Ga and In modified ceria as supports for cobalt-catalyzed Fischer-Tropsch synthesis

Article abstract of DOI:10.1016/j.apcata.2017.08.026

Ga- and In-modified ceria (Ce_0.8Ga_0.2O₂, Ce_0.8In_0.2O₂) materials were used as supports for cobalt-catalyzed Fischer-Tropsch synthesis (FTS). The addition of Ga to ceria was found to improve CO conversion for cobalt-catalyzed FTS, while the addition of In tended to decrease it. A similar trend was observed with the Ag-promoted cobalt/ceria catalysts. Doping of ceria with Ga or In decreased methane and increased the selectivity to olefins and alcohols for Ag-promoted cobalt/ceria. The sum of the products of olefins and alcohols for various catalysts exhibited a decreasing trend as follows: Ag-Co/Ce-Ga > Ag-Co/Ce-In > Ag-Co/Ce. Results of H₂-TPR-XANES showed that adding of Ga or In to ceria increases the fraction of Ce³⁺ in the surface shell for both unpromoted and Ag-promoted catalysts in the range of temperature typical of catalyst activation. This partially reduced ceria plays an important role in controlling the product selectivity of cobalt-catalyzed FT synthesis.

Full text of DOI:10.1016/j.apcata.2017.08.026

Accepted Manuscript
Title: Ga and In modified Ceria as supports for
cobalt-catalyzed Fischer-Tropsch synthesis
Authors: Muthu Kumaran Gnanamani, Gary Jacobs, Wilson
D. Shafer, Michela Martinelli, Donald C. Cronauer, A. Jeremy
Kropf, Christopher L. Marshall, Burtron H. Davis
PII:
S0926-860X(17)30406-4
DOI:
Reference:
http://dx.doi.org/10.1016/j.apcata.2017.08.026
APCATA 16381
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
1-6-2017
15-8-2017
19-8-2017
Please cite this article as: Muthu Kumaran Gnanamani, Gary Jacobs,
Wilson D.Shafer, Michela Martinelli, Donald C.Cronauer, A.Jeremy Kropf,
Christopher L.Marshall, Burtron H.Davis, Ga and In modified Ceria as
supports for cobalt-catalyzed Fischer-Tropsch synthesis, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.08.026
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Ga and In modified Ceria as supports for cobalt-catalyzed Fischer-Tropsch synthesis
Muthu Kumaran Gnanamani1, Gary Jacobs1,2, Wilson D. Shafer1, Michela Martinelli1,
Donald C. Cronauer3, A. Jeremy Kropf3, Christopher L. Marshall3, Burtron H. Davis1*
1Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive,
Lexington, Kentucky 40511, USA
2 University of Texas at San Antonio, Department of Biomedical and Chemical Engineering,
UTSA Circle, San Antonio, TX 78249
3Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL,
60439, USA
*Corresponding author. Tel.: +1-859-257-0251; email address: burtron.davis@uky.edu
1

Graphical abstract
Research Highlights
 Ga or In was introduced to ceria by co-precipitation using urea hydrolysis.
 H2-TPR XANES showed that Ga or In addition increased the Ce3+ concentration after H2
activation.
 Ag was used as a reduction promoter to cobalt for FT synthesis.
 HR-TEM analyses showed a similar distribution of cobalt particles among catalysts.
 Ce0.8Ga0.2O2 outperformed CeO2 and Ce0.8In0.2O2 for CO conversion and product
selectivity to olefins and oxygenates.
Abstract
Ga- and In-modified ceria (Ce0.8Ga0.2O2, Ce0.8In0.2O2) materials were used as supports for
cobalt-catalyzed Fischer-Tropsch synthesis (FTS). The addition of Ga to ceria was found to
improve CO conversion for cobalt-catalyzed FTS, while the addition of In tended to decrease it.
A similar trend was observed with the Ag-promoted cobalt/ceria catalysts. Doping of ceria with
Ga or In decreased methane and increased the selectivity to olefins and alcohols for Ag-
promoted cobalt/ceria. The sum of the products of olefins and alcohols for various catalysts
exhibited a decreasing trend as follows: Ag-Co/Ce-Ga > Ag-Co/Ce-In > Ag-Co/Ce. Results of
H2-TPR-XANES showed that adding of Ga or In to ceria increases the fraction of Ce3+ in the
2

surface shell for both unpromoted and Ag-promoted catalysts in the range of temperature typical
of catalyst activation. This partially reduced ceria plays an important role in controlling the
product selectivity of cobalt-catalyzed FT synthesis.
Keywords: Fischer-Tropsch synthesis; Ag promoter; CeO2; CeO2-Ga2O3; CeO2-In2O3;
oxygenates
1.0 Introduction
The Fischer-Tropsch synthesis (FTS) is an attractive process used to convert syngas
derived from coal, biomass, and natural gas to clean transportation fuels and chemicals [1-4].
Cobalt and iron are the two elements most used as catalysts by industry [5-8]. The cobalt-based
FT catalysts are often used to produce longer straight-chain hydrocarbons with minimal olefin
and oxygenate contents. On the other hand, iron catalysts produce a significant amount of
olefins and oxygenates along with n-alkanes. This difference in FT selectivity between iron and
cobalt is generally explained by the nature of active sites and their adsorption behavior of CO
and H2 as well as the influence of reaction conditions [9-12]. The active form of cobalt in FT
synthesis is metallic in nature [13], whereas for iron it is the iron carbide [14]. Due to intrinsic
water-gas shift activity, the Fe-FT catalyst can be operated at a relatively low H2/CO ratio (~1.0)
in comparison with cobalt catalyst which requires a H2/CO ratio of ~2.0 [14].
Many factors affect the FT activity and the product selectivity of cobalt catalysts. In this
regard, the support plays a vital role in controlling cobalt metal dispersion through the metal-
support interaction [15-18]. In general, supports like Al2O3, SiO2, TiO2, ZrO2, and zeolites [19-
23] are commonly used. Very recently partially reducible oxides such as CeO2 also showed
promising results as a support for cobalt FT synthesis [24-26]. Ceria is a versatile oxide that can
generate oxygen vacancies under reducing conditions. In many instances, oxygen vacancies
3

serve as adsorption sites for reactants (e.g., water-shift reaction [27], dehydration [28]). On the
other hand, the metal-support interface is also very important in the sense that the intermediates
formed on the metal surface can interact with another adsorbate at the metal-metal oxide
interface to yield different products [29]. Promoters such as Pt, Ag, Pd, Ru, and Re [30-34] were
added along with cobalt on to the support in order to increase the fraction of reduced cobalt
nanoparticles after activation in H2. In comparison with Pt, Ag has an added advantage as a
promoter to cobalt for FT synthesis [35]. A recent study suggests that Ag is beneficial compared
to Pt in terms of producing less methane and higher C5+ hydrocarbons using alumina supported
cobalt catalysts [34]. In addition, Ag is less expensive than Pt from an economic point of view.
For these reasons, Ag was used as a promoter in the current investigation.
In our earlier investigations, cobalt supported on CeO2 [25] and higher rare earth oxide
(REO) modified CeO2 [26] catalysts were shown to have higher selectivity for alcohols in
comparison with a conventional alumina supported cobalt catalyst. We offered an explanation
that the selectivity control is achieved by terminating the FT growth either through the addition
of mobile bridging OH groups located on ceria or through termination by molecular CO from
surface species (e.g., formates formed from the reaction of CO with bridging OH groups) located
on ceria. We concluded that the metal-support interface of supported cobalt catalysts may be
involved in the synthesis of alcohols. Johnston and Joyner [29] observed for the first time the
possible nucleophilic attack by adsorbed hydroxyl groups in oxygenates synthesis from syngas,
and indicated the existence of common intermediates to both oxygenate and hydrocarbon
synthesis, possibly through an adsorbed ethylidyne species. On the other hand, Baker et al. [36]
claimed that unreduced cobalt ions were responsible for producing a significant amount of higher
oxygenates using silica supported Co-Cu bimetallic catalysts.
4

In the present investigation, we have studied the effect of Ga or In addition on the
reducibility of ceria by using the H2-TPR XANES technique. The dispersion of cobalt particles
on ceria modified supports was evaluated by using the HRTEM technique. Both cobalt and Ag-
promoted cobalt catalysts supported on ceria and Ga- or In-modified ceria were tested for
Fischer-Tropsch synthesis using a 1L CSTR. The aim was to assess the reducibility of ceria and
its interrelation with activity and the product selectivity of cobalt for FT synthesis.
2.0 Experimental
2.1 Catalyst preparation
The supports, ceria and Ga- or In-modified ceria were prepared by following the
homogeneous precipitation method using urea decomposition. In this method, an appropriate
amount of Ce(NO3)3.6H2O (Alfa Aesar, 99.5%) and Ga(NO3)3.xH2O (Alfa Aesar, 99%) or
In(NO3)2.xH2O with Ce and Ga or Ce and In molar ratios of 80:20 were first dissolved
simultaneously in deionized water at room temperature (~25°C). In a separate beaker, the
required quantity of urea (usually ~4 times that of the molar equivalent of metal nitrates) was
dissolved in deionized water and then mixed with the metal nitrates solution. The resulting
solution was refluxed at 85-90°C with constant stirring (500 rpm) for 15 h. The precipitate
obtained was filtered and washed three times using deionized water. It was then dried in a static
air oven at 120°C and calcined in air in a muffle furnace at 500°C for 3 h. To the supports, 5
wt% Co with and without Ag (0.55 wt%) were added by sequential incipient wetness
impregnation (Co-first), followed by drying and calcining at 350°C for 3 h. Hereafter, the
catalysts will be referred as Co/Ce, Co/Ce-Ga, Co/Ce-In, Ag-Co/Ce, Ag-Co/Ce-Ga, and Ag-
Co/Ce-In.
2.2 Catalyst characterization
5

BET surface area and porosity characteristics of the calcined catalysts were measured
using a Micromeritics 3-Flex system. Before performing the test, the temperature was gradually
ramped to 160 °C and the sample was evacuated at this temperature for 12 h to approximately 50
mTorr. The BET surface area, single point pore volume, and single point average pore diameter
were obtained. The Barrett-Joyner-Halenda (BJH) method was also used to estimate pore
volume and average pore diameter, as well as to provide pore size distribution as a function of
pore radius.
In-situ H2-TPR XAFS studies were performed at the Materials Research Collaborative
Access Team (MR-CAT) beamline at the Advanced Photon Source, Argonne National
Laboratory. A cryogenically cooled Si(1 1 1) monochromator was used to select the incident
energy and a rhodium-coated mirror was used to reject higher order harmonics of the
fundamental beam energy. The experimental setup was as outlined by Jacoby [37]. A stainless
steel multi-sample holder (4.0 mm i.d. channels) allowed for testing the in-situ reduction of six
samples during temperature programmed heating. Catalyst samples were diluted with silica gel
at a weight ratio (silica/sample) of approximately 12. Approximately 5 mg of each sample was
loaded as a self-supporting wafer in each channel. The holder was placed in the center of a
quartz tube, equipped with gas and thermocouple ports and Kapton windows. The amount of
sample used was optimized for the Ce LIII edge (5723 eV). The quartz tube was placed in a
clamshell furnace mounted on a positioning table. Each sample cell was positioned relative to
the beam by finely adjusting the position of the table to an accuracy of 20 μm for repeated scans.
Once the sample positions were fine-tuned, the reactor was purged with helium for more than 5
min at 30 ml/min. Then, the reactant gas (3.5% H2/He) was flowed through the samples (95
6

ml/min) and a temperature ramp of ~1.2 oC/min was initiated for the furnace. The final
temperature was ~875oC.
The Ce LIII-edge spectra were recorded in transmission mode and a Cr metallic foil
spectrum was measured simultaneously with each sample spectrum for energy calibration. X-ray
absorption spectra for each sample were collected from 5575 to 6136 eV, with a step size of:
from -150 to -35 eV, 10 eV; from -35 eV to -15 eV, 3 eV; from -15 to 25 eV, 0.7 eV; from 25 eV
to 35 eV, 1 eV; and from 35 eV to 10.4k, 0.08k. The acquisition time was ~4.5 min per
spectrum. By measuring each sample, in turn, and repeating, this allowed for approximately 25
scans to be collected for each sample over ~11 h. A sample’s temperature change from the
absorption edge through the end of the scan was then about 18 oC, while a spectrum was
measured approximately every 33 oC. Data reduction of the XAFS spectra was carried out using
the WinXAS program [38], and raw data were processed to give the normalized XANES spectra.
Linear combination fitting of spectra with appropriate reference compounds was carried out.
The initial spectrum of each catalyst was used as a fingerprint for the Ce4+ oxidation state, while
the final spectrum of a 0.5%Pt/Ca0.50Ce0.50OX catalyst following TPR (30 ml/min of 4%H2/He,
820oC, 1.5 h hold time) from [39] was used as a fingerprint of the Ce3+ oxidation state for all
samples, since its lineshape resembled Ce3+ compounds, such as Ce(NO3)3 or Ce(Cl3)3. EXAFS
data reduction and fitting were carried out using the WinXAS program [38], Atoms [40], FEFF
[41] and FEFFIT [42] programs. The k- and r-ranges were chosen to be approximately 2.5–10
Å–1 and 1.1–3.2 Å, respectively.
Transmission electron microscopy (TEM) imaging was performed using a JEOL 2010F
field-emission gun TEM with an accelerating voltage of 200 keV and magnification ranging
from 50 to 1000 K. High-resolution imaging was performed using a symmetrical multibeam
7

illumination with a beam resolution of 0.5 nm. Images were recorded using a Gatan Ultrascan
4k × 4k CCD camera, and data were analyzed using Gatan Digital Micrograph software.
2.3 Fischer-Tropsch synthesis
Fischer–Tropsch synthesis was performed using a 1 L CSTR. Typically, 15.0 g of
calcined catalyst (60-100 microns) was reduced ex-situ using a H2:He (1:3) mixture at 350°C for
20 h. The pre-reduced catalyst was transferred to a 1 L CSTR which already contained 310 g of
melted Polywax 3000 (polyethylene having an average carbon number of 200) under flowing
nitrogen. The catalyst then subjected to H2 polishing with pure H2 (30 slph) at 230°C for 24 h.
The FT synthesis was performed at a temperature of 220°C and a syngas pressure of 1.99 MPa at
a constant H2/CO feed of 2. The effluent gases were analyzed online using a Micro GC (HP,
Quad series, Refinery Gas analyzer) equipped with a TCD detector, while the liquid products
condensed in the 0° and 100° traps were analyzed separately using an HP 6890 GC with DB-5
capillary column. The aqueous product mixture was analyzed by using SRI-GC (model no.
8610C) with TCD detector and Porapak Q packed column. The conversion and selectivity
reaction parameters are defined as:
푛퐶푂 − 푛퐶푂
표푢푡
푖푛
푐표푛푣푒푟푠푖표푛 = 100 푥
푛퐶푂
푖푛
푛푝푟표푑푢푐푡표푢푡. 푐푎푟푏표푛 푛푢푚푏푒푟
푠푒푙푒푐푡푖푣푖푡푦 = 100 푥
푛퐶푂 − 푛퐶푂
푖푛
표푢푡
where 푛퐶푂 푎푛푑 푛퐶푂 are the number of moles of CO fed and not-consumed, respectively.
표푢푡
푖푛
The selectivity is defined as the percent mole of CO consumed to form a particular Cn product
(hydrocarbon, CO2 or oxygenate), normalized by the amount of CO consumed.
3.0 Results and discussion
8

Figure 1 shows the CO conversion for the unpromoted (left) and Ag-promoted (right)
cobalt. The Co/Ce and Co/Ce-Ga catalysts both displayed CO conversions at about 17%. The
Co/Ce-Ga catalyst exhibited a relatively lower rate of deactivation compared to Co/Ce while
Co/Ce-In displayed CO conversions of only about 6%. The promotional effect of Ag on the CO
conversion is noted only for Ce-Ga and Ce-In supported cobalt catalysts. Co/Ce-Ga and Ag-
Co/Ce-Ga catalysts outperformed Ce and Ce-In supported cobalt catalysts for CO conversions.
The product selectivity to methane for unpromoted and Ag-promoted cobalt catalysts are
shown in Figure 2. The addition of Ga or In tended to decrease the selectivity of methane for
cobalt and they followed the decreasing trend Co/Ce > Co/Ce-Ga > Co/Ce-In irrespective of
whether Ag promoter was present or not. Figure 3 shows that the selectivity to CO2 for Co/Ce
and Co/Ce-Ga catalysts remained more or less the same at about 4% while with Co/Ce-In the
CO2 selectivity was higher (~6.5%). A similar trend in the selectivity to CO2 is observed with
Ag-promoted catalysts as that of the unpromoted counterparts. The higher CO2 selectivity has
been reported in our previous work using ceria supported cobalt catalysts [25,26]. In general,
CO2 is assumed to be formed on Fe through the water-gas shift reaction during FT synthesis.
The selectivity to CO2 on cobalt is low due to a lack of a water-gas shift activity. It is reported
that cobalt could produce higher formation of methane and CO2 if it is present as surface CoO
and/or CoCx [13,43]. In this study, we obtained a higher selectivity to methane (10-15%) and
CO2 (4-8%) on cobalt supported on ceria and modified ceria catalysts. This is in line with our
earlier work showing that CoO and/or CoCx formation became more pronounced when a partially
reducible oxide such as La2O3, CeO2 or modified CeO2 was used to support cobalt nanoparticles
for FT synthesis [25,26]. The presence of CoO/CoCx phases after activation or during Fischer-
9

Tropsch synthesis along with cobalt metal particles can increase methane and CO2 selectivities
from syngas.
It is interesting to see if any changes occurred in the product distribution of olefins and
alcohols with the addition of Ga or In on to CeO2. For this purpose, the total olefin and alcohol
products formed during FT synthesis was summed and compared to all the three Ag-promoted
catalysts at various times-on-stream (Figure 4). The Ag-Co/Ce-Ga catalyst showed a
significantly higher selectivity for olefin and alcohol at about 30% in comparison with Ag-Co/Ce
and Ag-Co/Ce-In catalysts, which showed only about 20%. It is also noted that the sum of
olefins and alcohols selectivity for all the three catalysts increased with increasing time-on-
stream. Ribeiro et al. [26] observed such an increasing trend in alcohols selectivity with reaction
time for cobalt. The authors explained this behavior based on HRTEM and XANES analyses of
the used catalysts in that the metallic character of cobalt was partly transformed into cobalt
carbide which could possibly influence alcohol and CO2 formation from syngas. Hence, we
cannot rule out this possibility in the present system.
The lower olefins are important raw materials for the petrochemical industry. In this
regard, we made a comparison graph of C2-C4 olefins selectivity with time-on-stream for the
three Ag-promoted Co/Ce catalysts as shown in Figure 5. The olefins selectivity ranged from 8
to 10%, from 12 to 14%, and from 5 to 8% for Ag-Co/Ce, Ag-Co/Ce-Ga, and Ag-Co/Ce-In
catalysts, respectively. Overall, the olefinic content decreased with increasing time. A decrease
in the selectivity of lower olefins obtained with reaction time might be due to changes in the
readsorption behavior of olefins. Earlier, Schulz showed from isotopic tracer studies that the
active sites for olefin hydrogenation are different from those sites responsible for CO
hydrogenation [44]. Other studies suggest that changes in selectivities of secondary reactions
10

depend on the conversion of CO [45-47]. In the present system, we do observe a decreasing
trend in the conversion of CO with time for Ag-Co/Ce and Ag-Co/CeGa catalysts; on the other
hand, the selectivity of olefins also decreased, indicating that for these catalysts, the changes in
the selectivity to olefins do not solely depend on the partial pressure of CO but also on the nature
of active sites involved in secondary reactions of olefins.
Table 1 displays the results of nitrogen physisorption at 77 K. The BET surface area of
the starting support materials Ce, Ce-Ga, and Ce-In were 90, 104, and 79 m2/g, respectively.
After impregnation of cobalt (5 wt%), the BET surface area of the support decreased to 71, 84,
and 61 m2/g for Ce, Ce-Ga, and Ce-In, respectively. If the Co3O4 does not contribute
significantly to the surface area, then assuming that 5%Co corresponds to 6.75%Co3O4, the
expected decrease would be to 83.9 m2/g for Co/Ce, 97 m2/g for Co/Ce-Ga, and 74 m2/g for
Co/Ce-In catalysts. Since we obtained slightly lower values for all three catalysts, this indicates
that some minor pore blocking occurred. However, further addition of 0.55 wt% Ag did not
change the surface area of Co/Ce (Ce, Ce-Ga, Ce-In) appreciably. The single point pore volume,
as well as the BJH pore volume (desorption branch) of the catalysts, also followed a similar trend
to that of the BET surface area. The BJH pore size distributions of unpromoted as well as Ag-
promoted Co/Ce (Ce, Ce-Ga, Ce-In) catalysts are shown in Figure 6. The distributions remained
narrow for all the catalysts indicating that cobalt particles do not significantly alter the porous
structure of the support.
To better interpret the standard TPR profiles, TPR-XANES experiments at the Ce LIII
edge (5723 eV) were carried out using H2. The XANES lineshapes of Ce3+ and Ce4+ oxidation
states exhibit distinct differences [48-51], as shown in Figure S1, where spectra of reference
compounds as described in the experimental section are provided. The high energy XANES
11

peak (often denoted Peak C) for Ce4+ is typically assigned to absorption into the 5d level with no
occupancy in 4f for either the initial or final state, and is present in completely oxidized CeO2,
but absent for Ce3+ (final state configuration: Ce [2p54f 05d1] O [2p6]). With surface shell
reduction, the high energy peak decreases; further decreases are observed upon reduction of the
bulk at a higher temperature. The low energy peak (labeled B) is split into at least two separate
assignments [48,49]. Peak B1, also present in CeO2 with Ce4+, has been assigned to absorption
into the 4f level in the final state. Thus, in addition to the excitation of an electron from the Ce
2p shell to the 5d shell, another excitation occurs, coming from the valence band (O 2p shell) to
the Ce 4f shell, leaving a hole in the valence band (final state configuration Ce [2p54f 15d1] O
[2p5]). As the reduction of Ce occurs, peaks B1 and C diminish and a new peak, B0, develops as
a shoulder. Located just below peak B1, B0 is associated with absorption into the 5d level, with
4f occupancy in the initial state (final state configuration Ce [2p54f 15d1] O [2p6]). However, the
reduction of ceria occurs in two steps, a surface shell reduction step occurring at lower
temperature and a bulk reduction step occurring at higher temperature. Qualitatively, the most
clear indication of this is by examining peak C in Figure 7; the intensity of peak C (lighter gray
color) first decreases with surface shell reduction of ceria in the temperature range of 200oC to
350oC for Co/CeO2, Co/CeO2-Ga2O3, and Co/CeO2-In2O3 catalysts, and then peak C decreases
again with bulk Ce reduction at 600oC and above. There is more of a decrease in peak C for
surface shell reduction of ceria for the Ga and In doped Co/CeO2 catalysts. Using the lineshape
differences of Ce3+ and Ce4+ reference compounds as a guide, the degree of reduction of ceria
was estimated by linear combination fitting of XANES spectra with Ce3+ and Ce4+ references
using WinXAS [38]. In Figure 7, each TPR begins with ceria being present as Ce4+. Both the
lower temperature surface shell reduction and higher temperature bulk reduction are easily
12

observed for the six catalysts. Extents of reduction as quantified by LC fittings in Figure 8 show
that doping Ce with In or Ga improves the extent of surface shell reduction from ~10% as Ce3+
for the undoped Co/Ce and Ag-Co/Ce catalysts, to ~15% with In-doping and 20% with Ga-
doping at the 20% level.
On the other hand, Ag promoter tends to facilitate surface shell reduction of ceria; in
Figure 7, this is manifested by a decrease in peak C that occurs at a lower temperature in
comparison to the corresponding catalysts that do not contain Ag. Moreover, as observed in
Figure 8, linear combination fittings reveal a shift in the temperature of surface shell reduction
of ceria from ~300oC to a lower temperature below 250oC, perhaps due to a H2 dissociation and
spillover mechanism. A comparison at 235oC is provided as Figure S2 to highlight the line
shape differences. Reduction of ceria may occur by converting surface capping O to OH, or by
generating surface O vacancies [52].
Some information regarding the impact of adding In and Ga dopants, as well as Ag
promoter, to the reducibility of ceria, can be obtained from EXAFS by examining the influence
of each additive on the average coordination number of oxygen in the local environment of
cerium. To highlight the effect of In and Ga doping of ceria on Ce-O coordination, EXAFS
fittings were conducted on spectra recorded at the initial condition and at 235oC, as shown in
Figure S3, S4, and S5 for the undoped, Ga-doped, and In-doped catalysts, respectively. Table 2
summarizes the results of fitting the first shell of Ce-O coordination. Using the initial spectrum
taken at 25oC as a basis for ceria that is fully saturated with oxygen (i.e., Ce4+), the changes in
Ce-O coordination following H2 activation at 275oC are not significant for the catalysts that do
not contain Ag promoter; however, Ce-O coordination for those catalysts containing the Ag
promoter has decreased to an extent that demonstrates that surface shell reduction of Ce has
13

occurred for H2 activation at 275oC. Moreover, the decreases in Ce-O coordination are
somewhat higher for the catalysts containing Ga and In. Note, however, that the uncertainties
are quite high. Thus, the results only lend support to the conclusions obtained from XANES
spectroscopy.
The HR-TEM images of the reduced sample of Ag-Co/Ce, Ag-Co/Ce-Ga, and Ag-Co/Ce-
In are shown in Figure 9. In the reduced Ag-Co/Ce catalyst (9A), the magnified image shows
(right) uniformly distributed cobalt particles and the particle sizes of cobalt ranged between 5
and 8 nm. The reduced sample of Ag-Co/Ce-Ga (9B) and Ag-Co/Ce-In (9C) contain cobalt
particles just located at the edges of Ce-Ga or Ce-In particles. The EDS performed over the
circled region as shown in Figure 9 indicates that the particles are indeed cobalt. The data reveal
that the dispersion of cobalt on these supports remained more or less the same.
The mechanism of oxygenates formation in Fischer-Tropsch synthesis has been under
scrutiny for many years [53]. In cobalt-based FT synthesis, the amount of oxygenates formed
typically varies from 1 to 3 mol% whereas Fe produced a considerable amount of oxygenates,
typically between 5 and 30 mol% or above depending on the synthesis conditions and the
presence of alkali promoters. Linear alcohols are the primary oxygenate products found in both
Fe- and Co-based FT synthesis. Many argue that CO insertion across a metal-R (R=alkyl,
alkenyl) bond is the plausible route to the formation of alcohols from syngas using a Fe or Co
catalyst. However, the adsorbed hydroxyl groups on partially reducible oxides such as
lanthanides could also be involved in the formation of alcohols by nucleophilic addition. Our
previous studies indicated that ceria enhanced the formation of alcohols from syngas under a
typical FT synthesis conditions; moreover, the particle size of cobalt as well as the partially
14

reduced nature of the cerium oxide both played a vital role in controlling FT product selectivity
[25,26].
In general, Co metal sites are responsible for the FT chain growth. The metal-oxide
support interface could act as a termination site for the FT synthesis where hydroxyl groups or
molecular CO (e.g., from formate, etc.) on the oxide support may be involved in chain
termination. Therefore, it is desired to have more metal-support interface to promote alcohol
formation from syngas. In this regard, we have made an attempt to promote the surface
reduction of ceria (i.e., increasing the number of bridging OH groups associated with oxygen
vacancies) by doping ceria with Ga or In. We found that the addition of Ga or In to Co/Ce
increased the partial reduction of surface ceria as evidenced from H2-TPR XANES in which the
Ce3+ concentration was relatively higher in the case of Co/Ce-Ga and Co/Ce-In catalysts
compared to Co/Ce at a given temperature. The same trend was observed using the Ag-
promoted catalysts. The selectivity to oxygenates for Co/Ce-Ga and Co/Ce-In catalysts followed
a similar trend as the extent of reduction of the support. After 160 h of time-on-stream, the sum
of oxygenates and olefins selectivity was about 30% for Ag-Co/Ce-Ga, whereas Ag-Co/Ce and
Ag-Co/Ce-In catalysts both only showed about 20%. This difference in the selectivity observed
between Ag-Co/Ce and Ag-Co/Ce-Ga or Ag-Co/Ce-In could be due to the presence of more
partially reduced ceria in the case of Ag-Co/Ce-Ga catalyst after activation in H2 as inferred by
the H2-TPR XANES studies.
4.0 Conclusions
CeO2, CeO2-Ga2O3, and CeO2-In2O3 were prepared by following the homogeneous
precipitation method using urea decomposition and used as supports for cobalt FT synthesis.
The addition of Ga or In was found to increase the BET surface area and the pore volume of the
support. The H2-TPR XANES data indicate that Ga or In addition tended to increase the surface
15

partial reduction of ceria. The HR-TEM analyses on the reduced catalysts indicate that the
distribution of cobalt particle sizes remained the same for all the supports. The addition of Ga
was found to decrease methane formation and increase olefins and oxygenates selectivity for
cobalt. However, the promotional effect by addition of indium to Co/CeO2 or Ag-Co/CeO2 is
marginal. The increase in metal-oxide support interface is key to the production of olefins and
alcohols for cobalt FT catalysts using ceria.
Acknowledgements
The work carried out at the CAER was supported in part by funding from the
Commonwealth of Kentucky. Argonne’s research was supported in part by the U.S. Department
of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (NETL).
The use of the Advanced Photon Source was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
MRCAT operations are supported by the Department of Energy and the MRCAT member
institutions.
16

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Figure 1. CO conversion over the unpromoted (left) and Ag-promoted (right) Co/Ce (Ce, Ce-Ga,
Ce-In) catalysts. (Reaction condition: T=220°C, P=300 psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
21

Figure 2. Methane selectivity as a function of time on-stream for FT synthesis over the
unpromoted (left) and Ag-promoted (right) Co/Ce (Ce, Ce-Ga, Ce-In) catalysts. (Reaction
condition: T=220°C, P=300 psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
22

Figure 3. CO2 selectivity as a function of time on-stream for FT synthesis over the unpromoted
(left) and Ag-promoted (right) Co/Ce (Ce, Ce-Ga, Ce-In) catalysts. (Reaction condition:
T=220°C, P=300 psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
23

Figure 4. The sum of olefins and alcohol products selectivity as a function of time on-stream for
FT synthesis over the Ag-promoted Co/Ce (Ce, Ce-Ga, Ce-In) catalysts. (Reaction condition:
T=220°C, P=300 psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
24

Figure 5. The lower olefins (C2-C4) selectivity as a function of time on-stream for FT synthesis
over the Ag-promoted Co/Ce (Ce, Ce-Ga, Ce-In) catalysts. (Reaction condition: T=220°C,
P=300 psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
25

Figure 6. BJH pore size distributions (desorption branch) for the unpromoted (left) and Ag-
promoted (right) Co/Ce (Ce, Ce-Ga, Ce-In) catalysts. (Reaction condition: T=220°C, P=300
psig, H2/CO=2.0, SV=0.75 nL/h/g cat.).
26

Figure 7. Hydrogen TPR-XANES profile for the unpromoted and Ag-promoted Co/Ce (Ce, Ce-
Ga, Ce-In) catalysts as a function of temperature.
27

Figure 8. The percentage fraction of Ce4+ and Ce3+ found in the unpromoted and Ag-promoted
Co/Ce (Ce, Ce-Ga, Ce-In) catalysts as a function of reduction temperature using H2-TPR
XANES.
28

(A)
(B)
29