Advances in Ma terials Physics and Che mist ry, 2012, 2, 212-215
doi:10.4236/ampc.2012.24B054 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Yttria Promoted Nickel Nanowire Catalyst for the Partial
Oxidation of Methane to Synthesis Gas
Xuebin Hong, Bingbing Li, Cong Zhang
Rena i College of Tianjin University, Tianjin 301636, P. R. of China
Received 2012
ABSTRACT
A yttria promoted nickel nanowire catalyst was prepared by a hard templating method, and characterized by transmission electron
microscopy (TEM) and N2 physical adsorption. The catalytic properties of the yttria promoted nanowire catalyst in the partial oxida-
tion of methane to syngas were compared with a metallic Ni catalyst which was prepared with nickel sponge. The characterization
result s showed th at the yttria promoted nickel nano wire catalyst had high specific su rface area and th ere was more NiO ph ase in the
nickel nanowire cat alyst than in th e metall ic Ni catalyst. The r eaction results sho wed that the yttri a promot ed nickel nanowire catalyst
had high CH4 conversion and selectivities to H2 and CO.
Keywords: Yttria; Nanowire; Methane; Partial Oxidation; Syn gas
1. Introduction
The conversion of natural gas into liquid fuels is commonly
performed via an indirect route through synthesis gas, a mixture
of H2 and CO [1-3]. Industrially, synthesis gas is mainly pro-
duced from methane steam reforming process [4-6]. Such a
process produces a high H2/CO ratio [7] . Furthermor e, methan e
steam reforming is highly endothermic and heat-transfer limited
[8]. Catalytic partial oxidation of methane (CPOM) is an attrac-
tive alternative for the syngas production [9-12] as t he reaction
is mildly exothermic and a H2/CO ratio of 2 can be achieved,
which is desirable for Fischer-Tropsch synthesis [13-15], me-
thanol synthesis, etc.
The first row of transition metals (Ni, Co) and precious met-
als (Ru, Rh, Pd, Pt, and Ir) have been reported as active cata-
lysts for CPOM [16,17]. Among these, Ni has been intensively
studied. Recently, the synthesis of nickel oxide with controlled
nanostructures, such as mesoporous solids, nanotubes or nanowires,
has attracted considerable attention because such material may
exhibit better catalytic properties and be more read ily avail able
[18-23]. Kim et al [24] synthesized a mesoporous Ni-Alumina
catalyst and compared the performance with a nickel catalyst
impregnated on a commercially available alumina support
(Ni-IMP) in CPOM. They found that the Ni-Alumina catalyst
showed a relatively high surface area with a narrow pore size
distribution. And the Ni-Alumina catalyst h aving s mall er n ickel
particles and lower levels of carbon deposition had a more sta-
ble catalytic activity than the Ni-IMP catal yst.
The reaction of CPOM over a metallic Ni catalyst prepared
with nickel sponge has been studied [25]. The results showed
that the metallic Ni catalyst has s ome adv antages over th e sup-
ported nickel or nickel coated catalysts. For example, in the
supported nickel or nickel coated catalysts, the fine nickel par-
ticles ten d to aggregat e at high t emperatures and lo se the activ-
ity [26,27]. However, in the metallic Ni catalyst, the nickel acts
as both active component and the support, so it would not ag-
gregate furt her [25].
In this work, we prepared a yttria promoted nickel nanowire
catalyst by a hard templating method. The catalyst consists of
nickel nanowires, which has higher specific surface area than
the one prepared with nickel sponge. It is expected that the
yttria promoted nickel nanowire catalyst should have higher
activit y for CP OM, while keepi ng the advantages o f the metal-
lic ni ckel catalyst.
2. Experimental
2.1. Catalyst Preparation
Thr e e-dimensional mesoporous silica (KIT-6) was synthesized
according to references [28-31], and used as the hard template
for the preparation of yttria promoted nickel nanowires. For the
preparation of the yttria promoted nickel nanowires, 1.5 g of
Ni(NO3)2⋅6H2O (98.0%) and 1.0 g Y(NO3)3 were dissolved in
1.0 cm3 distilled water forming a saturated solution, followed
by addition of 2.0 g of KIT-6, which resulted an incipient im-
pregnation. After drying at 373 K until all the water had been
vaporized and a dry powder obtained, the sample was heated
slowly to 823 K in air and calcined in a muffle furnace at that
temperature for 5 h. Then, in the presence of hydrogen, the
sample was heated at 1 K/min from ambient temperature to
1123 K, kept at the final temperature for 2 h, and then cooled
down to ambient temperature. The above process was repeated
for four times. Then, the resulting sample was twice treated
with a hot 4.0 mol/L NaOH solution to remove the silica tem-
plate, followed by washing with distilled water and ethanol
several t imes, and then drying at room temperature. The sample
was triturated into 40-60 mesh.
The preparation of the metallic Ni catalyst has been de-
scribed before [25]. Briefly, a piece of metallic Ni sponge
(80 % porosity, Changsha Liyuan Material Co., Ltd.) was cut
into 40-60 mesh, treated with a mixture of 500 cm3 containing

X. B. HONG ET AL.
213
0.01 wt. % HCl and 0.2 wt. % H2SO4 for 24 h, and then tho-
roughly washed with distilled water and dried. This pretreat-
ment results in the formation of 0.18~0.92 μm wide channels
across the surface [25].
2.2. Catalyst Characterization
Transmission electron microscopy (TEM) investigations were
carried out using a FEI Tecnai G2 F20 apparatus, operated at an
accelerating voltage of 200 kV. The sample powders were dis-
persed in ethanol by ultrasonic radiation and the solution was
dropped on the sample holder, which is a copper grid coated
with a carbon film.
The specific su rface ar eas o f th e sampl es were d etermin ed b y
nitrogen physical adsorption at liquid nitrogen temperature
using a Mike TriStar 3000 instrument. All samples were de-
gassed at 573 K for 5 h prior to analysis. The specific surface
areas were calculated according to the method of Brunauer,
Emmett and Teller (BET).
2.3. Experimental Procedures
CPOM was studied with a quartz reactor with 10 mm internal
diameter, which was heated b y an electric furn ace. The catalyst
temperature was measured by a chromel-alumel thermocouple
which was inserted into a quartz thermocouple well, with the
thermocouple tip being placed in the middle of the catalyst bed.
In a typical run, the catalyst (diluted with double portions of
quartz silica of the same size as the catalyst), with the total
volume of 0.39 cm3, was packed in the reactor with a layer of
silica wool below. The reactant gases of CH4 (99.8 %) and O2
(99.9 %), controlled by mass flow controllers, were passed
through the reactor and the temperature was increased to the
required value with the electric furn ace.
Reaction products were analyzed by a 3420 Gas Chromato-
graph equipped with a TCD detector and two columns, a 5A
molecular sieve column for the separation of O2, CH4 and CO,
and a carbon molecular sieve column for the separation of H2
and CO2. Quan tification was perf ormed by injecti ng a gas mix-
ture with known compositions for the calibration.
The equations for the calculation of the conversion of CH4,
CONCH4, and the selectivities to H2 and CO, SH2 and SCO, are
given as follows:
CONCH4 = (FC O ,outlet + FC O 2,outle t) / (FCO ,o u t let + FCO2 ,o ut let
+ FC H 4,ou tle t) × 100 % (1)
SH2 = FH2,outlet / (2 × ( FC O , ou tle t + FCO2,outlet)) × 100 % (2)
SCO = FCO,outlet / (FC O , ou tle t + FCO 2,o u t le t) × 100 % (3)
where Fx is the mole number of substance x. No oxygen break-
through was found in the CPOM reaction.
3. Results and Discussion
3.1. Characterization of Catalysts
Morphology of the yttria promoted nickel nanowire catalyst
was determined by TEM and is shown in Figure 1. It is seen
that the catalyst consists of nanowires. The nanowires are
stacked together, probably because that they are paramagnetic
and cannot be dispersed by ultrasonic radiation. But the nanowires
are not stru ctu ral ly conn ected . B y measuri ng at h igh magnifica-
tion, the diameter of the nickel nanowires was measured to be
approximately 8 nm.
The specifi c surface area of th e yttria promoted nickel n ano-
wire catal yst (9.7 7 m2/g) is much h igher than that of the metal-
lic Ni catalyst (0.25 m2/g).
3.2. Results of the Reaction of CPOM
Changes of methane conversions and H2 and CO selectivities
on the yttri a pr omoted n ickel n an owire cat al yst an d t he metalli c
Ni catalyst with CH4/O2 ratios, reaction temperature, and
GHSV are shown in Figures 2 to 4, resp ectivel y. It can be seen
that with the increase of CH4/O2 ratios, the methane conver-
sions on both catalysts decrease and the selectivities to synthe-
sis gas increase (Figure 2). With the increase of the reaction
temperature, the methane conversions and the selectivities to H2
and CO on both catalysts also increase (Figure 3). With the
increase of GHSV, the CH4 conversion and H2 and CO selec-
tivities on the metallic Ni catalyst increase, but those on the
yttria promoted nickel nanowire catalyst decrease (Figure 4).
These tend encies were agreed with the persp ectives of what are
already known in literatures [32-38], except the changes of the
CH4 conversion and the selectivities to syngas on the yttria
promoted nickel nanowire catalyst with the increase of GHSV.
This will be explained below.
Figure 1. TEM images of the yttria promoted nickel nanowire cat-
alyst at different magnifications, 5 nm; 50 nm.
Figure 2. Comparison of CH4 conversions and H2 and CO selecti vi-
ties between metallic Ni catalyst (solid lines) and the yttria pro-
moted nickel nanowire catalyst (dashed lines) at different CH4/O2
ratios, (◊) methane co nversion; (∆) H2 selectivity; (□) CO selectivity.
Reaction conditions: Temperature = 1123 K, GHSV = 2.0 × 104 h-1.

X. B. HONG ET AL.
214
Figure 3. Compariso n o f C H4 conversions a nd H2 and CO sel ectivi-
ties between metallic Ni catalyst (solid lines) and the yttria pro-
moted nickel nanowire catalyst (dashed lines) at different reaction
temperatures, (◊) methane conversion; (∆) H2 selectivity; (□) CO
selectivity. Reaction conditions: CH4/O2 = 2.0, GHS V = 2.0 × 104 h-1.
Figure 4. Comparison of CH4 conversions and H2 and CO selecti vi-
ties between metallic Ni catalyst (solid lines) and the yttria pro-
moted nickel nanow ir e catalyst (dashed lines) at different GHSV , (◊)
methane conversio n; (∆) H2 selectivity; (□) CO selectivity. Reaction
condit ions : Temperatu r e = 1 123 K , C H4/O2 = 2.0.
However, it is noted that on the yttria promoted nickel nano-
wire catal yst, th e methan e con ver sio n and the s electi viti es to H2
and CO are much higher than those on the metallic Ni catal yst
under the same reaction conditions. For example, as shown in
Figure 2, on the yttria promoted nickel nanowire catalyst, at
reaction temperature 1123 K, GHSV 2.0 × 104 h-1, and CH4/O2
ratio 2.0, the conversion of methane and the selectivities to
hydrogen and carbon monoxide are 90 %, 99 %, and 97 %,
respecti vely, much h igher t han those on t he metallic Ni catal yst,
which are 58 %, 62 %, and 82 %, respectively. The value of the
conversion on the yttria promoted nickel nanowire catalyst is a
little lower than the thermodynamic equilibrium value, which is
95 %, but the values of the selectivities to syngas ar e near to the
thermodynamic equilibrium values, which are 98 % and 98 %,
respecti vely.
In general , i t is kno wn that defect s, su ch as oxygen vacan cies,
are important in the surface chemistry and catalysis of metal
oxides [39]. And the improved catalytic performance in oxida-
tion catalysis has been attributed to a high concentration of
oxygen vacan cies [ 40-43]. Lattice oxygen ions often involve in
reactions over oxide catalysts. Most of the partial oxidation
reactions proceed via the Mars-van Krevelen mechanism,
which is a redox model [44-47]. In this model, hydrocarbons
react with surface lattice oxygen ions to form oxidized products,
leaving a series of oxygen vacancies which are pending to be
recruited by new formed lattice oxygen ions. The cycle for
catalytic partial oxidation is closed via replenishment of the
extracted lattice oxygen ions through the dissociative adsorp-
tion of molecular oxygen on the surface [48].
In the present work, the yttria promoted nickel nanowire cat-
alyst had higher activity and select ivity. We infer that the r eac-
tion might proceed through the Mars-van Kre velen mechanism.
The yttria promoted nickel nanowire catalyst had higher spe-
cific sur face area, which sho ws promotio n effect on the acti vity
of catalyst, because the activity of catalyst was directly related
to its surface area [49]. Higher surface area results in higher
activity. Therefore, methane conversion and the selectivities to
syngas on the yttria promoted nickel nanowire catalyst were
much higher than those on the metallic Ni catalyst under the
same reaction conditions.
From the reaction results (Figure 4), it is seen that the con-
version and the selectivities on the yttria promoted nickel na-
nowire catalyst decreased with the increase of GHSV, while
those on the metallic Ni catalyst increased. The difference in
convecti ve heat transfer co efficients fo r the two catalysts might
be the most important reason to explain the differences in cata-
lytic results. When heat was removed from the surface faster
than it was generated by reaction, the temperature would fall.
When the temperature fell below the ignition temperature of
methane oxidation, reaction no longer occurred on that portion
of the catalyst. This behavior was known as blowout. Blowout
would occur easi er on a catalyst geometry that had a high con-
vective heat transfer coefficient [50]. Convective heat transfer
occurs axially in the direction of flow, acting to transfer heat
from the su rface to the coo ler gases. The convective h eat trans-
fer was much more efficient at removing heat from the yttria
promoted nickel nanowire catalyst because of the much higher
surface area an d the tortuo us flow passages in t he catalyst [51].
With th e increase of GHSV , the reactants increased in th e feed
which could result in the reaction blowout on the yttria pro-
moted nickel nanowire catalyst. This led to the decrease in
methane co nversion and th e selectivities t o syngas on the yttria
promot ed nickel nanowire catalyst with the increase of GHSV.
4. Conclusions
The yttria promoted nickel nanowire catalyst has higher BET
surface area than the metallic Ni catalyst. There is more NiO
phase i n the yttria p romoted nickel nanowire catal yst than in the
metallic Ni catalyst, whi ch brin gs on more active cen ters in th e
CPOM.
The yttria promoted nickel nanowire catalyst had higher CH4
conversion and H2 and CO selectivities than the metallic Ni
during CPOM. With the increase of CH4/O2 ratios, the methan e

X. B. HONG ET AL.
215
conversions on both catalysts decrease and the selectivities to
syngas increase. With the increase of the reaction temperature,
the methane conversions and the selectivities to H2 and CO on
both catalysts increase. With the increase of GHSV, the me-
thane con versio n and H2 and CO selectivities on the metallic Ni
catalyst increase, but those on the yttria promoted nickel nano-
wire catalyst decre ase.
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