Rh-promoted mixed oxides for "low-temperature" methane partial oxidation in the absence of gaseous oxidants

Article abstract of DOI:10.1039/c7ta01398a

Compared to conventional reforming, chemical looping reforming (CLR), which partially oxidizes methane in the absence of gaseous oxidants such as steam or oxygen, offers a simpler and potentially more efficient route for syngas generation. This is achieved by cyclic removal and replenishment of active lattice oxygen in oxygen carrier particles, a.k.a. redox catalysts. With redox catalysts being at the heart of CLR, their activity and selectivity are crucial for the CLR performance. While many redox catalysts have been developed, their activities toward methane partial oxidation (POx), especially at relatively low temperatures, are often limited due to the high activation energy for the migration and removal of lattice oxygen. Moreover, syngas selectivity is often less than ideal due to the non-selective nature of the surfaces for many oxides. To address these limitations, we investigated the effects of promoting the catalytic activity of oxide surfaces for two redox catalysts: CaMnO₃ and LaCeO₃. Our findings indicate that promoting mixed oxides with a small amount of Rh can lower the onset temperature of methane POx by as much as 300 °C (0.5 wt% Rh loading). Over 93% syngas selectivity and 7.9 mmol of syngas per gram of redox catalyst were obtained for a highly stable, Rh promoted CaMnO₃ at 600 °C, making it a promising redox catalyst for methane POx under a cyclic redox scheme.

Full text of DOI:10.1039/c7ta01398a

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Rh-promoted mixed oxides for “low-temperature”
methane partial oxidation in the absence of
gaseous oxidants†
Cite this: DOI: 10.1039/c7ta01398a
*
Arya Shafiefarhood,‡ Junshe Zhang,‡ Luke Michael Neal and Fanxing Li
Compared to conventional reforming, chemical looping reforming (CLR), which partially oxidizes methane
in the absence of gaseous oxidants such as steam or oxygen, offers a simpler and potentially more efficient
route for syngas generation. This is achieved by cyclic removal and replenishment of active lattice oxygen in
oxygen carrier particles, a.k.a. redox catalysts. With redox catalysts being at the heart of CLR, their activity
and selectivity are crucial for the CLR performance. While many redox catalysts have been developed, their
activities toward methane partial oxidation (POx), especially at relatively low temperatures, are often limited
due to the high activation energy for the migration and removal of lattice oxygen. Moreover, syngas
selectivity is often less than ideal due to the non-selective nature of the surfaces for many oxides. To
address these limitations, we investigated the effects of promoting the catalytic activity of oxide surfaces
for two redox catalysts: CaMnO3 and LaCeO3. Our findings indicate that promoting mixed oxides with
a small amount of Rh can lower the onset temperature of methane POx by as much as 300 ꢀC (0.5 wt%
Rh loading). Over 93% syngas selectivity and 7.9 mmol of syngas per gram of redox catalyst were
obtained for a highly stable, Rh promoted CaMnO3 at 600 ꢀC, making it a promising redox catalyst for
methane POx under a cyclic redox scheme.
Received 15th February 2017
Accepted 11th April 2017
DOI: 10.1039/c7ta01398a
rsc.li/materials-a
Compared to conventional methane reforming, chemical
looping reforming (CLR) offers a potentially more efficient route
that eliminates the need for ASU.7,8 This is realized through
Introduction
Methane, the primary component of natural gas, is the most
abundant organic compound on earth.1 Its relative abundance
and lower CO2 emission compared to petroleum and coal have
led to renewed interest in the conversion of methane to value-
added products such as liquid transportation fuels and chem-
icals. Although the direct conversion of methane to chemicals
such as oxygenates, olens, and aromatics has been extensively
investigated within the scientic community, it has yet to
demonstrate feasible product yields from an economic view-
point.2 To date, commercial methane valorization processes are
exclusively based on indirect approaches: methane is rst
reformed into syngas, i.e. a mixture of CO and H2; subsequently,
the syngas stream is conditioned and converted into desired
products.3–6 As a crucial rst step, methane reforming is both
capital intensive and inefficient due to coke formation, catalyst
deactivation, high endothermicity, signicant steam require-
ments, and/or the needs for cryogenic air separation units
(ASU).
redox active oxygen carriers, a.k.a. redox catalysts, which
incorporate oxygen from air into their lattice. The active lattice
oxygen is subsequently donated for methane partial oxidation
(POx). Two interconnected reactors are used to complete such
a cyclic redox process. Due to their critical importance for CLR,
numerous research efforts have been devoted to selection and
synthesis of more active and selective redox catalysts.7–13
A
typical redox catalyst is composed of an oxygen reservoir, which
is commonly a rst row transition metal oxide, and an inert
support to increase its stability and oxygen mobility.7,8 Although
proven to be effective, the aforementioned oxides face such
challenges as high cost, low activity, and/or limited selectivity.
Nickel based oxides are one of the more extensively studied
materials. Their application, however, is hindered due to high
cost, health and environmental concerns, and coking and sulfur
poisoning issues.14–19 The use of iron and manganese based
oxides can be advantageous as they are abundant and envi-
ronmentally benign, but they exhibited low syngas selectivity.7,8
Our recent studies showed that mixed metal oxides, such as
perovskites, can be used as supports to both increase the
mechanical integrity of the oxygen reservoir and provide
metallic catalytic sites for methane partial oxidation.20–28 It is
noted that such catalytic sites are only present in (partially)
reduced redox catalysts.22,24 Moreover, partially exposed iron
North Carolina State University, Chemical and Biomolecular Engineering, USA.
E-mail: i5@ncsu.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ta01398a
‡ First authors.
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oxide phases negatively impact the methane to syngas the Rh-promoter signicantly increases the activity of the
selectivity. catalysts at lower temperatures. For instance, the Rh-promoter
To improve the selectivity of redox catalysts, two potential reduces the onset temperature of the POx reaction by up to 300
strategies can be adopted. The rst approach is thermody- ꢀC. While un-promoted CaMnO3 exhibits very low activity/
namically inhibiting over-oxidation by designing redox catalysts selectivity below 800 ꢀC, the promoted sample exhibited
ꢀ
with suitable equilibrium oxygen partial pressures (PO ). Several a maximum CO yield at 600 C with CO selectivity above 86%
2
recent studies reported improved syngas selectivity using and remained active at 500 ꢀC. Although LaCeO3.5 exhibits high
oxygen carriers and reactor congurations that are thermody- syngas selectivity at high temperatures (900 ꢀC), it suffers from
namically selective.27,29 The second approach is to prepare redox slow kinetics and becomes signicantly less active below 800 ꢀC.
catalysts with more selective surfaces. One example is core–shell Promoting the LaCeO3.5 with Rh not only improves the kinetics
structured redox catalysts with a nonselective oxygen reservoir at high temperatures, but also reduces the onset temperature
as the core and a selective perovskite phase as the shell.21,26 The for methane conversion to 600 C.
ꢀ
selectivity of such redox catalysts, however, is limited by their
surface oxidation state, which is determined by relative rates of
bulk lattice oxygen diffusion and surface oxygen removal. The
Experimental
Redox catalyst synthesis
surface (or loosely bound) oxygen species of oxidized redox
catalysts were found to be responsible for the non-selective
combustion reactions.22 This leads to a non-selective region at
the beginning of the methane oxidation reaction.22,24,27
Although partially reoxidizing the core–shell redox catalyst can
signicantly enhance the syngas selectivity,24 it reduces the
overall oxygen carrying capacity and increases the complexity of
the process. It is also noted that most redox catalysts reported to
date require high operating temperatures (>800 ꢀC); this is
primarily due to the low surface activity for methane activation
and the high activation energy required for lattice oxygen and/or
cation migration.7,8
The current work investigates an alternative strategy to
improve the surface catalytic activity of mixed metal oxides and
to enhance the redox activity of these oxides at signicantly
lower temperatures. Rh is selected as the promoter since it is
readily reducible and highly effective for methane activa-
tion.30–35 Although Rh is active for methane combustion under
oxidizing environments, the O2 free environment in CLR allows
for high methane conversion activity and syngas selectivity.36–38
In terms of oxygen carrying materials, two mixed-oxides with
Two redox catalysts were synthesized for this study: CaMnO3 and
LaCeO3.5. The samples were prepared using a modied Pechini
method. Details of the method are described elsewhere.21 In short,
stoichiometric amounts of nitrate salts of lanthanum and cerium,
or calcium and manganese were dissolved in deionized water and
stirred for 30 minutes at room temperature to form a homoge-
neous solution. Citric acid is added to the solution with a molar
ratio of 2.5 : 1 of citric acid and total cations, and the solution is
ꢀ
kept under stirring for another half an hour at 60 C. Ethylene
glycol with a molar ratio of 1.5 : 1 with respect to citric acid was
then added to the solution, and the temperature of the solution is
increased to 80 ꢀC under stirring until a homogeneous gel is
formed. The gel is then dried overnight at 100 ꢀC and annealed at
950 ꢀC for 8 hours.
The surface of the synthesized redox catalysts was also
promoted with rhodium using an incipient wetness method (0.5
wt% Rh). Rhodium nitrate salt was dissolved in deionized water
and appropriate amounts of the solution were added to the
samples in multiple steps. The impregnated ꢀsamples were then
ꢀ
dried at 100 C for 4 hours and red at 950 C for 6 hours. We
different equilibrium PO s, i.e. CaMnO3 and LaCeO3.5, are
2
note that although the cost of Rh can be prohibitive for
conventional, circulating uid bed based CLR; the current study
focuses on mechanistic investigations of the effects of surface
promotion. Moreover, the high activity exhibited by the Rh-
promoted redox catalysts at low temperatures offers the
opportunities to use such redox catalysts in simplied xed bed
systems to minimize attrition and particle loss.
selected. CaMnO3 has “oxygen uncoupling” properties and
spontaneously releases oxygen at elevated temperatures due to
its high equilibrium PO . LaCeO3.5, on the other hand, has low
2
equilibrium PO which is desirable for POx reactions from
2
a syngas selectivity standpoint. Since these mixed oxides
possess both redox activity and mixed conductive properties,
they are highly recyclable without an inert support.
Our results indicate that the relative rates of lattice oxygen
(O2ꢁ) conduction to the surface and the surface oxygen removal
Sample characterization
by the gas–solid reactions determine the selectivity of the redox Powder X-ray diffraction (XRD) experiments were performed.
catalysts. This is particularly the case at the early stage of the XRD patterns were obtained using a Rigaku SmartLab X-ray
reaction. The presence of Rh on the surface increases the diffractometer with Cu-Ka (l ¼ 0.1542) radiation operating at 40
oxygen removal rate from the surface and hence inhibits the kV and 44 mA. A stepwise approach with a step size of 0.1ꢀ and
formation of nonselective surface oxygen species. This is real- residence time of 2.5 seconds at each step in the 20–80ꢀ angle
ized by the enhancement of methane activation and hence more range (2q) was used to generate the XRD patterns. Similar
effective oxygen removal from the surface. It is also noted that experiments were also performed on the spent samples to
the rate of bulk lattice oxygen diffusion controls the overall conrm their phase stability. X-ray photoelectron spectroscopy
reaction rate for later stages of the reaction. Such a diffusion (XPS, Thermo Fisher Scientic Inc.) with an Al-Ka X-ray source
rate can be notably affected by the surface of the redox catalysts, at an operating voltage of 20 kV and a current of 10 mA is used
which provides the driving force for O2ꢁ conduction. As such, to analyze near surface elemental compositions. Survey spectra
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and the single element spectra were collected with 20 eV and
100 eV pass energy. The BET surface areas of the samples are
determined using a Micromeritics ASAP 2020 physi-chemi-
sorption system with krypton as the adsorption gas. Diffuse
reectance infrared Fourier transform spectroscopy (DRIFTS)
is also performed on the selected samples to distinguish their
different surface activation and intermediate species.
DRIFTS measurements are performed in an in situ reaction
cell (Pike Technologies DiffuseIR cell) that allowed contin-
uous gas ow through the samples. The spectra are collected
on a Thermo Scientic Nicolet iS50 FT-IR spectrometer
equipped with a MCT/A detector and a KBr beam splitter and
optics using 128 scans at 4 cmꢁ1 resolution. The CO
desorption experiments were performed on the reduced
samples (50% H2 balance Ar, 700 ꢀC, 1 hour). The samples
were exposed to CO (grade 5.0, 50% balance Ar) at room
temperatures and spectra were collected aer purging the
system with argon.
Results and discussion
Redox catalyst selection and characterization
Two redox catalysts are chosen for the current study: CaMnO3
and LaCeO3.5. Calcium manganate is one of the most studied
mixed oxides for chemical looping applications.7,8,39 Most of the
previous studies, however, focused on its CLOU properties as it
exhibits poor syngas selectivity, as one would anticipate from its
high equilibrium PO and catalytic activity for methane
2
combustion. The low selectivity of un-promoted CaMnO3 makes
it an excellent candidate to investigate the effects of the Rh
promoter.
Ceria (CeO2) is another well-known material as both an
oxygen carrier and support in chemical looping applica-
tions.40–43 Ceria is a particularly interesting material as it is
redox active and has excellent syngas selectivity and mixed
conductivity.44,45 However, it suffers from poor reduction
kinetics. A recent study also showed that it can trigger deacti-
vation when used as a support for iron oxide.25 Addition of
lanthanum to ceria can potentially enhance the reduction
kinetics of ceria. It can also hinder coke formation by increasing
the surface basicity. In this study, LaCeO3.5 is chosen as addi-
tion of more than 50% lanthanum to CeO2 is reported to cause
phase segregation whereas LaCeO3.5 forms a stable structure
Redox experiments
The reducibility of the samples in methane was tested through
temperature programed reduction (TPR) experiments. The
experiments were performed in a thermal gravimetric analyzer
(TGA, Q600 TA Instruments) and in 10 vol% methane (grade 5.0,
balance Ar). The temperature was increased from room
with desirable PO for methane partial oxidation.46 Our prelim-
2
inary results conrm excellent methane partial oxidation
selectivity and coke resistance on LaCeO3.5. However, it shows
very slow kinetics. This makes it ideal for probing the kinetic
improvements with the Rh promoter. Both redox catalysts are
prepared through a modied Pechini method. XRD results
(Fig. S2†) conrmed the formation of cubic uorite and ortho-
rhombic perovskite structures for LaCeO3.5 and CaMnO3
samples respectively. The surface of the catalysts is then
impregnated with 0.5 wt% rhodium (metallic basis) using a wet
impregnation method. The XRD patterns of the Rh-promoted
samples are nearly identical to those of unpromoted samples
(Fig. S2†). Rietveld renement analysis indicates very minor
changes in the lattice parameters of the samples (Table S1†).
Similar experiments were also performed on the spent samples
to conrm their phase stability. Fig. S3† shows the XRD pattern
of the Rh-promoted CaMnO3 aer 5 redox cycles at 900 ꢀC and
10% CH4/O2. No major structural change was observed on the
sample aer redox cycles.
ꢁ1
ꢀ
ꢀ
temperature to 1000 C with a 20 C min ramping rate and
kept isothermal for 30 minutes. The samples were oxidized at
900 ꢀC in 20% oxygen (ultra-dry, balance Ar) for 30 minutes
prior to the experiments.
Redox experiments were performed in a xed bed congu-
ration and in a quartz U-tube reactor (1/800 ID) with 50 mg
sample loading, 100 ml minꢁ1 total ow rate, and 10 vol%
reactive gases (CH4 and O2) approximating a differential bed
reactor. The reactor was purged with argon aer each half cycle
to prevent mixing of the reactive gases. The samples were held
in place using quartz wool to prevent uidization of the parti-
cles. The reactor effluent was analyzed using a quadrupole mass
spectrometer (Cirrus 2, MKS). To ensure the accuracy of the
qualitative analysis, a single point calibration was performed
using a standard calibration gas (1% H2, 1% CH4, 1% CO, and
1% CO2, balance Ar) before each experiment.
The same reactor conguration was used to perform the pulse
injection and co-feed (CH4 and O2) experiments. The detailed gas
delivery conguration was presented elsewhere.22 Co-feed experi-
ments were done with 5 different methane to oxygen ratios (CH4/
Methane TPR experiments
O2 ¼ 1.5, 1, 0.5, 0.2, and 0.1) to investigate the selectivity and TPR experiments were performed to investigate the effect of Rh
methane conversion of the samples at different surface oxidation on the reactivity of the redox catalysts in methane. Differential
states. The concentrations of methane were kept below the thermal gravimetric (DTG) analysis was used to determine the
ammability limit in all experiments. The pulse injection reduction temperatures of the samples in methane. Fig. 1
conguration is demonstrated in Fig. S1† and explained in detail compares the DTG data of the promoted and un-promoted
elsewhere.22 This conguration allows injection of small amounts redox catalysts (weight loss data in Fig. S4†). It is obvious that
of reactive gases on the catalyst to observe the behavior of the the presence of Rh changes their reduction properties. The
catalyst with minimal change in its bulk properties. This also initial reduction temperature of CaMnO3 decreased by almost
allows conducting methane isotope exchange experiments by co- 200 ꢀC when Rh is present. Similarly, the Rh promoted LaCeO3.5
injection of methane and deuterated methane and monitoring started to reduce at around 600 ꢀC whereas the unpromoted
the exchange products.
sample did not show any signicant reduction until 950 ꢀC. The
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signicantly higher than those of their LaCeO3.5 counterparts
(Fig. S4†).
Methane–oxygen redox testing
The activity, selectivity, and coke resistance of the redox catalysts
were tested in a U-tube reactor. Redox experiments were per-
formed by alternating the reactor feed at 900 ꢀC. A relatively low
gas residence time was used to ensure low (#15%) methane/
oxygen conversion, in order to approximate a differential bed
operation. Coke formation was quantied by calculating the
amount of CO and CO2 produced during the oxidation half cycle.
Fig. 2 compares the CO selectivity and oxygen donation rate of
the samples during the reduction half cycle in the 5th redox cycle
(the product concentration proles are shown in Fig. S5†). The
CO and H2 selectivity, CH4 conversion, and coke formation data
for the same cycle are summarized in Table 1.
As anticipated, the two unpromoted mixed oxides behave
substantially different. Calcium manganate shows a faster but
less selective conversion of methane (Fig. S5a†) while
lanthanum ceria is more selective but with a much lower
conversion rate (Fig. S5c†). Both samples showed a non-selec-
tive region at the beginning of the POx reaction. This non-
selective region is considerably more prominent for the
CaMnO3 sample and is typically attributed to surface and/or
loosely bound oxygen species. Aer the initial CO2 peaks, both
samples become selective towards methane POx.
The presence of Rh signicantly affects both selectivity and
methane conversion. Lanthanum ceria undergoes a substantial
kinetic improvement when promoted with Rh (Fig. S5d†). The
presence of Rh increases the maximum oxygen donation rate by
more than 11 times. A slight increase in CO selectivity is also
observed. The kinetic improvement is less signicant for calcium
manganate, which is already highly active at 900 ꢀC without
promoters. Nevertheless, 40% improvement in the rate of oxygen
removal was observed (Fig. 3). A slight increase is observed in
both CO and H2 selectivities. This is attributed to the presence of
metallic rhodium on the surface as will be discussed later.
Addition of Rh increases the amounts of methane conver-
sion and oxygen donation by about 20 and 8% respectively for
these oxides. This increase in oxygen donation cannot be from
the rhodium oxide reduction as reduction of Rh2O3 to Rh would
have accounted for no more than 0.02 wt% oxygen, which is
signicantly lower than the observed oxygen capacity increase.
One potential explanation is that Rh is incorporated into the
bulk structures of the mixed oxides, thereby changing their
Fig. 1 DTG results of redox catalysts in methane TPR experiments (20
ꢀC minꢁ1 ramping rate).
larger weight loss at lower temperatures can be attributed to
changes in the surface and/or bulk properties of the redox
catalyst. Rh is known to promote methane activation.30,33
Therefore, the presence of Rh on the surface of the redox
catalyst can signicantly enhance methane activation, leading
to more effective oxygen removal at lower temperatures. In
addition to surface enhancement, Rh could also be incorpo-
rated into the perovskite bulk structure;47 this can alter the
thermodynamic and/or ionic conduction properties of the redox
catalyst. The peaks observed at lower temperatures (ꢂ420 ꢀC)
are attributed to rhodium oxide reduction as they appear only in
the case of the promoted samples. It is also evident that the
oxygen carrying capacities of CaMnO3 based redox catalysts are
Fig. 2 Comparison of the CO selectivity and maximum oxygen
donation rate of the redox catalysts at 900 ꢀC.
Table 1 Methane conversion, CO/H2 selectivity, oxygen donation, and coke formation of the redox catalysts during the reduction half cycle at
900 ꢀ
C
H2 selectivity%
(excluding H2 from coke)
H2 selectivity%
(including H2 from coke)
CH4 converted
(ml)
O2 extracted
(wt%)
Coke formation
(wt%)
Sample
CO selectivity%
CaMnO3
CaMnO3 + Rh
LaCeO3.5
40.08
44.93
98.21
99.46
29.34
40.30
86.76
90.11
37.18
46.47
87.69
91.26
2.13
2.59
2.13
2.54
9.18
10.17
3.91
0.29
0.32
0.17
0.36
LaCeO3.5 + Rh
4.36
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oxygen to methane ratios were used (O2/CH4: 1.5, 1, 0.5, 0.2, and
0.1). The results of LaCeO3.5-based samples are shown in Fig. 4.
As expected, the CO selectivity on the un-promoted LaCeO3
increased from 9.5% to 44.7% with decreasing O2 : CH4 ratio. In
comparison, CO selectivity is 98.2% in the O2-free redox mode.
This indicates that surface/loosely bound oxygen species are
likely to be responsible for methane combustion. In addition,
the redox reaction is likely to be limited by bulk lattice oxygen
conduction as evidenced by signicantly higher methane
conversion in the presence of gaseous oxygen. The immediate
shi in selectivity observed on the un-promoted samples in the
redox mode indicates that the formation of non-selective
surface oxygen species is inhibited in the absence of gaseous
oxygen. Similar trends are observed with the CaMnO3-based
samples (Fig. S6†).
Fig. 3 Oxygen extracted from the redox catalysts during the initial
stages of the reduction at 900 ꢀC in 10% methane.
Rh-promoted catalysts show notably different results.
Methane conversion increases with oxygen concentrations and
plateaus around 70% at O2 : CH4 ratios above 1 : 1. Higher
methane conversion is observed at low O2 : CH4 ratios
compared to the un-promoted sample. This indicates that the
presence of reduced Rh on the surface catalyzes methane acti-
vation. The promoted sample also showed improved CO selec-
tivity, especially at lower O2 : CH4 ratios (Fig. S7a and S8a†).
These observations illustrate that the presence of Rh on the
surface substantially changes the methane activation rate and
mechanism. This is also corroborated by the methane isotope
exchange studies below.
thermodynamic properties. This is not likely due to the very
similar crystal structures of the samples (Table S1†). The other
possible explanation is the kinetic effect as the presence of Rh
on the surface can improve methane activation. Enhancements
in surface reaction rates can in turn accelerate the rates of
lattice oxygen removal and increase oxygen carrying capacity of
the redox catalysts under the 15 min reduction half cycle.
To shed light on the role of Rh, XPS experiments were per-
formed. Table 2 summarizes the near surface concentrations of
the elements in both oxidized and reduced forms aer ve
complete redox cycles at 900 ꢀC. The near surface concentration of
Rh on LaCeO3.5 and CaMnO3 remained 3–5 times higher than the
nominal amount of Rh added. This conrms that rhodium is
enriched on the surface. Hence, improving the surface methane
activation is likely to be the primary reason for the enhanced
redox performance. Due to the low surface areas of the samples
(0.29 and 0.39 m2 gꢁ1 for the as-prepared and cycled Rh-promoted
CaMnO3), it is difficult to obtain reliable data on Rh dispersion on
the redox catalysts using chemisorption-based approaches.
Methane isotope exchange
Isotope exchange experiments were performed to investigate
methane activation on the promoted and un-promoted
samples. An equimolar mixture of CH4 and CD4 (25% each,
balance Ar) was injected to the redox catalysts at elevated
temperatures. The ratios of CHD3 (m/z ¼ 19) and CD4 (m/z ¼ 20)
signals are used to quantify the degree of methane activation on
redox catalysts. These two mass/charge signals are chosen as
they do not have partial contributions from the other exchange
Methane conversion in the presence of gaseous oxygen
To determine the effect of Rh on surface methane activation, products. Fig. 5 summarizes the results of the rst 10 pulse
methane–oxygen co-feed experiments are conducted. Five injections at 800 ꢀC. It is evident that the un-promoted and Rh-
Table 2 Near surface atomic elemental composition of the redox catalysts in the reduced and oxidized state after 5 methane/oxygen cycles
based on XPS analysis
Average based
Element
on Rh-loading
Un-promoted-Ox
Rh-promoted Ox
Un-promoted Red
Rh-promoted Red
LaCeO3.5
Ce
La
Rh
O
18.13
18.13
0.30
5.27
12.65
0.00
5.62
11.58
1.52
3.49
14.16
0.00
4.26
12.26
1.18
63.45
82.08
81.28
82.35
82.31
CaMnO3
Mn
Ca
19.97
19.97
0.14
8.97
23.52
0.00
8.66
23.88
0.43
5.73
24.23
0.00
5.98
23.07
0.67
Rh
O
59.92
67.51
67.03
70.03
70.27
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Fig. 4 Comparison of (a) CO selectivity and (b) methane conversion during methane–oxygen co-feed experiments using un-promoted and Rh-
promoted LaCeO3.5-based redox catalysts at 900 ꢀC.
comparison, also forms carbonyl species (1980–2110 cmꢁ1).
ꢀ
These carbonyl groups are stable until above 200 C and give
rise to gaseous CO2 species (2300–2400 cmꢁ1) at higher
temperatures.
Transient pulse experiments
Pulse experiments were performed to investigate the effect of
bulk oxygen conduction and surface evolution on the conver-
sion and selectivity of the catalysts. Two different relaxation
times of 30 seconds and 2 minutes were used between the 20
methane (50 vol% balance Ar) pulses injected. The relaxation
allows identication of potential O2ꢁ conduction limitations.22
Fig. 5 Comparison of methane isotope exchange on the redox
catalysts at 800 ꢀC.
Fig. 6 and 7 show the CO selectivity and oxygen removal at 900
ꢀC during each pulse for CaMnO3-based and LaCeO3.5-based
redox catalysts respectively. The un-promoted CaMnO3 sample
demonstrates non-selective behavior regardless of the relaxa-
tion time. This indicates a faster O2ꢁ supply from the bulk when
promoted samples behave signicantly different. While the
methane exchange remains relatively constant on the un-
compared to the rate of oxygen removal from the surface. The
promoted samples during 10 pulse injections, it increased by up
Rh-promoted sample, however, shows a transition to selective
to 400% on the Rh-promoted samples, especially during the
rst 5 pulses. This indicates that reduced Rh signicantly
enhances methane activation and surface coverage of interme-
partial oxidation aer the rst few pulses. This transition is
consistent with the generation of reduced Rh on the surface.
Pulse injection on the LaCeO3.5-based redox catalysts clearly
diate species from methane dissociation. The presence of
shows that a higher relaxation time leads to a less selective
metallic Rh on the surface is further conrmed by DRIFTS with
behavior on the un-promoted sample. Signicantly less oxygen
CO temperature programmed desorption. The un-promoted
extraction and methane conversion were also observed during
and Rh-promoted LaCeO3.5 samples showed markedly different
IR-spectra for adsorbed CO species (Fig. S9†). The unpromoted
the pulse injection on the un-promoted catalyst (Fig. 7). This is
consistent with the poor methane activation and slow overall
kinetics of the LaCeO3 redox catalyst combined with low
methane concentration in the pulse injection conguration.
sample only shows the formation of carbonate species (1300–
1600 cmꢁ1) at room temperature. The Rh-promoted sample, in
Fig. 6 Comparison of CO selectivity at each methane pulse injection on (a) CaMnO3-based and (b) LaCeO3.5-based redox catalysts at 900 ꢀC.
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Fig. 7 Comparison of cumulative oxygen donation during 20 methane pulse injections on (a) CaMnO3-based and (b) LaCeO3.5-based redox
catalysts at 900 ꢀC.
Further analysis of the pulses shows that at a similar oxygen catalysts enter the selective region much faster than unpro-
content (aer the 5th pulse with 2 min relaxation and 10th pulse moted redox catalysts (Fig. 6).
with 30 s relaxation), >250% more oxygen atoms are removed
from LaCeO3.5 in each pulse under a longer relaxation time,
suggesting that O2ꢁ diffusion is rate limiting. Lower CO selec-
tivity aer longer relaxation suggests that the non-selective
pathway is more dependent on the type and concentration of
oxygen species at the surface. Assuming that the injected
methane pulse completely depletes the accessible surface
oxygen species, the average rate of oxygen conduction to the
surface is calculated to be 2.38 ꢃ 10ꢁ2 and 1.52 ꢃ 10ꢁ2 mmol sꢁ1
during the rst 30 seconds and two minutes respectively. These
values are an order of magnitude lower than the initial rate of
oxygen release from LaCeO3.5 in a redox mode (0.343 mmol sꢁ1).
This also explains the quick shi to a selective pathway at the
initial stage of the LaCeO3.5 reduction in methane, i.e. non-
selective surface oxygen species are consumed quickly due to
the slow O2ꢁ replenishment from the bulk.
The presence of Rh signicantly enhances the performance
of the redox catalyst. Not only does it increase the methane
conversion, it also keeps the catalyst selective aer the rst
pulse. Based on isotope exchange results, Rh on the surface for
both redox catalysts increases the number of activated methane
reaction intermediates. Such reaction intermediates are likely
to be highly effective for oxygen removal from the surface. The
oxygen-depleted surface in turn enhances O2ꢁ conduction from
the bulk as evident in Fig. 7. As a result, Rh promoted redox
Effect of reaction temperature on redox properties
To further illustrate the effect of Rh, redox experiments were
performed at lower temperatures (Fig. 8). Product proles are
provided in Fig. S10.† As expected, the CaMnO3 redox catalyst
exhibits lower activity for methane conversion at lower
temperatures. A signicant drop in CO selectivity is also
observed (Table 3). At lower temperatures, slower oxygen
extraction may be expected to contribute to a higher POx
selectivity. However, CaMnO3, which is a known low tempera-
ture chemical looping combustion material,7,39 appears to acti-
vate methane for deep oxidation until the surface and loosely
bound oxygen is depleted. Aer that, the redox catalyst doesn't
show any activity. This implies that the surface of the catalyst
cannot be replenished with bulk lattice oxygen either because of
the O2ꢁ diffusivity at these temperatures, or smaller driving
force for O2ꢁ conduction.
Redox experiments on the Rh-promoted samples showed
more signicant effects at lower temperatures (Fig. 8). As shown
in Table 3, unlike the un-promoted CaMnO3, the CO selectivity
of the promoted samples meaningfully increases with
decreasing temperature. Decreasing the temperature
suppresses the initial CO2 formation and elongates the selective
region (Fig. S11†). While the un-promoted CaMnO3 sample did
not show noticeable activity at temperatures lower than 700 ꢀC,
Fig. 8 Comparison of the CO selectivity and maximum oxygen donation rate of the (a) CaMnO3-based and (b) LaCeO3.5-based redox catalysts at
different temperatures.
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Table 3 Methane conversion, CO/H2 selectivity, oxygen donation, and coke formation of the CaMnO3-based redox catalysts during the
reduction half cycle
H2 selectivity%
(excluding coke)
H2 selectivity%
(including coke)
CH4 converted
(ml)
O2 extracted
(wt%)
Coke formation
(wt%)
Temperature
Un-promoted
CO selectivity%
900
800
700
900
800
700
600
500
40.08
26.85
12.28
44.93
48.23
56.63
86.10
87.90
29.34
15.71
10.94
40.30
58.49
74.70
97.14
100.00
37.18
20.73
13.24
46.47
62.54
76.64
97.17
100.00
2.13
1.80
0.80
2.59
2.57
2.86
3.68
1.39
9.18
8.79
4.14
10.17
8.62
7.93
6.29
2.02
0.29
0.12
0.02
0.32
0.30
0.25
0.04
0.00
Rh-promoted
Table 4 Methane conversion, CO/H2 selectivity, oxygen donation, and coke formation of the LaCeO3.5-based redox catalysts during the
reduction half cycle (the selectivities of un-promoted samples at 800 and 700 ꢀC are not very accurate due to low signal to noise ratios)
H2 selectivity%
(excluding coke)
H2 selectivity%
(including coke)
CH4 converted
(ml)
O2 extracted
(wt%)
Coke formation
(wt%)
Temperature
Un-promoted
CO selectivity%
900
800
700
900
800
700
98.21
62.10
52.93
99.46
99.60
99.34
86.76
59.16
36.50
90.11
100.00
100.00
87.69
63.66
44.43
91.26
100.00
100.00
2.13
0.32
0.31
2.54
2.10
1.85
3.91
0.99
1.2
4.36
2.60
1.94
0.17
0.02
0.03
0.36
0.21
0.19
Rh-promoted
the Rh-promoted sample exhibits high methane conversion and catalysts with the Arrhenius equation, the apparent activation
selectivity at 600 ꢀC. The redox catalyst remains active and energies for redox experiments are obtained. In the pulse
selective at 500 ꢀC (Fig. S12†). At lower than 500 ꢀC, slow injection experiments, the reaction rates are quantied by the
kinetics hindered the reaction and reduced methane conver- total oxygen removal or methane conversion during the rst
sion. The lower reaction temperature and less reduced surface pulse. These activation energies are summarized in Table 5.
lead to less coke formation. The above results show that at 700
ꢀC and lower, the presence of Rh signicantly increases the effect on the apparent activation energies in pulse experiments.
oxygen release of CaMnO3. While the apparent activation energy reduces by about 33–50%
Table 5 clearly indicates that Rh has a more pronounced
A similar trend is observed on the LaCeO3.5-based samples in redox experiments, apparent activation energies derived from
with more signicant kinetic improvement (Fig. S13†). While pulse experiments decrease by more than 95%. This strongly
the un-promoted samples show very little activity at 800 ꢀC and indicates that Rh addition signicantly increases the activity for
ꢀ
below, the promoted samples remained active down to 600 C surface oxygen removal, by way of enhanced C–H bond activa-
with the initial oxygen release rate higher than that of the un- tion. The increase in the overall redox activity of the metal
ꢀ
promoted sample at 900 C (Fig. 8b and Table 4). This, again, oxides likely results from the enhanced surface oxygen removal.
conrms that providing methane activation sites on the surface This indicates that the higher oxygen extraction rate is a direct
allows for more oxygen extraction from the redox catalysts even result of a larger driving force for lattice oxygen removal, i.e.
at lower temperatures. It is concluded that activation of the C–H a higher oxygen chemical potential gradient between the bulk
bond enhances oxygen removal from the oxide surface.
and the surface of the redox catalyst.
In order to differentiate the bulk and surface effects of
rhodium on the redox performance of the catalysts, apparent
activation energies of the lattice oxygen removal reaction and
methane activation reaction are calculated for both continuous
redox and pulse experiments. To do so, pulse experiments are
conducted on CaMnO3-based redox catalysts at 700, 800, and
900 ꢀC (Table S3†). While the removal of surface oxygen is
dominant in the transient pulse experiments, the continuous
redox reactions can be affected by both bulk thermodynamics
and surface properties of the catalyst. Comparing the effects of
Rh on activation energies in these two reaction modes enables
us to infer the primary role of rhodium. By tting the maximum
oxygen donation or methane conversion rate of the redox
Table 5 Activation energies of the CaMnO3-based catalysts in both
redox and pulse injection configurations
Activation energy (kJ molꢁ1
)
Redox
(bulk + surface)
Pulse (surface)
Sample
O2ꢁ
CH4
O2ꢁ
CH4
Un-promoted
Rh-promoted
110.3
72.9
108.9
53.9
115.9
4.5
110.9
4.9
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increased oxygen donation and selectivity were observed for Rh
promoted redox catalysts at temperatures as low as 500 ꢀC.
When compared to unpromoted CaMnO3 which demonstrates
2.4 mmol gꢁ1 syngas productivity at 900 C, the Rh-promoted
ꢀ
CaMnO3 produces syngas at 7.9 mmol gꢁ1 at 600 ꢀC with
satisfactory long-term redox performance.
Acknowledgements
This work was supported by the U.S. National Science Foun-
dation (Award No. CBET-1254351) and Kenan Institute. The
authors acknowledge the use of the Analytical Instrumentation
Facility (AIF) at North Carolina State University, which is sup-
ported by the State of North Carolina and the National Science
Foundation. The authors also would like to acknowledge the
assistance from Dr Tianyang Li in XRD and Dr Fang He and Kun
Zhao at the Guangzhou Institute of Energy Conversion, Chinese
Academy of Sciences for XPS analysis.
Fig. 9 Selectivity, converted methane, and O2 extracted during 100
redox cycles at 600 ꢀC on the 0.5 wt% Rh-promoted CaMnO3.
Long term redox experiments
The aforementioned ndings indicate that signicantly higher
syngas selectivity, lower coke formation and lower operating
temperatures can be achieved by surface modication of the
redox catalysts through addition of platinum group metals on
their surfaces. To validate the long term performance of such
redox catalysts, Rh-promoted CaMnO3 is tested for extended
redox cycles at 600 ꢀC. Fig. 9 shows the selectivity and methane
conversion of the catalyst through 100 complete redox cycles.
The redox catalyst exhibits good redox activity throughout the
100 redox cycles. Syngas selectivity was maintained at above
94% throughout the 100 cycles. A slight decrease in syngas
productivity and oxygen carrying capacity was observed, which
may have resulted from sintering the redox catalyst. At the 100th
cycle, the sample exhibited a syngas productivity of 7.9 mmol
syngas per gram redox catalyst at 600 ꢀC. To compare, a recently
reported perovskite based redox catalyst exhibits 4.8 mmol gꢁ1
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