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Catalytic activity and characterizationsof Ni/K2TixOyeAl2O3 catalyst for steam methanereforming

So Yun Lee a, Hankwon Lim b, Hee Chul Woo a,*

a Department of Chemical Engineering, Pukyong National University, San 100, Yongdang-dong, Nam-gu,

Busan 608-739, Republic of Koreab Department of Chemical Systematic Engineering, Catholic University of Daegu, 13-13 Hayang-ro, Hayang-yep,

Gyeongsan-si, Gyeongsangbuk-do 712-702, Republic of Korea

a r t i c l e i n f o

Article history:

Received 14 December 2013

Received in revised form

2 August 2014

Accepted 10 August 2014

Available online 22 September 2014

Keywords:

Steam methane reforming

Hydrogen production

Nickel catalyst

Ni/K2TixOyeAl2O3

Deactivation

Catalytic stability

* Corresponding author. Tel.: þ82 51 629 643E-mail addresses: woohc@pknu.ac.kr, ho

http://dx.doi.org/10.1016/j.ijhydene.2014.08.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Nickel catalysts supported on the K2TixOyeAl2O3 were prepared by the wet impregnation

method for steam methane reforming to produce hydrogen. X-ray diffraction, N2 phys-

isorption, scanning electron microscopy with energy dispersive spectroscopy, the H2

temperature-programed reduction technique, and X-ray photoelectron spectroscopy were

employed for the characterization of catalyst samples. The results revealed that the per-

formance of the Ni/K2TixOyeAl2O3 catalysts was comparable to that of commercial FCR-4

for steam methane reforming under the mild condition. In particular, a catalytic stability

test at 800 �C and in the reactant flow with the steam-to-carbon (S/C) feed ratio of 1.0

indicated that the Ni/K2TixOyeAl2O3 catalysts were more active, thermally stable and

resistant to deactivation than the non-promoted Ni/Al2O3. It is considered that the

appropriate interaction strength between nickel and the modified support and proper

K2TixOy phases with a surface monolayer coverage achieved at ca. 15 wt.% loading in the

support play important roles in promoting the steammethane reforming activity as well as

suppressing the sintering of the catalyst.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Fuel cells which convert chemical energy directly into elec-

trical energy with high efficiency and low pollutants have

been considered as a commercially renewable energy device.

The development of fuel processing for the fuel cell system

has focused largely on hydrogen or synthesis gas production

from hydrocarbons and alcohols, and the conversion of fuels

6; fax: þ82 51 629 6429.kies0301@gmail.com (H.C14gy Publications, LLC. Publ

to hydrogen has been performed using one of four major

techniques such as steam reforming, partial oxidation, auto-

thermal reforming, or carbon dioxide reforming [1]. Among

these techniques, steam reforming is the most prevalent

technology for hydrogen production and steam methane

reforming produces a hydrogen-rich gas with the order of

70e75% hydrogen on a dry mass basis. The overall reaction of

methanewith steam can be represented by Eq. (1), followed by

the water gas shift reaction (Eq. (2)). A second route is

. Woo).

ished by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 517646

described by Eq. (3), where carbon dioxide is formed as a pri-

mary product [2].

CH4 þH2O4COþ 3H2

�DH

�298 ¼ 206 kJ$mol�1� (1)

COþH2O4CO2 þH2

�DH

�298 ¼ �41 kJ$mol�1� (2)

CH4 þ 2H2O4CO2 þ 4H2

�DH

�298 ¼ 165 kJ$mol�1� (3)

In addition, carbon-forming reactions such as methane

cracking (CH4 4 C þ 2H2) and Boudouard reaction

(2CO4 C þ CO2) may be accompanied and carbon from these

reactions can deactivate a catalyst for steam methane

reforming.

Methane reaction with water on supported metals (Pt, Pd,

Ru, and Ni) catalysts leads to a hydrogenecarbon monoxide

rich in hydrogen [3e6]. Although the supported Ni catalyst is

not the most active, it appears to be the most attractive due to

its acceptably high activity and significantly low cost,

compared to precious metal-based catalysts [5,7]. However,

nickel-based catalysts are susceptible to deactivation from

carbon deposition, evenwhen operated at steam-to-carbon (S/

C) ratios predicted to be thermodynamically outside of the

carbon-forming regime [7]. In the condition, where is close to

the stoichiometric S/C ratio, the graphitic carbon is formed on

unpromoted Ni catalysts and it results in increasing pressure

drop, a reactor blockage, and catalyst deactivation [6]. In order

to improve the catalytic stability of these materials, two

different approaches of modifying a conventional support

(Al2O3) by combining it with an additive and switching to an

entirely different support have been tried. Recently, elements

such as Mg [8,9], Fe [10], K [11], Ag [6], Pt [12], Pd and Rh [13],

and Ru [14] have been utilized as additives to suppress the

deactivation of catalysts.

Roh et al. [9] combined a gamma-alumina support with

30 wt.% MgO obtained from Sasol as a promoter. They found

that nickel dispersion was 12% with a constant activity

maintained for one day, and the methane conversion for

steam methane reforming (SMR) over Ni/MgOeAl2O3

improved by more than 25% compared to the non-promoted

Ni/Al2O3. Temperature-programed reduction (TPR) results of

the catalyst indicated a strong metal-support interaction in

the NieMgOeAl2O3 system as shown by high temperatures

required to reduce the nickel species. Several studies inves-

tigated the properties of Au as a promoter for Ni catalysts

[15e17].

On the other hand, different support materials like CeO2

[10,18], MgAl2O4 [19], MgO [20], hydrotalcite-like compounds

[21], NiAl2O4 [2], ZrO2 [10,22], and perovskites [23] were used

and investigated. Matsumura and Nakamori [24] studied SMR

on Ni/Al2O3, Ni/SiO2, and Ni/ZrO2 and found that a Ni/ZrO2

catalyst showed the highest methane conversion and least

coking. Additionally, they observed that steam gradually

oxidized Ni to NiO on Ni/Al2O3 and Ni/SiO2 and this resulted in

catalyst deactivation. A temperature higher than 500 �C was

required to reduce the nickel species on ZrO2 compared to

those on SiO2 and Al2O3 indicating a strong metal-support

interaction on the Ni/ZrO2 catalyst.

Although many recent studies have shown that the

development of various catalysts for the technology, the

catalyst deactivation has been still regarded as a major

drawback of technical advancement [25]. Sehested [26] re-

ported that carbon formation, sulfur poisoning, and sintering

are the main causes for deactivation of nickel-based catalysts

used in the steammethane reforming and further work needs

to be done to overcome these barriers and improve a catalytic

stability.

The most commonly used support for commercial Ni

steam reforming catalysts is an Al2O3 support and it works

well for natural gas (methane) steam reforming although a

fuel containing a wide range of impurities negatively in-

fluences the activity of a conventional reforming catalyst [27].

Therefore, it is worthwhile to focus on modifying an Al2O3

support to make it more resistance to impurities. It has been

widely known that potassium and TiO2 are resistant to sulfur

poisoning of the catalyst for various catalytic reactions such

as NO adsorption and NOx storage reduction [28,29]. Although

potassium has been regarded as a good promoter to prevent

deactivating catalysts, its severe volatility for long operations

at high temperatures causes an abrupt deactivation due to

potassium loss. As a solution to this problem, Kim et al. [30]

reported that the addition of potassium titanate on a con-

ventional catalyst, Ni/YSZ, showed a stable maintenance of

activity without potassium loss, as well as resistance to coke

formation during a long time-on-stream tests.

The objective of this research was to examine the pro-

moting effect of potassium titanate in a nickel catalyst for

steam methane reforming and the activities of nickel-based

catalysts supported by alumina based mixed oxide including

potassium titanate were investigated in this paper.

Experimental configuration

Materials

A support used in this study was Al2O3 (Aldrich, activated,

acidic, Brockmann I, standard grade, SBET ¼ 173.7 m2 g�1,

gamma-phase) and the chemicals used in the synthesis of the

catalysts were TiO2 (Degussa, P-25), K2CO3 (Katayama, 99.5%),

and Ni(NO3)2$6H2O (Junsei, 97%). The gases utilized in the

reactivity study were CH4 (>99.5%), H2 (>99.999%), and He

(>99.999%).

Catalyst preparation

Ni/K2TixOyeAl2O3 catalysts were synthesized using the wet

impregnation method. Different Ni catalysts with a fixed

10 wt.% nickel loading were prepared by varying the amounts

of potassium titanate, K2TixOy, from 5 to 50 wt.% of the sup-

port. In order to investigate the potassium titanate effect on

the Ni/Al2O3 catalyst, catalysts with different amounts of

K2TixOy denoted as Ni/(wt.%)K2TixOyeAl2O3: Ni/(5)K2Tix-OyeAl2O3, Ni/(11)K2TixOyeAl2O3, Ni/(20)K2TixOyeAl2O3, Ni/(30)

K2TixOyeAl2O3, and Ni/(50)K2TixOyeAl2O3 and their samples

before reduction denoted as NiO/(wt.%)K2TixOyeAl2O3 were

characterized and tested. The catalysts were synthesized in

two steps: the modification of alumina support with potas-

sium titanate and the loading of nickel. In the first step, to

obtain the support (20)K2TixOyeAl2O3, K2CO3 of 2.19 g was

Fig. 1 e X-ray diffraction patterns of (a) g-Al2O3, (b) NiO/Al2O3 and NiO/K2TixOyeAl2O3 catalysts with K2TixOy loading: (c) NiO/

(5)K2TixOyeAl2O3, (d) NiO/(11)K2TixOyeAl2O3, (e) NiO/(20)K2TixOyeAl2O3, (f) NiO/(30)K2TixOyeAl2O3, and (g) NiO/(50)

K2TixOyeAl2O3.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 5 17647

dissolved in a distilled water of 200 cm3 and TiO2 of 2.52 g was

added to form a sol. Al2O3 of 16 gwas then added in the sol and

the sol was maintained for 10 min under stirring. By control-

ling the concentration of the K and Ti precursor and the

amount of Al2O3 in the sol, modified supports with different

amount of potassium titanate were obtained. In order to

remove the water, the sol mixture was heated and evacuated

for 3 h using a rotary evaporator (Eyela, N-1110S-W). After

evaporation, the collected wet powder was dried at 110 �Covernight and calcined at 850 �C for 6 h. In the second step,

Ni(NO3)2$6H2O of 5.11 g was dissolved in a distilled water of

100 cm3 and a calcined powder of 9 g in the former step was

added to this solution and the mixture was stirred for 10 min.

The water in the mixture was then removed using the rotary

evaporator in the same manner. The collected powder was

dried at 110 �C overnight and calcined at 850 �C for 6 h. The

obtained materials were denoted as NiO/K2TixOyeAl2O3 and

materials after reduction were denoted as Ni/K2TixOyeAl2O3.

The reduction procedure to convert NiO/K2TixOyeAl2O3 into

Ni/K2TixOyeAl2O3 catalysts was carried out at 800 �C for 2 h

under a H2 flow of 100 cm3/min as a pretreatment condition of

the catalytic activity test.

A NiO/Al2O3 catalyst prepared by the same method

mentioned above using Al2O3 of 9 g instead of K2TixOyeAl2O3

and a commercial catalyst FCR-4 (Sud-Chemie Co., ~12 wt.%

Ni/a-Al2O3) were used for comparison studies.

Characterizations

X-ray diffraction (XRD) patterns of both the calcined and spent

samples were measured with a X-ray diffractometer (Rigaku,

D/MAX2500) operated at 45 kV and 40 mA, using CueKa

monochromatized radiation (l ¼ 0.154178 nm). From the XRD

result, a crystallite size of a catalyst was calculated by

Scherrer equation: d ¼ Kl/bcosq; b, full width at half

maximum; K ¼ 0.94 and l ¼ 1.542 Å. The surface area of the

catalysts was measured in a surface area analyzer (Micro-

meritics, TriStar Ⅱ) using nitrogen physisorption after

degassing at 300 �C for 3 h under vacuum. The specific surface

area was determined according to the Brunauer-Emmett-

Teller (BET) equation in the relative pressure range of

0.05e0.25. The temperature-programed reduction (TPR) tech-

nique was used to investigate the reduction characteristics of

the calcined catalyst samples with hydrogen flow. For the H2-

TPR test, a sample of 0.1 g was loaded in a quartz U-tube and

heated from 30 to 900 �Cwith a heating rate of 10 �C/min and a

10% H2/He flow of 60 cm3/min. During the TPR procedure, a

portion of the exiting gas flow was sampled through a leak

valve into a mass spectrometer (HIDEN, HPR-20), and the

masses of 2(H2), 4(He), and 18(H2O) were monitored. The

microscopic feature and element distribution of the samples

were observed using a scanning electron microscopy (SEM)

(Hitachi, S-2700) coupled with energy dispersive spectroscopy

(EDS) (Horiba, EDXS). X-ray photoelectron spectra were

measured with a Thermo VG Scientific MultiLab2000 spec-

trometer equipped with an electrostatic analyzer. The anal-

ysis was carried out under a vacuum of at least 1 � 10�10 torr.

The X-ray source was the AleKa radiation. Survey scans were

conducted between 0 and 1300 eV binding energy at a rate of

1.0 eV s�1. Binding energies were recorded for the Ni 2p, K 2p,

Ti 2p, Al 2p, O 1s, and C 1s regions.

Catalytic activity

The activity test of the catalysts for methane steam reforming

was performed using the system including a continuous flow

fixed-bed reactor made of quartz or Inconel material and

reactivity tests were carried out at 750e850 �C under

Table 1 e Physical properties of NiO/Al2O3 and NiO/K2TixOyeAl2O3 catalysts.

Catalyst BET surfacearea (m2 gcat

�1 )Crystallitesizea (nm)

NiO/Al2O3 106.1 e

NiO/(5)K2TixOyeAl2O3 68.6 9.6

NiO/(11)K2TixOyeAl2O3 73.1 11.1

NiO/(20)K2TixOyeAl2O3 67.9 14.0

NiO/(30)K2TixOyeAl2O3 48.8 18.5

NiO/(50)K2TixOyeAl2O3 11.5 19.5

a Calculated from NiO(220) plane using Scherrer equation from

XRD for calcined samples.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 517648

atmospheric pressure. The steam-to-carbon ratio (S/C) of

1.0e3.0 and a gas hourly space velocity (GHSV) of 15,000 to

200,000 h�1 were used for this study. The inlet and outlet gas

after cooled using a cold trapwere analyzed on line using a gas

chromatograph (Donam Instrument, DS6200). After one hour

of steady-state operation at each temperature, the concen-

trations of H2, CH4, CO, and CO2 in product streams were

analyzed using a thermal conductivity detector (TCD) with a

HayeSep DB column and a flame ionized detector (FID) with a

OV-101-10% column. Conversion of CH4 (4) and selectivity of

CO (5), H2 (6), and CO2 (7) were calculated from the following

equations [30,31], in which the selectivity was defined as the

molar ratio of a product and a converted reactant compared to

a conventional method defined as an available maximum

amount of product from a reaction equation.

X ¼ ðFCH4 ;0 � FCH4Þ

FCH4 ;0(4)

SCO=CH4¼

�FCH4 ;0X� FCO2

�

FCH4 ;0X(5)

SH2=CH4¼

�3FCH4 ;0Xþ FCO2

�

FCH4 ;0X(6)

SCO2=CH4¼ FCO2

FCH4 ;0X(7)

FCH4 ;0: molar flow rate at which CH4 is fed to the system

FCH4 : molar flow rate at which CH4 leaves the system

FCH4 ;0X: molar flow rate at which CH4 is consumed within

the system

FCO2 : molar flow rate at which CO2 leaves the system

Fig. 2 e H2 temperature-programed reduction (TPR) profiles

for (a) NiO/Al2O3 and NiO/K2TixOyeAl2O3 catalysts with

K2TixOy loading: (b) NiO/(5)K2TixOyeAl2O3, (c) NiO/(11)

K2TixOyeAl2O3, (d) NiO/(20)K2TixOyeAl2O3, (e) NiO/(30)

K2TixOyeAl2O3, and (f) NiO/(50)K2TixOyeAl2O3.

Results and discussion

Characterization of the catalysts

X-ray diffraction patterns of g-Al2O3, NiO/Al2O3, and NiO/

K2TixOyeAl2O3 catalysts are presented in Fig. 1. The patterns

displayed several distinctive features. First, the NiO/Al2O3

sample shows the appearance of a single phase, which is

assigned to spinel NiAl2O4 (JCPDS 10-0339). The angles of the

XRD of the sample are slightly shifted from those of g-Al2O3

(JCPDS 10-0425). Secondly, for the samples with lower con-

tents of K2TixOy (Fig. 1(c and d)), the nickel oxide phase is well-

dispersed and the titanium component is isolated on the

catalysts. Thirdly, for the catalysts with higher contents of

K2TixOy (Fig. 1(eeg)), these samples have mixed phases

derived from the KeTieO system such as K2Ti2O5, K2Ti6O13,

and K3Ti8O17. With an increase of the K2TixOy amount, the

peak intensities of the nickel oxide phase noticeably

increased, indicating the agglomeration of NiO particles and

their isolation on the supports.

Table 1 shows the surface areas and crystallite sizes of the

NiO/Al2O3 and NiO/K2TixOyeAl2O3 catalysts. The crystalline

diameters of the NiO on K2TixOyeAl2O3 catalysts are calcu-

lated to be 9.6e19.5 nm and the size of NiO increases with

increasing K2TixOy content in the samples. Table 1 compares

the surface areas of the reference catalyst, NiO/Al2O3 to

modified NiO/K2TixOyeAl2O3 catalysts. After the K2TixOy

loading, the surface areas (11e73 m2 g�1) of the nickel cata-

lysts were smaller than that (106 m2 g�1) of NiO/Al2O3 mainly

because the density of the K2TixOy particles is larger than that

of the porous Al2O3 and some of the pores may be blocked by

the K2TixOy nanoparticles. Therefore it can be concluded that

NiO particles become isolated from the support for the cata-

lysts with higher contents of K2TixOy. Similar to the case of

any promoters [6,11e14], it is likely that the appropriate

amount, not high, of K2TixOy can induce dispersing nickel

component onto a support. Therefore, we confirmed from

combined results presented in Fig. 1 and Table 1 that NiO

particles are not isolated and can be well-dispersed onto only

the K2TixOyeAl2O3 supports when the content of K2TixOy is

below 20 wt.%.

Since the TPR curves of the supported metal catalysts are

strongly affected by the characteristics of the metal-support

Table 2 e Quantitative data from H2 temperature-programed reduction profiles of NiO/Al2O3 and NiO/K2TixOyeAl2O3

catalysts.

Catalysts Tm (�C) Fraction of total area (%)

a-Type NiO b-Type NiO g-Type NiO a-Type NiO b-Type NiO g-Type NiO

NiO/Al2O3 e e 790 e e 100.0

NiO/(5)K2TixOyeAl2O3 367 665 776 9.9 55.9 34.2

NiO/(11)K2TixOyeAl2O3 380 667 788 8.1 69.4 22.5

NiO/(20)K2TixOyeAl2O3 381 603, 672 788 7.8 66.6 25.6

NiO/(30)K2TixOyeAl2O3 414 665, 724 824 30.2 55.6 14.2

NiO/(50)K2TixOyeAl2O3 434 598 857 34.4 65.6 e

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interaction [32], TPR measurements were carried out to

examine the interaction between nickel species and the

K2TixOyeAl2O3 support. Fig. 2 shows the H2-TPR profiles of

calcined catalyst sampleswith different K2TixOy contents. The

H2O (amu ¼ 18) generating curves derived from the H2 con-

sumption were fitted using Gaussian-type functions and the

quantitative results are listed in Table 2. NiO/Al2O3 (Fig. 2(a))

exhibits single reduction band at around 790 �C indicating the

existence of the nickel aluminate phase. It is well known that

NiO can react with Al2O3 to form a highly stable NiAl2O4 phase

which has a lower reducibility than that of NiO [33].

All results from Fig. 2 and Table 2 show that NiO/K2Tix-OyeAl2O3 catalysts have more than two reduction peaks and

their dominant reduction peaks are located at a lower tem-

perature with higher K2TixOy contents. As Zhang et al. [34] and

Zhao et al. [35] reported, the reducible NiO peaks of the sup-

ported catalysts can be approximately classified into three

types: a-type (weak interaction between the NiO and support,

300e475 �C), b-type (medium interaction, 475e755 �C), and g-

type (strong interaction, 755e900 �C). Reducible NiO peaks of

NiO/K2TixOyeAl2O3 are widely located in the range of

400e850 �C as shown in Fig. 2(bef) including three sub-peaks

corresponding to the a-, b-, and g-type. In particular, b-type

NiO becomes dominant with the K2TixOy presence

(55.6e69.4%) and the fraction of g-type NiO decreases with the

K2TixOy loading (34.2% for NiO/(5)K2TixOyeAl2O3 to 0% for NiO/

(50)K2TixOyeAl2O3). In short, the metal-support interaction of

NiO/K2TixOyeAl2O3 is weaker than that of NiO/Al2O3. In other

words, K2TixOy in a Al2O3-supported Ni catalyst plays a role in

varying NiO species with weaker interaction between the

metal and the support. This is consistent with a previous

finding that NiO can be reduced easily in the presence of an

additive [10].

SEM images and Ni mapping using EDS for NiO/Al2O3 and

NiO/K2TixOyeAl2O3 catalysts are shown in Figs. 3 and 4.

Accordingly, NiO/Al2O3 (Fig. 3(a)) shows larger particles with

uniform size (about 100 mm in diameter). In contrast, NiO/

K2TixOyeAl2O3 catalysts (Fig. 3(bef)) consisted of small

particles distributed with various sizes of 10e100 mm in

diameter. Fig. 4 shows the distribution of Ni element on

Al2O3 and K2TixOyeAl2O3 supports. The mapping results in

Fig. 4 show that the NiO particles are uniformly dispersed

on the support. The NiO/(50)K2TixOyeAl2O3 sample (Fig. 4(f))

presents some inhomogeneity in the dispersion of Ni spe-

cies, which is consistent with the XRD results that the

sample shows strong intensity of NiO peak (Fig. 1(g)) and

large NiO particle size (Table 1), indicating agglomeration of

metal species.

Fig. 5 shows the XPS analysis of the K2TixOy/Al2O3 system

as a function of K2TixOy loading. At low K2TixOy loading, the

surface titanium signal, which is represented by the XPS

Ti(2p)/Al(2p) ratio referenced against the Al2O3 support signal,

increases linearly with the bulk titanium content, which is

represented by the K2TixOy loading, because every titanium

atom in this two-dimensional structure is detected by the XPS

measurement. However, at K2TixOy loadings above 11 wt.%,

the surface titanium signal deviates from linearity with

increasing bulk titanium content because of the formation of

the three-dimensional K2TixOy crystals. The XPS measure-

ments reveal that the surface potassium titanate monolayer

coverage is achieved at approximately 15% K2TixOy loading on

this alumina support, which corresponds to the loading value

at the intersection of the linear fit of two curves shown in

Fig. 5. Additional K2TixOy (higher than 15%) beyond the

monolayer coverage usually results in the formation of

K2TixOy crystallite because Al2O3 cannot accommodate any

additional K2TixOy in the two-dimensional overlayer. For NiO/

(50)K2TixOyeAl2O3 sample, those K2TixOy crystallites formed

by excess loading of K2TixOy interrupt interaction between

nickel and Al2O3 and then make aggregation of nickel species,

in agreement with the above XRD and SEM-EDS.

Activity of Ni/K2TixOyeAl2O3 catalysts

In order to investigate the effect of the reaction condition over

Ni/K2TixOyeAl2O3 catalysts, the Ni/(11)K2TixOyeAl2O3 catalyst

was tested under different S/C ratio and GHSV. The sample

was selected among Ni/K2TixOyeAl2O3 catalysts with different

K2TixOy content because it had not only well-known charac-

teristics for good catalysts such as larger surface area

(73 m2 g�1) and a good dispersion of active metal but also

K2TixOy amount closest to the 15% loading in the support for

the surfacemonolayer coverage. Thermodynamic equilibrium

calculations were carried out using HSC Chemistry 5 software

(Outokumpu Research Oy), and compared to the experimental

data as shown in Fig. 6 and Fig. 7(a). While thermodynamic

methane conversion increases with the S/C ratio at a tem-

perature of 750 �C, the experimental methane conversion over

the catalyst increased until the S/C ratio reached 2.5 and then

declined. In SMR over Ni/(11)K2TixOyeAl2O3 catalyst, a part of

active component Ni may be oxidized to NiO due to the effect

of excess steam at high temperature and this oxidation of

activemetal canmake low activity [7]. The activity test results

of the Ni/(11)K2TixOyeAl2O3 catalyst were close to the equi-

librium limit and was superior to those of Ni/perovskite

(77e80%) reported in previous research [23]. In addition, the

Fig. 3 e Scanning electron microscopy (SEM) images of the Ni catalysts: (a) NiO/Al2O3, (b) NiO/(5)K2TixOyeAl2O3, (c) NiO/(11)

K2TixOyeAl2O3, (d) NiO/(20)K2TixOyeAl2O3, (e) NiO/(30)K2TixOyeAl2O3, and (f) NiO/(50)K2TixOyeAl2O3.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 517650

effects of reaction temperature and GHSV on the catalyst are

shown in Fig. 7. The experimental methane conversion

increased with increasing reaction temperature like the

thermodynamic equilibrium conversion increases and the gap

between experiment and thermodynamic equilibrium con-

version is smaller with a higher temperature. Both the

methane conversion and hydrogen selectivity sharply

decreased in the highest GHSV of 150,000 h�1. The reason

might be that the Ni/(11)K2TixOyeAl2O3 catalyst cannot supply

adequate active sites at a very short residence time of re-

actants due to harsh atmosphere for reduction of Ni species or

oxidation of metallic nickel.

The activity for steam methane reforming over the Ni/

K2TixOyeAl2O3 catalysts was tested under the mild reaction

condition and the average results in a steady-state during 10 h

are listed in Table 3. With increasing the K2TixOy content,

catalytic activities over the Ni/K2TixOyeAl2O3 catalysts

showed no clear trend while a Ni/(20)K2TixOyeAl2O3 catalyst

showed both maximum conversion of methane (97.2%) and

selectivity of hydrogen (3.01), which were close to the ther-

modynamic equilibrium values (98.8% and 3.32, respectively).

Compared to the FCR-4 and Ni/Al2O3 catalysts, the Ni/K2Tix-OyeAl2O3 catalysts exhibited similar performances for the

steam methane reforming under the reaction condition.

Catalytic stability of Ni/K2TixOyeAl2O3 catalysts

A catalytic stability test for steammethane reforming over Ni/

K2TixOyeAl2O3 catalysts was performed at 800 �C with severe

reaction conditions of low steam-to-carbon ratio of 1.0 and

time-on-stream of 100 h (Fig. 8) that can derive deactivation in

a steam methane reforming reaction. We set FCR-4 and Ni/

Al2O3 as reference catalysts for this stability test and

compared their activities to those of the Ni/K2TixOyeAl2O3

Fig. 4 e Scanning electron spectroscopy (SEM) and electron dispersive spectroscopy (EDS) mapping images of the Ni

catalysts: (a)e(f) are the same as those in Fig. 3.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 5 17651

catalysts. In this long-term stability test, we focused on the

decreasing degree of activity to deactivation degree, to eval-

uate all catalysts applied. The deactivation degrees of the

catalysts were estimated from the difference between the

initial average activity (during the initial time period from 0 to

3 h) and the final average activity (during the terminal time

period from 97 to 100 h). According to the results, respective

deactivation degrees for the FCR-4 andNi/Al2O3 catalysts were

18.5 and 9.9%. Although the FCR-4 showed a big difference

between initial and terminal activity, its stable activity was

maintained after 20 h. On the other hand, the deactivation

degrees for Ni/(5)K2TixOyeAl2O3, Ni/(11)K2TixOyeAl2O3, Ni/(20)

K2TixOyeAl2O3, and Ni/(30)K2TixOyeAl2O3 were 4.9, 3.4, 9.9,

and 11.4%, respectively. It can be seen from the results that

the deactivation of Ni/K2TixOyeAl2O3 increased with

increasing K2TixOy loading. Particularly, the Ni/K2TixOyeAl2O3

catalysts that included the appropriate amount of K2TixOy

(11 wt.% or less) showed resistance to deactivation, which

may be derived from the carbon coking or sintering of the

catalyst.

Fig. 9 shows the XRD patterns of reduced Ni/K2TixOyeAl2O3

catalysts before and after the stability test. The crystalline

phase change and nickel crystallite size of Ni/K2TixOyeAl2O3

catalysts and reference catalysts are listed in Table 4. After

reaction, the diffraction peaks of nickel metal (2Theta ¼ 44.5,

51.7, and 76.7�) had almost disappeared for Ni/(5)K2Tix-OyeAl2O3, Ni/(30)K2TixOyeAl2O3, and FCR-4 catalysts,

oxidizing onto NiO. For the Ni/Al2O3 and Ni/(11)K2TixOyeAl2O3

catalysts, the nickel metal phases were retained and their

amount decreased, indicating larger Ni particles than those

before the reaction. The amount of nickel metal increased

only for Ni/(20)K2TixOyeAl2O3 catalyst. Based on these XRD

Fig. 5 e X-ray photoelectron spectroscopy analysis of NiO/

K2TixOyeAl2O3 catalyst as a function of K2TixOy loading.

Fig. 7 e Reaction temperature (a) and gas hourly space

velocity (b) effects for steam methane reforming over Ni/

(11)K2TixOyeAl2O3 catalyst for time-on-stream ¼ 20 h (the

reaction conditions: (a) S/C ¼ 2.5 and GHSV ¼ 15,000 h¡1;�

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 517652

results (Figs. 1 and 9, Tables 1 and 4), these stability test results

agree with a previous researcher's suggestion [36] that the rate

of deactivation increases with increasing Ni particle size. In

particular, fresh and spent Ni/(11)K2TixOyeAl2O3 catalysts

have a dominant phase, which is assigned to KTi8O13 (JCPDS

41-1097). In addition, the nickel particles of the spent Ni/(11)

K2TixOyeAl2O3 were well-dispersed on the support without

strong sintering and oxidizing. Therefore, it can be concluded

that the alumina-supported nickel catalyst with 11 wt.%

K2TixOy plays an important role in suppressing the sintering

and oxidizing of active metal.

Comparing the stability test results (Fig. 8) of the catalysts

with their H2-TPR features (Table 2), catalysts having both b

and g type reducible NiO higher than 90% showed better cat-

alytic stability. This means that catalysts having a mainly

Fig. 6 e Comparison of thermodynamic equilibrium value

and catalytic activity of Ni/(11)K2TixOyeAl2O3 catalyst for

steam reforming under the reaction condition:

GHSV¼ 15,000 h¡1, T¼ 750 �C, and time-on-stream¼ 20 h.

(b) S/C ¼ 2.5 and T ¼ 750 C).

weaker interaction between Ni and the support such as NiO/

(50)K2TixOyeAl2O3 and NiO/(30)K2TixOyeAl2O3 can not lead to

a stable performance. Previously, much efforts have been

made to understand the interaction between the metal

component and the support, and the strong metal-support

interaction (SMSI) was often used to explain experimental

Table 3 e Activities of steam methane reforming over Ni/K2TixOyeAl2O3 catalysts.

Catalyst XCH4 (%) SH2=CH4SCO=CH4

SCO2=CH4

FCR-4 97.5 3.06 0.73 0.22

Ni/Al2O3 96.9 2.97 0.79 0.15

Ni/(5)K2TixOyeAl2O3 95.6 2.90 0.76 0.15

Ni/(11)K2TixOyeAl2O3 94.6 2.84 0.74 0.16

Ni/(20)K2TixOyeAl2O3 97.2 3.01 0.77 0.18

Ni/(30)K2TixOyeAl2O3 91.9 2.69 0.69 0.16

Ni/(50)K2TixOyeAl2O3 94.7 2.86 0.73 0.17

The reaction conditions: S/C ¼ 2.5, GHSV ¼ 15,000 h�1, T ¼ 750 �C,and time-on-stream ¼ 10 h.

Fig. 8 e Change in activity of Ni/K2TixOyeAl2O3 catalysts

and reference catalysts with time-on-stream. (Reaction

condition: S/C ¼ 1.0, GHSV ¼ 200,000 h¡1, T ¼ 800 �C, andtime-on-stream ¼ 100 h).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 5 17653

phenomena [37e39]. However, TPR characterization results

demonstrate that NiO/K2TixOyeAl2O3 catalysts have weaker

metal-support interactions compared with the NiO/Al2O3, but

they show better catalytic stability. Therefore, the SMSImodel

is not enough to understand the phenomena mentioned

above. We thus can consider that the utilization of Ni species

Fig. 9 e X-ray diffraction patterns of fresh and spent samples of

(c) fresh and (d) spent Ni/(5)K2TixOyeAl2O3, (e) fresh and (f) spen

K2TixOyeAl2O3, (i) fresh and (j) spent Ni/(30)K2TixOyeAl2O3. (Fres

spent samples were taken after the stability test with the same

leading to the proper strength of the interaction between Ni

and the support can contribute to good catalytic stability of

the catalyst.

It is believed that there are several factors to cause deac-

tivation of reforming catalysts such as carbon deposition and

sulfur poisoning [26]. Therefore, it is necessary to investigate

the resistance of Ni/K2TixOyeAl2O3 catalysts to sulfur com-

pounds that can be easily found in a gaseous fuel like city gas.

In our future work, catalytic performance and further char-

acteristics of the Ni/K2TixOyeAl2O3 catalysts under these se-

vere conditions will be studied.

Conclusions

In this study, we report the characterization and catalytic

performance of K2TixOyeAl2O3 supported Ni catalysts for

steam methane reforming. The Ni/K2TixOyeAl2O3 catalysts

showed comparable catalytic activity to commercial FCR-4

under a mild experimental condition: at 750 �C, in a reactant

flow with S/C feed ratio of 2.5, at a GHSV of 15,000 h�1, and

under atmospheric pressure. A stability test result for Ni/

K2TixOyeAl2O3 catalysts under a severe reaction condition: at

800 �C, in a reactant flow with S/C feed ratio of 1.0, and at a

GHSV of 200,000 h�1 demonstrated that Ni/K2TixOyeAl2O3

catalysts with less than 11 wt.% K2TixOy in the support

maintaine their good stability for the reaction time of 100 h. It

is also found that the main factors accounting for the better

catalytic performance over the Ni/K2TixOyeAl2O3 catalysts

than Ni/Al2O3 are the medium interaction strength between

Ni/K2TixOyeAl2O3 catalysts: (a) fresh and (b) spent Ni/Al2O3,

t Ni/(11)K2TixOyeAl2O3, (g) fresh and (h) spent Ni/(20)

h samples were taken after the reduction pretreatment and

reaction conditions as those shown in Fig. 8).

Table 4 e Crystalline phases change and Ni metal sizes of the catalysts before and after the stability test.

Sample Crystalline phases Crystalline size of Ni (nm)c

Retaineda Changedb Before test After test

Ni/Al2O3 Ni, NiAl2O4, g-Al2O3 [NiO, YNi 12.6 23.9

Ni/(5)K2TixOyeAl2O3 Ni, TiO2, NiAl2O4, g-Al2O3 [NiO, YNi, YTiO2 17.4 e

Ni/(11)K2TixOyeAl2O3 Ni, KTi8O13, TiO2, NiO, NiAl2O4, g-Al2O3 [NiO, YNi, YKTi8O13 18.8 26.0

Ni/(20)K2TixOyeAl2O3 Ni, K3Ti8O17, K2Ti6O13, K2Ti2O5, NiAl2O4,

g-Al2O3

[Ni 16.1 30.5

Ni/(30)K2TixOyeAl2O3 Ni, K3Ti8O17, K2Ti6O13, K2Ti2O5, NiAl2O4,

g-Al2O3

[NiO, YNi 21.1 e

a For reduced samples.b After the stability test with reaction conditions the same as those in Fig. 8.c Calculated from Ni metal (111) plane using Scherrer equation by XRD.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 6 4 5e1 7 6 5 517654

Ni and the support, the proper amount of K2TixOy additive,

and their superior stability. This work demonstrates that

K2TixOy in the proper amount would be promising additive

material for alumina-supported nickel catalyst used in a

steam methane reforming process.

Acknowledgments

Following are results of a study on the “Leaders Industry-

University Cooperation” Project (C-D-2013-0806), supported

by the Ministry of Education, Korea.

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