Single and bimetallic catalyst screenings of noble metals for methane combustion

Article abstract of DOI:10.1016/j.catcom.2018.03.022

Single metals and bimetallics impregnated on γ-Al₂O₃ were screened for methane combustion. A screening of 20 noble metals was first performed in a fuel-rich condition, revealing the top six active metals: Rh, Ru, Pd, Pt, Cu, and Cr, in order of decreasing activity. Then, the screening of the combinations of these six metals delivered several catalysts that offered a higher methane conversion when compared to the most active single catalysts. (4 wt% Rh + 1 wt% Cr)/γ-Al₂O₃ achieved the highest methane combustion and provided excellent stability. NH₃-TPD analysis revealed strong acid sites playing a key role in the activation of methane.

Full text of DOI:10.1016/j.catcom.2018.03.022

Accepted Manuscript
Single and bimetallic catalyst screenings of noble metals for
methane combustion
Weerit Kumsung, Metta Chareonpanich, Paisan Kongkachuichay,
Selim Senkan, Anusorn Seubsai
PII:
S1566-7367(18)30114-6
doi:10.1016/j.catcom.2018.03.022
CATCOM 5363
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
28 December 2017
16 March 2018
19 March 2018
Please cite this article as: Weerit Kumsung, Metta Chareonpanich, Paisan
Kongkachuichay, Selim Senkan, Anusorn Seubsai , Single and bimetallic catalyst
screenings of noble metals for methane combustion. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2018), doi:10.1016/j.catcom.2018.03.022
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ACCEPTED MANUSCRIPT
Single and bimetalliccatalystscreenings of noble
metals for methane combustion
a
a,b,c,d
a,b,c,d
f
Weerit Kumsung , Metta Chareonpanich
, Paisan Kongkachuichay
, Selim Senkan ,
a,b,c,e,*
Anusorn Seubsai
a
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok
0900, Thailand
1
b
Center of Excellence on Petrochemical and Materials Technology, Kasetsart University,
Bangkok 10900, Thailand
c
NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology, Kasetsart
University, Bangkok 10900, Thailand
d
Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and
Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand
e
Center for Advanced Studies in Industrial Technology and Faculty of Engineering, Kasetsart
University, Bangkok 10900, Thailand
f
Department of Chemical and Biomolecular Engineering, University of California Los Angeles,
Los Angeles, CA 90095, United States

ACCEPTED MANUSCRIPT
Abstract
Single metals and bimetallics impregnated on γ-Al O were screened for
2
3
methane combustion. A screening of 20 noble metals was first performed in a fuel-
rich condition, revealing the top six active metals: Rh, Ru, Pd, Pt, Cu, and Cr, in
order of decreasing activity. Then, the screening of the combinations of these six
metals delivered several catalysts that offered a higher methane conversion when
compared to the most active single catalysts. (4wt% Rh + 1wt% Cr)/γ-Al O3
2
achieved the highest methane combustion and provided excellent stability. NH3-
TPD analysis revealed strong acid sites playing a key role in the activation of
methane.
Keywords: Alumina support; Bimetallic catalyst; Chromium; Methane
combustion; Rhodium

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1
. Introduction
In freezing-cold places, houses need a heater to keep the temperature warm. Such
heaters can be classified into two types: that are namely; electrical heater and gas
catalytic heater. For the latter, the heat is generated by the combustion of fuel gases
such as methane (CH ) and propane, with combustion of the fuel gases in air
4
occurring in a catalytic pad. The products from the combustion are CO , CO, and
2
H O, depending on the processes of combustion: (1) complete or (2) incomplete
2
combustions. Reaction equations the equation reactions for CH combustion are as
4
follows:
CH + 2O
CO + 2H O
; complete combustion (1)
4
2
2
2
2
CH + 3O
2CO + 4H O ; incomplete combustion (2)
4
2
2
Generally, Pt catalyst ceramic composite is commercially used for this operation.
However due to the high cost of Pt, the fact that it has poor-stability at high
reaction temperatures [1], and the fact that it has lower activity compared to some
other catalysts [2-4], the new development of new economical and highly active
catalysts for the combustion process is essential.
Various catalysts in CH combustion have been explored to achieve better
4
activity and higher catalytic stability. These catalysts are mostly from transition
metals such as Pt, Rh, and Pd. In a study of these three metals impregnated on γ-

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Al O under a fuel-rich condition, the CH conversions of Pd and Rh catalysts were
2
3
4
found to be higher than those of Pt catalyst in a temperature range of 300–800 °C
2]. In other work, Pd and Rh were mixed and embedded on γ-Al O , giving a CH
[
2
3
4
conversion of 87 % at 550 °C. The addition of Pd 1 wt% to the Rh catalyst could
improve the activity when compared to the single Pd or Rh catalysts, because the
formation of Rh O inhibited the reduction of PdO to Pd(0) and thus the Pd oxide
2
3
form enhanced combustion of CH [5]. When Pd and Rh were combined and
4
embedded on a metal-foam support, a CH conversion of 96.7 % was attained and
4
a stability test at atmospheric pressure and 750°C for 200 h showed a steady CH4
conversion [6]. In a similar study of Rh impregnated on ZrO , a CH conversion of
2
4
60 % was achieved at 600 °C. This study suggested that the metallic dispersion and
oxidation state of Rh improved the catalytic activity of CH combustion [7]. Rh
4
was also used with LaMnO /Al O support; approximately 90 % CH conversion
3
2
3
4
and 80 % CO selectivity were achieved at 600 °C. The manganese from
LaMnO /Al O support was found to reduce the light-off temperature, while the
3
2
3
perovskite was found to improve the stability of Rh particles [8]. In other research,
Cr catalyst for CH combustion has been studied due to its low cost. In a study
4
wherein Cr catalyst was used with Ce added by wet impregnation, Cr-based
catalyst with 3 wt% of Ce was the most active catalyst, giving 70 % conversion
CH at 400 °C [9]. Finally in some recent works, three-dimensionally-ordered
4

ACCEPTED MANUSCRIPT
macroporous (3DOM) materials possessing a high surface area were synthesized
and were found to deliver an improvement in CH combustion [10]. Among these
4
materials are 1D single-crystalline perovskite [11], 3DOM perovskites [12], Pt
nanoparticles on 3DOM Ce Zr Y O [13], and Ag on 3DOM La Sr MnO
0
.6 0.3 0.1
2
0.6 0.4
3
[14], all of which have been become increasingly of interest in the field.
Although there have been several noble metal catalysts investigated for used
in CH combustion, tests of the above-mentioned single metal catalysts have never
4
been reported in terms of catalyst activity under the same testing conditions.
Furthermore, bimetallic catalysts of those single noble metals have never been
systematically explored and identified for CH combustion. Herein, we used a
4
combinatorial approach [15, 16] to explore the top-most active single and
bimetallic catalysts for CH combustion from 20 selected noble elements. The most
4
active catalyst was then characterized to understand its physicochemical properties
in relation to the catalyst performance.
2
. Experimental
2.1 Catalyst preparation
Single and bimetallic catalysts were prepared by impregnation. In brief, the
catalysts were prepared as follows. 20 metal precursors (see Table S1) were

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dissolved in DI water as a stock solution. First, each stock solution was determined
and pipetted into commercial γ-Al O (Alfa Aesar, 99.97%, 3 µm APS powder) to
2
3
achieve 5wt% of single noble metal supported on γ-Al O . The solution was stirred
2
3
at room temperature for 1 h, then heated to 170 ºC and stirred until dried. A fine
powder obtained was taken to calcine at 750 ºC for 6 h. Second, the bimetallic
catalysts were prepared by selecting the six elements showing the highest CH4
conversion from the catalyst screening of the single catalysts. Five metal ratios
were examined for each pair and the total metal loading of each pair, loaded on γ-
Al O was 5 wt% , i.e. 5wt% A + 0wt% B, 4wt% A + 1wt% B, …, 0wt% A + 5
2
3
wt% B, where A and B are each selected elements. The impregnation, drying, and
calcination procedures were the same as for the preparation of the single catalysts.
2.2 Catalyst characterization
X-ray powder diffraction (XRD, Bruker AXS/Diffraktometer D8) was used
to determine crystallization of the catalysts. The operating parameters were
monochromatic Cu–K radiation, 40kV, 40 mA, a step size of 0.02º, and a scan
α
speed of 0.5 sec/step. N physisorption using a Quantachrome Autosorp-1C
2
instrument with BET method at −196 °C was used to determine the specific surface
area of samples. Elemental compositions of the samples were analyzed by using a
scanning electron microscope with an energy dispersive X-ray spectrometer (FE-

ACCEPTED MANUSCRIPT
SEM/EDX, FE-SEM: JEOL JSM7600F). Ammonia-temperature programmed
desorption (NH -TPD) was used to determine the acidic sites of the catalysts
3
(
Thermofinnigan model 1100). 0.25 g of the catalyst was pretreated under N flow
2
at 400 °C for 1 h and cooled down to 40 °C before 10% NH /He mixed gas was
3
flowed over the catalysts for 30 min to adsorb on the acid sites. The excess
ammonia was eradicated by flowing N at 40 °C for 30 min. Then the catalysts
2
were heated to 800 °C at a heating rate of 10 °C /min, while He was passed over
the catalysts at 30 mL/min. All of the gas flow rates in each step were 30 mL/min.
2.3 Catalytic activity test
A sketch of the testing system is shown in Fig. S1. 10 mg of each prepared
catalyst was packed in a quartz tube reactor (0.5 cm inner diameter) and
sandwiched between layers of quartz wool. If not otherwise indicated, the reactor
temperature was set at 475 °C and atmospheric pressure was used. After the reactor
temperature reached the setting point, the tubular reactor containing the catalyst
was introduced into the reactor system. Then, a feed gas containing O (37.5%) and
2
CH (62.5%) at a volume ratio of O /CH = 28.0/46.7, and without any inert gas,
4
2
4
was introduced into the tubular reactor at a total feed flow rate of 5 mL/min,
-
1
corresponding to a gas hourly space velocity (GSHV) of 15,000 h . It should be
noted that the fuel-rich condition was investigated in this work because the

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condition can better approximate or simulate the situation of the fuel gas meeting
the catalytic pad when using a catalytic heater. Mass flow controllers (KOFLOC
3810 DSII) were used to control all of the feed gas flow rates in the system. The
system was left to sit for 1 h without any catalyst pretreatment or activation prior to
sampling the effluent gas. Gas chromatography was an online Gas Chromatograph
(SHIMADZU, GC-14A) equipped with a thermal conductivity detector (TCD).
The detected products were CO, CO , and H O. Some catalysts also produced a
2
2
trace amount of H . The activity of the catalyst was evaluated after the system was
2
left at the setting condition for 1 h and was determined using the amount of the
CH conversion, CO selectivity, and CO selectivity, as expressed in equations (3),
4
2
(4), and (5), respectively:
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂 +푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂
2
%
퐶퐻 푐표푛푣푒푟푠푖표푛 = 100×
(3)
4
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂 +푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂+푚ꢀ푙ꢁꢂ ꢀ푓 ꢃꢄ 푙ꢁ푓푡
2
ꢅ
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂
2
%
%
퐶ꢆ 푠푒ꢈ푒푐ꢉ푖푣푖ꢉ푦 = 100 ×
(4)
(5)
ꢇ
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂 +푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂
2
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂
퐶ꢆ 푠푒ꢈ푒푐ꢉ푖푣푖ꢉ푦 = 100 ×
푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂 +푚ꢀ푙ꢁꢂ ꢀ푓 ꢃ푂
2
3
. Results and discussion
Fig. 1 presents the CH conversions using 5wt% of the single catalysts;
4
results fall in a range of 0.1–11.4 % . The top six active catalysts were (in order)
Rh > 5Ru > 5Pd > 5Pt > 5Cu > 5Cr, yielding 11.4-4.6 % CH conversion. The
5
4

ACCEPTED MANUSCRIPT
other catalysts provided CH conversions of less than 2.5 % . Our results are in a
4
good agreement with experiments done by Lyubovsky et al., in which the single
catalysts of Pd, Pt, or Rh on γ-Al O were used for CH combustion; this work
2
3
4
found that in a fuel-rich condition (O /CH = 2/3 by volume) and at constant
2
4
temperature (475%), Rh catalyst showed the highest activity for the combustion
products, followed by the Pd and Pt catalysts. When the reaction temperature was
varied, the Rh and Pd catalysts had a light-off temperature of 400 ºC, lower than
that of the Pt catalyst at 500 ºC [2]. Similarly, Hickman et al. found that Rh-coated
monolith had an activation energy less than that of Pt-coated monolith catalyst
[
17]. Another study of CH reaction for Ru, Rh, and Pt catalysts supported on
4
Al O showed that, at 500 °C, the highest CH conversion was obtained for the
2
3
4
Rh/Al O catalyst, followed by Ru and Pt catalysts. The Rh catalyst achieved
2
3
complete CH conversion (i.e. 100%) at 660 ºC, while the Ru and the Pt catalysts
4
achieved complete CH conversion at 699 ºC and over 800 ºC, respectively [4].
4
Reactions with the other metals presented in Fig. 1 have not been reported under
this fuel-rich condition. However, some transition metals including Co, Mn, Cr,
Cu, and Ni supported on Al O were investigated by Paredes et al. under a fuel-
2
3
lean condition; they found that the catalytic activity at 550 ºC was ranked as
follows: Co > Mn > Cr > Cu > Ni [18], results which differ from our findings due

ACCEPTED MANUSCRIPT
to the difference in the fuel composition. Discussion of activity in relation to the
catalyst properties will be presented later in this paper.
1
2
0
8
6
4
2
0
1
RhRuPd PtCuCr Ir Bi FeReAgWCe In GaNiSmZn VMn
Single catalysts
Fig.1. CH conversion of γ-Al O -supported single catalysts.
4
2
3
The top six active single metals that achieved the highest % CH combustion
4
(
i.e. Rh, Ru, Pd, Pt, Cu, and Cr) were chosen for the preparation of bimetallic
catalysts, which were made by combining each one with the other five in turn.
Each pair of the metal combinations was prepared at 5 different metal ratios, and
the total metal loading of each one was fixed at 5 wt%. Fig. 2 shows the % CH4
conversions of all possible bimetallic catalysts. The % CH conversions were in the
4

ACCEPTED MANUSCRIPT
range of 0.1%-15.9%, and the average was 8.9%. This clearly showed that the
activity of several bimetallic catalysts was much higher than that of the top six
single catalysts. 17 of these bimetallic catalysts achieved a CH conversion more
4
than 11.4% and gave an activity for CH combustion higher than of the single Rh
4
catalyst (the single catalyst with the highest conversion). The maximum CH4
conversion was obtained using the combination of 4wt% Rh and 1 wt% Cr
(4Rh1Cr): 15.9%, that is, approximately 1.4 or 3.4 times the conversion of the
single Rh and Cr catalysts, respectively. Note that when the repeatability of the
best five active bimetallic catalysts was also investigated by performing the testing
procedure for each chosen catalyst in tuplicate, standard deviations were found in a
range of 0.03-0.53 (see Table S2).
In a previous study of CO reforming of CH over Ni-Cr/yttria-doped ceria
2
4
catalysts, the role of Cr was suggested to be in the enhancement of the metallic
dispersion and provision of surface oxygen vacancies on the catalyst, and thus the
promotion of CH combustion [19]. In addition, Cr was found to reduce the light-
4
off temperature of the reaction, which implies that the products were easily
generated at lower reaction temperatures [20]. Pecchi et al. studied CH4
+
combustion using Rh/ZrO and found that the Rh state and the metallic dispersion
2
of the catalyst played a key role in the improvement of the activity for CH4
combustion [7]. These studies imply that the physicochemical properties of the Rh

ACCEPTED MANUSCRIPT
and Cr components support each other, explaining why the best activity for CH4
conversion in this work is given by 4Rh1Cr. The 4Rh1Cr catalyst exhibits a
synergistic catalyst effect, which will be discussed in the analysis of catalysts using
NH -TPD.
3
Cr
4.6
Cu
4.8
Pt
6.7
Pd
7.6
Ru
9.3
Rh
11.4
5
X
4
3
2
1
4
3
2
1
4
3
2
1
X + 1Rh
X + 2Rh
X + 3Rh
X + 4Rh
X + 1Ru
X + 2Ru
X + 3Ru
X + 4Ru
X + 1Pd
X + 2Pd
X + 3Pd
X + 4Pd
10.6
13.8
15.1
15.9
9.7
8.3
10.5
8.6
0.1
9.1
1.9
13.9
5.9
4.1
3.6
11.2
11.3
7.4
7.2
8.1
7.1
3.7
7.9
9.0
7.7
3.4
8.7
4.9
4.4
13.0
13.2
14.4
12.3
9.2
9.1
11.0
6.3
12.3
14.9
13.4
14.0
12.0
12.1
1.8
9.0
12.3
11.8
13.0
>
14.1
8.5
12.0
6.9
7.4
1
2.1 - 14.0
9.9
1
0.1 - 12.0
4
3
2
1
X + 1Pt
X + 2Pt
X + 3Pt
X + 4Pt
7.9
5.6
9.1
8
.1 - 10.0
4
3
2
1
X +1 Cu
X + 2Cu
X + 3Cu
X + 4Cu
4.3
5.0
8.1
2.6
<
8.0
Fig. 2. CH conversion for γ-Al O -supported bimetallic catalysts, showing all the
4
2
3
combinations of Rh, Ru, Pd, Pt, Cu, and Cr. The total metal loading was fixed at 5
wt%.

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Furthermore, the effect of varying the reactor temperature used with
4Rh1Cr, 5Rh, and 5Cr catalysts was investigated (see Fig. S2). Note that the
possible maximum CH conversion in theory under this condition was 40.0%. As
4
observed, at each temperature, the 5Cr catalyst achieved the lowest CH4
conversion. Above 400 ºC, the 4Rh1Cr catalyst evidently gave CH conversions
4
higher than those of the 5Rh catalyst. The light-off temperature of the 4Rh1Cr
catalyst was established at approximately 545 ºC, about 37 ºC lower than that of
the 5Rh catalyst. For the 5Cr catalyst, CH conversion slowly increased and the
4
light-off temperature was not clear. When increasing the of O /CH feed gas ratio
2
4
from 0.6–2.0 with a constant total feed flow rate of 5 mL/min (see Fig. S3), CH4
conversion and CO selectivity sharply increased to nearly 100% at a O /CH feed
2
2
4
gas ratio of 2.0, while the CO selectivity decreased to 0%. This suggests that
complete combustion can occur in an excess supply of O , i.e. O /CH feed gas
2
2
4
ratio > 2.0.
Fig. 3 shows PXRD patterns of the single 5Rh, 5Cr, and bimetallic 4Rh1Cr
catalysts. The γ-Al O support shows diffraction peaks at 33.7, 36.3, 45.8, 61.7,
2
3
and 67.3 (ICDD 00-010-0425) appearing for all catalysts. In particular, the
diffraction peaks at 19.2 and 54.3 (ICDD 00-041-0541) were assigned to the Rh O3
2
component in the 5Rh and 4Rh1Cr catalysts. It seems that the diffraction peaks of

ACCEPTED MANUSCRIPT
the Cr component (i.e. Cr O ) in the 5Cr and 4Rh1Cr catalysts were undetectable
2
3
because Cr might be in the amorphous phase. In addition after being used for 1 h,
the three catalysts were taken to be analyzed by PXRD. Interestingly, the peak
intensities of Rh O in 4Rh1Cr and 5 Rh catalysts were found to decrease, and a
2
3
new peak at 2θ = 41.0 and 48.0 (ICDD 00-005-0685), indicating the presence of
the Ru(0), appeared. Similarly, the Ru(0) phase was also seen in the 4Rh1Cr
catalyst used for 8 h. For 5Cr catalyst, no clear PXRD peaks of Cr(0) were
observed. This agrees with a study of oxidative dehydrogenation of isobutene to
3
+
isobutene under a fuel-rich condition, wherein it was found that Cr (i.e. Cr O )
2
3
species was dominant during the reaction [21]. The preceding results can imply
that the Rh(0) phase is the co-active component with Cr O during the reaction.
2
3
The BET surface areas of the 5Rh, 5Cr, and 4Rh1Cr catalysts were
measured and found to be 70, 57, and 58 m²/g, respectively, which were slightly
2
lower than that of the γ-Al O support (80 m /g), as a result of the preparation
2
3
effects (i.e. calcination temperature, metal loading). The elemental compositions of
these three catalysts were also determined by SEM/EDX, giving results in a good
agreement with theoretical values (see Table S3). Moreover, Rh O nanoparticles
2
3
with an average size of 17 nm for both 4Rh1Cr and 5Rh catalysts were found using
TEM images (see Fig. S4–S5). Again, Cr O crystals could not be seen in the TEM
2
3

ACCEPTED MANUSCRIPT
images because they were in the amorphous phase and could not be distinguished
from the support (see Fig. S6).

Rh O3
2

Al O3

2

8h used

Rh





4Rh1Cr

1h used


4Rh1Cr





4Rh1Cr


1
h used 5Rh

5Rh
1h used 5Cr
5Cr
10
20
30
40
50
60
70
80
2
Theta (degree)
Fig. 3. XRD patterns of catalysts: 5Rh, 5Cr, and 4Rh1Cr.
Fig. 4 shows NH -TPD spectra and thus the acidity of the 5Rh, 5Cr, and
3
4Rh1Cr catalysts after being used for 1 h. Each peak area of the catalysts is
quantified as presented in Table S4. For all catalysts, there were some weak-

ACCEPTED MANUSCRIPT
intensity peaks of around 50–250 ºC, which were assigned to the NH desorption
3
from weak acidic sites. The 5Rh and 4Rh1Cr catalysts had a similar peak at about
4
00–650 ºC, which was assigned to NH desorption of the strong acidic sites.
3
Meanwhile, the strong acidic sites of the 5Cr catalyst did not appear. Interestingly,
with the addition of Cr, the strong acidic sites of the 5Rh catalyst at 560 ºC shifted
toward a higher temperature (585 ºC). Moreover, the 4Rh1Cr/5Rh peak area ratio
of the strong acidic sites was 1.43. These results indicate that the strong acidic sites
play an important role in the combustion of CH , and a higher number of strong
4
acidic sites results in an increase in the catalytic activity for CH combustion.
4
Generally, CH molecules do not easily absorb on a solid surface and
4
continue to react unless the surface possesses highly active sites to accommodate
them. In a study by Lu et al. on the role of acidity of Pd/ZSM-5 catalyst in CH4
combustion, the presence of a great amount of Brønsted acid sites, referring to the
acidic hydroxyl species attached to the ZSM-5 supports, strongly influenced the
catalytic activity for CH activation [22]. Some other studies have also suggested
4
that an increase of the acidity of catalyst surface favors the breaking of C-H bonds
in CH molecules [23-25]. These studies can potentially be applied to our 4Rh1Cr
4
catalyst, over which the CH is activated by the Brønsted acid sites (i.e. strong acid
4
sites) where the oxygen species attach to the proton. After CH is adsorbed by
4
oxygen, the two together can further create CO and H O [26]. The absorbed
x
2

ACCEPTED MANUSCRIPT
oxygen species, which could be a part of either the active metal oxides [27] or the
active metal oxides promoted by supports [28], must desorb from the surface as
CO , such that oxygen vacancies on the catalyst surface are created and
x
subsequently refilled with the feed gas oxygen [29]. A new cycle then occurs. The
identification of active sites, as well as the mechanism, needs further study.
1
h used 4Rh1Cr
1
h used 5Rh
1
h used 5Cr
0
100 200
300 400
500 600
700 800
Temperature (deg C)
Fig. 4. NH -TPD profiles of catalysts: 5Rh, 5Cr, and 4Rh1Cr.
3
Since the catalyst stability is important, a time-on-stream experiment of the
4
Rh1Cr catalyst was performed. Fig. 5 charts CH conversion, CO selectivity, and
4

ACCEPTED MANUSCRIPT
CO selectivity over 97 h of continuous operation at 475 ºC and a O /CH ratio of
2
2
4
28.0/46.7 by volume. As indicated in the figure, the CO selectivity is much higher
than the CO selectivity; this is because the production of CO is normally favored
2
under the fuel-rich condition, owing to there being insufficient oxygen for
complete combustion. In these experiments, the CH conversion gradually
4
increased for the first 2 h, and then remained steady at approximately 17%.
Likewise, CO and CO selectivities were virtually constant during hours 3-97: 86%
2
and 14%, respectively. Note that a trace amount of H (<1.0% selectivity) was also
2
found in the effluent gas analysis by GC, suggesting that coke formation could
have occurred. However, observing the quartz tubular reactor, no black film was
seen on the wall after the 97-h experiment (see Fig. S7) and the catalyst stability
was excellent within the time period. These latter findings implied that
carbonaceous materials did not deposit on the catalyst’s surface. This implies that
the 4Rh1Cr catalyst is quite durable for the reaction. When comparing this catalyst
to the other catalysts (see Table S5), it can be established that 4Rh1Cr is a catalyst
of high activity at low reactor temperature.

ACCEPTED MANUSCRIPT
25
20
15
10
5
0
100
90
80
70
60
50
40
CH conversion
4
CO selectivity
2
CO selectivity
30
20
10
0
0
20
40
Time (h)
60
80
100
Fig. 5. Time-on-stream testing results of 4Rh1Cr catalyst over 97 h.
. Conclusion
The top six active noble metals supported on γ-Al O –Rh, Ru, Pd, Pt, Cu,
4
2
3
and Cr–were obtained from a achieved from the single catalyst screening. A
bimetallic catalyst screening from the combinations of these six metals later led to
the discovery of several bimetallic catalysts that were more active than the
commercial Pt/γ-Al O catalyst. Among these, the greater activity was attained by
2
3
the 4Rh1Cr catalyst because of the increase in the strong acid sites on this
catalyst’s surface, which enhanced the activation of C-H bonds in CH . Moreover,
4

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in monitoring the activity over 97 h, the 4Rh1Cr catalyst was found to be quite
stable, which means that it can potentially be considered for commercial use.
Acknowledgements
This research was supported in part by the Graduate Program Scholarship
from Graduate School, Kasetsart University; the Kasetsart University Research and
Development Institute (KURDI); the Center of Excellence on Petrochemical and
Materials Technology; the National Science and Technology Development Agency
(NSTDA); and the Thailand Research Fund (TRF).

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Graphicalabstract

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Highlights





20 metals supported on γ-Al
The top 6 active metals were Rh, Ru, Pd, Pt, Cu and Cr.
The most active amongthe 60 tested bimetalliccatalystwas (4wt% Rh + 1wt% Cr)/γ-Al
The strong acid site plays akey role in the activation of methane.
The most active bimetalliccatalystpresented an excellent stability over97 hours.
2
O werescreened for methane combustion.
3
2
3
O .