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Catalysis Today 228 (2014) 145–151

Contents lists available at ScienceDirect

Catalysis Today

j our na l ho me page: www.elsev ier .com/ locate /ca t tod

ydrogen production by steam reforming of biomass-derivedlycerol over Ni-based catalysts

im Seung-hoona,b, Jung Jae-suna,c, Yang Eun-hyeoka,c,ee Kwan-Youngb,d, Moon Dong Jua,c,∗

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, Republic of KoreaDepartment of Chemical and Biological Engineering, Korea University, Seoul, Republic of KoreaClean Energy & Chemical Engineering, University of Science and Technology, Daejeon, Republic of KoreaGreen School, Korea University, Seoul, Republic of Korea

r t i c l e i n f o

rticle history:eceived 4 July 2013eceived in revised form4 November 2013ccepted 18 November 2013vailable online 19 December 2013

eywords:

a b s t r a c t

The steam reforming of bio-glycerol for the production of hydrogen has been investigated in a fixed-bedreactor containing spherical particle of Ni supported on �-Al2O3 as catalyst. Several catalysts were pre-pared by the dry impregnation method at different calcination temperatures (750–950 ◦C) with/withoutthe addition of various alkaline promoters (K, Ca, Sr). The physical and chemical properties of the preparedcatalysts were characterized by nitrogen (N2) physisorption, X-ray diffraction (XRD), temperature-programmed reduction, thermogravimetric analysis, scanning electron microscopy-energy dispersiveX-ray spectroscopy and transmission electron microscopy techniques. The Ni/�-Al2O3 catalysts showed

lycerolteam reformingickel catalystalcinationlkali promoterarbon formation

a high glycerol conversion and hydrogen selectivity, however they gradually deactivated due to theformation of carbon over those catalysts.

Nevertheless, it was found that the catalysts calcined at high temperatures could inhibit the formationof carbon without affecting the glycerol conversion and hydrogen selectivity. The addition of alkalinepromoters caused a slight decrease in the glycerol conversion, but also reduced the formation of carbonand thereby increased the long-term stability of the catalyst.

. Introduction

With an ever-increasing demand for energy and problems asso-iated with global warming due to the use of fossil fuels, the questor alternative and renewable fuels has become paramount [1]. Inhis respect, biodiesel is being gradually accepted as a replacementor mainstream diesel. Biodiesel is composed of fatty acid methylsters (FAMEs), which are produced by an alkali-catalyzed trans-sterification reaction between biomass-derived triglycerides andethanol (Fig. 1). As can be seen in the reaction scheme, glycerol is

roduced as a by-product at about 10% of the amount of biodiesel,aking it sufficiently significant.Glycerol is an important feedstock in food, cosmetics, pharma-

euticals and other industries. Among the possible uses of glycerol,

he production of hydrogen by steam reforming (SR) is consid-red attractive because it makes glycerol (and its raw materials)

potential source for a renewable zero-emission fuel [2].

∗ Corresponding author at: Clean Energy Research Center, Korea Institute of Sci-nce and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, Republic of Korea.el.: +82 2 958 5867.

E-mail address: djmoon@kist.re.kr (M. Dong Ju).

920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2013.11.043

© 2013 Elsevier B.V. All rights reserved.

The dominant reaction in the glycerol steam reforming processis as follows:

C3H8O3 + 3H2O → 3CO2 + 7H2 (1)

Eq. (1) is further constituted of the glycerol decomposition reac-tion (Eq. (2)) and the water–gas shift reaction (Eq. (3)).

C3H8O3 → 3CO + 4H2 (2)

CO + H2O → CO2 + H2 (3)

However, several side reactions also occur as shown in Eqs.(4)–(10)

C3H8O3 + 5H2 → 3CH4 + 3H2O (4)

CO + 3H2 → CH4 + H2O (5)

CO2 + 4H2 → CH4 + 2H2O (6)

CO2 + CH4 → 2CO + 2H2 (7)

CH4 → 2H2 + C (8)

2CO → CO2 + C (9)

C + H2O → CO + H2 (10)

146 K. Seung-hoon et al. / Catalysis Today 228 (2014) 145–151

diesel

omiDsdr

bttStotdtrs

at

teotd

met

2

2

tnn�fds

Fig. 1. Main scheme of bio

Amongst the side reactions, Eqs. (8) and (9) show the two typesf mechanisms for the formation of carbon. The former is theethane decomposition reaction and the latter is a carbon monox-

de disproportionation reaction called the “Boudouard reaction.”espite the mechanism involved, the formation of carbon on the

urface of the catalyst causes a loss of the effective surface area,ecreases the mass and heat transfer rate in the catalyst bed, andeduces the lifetime of the catalyst.

Many studies have been conducted to limit the formation of car-on during the steam reforming process. Urasaki et al. [3] suggestedhe use of perovskite-type oxides as a catalytic support becausehey reduced the formation of carbon with their lattice oxygen.ánchez-Sánchez et al. [4] reported that the presence of basic addi-ives or promoters on an alumina (Al2O3) support lowered the ratef carbon deposition on the catalyst surface because they favoredhe adsorption of water and the surface mobility of hydroxyl ionsuring the steam reforming of ethanol. Sahli et al. [5] found thathe nickel aluminate (NiAl2O4) spinel having an optimized Ni/Alatio exhibits a good performance in the reforming of methane intoynthetic gas with limited formation of carbon.

Many researchers reported that, the effect of the addition oflkaline earth promoter to a nickel based catalyst will enhanceshe resistivity toward carbon formation.

Sekine et al. [6] showed that the addition of strontium enhancehe active oxygen species under steam reforming of tolune. Haddent al. [7] proved that the potassium promoted the adsorption abilityf water under steam reforming reaction. Elias et al. [8] showedhat calcium oxide having less acidic sites showed the less carboneposition under steam reforming reaction of ethanol.

In this study, we have investigated the reduction of carbon for-ation by changing the calcination temperatures and studied the

ffect of alkali earth metal as promoters to Ni/�-Al2O3 catalysts forhe steam reforming of glycerol.

. Experimental

.1. Catalysts preparation

The Ni/�-Al2O3 catalysts were prepared by the dry impregna-ion method. In a typical experiment, aqueous solution of nickelitrate [Ni(NO3)2·6H2O 98.0% Samchun Chemical] was impreg-ated (15 wt.%) on the calcined alumina support (Alumina spheres

1.0 SASOL). After impregnation, the catalyst was calcined at dif-

erent temperatures (750, 850 and 950 ◦C) for 5 h. The catalysts areenoted as A, B and C, respectively. For the comparison, NiAl2O4pinel phase catalyst was also prepared by sol–gel method [9].

production from biomass.

The alkali promoter modified M–Ni/�-Al2O3 catalysts (M: K, Ca,Sr) were prepared by the same method used for the Ni/�-Al2O3 cat-alysts. Aqueous solutions of the respective alkali and alkaline-earthmetal nitrate, which include potassium nitrate (KNO3, 98.0%, Sam-chun Chemical), calcium nitrate (Ca(NO3)2·4H2O, 98.5%, SamchunChemical), and strontium nitrate (SrNO3, 98.0%, Samchun Chem-ical) were impregnated (1 wt.%) on the Ni/�-Al2O3 catalyst. Afterimpregnation of promoters, catalysts were calcined at 950 ◦C for5 h.

2.2. Catalysts characterization

The specific surface area, pore sizes and pore volumes of the cat-alysts were calculated by applying the Brunauer–Emmett–Teller(BET) method to the nitrogen adsorption/desorption isotherms,measured at 77 K (liquid nitrogen temperature) on a Moonsorp-IIapparatus (BEL Japan). The catalyst samples were pretreated withnitrogen gas at 200 ◦C for 2 h.

The temperature-programmed reduction (TPR) experimentswere carried out with an Autochem II apparatus (Micromeritics)equipped with a thermal conductivity detector (TCD). Prior to theTPR experiments, about 0.1 mg of the samples was washed under ahelium stream (50 mL/min) at room temperature for 2 h to removewater and other contaminants. TPR profiles were obtained by heat-ing the samples under a 5% H2/Ar flow (50 mL/min) from 50 to980 ◦C at a linearly programmed rate of 10 ◦C/min.

X-ray diffraction (XRD) patterns of the calcined and used (afterSR) catalysts were recorded using an XRD-6000 apparatus (Shi-madsu Co.) with a monochromatized CuK� radiation (� = 1.5404 A)as the source over the diffraction angle, 2� scan range of 20–80◦

with a scan rate of 4◦/min.Temperature-programmed oxidation analysis of the used cat-

alysts was carried out on a thermogravimetric analyzer (TGA)SDT-Q600 apparatus (TA Instruments) to determine the amountof carbon deposited on the catalysts. In the procedure, 20 mg ofthe used catalysts were subjected to a flow (100 mL/min) of dry airfrom room temperature to 1000 ◦C at a heating rate of 5 ◦C/min.

The composition and morphology of the calcined and usedcatalysts were examined by field emission scanning electronmicroscopy (FE-SEM) and energy dispersive X-ray spectroscopy(EDX) using an S-4200 apparatus (Hitachi). The catalyst sampleswere ground and mechanically dispersed on an electrically con-

ductive carbon tape placed on an aluminum stub.

The morphology of the catalysts before and after the reac-tion was also investigated by transmission electron microscopy(TEM) using a Tecnai F20 G2 microscope (FEI Company). For samp-

K. Seung-hoon et al. / Catalysis T

ls

2

(so

pDHo

Nha

TP

S

Fig. 2. Schematic diagram of the glycerol steam reforming system.

ing, finely ground catalyst samples were dispersed in ethanol andprayed on a copper grid.

.3. Catalytic activity test

The catalytic activity tests were performed in a 3/8 in.9.525 mm) Inconel tubular fixed bed reactor (FBR). Fig. 2 shows thechematic diagram of the reaction system used for steam reformingf glycerol.

The feedstock used as a 36 wt.% aqueous glycerol solution pre-ared by mixing deionized water with bio-glycerol (glycerol, 99.0%,ansuk Industry). It was charged from a feed vessel (3) using anPLC pump (4), vaporized in a pre-heater at 350 ◦C (5) and passedver the catalyst bed (6) at the desired temperature.

The catalyst bed was kept between two pieces of quartz wool.itrogen, which was used as a carrier gas in all experiments, andydrogen, which was used as a reducing gas were introduced at

predetermined flow rate using a needle valve (1) and a mass

able 1hysical and chemical properties of the catalysts prepared.

Catalyst Calcination temperature (◦C) Metal loading (wt.%) BET

NiAl2O4 750 – 46

Ni/�-Al2O3 (A) 750 Ni (15) 111

Ni/�-Al2O3 (B) 850 Ni (15) 106

Ni/�-Al2O3 (C) 950 Ni (15) 87

K–Ni/�-Al2O3 950 Ni (15), K (1) 88

Ca–Ni/�-Al2O3 950 Ni (15), Ca (1) 86

Sr–Ni/�-Al2O3 950 Ni (15), Sr (1) 90

.A. stands for the surface area.

oday 228 (2014) 145–151 147

flow controller (2). The gaseous products leaving the reactor passedthrough a back pressure regulator (7) and a cold trap (8). Thenon-condensable gases were analyzed by an on-line 6890N gaschromatography apparatus (Agilent Technologies) (9) using anautomatic valve injection system.

The steam reforming of hydrocarbons typically takes place inthe vapor phase at atmospheric pressure and temperatures above700 ◦C [10–18]. Wang et al. [19] reported that based on Gibbs freeenergy minimization, the optimum conditions for hydrogen pro-duction was: temperatures between 650 and 700 ◦C, water/glycerolratios between 9 and 12, and pressure around the atmosphericpressure. They also found that the amount of hydrogen increasedwith the reaction temperature and maximized at 800 ◦C. In thisstudy, the performance of each catalyst was tested at the followingconditions: temperature of 800 and 600 ◦C, atmospheric pressure,24–100 h and a gas hourly space velocity (GHSV) of 10,000 h−1.Prior to the catalytic reaction, each catalyst was reduced with amixed stream of hydrogen (10 mL/min) and nitrogen (40 mL/min)at 850 ◦C for 6 h [20].

3. Results and discussion

3.1. Catalyst characterization

Table 1 shows the physical and chemical properties of the cat-alysts. It can be seen that with the increase in the calcinationtemperature, the BET surface area and pore volume of the cata-lysts decreased in good agreement with these references [21–23],while the particle size increased due to particle growth [24]. Theseresults were identical to those obtained by Tangchupong et al. [25].The primary cause for the change in morphology with temperaturecould be due to the phase transformation of the alumina support,from � to ı or the � phase [24].

The other cause could be addition of alkali promoters whichleads to an increase of the catalyst particle size. In addition, thedeposition of promoters could have also plugged the pore cavitiesof the �-Al2O3 support [26].

3.2. TPR profiles

Fig. 3a and b illustrates the TPR profiles of bare and promotedNi/�-Al2O3 catalysts, respectively. All catalysts showed only onemain peak between 820 and 860 ◦C. As the calcination tempera-ture increased, that peak shifted to a higher temperature due to anincrease in the formation of NiAl2O4. Cheng et al. [27] also reportedthat NiAl2O4 was formed due to a solid-solid reaction betweennickel oxide and the alumina support above 500 ◦C. Nevertheless,an increase in the formation of NiAl2O4 led to a decrease in the

reducibility of the Ni/�-Al2O3 catalysts.

In Fig. 3b, the effects of alkaline promotion on the reductionbehavior of the nickel species can be observed. The main peak inthe promoted Ni/�-Al2O3 catalysts shifted to slightly lower tem-

S.A. (m2/g) Pore size (nm) Pore volume (cc/g) Particle size (nm)

XRD TEM

7.0 0.29 15.0 –11.8 0.32 4.4 6.812.1 0.32 5.5 7.713.2 0.29 6.6 8.113.3 0.30 19.2 24.513.8 0.30 18.3 24.713.2 0.30 19.1 24.6

148 K. Seung-hoon et al. / Catalysis Today 228 (2014) 145–151

Fig. 3. TPR profiles of (a) bare Ni/�-Al2O3 catalysts and (b) promoted M–Ni/�-Al2O3 (M = K, Ca, Sr) catalysts. A, B, and C stand for calcination temperatures of 750, 850, and950 ◦C, respectively.

F t 8009

pttMtc

3

cbrmdtNsstf

ig. 4. XRD patterns of the catalysts (A) NiAl2O4, (B) fresh, (C) after the reaction a50 ◦C, respectively.

eratures as compared to the bare catalyst. It is generally acceptedhat alkali and alkaline earth promoters on Al2O3 support can neu-ralize the acidic site and increase reducibility of the catalysts [28].

oreover, promoters can also decrease the metal-support interac-ion between nickel and Al2O3 [29]. It was found that Sr–Ni/�-Al2O3atalyst has the lower reduction peak among the prepared catalysts.

.3. XRD patterns

Fig. 4(A)–(D) shows the XRD patterns of the Ni/�-Al2O3 catalystsalcined at different temperatures. Fig. 4(A) and (B) represents theefore reaction and (C) and (D) after the reaction at 800 and 600 ◦C,espectively. In Fig. 4(A), the major diffraction peaks were wellatched with JCPDS record of NiAl2O4 spinel. In Fig. 4(B), the major

iffraction peaks are due to Al2O3, NiO and NiAl2O4. The calcina-ion at higher temperatures leads to increase in the formation ofiAl2O4 and increase in intensity of diffraction peaks. Fig. 5(A)–(C)

hows the XRD patterns of the promoted Ni/�-Al2O3 catalysts. Noignificant difference was observed between the bare catalysts andhe promoted catalysts. However in Fig. 5(B), the carbon peak whichound in Fig. 4 was not detected at all. But the carbon formation was

◦C, and (D) 600 ◦C. A, B, and C stand for calcination temperatures of 750, 850, and

increased under severe reaction conditions as shown in Fig. 5(C).It was considered that Sr–Ni/�-Al2O3have the resistance to carbondeposition.

3.4. Catalytic activity tests

Catalytic activity studies of the prepared catalysts for steamreforming of glycerol were studied and the results were repre-sented in Table 2. It was observed that, with the increase in thecalcination temperature, the methane selectivity and the amountof carbon formed decreased, although the glycerol conversionand hydrogen selectivity are almost similar. The NiAl2O4was notreducible fully as observed in TPR profiles. Although the activity ofNiAl2O4 is lower than other catalysts, NiAl2O4 was active speciesunder the reaction conditions studied [30]. The formation of theNiAl2O4 phase affects the deposition of carbon but does not affectthe activity of the catalyst for the steam reforming of glycerol.

Because the steam reforming of glycerol is affected by the decom-position of glycerol, which was controlled by pre-heater [31].

However, the addition of alkali promoters lowered hydrogenand methane selectivity and but also the formation of carbon.

K. Seung-hoon et al. / Catalysis Today 228 (2014) 145–151 149

Fig. 5. XRD patterns of promoted M–Ni/�-Al2O3 catalysts (M = K, Ca, Sr) (A) fresh, (B) after the reaction at 800 ◦C, and (C) 600 ◦C.

Table 2Catalytic activity tests for steam reforming of glycerol. A, B and C stand for calcination temperatures of 750, 850, and 950 ◦C, respectively.

Catalysts Reaction temperature (◦C) Reaction time (h) Glycerol conversion (%) Hydrogen selectivity (%) Methane selectivity (%) Coke (g/gcat h)

NiAl2O4 800 24 75.2 60.12 8.24 0.5212Ni/�-Al2O3 (A) 800 24 94.1 65.25 6.22 0.1572Ni/�-Al2O3 (B) 800 24 94.3 63.79 5.90 0.1485Ni/�-Al2O3 (C) 800 24 95.0 65.26 5.69 0.1043Ni/�-Al2O3 (A) 600 24 86.5 63.70 4.12 0.3561Ni/�-Al2O3 (B) 600 24 88.5 63.89 4.08 0.2840Ni/�-Al2O3 (C) 600 24 90.1 64.48 2.10 0.1875K–Ni/�-Al2O3 800 100 93.4 60.01 4.61 0.0081Ca–Ni/�-Al2O3 800 100 94.1 60.07 4.11 0.0431Sr–Ni/�-Al2O3 800 100 93.6 64.79 0.40 0.0006K–Ni/�-Al2O3 600 24 83.7 62.70 4.22 0.2174Ca–Ni/�-Al2O3 600 24 84.5 63.09 3.90 0.1484Sr–Ni/�-Al2O3 600 24 87.2 63.38 3.69 0.1358

Table 3Results of the TGA analysis on the catalysts after the steam reforming reaction. A, B, and C stand for calcination temperatures of 750, 850, and 950 ◦C, respectively.

Ni/�-

1.765

Ayt[ptit

3

Cctm

Catalysts Ni/�-Al2O3 (A) Ni/�-Al2O3 (B)

Weight change % (g/gcat) 2.027 1.895

raque et al. [32] found that condensable products such as hydrox-acetone, acetaldehyde, and acrolein were also produced duringhe steam reforming of glycerol, but they were a precursor of coke33,34]. On the other hand, Frusteri et al. [35] reported that basicromoters could prevent the dehydration of ethanol which leads tohe formation of ethylene, a well-known coke precursor. Therefore,t might be possible that alkaline promoters in this study decreasedhe formation of carbon by preventing a dehydration reaction.

.5. TGA analysis

Table 3 shows the results of the TGA analysis of used catalysts.

ommensurate with the activity test results listed in Table 2, theatalyst calcined at higher temperatures were more durable, andhe promoted catalysts had a greater resistance against carbon for-

ation although their steam reforming reaction time was longer. Of

Al2O3 (C) K–Ni/�-Al2O3 Ca–Ni/�-Al2O3 Sr–Ni/�-Al2O3

1.198 1.580 1.056

all the catalysts, Sr-Ni/�-Al2O3 catalyst had the best stability againstthe formation of carbon during the steam reforming of glycerol.

Sekine et al. [6] have reported that the addition of strontiumto the surface of the catalyst can increase the adsorption of water,gasification of carbon and inhibit the adsorption of coke during thesteam reforming of toluene. Therefore, it is possible that surfacestrontium exhibits the same behavior in the steam reforming ofglycerol.

3.6. SEM-EDX results

Table 4 shows the data obtained by the SEM-EDX analysis of the

Ni/�-Al2O3 catalysts. As found in the activity and TGA tests whoseresults are consolidated in Tables 2 and 3, the formation of carbonon the surface of the catalysts after the steam reforming reactiondecreased with the increase in the calcination temperature.

150 K. Seung-hoon et al. / Catalysis Today 228 (2014) 145–151

Table 4Surface atom distribution on the Ni/�-Al2O3 catalysts before and after the steam reforming reaction.

Catalysts Ni/�-Al2O3 (A) Ni/�-Al2O3 (B) Ni/�-Al2O3 (C)

Reaction Before After Before After Before After

Surface elements (wt.%) C 4.01 11.87 5.19 8.46 4.22 4.74O 48.68 47.16 49.19 47.66 50.55 51.03Al 38.35 34.54 35.01 34.79 38.09 37.09Ni 8.96 6.43 10.61 9.09 7.15 7.14

A, B, and C stand for calcination temperatures of 750, 850, and 900 ◦C, respectively.

(C) N

3

tll

4

otmicaoh

Fig. 6. TEM images of (A) Ni/�-Al2O3 (750 ◦C), (B) Ni/�-Al2O3 (850 ◦C) and

.7. TEM results

Fig. 6 shows the TEM images and particle size distributions ofhe Ni/�-Al2O3 catalysts. Calcination at higher temperature led toarger particles of Ni, higher amount of the NiAl2O4 phase, andower amount of carbon.

. Conclusions

Several catalysts containing spherical particles of Ni supportedn �-Al2O3 using the dry impregnation method at different calcina-ion temperatures with/without the promotion of various alkaline

etals (K, Ca, Sr) and successfully employed for the steam reform-ng of glycerol. The bare Ni/�-Al2O3 catalysts prepared at high

alcination temperatures showed a higher formation of NiAl2O4,nd a lower formation of carbon after the steam reforming reactionf glycerol, determined by XRD, SEM-EDX, and TGA. On the otherand, the strontium promoted Sr-Ni/�-Al2O3 catalyst exhibited a

i/�-Al2O3 (950 ◦C) catalysts before (top) and after (bottom) the reaction.

long-term stability (>100 h) against the formation of carbon. This isbecause strontium on the surface of the Ni/�-Al2O3 catalyst mighthave increased the basicity, increased the adsorption of steam, anddecreased the adsorption of carbon.

Therefore, we can conclude that the calcination temperatureand addition of alkaline metal has a strong influence on the sta-bility of the catalyst against the formation of carbon of the catalystduring the steam reforming of glycerol.

Acknowledgement

We appreciate the financial support from the Ministry of Trade,Industry and Energy (MOTIE), Republic of Korea via the Project No.

10033687.

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