@SiO2

TGA

(Der

ivat

ive,

a.u

.)

TGA

(Der

ivat

ive,

a.u

.)

Fig. 6. CO- (A) and H2O-TPD (B) profiles of the reduced samples, NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, andNi1Ba0.3O2.66@SiO2.

300 400 500 600 700 800

NiO@SiO2Ni1Mg0.3O2.66@SiO2Ni1Ca0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2Ni1Ba0.3O2.66@SiO2

0102030405060708090

100

EtO

H c

onve

rsio

n (%

)

300 400 500 600 700 800

10

3040506070

H2

sele

ctiv

ity (%

)

0

20

8090

100NiO@SiO2Ni1Mg0.3O2.66@SiO2Ni1Ca0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2Ni1Ba0.3O2.66@SiO2

Temperature (oC)

Temperature (oC)

Fig. 7. Catalytic activities for ethanol conversion (a) and hydrogen selectivity(b) over the reduced five catalysts, NiO@SiO2, Ni1Mg0.3O2.66@SiO2,Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalystsaccording to the temperature-on-steam at GHSV 4500 h�1.

S. Kang et al. / Ceramics International 40 (2014) 14197–14206 14203

reformed entirely to H2, CO2, CH4, and CO at approximately350 1C. The steam reforming of methane and the water gasshift reaction of CO are the major processes above 500 1C. Inthe other pathway, an ethanol molecule is transformed to areaction intermediate of ethylene by dehydroxylation, and isreformed to CH4 and CO. The steam reforming of methane andthe water gas shift of CO progresses to form CO, CO2, and H2,and finally CO transforms to CO2. In Fig. 7A, the ethanol

conversion over all catalysts exceeded 80% from 400 1Cexcept for Ni1Ba0.3O2.66@SiO2, and their high performancewas maintained at 800 1C without catalytic deactivation.Generally the catalytic performance was reduced at hightemperature due to sintering of metallic nickel particles undermore severe reaction conditions, which finally led to decreasedethanol conversion. However the catalytic performance ratherincreased in this study with increasing reaction temperatures.The reason maybe put on the next; the presences of reformingproducts such as carbon oxides, methane and hydrogen wereobserved in complete conversion of ethanol. Particularly theevolved methane amount was slightly increased in NiMO@-SiO2 catalysts around 550 1C. The steam reforming of methaneas an intermediate were perhaps thermodynamically favorableprocesses at high temperature, leading to high levels ofhydrogen production as well as reduced methane and cokeformation. A more pronounced decrease in coke formation wasobserved in the case of alkaline metal-containing catalysts.Therefore, these results highlight the synergy of the alkalinemetals and Ni components on the catalytic performance, alongwith the enhanced methane steam reforming, the depression ofaggregation between Ni particles, and the subsequent decreasein catalytic deactivation. The ethanol conversion reached morethan 90% above 900 1C when the Ni1Mg0.3O2.66@SiO2

catalyst was used, but it was very low over the Ni1Sr0.3O2.66@-SiO2 and Ni1Ba0.3O2.66@SiO2 catalysts, which containedstrong alkaline metals. In general, the basicity is strongertoward the down at the same group in periodic table. Thus, theorders of basicity are expected to be MgoCaoSroBa. Onthe other hand, hydrogen production over all five catalystsincreased gradually with increasing over the temperaturerange, 300–800 1C. The reforming temperature was lower forthe NiO@SiO2 catalyst than for those incorporated with thealkaline earth metals, but significantly less hydrogen gas wasproduced at high temperatures compared to that on theNi1Mg0.3O2.66@SiO2 catalyst. It is possible to explain in

20 30 40 50 60 70 80 90 1002theta/CuKα

Ni : CubicBaSiO3

SrSiO3

a) NiO@SiO2b) Ni1Mg0.3O2.66@SiO2c) Ni1Ca0.3O2.66@SiO2d) Ni1Sr0.3O2.66@SiO2e) Ni1Ba0.3O2.66@SiO2

Inte

nsity

(a.u

.)Fig. 8. XRD patterns of the used NiO@SiO2, Ni1Mg0.3O2.66@SiO2,Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalystsafter ethanol reforming.

S. Kang et al. / Ceramics International 40 (2014) 14197–1420614204

ESR mechanism which formed intermediates as like acetalde-hyde and methane. The Ni gradients are a good catalyst forproducing methane at high temperature, thus the steamreforming reaction for methane as intermediate in the Ni/SiO2 catalyst could be accelerated at high temperature in thisstudy, resulted that the hydrogen production by methanecracking increased in Ni/SiO2 catalyst at high reactiontemperature. The hydrogen evolution was maximized overNi1Mg0.3O2.66@SiO2 to reach 88% at 700 1C. Here, �9.0%CO2 and CH4 were the main products obtained with smallquantities of acetaldehyde (o1.0%) and trace amounts ofmethane. The CO selectivity was close to 0% over theNi1Mg0.3O2.66@SiO2 catalyst over the entire temperaturerange. The presence of CO degrades the active catalyst dueto catalyst poisoning by CO molecules. As mentioned above,the first mechanism might be more important in the Ni1Mg0.3O2.66@SiO2 catalyst. A H2 molecule is removed easilyfrom ethanol over the reduced Ni sites in Ni1Mg0.3O2.66@-SiO2, and the aldehyde formed is transformed to methane andCO, which are then reformed to CO2 and H2 on the NiO andNiMgO surfaces. The catalytic performance decreased to atleast 40% over the Ni1Sr0.3O2.66@SiO2 and Ni1Ba0.3O2.66@-SiO2 catalysts compared to that over the Ni1Mg0.3O2.66@SiO2

catalyst. These results indicate synergy between Mg and Ni inNi1Mg0.3O2.66@SiO2 regarding the catalytic performance,along with the inhibition of sintering and the consequentdecrease in catalytic deactivation. Note that NiO and Nispecies in the Ni1Mg0.3O2.66@SiO2 catalyst were present. Thismeans that methane and CO, as intermediates absorbed on theNi species, were oxidized easily by oxygen in NiO in theNi1Mg0.3O2.66@SiO2 catalyst. In addition, the introduction ofMgO during ethanol steam reforming had a favorable effect asan oxygen donor to reduced Ni.

3.3. Characteristics of the catalysts after ethanol reforming

The XRD patterns of the five used catalysts. NiO@SiO2,Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@-SiO2, and Ni1Ba0.3O2.66@SiO2, were analyzed after ethanolreforming to examine the structural changes as well as thechanges in the oxidation state of the metals after ethanol steamreforming, as shown in Fig. 8. The XRD patterns were assignedto the reduced metallic cubic Ni in all the catalysts. On the otherhand, the peak intensities decreased in the order of NiO@SiO24Ni1Mg0.3O2.66@4Ni1Ca0.3O2.66@SiO24Ni1Sr0.3O2.66

@SiO24Ni1Ba0.3O2.66@SiO2. Reduced Ba3SiO5 and Sr3SiO5

were observed on the Ni1Sr0.3O2.66@SiO2 and Ni1Ba0.3O2.66@-SiO2 catalysts. According to Scherrer's equation, the crystalsizes in the grown Ni metal in the used catalysts after ethanolsteam reforming were determined. The estimated crystallitesizes of the Ni metal in the used NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 using the 44.371 (111) peak were 33.79, 25.24,33.66, 32.15, and 31.51 nm, respectively.

To determine the amounts and shapes of carbon depositedon each catalyst, the surfaces of the used catalysts wereobserved by TEM, as shown in Fig. 9. Generally, catalytic

deactivation during ethanol steam reforming is related to theappearance of coke formation from CO and C2H4 [34]. If thenumber of cleaved C–C bonds decreases gradually over thecatalyst, the accumulated acetaldehyde would be converted toCH4 and then to CO, which acts as a precursor for coke. Overthe NiO@SiO2 catalyst, only carbon nanofibers (CNF) wereobserved. This was attributed to the production of ethylene asan intermediate, which reacts with hydrogen at high tempera-tures to form carbon nanotubes (CNT) or CNFs over thetransition metal: coke to CNF formation generally occurs inthe following order [35]: CH3CHO thermal cracking-CH4

methanation-C2H6 dehydrogenation-C2H4 polymeriza-tion-CNFs. Otherwise, two types of coke would have beenobserved in the used Ni1Mg0.3O2.66@SiO2 catalyst; carbonlumps and CNFs. In the Ni1Mg0.3O2.66@SiO2 catalyst, if anethanol molecule is decomposed through a CH3CHO inter-mediate on the surface of the metallic Ni sites, the carbonlumps would have also been produced using the followingprocess: CH3CHO-CH4-CO-C lumps. In addition, thecarbon lumps were produced more predominantly over theNi1Ca0.3O2.66@SiO2 catalyst. In contrast, on the used Ni1S-r0.3O2.66@SiO2 and Ni1Ba0.3O2.66@SiO2 catalyst the carbondeposits were quite small because the ethanol steam reformingreaction had not been properly conducted.

4. Conclusions

This study examined the role of the assistant promoter, alkalineearth metal oxides, in improving the Ni-based catalytic stabilityusing their oxygen storage capacities. The 30 wt% alkaline earthmetal oxides were mixed Ni oxides capsulated by 70 wt%porous SiO2. The catalytic performance was improved overNi1Mg0.3O2.66@SiO2, and catalytic deactivation was retardedduring ESR. Hydrogen evolution was maximized over

NiO@SiO2 Ni1Mg0.3O2.66@SiO2

Ni1Ca0.3O2.66@SiO2 Ni1Ba0.3O2.66@SiO2Ni1Sr0.3O2.66@SiO2

100 nm 100 nm

100 nm 100 nm 100 nm

Fig. 9. TEM images of the used NiO@SiO2, Ni1Mg0.3O2.66@SiO2, Ni1Ca0.3O2.66@SiO2, Ni1Sr0.3O2.66@SiO2, and Ni1Ba0.3O2.66@SiO2 catalysts after ethanol reforming.

S. Kang et al. / Ceramics International 40 (2014) 14197–14206 14205

Ni1Mg0.3O2.66@SiO2 to reach 88% at 700 1C: �9.0% CO2 andsmall quantities of acetaldehyde (o1.0%) were also obtained. Inparticular, the CO selectivity was close to 0% over the catalystacross the entire temperature range. Overall, these results show thatthe introduction of Mg oxides to ESR has a favorable effect on theNiMO@SiO2 catalytic performance by preventing catalytic deac-tivation caused by sintering among nickel particles and facilitatingthe WGS by allowing more CO adsorption on the surface ofNi1Mg0.3O2.66.

Acknowledgments

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (NRF-2012R1A1A3005043), for which the authors are very grateful.

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