Partial Oxidation of Methane to Synthesis Gas Using Supported Ga-Containing Bimetallic Catalysts and a Ti-Promoter

Article abstract of DOI:10.1002/cctc.201801030

A series of bimetallic Ga-containing materials using TiO₂ and TiO₂-promoted SiO₂ supports have been prepared. Rhodium, palladium, and platinum have been used as additional metals in this system. The materials are characterized and used as catalysts for the partial oxidation of methane into synthesis gas (H₂ and CO). The presence of a low quantity of titanium in the form of anatase TiO₂ was shown to improve the overall activity of catalytic methane oxidation and to strongly increase the selectivity of partial oxidation products over the total oxidation of methane to carbon dioxide and water. Particular attention is paid to the formation of gallium-metal alloys on the surface of the catalyst supports. Rh-Ga-Ti-SiO₂ was found to be the most active and selective catalyst, giving 89 % conversion of methane and 99 % selectivity to synthesis gas at 750 °C, as well as exhibiting catalytic activity and preferential conversion to partial oxidation products at temperatures as low as 350 °C.

Full text of DOI:10.1002/cctc.201801030

DOI: 10.1002/cctc.201801030
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Partial Oxidation of Methane to Synthesis Gas Using
Supported Ga-Containing Bimetallic Catalysts and a
Ti-Promoter
[a]
[b]
[c]
[c]
[c]
David A. Kriz, Quddus A. Nizami, Junkai He, Tahereh Jafari, Yanliu Dang,
Peter Kerns, Andrew G. Meguerdichian, Steven L. Suib,* and Partha Nandi*
[a]
[c]
[a, c]
[b]
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A series of bimetallic Ga-containing materials using TiO and
selectivity of partial oxidation products over the total oxidation
of methane to carbon dioxide and water. Particular attention is
paid to the formation of gallium-metal alloys on the surface of
2
TiO -promoted SiO2 supports have been prepared. Rhodium,
2
palladium, and platinum have been used as additional metals in
this system. The materials are characterized and used as
catalysts for the partial oxidation of methane into synthesis gas
the catalyst supports. Rh-Ga-Ti-SiO was found to be the most
2
active and selective catalyst, giving 89% conversion of methane
and 99% selectivity to synthesis gas at 7508C, as well as
exhibiting catalytic activity and preferential conversion to partial
oxidation products at temperatures as low as 3508C.
(H and CO). The presence of a low quantity of titanium in the
2
form of anatase TiO was shown to improve the overall activity
2
of catalytic methane oxidation and to strongly increase the
Introduction
Steam reforming
CH þ H O ! CO þ 3H DH ¼ 206 kJ=mol
ð1Þ
ð2Þ
4
2
2
Methane is the main component of natural gas. According to
the International Energy Agency (IEA), international production
rate of natural gas reached a record high, while demand is
Dry reforming
[1]
projected to continue to increase. Abundant deposits of
natural gas exist, but many are located in remote areas of the
world and therefore transportation of these materials over long
distances is often required. There is therefore significant interest
in developing methods for converting methane to liquid
chemicals, such as methanol or larger hydrocarbons, which are
more easily transported. One such method involves the
conversion of methane into syngas, a mixture of hydrogen and
carbon monoxide, which in turn is a feedstock for Fischer-
Tropsch hydrocarbon synthesis and an intermediate step in
methanol synthesis from methane. Syngas is mainly produced
from methane by steam reformation, carbon dioxide reforma-
tion, autothermal oxidation, and partial oxidation [Eqs. (1)–
CH þ CO ! 2CO þ 2H DH ¼ 247 kJ=mol
4
2
2
Autothermal reforming
x
CH þ O þ ð1 ꢀ xÞH O ! CO þ ð3 ꢀ xÞH
4
2
2
2
2
ð3Þ
ð4Þ
DH ¼ 206 ꢀ 242x kJ=mol ð0 < x < 1Þ
Partial oxidation
1
CH þ O ! CO þ 2H DH ¼ -36 kJ=mol
4
2
2
2
[2,3]
Of these, the partial oxidation route has distinct advantages
(4)].
including a lower energy requirement, lower process temper-
Syngas Production from Methane:
ature, and the capability to adjust the H :CO product ratio.
2
However, the conversion of methane into valuable products
[
[
a] D. A. Kriz, P. Kerns, Prof. S. L. Suib
Unit 3060
Department of Chemistry
University of Connecticut
Storrs CT 06269 (USA)
E-mail: steven.suib@uconn.edu
b] Q. A. Nizami, Dr. P. Nandi
Corporate Strategic Research
ExxonMobil Research and Engineering
Annandale NJ 08801 (USA)
E-mail: partha.nandi@exxonmobil.com
remains difficult due to the high CH -H bond dissociation
3
[4]
energy (436.3 kJ/mol). Improved heterogeneous catalysts are
thus called for.
Our approach involves the use of bimetallic gallium-
containing catalysts on titanium dioxide and titanium-pro-
[
4]
moted silicon oxide supports. A wide variety of supported
transition metal catalysts have been tested for the methane
partial oxidation reaction, including other bimetallic systems
including the noble metals rhodium, palladium and platinum.
In many cases, an increase in catalytic activity attributed to the
formation of an alloy has been demonstrated in similar
[c] J. He, Dr. T. Jafari, Y. Dang, A. G. Meguerdichian, Prof. S. L. Suib
Institute of Materials Science
University of Connecticut
Storrs CT 06269 (USA)
[5–10]
materials.
Gallium is chosen as an alloy component in our
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201801030
system due to its demonstrated capability to bind methane
ChemCatChem 2018, 10, 4300–4308
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molecules and aid in the initial dissociation of the CꢀH
bond. The reaction of methane with gallium is shown to
occur by oxidative addition to form surface methyl and
presented herein to be good catalysts for the partial oxidation
of methane to synthesis gas.
[
11,12]
Although the present focus of methane conversion by
chemists is largely motivated by the current increase in use of
natural gas from fossil fuel resources, methane can also be
obtained from the decomposition of biomass and organic
waste materials. The technologies developed for methane
conversion are therefore also able to contribute to sustainable
carbon-neutral energy production systems which continue to
[13,14]
hydroxide or CH MH moieties.
Ga-species have been shown
3
to promote the recombination and desorption of dihydrogen
from alkanes and is therefore a useful material for alkane
[15]
aromatization via hydrogenation. . Choudhary et al. have
shown conversions of methane with alkanes or methanol over
gallium-zeolite catalysts to the form of higher alkyl chains and
[16,17]
[19]
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aromatic hydrocarbons.
Gallium also plays a critical role in
grow.
BP’s Cyclar process, wherein a continuously-regenerated gal-
lium-doped catalyst, tailored for size selectivity, steadily con-
verts short-chain hydrocarbons into aromatics. In lieu of a
titania support, Chen et al. demonstrated a strong increase in
the stabilization of Rh-adsorbed CO induced by the addition of
a small amount of a Ti promoter (0.0025–0.01 wt%) on their
Results
Figure 1A shows the X-ray diffraction (XRD) patterns for TiO2-
supported materials. The diffraction peaks correlate to a pure
anatase phase of the support material, while the Ti-SiO2-
supported materials are X-ray amorphous (Figure 1B). An addi-
tional peak manifests in the diffraction patterns of both Pd-
containing materials at 348 2-theta, which correlates to the
[18]
silica-supported catalyst. Therefore, the demonstrated capa-
bility for gallium to activate methane and generate dihydrogen
from hydrocarbons, combined with the stabilization of CO from
a trace-level Ti-promoter led us to expect the materials
Figure 1. XRD patterns of (A) TiO
2
2
-supported materials and (B) Ti-SiO -supported materials. (C) Raman spectra of the catalysts. (D) DSC results of the catalysts.
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(101) planes of tetragonal PdO. Raman spectra are shown in
These nano-sized features are more easily seen in the
transmission electron microscopy (TEM) images (Figure 3), and
the high porosity of the materials is apparent. The blank dots
on the surfaces of mesoporous substrates observed from the
TEM images are attributed to the noble metal nanoparticles.
The scanning transmission electron microscopy (STEM) and EDS
mapping images of the Rh-Ga-Ti-SiO2 sample after CH4
oxidation reaction are shown in Figure 4. All the measured
elements are evenly distributed throughout the material. The
merged image (Figure 4B) displays the homogenous intermix-
ing of Rh and Ga. The even elemental distribution and lack of
visible discrete metal particles in the TEM and EDS maps
indicates that the alloyed material exists as a thin layer
distributed over the surface of the material.
ꢀ1
Figure 1C. All six materials exhibit a Raman shift at 150 cm
corresponding to the Eg transition of anatase. The TiO2-
supported materials exhibit three additional signals at 400, 515,
ꢀ1
and 640 cm corresponding to B , A and E transitions. Due
1g
1g
g
to the relatively much lower Ti concentration present in the Ti-
ꢀ1
SiO -supported materials, only the strongest signal at 150 cm
2
is observable but is sufficient to confirm the presence of
anatase TiO in these samples. Differential scanning calorimetry
2
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(DSC) results (Figure 1D) for the as-synthesized samples show
transitions between 25 and 1008C corresponding to endother-
mic melting of gallium and its formation of an alloy with
another metal present on the surface of the material. From
these data, enthalpies of melting (DHmelt) were obtained.
[20]
Comparing these values to that of the bulk gallium (81.097 J/
g) allows us to gain some insight to the distribution of metals,
the effect of particle size, and the interaction of the metals with
the support materials., as summarized in Table 1.
Secondary-ion mass spectrometry (SIMS) results of Ga and
Rh for the fresh Rh-Ga-Ti-SiO2 material are presented in
Figures 5A and 5B, respectively, both displayed uneven distribu-
tion for the selected area. However, after the CH oxidation
4
Surface textural properties of the materials as determined
by N2 adsorption-desorption studies are outlined in Table 1.
Surface areas were determined using the Brunauer-Emmett-
Teller (BET) method, while pore size metrics were calculated
using the Barrett-Joyner-Halenda (BJH) analysis method. Pd-Ga-
TiO , Pt-Ga-TiO , and Rh-Ga-TiO had surface areas of 165, 144,
reaction at 7508C, Ga and Rh are found uniformly intermixed
and homogeneously distributed in the tested area, indicating
the formation of metal alloys. Further, hydrogen temperature
programmed desorption (H -TPR) experiments (Figure 5E) ex-
2
hibit a similar trend, where reduction of the material occurs at a
lower temperature for the Ti-SiO supported materials versus
2
2
2
2
2
and 154 m /g, respectively. All of the Ti-SiO -supported materi-
the TiO2 supported analogues. The lower reduction temper-
atures correspond with the lower melting enthalpies calculated
from the DSC data and indicates a more reactive, facile alloy
2
2
als had relatively higher surface areas of 196, 237, and 266 m /g
for Pd-Ga-Ti-SiO , Pt-Ga-Ti-SiO , and Rh-Ga-Ti-SiO , respectively.
2
2
2
A similar trend was observed for the average pore sizes of the
materials. Pd-Ga-TiO , Pt-Ga-TiO , and Rh-Ga-TiO had relatively
transition in the Ti-SiO materials. The Pt-containing materials
2
exhibit no reduction peak. The inherently low melting point of
gallium also results in greatly increased Brownian motion in the
catalytically active surface which we believe leads to enhanced
interaction between the catalyst and reactant components.
Although the melting point is low, the low vapor pressure of
gallium keeps the catalyst activity stable at the elevated
temperatures required for the reaction.
2
2
2
smaller average pore diameters of 9.62, 9.45, and 9.27 nm,
respectively, versus Pd-Ga-Ti-SiO , Pt-Ga-Ti-SiO , and Rh-Ga-Ti-
2
2
SiO , which had average pore diameters of 13.0, 12.9, and
2
12.3 nm, respectively. The loading amount of the noble metals
and Ga on each catalyst was determined using X-ray
fluorescence (XRF), as summarized in Table 1.
Scanning electron micrographs (SEM) of the materials reveal
the surface morphology of the materials (Figure 2). The TiO2
support materials are characterized by roughly 50 nm platelet-
like particles which aggregate to form micron-sized clusters.
The materials appear highly porous but poorly ordered. The
presence of the catalytic metals on the surface of the support is
The percent methane conversion (Figure 5F) was calculated
by measuring the CH4 concentration at the outlet and
comparing it to the reactor input concentration. The conversion
of methane generally trended up as a function of temperature
for all materials. At higher temperatures, the Ti-SiO supported
2
materials were significantly more active than the TiO -sup-
2
not readily observable. The Ti-SiO supported materials consist
ported materials. The outlet gas composition analysis, shown in
Figure 6, shows the selectivity toward hydrogen, carbon
monoxide, water, and carbon dioxide for each material.
Comparison of the TiO -supported and Ti-SiO -supported
2
of much smaller nanoparticles, on the order of 10 nm, which
coalesce to form much larger, micron-sized aggregates. On the
surface of the support, 100 nm particles of another phase can
be observed.
2
2
materials shows that not only were the Ti-SiO (Figure 6B, D, F)
2
materials more active, but also significantly more selective
Table 1. Nitrogen sorption, DHmelt, and chemical composition of catalysts.
Sample
Surface area
Average pore diameter
[nm]
DHmelt
[J/g]
DHmelt vs bulk metal
[%]
Noble metal atomic ratio
[%]
Ga atomic ratio
[%]
2
[
m /g]
PdꢀGa-TiO
2
165
144
154
196
237
266
9.62
9.45
9.27
13.0
12.9
12.3
49.1
61.3
89.7
74.6
49.4
88.9
57.2
3.6
2.2
4.5
3.8
1.6
3.3
4.6
4.5
5.3
4.6
3
PtꢀGa-TiO
2
71.87
59.76
39.59
71.24
45.79
RhꢀGa-TiO
2
PdꢀGa-Ti-SiO
PtꢀGa-Ti-SiO
2
2
2
RhꢀGa-Ti-SiO
2.3
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2 2 2 2 2 2
Figure 2. SEM images of (A) Rh-Ga-TiO , (B) Rh-Ga-Ti-SiO (C) Pd-Ga-TiO , (D) Pd-Ga-Ti-SiO , (E) Pt-Ga-TiO and (F) Pt-Ga-Ti-SiO .
Figure 3. TEM images of (A) Pt-Ga-TiO
2
, (B) Pd-Ga-TiO
2
, (C) Rh-Ga-TiO
2
2 2 2
, (D) Pt-Ga-Ti-SiO , (E) Pd-Ga-Ti-SiO , and (F) Rh-Ga-Ti-SiO .
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2 4
Figure 4. (A) HAADF and (B–F) element mapping images of Rh-Ga-Ti-SiO after CH oxidation.
Figure 5. SIMS profiles of (A) Ga and (B) Rh for Rh-Ga-Ti-SiO
2 4 2 4
before CH oxidation. SIMS profiles of (C) Ga and (D) Rh for Rh-Ga-Ti-SiO after CH oxidation. (E)
H
2
-TPR profiles for TiO and Ti-SiO -supported materials. (F) Percent methane conversion using supported bimetallic catalysts at 300–7508C.
2
2
toward partial oxidation products when compared to the TiO2
materials (Figure 6A, C, E), which favored the formation of total
oxidation products carbon dioxide and water. Among the TiO2-
supported catalysts, only the Rh-containing material generated
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Figure 6. Composition of the gas stream at the outlet of the partial methane oxidation reactor using (A) Rh-Ga-TiO , (B) Rh-Ga-Ti-SiO , (C) Pd-Ga-TiO , (D) Pd-
Ga-Ti-SiO , (E) Pt-Ga-TiO , and (F) Pt-Ga-Ti-SiO .
2 2 2
2
2
2
significant quantities of partial oxidation products, but only at
temperatures exceeding 5508C. A comparison of the Ti-SiO2-
supported materials shows that the Pd-Ga-Ti-SiO2 required
higher reactor temperatures above 4508C to generate partial
oxidation products. The Rh-Ga-Ti-SiO and Pt-Ga-Ti-SiO materi-
A summary of the highest methane conversion achieved
and the selectivity to partial oxidation products CO and H is
2
shown in Table 2. For each material, selectivity to partial
oxidation was favored at higher temperatures if they were
formed at all throughout the experiment. Among the TiO2-
supported materials, the Pt-Ga-TiO2 material exhibited the
highest methane conversion (52%) and partial oxidation
product selectivity (82%) at 6508C. This is followed by the
2
2
als produce these products at 350 and 4008C, respectively. At
lower temperatures, any methane conversion observed yields
CO and H O.
2
2
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Ti-SiO -supported materials (196–266 m /g), but also much
Table 2. Maximum methane conversion and partial oxidation product
selectivity.
2
lower average pore sizes and overall lower total pore volumes
(~9.4 nm vs 13 nm, and 0.35 cc/g vs 0.75 cc/g, respectively).
The particle size of the more active material support is also
smaller by an order of magnitude, perhaps improving dis-
persion of catalytically active sites.
Sample
Max. conver-
sion
Temperature CO selectivity at max. con-
version
[
%]
[8C]
[%]
RhꢀGa-TiO
2
2
44
43
52
89
750
750
650
750
46
28
82
99
PdꢀGa-TiO
In general, the TiO -supported materials had higher DH
2
melt
PtꢀGa-TiO
RhꢀGa-Ti-
SiO
2
values compared to Ti-SiO materials. This can be explained by
2
taking into account the relatively lower pore size values
2
PdꢀGa-Ti-
80
71
700
750
93
94
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obtained from N -sorption experiments. The smaller pore sizes
2
SiO
2
increase the degree to which individual metal particles are
confined in discrete locations prior to alloy formation, resulting
in a necessary increase in energy required to promote alloying.
Similarly, the relatively sharper transitions indicate a narrower
particle size distribution which would be expected with smaller
PtꢀGa-Ti-
SiO
2
RhꢀGa-TiO material which showed a maximum conversion of
2
44% and selectivity of 46% at 7508C, and PdꢀGa-TiO which
pores in the support material. The Ti-SiO supported materials,
2
2
showed maximum conversion of 43% and only 28% selectivity
on the other hand, have relatively lower DHmelt values. These
data show that the larger pore sizes are more conducive to the
formation of gallium-metal alloys in this system. An alternative
explanation for the decrease in DHmelt is that the alloy formation
is incomplete. However, TEM mapping data shows that the
metals are intimately mixed.
at the same temperature. Among the TiO -supported materials,
2
the activity and selectivity of the materials fall in the order Pt-
Ga-TiO >Rh-Ga-TiO >Pd-Ga-TiO . Although all of the Ti-SiO
2
2
2
2
materials significantly outperform the TiO -supported materials
2
in terms of both conversion and selectivity, this same trend is in
activity is not observed. In this case, the most active and
DSC studies provide insight to the behavior of the metals
on the support surface upon heating, specifically with respect
to gallium-metal alloy formation. In general, DHmelt is decreased
in the Ti-SiO supported materials versus the TiO supported
selective material is Rh-Ga-Ti-SiO , which was capable of 89%
2
conversion and 99% selectivity at 7508C. This is followed by
Pd-Ga-Ti-SiO which gave 80% conversion and 93% selectivity
2
2
2
at 7008C, and the Pt-Ga-Ti-SiO gave 71% conversion and 94%
materials. This is likely due to properties of the support such as
pore size and the distribution of the metal particles influencing
the alloying process upon heating of the material. Specifically,
2
selectivity at 7508C. In this set of materials, the activity and
selectivity fall in the order Rh-Ga-Ti-SiO >Pd-Ga-Ti-SiO >Pt-
2
2
Ga-Ti-SiO . During experimentation, CO chemisorption was
we believe that the pore size in the Ti-SiO material is more
2
2
attempted to determine the number of active sites on the
catalyst. These tests proved difficult, yielded inadequate data,
and therefore made the reliable calculation of turnover
numbers for methane conversion not possible.
suitable for alloy formation. A similar effect was observed by
Unruh et al., where DHmelt of indium was substantially affected
[21]
by the porosity of porous glass substrates. It is interesting to
note that the two most active materials, Rh-Ga-Ti-SiO and Pd-
2
Ga-Ti-SiO2 both showed the greatest decrease in DHmelt
compared to bulk Ga, indicating that alloy formation is critical
to these systems.
Discussion
The introduction of a trace amount of anatase TiO2 onto a
porous SiO support for MꢀGa alloy catalysts has been shown
Conclusion
2
to greatly improve activity and selectivity in the partial
oxidation of methane to CO and H2 versus a pure anatase
support. In a similar system, Rh has been shown to stabilize CO
A set of bimetallic Ga-containing materials using TiO and Ti-
2
promoted SiO supports have been synthesized and character-
2
[18]
molecules on solid surfaces, while Ga is known to activate
CꢀH bonds in methane and other hydrocarbons, and to
contribute the recombination of surface -H species to desorb
ized. Rhodium, palladium, and platinum have been incorpo-
rated into these systems to form catalytically active M-Ga alloys
on the support surface. The materials, in particular the Ti-
promoted materials, are active and selective in the partial
[
15–17]
molecular hydrogen.
Rh-Ga-Ti-SiO , the most active material, also showed the
oxidation of methane to synthesis gas (H and CO) at temper-
2
2
strongest exothermic transition in the DSC study (Figure 1D),
indicating the formation of a Rh-Ga alloy may play a critical role
in the system. The as-synthesized materials include metal and
gallium as separate particles on the surface but form a uniform
mixture upon heating under methane partial oxidation con-
ditions. Additional factors influencing the activity of the catalyst
may be the surface textural properties and the morphologies of
atures between 300–7508C. The trace level of titanium in the
catalyst was shown to not only improve the overall activity of
catalytic methane oxidation but also strongly increased the
selectivity toward partial oxidation products versus carbon
dioxide and water. Overall, Rh-Ga-Ti-SiO was found to be the
2
most active and selective catalyst, giving 89% conversion of
methane and 99% selectivity to synthesis gas at 7508C, and
showed activity for the partial oxidation reaction at temper-
atures as low as 3508C.
the materials. In general, the TiO -supported materials had
2
2
somewhat lower surface areas (144–165 m /g) compared to the
ChemCatChem 2018, 10, 4300–4308
www.chemcatchem.org
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Experimental
obtained using a TA Instruments DSC 2920 and a temperature
ramp rate of 108C/min in an Ar environment. H -TPR was
2
completed using a MKS e-Vision+ residual gas analyzer (quadru-
pole mass spectrometer). A 100 mg portion of material was loaded
into a 7 mm ID quartz tube between quartz wool plugs. This tube
was housed inside a programmable tube furnace. A mixture
Sample Preparation
Preparation of Ga-TiO . A 10% Ga loading on titania was prepared
by adding 2 g of elemental Ga to 18 g of TiO (HOMBIKAT TiO ,
2
2
2
2
8602 having a surface area of 850 m /g. The material was calcined
at 2008C for 16 hours in air, cooled to room temperature, followed
by subjecting to a tumbling Parr reactor at 1508C for 4 hours. The
material was subsequently used to prepare the following titania-
supported bimetallic systems with gallium.
containing 5% H in argon was flowed through the packed bed at
2
a rate of 50 sccm and the furnace temperature was ramped from
room temperature to 4008C. The outlet gas was continuously
sampled to the MS via a capillary line. Raman analysis was
completed using a Renishaw 2000 Raman microscope using a
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514 nm laser source. SIMS measurements of the samples were
Preparation of Pd-Ga-TiO . A 1 g portion of palladium nitrate
hydrate (99.8%) was transferred into a beaker and 3 mL of HNO3
2
done using a Hiden SIMS workstation. An IG-20 ion gun and the
MAXIM quadrupole mass spectrometer for the detection of
secondary positive ions and sputtered neutrals.
(
1
69–70%, J. T. Baker) was added and the solution was mixed well. A
0 g portion of Ga-TiO was added to the solution and the mixture
2
was stirred with a spatula for 15–20 minutes at room temperature.
The material was calcined in air at 4008C overnight.
Methane Partial Oxidation Tests
Preparation of Pt-Ga-TiO . A 1 mL portion of platinum tetraamine
solution was transferred into a beaker. A 5 g portion of Ga-TiO was
mixed with the Pt-containing solution for 15–20 minutes at room
temperature. The material was calcined in air at 4008C overnight.
2
Methane oxidation catalysis was carried out by preparing a fixed
bed catalyst by loading 50 mg of material into a 7 mm ID quartz
tube between quartz wool plugs. The quartz tube was housed
inside a tube furnace. The samples were degassed under flowing
helium (50 sccm) for 1 hour prior to analysis, then cooled to
ambient temperature. A mixture containing 5% methane and 2.5%
oxygen in helium was flowed through the packed bed at 50 sccm
2
Preparation of Rh-Ga-TiO . A 1 mL portion of rhodium nitrate
2
solution (10% Rh) was transferred into a beaker. A 5 g portion of
Ga-TiO2 was mixed with the Rh-containing solution for 15–20
minutes at room temperature. The material was calcined in air at
ꢀ
1
(
GHSV=60,000 h ). The catalyst bed was allowed to equilibrate at
temperatures between 300–7508C for 35 minutes prior to sampling
with a standardized on-line SRI 8610C gas chromatograph (GC)
equipped with Haysep and molecular sieve columns as well as
helium ionization and thermal conductivity detectors. All products
were well separated, identified based on retention time in the GC
column, and quantified using standardized integrated peak areas.
4008C overnight.
Preparation of Ti-SiO . A 20 g portion of silica (Davisil 646) was
modified with titania by reacting 1 mL of Ti(OPr) . The material was
4
calcined in air at 4008C overnight. The resulting material was
subsequently used to prepare the following bimetallic systems
with titania-promoted silica supports. Gallium was added following
the previously described procedure.
2
i
Preparation of Pd-Ga-Ti-SiO . A 1 g portion of palladium nitrate
hydrate (99.8%) was transferred into a beaker and 3 mL of HNO3
2
Acknowledgements
(
1
69–70%, J. T. Baker) was added and the solution was mixed well. A
0 g portion of Ti-SiO was added to the solution and the mixture
We would like to acknowledge ExxonMobil for their support of
this research. The TEM/STEM studies were performed using the
facilities in the UConn/Thermo Fisher Scientific Center for
Advanced Microscopy and Materials Analysis (CAMMA). The SEM
images were acquired at the Bioscience Electron Microscopy
Facility at the University of Connecticut, with financial support
from NSF grant #1126100. We thank the US Department of Energy,
Office of Basic Energy Sciences, Division of Chemical, Biological
and Geological Sciences under grant DE-FG02-86ER13622.A000
for partial support of this research.
2
was stirred with a spatula for 15–20 minutes at room temperature.
The material was calcined in air at 4008C overnight.
Preparation of Pt-Ga-Ti-SiO . A 1 g portion of palladium nitrate
hydrate (99.8%) was transferred into a beaker and 3 mL of HNO3
(69–70%, J. T. Baker) was added and the solution was mixed well. A
2
5
g portion of Ti-SiO was added to the solution and the mixture
2
was stirred with a spatula for 15–20 minutes at room temperature.
The material was calcined in air at 4008C overnight.
Preparation of Rh-Ga-Ti-SiO . A 1 mL portion of rhodium nitrate
2
solution (10% Rh) was transferred into a beaker. A 5 g portion of Ti-
SiO was mixed with the Rh-containing solution for 15–20 minutes
2
at room temperature. The material was calcined in air at 4008C
overnight.
Conflict of Interest
The authors declare no conflict of interest.
Characterization
XRD patterns were collected using a Rigaku Ultima IV X-ray
diffractometer using CuKa radiation operating at 40 kV and 44 mA.
Keywords: Bimetallic catalyst · Gallium · Methane · Partial
oxidation · Synthesis gas
N sorption experiments were carried out using a Quantachrome
2
Autosorb-1-1C automated adsorption system. The powders were
degassed for 6 hours prior to the measurements under vacuum.
The surface areas were calculated using the BET method and pore
sizes were determined using the BJH analysis method. The STEM
and TEM images were collected using a Talos F200X microscope
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Manuscript received: June 26, 2018
Version of record online: August 27, 2018
ChemCatChem 2018, 10, 4300–4308
www.chemcatchem.org
4308
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim