Catalytic performance of Co/Zn-Al2O3 Fischer-Tropsch catalysts: A comparative study of zinc introduction methodologies

Article abstract of DOI:10.1039/c5ra08965d

Zinc was introduced into γ-Al₂O₃ by either co-precipitation or impregnation methods. Zinc introduced by co-precipitation was homogeneously dispersed in the framework of the support, while it was aggregated on the surface of alumina u

Full text of DOI:10.1039/c5ra08965d

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PAPER
Catalytic performance of Co/Zn–Al2O3 Fischer–
Tropsch catalysts: a comparative study of zinc
introduction methodologies
Cite this: RSC Adv., 2015, 5, 60534
a
a
Juan Du, Junkun Yan, Jingping Hong, a Yuhua Zhang,a Sufang Chenb and Jinlin Lia
*
Zinc was introduced into g-Al2O3 by either co-precipitation or impregnation methods. Zinc introduced by
co-precipitation was homogeneously dispersed in the framework of the support, while it was aggregated
on the surface of alumina upon addition via impregnation. The co-precipitated prepared Zn–Al2O3
supported cobalt catalyst possessed appropriate pore structure, lower cobalt–support interaction and
improved cobalt reducibility, thus showing a significantly enhanced catalytic activity with good stability in
Fischer–Tropsch synthesis.
Received 13th May 2015
Accepted 3rd July 2015
DOI: 10.1039/c5ra08965d
www.rsc.org/advances
increase the cobalt reducibility and thus, improve catalyst
performance.
1 Introduction
Zinc is a commonly used promoter or support (ZnO) and has
been studied in a wide range of reactions, including FTS.23–30 As
an additive, zinc affects the structure and catalytic performance
of corresponding catalysts.27–29,31 Coville et al.31–33 reported that
zinc had a positive effect on the activity and the selectivity of Co/
TiO2 catalysts, however, related studies34 showed that low
content Zn (1–5 wt%) was acted as a poison and decreased the
activity of Co/Al2O3 catalysts during CO hydrogenation at
steady-state isotopic transient kinetic analysis conditions. The
controversial results led us to perform the comparative study of
zinc introduction methodologies (co-precipitation or impreg-
nation) in this work; the effects of zinc modication on the
catalyst structure and catalytic performance in FTS were inves-
tigated. Various characterization methods as well as FTS tests
were employed to provide in-depth information about the
catalysts structure and reactive performance.
Fischer–Tropsch synthesis (FTS) is a potential alternative to
petroleum for the production of ultraclean and sustainable
automotive fuels. It uses syngas as feedstock, which can be
derived from natural gas, coal, or renewable biomass
resources.1–6
Cobalt-based catalysts are one of the most important
candidates for FTS because of their acceptable cost, high
activity, low water–gas shi activity and high selectivity to long
chain paraffins.7,8 Cobalt precursors are usually dispersed on
porous supports to obtain a high density of cobalt sites.
Conventional supports such as titania,9 alumina,10 silica11 and
zirconia12 exhibit varying degrees of cobalt–support interac-
tions, which signicantly inuence the reducibility and
dispersion of supported cobalt catalysts, and thereby, affect the
nal catalytic activity and products selectivity. A controllable
catalyst design is able to ne-tune support–cobalt interactions,
and will be highly desirable to improve the catalytic perfor-
mance of FTS catalysts.
Alumina is a traditional commercial support due to its
favourable mechanical and controllable surface properties.13
However, a strong interaction between alumina and cobalt
oxides limits the reducibility of cobalt species.14 Modication
of the support surface by certain amounts of noble metals,15–17
other metal oxides,18–20 or organic compounds,21,22 could
modify the interaction between cobalt oxide and support,
2 Experimental
2.1 Catalyst preparation
Support preparation. The pure alumina support denoted as
Al1 was prepared by co-precipitation method through the
following procedure. A known amount of aluminum nitrate
aqueous solution was stirred by a magnetic stirrer, then pure
ammonia aqueous solution (25 wt%) was added drop-wise until
the pH value of the solution reached 9. Aer stirring for another
2 h, the generated precipitate was ltered and washed thor-
oughly by distilled water and anhydrous ethanol. The obtained
precipitate was dried at 383 K for 12 h, and calcined at 973 K for
5 h.
aKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-Central University for Nationalities,
Wuhan, Hubei 430073, China. E-mail: jingpinghong@mail.scuec.edu.cn; Fax: +86
27 67842752; Tel: +86 27 67843016
Zinc was introduced into alumina by two methods. One was
introduced by co-precipitation method and labelled as PZnAl1.
The support was synthesized following the same procedures as
bKey Laboratory for Green Chemical Process of Ministry of Education, School of
Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei
430073, China
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Al1, the only difference was during the rst step, mixed H2/Ar (constant ow rate of 30 ml minꢀ1) ow was introduced
aluminum nitrate and zinc nitrate aqueous solution was used into the reactor and the temperature was raised to 1073 K, with
instead of pure aluminum nitrate aqueous solution. The other a ramping rate of 10 K minꢀ1. The consumption of H2 was
was introduced by traditional incipient wetness impregnation. monitored by a thermal conductivity detector.
Zinc nitrate was impregnated on Al1 material, followed by
The dispersion and crystallite size of cobalt were measured
drying at 383 K for 12 h, and calcining at 973 K for 5 h. The by hydrogen temperature programmed desorption and oxygen
obtained support was marked as IZnAl1. The theoretic zinc titration, using the Zeton Altamira AMI-200 unit. The catalysts
loading in both nal samples is 10 wt%. Inductively coupled were rstly reduced at 723 K, for 12 h in a hydrogen ow. Then
plasma mass spectrometry (ICP-MS) analysis conrmed that the the catalysts were purged with argon at 373 K to drive away
content of Zn cations in PZnAl1 and IZnAl1 was approximately weakly bound physisorbed species. Aer that, the temperature
equal to the nominal value.
increased from 373 K to 723 K with a heating rate of 10 K minꢀ1
To eliminate the effect of support pore structure, another and held at 723 K, under owing argon, to desorb the remaining
alumina support (Al2) with similar specic surface area and chemisorbed hydrogen. Meanwhile, the TCD signal was recor-
pore size distribution as PZnAl1, was also synthesized by co- ded until it returned to the baseline. Subsequently, the reduced
precipitation method, following the same preparation process catalyst was re-oxidized at 723 K, by purging with oxygen pulses
of Al1, but using hexadecyl trimethyl ammonium bromide until no further consumption of O2 was detected by the TCD
(CTAB) instead of ammonia as precipitator.
located downstream. The detailed description of how to calcu-
Cobalt deposition. Cobalt was deposited on the supports by late the cobalt catalyst dispersion and reduction degree has
incipient wetness impregnation using solutions containing the been reported previously.14
desired amount of cobalt nitrate. The samples were dried at
Raman spectra were performed on a Confocal Renishaw RM-
393 K for 12 h, and then calcined at 623 K in air for 5 h, with a 1000 instrument with Ar ion laser of wavelength 514.5 nm. The
ramping rate of 2 K minꢀ1. The catalysts were labelled as Co/Al1, laser power was adjusted at 7 mW with an exposure of 30 s aer
Co/PZnAl1, Co/IZnAl1 and Co/Al2, respectively. The nominal 3 accumulations.
loading of cobalt in these four catalysts was 15 wt%.
2.3 Fischer–Tropsch synthesis tests
2.2 Catalyst characterization
The FTS reaction were carried out in a xed-bed stainless-steel
reactor (id ¼ 12 mm). Typically, 0.5 g catalyst was mixed with
5 g same-sized inert silicon carbide particles to improve the
temperature distribution. Aer reduction in owing H2 at
723 K, for 10 h under atmospheric pressure, the temperature of
catalyst bed was cooled down to 373 K in owing H2. The system
pressure was then increased to 1.0 MPa by a syngas ow with a
H2 : CO ratio of 2 : 1. The gas ow was set at a space velocity of
4 SL gꢀ1 hꢀ1 and the reaction temperature was slowly increased
to 493 K. The reaction products were collected aer more than
80 h of operation, to achieve a good mass balance at close to
steady state. Liquid products and wax were collected through a
cold trap keeping at 273 K and a hot trap keeping at 393 K,
respectively. Gaseous products were analyzed on line by an
Agilent MicroGC 3000A gas chromatograph (GC). The oil and
wax fractions were analyzed by ame ionization detector (FID)
with an Agilent GC7890 and Agilent GC6890, respectively. The
mass balance and carbon balance of the reaction were both in
the range of 94% to 103%. Detailed information can be found in
our previous report.35
The BET surface area, pore volume and pore size distribution
were obtained from nitrogen adsorption and desorption
isotherms, performed at 77 K, using a constant-volume
adsorption apparatus (Quantachrome Autosorb-1-C-MS). The
pore volume was determined at a relative pressure (P/P0) of 0.99.
The pore size distributions of the samples were determined by
the BJH (Barett–Joyner–Halenda) model from the data of
desorption branch of the nitrogen isotherms.
X-ray diffraction (XRD) spectra were recorded on a Bruker
Advanced D8 powder X-ray diffractometer, with monochromatic
CuKa radiation and a VANTEC-1 detector over a 2q range of 10–
80ꢁ, with a step size of 0.0167ꢁ. Crystallite phases were deter-
mined by comparing the diffraction patterns with those in the
standard powder XRD les (JCPDS). In situ XRD measurements
were carried out under a pure hydrogen gas ow, from 423 K to
723 K with a heating rate of 1 K minꢀ1, followed by several scans
taken in an interval of 1 h. The average Co3O4 crystallite size was
calculated by line broadening analysis of Co3O4 peak using the
Scherrer equation.
27Al solid-state magnetic angle spinning nuclear magnetic
resonance (27Al MAS NMR) spectra were recorded at room
temperature on a Bruker MSL-400 spectrometer, with zirconia
rotors spun at 5 kHz, using a commercial 4 mm MAS NMR
3 Results and discussion
3.1 Structure of supports and corresponding catalysts
probe. Data were acquired at 104.26 MHz, 14.5 ms pulse width The nitrogen adsorption–desorption isotherms for the supports
and 1 s recycle delay, using Al(NO3)3$9H2O as the reference. The and corresponding catalysts are shown in Fig. 1, and their
chemical shis were given in ppm.
surface area, pore diameter and pore volume data are listed in
H2-temperature-programmed reduction (H2-TPR) measure- Table 1. All the supports and catalysts presented representative
ments were performed on a Zeton Altamira AMI-200 unit. The type-IV isotherms with pronounced H2 hysteresis loops
calcined catalysts placed in a U-shape quartz reactor, aer according to the classication of Brunauer et al.,36 which was
removing the adsorbed water and other contaminants, a 10% typical of mesoporous materials. The pore diameter of the
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supports and corresponding catalysts were calculated using the
BJH method. The porosity of the supports was signicantly
affected by the introduction methods of zinc. The PZnAl1
support, in which Zn was introduced by co-precipitation
method, showed the highest BET surface area, highest pore
volume and widest average pore size, while IZnAl1 support (Zn
introduced by impregnation method) presented opposite values
(Table 1). It is thus suggested that in PZnAl1, Zn entered into the
framework of alumina and altered its structure, whereas in
IZnAl1 support, zinc was mainly existed on the surface of the
matrix g-Al2O3, causing some blocking of the pores in Al1. The
Al2 support, which had similar pore structure as PZnAl1, was
used as a reference to compare and better understand the effect
of support structure.
Fig. 2 Pore size distribution of supports (a) and catalysts (b).
Similar pore size distribution curves and nitrogen adsorp-
tion–desorption isotherms in the supported catalysts (Fig. 2)
suggested that impregnation of cobalt had no effect on the
structure of the corresponding supports. The drop of surface
area could be due both to plugging support pores with cobalt
oxide crystallites and to the effect of the support “dilution”
because of the presence of cobalt species.37
The properties of aluminum in the supports were investi-
gated by 27Al MAS NMR spectroscopy. The 27Al chemical shis
were easily distinguished among different coordination
numbers for aluminum species.38 The 27Al MAS NMR spectra of
PZnAl1, IZnAl1 and Al1 supports are shown in Fig. 3. For all
three samples, there are two main resonance peaks observed
around d ¼ 0 ppm and 68 ppm, which corresponded to
aluminum species in octahedral (Alocta) coordination and
tetrahedral (Altetra) coordination, respectively.39 It was demon-
strated that the modication of zinc on Al2O3 by both co-
precipitation and impregnation, led to an increase in the
concentration of Al in octahedral coordination. Since Al species
in pure ZnAl2O4 are mostly in the form of octahedral coordi-
nation,40 the increased ratio of Alocta to Altetra in the supports
Fig. 3 27Al solid-state MAS NMR spectra of Al1, PZnAl1 and IZnAl1, the
peak area ratios of Alocta to Altetra were labelled.
aer zinc modication suggested the formation of ZnAl2O4
species, and the higher Alocta/Altetra ratio in PZnAl1 indicated a
higher concentration of ZnAl2O4 species.
The supports and corresponding catalysts were also charac-
terized by X-ray diffraction. Wide-angle XRD patterns of the Al1,
Al2, PZnAl1 and IZnAl1 supports are presented in Fig. 4.
Diffraction peaks located at 37.6ꢁ, 39.5ꢁ, 45.7ꢁ and 67.0ꢁ were
attributed to the g-Al2O3 phase (JCPDS: 47-1308), while the
peaks at 31.2ꢁ, 36.8ꢁ, 44.8ꢁ, 55.6ꢁ, 59.3ꢁ and 65.2ꢁ in the PZnAl1
and IZnAl1 samples were assigned to the ZnAl2O4 phase, with
spinel structure (JCPDS: 05-0669). No characteristic diffraction
peak of ZnO was detected under experimental conditions.
Therefore, we draw the inference that zinc, doped by either co-
precipitation or impregnation, could react with a matrix of
g-Al2O3 and form zinc aluminate with spinel structure aer
high temperature calcination, the result is consistent with the
investigation made by Strohmeier et al.,41 the strong interaction
between zinc and g-Al2O3 led to the formation of ZnAl2O4 when
the zinc loading was not exceeded 20%. The diffraction peaks of
Al2O3 are completely overlapped with those of ZnAl2O4 in
Fig. 1 Nitrogen adsorption–desorption isotherms of supports (a) and
catalysts (b).
Table 1 Textural parameters of supports and catalysts
BJH pore size
(nm)
Pore volume
BJH pore size
(nm)
Pore volume
Support
SBET (m2 gꢀ1
)
(cm3 gꢀ1
)
Catalyst
SBET (m2 gꢀ1
)
(cm3 gꢀ1
)
Al1
182.0
136.3
234.1
222.8
7.2
6.1
12.5
11.4
0.59
0.49
0.86
0.78
Co/Al1
132.9
106.7
189.5
180.6
5.9
5.1
10.6
9.8
0.42
0.34
0.63
0.54
IZnAl1
PZnAl1
Al2
Co/IZnAl1
Co/PZnAl1
Co/Al2
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Scherrer equation. Aer subtracting the contribution of the
supports, the Co3O4 particle size of pure alumina supported
Co/Al1 and Co/Al2 catalysts is assessed from the width of (511)
diffraction peaks (Table 2), the Co/Al2 catalyst, with larger pores
in support, has larger Co3O4 particles.
In order to obtain the cobalt particle size data in zinc con-
taining catalysts, H2-temperature programmed desorption
(H2-TPD) and O2-titration experiments were carried out and the
derived data are shown in Table 2. With the same variation
tendency as XRD ndings, it conrmed that the particle size of
the cobalt species was related to the support pore size. The
particle size of Co3O4 in Co/PZnAl1 and Co/Al2 catalysts was
corresponding to the pore size of their counterpart supports.
While in Co/Al1 and Co/IZnAl1 samples, although smaller
Co3O4 crystallites were found, they were still much larger than
the pore size of supports, indicating that a fraction of cobalt
oxide might not be situated inside the pores but on the outer
surface.18
The inuence of zinc addition with different methods on the
cobalt reducibility of supported catalysts was rstly investigated
by temperature programmed reduction (TPR). Several hydrogen
consumption peaks are observed in the TPR proles of the four
catalysts (Fig. 6). Peaks below 550 K could be attributed to the
reductive decomposition of residual nitrate species.16,42 And the
peaks between 550 K and 673 K were assigned to the reduction
of Co3O4 to CoO (ref. 43) for all catalysts. The intensity and
position of this peak were not markedly changed by zinc
modication or difference in support structure, indicating that
the rst reduction step of supported Co3O4 to CoO, did not
signicantly depend on catalyst properties. Similar conclusion
was also illustrated by Castner et al.44 The second reduction step
(reduction of CoO into Co0) of four catalysts was taken place at a
relative wide temperature range (673–1000 K),45 corresponding
to the progressive reduction of CoO–alumina species with
different interaction strength, or of cobalt aluminate, which
might be formed by a reaction between smaller CoO particles
and alumina at elevated temperatures in the presence of water-
vapor, producing during the reduction process.46 In contrast
with the rst step reduction, the reduction of CoO to Co0 was
largely dependent upon the nature of the catalyst. It is known
that smaller cobalt oxide particles usually have stronger inter-
action with supports, and they are more difficult to reduce than
larger cobalt oxide particles. Hence, the Co/Al1 and Co/IZnAl1
catalysts with smaller Co3O4 crystallites have relative high
nal reduction temperatures. The highest reducibility of cobalt
Fig. 4 Powder XRD patterns of the supports.
PZnAl1, indicating the homogeneous dispersion of ZnAl2O4 in
the framework of g-Al2O3. Comparing with PZnAl1, the
diffraction peak of g-Al2O3 at 39.5ꢁ could be clearly distin-
guished in IZnAl1, and the shape of various overlapped peaks
was narrower and sharper, suggesting the formation of larger
ZnAl2O4 crystallites on the surface of g-Al2O3 matrix when zinc
was introduced by impregnation method. These larger particles
blocked the pore channels in original Al1 material, and led to a
decrease in surface area, pore volume and average pore size,
consistent with nitrogen physisorption data.
X-ray diffraction proles of the catalysts are shown in Fig. 5.
XRD patterns characteristic of Co3O4 were detected for Al2O3
supported catalysts. However, the diffraction peaks of Al2O3,
ZnAl2O4 and Co3O4 phases are all overlapped to a large extent in
zinc containing samples. Thus, it is difficult to calculate the
crystallite size of Co3O4 in Co/IZnAl1 and Co/PZnAl1 catalysts by
Fig. 5 X-Ray diffraction patterns of calcined catalysts.
Table 2 Particle size of cobalt species at different conditions
DCo O a (nm)
DCo-FC (nm)
DCo-FC (nm)
DCo-UC (nm)
RDe (%)
b
c
d
Catalyst
3
4
Co/Al1
9.8
—
—
11.8
8.0
7.9
9.8
9.3
6.0
5.4
7.2
6.6
6.6
8.2
7.6
7.2
42.1
36.3
61.7
53.6
Co/IZnAl1
Co/PZnAl1
Co/Al2
a
Average particle size of Co3O4 determined by XRD using Scherrer equation. b Cobalt particle size of fresh catalyst, determined by in situ XRD at 2q
¼ 44.8ꢁ. c Cobalt particle size of fresh catalyst, determined by H2-TPD and O2 titration. d Cobalt particle size of used catalyst, determined by H2-TPD
and O2 titration. e Reduction degree, data obtained from O2 titration.
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temperature, and thus, enhance the reduction stage of CoO to
Co0.
The reducibility of the four catalysts was also measured by
H2-temperature programmed desorption (H2-TPD) and O2-
titration experiments, the results are shown in Table 2. Elimi-
nating the inuence of support pore size (compared both with
Co/Al1 and Co/Al2), Co/PZnAl1 catalyst showed the highest nal
cobalt reducibility, in consistent with the above TPR and in situ
XRD ndings. The reducibility of cobalt catalysts was affected
by both the cobalt particle size47 and metal–support interac-
tion.48 The relative low reduction degree of Co/IZnAl1 could be
ascribed both to its smallest Co3O4 particles which are much
more difficult to reduce, and the aggregation of ZnAl2O4 spines,
which showed lower impact on the weakening of cobalt–
alumina interaction.
Fig. 6 TPR spectra of calcined catalysts.
species in Co/PZnAl1 catalyst was due to a combined effect of
larger Co3O4 particles, as well as a weakened cobalt–support
interaction.
3.2 Catalytic performance
To better understand the reduction stages of these catalysts,
in situ XRD measurements in pure hydrogen atmosphere during
heat treatment were also employed in this study. As shown in
Fig. 7, all the four catalysts showed two reduction steps and the
Co3O4 phase transformed into CoO when the temperature
reached 573 K, conrming TPR results that the reduction of
Co3O4 to CoO was regardless of the support composition and
structure. Nevertheless, the appearance temperature of metallic
Co diffraction peaks in the catalysts varied. Metallic cobalt
diffraction could be found as soon as the temperature reached
753 K in Co/PZnAl1, while in Co/Al1, Co/IZnAl1 and Co/Al2, the
temperatures were 833 K, 833 K and 793 K, respectively. The
appearance temperatures of metallic Co metal were in the
following order: Co/PZnAl1 < Co/Al2 < Co/IZnAl1 z Co/Al1.
Compared with the reduction process of two catalysts
(Co/PZnAl1 and Co/Al2) with similar cobalt particle size, the
improved reducibility in Co/PZnAl1 suggested that the existence
of co-precipitated Zn in support could weaken the interaction
between the support and cobalt, hinder the formation of cobalt
aluminates by the reaction of CoO and alumina at elevated
The catalytic performance of cobalt-based catalysts was evalu-
ated in a differential catalytic reactor at 493 K, under 1.0 MPa.
Since all of the catalysts had the same cobalt loading and are
performed under same reaction conditions, their catalytic
activity expressed as FTS reaction rate (in moles of converted CO
per second divided by the total amount of cobalt (in moles)
loaded into the reactor), were in direct proportion to CO
conversion, as shown in Fig. 8 and Table 3. The activity of the
catalysts at quasi-steady state was in the order of Co/IZnAl1 <
Co/Al1 < Co/Al2< Co/PZnAl1, while the selectivities of hydro-
carbons were not remarkably changed.
It is generally accepted that when the cobalt particle size is
larger than 6–8 nm, the activity of cobalt catalysts for FTS is
dependent upon the amount of the exposed active metal cobalt
sites, which are decided by both cobalt dispersion and reduc-
ibility. We show here that in Co/PZnAl1, zinc is entered into the
framework of alumina by co-precipitation method and forms
homogeneous dispersed zinc aluminate aer calcination. The
obtained Al2O3–ZnAl2O4 material (PZnAl1) conned the cobalt
particles inside its pores, and weakened the interaction between
the cobalt and support, thus, the reducibility of corresponding
catalyst was improved and an enhancement in FTS activity was
achieved.
Fig. 7 In situ XRD patterns of (a) Co/Al1, (b) Co/IZnAl1, (c) Co/PZnAl1 Fig. 8 CO conversion of the four catalysts as a function of time on
and (d) Co/Al2.
stream.
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Table 3 FTS performances of cobalt catalysts in a fixed bed reactora
Hydrocarbon selectivity (mol%)
Catalyst
XCOb (%)
FTS reaction rate (10ꢀ3 sꢀ1
)
SC1
SC2–4
SC5+
Carbon balance
Co/Al1
38.5
32.7
49.8
43.3
2.5
2.2
3.3
2.9
12.2
13.8
12.3
11.9
10.2
9.3
10.0
8.8
77.6
76.9
77.7
79.3
99.1%
95.3%
101.8%
98.3%
Co/IZnAl1
Co/PZnAl1
Co/Al2
a
Reaction conditions: 1.0 MPa, 493 K, H2/CO ¼ 2, 4 SL gꢀ1 hꢀ1, CO conversion and hydrocarbon selectivity were collected at 100 h. b Average CO
conversion, recorded at quasi-steady state.
due to the sintering of cobalt particles and wax deposition. Zinc
introduced by co-precipitation could weaken the cobalt–
alumina interaction and improve the reduction of CoO to Co0 in
the corresponding Co/PZnAl1 catalyst, therefore, the Co/PZnAl1
catalyst, which combined high reducibility as well as compa-
rable dispersion of cobalt species, showed the highest FTS
activity and good reaction stability.
Acknowledgements
This work was supported by National Natural Science founda-
tion of China (21203255, 21403158, 21473259), the Key Program
of Technology Innovation of Hubei Province (2013AGB002),
Technology Foundation for Selected Overseas Chinese Scholar,
Ministry of Personnel of China (BZY14037) and the Fund for
Basic Scientic Research of South-Central University for
Nationalities (YZZ12001).
Fig. 9 Raman spectra of used Co-based catalysts.
It is known that the catalytic deactivation in FTS could be
due to sintering of cobalt nanoparticles, cobalt oxidation,
carbon (wax) deposition, etc. Compared with the other three
catalysts, more signicant deactivation was presented on Co/
IZnAl1 (from initially 41.0% to 32.7%, declined by 20.2%).
The particle sizes of cobalt species in fresh reduced catalysts
and corresponding used ones are listed in Table 2, a much more
remarkable sintering of cobalt particles (from 5.4 nm of the
fresh catalyst to 8.2 nm of the used one, augmented by 51.9%)
was observed on Co/IZnAl1, while for the other three catalysts,
the augmentation of cobalt particle size aer reaction was less
than 10%.
In order to better analysis the deactivation mechanism of Co/
IZnAl1 catalyst, Raman spectra of used Co/IZnAl1 and Co/
PZnAl1 catalysts were recorded. As shown in Fig. 9, only
bands arose from C–H stretching mode in the range of 2850–
3100 cmꢀ1 were detected, which suggested the presence of
hydrocarbons on the catalysts. The huge and broad bands on
Co/IZnAl1 catalyst indicated that besides cobalt sintering, wax
deposition was also one of the possible factor in charge of the
deactivation phenomena on this catalyst.
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