Effects of Ir-doping on the transition from oxidative coupling to partial oxidation of methane in La2O3-CeO2 nanofiber catalysts: Spatial concentration and temperature profiles

Article abstract of DOI:10.1039/c7ra02616a

Spatially resolved temperature and species concentration profiles determined via microprobe sampling and online gas chromatography were used to examine the transition from oxidative coupling of methane (OCM) to catalytic partial oxidation (CPO) as a function of Ir-doping of a La₂O₃-CeO₂ nanofiber fabric catalyst. Experiments were performed at atmospheric pressure at a furnace temperature of 750 °C, feed gas CH₄/O₂ molar ratio of 5 and at a nominal space time of about 60 ms. Reaction products exhibited a dramatic shift from ethane and ethylene (OCM, C₂ selectivity of 54%), in the case of the un-doped catalyst, to exclusively synthesis gas (CPO, CO and H₂ selectivity 90+%), at 1% Ir doping. Species concentration profiles obtained in the latter case indicate the formation of H₂O, H₂ and CO first, followed by the steam reforming of CH₄ to produce the balance of the syngas concomitant with a sharp decrease in H₂O levels. These findings suggest a CPO mechanism that is different from the literature, calling for the revision of detailed chemical kinetic mechanisms proposed before.

Full text of DOI:10.1039/c7ra02616a

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Effects of Ir-doping on the transition from oxidative
coupling to partial oxidation of methane in La2O3–
CeO2 nanofiber catalysts: spatial concentration and
temperature profiles
Cite this: RSC Adv., 2017, 7, 26783
a
B. Zohour,a A. Bae,a A. Seubsaib and S. Senkana
*
D. Noon,
Spatially resolved temperature and species concentration profiles determined via microprobe sampling and
online gas chromatography were used to examine the transition from oxidative coupling of methane (OCM)
to catalytic partial oxidation (CPO) as a function of Ir-doping of a La2O3–CeO2 nanofiber fabric catalyst.
Experiments were performed at atmospheric pressure at a furnace temperature of 750 ꢀC, feed gas CH4/
O2 molar ratio of 5 and at a nominal space time of about 60 ms. Reaction products exhibited a dramatic
shift from ethane and ethylene (OCM, C2 selectivity of 54%), in the case of the un-doped catalyst, to
exclusively synthesis gas (CPO, CO and H2 selectivity 90+%), at 1% Ir doping. Species concentration
profiles obtained in the latter case indicate the formation of H2O, H2 and CO first, followed by the steam
reforming of CH4 to produce the balance of the syngas concomitant with a sharp decrease in H2O
levels. These findings suggest a CPO mechanism that is different from the literature, calling for the
revision of detailed chemical kinetic mechanisms proposed before.
Received 3rd March 2017
Accepted 8th May 2017
DOI: 10.1039/c7ra02616a
rsc.li/rsc-advances
The prevailing mechanism for OCM involves coupled surface
and gas phase reactions in which the critical steps are the
Introduction
surface generation, subsequent gas phase diffusion and
The oxidative coupling of methane (OCM) and catalytic partial
oxidation (CPO) of methane are closely related reaction processes
involving the transformation of CH4, the major component of
natural gas, and O2 into valuable chemical intermediates that are
commonly derived from petroleum or coal.1–6 Both were devel-
oped as an economic alternative to the highly endothermic steam
reforming process.6,7 Despite decades of research in OCM and
CPO, further developments in catalysis (activity, selectivity,
stability and mechanistic understanding), reactor design (selec-
tivity and product separation) and process safety are still needed
to exploit these processes industrially.
The catalytic materials for OCM are typically oxides of alkali
metals (e.g. Mg, Ca) and lanthanides (e.g. La, Ce) whereas those
used to accomplish methane CPO are transition and noble
metals (e.g. Ni, Co, Rh, Ir, Pt).3 In OCM the CH4/O2 ratio in the
feed is typically in the 3–7 range and the desired reaction
products are C2H4, C2H6 and C3+ hydrocarbons with H2 and CO
as co-products. On the other hand, methane CPO typically
employs a CH4/O2 ratio of 1–2, producing CO and H2 (synthesis
gas) as the target products.3–6
coupling of methyl radicals to form C2H6.8–13
O2 + 2[*] / 2[O]
CH4 + [O] / CH3 + [OH]
CH3 + CH3 / C2H6
(1)
(2)
(3)
(4)
[OH] + [OH] / [O] + [*] + H2O
The formation of C2H4, H2, H2O, CO and CO2 has been
proposed to occur via combined surface and gas-phase reac-
tions.9–13 Because OCM is highly exothermic, the resulting
temperature increase forces the reaction to proceed under
transport limitations. Nevertheless, gas phase reactions play an
important role in affecting product evolution.14 Recently, we
also reported that nanobers of La2O3–CeO2 exhibit excellent
+
OCM activities and selectivities, with combined C2 yields
(selectivity ꢁ conversion) in excess of 22%.15,16 In that work, 20–
100 nm diameter nanobers were prepared by electrospinning,
which is a versatile technique capable of controlling the surface
chemistry of catalysts that will be described further.
On the other hand, no uniformly applicable mechanism
seems to exist for the CPO of methane.3 Nevertheless, existing
literature can be grouped into two main categories. First is the
direct decomposition of methane on the catalyst surface
aDepartment of Chemical and Biomolecular Engineering, University of California, Los
Angeles, CA 90095, USA. E-mail: daniel.p.noon@gmail.com
bDepartment of Chemical Engineering, Faculty of Engineering, Kasetsart University,
Bangkok, Thailand 10900
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followed by the oxidation of surface moieties to form synthesis
Herein we report, for the rst time, detailed spatial
gas. This is well demonstrated in Pt and Rh catalyzed CPO,7,17–20 concentration and temperature proles in Ir-doped La2O3–CeO2
where the mechanism can be schematically summarized as nanober catalysts prepared by electrospinning. These investi-
below:
gations capture the dramatic change in product selectivities
from OCM to CPO even at small levels of Ir doping, thus
providing useful new data for the development and validation
detailed chemical kinetic mechanisms.
O2 + 2[*] / 2[O]
CH4 + 4[*] / [C] + 4[H]
[C] + [O] / 2[*] + CO
2[H] / 2[*] + H2
(5)
(6)
(7)
(8)
Experimental section
Catalyst preparation
Experiments were performed using a xed-bed tubular reactor
system as shown in Fig. 1.25 The reactor was packed with La2O3–
CeO2 nanober catalysts, both neat and doped with different
levels of Ir. Catalysts were prepared by electrospinning a viscous
solution of polyvinylpyrrolidone (PVP; 1.3 MDa, 0.60 g), water/
ethanol (ꢂ1 : 1 wt ratio, 9.5 g) in which the metal precursors,
as La(NO3)3$6H2O and Ce(NO3)3$6H2O (La/Ce wt ratio ¼ 3, 0.35
g) were dissolved.14,15 The required amount of precursor IrCl3
was also dissolved in co-spun samples. The electrospun mate-
In the second category, methane CPO has been advocated to
proceed via sequential combustion-reforming reactions, with
the formation of CO2 and H2O rst, followed by endothermic
CO2 and H2O reforming of CH4 reactions producing CO and H2
in the second step.21 This two-step process appears to be the
predominant mechanism of syngas production over transition
metal based catalysts,3,21–23 where the overall reactions are:
ꢀ
rial was dried and then calcined at 625 C to form metal oxide
nanober fabrics.
CH4 + 2O2 / CO2 + 2H2O
CH4 + CO2 / CO + H2
CH4 + H2O / CO + 3H2
(9)
(10)
(11)
Spatial prole acquisition
The fabric catalyst (20 mg) was loosely packed into a 6 mm
diameter quartz tube and sandwiched between two quartz wool
plugs (20 mg each, Fig. 1).25 The bulk density and void fraction
of the catalytic bed were approximately 0.3 g cmꢃ3 and 0.94,
respectively. The reactor was placed inside a cylindrical tubular
furnace, which preheated the feed gas and catalyst. Electronic
mass ow controllers (MFCs; MKS Billerica, MA) were used to
set the total reactant ow at 120 sccm, corresponding to
a nominal space time of about 60 ms. The experiments were
performed at atmospheric pressure with a furnace temperature
of 750 ꢀC and CH4/O2 ratio of 5.
Previous methane CPO studies with Ir supported on La2O3
indicate the presence of the latter mechanism.23,24 However, this
conclusion was indirectly based on separate methane combus-
tion and CH4–H2O and CH4–CO2 reforming studies, in
conjunction with CPO experiments that were all accomplished
through the measurement of reactor exit gas compositions.23,24
In addition, Ir catalysts were prepared by traditional impreg-
nation of a La2O3 support with little control over surface Ir
cluster size and dispersion.24
Since the La2O3–CeO2 nanober fabric is an excellent OCM
catalyst15 and Ir–La2O3 is the same for CPO,3 it is of scientic and
practical interest to explore how Ir doping of La2O3–CeO2 would
affect product selectivity. In particular, it is important to
understand how the temperature and detailed species concen-
tration proles evolve inside the catalyst bed at relevant oper-
ating conditions and how surface and gas phase kinetics are
coupled between OCM and CPO. With ample literature on the
materials, lanthanide supported Ir CPO catalysts are of consid-
erable interest for reassessing the relevant kinetic mechanisms
using our unique approach. Additionally, electrospun La2O3–
CeO2 nanobers were discovered and extensively studied by the
authors as novel, high-performing OCM catalysts. The potential
for bridging the OCM and CPO mechanisms in this manner was
rst postulated when Ir-doped La2O3–CeO2 nanobers were
found to have by far the highest selectivity for CO/H2 in
preliminary screenings of dopants, with the original intention of
improving OCM performance. Hence, the authors have the
expertise in synthesizing, testing and evaluating such catalysts
and can easily translate this to thin, uniform and incrementally
doped La2O3–CeO2–Ir nanobers with precisely controlled
nanostructures.25
Sampling was accomplished by withdrawing gases from
within the bed using a closed-end quartz capillary tube (800 mm;
Friedrick and Dimock, Millville, NJ) with a 80 mm diameter
sampling orice laser drilled on its side (see inset Fig. 1).25–27
Fig. 1 Packed bed reactor system and accompanying equipment used
for acquiring spatial concentration and temperature profiles within the
fabric catalysts.25
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Gas analysis was accomplished by on-line gas chromatography
˚
(Varian 4900 Mini GC, with 5 A molecular sieve and Poraplot U
columns). N2 (1.0 mol%) was used as an internal standard. The
location of the sampling orice and overall probe length were
designed such that the capillary tip always remained outside the
bed at any sampling position to avoid gas bypass. Samples were
withdrawn through the capillary probe at a rate of about 5 sccm;
thus, the ow within the reactor was minimally perturbed.
Temperature measurements were performed by placing a 250
mm diameter K-type thermocouple inside the capillary probe in
the absence of gas withdrawal. The tip of the thermocouple was
positioned at the sampling orice. The temperature and
concentration proles were obtained by moving the capillary
(with and without the thermocouple, respectively) in the axial
direction using a micropositioning device (Velmex, Bloomeld,
NY). Positional accuracy associated with the placement of the
capillary probe within the reactor was estimated to be ꢄ0.25
mm. Similar uncertainty would be expected to exist between the
temperature and concentration proles as well.
Fig. 2 SEM image of La2O3–CeO2 nanofibers doped with 1 wt% Ir on
a metals basis (secondary electron imaging, 7.6 mm working distance,
ꢁ30 000 magnification, 5.0 kV).
Catalyst characterization
Catalyst characterizations were performed as the following: BET
surface areas were determined using Micromeritics ASAP 2020
at 77 K on all fabrics tested in this work. Powder X-ray diffrac-
tion (XRD) patterns were obtained on an X-ray powder diffrac-
tometer (PANalytical X'Pert PRO) using Cu-Ka radiation, 45 kV
and 40 mA, tted with Ni lter and Soller slit collimator to
identify crystalline phases. A scanning electron microscope and
an energy dispersive X-ray spectrometer (FE-SEM/EDS, FE-SEM:
JOEL JSM-7600F) were used to image the catalysts' morphology.
X-ray photoelectron spectroscopic studies (XPS) were carried
out using Al Ka monochromatic radiation, hn ¼ 1486.74 eV with
SPECS spectrometer for the X-ray source to determine the
surface composition and identify electronic state of the mixing
elements.
Fig. 3 X-ray diffractograms of La2O3–CeO2 nanofibers, undoped and
with metal doping of 0.05 wt% and 1 wt% Ir. Based on Scherrer analysis,
the crystal sizes were about 5 nm for all samples.
Results and discussion
Characterization studies
sizes of the crystals formed remained at approximately 5 nm,
based on Scherrer analysis, regardless of the Ir doping level.
It is possible that metal ions can segregate during the solvent
evaporation in the electrospinning process and the subsequent
calcination steps, and can lead to the establishment of radially
non-uniform metal composition, especially Ir, across the ber.
To address this potential problem, we performed XPS studies to
determine the surface composition of a 5 wt% (4 atom%) Ir-
doped La2O3–CeO2 ber that has been calcined. We used
a higher Ir loading in this test to improve the accuracy of the
XPS measurements. The relevant XPS spectra are shown in
Fig. 4. The surface atomic metal compositions were estimated
by integrating the areas under the La 3d, Ce 3d and Ir 4f curves
and computing the fraction that each constitutes relative to the
total area, yielding 75% La, 18% Ce and 3% Ir. This measured
3% surface Ir composition is consistent with the calculated bulk
atomic composition of 4% Ir for the ber. Therefore, surface
In Fig. 2, a SEM image of the 1 wt% Ir doped La2O3–CeO2 fabric
is shown, which was virtually identical to the undoped fabric. As
evident from this image, the bers are long, straight and
uniform, possessing diameters of about 100 nm. The BET
surface area was about 10 m2 gꢃ1, a value that is close to what
would be expected for the specic surface area of 100 nm
diameter, straight, non-porous cylinders. This suggests that the
bers do not have internal porosity regardless of the level of Ir
doping.
In Fig. 3 the XRD spectra of the undoped,15 0.05 wt% and 1
wt% Ir-doped La2O3–CeO2 fabrics are presented. The XRD of the
undoped fabric exhibits much sharper peaks compared to the
doped fabrics but has an overall shape that is similar to that
with 0.05 wt% Ir. All three samples, including the 1 wt% Ir,
appear to have similar cubic crystal structures. Increasing the
level of Ir doping suppresses the XRD peaks. However, the mean
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evident from Fig. 5, the peak temperature of 970 ꢀC occurs 4
mm within the catalytic zone. It is worth noting that C2H6 and
C2H4 both peak at about same depth within the catalyst (3 mm)
at values of 3.4 and 1.9% respectively. At higher CH4/O2 ratios,
e.g. 7 or higher, substantial lag time is observed for the
formation of C2H4 relative C2H6.25
The level of H2 peaks at 5.8%, 2 mm deep within the catalyst
bed and levels off to about 5%, aer 5 mm. The CO2 concen-
tration increases and levels off at about 7.4%, 3 mm deep. The
prole of CO also peaks at 4.0% at 3 mm within the catalyst and
levels off at 2.4%, 11 mm into the catalyst. The water concen-
tration, as calculated by O balances, increases uniformly
reaching 15.6% at 2 mm, concomitant with the complete
consumption of O2. Here, methane consumption also ceases,
leveling at 62.1%.
Fig. 4 X-ray photoelectron spectra of the La2O3–CeO2 nanofibers
doped with 5 wt% Ir.
It is widely accepted that OCM is induced by a surface oxygen
atom (eqn (2)), activating CH4, generating a CH3 radical and
[OH].8–13 Ethane is then created in the gas phase by combination
of two CH3 radicals (eqn (3)). The catalyst is regenerated
through the formation and desorption of H2O (eqn (4)) and
subsequently by the dissociative chemisorption of O2 (eqn (1)).
The generation of C2H4, however, may be initiated on the
catalyst surface or in the gas phase, but either way is strongly
inuenced by the presence of O2. In the catalytic route, [O] may
abstract hydrogen from C2H6 in a manner similar to CH4 acti-
vation. Ethane may then interact with other species to form
C2H4. This is summarized in eqn (12)–(15) below:10,11
enrichment or depletion of Ir relative to the ber bulk can
substantially be ruled out. Evidently, the rapid precipitation of
the solid ber within the liquid jet formed in electrospinning
was fast enough to prevent substantial migration of the metal
salts across the ber diameter.
Reaction studies
Before the results are presented, an important issue related to
the catalyst selection must be noted. Although a large number
of Ir doped catalysts were prepared and tested, only the results
with undoped, 0.05% Ir and 1% Ir doped La2O3–CeO2 will be
presented for the following reasons. At 0.05% Ir loading the
reaction process exhibits both the features of OCM and CPO. At
lower Ir loadings OCM dominates the reaction process. At 1% Ir,
the reaction process becomes virtually CPO, with only trace
OCM products. CPO remains the only reaction process at Ir
loadings higher than 1%.
C2H6 + [O] / C2H5c + [OH]
C2H5c + HO2c / CH3c + CH2O + OHc
C2H5c + M / C2H4 + Hc + M
C2H5c + O2 / C2H4 + HO2c
(12)
(13)
(14)
(15)
Shown in Fig. 5 are the axial temperature and concentration
proles for the reactor packed with the undoped La2O3–CeO2
fabric catalyst. These proles exhibit behaviors similar to those
reported in our earlier OCM studies14,16,25 with the important
conclusion that C2 hydrocarbons are the principal products. As
C2H6 dehydrogenation is also feasible in the gas phase by
eqn (16):10
C2H6 + OHc / C2H5c + H2O
(16)
Clearly, the rates of above reactions are impacted by the
concentration of O2 in the feed gas, consistent with the lag of
C2H4 relative to C2H6 diminishing at lower CH4/O2 ratios.
In Fig. 6, the temperature and concentration proles are
presented for the La2O3–CeO2 catalyst doped with 0.05 wt% Ir,
the addition of which most dramatically impacts the concen-
tration prole for water. Instead behaving as an ultimate reac-
tion product as is the case for OCM (Fig. 5), H2O, here in Fig. 6,
exhibits a distinct peak of 12.7% at 3 mm in the catalyst bed and
then decreases sharply, ultimately leveling off at 7%. This
behavior supports the methane steam reforming reaction, an
expected result for Ir based catalysts as reported in the
literature.3
At 0.05 wt% Ir loading, some OCM products are also
observed, albeit at lower levels than the undoped case. The
reactor effluent concentrations of C2H6 and C2H4 were 1.80%
and 0.02% respectively in Fig. 6, compared to the 3.4 and 1.9%
Fig. 5 Spatial temperature and species mole percent profiles for the
undoped La2O3–CeO2 fabric catalyst.
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employs different mechanistic steps, in which gaseous CH3
radicals serve as precursors to syngas generation. For example,
the possible fragmentation of CH3 radicals and all other OCM
intermediates and products should not be ruled out and would
suggest the mechanism for CPO over Ir-based catalysts is not
exclusively explained by the proposed two-step combustion-
steam reforming path. Follow-up investigations are clearly
called for in which novel DCKMs for methane CPO are formu-
lated against the spatial proles presented in this work.
Fig. 7 shows the temperature and concentration proles for
the catalyst doped with 1 wt% Ir. As can be seen from this gure,
the product composition is exclusively that of CPO with no OCM
products. Exit H2 and CO levels are 42% and 20%, respectively,
which are in harmony with the reaction stoichiometry of
methane CPO (i.e. eqn (9)–(11)). Both H2 and CO are produced
early in the catalytic reaction zone, although there was some
delay in CO production. However, earlier H2 detection can be
attributed to diffusion due to its very steep gradient and high
diffusivity. In fact the measurement of signicant levels of H2
upstream outside of the catalytic zone is a clear manifestation of
such diffusion effects.
The concentration proles for H2O and CO2 exhibit several
new and signicant mechanistic insights. First, as seen in
Fig. 7, the H2O prole initially rises to a peak of about 9% at 2
mm and then sharply decreases to levels of about 0.2%, 3 mm in
the catalyst bed, which is consistent with the two-step
combustion-steam reforming reaction mechanism discussed
before.3,21–24 On the other hand the CO2 concentration prole
does not exhibit such a peak (Fig. 7). Instead, the CO2 concen-
tration prole exhibits an initial low level reaching a plateau of
about 1.7% at 2 mm in the catalyst bed, coincident with the H2O
peak. The absence of a peak in the CO2 prole creates questions
regarding the validity of the two-step mechanism for methane
CPO over Ir-based catalysts as reported earlier.23,24 The results in
Fig. 7 clearly suggest a hybrid mechanism, i.e. a composite of
the two previous reaction mechanisms suggested for methane
CPO.3,5,6,21–24 That is, the CPO of methane over Ir appears to
involve rst its catalytic fragmentation, concomitant with the
formation of abundant levels of H2O (combustion) and CO,
Fig. 6 Spatial temperature and species mole percent profiles for the
La2O3–CeO2 fabric catalyst doped with 0.05 wt% Ir.
values observed in the undoped catalyst experiments (Fig. 5).
This reduction in C2 products could also be due to their steam
reforming reactions. Carbon monoxide and H2 levels were 10%
and 15%, respectively, both of which are substantially higher
than the undoped experiments that were 2.4% and 5%,
respectively (Fig. 5); this again is an expected result for the
reforming process. Carbon dioxide increases and levels off at
6.2%, slightly lower than the 7.4% with the undoped experi-
ments. Unlike the water prole, the carbon dioxide prole does
not exhibit any discernable peak in the present experiments.
This could be due to continued CH4 combustion (eqn (9)) and
water gas shi reaction (eqn (17)):
CO + H2O 4 CO2 + H2
(17)
both of which can compensate for the CO2 consumption by the
reforming reaction (eqn (10)). Oxygen consumption is
completed at a depth of 4 mm, which is slightly later than the
undoped catalyst.
A comparison of temperature proles between the undoped
and 0.05 wt% Ir doped catalyst experiments reveals several
interesting features. First, the peak temperature reached only
ꢀ
950 C in the 0.05 wt% Ir catalyst bed (Fig. 6), which is about
20 ꢀC cooler than the undoped catalyst (Fig. 5). However, a close
inspection of the temperature proles at reactor entry reveals
a slightly higher temperature in the 0.05 wt% Ir catalyst bed
compared to the undoped catalyst bed. On the other hand, the
reactor exit temperatures of the 0.05 wt% Ir doped catalyst bed
is lower than the undoped catalyst bed. These measurements
are consistent with the combustion of methane rst followed by
endothermic reforming reactions within the 0.05 wt% Ir doped
bed. This phenomenon will be expanded upon later.
Pt-based catalytic materials have been shown to also produce
C2 hydrocarbons in methane CPO; however, their creation
appears to occur solely in the gas phase, facilitated by highly
exothermic heterogeneous steps.27 In other words, the surface
and gas phase mechanisms are coupled only by heat transfer, in
contrast to OCM in which CH3 radicals diffuse off of the catalyst.
Here, the La2O3–CeO2 fabric doped with 0.05 wt% Ir plausibly
Fig. 7 Spatial temperature and species mole percent profiles for the
La2O3–CeO2 fabric catalyst doped with 1.00 wt% Ir.
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Table 1 Peak temperature and peak and effluent species mole percents for the reaction products of Fig. 5–7. Locations of peaks are provided in
parentheses in mm measured in catalytic zone
Fig. 5 – undoped catalyst
Fig. 6 – 0.05 wt% Ir
Fig. 7 – 1 wt% Ir
890 (5)
0
Peak value, (location, mm)
Temperature, ꢀC
970 (4)
3.4 (3)
1.9 (3)
15.8 (3)
5.8 (2)
7.3 (2)
4.0 (2)
3.4
950 (4)
2.3 (3)
1.1 (3)
12.7 (3)
15.4 (6)
6.2 (8)
9.5 (5)
1.8
C2H6, mol%
C2H4
H2O
H2
CO2
CO
C2H6
C2H4
H2O
H2
0
9.2 (3)
42 (4)
1.7 (3)
20.4 (4)
0
0
1.2
42
Exit
1.9
15.8
5.0
0.02
7.2
15.4
CO2
7.3
6.2
1.7
CO
2.4
9.5
19.2
together with H2. This step is then followed by the steam levels of Ir doping, in a xed bed reactor fed by CH4 and O2
reforming of methane reaction producing the balance of the CO (CH4/O2 ¼ 5) indicated a clear transition from the oxidative
and H2. The low levels of CO2 subsequently produced in the coupling of methane (OCM) to the catalytic partial oxidation
reactor can be explained by the water gas shi reaction (eqn (CPO) of methane reaction scheme. With the undoped fabric
(17)). Also, the reappearance of H2O from between the 11–15 catalyst, OCM was dominant process with ethane and ethylene
mm positions may be due to temperature drop pushing steam as major products. The catalyst doped with 1 wt% Ir produced
reforming in the reverse direction. Although the needed exclusively synthesis gas (CO and H2). At 0.05 wt% Ir doping,
consumption of H2 appears to be absent in Fig. 7, it could easily both OCM and CPO processes were operational. Species
be masked by the high diffusivity of H2 effectively attening the concentration proles obtained in the 1 wt% Ir case indicate the
concentration prole. Methane also experiences a slight net rise formation of H2O, H2 and CO rst, followed by the steam
in concentration in this same region. Similar artifacts are also reforming of CH4 to produce the balance of the syngas
present in Fig. 6.
concomitant with the sharp decrease in H2O levels. These
As evident from Fig. 7, the temperature prole has several ndings suggest a CPO mechanism that is different than the
important features. First, the temperature prole exhibits two literature, calling for the revisions of detailed chemical kinetic
sequential but overlapping peaks. The second peak clearly indi- mechanisms proposed before.
cates the presence of a sequential exothermic process, one likely
prospect being the water gas shi reaction (eqn (17)). However,
without detailed chemical kinetic and reactor modeling studies,
Acknowledgements
denitive conclusions cannot be made regarding the specic
We thank I. Onal and Y. Kaya of Middle East Technical
reaction processes responsible for this behavior.
University (METU) and H. Yu of UCLA for their assistance in the
Second, the temperature prole in Fig. 7 along the entire
XPS characterization work. We credit H. Zhou, F. Liu and L.
catalyst bed is lower than the other catalyst beds discussed
Shen of UCLA for their assistance with the BET characterization.
above (Fig. 5 and 6). The peak temperature of the 1 wt% Ir
We also thank Laboratory Catalyst Systems, LLC for use of its
catalyst bed is 890 ꢀC, which is signicantly lower than the
facilities and database. B. Zohour acknowledges the National
undoped (970 ꢀC, Fig. 5) and the 0.05% Ir doped (950 ꢀC, Fig. 6)
Science Foundation (NSF) GRFP Support Grant No. DGE-
catalysts. Evidently the 1% Ir system is an excellent reforming
1144087 and the University of California, Los Angeles (UCLA)
catalyst; that is, as soon as H2O is produced it undergo endo-
Graduate Division Fellowship. D. Noon acknowledges the NSF
thermic reforming reactions thereby limiting the temperature
IGERT: Materials Creation Training Program (MCTP-DGE-
rise in the catalyst bed. It is also interesting to note that the
0654431) and the California Nano-Systems Institute.
gases leave the reactor at 733 ꢀC (Fig. 7), substantially lower
than the furnace temperature of 750 ꢀC, as a consequence of the
endothermic reforming reactions.
References
In Table 1, key summary features of Fig. 5–7 are provided,
1 M. C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas,
R. M. Navarro and J. L. G. Fierro, Catal. Today, 2011, 171,
15–23.
namely the peak temperatures and peak and exit product
concentrations, and their locations within the catalyst bed.
¨
2 U. Zavyalova, M. Holena, R. Schlogl and M. Baerns,
Conclusions
ChemCatChem, 2011, 3, 1935–1947.
3 B. C. Enger, R. Lødeng and A. Holmen, Appl. Catal., A, 2008,
346, 1–27.
Detailed species concentration and temperature proles ob-
tained over La2O3–CeO2 nanober fabric catalysts at varying
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This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
4 D. A. Wood, C. Nwaoha and B. F. Towler, J. Nat. Gas Sci. Eng., 16 B. Zohour, D. Noon and S. Senkan, ChemCatChem, 2014, 6,
2012, 9, 196–208. 2815–2820.
5 D. A. Hickman and L. D. Schmidt, Science, 1993, 259, 343– 17 A. B. Mhadeshwar and D. G. Vlachos, Ind. Eng. Chem. Res.,
346. 2007, 46, 5310–5324.
6 O. Korup, C. F. Goldsmith, G. Weinberg, M. Geske, 18 D. Dalle Nogare, N. J. Degenstein, R. Horn, P. Canu and
¨
T. Kandemir, R. Schlogl and R. Horn, J. Catal., 2013, 297,
1–16.
L. D. Schmidt, J. Catal., 2011, 277, 134–148.
19 R. Horn, K. A. Williams, N. J. Degenstein, A. Bitsch-Larsen,
D. Dalle Nogare, S. A. Tupy and L. D. Schmidt, J. Catal.,
2007, 249, 380–393.
7 K. A. Williams, R. Horn and L. D. Schmidt, AIChE J., 2007, 53,
2097–2113.
8 L. Mleczko and M. Baerns, Fuel Process. Technol., 1995, 42, 20 R. Horn, K. A. Williams, N. J. Degenstein and L. D. Schmidt,
217–248. Chem. Eng. Sci., 2007, 62, 1298–1307.
9 R. Quiceno, J. Perez-Ramırez, J. Warnatz and 21 M. Prettre, C. Eichner and M. Perrin, Trans. Faraday Soc.,
O. Deutschmann, Appl. Catal., A, 2006, 303, 166–176. 1946, 42, 335b.
10 J. W. Thybaut, J. Sun, L. Olivier, A. C. Van Veen, C. Mirodatos 22 J. R. Rostrup-nielsen and J. H. B. Hansen, J. Catal., 1993, 144,
and G. B. Marin, Catal. Today, 2011, 159, 29–36. 38–49.
11 J. Sun, J. W. Thybaut and G. B. Marin, Catal. Today, 2008, 23 G. Postole, K. Girona, J. Toyir, A. Kaddouri and P. Gelin, Fuel
137, 90–102. Cells, 2012, 12, 275–287.
12 M. Y. Sinev, Z. T. Fattakhova, V. I. Lomonosov and 24 K. Nakagawa, K. Anzai, N. Matsui, N. Ikenaga, T. Suzuki,
´
´
´
Y. A. Gordienko, J. Nat. Gas Chem., 2009, 18, 273–287.
13 Z. Stansch, L. Mleczko and M. Baerns, Ind. Eng. Chem. Res.,
1997, 36, 2568–2579.
14 D. Noon, B. Zohour and S. Senkan, J. Nat. Gas Sci. Eng., 2014,
18, 406–411.
Y. Teng, T. Kobayashi and M. Haruta, Catal. Lett., 1998, 51,
163–167.
25 B. Zohour, D. Noon and S. Senkan, ChemCatChem, 2013, 5,
2809–2812.
26 R. Horn, O. Korup, M. Geske, U. Zavyalova, I. Oprea and
¨
R. Schlogl, Rev. Sci. Instrum., 2010, 81, 064102.
27 M. Geske, K. Pelzer, R. Horn, F. C. Jento and R. Schlogl,
15 D. Noon, A. Seubsai and S. Senkan, ChemCatChem, 2013, 5,
146–149.
¨
Catal. Today, 2009, 142, 61–69.
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