Co@Ru nanoparticle with core–shell structure supported over γ-Al2O3 for Fischer–Tropsch...

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Co@Ru nanoparticle with coreeshell structuresupported over g-Al2O3 for FischereTropschsynthesis

Ali Haghtalab*, Amir Mosayebi

Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 14 March 2014

Received in revised form

17 August 2014

Accepted 16 September 2014

Available online xxx

Keywords:

Coreeshell structure

FischereTropsch synthesis

Co@Ru/g-Al2O3

Nanoparticles

Selectivity

* Corresponding author. Tel.: þ98 21 8288331E-mail addresses: [email protected].

Please cite this article in press as: HaghtAl2O3 for FischereTropsch synthesisj.ijhydene.2014.09.074

http://dx.doi.org/10.1016/j.ijhydene.2014.09.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Co@Ru/g-Al2O3 coreeshell structure catalysts with Co/Ru different weight ratios are suc-

cessfully prepared via surface displacement reaction. This novel route including reduction

of Co core by NaBH4 on the surface of g-Al2O3 and then substitution of Co species with Ru

species, the resultant of reduction of RuCl3 precursor with N2H4. These catalysts are

characterized with techniques X-ray diffraction (XRD), energy-dispersive X-ray spectros-

copy (EDX), high resolution transmission electron microscopy (HRTEM), N2 adsorption/

desorption (BET), temperature programmed reduction (TPR), X-ray photoelectron spec-

troscopy (XPS) and Fourier transform infra-red (FTIR) of CO adsorbed. The characterization

results confirm a uniform dispersion of Co@Ru nanoparticles with coreeshell structure

over g-Al2O3. The coreeshell Co@Ru/g-Al2O3 catalysts show the remarkable catalytic ac-

tivity towards FischereTropsch synthesis (FTS) in comparison with Co/g-Al2O3, which is

related to special coreeshell structure. These catalysts exhibited excellent abilities in the

cases of increasing formation of long-chain hydrocarbons and suppressing selectivity to

lighter hydrocarbons.

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

reserved.

Introduction

The conversion of natural gas to hydrocarbons (GTL) is

currently one of the most promising topics in the energy in-

dustry due to economic utilization of resources such as nat-

ural gas, coal or biomass to environmentally clean fuels,

especially waxes. FischereTropsch synthesis (FTS) reaction is

a well known way for production of hydrocarbons with

different chains from syngas (CO þ H2), that firstly was foun-

ded by Fischer and Tropsch in 1925 [1].

The numerous studies on FTS had been carried out, since

now. Usually in these studies the influence of different active

3; fax: þ98 21 82883381.ir, [email protected]

alab A, Mosayebi A, Co@, International Journa

74gy Publications, LLC. Publ

components, support species, methods of synthesis catalyst,

introduction of promoters, etc. had been investigated. Study

in the case of effect of catalyst structure on FTS performance

was less evaluated. The coreeshell structure due to higher

catalytic activity was considered in different processes.

Nevertheless, in this case few studies on FTS were reported

[2e5]. Xie et al. [2] solvothermally synthesized Co3O4@m-SiO2

nanocomposites and obtained better results (higher CO con-

version and long-chain hydrocarbon selectivity) in FTS in

comparison with Co3O4/m-SiO2. Yang et al. [3] founded the

addition of zeolite shell to Co/SiO2 and formation of a coree-

shell structure will improve catalytic performance of FTS.

Further, Yang et al. in another work [4] achieved the inverse

m (A. Haghtalab).

Ru nanoparticle with coreeshell structure supported over g-l of Hydrogen Energy (2014), http://dx.doi.org/10.1016/

ished by Elsevier Ltd. All rights reserved.

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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 x x x ( 2 0 1 4 ) 1e1 22

results when zeolite shell was added to Ru/SiO2 which resul-

ted in synthesis of Ru/SiO2@H-ZSM-5. Jiang et al. [5] prepared a

novel catalyst with a core (Fe/SiO2) e shell (silicalite-1), results

showed the production of light alkenes from syngas in FTS

substantially will increase in comparison with Fe/SiO2, while

CO conversion and production of heavier hydrocarbons de-

creases. All these results should be ascribed to the unique

coreeshell structure. Coreeshell structure have got attracted

much attention due to its higher potential in improvement of

the core with suitable shell components to reach particular

catalytic performance [6]. In the prior studies, a support (silica

or zeolite) played the shell role [2e5]. As mentioned, the type

of material used as shell has a critical impact in catalytic ac-

tivity. Thus, using a component as shell with higher catalytic

activity would participate in more intensification of FTS per-

formance. All group VIII metals (Fe, Co, Ru, Pt, etc.) have

notable activity in the CO hydrogenation [7]. Co (transition

metal) is the most common active metal used in FTS due to its

higher activity, higher selectivity for heavier hydrocarbons,

more resistant to deactivation and lower tendency to water-

gas shift reaction [8,9]. The alloy which is composed of tran-

sitionmetal and noblemetal has higher catalytic activity than

a metal, indicating the synergism effect between two metals

[10,11]. Noble metals usually used as promoter and have

strong influence on the structure and reducibility of Co, CO

conversion and hydrocarbons selectivity in FTS [7]. The results

had showed among noblemetals, Ru have themost synergism

with cobalt species [12]. Hereby, Ru was selected as shell and

Co as core. The formation of special coreeshell structure and

synergism between Co and Ru, led to more advantages in FTS.

In thepresent investigation, bimetallicCoeRunanoparticles

with a coreeshell structure supported over g-Al2O3 (Co@Ru/g-

Al2O3) in Co/Ru different weight ratios are synthesized by sur-

face displacement reaction method, which in previous litera-

tures had not been reported. For comparison of the results, Co/

g-Al2O3 and Ru/g-Al2O3 catalysts are also synthesized. The BET,

HRTEM,EDX,XPSandXRDcharacterizationanalysis areused to

evaluate the catalystsmorphology, elemental analysis, surface

chemical analysis, and crystal phase structure so that their re-

sults are confirmed the formation of coreeshell structure. The

TPR experiment carries out for consideration catalysts reduc-

tion behavior, the reducibility improved strikingly with coree-

shell structure formation. The FTIR characterization of CO

adsorption on the catalysts after reduction treatment is per-

formed and catalysts structural variation, as the resultant of

exposure to CO is discussed in details. Finally, effect of the

coreeshell structure formation is investigated on the efficiency

of the FTS catalyst. The FTS reaction performance of the cata-

lysts in terms of CO conversion, CH4 selectivity, long-chain

hydrocarbons selectivity and CO2 formation rate are analyzed

and compared with the other works.

Fig. 1 e The schematic diagram of the preparation of

Co@Ru nanoparticles on g-Al2O3.

Experimental

Preparation of Co/g-Al2O3 nanoparticle

Cobalt nanocrystals deposited on g-Al2O3 are prepared by co-

balt chloride reduction with NaBH4 solution in the presence of

water-ethanol. In detail, 0.5 g of g-Al2O3 (Merck) is dispersed in

Please cite this article in press as: Haghtalab A, Mosayebi A, Co@Al2O3 for FischereTropsch synthesis, International Journaj.ijhydene.2014.09.074

60 ml ethanolewater (Volume ratio ¼ 1:1) solution by ultra-

sonic at 323 K for 30 min. Then, 0.025 g of CoCl2.6H2O (Sigma

Aldrich) aqueous solution was added into the suspension,

stirred and purged byN2 for 90min. Suspension is saturated by

N2, NaBH4 prepared solution (0.005 g in 10 ml of 0.5 M NaOH

solution), as reducing, is added drop wise into the solution

under stirring at room temperature. After mixing of the sus-

pension for 12 h, a clear change from pink to brown was

observed, indicating the reduction process is taking place as

CoCl2$6H2Oþ 2NaBH4/CoYþ2NaClþ 2BH3þ6H2OþH2 (1)

Hereby, Co nanocrystals are formed over g-Al2O3 support.

Preparation of Co@Ru/g-Al2O3 catalyst

The Co@Ru/g-Al2O3 nano-structure catalysts are synthesized

using a surface displacement reaction. Briefly, the solution

including Co/g-Al2O3nanoparticles was heated to 513 K. 10 ml

of RuCl3 (Sigma Aldrich) aqueous solution is added into sus-

pensionunder stirring for 1 h. At this time, a color change from

brown to black is taken place. Afterward, 3 mL of hydrazine

hydrate solution as a reducing is added drop wise under stir-

ring. After 6 h, the solution is left to settle. Then, the solid

sample is filtrated using circle filtration paper and washes

several times with ethanol and deionized water. The catalysts

are dried in vacuum condition at 373 k for 12 h. In order to

remove impurities and residuals, the catalysts are calcined at

773 k for 6 h. The schematic illustration of the preparation

method of Co@Ru nanoparticles on g-Al2O3 is shown in Fig. 1.

In this study to comparing, the Co@Ru/g-Al2O3 catalysts at

the different Co/Ru ratios (from 9 to 1) are synthesized so that

the cobalt loading is 5 wt% for all the catalysts. To synthesis

the Co/g-Al2O3 catalyst, a solution containing Co/g-Al2O3 is

washed with DDI water and ethanol for several times, then,

dried at 373 k and finally calcined at 773 K for 6 h. The Ru/g-

Al2O3 catalyst is prepared directly in g-Al2O3 suspension slurry

through reducing the RuCl3 using hydrazine hydrate. Co/g-

Al2O3 and Ru/g-Al2O3 catalysts were noted as CA and RA,

respectively.

The loading of cobalt and ruthenium in the calcined cata-

lysts is verified by an Inductively Coupled Plasma Atomic

Emission Spectroscopy (ICP-AES) system after complete

dissolution of the catalysts (5mg) in 5ml of HNO3/HCl solution

(1/3 volumeratio). The theoreticalCoandRucontents in theCA

and RA catalysts are targeted as 5 and 0.55 wt%, respectively.

The ICP analysis shows the actual contents of Co and Ru are

4.93 and 0.64 wt%, respectively. The Co@Ru/g-Al2O3 catalysts

nomenclature and compositions are summarized in Table 1.




Table 1 e The composition of the Co@Ru/g-Al2O3

catalysts.

Sample A1 A2 A3 A4 A5

wt.% Co (theoretical) 5 5 5 5 5

wt.% Ru (theoretical) 0.55 1.25 2.14 3.33 5

Co/Ru ratio 9 4 2.33 1.5 1

wt.% Co (ICP) 5.22 5.18 4.81 5.03 4.9

wt.% Ru (ICP) 0.61 1.23 1.95 3.52 5.15

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 2 3

Characterization

The XRD characterization of the calcined catalysts is obtained

on a X'Pert MPD X-ray diffractometer (Philips Co.) using Cu/Ka

radiation (l ¼ 0.154 nm). The X-ray tube is operated at 40 kV

and 30 mA. The average CoO particle size (d) is calculated

using Sherrer equation as d ¼ 0.89l/b cosq where l is the

wavelength of X-ray and b is the full width half maximum of

the CoO diffraction peak with 2q ¼ 42.4 in radians.

The Fourier transform infra-red (FT-IR) spectra is carried

out using a Perkin Elmer FT-IR spectrometer frontier. The

catalyst (1mg) is placed in an infrared cell equippedwith ZnSe

windows and a heater for temperature control. The spectrum

was recorded in wavelength the range of 1700e2200 cm�1.

Before an IR spectrum record, the catalyst is reduced for 12 h

with a stream hydrogen (GHSV ¼ 900 h�1) at 450 �C under at-

mospheric pressure. Then, the system is cooled to 30 �C so

that a stream CO is introduced for 1 h. At the end, the catalyst

is purged using nitrogen for 30 min to remove CO gaseous.

The surface area, pore volume and average particle size of

the catalysts are measured by nitrogen adsorption/desorption

using a Belsorp mini II apparatus. The TPR experiment is

carried out using QuantachromeChemBET-3000 so that the

0.05 g of catalyst is placed in a quartz tubular reactor. Prior to

the temperature-programmed reduction measurement, the

calcined catalysts are heated to 150 �C in a pure N2 steam and

held at this temperature for 30 min in order to remove water

or impurities. The sample is then cooled down to the room

temperature in N2 and the gas is switched to 5% H2/Ar, then

catalyst temperature increases to 1073 k at 10 k min�1. To

measuring the H2 consumption amounts, equipment is pre-

viously calibrated using AgO2 reduction as the reference. The

reduction degree of catalysts in range of 150e400 �C is evalu-

ated based on the hydrogen amount consumption. The

morphology of catalyst is specified using Philips CM30 high

resolution transmission electron microscopy (HRTEM). The

compositional analysis of catalyst is measured by the energy-

dispersive X-ray spectroscopy (EDX) (JEOL, JED-2300). The X-

ray photoelectron spectroscopy (XPS) tests are performed to

surface chemical analysis on a DLD (Kratos AXis Ultra) spec-

trometer, employing Mg Ka radiation. The dispersion values

for CO and Ru are calculated using the XPS test data as

Dispersion ð%Þ

¼ number of metallic atoms on the surfacetotal number of metallic atoms

� 100(2)

FischereTropsch synthesis reaction

The performance of the catalysts for FTS is tested in a setup

that is shown in Fig. 2. The samples (0.3 g) are loaded in a


fixed-bed reactor (i.d. ¼ 10 mm). Prior to FTS reaction, the

catalysts are reduced at 450 �C in atmospheric pressure for

12 h by a flow of pure hydrogen. Then, the reactor temperature

decreases to 220 or 230 �C under hydrogen flow. The syngas

with CO/H2 ¼ 2 and GHSV ¼ 900 h�1 flows in the reactor while

the pressure increases slowly to 2 MPa so that the reaction is

carrying out for 10 h. The products are collected in a heat trap

(90 �C) and a cold trap (0 �C). The outlet gases and liquids are

analyzed with Agilent 7890A refinery gas analyzer and Agilent

DHA analyzer, respectively.

The main overall reactions of the FTS over a coreeshell

structure Co@Ru/g-Al2O3 catalyst can be written as

nCoþ ð2nþ 1ÞH2/CnH2nþ2þnH2O ðParrafins formationÞ (3)

nCoþ ð2nÞH2/CnH2nþnH2O�Olefins formation

�(4)

The CO conversion and hydrocarbons selectivity are

calculated for depicting the FTS reaction performance as

follows:

CO conversion ðmol%Þ ¼ COin � COout

COin� 100% (5)

Hydrocarbon selectivity ðg%Þ

¼ Amount of desired hydrocarbonTotal amount of hydrocarbons

� 100%(6)

It should be noticed that the water-gas shift reaction is

ignored here so that production of CO2 is negligible in the

presence of the Co catalyst.

Results and discussion

Structural features

Before reduction treatment, the crystal structure of catalysts is

determined by XRD as illustrated in Fig 3A. In the XRD of cat-

alysts except RA, the peaks at 2q ¼ 36.5�, 42.4� and 61.52�

ascribed to CoO with cubic structure. Based on the previous

studies, the cobalt oxide phase is presented in XRD pattern as

Co3O4 form. In this work due to using reducer in catalysts

preparationmethod, one stepof oxidationcobalt is passed that

is matched with the result of TPR test [13e20]. The diffraction

patterns show the broad peaks at 2q ¼ 25.57�, 37.78�, 46.4� and66.2� corresponding to g-Al2O3. The 48.4� peak in the spectrum

of CA catalyst is ascribed to formation the cobalt aluminate,

this peak was not observed in the XRD patterns of other cata-

lysts [21,22]. However, the intensity of this peakwas low. In the

XRD spectrum of RA, the most intense peaks at 2q ¼ 28.18�,35.29�, 54.7� are observed that is attributed to RuO2 with

tetragonal structure. In agreement with the literature [23,24],

positive shift of the RuO2 peaks is occurred obviously on the

Co@Ru/g-Al2O3 catalysts comparing with the RA, that are

shown in Fig. 3BeD. The RuO2 peaks for A1eA5 catalysts are

located at 2q ¼ 28.2�, 35.32�, 54.76�.Zhanget al. [23] also founded that Pd peaks positions at

Ni@Pd/MWCNTs are shifted in comparison with the Pd/

MWCNTs catalyst, whiles the peaks positions are attributed to

Ni (core) at Ni@Pd/MWCNTs so that the Ni/MWCNTs catalysts




Fig. 2 e The schematic diagram of experimental setup.


are exactly the same. Comparing with Pt/C catalyst, there is a

noteworthy positive shift (0.87�) of the XRD peaks related to Pt

in Co@Pt/MWCNTs [24]. The shift of ruthenium oxide peaks as

a result of coreeshell structure formation would be related to

CoO atoms diffusion into ruthenium oxide crystals that is

caused the slight transformation of ruthenium oxide crystal

lattice distance. The change in crystal lattice structure of the

noble metals would improve its catalytic performance

because of synergism effect [25]. After the introduction of Ru,

the A5 catalyst had most intensity, while the least XRD peak

intensity of CoO is for A1. Therefore, with formation of the

coreeshell structure and enhancing Ru loading, intensity of

CoO peaks increased and it was narrowed. Li et al. [26] foun-

ded by increasing of Ru loading (1e4 wt%) in RueCo/SG/Z

catalyst lead to reduction of the Co3O4 peaks intensity. On the

other hand, as it is displayed in Fig. 3A, decrease of Co/Ru ratio

in Co@Ru/g-Al2O3 catalysts leads to enhancing of the intensity

of the peaks as ascribed to RuO2. The average CoO particle size

of the catalysts is measured by the XRD pattern using scherrer

equation at 42.4� that is found to be in the range of

7.3e10.7 nm as shown in Table 2. The broad and small

intensive CoO peak for CA is a result of the least size of 7.3 nm

that is proved by the XRD spectrum.


With formation of the coreeshell structure, size of cobalt

oxide nanocrystal increases to 0.8 nm and with enhancing

ruthenium loading, the average size of cobalt oxide nano-

particle increases from 8.1 to 10.7 nm so that this enhance-

ment could be attributed to a lower number of cobalt oxide

crystallization sites by addition of Ru and formation of the

coreeshell structure. Park et al. [19] observed that the Co3O4

particle diameter enhanced through increasing of catalyst

synthesis temperature. Also in this work, for preparation of

the coreeshell structure catalysts in comparison with CA

catalyst, the synthesis temperature increases to 240 �C.Following addition of the noble metals such as Ru, Re, Pt, the

cobalt oxide nanocrystal size reduces as was observed in the

previous works [27e30]. De la osa et al. [31] founded by adding

calcium to Co/Al2O3 catalyst leads to enhancing remarkably

cobalt oxide nanoparticle diameter.

The HRTEM images describe the crystallite nature of some

selected catalysts as shown in Fig. 4. As it can be seen in

Fig. 4(A), in the case of the CA catalyst, the spherical shape of

CoO nanocrystals uniformly dispersed on the support

external surface that it indicates the advantage of this syn-

thesis method compared to conventional ones. The average

particle size is measured as follows:




Fig. 3 e (A) The XRD patterns of synthesized catalysts before reduction behavior, (B)e(D) The enlarged XRD patterns of RA

and A1 catalysts at different diffraction.


dTEM ¼ nid3iP

n d2 (7)

P

i i

where ni is the number of particles with given diameter di [26].

The CoO average particle size for the CA catalyst is 8.1 nm that

is almost coincided with the value calculated by XRD as given

in Fig 4(B). Fig 4(CeD) showed that the CoO core is well covered

by RuO2 shell over g-Al2O3 so that is confirmed formation of

the coreeshell structure for A1 and A3. Using this synthesis

method, no damage is occurred at the CoO core following

RuO2 deposition. The average particle size (coreþ shell) for the

A1 and A3 catalysts are about 16.3 nm and 22.1 nm, respec-

tively, so that their average CoO particle sizes are 12.7 and

15.1, respectively. Therefore, by decreasing of Co/Ru ratio the

HRTEM results show the CoO particle size enhances that is in

agreement with the XRD results; however the calculated

values, which are obtained byHRTEM, aremore than those are

given by XRD. This indicated the agglomeration of particles is

emerged during the synthesis method of coreeshell structure

Table 2 e The textural parameters of the preparedcatalysts.

Catalyst Surfacearea(m2/g)

Total porevolume(cm3/g)

Averagepore size

(nm)

Average CoOcrystal size(nm) XRD

g-Al2O3 219 0.515 8.5 e

CA 181 0.48 7.8 7.3

A1 178.3 0.432 6.9 8.1

A2 174.1 0.429 6.8 8.7

A3 171.5 0.445 7.5 9.2

A4 165.7 0.445 7.4 9.6

A5 163.2 0.441 7.2 10.7


catalysts. Also by reducing of Co/Ru ratio, an increase in the

shell thickness from 3.6 nm to 7 nm is visible as seen in

Fig. 4(CeD). The EDX results for selected catalysts are shown

in Fig. 5. The elemental analysis by EDX proved that CA

catalyst composed of Co and Al, while A1 and A3 included Co,

Al, Ru. The O signal in patterns came from the oxidation of

surface metal atoms.

The results of BET surface area and porosity measurements

are shown inTable 2.TheBETsurfaceareaandporosityamount

of the loaded catalysts are clearly lower than g-Al2O3. The

dilution effect of the support andpartial blockage of its pores by

active components (Co andRu) during thecatalysts preparation

are responsible for reducing in the surface area and porosity

amounts [32]. An increase in Ru loading at the range 0.55e5%,

differences in pore volume and average pore size are not

observed significantly so that the surface area decreases from

178.3 to163.2m2/g.AmongCo@Ru/g-Al2O3 catalysts,A3 catalyst

presents the highest values for pore volume and average pore

size that are 0.445 cm3/g and 7.5 nm, respectively.

The results were obtained using the XPS experiments for

calcined catalysts as are shown in Table 3. The notable

photoelectron peaks were Co2p3/2, Co2p1/2, C1s (Ru3d3/2),

Ru3p3/2, Ru3p1/2, O1s and Al2p. Of course, the intensity of

peaks attributed to Ru3p3/2 and Ru3p1/2 were weak. The peak

at 780.7 eV with spineorbital splitting of 15.1 eV (Co2p) for CA

catalyst was a characteristic of dominant CoO phase on

catalyst surface [33]. In A1 and A3 catalysts compared with CA

catalyst, positions of Co2p3/2, and Co2p1/2 peaks shifted to

higher values. Existence of Co2p energy bindings in A1 and A3

catalysts, showed the presence of Co2þ ions on these catalysts

surface [16]. By adding Ru to CA catalyst, a decrease in Co/Al

surface atomic ratio from 0.0033 to 0.0001 than bulk value was

observed (see Table 4). It implied the migration of Co2þ from




Fig. 4 e HRTEM image of CA (A), cobalt oxide particle size histogram of CA (B), HRTEM image of A1 (C), HRTEM image of A3 (D).

Fig. 5 e EDX spectrums for catalysts A) CA, B) A1, C) A3.


Please cite this article in press as: Haghtalab A, Mosayebi A, Co@Ru nanoparticle with coreeshell structure supported over g-Al2O3 for FischereTropsch synthesis, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.074



Table 3 e XPS data for calcined catalysts.

Catalysts Co2p3/2

bindingenergy (eV)

Co2p1/2

bindingenergy (eV)

Ru3d3/2 þ C1sbinding

energy (eV)

Ru3p3/2

bindingenergy (eV)

Ru3p1/2

bindingenergy (eV)

O1s bindingenergy (eV)

Al2p bindingenergy (eV)

CA 780.7 795.8 285.6 e e 530.2 73.76

A1 780.93 795.91 285.9 463.7 483.9 530.25 73.79

A3 781.56 796.22 285.95 463.2 483.3 530.27 73.77

Table 4 e Comparison between bulk atomic ratio and surface atomic ratio for calcined catalysts.

Catalysts Bulk atomic ratio (EDX) Surface atomic ratio (XPS) Dispersion (%)

Co/Al Ru/Al Ru/Co Co/Al Ru/Al Ru/Co Co Ru

CA 0.023 e e 0.0033 e e 14.7 e

A1 0.023 0.0016 0.069 0.0029 0.0015 0.51 12.6 93.7

A3 0.023 0.006 0.26 0.0001 0.0013 1.3 0.4 12.6


external surface to support during synthesis of Co@Ru/g-Al2O3

[16]. Of course, it can be expressed by coverage of CoO parti-

cles with RuO2 particles, so that reaching the Co photoelec-

trons into detector became more difficult. This coverage can

lead to decrease in CoeAl interaction and inhibition from

forming cobalt aluminate, which was consistent with XRD

and TPR patterns (see Figs. 3 and 6). Moreover, by enhancing

Ru loading, the dispersion of Co remarkably decreases from

14.7% to 0.4% that indicated enhancement in cobalt oxide

particle size, whichwas in consistencewith the XRD results as

seen in Table 2. C1s resulted from the unavoidable impurities,

where had an overlap with Ru3d3/2 peak [16]. The binding

energies of Ru3d3/2, Ru3p3/2, Ru3p1/2 peaks for A1 and A3 were

assigned to RuO2 [34]. Binding energies of Ru3p shifted to

lower position bymore Ru loading, it indicated higher electron

density or lower oxidation states of Ru [16]. As it is shown in

Table 4, the surface Ru/Co atomic ratios of A1 and A3 (0.51 and

1.3) were far higher than bulk values (0.069 and 0.26), which

described uniformly dispersion of Ru particles on the external

surface and formation of higher values of surface concentra-

tion. It was clear in Table 4 that surface Ru/Al atomic ratio

decreases from A1 to A3, which indicates Ru higher dispersion

of A1 (93.7%) on the support than A3 (21.6%). Hereby, the Ru

Fig. 6 e H2-TPR profiles of the sy


particle size increases with more Ru loading. This conclusion

is in agreement with the XRD results, where the intensities of

Ru peaks for A1 was weak and then increase in Ru amount led

to higher peaks intensities. It should be noted that HRTEM

results confirmed an increase in shell thickness by the

enhancement of Ru loading as seen in Fig. 4. According

to Table 4, the dispersion values of Ru for A1 and A3 were very

higher than the Co dispersion values, which indicated that

higher percentage of the surface is covered by Ru compared to

Co. According to the present results with XPS data, it can be

explained that during synthesis of A1 and A3 via surface

displacement reaction method, higher percentage of CoO

migrated to support interior. And it was covered by RuO2,

where its higher percentage located on the support external

surface. Therefore, core was rich in cobalt oxide, while shell

was rich in ruthenium oxide, suggesting formation of CoeRu

alloy with coreeshell structure.

TPR experiments

Fig. 6 shows the reduction behavior of the catalysts that are

characterized through the TPR experiments. For the TPR

spectrum of RA catalyst, a small reduction peak at 149 �C is

nthesized catalysts for FTS.




Table 5 e The H2 consumption and reduction degree at150e400 �C for the prepared catalysts.

Sample H2 consumption (mol) Reduction degree (%)

CA 0.000115 68.0

A1 0.000134 80.1

A2 0.000145 86.2

A3 0.000157 93.2

A4 0.000162 96.4

A5 0.000166 98.7

Fig. 7 e FTIR spectrum of CO adsorbed at temperature 30 �Con selected catalysts pretreated with H2 for 12 h at 450 �Cand subsequent desorption with N2 for 30 min at 30 �C.


observed that is attributed to the reduction of ruthenium

oxide at metallic ruthenium. For CA catalyst, the two reduc-

tion peaks are observed that are located at 366 and 525 �C.Although the different studies on TPR behavior of Co/Al2O3

catalysts have been made, however there are the different

interpretations about the attribution of the reduction peaks so

that at the present catalyst, the first reduction peak at 366 �Ccould be attributed to reduction of CoO to metallic cobalt. The

high temperature reduction peak at 525 �C would be assigned

to reduction of cobalt aluminate formation due to a strong

interaction between cobalt and the alumina support [35]. In

most works demonstrated that cobalt aluminates species are

formed through the reaction of dispersed CoO with alumina,

resulted in TPR spectrum shifted to higher temperature so

that the cobalt oxide reduction is more difficult [35]. Of course

in this investigation [19,35], the reduction temperature of co-

balt aluminate is lower in compared to the other works that

this would be related to weak interaction between cobalt and

alumina. For the coreeshell catalysts, the peak position of the

first reduction (CoO/Co�) is shifted to lower temperature in

compared with the CA catalyst. Furthermore, the peak posi-

tion of the second reduction is disappeared due to that there

was no interaction between cobalt and support with addition

of ruthenium that is in accordance with XRD patterns [36].

For Co@Ru/g-Al2O3 catalysts with the different ratio of Co/

Ru, hydrogenation of the CoO nanoparticles isn't straightfor-ward due to the protective effect of the ruthenium shell,

although, the presence of the ruthenium leads to the reduc-

tion of cobalt oxide to metallic cobalt. At first, H2 adsorbed at

Ru and consequently, it is dissociated to active hydrogen atom

that could diffuse in Ru shell to react with cobalt oxide (core)

and contribute to facilitate its reduction [37]. Generally, the

reduction of Co@Ru/g-Al2O3 catalysts ismore convenient than

the CA catalyst. In contrast with this work, the formation of

the coreeshell structure for Co3O4/m-SiO2 catalyst led tomore

difficult reduction than Co3O4/m-SiO2 catalyst which was

synthesized by impregnation method [2]. By decreasing the

ratio of Co/Ru from 9 to 1, the first reduction peaks are located

at 335 �C, 323 �C, 317.5 �C, 314.4 �C and 312.2 �C. Referring to

the different works [29,38e41], ruthenium presented more

significant influence on the reduction peak of Co3O4 to CoO

than CoO to metallic cobalt that is in accordance with the TPR

results in this work, however, the protective role of the

ruthenium shell is effective. Nevertheless, Jacobs et al. [42]

investigated presence of Re promoter on the reduction

behavior of Co/alumina and founded Re only affected on

second reduction peak of cobalt oxide (CoO/Co�). For the

catalysts with the coreeshell structure, the intensity of the

reduction peak increases with enhancing in the CoO particle

size from 8.1 to 10.7 nm (XRD results) so that this implies the

easier reduction by increasing in Ru loading. Park et al. [19]

observed that the intensity of reduction peak increases with

enhancing Co3O4 particle size in Co/g-Al2O3 catalysts.

The H2 consumption amount and reduction degree at

150e400 �C are calculated by the TPR test, as depicted in Table

5. The reducibility in the range of 150e400 �C is very significant

that is found to be related with the catalyst reducibility during

FTS reaction [26]. The formation of the coreeshell structure,

firstly leads to considerable growth in the reduction degree

from 68% to 80.1%. By enhancing ruthenium loading, the


reduction degree increases from 80.1% to 98.7%. At the same

time, the H2 consumption presents a similar behavior how-

ever variation in H2 consumption shows not as much as

reduction degree. The growth in reduction degree leads to an

increase of cobalt oxide particle size that is observed by the

others [18,19]. The highest reducibility and H2 consumption

are 0.000166 mol and 98.7% for A5, respectively. Using ruthe-

nium as shell lead to decrease reduction temperature of cobalt

species by Hads spillover from Ru to Co, that is the result of

high activation and dissociation of H2 by Ru [43]. Therefore,

the cobalt oxide nanoparticle would be activated easier to

provide cobalt active sites over g-Al2O3 for FTS, which results

in higher reduction degree for the coreeshell structure

catalysts.

FTIR analysis

Following reduction with H2 at 450 �C for 12 h, CO chem-

isorbed at the CA, A1 and A3 catalysts surfaces by demon-

stration using the FTIR test as shown in Fig. 7. The bands

around 2169 and 2120 cm�1 could be assigned to CO adsorbed

on cations (Co2þ, Co3þ and Al3þ) [7]. The CO wavelength for

these complexes was very close to CO wavelength in the

gaseous phase; however, these complexes had a significantly

stability comparing with the physisorbed CO species [7]. The

peaks at 2057 and 2035 cm�1 may be attributed to linear-type

CO adsorbed on metallic cobalt [7]. Moreover, the peaks be-

tween 1900 and 2000 cm�1 are assigned to bridge-type CO

adsorbed at themetallic cobalt sites. Finally, one can conclude

that the peaks at 1978 and 1936 cm�1 could be assigned to the

bridged CO complexes (CoeCoeCo) [44,45]. Song et al. [46]




Fig. 8 e The FTS performance of the synthesized catalysts.

Table 6 e The product distributions of FTS for thesynthesized catalysts.

Catalysts Reactiontemperature (�C)

Hydrocarbonselectivity (%)

CH4 C2eC4 C5þ

CA 220 24.06 51.59 24.33

230 24.47 50.12 25.39

A1 220 13.43 33.00 53.55

230 13.12 32.24 54.62

A2 220 13.31 27.93 58.75

230 12.49 26.21 61.28

A3 220 11.48 22.43 66.07

230 11.20 21.90 66.88

A4 220 17.02 15.08 67.94

230 16.86 14.94 68.27

A5 220 18.37 13.75 67.86

230 17.86 13.36 68.76

Reaction conditions: 2 MPa, CO/H2 ¼ 2, GHSV ¼ 900 h�1.


observed the peaks around 1974 and 2054 cm�1 that could be

related to CO adsorption at cubic metallic cobalt while the

peaks around 1937 and 2035 cm�1are assigned to CO adsorp-

tion on hexagonal cobalt phase. The adsorption peaks at 2051

and 1905 cm-1 are observed for the catalyst Co/ITQ-2, which

are ascribed to CO adsorption in linear and bridge forms on

metallic cobalt sites, respectively [47]. The FTIRmaxima peaks

positions depend on the nature of cobalt species and support.

Using ruthenium in the Co/g-Al2O3 catalyst, the position of

bands assigned to CO adsorption at different states of cobalt

are shifted from 1936, 1978, 2035, 2057, 2169 and 2120 cm�1 to

1933, 1977, 2033, 2055, 2170 and 2122 cm�1. These shifted

peaks could be evidence of an electronic interaction between

metallic cobalt and ruthenium in the coreeshell structure.

Following ruthenium introduction, a sharply decrease in peak

intensity located at 2170 and 2122 cm�1 is observed that this

reduction can be due to enhancing of reducibility.

Enhancing number of cobalt active sites leads to increase

of peak intensities in the CO adsorption for both linear and

bridge types. The cobalt reducibility significantly improved by

Ru, this led to increase in number of cobalt active sites in the

A1 and A3 catalysts than CA catalyst. Of course, Ru presents a

protective role on Co and resistance in formation the different

CO bands formation with Co. As a result, the intensity of the

linear and bridge types adsorbed CO for the A1 andA3 catalysts

aremore than the CA catalyst and by increasing of Ru content,

this peak intensity increased. Tsubaki et al. [48] observed that

using Ru promoter leads to enhancement the intensity of

bridge-type adsorbed CO. Xiong et al. [36] found for the Co/Ru/

SBA-15 catalyst, the band intensities of CO adsorption in-

crease for both the linear and bridge types using ruthenium

promoter and no frequency shift is observed. For the A1 and A3

catalysts, the band at 2041 cm�1 may be assigned to CO

adsorbed linearly on metallic Ru [49] and peaks at 2136 and

2073 cm�1 could be assigned to the stretching vibration of

tricarbonyl species adsorbed on oxidized Ru (Runþ(CO)3)

[50e52]. In contrast with this work, no adsorbed CO peak on

Ru for Co/Ru/SBA-15 is observed, however for Ru/SBA-15,

there are three significant bands at 2036, 2074, 2138 cm�1

[36]. Elmasides et al. [53] observed high intensity peaks of CO

adsorbed on Ru for Ru/Al2O3 at 2005, 2138, 2070 cm�1. With

formation of the coreeshell structure and decreasing the Co/

Ru ratio, intensity of the bands attributed to adsorbed CO on

ruthenium in metallic state increases as a result of enhancing

catalyst reducibility and ruthenium loading. However, the

peaks intensities related to Runþ(CO)3 complexes present very

little decrease.

Catalytic tests

Using the present catalysts, the FTS is carried out at the

different temperatures and the results are shown in Fig. 8. The

reduction degree is low for CA catalyst that leads to decrease

in the number of accessible cobalt active sites for CO and thus,

the CO conversion reduces [54]. By loading Ru to CA catalyst,

the CO conversion increases from 10.81% to 23.83% and from

12.4% to 28.56% at 220 �C and 230 �C, respectively. This

ascribed to enhancement in reduction degree from 68% to

80.1% in which higher metallic Co sites were obtained for FTS

and of course, the main reason was formation of the


coreeshell structure for A1 catalyst. A strong electronic

interaction of core atoms on shell surface ones, leads to the

higher catalytic activity of Co@Ru/g-Al2O3 [55]. Moreover, by

increasing Ru loading in A1-A5 catalysts, the CO conversion

enhances significantly from 23.83% to 57.27% and from 28.56%

to 58.08% at 220 �C and 230 �C, respectively. This indicates

increase in the shell thickness from A1 to A5, besides, the in-

crease in reduction degree from 80.1% to 98.7% is also effec-

tive. In the different works [26,39,43], Ru plays a promoter role

for the Co catalyst so that by H2 spillover from Ru to Co cat-

alytic activity in FTS is improved. Iglesia et al. [56] observed

that catalytic activity is proportional to the metallic Co

dispersion and is independent of the support. The small cobalt

particle size with high metallic surface area causes an in-

crease in dispersion ability of metallic Co on the RueCo/

HZSM-5 that resulted in higher CO conversion [39].

The CO conversion increases from 44.6% to 75.1% at 220 �C[26] through increasing Ru loading in range of 0e5% for the

Rue20%Co/SG/Z catalysts. In this study with the same Ru

loading, the CO conversion increases from 10.81% to 57.27% at

220 �C that this could show considerable effect of the coree-

shell structure on catalytic activity in CO hydrogenation. On

the other hand, a decrease in CO conversion is observed using

Ru/SiO2@H-ZSM-5 catalyst in comparison with the Ru/SiO2 so

that thismight be related to covering the Ru active sites during

synthesis reaction [4]. Using the synthesized catalysts, the FTS





product distributions are illustrated in Table 6. Clearly under

the same process conditions, the A1-A5 catalysts produce

higher C5þ selectivity than CA catalyst with the lower selec-

tivity toward CH4 as shown in Table 6. These are closely

related to the particular coreeshell structure of the catalysts.

In the coreeshell structure catalysts, the Ru shell metal plays

a principal role in the catalytic properties which could be

modified by secondarymetal that is located in the Co core [57].

Moreover, Ru compared with Co have higher tendency to

production of the heavier hydrocarbons so that this tendency

may be improved by Co in which plays as core role [57].

Therefore, it presented higher C5þ selectivity (more than 2

time) and slighter CH4 selectivity (about half) using the A1eA5

in comparison with CA. Of course, these could be attributed to

the larger particles sizes of coreeshell structure catalysts. In

the different works it has been observed that the smaller co-

balt cluster size is more favored for higher selectivity to

methane, while the large cluster show higher tendency to

heavier hydrocarbons [58,59]. Moreover, higher selectivity for

heavier hydrocarbons and lower CH4 selectivity are obtained

using Co@m-SiO2 in compared with Co/SiO2, though these

changes are poor [2]. Yang et al. [3] demonstrated that using

the Co/SiO2@H-ZSM-5 catalyst show a relative improvement

in C5þ selectivity respect to using Co/SiO2, while CH4 selectivity

is diminished. In contrast with this work, the higher CH4

selectivity and lower C5þ selectivity are obtained using Ru/

SiO2@H-ZSM-5 catalyst [4]. When Ru loading increases from

0.55% to 2.14%, the CH4 selectivity decreases from 13.43% to

11.48% and from 13.12% to 11.2% at 220 �C and 230 �C,respectively, while heavier hydrocarbons selectivity increases

about 25% at the different reaction temperatures. These

would be explained by an increase in shell (Ru) thickness from

3.6 nm to 7 nm. However, a slight decrease in surface Ru/Al

atomic ratio from A1 (0.0015) to A3 (0.0013) may decrease the

effect of shell thickness on the product distribution as seen

Table 4.

By increasing Ru content from 2.14% to 5%, the CH4

selectivity increases, while there is no improvement in C5þ

selectivity. These phenomena could be ascribed to more

diminution of surface Ru atoms. The C2eC4 selectivity de-

creases from 48.22% to 13.75% and 45.12%e13.36% at 220 �Cand 230 �C using Ru in the range of 0e5%. A decrease in the

tendency of heavier hydrocarbons and enhancement toward

lighter ones is demonstrated due to H2/CO ratio improvement

over catalyst when zeolite shell thickness increases from 3.1

to 5.1 mm, which was observed by Yang et al. [4]. In the pre-

sent work, the C5þ selectivity maximum values are 67.94% at

220 �C for A4 and 68.76% at 230 �C for A5, respectively. For all

synthesized catalysts by enhancement of FTS temperature,

CH4 production decreases and C5þ selectivity increases;

nevertheless, the changes are little that is in agreement with

literature [2].

Conclusions

The Co@Ru/g-Al2O3 catalysts with coreeshell structures at

different Co/Ru ratios were prepared using substitution reac-

tion method between Co and Ru metals over g-Al2O3 surface.

Characterization results emphasized that this new synthesis


method successfully produced the coreeshell structure for-

mation. The synthesis of Co@Ru/g-Al2O3 coreeshell catalysts

and enhancement of Ru loading led to the decrease in CoO

species reduction temperature so that the notable enhance-

ment of reduction degree from 68% to 98.7% is obtained

through TPR at 150e400 �C. A strong electronic interaction of

core atoms on the shell surface ones in special Co@Ru cor-

eeshell structure led to great increment in FTS performance of

catalysts with the coreeshell structure. The percentage of CO

conversion and selectivity toward C5þ (desirable products) are

greatly improvedwhile, the selectivity of lighter hydrocarbons

were suppressed using the coreeshell structure. By

enhancement of the shell (Ru) thickness, percentage of CO

conversion and selectivity to heavier hydrocarbons consider-

ably were increased. However, CH4 selectivity was passed

through a minimum value that attributed to A3.

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