Oxidative coupling of methane (OCM): Effect of noble metal (M?=?Pt, Ir, Rh) doping on the performance of mesoporous silica MCF-17 supported MnxOy-Na2WO4 catalysts

Article abstract of DOI:10.1016/j.apcata.2017.07.017

Noble metal doped M-Mn_xO_y-Na₂WO₄/SiO₂ (M?=?Pt, Ir, Rh) catalysts were prepared via a wet impregnation method using mesoporous silica MCF-17 as support, and their performance in the oxidative coupling of methane (OCM) was studied in a fixed-bed flow reactor. The reaction was carried out at 750?°C under atmospheric pressure with a gas feed composition of CH₄:O₂?=?4:1. The incorporation of the noble metals yielded an enhanced selectivity towards both C₂ and C₃ hydrocarbons as compared to the undoped Mn_xO_y-Na₂WO₄/MCF-17 catalyst in the order of Rh-doped > Ir-doped > Pt-doped samples together with a lower olefin to paraffin ratio. On the other hand, the Ir-doped catalyst showed the highest overall yield for C₂ production. Elemental analysis, N₂-adsorption, and X-ray powder diffraction (XRD) measurements confirmed similar metal loading, surface area, and phase composition for both the undoped and doped catalysts. Electron microscopy analysis showed a near-homogeneous distribution of Na and W but a higher tendency to form aggregates for Mn, with the Rh-doped catalyst being the best in overall elemental dispersion. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H₂-TPR) results indicate a more reduced nature of the surface oxide species in the noble metal doped catalysts as compared to the undoped one. They also suggest a more optimized strength of interaction between the carbon intermediates and the surface of the noble metal doped catalysts, which in combination with the improved reducibility of Mn and W species and metal dispersion, accounted for the enhanced C₂ production on the noble metal doped Mn_xO_y-Na₂WO₄/MCF-17 catalysts.

Full text of DOI:10.1016/j.apcata.2017.07.017

Accepted Manuscript
Title: Oxidative Coupling of Methane (OCM): Effect of Noble
Metal (M = Pt, Ir, Rh) Doping on the Performance of
Mesoporous Silica MCF-17 Supported MnxOy-Na2WO4
Catalysts
´ ˆ
Authors: Wen-Chi Liu, Walter T. Ralston, Gerome Melaet,
Gabor A. Somorjai
PII:
S0926-860X(17)30317-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.017
APCATA 16322
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-4-2017
23-6-2017
12-7-2017
´ ˆ
Please cite this article as: Wen-Chi Liu, Walter T.Ralston, Gerome Melaet,
Gabor A.Somorjai, Oxidative Coupling of Methane (OCM): Effect of
Noble Metal (M = Pt, Ir, Rh) Doping on the Performance of Mesoporous
Silica MCF-17 Supported MnxOy-Na2WO4 Catalysts, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.017
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Oxidative Coupling of Methane (OCM): Effect of Noble Metal (M = Pt, Ir,
Rh) Doping on the Performance of Mesoporous Silica MCF-17 Supported
MnxOy-Na2WO4 Catalysts
Wen-Chi Liu†,‡, Walter T. Ralston†,§, Gérôme Melaet†,‡, Gabor A. Somorjai*,†,‡
†Department of Chemistry, University of California, Berkeley, California 94720, United States
‡Materials Sciences Division and §Chemical Science Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States
Graphical abstract
Highlights
 M-MnxOy-Na2WO4 (M = Pt, Ir, Rh) catalysts were prepared on mesoporous silica MCF-17.
 Enhanced C2 selectivity in OCM was obtained in the order of Rh > Ir > Pt-doped catalysts.
 Reducibility of W and Mn species was modified by noble metal additives
 Optimized strength of interaction between the surface and intermediates was suggested.
Abstract
Noble metal doped M-MnxOy-Na2WO4/SiO2 (M = Pt, Ir, Rh) catalysts were prepared via a wet impreg-
nation method using mesoporous silica MCF-17 as support, and their performance in the oxidative cou-
pling of methane (OCM) was studied in a fixed-bed flow reactor. The reaction was carried out at 750 oC
under atmospheric pressure with a gas feed composition of CH4:O2 = 4:1. The incorporation of the noble
metals yielded an enhanced selectivity towards both C2 and C3 hydrocarbons as compared to the undoped
MnxOy-Na2WO4/MCF-17 catalyst in the order of Rh-doped > Ir-doped > Pt-doped samples together with
a lower olefin to paraffin ratio. On the other hand, the Ir-doped catalyst showed the highest overall yield
1

for C2 production. Elemental analysis, N2-adsorption, and X-ray powder diffraction (XRD) measurements
confirmed similar metal loading, surface area, and phase composition for both the undoped and doped
catalysts. Electron microscopy analysis showed a near-homogeneous distribution of Na and W but a
higher tendency to form aggregates for Mn, with the Rh-doped catalyst being the best in overall elemental
dispersion. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H2-TPR)
results indicate a more reduced nature of the surface oxide species in the noble metal doped catalysts as
compared to the undoped one. They also suggest a more optimized strength of interaction between the
carbon intermediates and the surface of the noble metal doped catalysts, which in combination with the
improved reducibility of Mn and W species and metal dispersion, accounted for the enhanced C2 produc-
tion on the noble metal doped MnxOy-Na2WO4/MCF-17 catalysts.
Keywords: Oxidative coupling of methane; MnxOy-Na2WO4/SiO2; Noble metal additives; C2 produc-
tion;
1. Introduction
Oxidative coupling of methane, in which CH4 molecules are catalytically transformed into C2 hydro-
carbons in the presence of an oxidizing reagent, possesses the potential capability of directly utilizing
natural gas for the production of value added chemicals. [1–4] Different types of catalysts and combina-
tions thereof have been examined for this reaction, and the MnxOy-Na2WO4/SiO2 system is among the
very few of those that provide high C2 yield and excellent stability at the same time. [5,6] Extensive
research has been performed on this catalyst system, in which the effect of the preparation methods, ele-
mental concentrations, and the variation of each of its components have been reported. [7] For catalyst
preparation, the wet impregnation and the mixture slurry methods are proven to be more efficient in pro-
ducing high-performance catalysts. [8] As for the catalyst composition, results have indicated that there
exist a range of combinations, i.e., 0.4–2.3% for Na, 2.2–8.9% for W, and 0.5–3.0% for Mn, that would
allow an optimized C2 yield. [9] In addition, Yildiz et al. showed that by simply utilizing a mesoporous
silica support, the elemental dispersion is improved, resulting in a higher overall C2 production. [10,11]
In spite of the large amount of work performed on this catalytic system in the OCM reaction, the effect
of noble metal additives is experimentally less explored. In liquid phase homogenous catalysis, noble
metal complexes including that of Pt, Rh, and Ir are widely used as catalysts for C-H bond activation.
[12,13] In addition, there have been several theoretical investigations suggesting a potential promotional
2

effect on the CH4 activation and C-C bond coupling on the surface of IrO2. [14–16] Therefore, it is rea-
sonable to explore the effect of noble metal doping on the performance of the MnxOy-Na2WO4/SiO2 sys-
tem.
Herein, we report the investigation on the effect of noble metal doping (M = Pt, Ir, Rh) on the perfor-
mance of the MnxOy-Na2WO4/SiO2 system. The catalysts were synthesized via a wet impregnation
method with mesoporous silica material MCF-17 as the support. Their structural and chemical properties
were characterized using techniques including X-ray powder diffraction (XRD), transmission electron
microscopy (TEM), scanning transmission electron microscopy (STEM)/energy dispersive spectroscopy
(EDS), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy
(ICP-AES), X-ray photoelectron spectroscopy (XPS), N2-adsorption, and H2-TPR. The catalytic perfor-
mance of the M-MnxOy-Na2WO4/SiO2 system (M = Pt, Ir, Rh) was studied in a continuous flow reactor,
of which the results are compared and discussed in combination with the information obtained from the
characterization studies.
2. Experimental
2.1. Preparation of MnxOy-Na2WO4/MCF-17 catalysts
The mesoporous silica material MCF-17 was prepared beforehand using a method reported previously.
[17] It has cell-like mesopores with tunable sizes that are connected by windows, large surface area and
pore volume, and has been used as the catalyst support in various applications, such as Fischer-Tropsch
synthesis and alkane isomerization reactions. [18–20] In a typical synthesis, a desired amount of MCF-17
was firstly impregnated with an aqueous solution of manganese (II) acetate (Sigma-Aldrich, ≥ 99 %) at
room temperature and then dried in air at 80 °C overnight. Afterwards, the mixture was impregnated with
an aqueous solution of sodium tungstate (Sigma-Aldrich, ≥ 99 %) at room temperature followed by drying
in air at 80 °C overnight. The obtained material was calcined in air at 750 °C for 1 h with a heating rate
of 1 °C/min. The obtained catalyst is denoted as ‘Original’.
2.2. Preparation of M-MnxOy-Na2WO4/MCF-17 (M = Pt, Ir, Rh) catalysts
The preparation of noble metal doped MnxOy-Na2WO4/MCF-17 catalysts follows a similar procedure
described above. After impregnation with manganese (II) acetate and sodium tungstate and dried at 80 °C
overnight, the obtained material was then divided into three portions with each being impregnated with a
certain metal precursor (Sigma-Aldrich, rhodium(III) acetylacetonate [Rh(acac)3, 97%], platinum(II)
acetylacetonate [Pt(acac)2, 97%], or Iridium(III) acetylacetonate [Ir(acac)2, 97%]) dissolved in chloroform
[Fisher Scientific, ≥ 99%]. The calcination process was carried out in air at 750 °C for 1 h with a heating
3

rate of 1 °C/min. The obtained catalysts are denoted as ‘Pt-doped’ for M = Pt, ‘Ir-doped’ for M = Ir, and
‘Rh-doped’ for M = Rh.
2.3. Characterizations of M-MnxOy-Na2WO4/MCF-17 (M = Pt, Ir, Rh) catalysts
XRD measurements of the fresh and spent catalysts were performed using a Bruker D2 Phaser Bench-
top X-ray powder diffractometer with a Cu x-ray source (Kα 1.54184 Å). 2 theta value was varied from
10° to 60° with a step size of 0.02°.
TEM and STEM/EDS analysis were performed using a JEOL JEM2100F TEM equipped with an Inca
Energy Dispersive Spectrometer operated at 200 kV. Samples for TEM analysis were prepared by first
dispersing the catalysts in ethanol and then dropping the dispersion on ultrathin lacey carbon TEM grids
(Ted Pella, Inc.).
SEM images of the fresh catalysts were obtained with a FEI/Philips XL-30 Field Emission ESEM
operated at 15 kV. Samples for SEM analysis were prepared by drop-casting the dispersion of the catalysts
in isopropanol on a piece of silicon wafer.
The loading of the prepared catalysts was determined using ICP-AES on a PerkinElmer ICP Optima
7000 DV spectrometer. Samples for ICP-AES measurements were prepared by first digesting the catalysts
powder with nitric acid and hydrofluoric acid and then diluting with deionized water.
The surface composition and oxidation states of the catalysts were examined using PHI 5400 XPS
System with an Al X-ray source (Kα 1486.6 eV). A pass energy of 35.75 eV with a step size of 0.05 eV
was used. Samples for XPS measurements were prepared by drop-casting the dispersion of the catalysts
o
in isopropanol on a piece of silicon wafer. All samples were dried in a vacuum oven at 85 C overnight
before testing. The binding energies were corrected for charging effect with respect to the Si 2p line at
103.4 eV. [21,22] The spectra were fitted with a Shirley background and the area under the peaks is cali-
brated using the Relative Sensitivity Factor provided in the CasaXPS library.
The specific surface area and the pore structure of the catalysts were verified using N2-adsorption at
liquid nitrogen temperature on an ASAP 2020 Surface Area and Porosimetry System (Micromeritics).
BET and BJH models were used for the determination of surface area and pore size distribution, respec-
tively.
H2-TPR measurements were carried out to assess the reducibility of the catalysts. A high purity mixture
of 5% H2/Ar (Praxair) flowing at 30 sccm was used as the reducing gas. The reactor was heated to 850 oC
with a heating rate of ~6°C/min and the composition of the outlet gas was monitored by an online quad-
rupole mass spectrometer (SRS RGA 200).
2.4. Catalytic measurements
4

OCM reaction was performed in a home-built fixed bed flow reactor under atmospheric pressure. The
catalysts were sieved to an average grain size of 150-250 um. Approximately 60 mg of the sieved catalyst
was mixed with quartz sand (Umicore, 99.99%, grain size: 0.2-0.7 mm) and then packed in the center of
a quartz reactor tube (Quartz Scientific, 16 inches, 4mm ID, 6.35mm OD) with quartz wool. Additional
quartz sand was packed on both sides of the catalytic bed to help reduce the empty space of the reactor
and promote heat dissipation. Methane (Praxair, 5.0 Research Grade) and synthetic air (Praxair, Ultra
Zero) were fed into the reactor using carefully calibrated mass flow controllers (Parker) with a total flow
rate of 60 sccm and a composition of CH4:O2:N2 = 4:1:4. The reactor was heated to 750 oC with a heating
rate of 8 oC/min by an electric tube furnace. Outlet gas was sampled and separated using an on-line GC-
MS (Agilent 7890A-5975C) equipped with two capillary columns (HP-PLOT Q and HP-PLOT Mole-
Sieve). The gaseous composition was analyzed by a thermal conductivity detector (TCD) and a flame
ionization detector (FID) detector equipped with a methanizer. The equations used to determine the reac-
tion conversion, selectivity, and carbon balance are provided in the supporting information.
3. Results and discussion
3.1. Characterizations
3.1.1. Surface area and porosity measurements
The BET surface area and pore structure of the MCF-17 support material and the supported catalysts
are reported in Table 1. The pristine MCF-17 exhibits a surface area of 566 m2∙g-1, owing to its porous
nature. The total pore volume is 1.6 cm3∙g-1 with an average pore size of 16 nm, which are typical among
its category. After impregnation and calcination, the catalysts showed a significant loss of surface area
and porosity (Table 1). The measured surface areas are among 7-9 m2/g and the total pore volumes are in
the vicinity of 0.020 cm3∙g-1 for all the prepared catalysts. This could be explained by the structural change
of the silica support induced by the phase transformation from amorphous SiO2 to others forms, such as
α-cristobalite and tridymite (Figure 2a), causing the collapse of the porous structure.
BET measurements of the spent samples were also performed and the measured surface area are typi-
cally around 4-5 m2/g for all the catalysts tested, indicating a slight decrease in the surface area after the
reaction.
3.1.2. Composition analysis
The elemental compositions of the prepared catalysts as obtained from ICP-AES are shown in Table 2.
The intended loading of each of the components were chosen to be within their optimal range, as men-
tioned in the introduction. It can be seen from Table 2 that the actual loading of the catalysts agrees well
5

with the theoretical values except for a slightly lower loading for W, yet there are no significant differ-
ences among the four catalysts. The determined loadings are 0.72 to 0.79 wt.% for Na, 2.29 to 2.51 wt.%
for W, and 2.04 to 2.37 wt.% for Mn. ICP results also confirmed that Rh, Ir, and Pt are successfully
incorporated into the catalysts, of which the metal loadings are in the vicinity of 0.12 wt.%.
3.1.3. TEM, SEM and STEM/EDS analysis
TEM and STEM measurements were performed to study the morphology and texture of the catalysts.
The first column of Figure 1a-d shows the TEM images for the fresh catalysts, next to which their dark
field (HAADF) images are also presented. The catalysts are generally in fibrous or rod-like shapes with
small spherical and cubic aggregates distributed across the support. The SEM images (Figure S1) also
indicate the existence of the particles on the surface of the catalysts. EDS point scans (Figure S2) revealed
that the particles are mostly rich in Mn, with some of them also containing notable amount of W. TEM
analysis of the spent samples (Figure S3) revealed plate-like shapes with particles spread across the sup-
port with sizes of tens of nanometers. In addition, both TEM and SEM images confirm the lack of porosity
in the calcined catalysts, in agreement with the BET measurements.
The catalysts were further studied with EDS mapping technique to gain information on the elemental
dispersions. In general, the utilization of MCF-17 achieved good overall elemental dispersion, owing to
its high surface area and pore volume as well as the ordered pore structure, which could provide signifi-
cantly more sites than the regular silica-gel to homogeneously distribute the supported metals and inhibit
their aggregation to achieve better fixation. More specifically, elemental mapping results (Figure 1, third
to sixth columns) indicate a near-homogeneous distribution of W and Na with minor formation of W-rich
or Na-rich areas. On the other hand, Mn shows poorer dispersion and has a higher tendency to form small
aggregates, particularly for the original and Pt-doped samples. Since the loadings of the noble metals are
lower than Mn and W, the signal level is weaker. Nevertheless, results indicate homogeneous distributions
of the noble metals, with no obvious aggregation or particle formation of significant sizes observed. This
is also in accordance with the fact that no noble metal containing species were observed in the XRD
spectra of the doped catalysts. Among the four catalysts, the Rh-doped sample shows the best overall
elemental dispersion, especially for Mn.
3.1.4. XRD measurements
In order to study the phase composition, XRD measurements were conducted on both the fresh and
spent catalysts. The diffraction patterns of the fresh samples are shown in Figure 2a. Na2WO4 and Mn2O3
phases were clearly identified and are the main form of tungsten and manganese oxide, respectively.
While most of the tungsten species exist as Na2WO4, features due to Na4WO5 were also detected. Unlike
6

the amorphous nature of the silica support MCF-17, strong diffraction peaks associated with α-cristobalite
were observed in the calcined catalysts. In addition, a small amount of tridymite was also detected, as
indicated by the diffraction peak at 20.6 ֯. The transformation of amorphous silica to α-cristobalite during
the high temperature calcination process is facile with Na present, especially aided with W, and is well
known for the MnxOy-Na2WO4/SiO2 system. [23] The phase transformation is also accompanied by a
significant loss of pore volume and surface area, as shown in Table 1. It is also noted that no noble metal-
containing crystal phases were observed for the doped catalysts, which could be explained by the low
degree of metal loading and the insensitivity of X-ray diffraction to small clusters. Despite the slight
variation in the intensity of the diffraction peaks, no significant difference in the phase composition was
observed among the four catalysts.
In the spent samples (Figure 2b), MnWO4 was identified in addition to Mn2O3, while peaks due to
Na4WO5 disappeared after the reaction. The transformation of Mn2O3 to MnWO4 indicates a partial re-
duction of Mn3+ to the more stable Mn2+, which could be involved in the total catalytic cycle. However,
its exact contribution to the catalytic performance is still unclear. One plausible mechanism involves the
activation of oxygen and methane on Mn2+ and WO42- sites, respectively. The spillover of the activated
surface oxygens from Mn sites to W sites replenishes the active centers for methane activation and keeps
the catalyst active. [7] [24] In addition, α-cristobalite remains as the primary silica phase. It should be
noted that the appearance of the quartz phase in the spent samples could have mainly originated from the
quartz sand residue, which was used to dilute the catalyst in the catalytic bed, and is therefore not the
result of the transformation from other forms of silica during the reaction. Nonetheless, its formation could
not be completely ruled out.
Using the Scherrer equation, the average sizes of the catalyst particles could be estimated (Table S1).
The results indicate the growth of both Mn2O3 and Na2WO4 particles after the catalytic measurements.
Among the four catalysts tested, the Rh-doped catalyst exhibited the biggest change in the Mn2O3 particle
sizes before and after the reaction, with about 12 nm growth being observed.
3.1.5. XPS analysis
Surface analysis of the prepared catalysts is carried out using XPS. Figure 3 shows the XPS spectra of
W 4f and Mn 2p level of the fresh catalysts. Binding energies of W 4f7/2 and Mn 2p3/2 peaks are in the
range of 35.0-35.2 eV and 641.7-642.1 eV, which are assigned to the corresponding species in Na2WO4
and Mn2O3, respectively. [22,25] It is noted that the W 4f and Mn 2p spectra of the doped catalysts (35.0
eV for W 4f7/2 and 641.7 eV for Mn 2p3/2 peaks) shifted ~0.2 eV and ~0.4 eV, respectively, towards lower
binding energy as compared to those of the original sample (35.2 eV for W 4f7/2 and 642.1 eV for Mn
7

2p3/2 peaks), which indicates a more reduced nature of the W and Mn species on the surface of the doped
catalysts.
Information on the surface compositions are shown in Table 3. For the fresh catalysts, comparable
amounts of Na were found on the surface of both the doped and undoped samples. As for the Mn concen-
tration, the Rh-doped catalyst showed the highest value, while similar loadings were observed for the
others. The Rh-doped catalyst also displayed the highest W concentration among the four catalysts, fol-
lowed by the Pt-doped and undoped samples, with the Ir-doped one exhibited the lowest surface W content.
This is in accordance with the finding from other researchers that the Mn surface concentration increases
with the W concentration. [9][26] The most noticeable difference in the surface concentration is observed
for Mn, while the variation in the surface W content is less than 10% among the four catalysts. S.-f. Ji et
al. reported an optimum surface Mn content of 1.1% for C2 production on MnxOy-Na2WO4/SiO2 catalysts
with similar bulk composition, below which a higher Mn surface concentration would be beneficial. [9]
This agrees with the high C2 selectivity (Figure 5b) for the Rh-doped catalyst, which has the highest
surface Rh concentration of 0.84%. Again, due to the quartz sand residue, the surface concentrations
reported for the spent catalysts should be taken as their lower limit, and therefore only the change in the
surface C concentration could be discussed without ambiguity. The spent catalysts showed significantly
more amount of carbon on their surfaces after the reaction, with the increase in the order of Original ≈ Pt-
doped > Ir-doped > Rh-doped samples. This is also in agreement with the degree of carbon balance during
the reaction. The Rh-doped catalyst showed the best carbon balance of ~98%, followed by the Ir-doped
catalyst at ~94%, while the original and Pt-doped catalysts have the lowest value of ~91%.
3.1.6. H2-TPR analysis
Figure 4 shows the H2-TPR results for the fresh catalysts. The reduction peaks in the range of 300 oC
o
to 650 C are associated with the reduction of Mn. The broadness of the peak and the shoulder feature
o
indicate the coexistence of different Mn species. The broad feature starting from 700 C represents the
reduction of the W species in the catalysts. [23,27] As compared to the undoped MnxOy-Na2WO4/SiO2
sample, the incorporation of noble metals facilitates the reduction of Mn, as indicated by the higher in-
o
o
tensity of the Mn reduction features between 300 C to 650 C and their shift to lower temperatures,
especially for the Rh-doped catalyst. The change in the reducibility could also be due to a better dispersion
of the MnOx species, [28] as is in accordance with the EDS mapping results. On the other hand, the Pt-
doped and Ir-doped catalysts showed the W reduction feature at more than 20 °C lower in temperature
than the Rh-doped sample.
3.2. Catalytic measurements
8

The catalysts, after being pelletized and sieved, were utilized in the OCM reaction. Figure 5a shows the
reaction conversion as a function of time on stream. Among the four catalysts tested, the undoped MnxOy-
Na2WO4/MCF-17 catalyst showed the highest conversion at 15.6%. The introduction of noble metal do-
pants led to a slight decrease in the CH4 conversion, with that of the Pt-doped, Ir-doped, and Rh-doped
catalysts being at 14.1%, 14.4%, and 9.3% (Table 4). No significant deactivation was observed for the
catalysts during the time period (~12 h) tested.
The product distribution for all the catalysts are shown Figure 5b. C2 hydrocarbons (C2H4 and C2H6)
and COx (CO and CO2) make up the majority of the reaction products (>90%). A small amount (~4-6%
in selectivity) of C3s (C3H6 and C3H8), which are due to the further chain growth of C2 hydrocarbons, are
also observed. The incorporation of noble metals improved the selectivity towards both C2 and C3 hydro-
carbons in the order of Rh-doped > Ir-doped > Pt-doped catalyst. The doped catalysts also showed lower
C2H4/C2H6 and C3H6/C3H8 ratios as compared to their undoped counterpart. The olefin to paraffin ratio
for both C2s and C3s follows the same trend, in which the Rh-doped catalyst offers the lowest value,
followed by the Ir-doped and Pt-doped catalysts, while the original sample shows the highest percentage
of unsaturated products. The decrease in the olefin to paraffin ratio for the noble metal doped catalysts
indicates a less favorable ethane dehydrogenation process.
The yield of different products are shown in Figure 6b. It can be seen from Figure 6b that the Ir-doped
catalyst showed the highest overall yield for C2 and C3 hydrocarbons. The Rh-doped sample, having the
highest selectivity, exhibited a smaller degree of improvement on the C2 yield due to a slightly lower
conversion. The Pt-doped and the undoped samples displayed similar activity towards overall C2 produc-
tion. Since the catalysts were tested under the same conditions, non-catalytic gas phase reactions should
contribute in a similar way. Therefore the difference in the dehydrogenation ability is more likely due to
the variation in the surface properties of the catalysts. All the catalytic results are also summarized in
Table 4.
Since the ICP-AES (Table 2), BET (Table 1), and XRD (Figure 2) measurements all showed similar
results, the difference in the catalytic behavior, especially the enhancement of C2 selectivity for the doped
catalysts could hardly be explained by the difference in their structural and compositional characteristics.
It is commonly agreed that the OCM reaction proceeds via a heterogeneous-homogeneous mechanism,
where ethane is primarily formed through the gas-phase recombination of methyl radicals that are gener-
ated through surface reactions. Ethylene is then formed via the dehydrogenation of ethane either homo-
geneously or heterogeneously. COx, on the other hand, is produced from the consecutive oxidation of
either the methyl radicals or the C2 products. [29] The enhancement of C2 selectivity implies a restraint
9

on these deep oxidation reactions, which could result from a more optimized strength of interaction be-
tween the carbon intermediate species and the surface of the catalyst, which allows them to desorb readily
into the gas phase without being further consumed by the surface oxygen. [30] This is evidenced by the
fact that there is significantly less amount of carbon left on the surface of the noble metal doped catalysts
after the reaction (Table 3) and also lower olefin to paraffin ratios obtained on these catalysts (Table 4).
In addition, the change in the redox property of the oxide species (Figure 4) would alter the electronic
structure at the surface of the catalysts, thus affecting the strength of their interaction with the reaction
intermediates, which could also be part of the reason for the enhanced performance. As has been men-
tioned in section 3.1.4, the reaction involves the activation of oxygen on the Mn2+ sites. The improvement
of the reducibility of Mn species would imply an increased ability to activate oxygen. On the other hand,
the activation of methane occurs mainly on the WO42- sites, to which a facile reduction of the W species
would be beneficial. The Rh-doped catalyst showed the Mn reduction feature at the lowest temperature,
followed by the Ir-doped sample. On the other hand, the Ir-doped and Pt-doped catalysts showed higher
degree of reduction of W than the Rh-doped sample at the temperature (750 °C) where the catalytic meas-
urements were carried out. These two factors together result in a higher overall C2 yield for the Ir-doped
catalyst. The Rh-doped sample, although having Mn reduction peak at the lowest temperature, showed
smaller amount of enhancement in the overall C2 yield due to a more difficult reduction of WO42- species,
which would lead to a weaker methane activation process.
4. Conclusions
To summarize, noble metal doped M-MnxOy-Na2WO4/MCF-17 (M = Pt, Ir, Rh) catalysts with similar
elemental compositions were prepared via a wet impregnation method, and their performance in the OCM
reaction was tested. When compared to the undoped MnxOy-Na2WO4/MCF-17 catalyst, the incorporation
of the noble metals enhanced the selectivity towards both C2 and C3 hydrocarbons in the order of Rh-
doped > Ir-doped > Pt-doped catalyst and offered lower olefin to paraffin ratios. On the other hand, the
Ir-doped catalyst showed the highest C2 yield, followed by the Rh-doped sample. Since no significant
difference between the original and the doped catalysts was observed in terms of elemental loading, sur-
face area, and phase composition, the enhanced OCM performance was attributed to a more optimized
strength of interaction between the carbon intermediate species and the surface of the noble metal doped
catalysts in combination with an improved reducibility of W and Mn species and metal dispersion.
10

Acknowledgment
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials
Sciences and Engineering Division, U.S. Department of Energy, under Contract DE-AC02-05CH11231,
through the Chemical and Mechanical Properties of Surfaces, Interfaces and Nanostructures program
(FWP KC3101). The XPS and TEM/EDS measurements were conducted in Molecular Foundry at Law-
rence Berkeley National Laboratory under user proposal #4435, which is supported, by the Office of
Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-
AC02-05CH11231. Authors also acknowledge Zeqiong Zhao for her generous help with the SEM meas-
urements.
11

Reference
[1] M.C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R.M. Navarro, J.L.G. Fierro, Catal. Today 171
(2011) 15–23.
[2] A. Galadima, O. Muraza, J. Ind. Eng. Chem. 37 (2016) 1–13.
[3] C. Hammond, S. Conrad, I. Hermans, ChemSusChem 5 (2012) 1668–1686.
[4] J.H. Lunsford, Angew. Chemie Int. Ed. English 34 (1995) 970–980.
[5] V.I. Alexiadis, M. Chaar, A. van Veen, M. Muhler, J.W. Thybaut, G.B. Marin, Appl. Catal. B
Environ. 199 (2016) 252–259.
[6] U. Zavyalova, M. Holena, R. Schlögl, M. Baerns, ChemCatChem 3 (2011) 1935–1947.
[7] S. Arndt, T. Otremba, U. Simon, M. Yildiz, H. Schubert, R. Schomäcker, Appl. Catal. A Gen. 425–
426 (2012) 53–61.
[8] J. Wang, L. Chou, B. Zhang, H. Song, J. Zhao, J. Yang, S. Li, J. Mol. Catal. A Chem. 245 (2006)
272–277.
[9] S. Ji, T. Xiao, S. Li, C. Xu, R. Hou, K.S. Coleman, M.L.. Green, Appl. Catal. A Gen. 225 (2002)
271–284.
[10] M. Yildiz, Y. Aksu, U. Simon, K. Kailasam, O. Goerke, F. Rosowski, R. Schomäcker, A. Thomas,
S. Arndt, Chem. Commun. 50 (2014) 14440–14442.
[11] M. Yildiz, Y. Aksu, U. Simon, T. Otremba, K. Kailasam, C. Göbel, F. Girgsdies, O. Görke, F.
Rosowski, A. Thomas, R. Schomäcker, S. Arndt, Appl. Catal. A Gen. 525 (2016) 168–179.
[12] J.A. Labinger, J.E. Bercaw, Nature 417 (2002) 507–14.
[13] J. Wencel-Delord, T. Dröge, F. Liu, F. Glorius, Chem. Soc. Rev. 40 (2011) 4740–4761.
[14] A.A. Latimer, A.R. Kulkarni, H. Aljama, J.H. Montoya, J.S. Yoo, C. Tsai, F. Abild-Pedersen, F.
Studt, J.K. Nørskov, Nat. Mater. 1 (2016) 1–6.
[15] C.-C. Wang, S.S. Siao, J. Jiang, J. Phys. Chem. C 116 (2012) 6367–6370.
[16] T.L.M. Pham, E.G. Leggesse, J.C. Jiang, Catal. Sci. Technol. 5 (2015) 15–18.
12

[17] Y. Han, S.S. Lee, J.Y. Ying, Chem. Mater. 19 (2007) 2292–2298.
[18] N. Musselwhite, K. Na, K. Sabyrov, S. Alayoglu, G.A. Somorjai, J. Am. Chem. Soc. 137 (2015)
10231–10237.
[19] W.-C. Liu, G. Melaet, W.T. Ralston, S. Alayoglu, Y. Horowitz, R. Ye, T. Hurlburt, B. Mao, E.
Crumlin, M. Salmeron, G.A. Somorjai, Catal. Letters 146 (2016) 1574–1580.
[20] G. Melaet, W.T. Ralston, C.-S. Li, S. Alayoglu, K. An, N. Musselwhite, B. Kalkan, G.A. Somorjai,
J. Am. Chem. Soc. 136 (2014) 2260–2263.
[21] S.-F. Ji, T.-C. Xiao, S.-B. Li, C.-Z. Xu, R.-L. Hou, K.S. Coleman, M.L.H. Green, Appl. Catal. A
Gen. 225 (2002) 271–284.
[22] Y. Kou, B. Zhang, J. Niu, S. Li, H. Wang, T. Tanaka, S. Yoshida, J. Catal. 173 (1998) 399–408.
[23] A. Palermo, J.P. Holgado Vazquez, A.F. Lee, M.S. Tikhov, R.M. Lambert, J. Catal. 177 (1998)
259–266.
[24] P. Wang, G. Zhao, Y. Wang, Y. Lu, Sci. Adv. 3 (2017) e1603180.
[25] H.W. Nesbitt, D. Banerjee, Am. Mineral. 83 (1998) 305–315.
[26] T.W. Elkins, H.E. Hagelin-Weaver, Appl. Catal. A Gen. 497 (2015) 96–106.
[27] R. Koirala, R. Büchel, S.E. Pratsinis, A. Baiker, Appl. Catal. A Gen. 484 (2014) 97–107.
[28] Y. Liu, M. Luo, Z. Wei, Q. Xin, P. Ying, C. Li, Appl. Catal. B Environ. 29 (2001) 61–67.
[29] E. V. Kondratenko, M. Baerns, in:, E. Ertl, G., Knozinger, H., Schuth, F., Weitkamp, J (Ed.),
Handb. Heterog. Catal., 2nd ed., Wiley-VCH, Weinheim, 2008, pp. 3010–3023.
[30] G. Kumar, S.L.J. Lau, M.D. Krcha, M.J. Janik, ACS Catal. 6 (2016) 1812–1821.
13

Figure 1. Column 1: TEM images; Column 2: high-angle annular dark-field (HAADF) images; Columns
3-6: EDS elemental mapping results (Na: green, Mn: red, W: blue, and noble metals: yellow) of the fresh
M-MnxOy-Na2WO4/MCF-17 catalysts: a) Original; b) M = Rh; c) M = Ir; d) M = Pt.
Figure 2. XRD patterns of the a) fresh and b) spent catalysts. Sample names are listed under their corre-
sponding spectra. The file numbers in the PDF database used for phase identification are listed after the
name of the compound.
14

Figure 3. XPS spectra of a) W 4f and b) Mn 2p level for the fresh M-MnxOy-Na2WO4/MCF-17 catalysts:
i) Original; ii) M = Rh; iii) M = Ir; iv) M = Pt. A spin-orbital splitting of 2.17 eV and an area ratio for W
4f7/2 (red) and W 4f5/2 (blue) peaks of 4:3 were used when fitting the W 4f spectra. Due to the low signal-
to-noise ratio, fitting was not performed on the Mn 2p spectra.
15

Figure 4. H2-TPR profiles of the fresh catalysts. Sample names are listed under their corresponding scans.
A high purity mixture of 5% H2/Ar was used as the reducing gas with a flow rate of 30 sccm. The reactor
was heated to 850 oC with a heating rate of ~6°C/min.
Figure 5. a) Reaction conversion as a function of time on stream and b) reaction selectivity of C2, C3, and
COx for all the catalysts in the OCM reaction. Reaction conditions: feed composition of CH4:O2:N2 =
4:1:4, total flow rate of 60 sccm, and reaction temperature of 750 oC. Selectivity is in terms of carbon.
Figure 6. a) Conversion-Selectivity plot and b) Reaction yield for all the catalysts in the OCM reaction.
Reaction conditions: feed composition of CH4:O2:N2 = 4:1:4, total flow rate of 60 sccm, and reaction
temperature of 750 oC.
16

Table 1. BET surface area and total pore volume of MCF-17 and the prepared catalysts
MCF-17 Original Rh-doped Ir-doped
566.3 ± 4.9 9.18 ± 0.04 8.91 ± 0.02 8.19 ± 0.03 7.59 ± 0.02
Pt-doped
BET surface area/m2∙g-1
Total pore volumea/cm3∙g-1
Average pore sizeb/nm
1.598
16.6
0.024
-
0.024
-
0.021
-
0.017
-
aBJH adsorption cumulative volume of pores; bBJH adsorption pore size
Table 2. Theoretical and determined loadings of the prepared catalysts
Determined Loadinga/wt.%
Elemental Composition
Theoretical Loading/wt.%
Original
Rh-doped Ir-doped Pt-doped
Na
W
Mn
Rh
Ir
0.78b
3.13b
2.00c
0.10
0.73
0.78
2.23
2.04
0.13
-
0.72
2.29
2.23
-
0.79
2.48
2.37
-
2.51
2.04
-
-
-
0.10
0.12
-
-
Pt
0.10
-
0.12
b
c
aAs determined from ICP-AES; Corresponds to 5 wt.% of Na2WO4; Calculated in terms of elemental
Mn
17

Table 3. Surface compositionsa of the fresh and spent catalysts.
C 1s/%
Fresh Spentb
9.6 52.3
Na 1s/%
Fresh Spentb
0.27 0.14
0.28 0.20
0.26 0.13
0.32 0.21
Mn 2p/%
Fresh Spentb
0.53 0.13
0.84 0.27
0.56 0.16
0.56 0.13
W 4f/%
Fresh Spentb
0.89 0.65
1.03 0.71
0.91 0.50
0.83 0.68
Original
Rh-doped
Pt-doped
Ir-doped
9.3 37.8
8.3 50.1
12.4 42.7
aAs obtained from XPS; bDue to the quartz sand residue, the surface concentrations reported for the spent
catalysts should be taken as their lower limits
Table 4. OCM activity and selectivity after ~12h of time on streama.
Selectivityb/%
CH4
Catalysts
C2H4/C2H6 C3H6/C3H8
CO2/CO
Conversion/%
C2
C3
COx
57.3
36.7
46.0
57.1
Original
Rh-doped
Ir-doped
Pt-doped
15.6
9.3
33.0
55.6
46.7
36.1
4.0
5.6
4.8
4.0
3.6
0.93
1.5
2.8
4.4
1.4
3.0
4.7
0.11
1.1
14.4
14.1
0.30
0.12
aReaction conditions: feed composition of CH4:O2:N2 = 4:1:4, total flow rate of 60 sccm, and reaction
temperature of 750 oC; bSelectivity is in terms of carbon.
18