Nickel nanoparticles supported on silica for the partial oxidation of isooctane

Article abstract of DOI:10.1016/j.apcata.2017.08.015

A Ni-based nanoparticle catalyst was synthesized over a silica support via wet impregnation using either ethylene glycol or water. X-ray diffraction and transmission electron microscopy analysis showed that the particle size of the Ni catalyst prepared using ethylene glycol as the solvent tended to be smaller than that obtained when water was used. The resulting catalysts were tested for performance toward partial oxidation (POX) of isooctane at high weight hourly space velocities (WHSV). The results were compared with those from a Rh-based catalyst, which is commonly used for the same reaction. At a WHSV of 13.8 h^?1, a Ni/SiO₂ catalyst with an average Ni particle size of 6.8 nm exhibits higher catalytic activity and stability with an improved resistance to carbon formation than a Ni/SiO₂ catalyst with an average Ni particle size of 16.6 nm. The catalyst with the smaller Ni particles outperformed the supported Rh catalyst for the POX of isooctane. To further improve the Ni dispersion and enhance its ability to run at even higher WHSVs with milliseconds of residence time, ceria was used as a promoter. The ceria-promoted Ni catalyst was also prepared by the wet impregnation method using ethylene glycol as the solvent. The addition of ceria corresponded with a further reduction in the size of the Ni nanoparticles and improved Ni dispersion. Using the Ni-Ce/SiO₂ catalyst the WHSV could be increased up to 20.0 h^?1 while still maintaining its good catalytic activity. At this high WHSV of 20.0 h^-1, the Ni/SiO₂ catalyst without the ceria promoter and the Rh catalyst showed severe deactivation due to the formation of surface carbon deposits.

Full text of DOI:10.1016/j.apcata.2017.08.015

Accepted Manuscript
Title: Nickel Nanoparticles Supported on Silica for the Partial
Oxidation of Isooctane
Authors: Qusay Bkour, Oscar G. Marin-Flores, Trent R.
Graham, Parissa Ziaei, Steven R. Saunders, M.Grant Norton,
Su Ha
PII:
S0926-860X(17)30385-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.08.015
APCATA 16370
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
31-5-2017
27-7-2017
10-8-2017
Please cite this article as: Qusay Bkour, Oscar G.Marin-Flores, Trent R.Graham,
Parissa Ziaei, Steven R.Saunders, M.Grant Norton, Su Ha, Nickel Nanoparticles
Supported on Silica for the Partial Oxidation of Isooctane, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.08.015
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Nickel Nanoparticles Supported on Silica for the Partial Oxidation of
Isooctane
Qusay Bkour1, Oscar G. Marin-Flores2 , Trent R. Graham1, Parissa Ziaei2 , Steven R.
Saunders1, M. Grant Norton1,2*, Su Ha1*
1Voiland School of Chemical Engineering and Bioengineering, Washington State University,
Pullman, WA, 99164, USA
2School of Mechanical and Materials Engineering, Washington State University, Pullman, WA,
99164, USA
*Corresponding Authors:
suha@wsu.edu (S. Ha) and mg_norton@wsu.edu (M.G. Norton)
GRAPHICAL ABSTRACT

Highlights:
 Ni-Ce supported on SiO2 was synthesized with ethylene glycol (EG) as solvent.
 Ni particle size prepared using EG and ceria promoter tend to be small.
 Ni/SiO2-EG showed 97% conversion and 80% H2 yield toward POX of isooctane.
 Ni-Ce/SiO2-EG showed much improved coking resistance at high WHSV of 20.01 h-1.
 Rh/SiO2 showed severe deactivation at the same high WHSV of 20.01 h-1.
A Ni-based nanoparticle catalyst was synthesized over a silica support via wet
impregnation using either ethylene glycol or water. X-ray diffraction and transmission
electron microscopy analysis showed that the particle size of the Ni catalyst prepared
using ethylene glycol as the solvent tended to be smaller than that obtained when water
was used. The resulting catalysts were tested for performance toward partial oxidation
(POX) of isooctane at high weight hourly space velocities (WHSV). The results were
compared with those from a Rh-based catalyst, which is commonly used for the same
reaction. At a WHSV of 13.8 h-1, a Ni/SiO2 catalyst with an average Ni particle size of 6.8
nm exhibits higher catalytic activity and stability with an improved resistance to carbon
formation than a Ni/SiO2 catalyst with an average Ni particle size of 16.6 nm. The catalyst
with the smaller Ni particles outperformed the supported Rh catalyst for the POX of
isooctane. To further improve the Ni dispersion and enhance its ability to run at even
higher WHSV with milliseconds of residence time, ceria was used as a promoter. The
ceria-promoted Ni catalyst was also prepared by the wet impregnation method using
ethylene glycol as the solvent. The addition of ceria corresponded with a reduction in the
size of the Ni nanoparticles and improved Ni dispersion. Using the Ni-Ce/SiO2 catalyst the

WHSV could be increased up to 20.01 h-1 while still maintaining its catalytic activity. The
Ni/SiO2 catalyst without the ceria promoter and the Rh catalyst showed severe
deactivation due to the formation of surface carbon deposits.
Keywords: partial oxidation, isooctane, hydrogen, ethylene glycol, ceria
Introduction
Fuel cells can directly convert the chemical energy of gas and liquid fuels into
electrical energy with a higher efficiency than conventional gas turbine engines (1,2). Fuel
cells can operate using various types of conventional logistic fuels such as gasoline via
fuel reforming processes, which enables the use of existing energy distribution networks.
The production of hydrogen from heavy hydrocarbons is achieved by three main methods:
steam reforming (SR), partial oxidation (POX), and auto-thermal reforming (ATR). Steam
reforming is the most widely employed method in industry for hydrogen production from
methane (3). The major drawback of SR for fuel cell applications is that it is a highly
endothermic reaction, which leads to a slower startup and complex overall fuel cell system
design. Furthermore, supplying water to the system requires extra energy for steam
generation. POX and ATR are more attractive for practical fuel cell applications because
POX reaction is mildly exothermic while ATR is energy neutral (4). Because POX
generates heat it enables a faster startup and simpler system design, which makes it very
suitable for distributed SOFC power generation and transportation (5,6).

Noble metals (e.g., Ru, Rh, Pd, Pt and Ir) are highly active and coking-resistant
catalytic materials for hydrocarbon reforming (7,8). Nonetheless, their cost limits their
application as catalysts. In contrast, nickel (Ni)-based catalysts have become promising
alternatives to expensive noble-metals because of their availability, low cost, and high
reforming activity (9,10). However, high operating temperatures, like those employed in
reforming, promote unwanted side reactions and undesirable physical degradation that
eventually lead to the deactivation of Ni catalysts. For example, Ni nanoparticles may
aggregate via sintering to form larger metal clusters with a loss of a significant number of
active sites. In addition, Ni can also deactivate via carbon formation resulting from
undesirable hydrocarbon cracking reactions (11).
Catalyst supports such as YSZ (12), ZrO2 (13), and GDC (14), and promoters such
as cerium-zirconium (10) and boron (15) have been employed to improve the
performance of Ni-based catalysts operating at high temperatures. However, most of
these cited studies investigated supported Ni catalysts for reforming of simple
hydrocarbons such as methane and not for reforming of heavy liquid hydrocarbons such
as isooctane. Despite the introduction of supports and promoters, researchers still found
significant amounts of coke deposition at high space velocities for these supported Ni
catalysts toward the reforming of heavy liquid hydrocarbons. Corbo et al. (16) studied the
performance of supported Ni-based catalysts for SR of various liquid fuels for only 5h at
a WHSV of 20,000 h-1 and found that the catalysts deactivated because of carbon
deposition. Dong et al. (17) investigated various types of supported transition metals
including Ni for partial oxidation of isooctane and concluded that the hydrogen

concentration decreased significantly as the WHSV increased beyond 20,000 h-1 due to
severe coking.
Small Ni particle sizes are able to limit carbon nucleation and subsequent growth,
which decreases the rate of coking (11). There are several approaches that can be used
to control particle size as well as to improve Ni metal dispersion. These include:
strengthening metal-support interactions by employing suitable carriers and promoters
(18–20), alloying with a metal of higher melting point (21), and physical confinement of
nanoparticles on porous catalytic support (22). The last approach is believed to be the
most effective. For example, many researchers have reported the catalytic activity of
encapsulated Ni nanoparticles in mesoporous hosts and porous inorganic shell structures
(23–26). Xie et al. (27) and Din et al. (28) prepared highly dispersed Ni nanoparticles
immobilized within a mesoporous silica matrix using a polyol-assisted route as a
removable carbon template for improving sintering and coking resistant. The
encapsulated metal nanoparticle design demonstrated improved Ni dispersion, inhibited
nanoparticle sintering, and hindered Ni migration from the mesoporous channels to the
external surface.
Promoters are another key factor in the formulation of industrial catalysts as they
can enhance catalytic performances in significant ways (29). For example, Ni catalysts
are more stable, active and form less carbon during reforming processes when ceria is
used as promoter (30,31). Zhenxing, et al. (32) studied the promoting effect of ceria in
Ni/ᵞ-Al2O3 for dry reforming of methane, and found that ceria can greatly enhance the
activity.

In this work, Ni nanoparticles with different particle sizes supported by mesoporous
SiO2 were synthesized via a wet impregnation method using both aqueous and non-
aqueous solvents, and tested for their catalytic activities toward the partial oxidation of
isooctane at high WHSV. The Ni catalyst was promoted with ceria to further improve its
dispersion over the support in order to increase its effective operating range. Moreover,
in this research the fuel was vaporized when it was introduced through the top of the
reactor without the need for an external vaporizer, which makes the system more
compact. The results obtained demonstrate that Ni/SiO2 catalyst samples prepared with
the non-aqueous solvent have a smaller average particle size than those prepared using
the aqueous solvent. Incorporation of ceria as the promoter appears to further decrease
the average particle size and its dispersion over the support. As the average Ni particle
size decreases, both the reforming activity and stability toward the POX of isooctane
significantly increases with improved coking resistance.
Experimental
Catalyst Preparation Several silica (SiO2)-supported Ni catalysts were prepared in this
work using aqueous and non-aqueous wet impregnation of a Ni precursor on a
commercial silica support. Based on the N2 adsorption-desorption experiments, the
commercial silica support (lot # G02M44, Alfa Aesar) possesses BET surface area, pore
volume, and pore diameter range of 101.6 m2/g, 0.25 cm3/g, and 5-65 nm, respectively.
In a typical preparation, 0.6 g of Ni(NO3)2∙6H2O (Alfa Aesar) with or without ceria
precursor (0.1 g of cerium nitrate hexahydrate (Ce(NO3)3∙6H2O), Alfa Aesar, 99.99%) was
dissolved in 5 mL of Millipore water for aqueous impregnation or ethylene glycol

(anhydrous, Sigma Aldrich, 99.8%) for non-aqueous impregnation. Nickel and ceria
contents were kept at 11 and 3 wt.%, respectively. The resulting solutions were mixed
with 1 g of silica support (Alfa Aesar) and ultrasonicated before stirring at 70 °C for 1 h.
The aqueous suspensions were dried overnight and then calcined at different
temperatures (500-700 °C) in static air and reduced in hydrogen (H2) at 700 °C for 3 h.
With the non-aqueous solvent, the suspensions were first vacuum-dried for 3 h at 170 °C
and then annealed in flowing helium (He) at 700 °C for 3 h to produce a carbon template
by carbonization of ethylene glycol. The resulting carbon template was later removed
during calcination in static air at 700 °C for 2 h. The samples were then reduced in H2 at
700 °C for 3 h in a tube furnace. All reduced samples were cooled in flowing He to room
temperature before removal from the tube furnace and were used without any further
pretreatments prior to the reaction tests. For comparison purposes, a sample of 1%
Rh/SiO2 was prepared by dissolving an appropriate amount of rhodium chloride hydrate
(Rh 38-40%, Aldrich) in 1:3 HNO3 and HCl mixture, which was then impregnated on a
SiO2 support. The sample was calcined and reduced in H2 for 3 h for each step at 700 °C
and 500 °C, respectively, after drying for 24 h at 110 °C. The composition of the samples
prepared in this study are summarized in Table 1.
Characterization: X-ray diffraction (XRD) patterns from Ni and Rh catalysts were obtained
using a Rigaku Miniflex 600 diffractometer with Cu Kα radiation. Particle size
measurements were made using transmission electron microscopy (TEM) with a FEI
Technai G2 20 Twin. The morphology of the post-test catalysts was examined by
scanning electron microscopy (SEM) using a FEI Quanta 200F SEM. Nitrogen
adsorption-desorption isotherms were measured at -196 °C using a TriStar II 3020

automotive physisorption analyzer to obtain the BET surface area, total pore volume, and
pore diameter for the SiO2 support. Fourier transform infrared spectroscopy (FTIR) was
used to characterize the presence of carbon after the carbonization step. FTIR spectra
were obtained in the range of 1800-1300 cm-1 during 64 scans, with 2 cm-1 resolution,
using attenuated total reflection (ATR) mode (ThermoScientific Nicolet IS10).
The amount of carbon deposited on the surface of the post-test catalysts was
quantified using thermal gravimetric analysis (TGA) under air using TA instruments Q50.
Samples were loaded in platinum crucibles and then heated with air from room
temperature to 800 oC at a rate of 5 oC min1.
Activity tests: The catalytic activity for the partial oxidation of isooctane was measured in
a simplified quartz tube reaction system (see Figure 1). The catalyst samples were
supported on a quartz wool plug inside a 7-mm quartz tube, which was heated by an
electrical furnace. A K-type thermocouple was placed at the center of the catalyst bed to
record and control its temperature. The temperature of the catalyst bed was maintained
at the set point of 750 oC for our reforming tests. We also measured the inlet and outlet
temperatures of the catalyst bed during the reforming reaction of isooctane at 750 °C and
we did not observe any temperature changes. Isooctane and air were introduced through
the top of the quartz tube, which allowed rapid vaporization of the liquid. A silicon carbide
bed was placed between the inlet and the catalyst bed to enhance the mixing of reactants.
A calibrated syringe pump and a mass flow controller were used to control the flow rates
of liquid fuel and air, respectively. The exit stream was cooled to 5 oC to separate water,
non-reacted fuel, and other possible condensable compounds. The dry gaseous product

thus obtained was analyzed using a gas chromatograph equipped with two packed
columns (Molecular Sieve 13X and HayeSep D) and a TCD detector.
The amounts of fuel and air were adjusted to obtain different space velocities at a
constant oxygen-to-carbon molar ratio (O2/C) of 0.5. The catalyst performance was
evaluated in terms of H2 yield, CO yield, and carbon conversion, which were calculated
as shown below:
2×moles of H2 produced
H2 Yield =
×100%
(1)
(2)
(3)
18×moles of isooctane fed
moles of CO produced
CO Yield =
×100%
8×moles of isooctane fed
(
)
moles of CO+CO2+CH4 produced
Carbon Conversion =
×100%
8×moles of isooctane fed
Prior to testing, the catalysts were subjected to in-situ activation. To do so, the samples
were reduced by a H2/N2 mixture at 750 oC for 30 min.
Thermodynamic considerations
The partial oxidation of isooctane is believed to be the result of a multitude of
reactions including total oxidation, steam reforming, dry reforming, and water-gas shift,
which all occur to varying degrees. Secondary reactions may also take place between
reactants and products, for instance, methanation and carbon deposition through the
Boudouard reaction (Table 2). These reactions occur on the surface of the catalyst as
well as in the gas phase (33,34). Equilibrium concentrations were calculated using Gibbs
free energy minimization and the results are reported in Figure 2. Under the operating

conditions used in these calculations, the formation of C2 compounds is almost negligible
and the carbon conversion of isooctane is ~100%. Figure 2 (A) shows the effect of the
O2/C ratio on the composition of the product stream at 750 oC. For values of O2/C between
0.5 and 0.7 (the shaded area of Figure 2(A)), the carbon formation can be minimized with
high concentrations of H2 and CO and low amounts of CO2 and H2O. If O2/C values are
higher than 0.7, the carbon formation can be completely suppressed. However,
considerable amounts of H2O and CO2 are produced at the expense of syngas as
complete combustion becomes dominant. Figure 2 (B) shows the effect of the reaction
temperature at O2/C of 0.5. Temperatures higher than 750 oC favor the production of H2
and CO over CO2 and H2O with a minimum amount of the carbon formation. Based on
this thermodynamic result, the catalytic activity tests were performed at 750 oC and O2/C
ratio of 0.5 for our studies.
Results and discussion
The XRD patterns of the as-prepared samples are shown in Figure 3. The nickel oxide
(NiO) phase was entirely transformed into metallic nickel (Ni) after the reduction step, as
deduced from the absence of peaks associated to the NiO phase and the appearance of
the metallic Ni planes (111) and (200) at 44.7° and 52.0°, respectively (JCPDS No. 04-
0850). The Scherrer equation was used to calculate the average crystallite size of the
metallic Ni phase and the values are reported in Table 3. As seen, the average crystallite
sizes for the samples prepared using aqueous impregnation were larger than those using
non-aqueous impregnation. These results agree with previous reports (28), which indicate

that the Ni crystallite size of catalysts prepared using ethylene glycol tend to be smaller
than those prepared with water.
In aqueous impregnations, water is completely evaporated during drying. In the
calcination process, nickel nitrate decomposes to NiO, which easily agglomerates to form
larger particles (35). If the solvent is ethylene glycol, the carbon template is formed from
the carbonization of ethylene glycol by heating it under He conditions at 700 °C. Nickel
nitrate decomposes to NiO under the confinement of the carbon template during the air
calcination process, resulting in well-dispersed and small NiO nanoparticles. The carbon
template would be removed during calcination. However, we believe that the time of air
exposure without the carbon template was sufficiently short to prevent the severe
aggregation of NiO nanoparticles. The presence of these small NiO nanoparticles
eventually leads to the formation of highly dispersed Ni nanoparticles following the
reduction process. Many researchers also studied the carbonization of ethylene glycol
and the formation of a carbon template. Miao et al. (36) used ethylene glycol as a carbon
source to fill an aluminum oxide template and they found that high temperature
carbonization for 30 min resulted in the formation of carbon nanotubes. Xie et al. (27)
reported that the nickel nitrate can coordinate with ethylene glycol and enters the silica
channels by the capillary force. During the inert gas heat treatment, ethylene glycol then
decomposes to carbon and forms the carbon template that hinders NiO nanoparticles
from sintering by preventing the migration of nanoparticles from the internal surface of
silica channels to the outside. TEM analysis performed after the pyrolysis step showed
that the mesoporous channels of SiO2 were filled with the carbon templates under the
inert gas heat treatment and NiO nanoparticles were immobilized under the carbon

template without sintering during the calcination step. They also collected Raman spectra
and found distinct peaks that were assigned to carbonaceous matrix after the
carbonization of ethylene glycol. After calcination, the peaks vanished, indicating the
removal of the carbon template. In this work, the presence of carbon template before the
calcination process was determined using FTIR (Figure S1) for NS-EG sample. A distinct
peak at 1580 cm-1 before exposure to air was detected, which is assigned to C=C
stretching, indicating the presence of a carbonaceous matrix from the carbonization of
ethylene glycol. After being exposed to air during the calcination process at 700 °C, the
peak almost completely disappeared indicating the removal of the carbon template (37).
Figure 4 (A, B, and C) shows TEM images combined with particle size distributions for
all the Ni-containing samples. As seen, the NS-EG exhibits a particle size distribution with
a peak at ~8 nm and average particle size of 6.8 nm, while the samples that were
prepared using water as a solvent display larger average particle sizes of 16.6 and 12.7
nm for NS-W700 and NS-W500, respectively. Table 3 compares the TEM results with
those obtained using XRD. Standard deviations for TEM particle size measurements is
also provided in Table 3.
Beside the confinement effect of the carbon template for the ethylene glycol solvent, there
could be other possible solvent effects improving Ni dispersion and catalytic properties
such as chemical modification of the silica surface and improvement of the reducibility of
supported metals. For example, Zhang et al (38) used different solvents (acetic acid or
ethanol) to modify the surface properties of silica to improve the reducibility and
dispersion of supported cobalt, which resulted in excellent stability for the Fischer-
Tropsch synthesis reaction. Furthermore, the physical and chemical properties of the

metal species on the silica surface can be also strongly affected by the pH of the solvent
as the charge on the silica surface changes as a function of solvent pH. At lower pH
values (pH <2), the silica surface is positively charged and, consequently, metal ions with
a positive charge are poorly dispersed (39). On the other hand, the surface of silica is
negatively charged at higher pH values than 2, leading to greater dispersion of metal ions
and formation of smaller supported metal nanoparticles (38,40,41). Miao et al. (42)
reported that Ni/SBA-15 prepared from ethanol solution would promote the fixation of the
precursor onto the silica support and increase Ni dispersion. As a result, the activity of
catalysts for CO methanation was improved. They found that the effect of impregnation
solvents could be related to the solubility of nickel nitrate and the polarities of solvents.
The decreased polarity of the ethanol solvent caused an increase in the interaction
between the metal complex and silanol group on the silica surface, promoting the fixation
of the precursor onto the support and increased nickel dispersion (43). Lucredio et al.
(44) prepared cobalt catalysts on SiO2 and Al2O3 supports by impregnation using
methanol. They found that a methanol solution led to smaller crystallites compared to that
obtained with an aqueous solution because the methanol hydroxyl groups have a strong
interaction with the silica surface. In addition to the carbon template effect, these other
possible solvent effects could play an important role for improving the Ni dispersion for
the Ni/SiO2 catalyst synthesized using ethylene glycol.
A series of isooctane partial oxidation activity tests for NS-EG, NS-W500, and NS-W700
(with particle sizes of 6.8, 12.7, and 16.6 nm) were performed at different WHSVs to
determine the maximum WHSV value that each catalyst sample can process without
severe coking. The reforming tests were performed at fixed temperature and O2/C of 750

°C and of 0.5, respectively. The fuel flow rate was changed to obtain different WHSV
values. The maximum WHSV was determined by catalyst deactivation due to the severe
carbon formation, at which point there was no syngas in the effluent stream and the
performance drops significantly (i.e., the conversion drops down to that of the blank run,
which is about 30%). The results are shown in Figure 5. As the Ni particle size
decreases, there are more active sites available for processing higher amount of fuel per
unit time. Thus, higher dispersion and smaller Ni particles lead to higher maximum WHSV
values as shown in Figure 5. If the WHSV is too high (i.e., beyond the maximum WHSV
value), then there would be insufficient surface area available for the catalyst to process
all of the incoming fuel. In other words, there would be a large amount of fuel and their
fragments that would not interact with the catalyst. Thus, the chance of coke formation
increases.
A second set of fuel reforming experiments was carried out under the fixed high fuel flow
rate (WHSV of 13.8 h-1) for 2 h in order to compare the degree of coke formation in each
sample. TGA was used to obtain quantitative information regarding the deposited carbon
on the catalyst surface. According to Figure 6, the TGA profiles initially experienced an
increase trend starting at 250 oC, which is derived from the oxidation of metallic Ni (45).
o
The weight losses after 400 C are caused by the oxidation of carbon (46,47). The NS-
W700 sample with average particle size of 16.6 nm shows the highest weight loss, which
is attributed to the presence of the largest amount of coke formation (~17 wt.%). On the
other hand, the NS-EG sample with average particle size of 6.8 nm exhibited superior
activity at this high space velocity with less than 3.3 wt.% coke formation. It has been
shown that the coke depositions on Ni catalysts may be present in different morphologies

such as amorphous, filamentous, or graphitic carbon (48). Filamentous carbon over Ni
particles, which originates from the polymerization reaction of hydrocarbons (49), is
generally oxidized at temperatures between 300 and 530 oC (50). More stable graphitized
carbon is expected to be oxidized at temperatures higher than 530 oC (51). The oxidation
weight loss curves as a function of temperature measured by TGA are shown in Figure
S2 (black line) together with the corresponding derivative curves of the weight loss to
temperature (red line). A major derivative peak is observed in the temperature range of
o
450 – 680 C for all samples. These peaks are ascribed to the oxidation of both
filamentous carbon as well as graphitic carbon. The peaks with large particle sizes (16.6
and 12.7 nm) were shifted to higher temperatures, due to higher degree of graphitization.
The TGA results show that the sample with the smaller particle size exhibits higher
resistance to carbon formation. The cleavage of C-H bonds and polymerization of C
species on the Ni surface are dependent on the variation of Ni particle size and dispersion
(35,52). It has been reported in the literature that carbon formation is not able to proceed
when the crystals are below a critical ensemble size (53). carbon can grow only on Ni
crystals larger than this critical ensemble size at certain temperatures and fuel flow rates
(54). Kim et al. (55) observed that the formation of carbon proceeded over Ni particles >
7 nm. Others consider the critical ensemble size of Ni particles to be 10 nm (56,57). Our
results agree with those reported in the literature, which shows that smaller and well
dispersed Ni crystals have higher resistance to carbon formation (58). Thus, the NS-EG
catalyst that possesses the smallest particle size shows the best reforming performance
toward POX of isooctane.

It is also important to maintain the high dispersion and to prevent the Ni
nanoparticles from aggregation during the reforming reaction at high temperatures in
order to achieve long-term stability. TEM images and particle size distribution of spent
NS-EG (Figure S3 A) showed that its average particle size only increases from 6.8 nm
to 8.3 nm after the reforming reaction, while maintaining good Ni nanoparticle dispersion.
TEM images indicates no carbon deposition. On the other hand, NS-W700 and NS-W500
spent samples (Figure S4) showed a significant increase of their average particle size
from 16.6 to 32 nm and from 12.7 to 25 nm, respectively. Corresponding TEM images
show a high degree of carbon deposition with a formation of carbon filaments.
To further investigate the stability of the NS-EG catalyst, activity tests were conducted
over 12 h at 750 oC and 1 atm, with a WHSV of 13.8 h-1, a residence time (RT) of 35 ms,
and O2/C ratio of 0.5. For our current study, a Rh/SiO2 catalyst with 1% Rh loading (with
particle size of 2.8 nm (Figure S5 A)) was chosen as a reference because it has been
extensively used as an active catalyst for complex fuel reforming applications. The blank
run shows a performance of 32 % conversion, 22% CO yield, 5% H2 yield and 3 sccm of
H2 production. As seen in Figure 7, NS-EG catalyst showed stability for 12 h under these
conditions and produced a conversion of 97%, H2 yield of 80%, CO yield of 85%, CO2
yield of 6% and syngas production of 70-75 sccm, while the Rh catalyst showed a much
lower performance (conversion of 65%, H2 yield of 20%, CO yield of 50%, CO2 yield of
9% and syngas production of 30 sccm). Rh/SiO2 spent sample (Figure S5 B) showed a
particle size of 5.4 nm, which is even smaller than that of the NS-EG sample. Since the
smaller particle size generally leads to both higher surface area and improved coking

resistance, our results shown in Figure 7 suggest that our NS-EG catalyst might possess
higher intrinsic catalytic properties than that of the Rh/SiO2 catalyst.
Effect of ceria addition on the catalytic performance: The NS-EG sample with the particle
size of 6.8 nm has a good catalytic activity and stability at the maximum WHSV of 13.8 h-
1 (Figure 5). For practical applications in SOFC systems, it is crucial to design a catalyst
that can handle the high WHSV with milliseconds of residence time and without
deactivation due to the carbon formation. Ceria (Ce) is a good promoter for hydrocarbon
reforming on Ni-based catalysts (30,31), and can promote carbon removal, enhance the
dispersion of Ni on the support, and enhance catalytic activity (59,60). Ni-Ce supported
on SiO2 was synthesized using the wet impregnation method with ethylene glycol as
solvent. The XRD diffractogram (Figure 3 (D)) shows that the incorporation of Ce as the
promoter appears to further decrease the average crystallite size of Ni, which suggests
that the presence of Ce may increase the metal dispersion. The absence of any of Ce
diffraction peaks for the Ni catalysts suggests the high dispersion of promoter. Figure 4
(D) shows that NCS-EG sample with a narrow distribution of Ni particle sizes with a peak
at ~5 nm (Table 3).
To investigate the effect of the Ce addition on the catalytic performance, POX of isooctane
was carried out under harsher conditions than those used to obtain the data in Figure 7
(WHSV=20.01, and residence time =23 ms) for 4 h or until excessive carbon formation
led to reactor plugging. As observed in Figure 8, NS-EG and RS-AR catalysts showed a
rapid drop in performance under this high WHSV value caused by reactor plugging with
an excessive amount of carbon deposition. On the other hand, the performance of the

NCS-EG catalyst exhibited reasonable stability with carbon conversion and syngas
production of 78% and 70 sccm, respectively.
Effect of Ce addition on carbon formation: Carbon formation is a major problem in liquid
hydrocarbon reforming since it can cause a loss of activity when it blocks the active sites.
The coke can be formed by the Boudouard reaction (Equation 1) or by hydrocarbon
cracking (Equation 2) (61).
2CO⇄ CO2 + C
(Equation 1)
(Equation 2)
CnH2m⇄ nC + mH2
The quantification of the total carbon formation on NS-EG and NCS-EG samples was
obtained by thermogravimetric analyses (TGA) after the activity tests at accelerated
conditions at a WHSV of 20.01 h-1 (Figure 9). Weight losses before 130 oC were excluded
as they are caused by the loss of adsorbed water. The weight gain observed for both
samples is caused by the oxidation of Ni metal to NiO. The subsequent weight losses are
caused by the removal of carbonaceous species (62). The weight loss of NS-EG (6.40
%) was found to be much higher than that of NCS-EG (0.66%) suggesting that the
addition of the Ce promoter significantly enhances the coking tolerance of Ni catalysts.
In Figure S6, we differentiated the weight loss data of TGA shown in Figure 9 as a
function of temperature. As indicated in Figure S6, the derivative peak with NS-EG spent
catalyst was shifted to higher temperature compared to that of the NCS-EG spent catalyst
due to higher degree of graphitization of the coke deposit. These TGA results show that
the NCS-EG sample with the smaller particle size exhibits higher resistance to carbon
formation compared to that of NS-EG sample with the larger particle size. Figure 10

shows SEM images of the post-test catalysts. As seen, carbon whiskers are clearly
observed on the spent NS-EG catalyst. While the spent NCS-EG shows negligible carbon
formation. Particle size distribution and dispersion of spent NCS-EG (Figure S3 A)
showed a good dispersion after the reforming reaction with average particle size of 5.8
nm and without any indication of carbon deposition. In summary, the results of our study
show that (1) utilizing the ethylene glycol solvent for the supported Ni nanoparticle
synthesis and (2) the addition of Ce to the Ni catalyst result in a NiCe/SiO2 catalyst with
a reduced Ni particle size and an increased Ni dispersion on the support, lead to superior
performance for POX of isooctane, the improved coking resistance, and the increased
operating WHSV window.
These results agree with other reports that studied the effect of Ni dispersion and particle
size to suppress carbon deposition during reforming of heavy hydrocarbons, such as
isooctane and dodecane. Some of these reports added different promoters and/or non-
noble metals to prevent Ni growth in order to mitigate carbon formation and strengthen
the interaction between Ni nanoparticles and the support (63,64). Bkour et al (10) found
that the presence of Mo maintains the small size of the Ni particles and lowers the
tendency for Ni nanoparticle growth, which hence prevent surface coking during POX of
isooctane at the high WHSVs. Tin (Sn) is another element used to improve the stability
during liquid hydrocarbon reforming. Nikolla et al (65) demonstrated that the Ni/Sn
bimetallic system has much better carbon tolerance than monometallic Ni during steam
reforming of isooctane. They found that the presence of Sn can decrease the ensemble
size of low-coordinated Ni atoms that bind carbon strongly (66,67). Other studies have
attempted to improve the catalytic activity of liquid fuel reforming by noble metal doping.

Gue at al. (68) found that the addition of Ru keeps small and well dispersed metallic Ni
particles improving their catalytic activity for steam reforming of kerosene. Most of these
studies used low space velocities and/or excess amounts of steam, which are not
practical operating conditions for fuel cell systems. Ibrahim et al. (69) studied the effects
of reaction operation conditions on the performance of the Ni/Al2O3 catalysts for the partial
oxidation of isooctane. They confirmed that the high nickel dispersion helps to enhance
the catalyst performance. Furthermore, the confinement of Ni nanoparticles to porous
supports for improving Ni dispersion and suppressing aggregation is an effective strategy
to operate catalytic reforming at high fuel space velocities (70). Xiao et al (71) found that
alloying of Ni-Co along with the addition of Ce promoter and confinement of Ni
nanoparticles are needed to improve Ni catalytic performance during steam reforming of
n-dodecane at the high space velocities.
Conclusion
Controlling the metal particle size and improving its dispersion over the support can be
achieved by utilizing the appropriate impregnation solvent and promoter. In this study, Ni-
based catalysts were synthesized via the wet impregnation method using either water or
ethylene glycol as solvents and were analyzed for the isooctane POX reaction at high
space velocities. Based on TEM and XRD analysis, Ni nanoparticles supported on SiO2
(i.e., Ni/SiO2 catalyst) synthesized using ethylene glycol show the smaller and more highly
dispersed Ni nanoparticles than Ni/SiO2 catalyst synthesized using water. When ethylene
glycol is used it creates a carbon template that confines the Ni metal nanoparticles and
prevents agglomeration, which leads to improved Ni metal dispersion over SiO2 support.

POX of isooctane was carried out at 750 °C, O2/C ratio of 0.5, and WHSV of 13.8 h-1.
Ni/SiO2 catalyst prepared in ethylene glycol with the average Ni size of 6.8 nm exhibit
higher catalytic activity and stability with an improved resistance to carbon formation
compared with Ni/SiO2 catalyst prepared in aqueous solution with the average Ni size
greater than 12 nm. The Ni/SiO2 catalyst prepared with ethylene glycol provides isooctane
conversion of 95%, CO and H2 yields of 84% and 81%, respectively, and syngas
production of 73 sccm for 12 h, which outperforms a supported Rh/SiO2 catalyst at the
same operating conditions. To further improve the Ni dispersion and enhance its
capability to run at even higher WHSV with milliseconds of residence time, a Ce promoter
was introduced. The Ni-Ce/SiO2 prepared in ethylene glycol has a smaller crystallite size
and higher metal dispersion than in the absence of Ce. For both the Ni/SiO2 catalyst
without the Ce promoter and the Rh/SiO2 catalyst, our results show that severe
deactivation occurs due to coking during the reforming reactions of liquid hydrocarbons
at the accelerated coking condition (WHSV of 20.01 h-1). On the other hand, under this
very harsh operating condition, Ni-Ce/SiO2 catalyst exhibits high catalytic activity with
improved coking resistance.
Acknowledgements
This work was financially supported by the Office of Naval Research (Grant No. N00014-
15-1-2416). We thank the Franceschi Microscopy and Imaging Center at Washington
State University for use of their facilities. We also would like to show our gratitude to Jake
Gray at Washington State University for taking the TEM images.

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