ECN-RX--05-050
REFORMING OF METHANE OVER
CIRIA AND PEROVSKITE-BASED
CATALYSTS
Subtitle
J. Boon
F.P.F. van Berkel
E.R. Stobbe
Posterpresentatie
15th World Hydrogen Energy Conference
June 27-July 2, 2004
Yokohama, Japan
JANUARY 2005
Reforming of Methane over Ceria and Perovskite-Based Catalysts
Jurriaan Boon, Frans P.F. van Berkel, Erwin R. Stobbe
Unit Fuel Cell Technology
Energy Research Centre of the Netherlands
P.O. Box 1, 1755ZG Petten
THE NETHERLANDS
E-mail: boon@ecn.nl
Abstract
The influence of the nature of the support on the activity of rhodium-based catalysts in the
catalytic reforming of methane has been studied. Based on the variety of applications of
methane reforming a selection of supports was made including fluorites, perovskites and
aluminum oxide. In the absence of rhodium, the perovskite and fluorite supports, i.e.
SrFe0.67Co0.33O3+•, La0.6Sr0.4Co0.2Fe0.8O3+•, Ce0.8Gd0.2O1.9+•, and CeO2-ZrO2 all showed
activity for the total combustion of methane but proved to be poor reforming catalysts.
Impregnation of the fluorite and alumina supports with rhodium provided active catalysts
for the reforming of methane. The activity of perovskite-supported rhodium appeared to
depend strongly on the reducibility of the perovskite. Rhodium supported on
La0.77Sr0.13MnO3+• is a reasonably active catalyst, but it deactivates rapidly. Rhodium
supported on mixed valence oxides SrFe0.67Co0.33O3+• or La0.6Sr0.4Co0.2Fe0.8O3+• is hardly
more active than the bare support. Application of other metals active in reforming such as
platinum, ruthenium, nickel, or palladium on La0.6Sr0.4Co0.2Fe0.8O3+• lead to surprisingly
low catalytic activity. For supported rhodium catalysts a relation is suggested between
reducibility of the support materials and catalytic activity for the reforming of methane,
which is presently under further investigation.
Keywords: methane catalytic partial oxidation, methane steam reforming, ceria,
perovskite, rhodium-based catalysts
1. Introduction
A significant part of the fuel cell and membrane research activities at ECN (Energy
Research Centre of the Netherlands) involves the reforming of fossil fuels and related
hydrocarbons. These research areas include the production of nitrogen-free synthesis gas
by means of oxygen transport membranes, fuel processing to provide clean hydrogen for
fuel cell applications, and internal reforming of hydrocarbons in solid oxide fuel cells
(SOFCs).
Reforming is used here as a comprehensive term, referring to the conversion of
hydrocarbons with oxygen, steam or carbon dioxide to synthesis gas. The conversion of
hydrocarbons to synthesis gas comprises a network of reactions. For methane, several
reactions can be involved, i.e. the direct catalytic partial oxidation (1), complete
combustion (2), methane steam reforming (3) and CO2 reforming (4) followed by the
water-gas shift reaction (5).
2CH4 + O2 • 2CO + 4H2 (CPO)
(1)
CH4 + 2O2 • CO2 + 2H2O (‘total combustion’)
(2)
CH4 + H2O • CO + 3H2
(SR)
(3)
CH4 + CO2 • 2CO + 2H2 (CDR)
(4)
CO + H2O • CO2 + H2
(WGS)
(5)
CPO
SR
100%
conversion CH4 (%)
conversion CH4 (%)
100%
80%
60%
40%
20%
0%
80%
thermodynamics
LSCF
CZ
CGO
SFC
60%
40%
20%
0%
300
400
500
600
700
temperature (°C)
800
900
300
400
500
600
700
800
900
temperature (°C)
Figure 1. Methane conversion for CPO and SR over bare carriers
With respect to methane CPO there is still debate on the question whether syngas is formed
by the direct reaction (1) or through a sequence of combustion (2) and reforming reactions
(3) and (4).
The partial oxidation reaction is catalysed by supported noble metals, [1-4] nickel, [2-7]
cobalt, [7] iron, [7] ceria, [4] pyrochlores [2] and perovskites. [2] Many metals, including
nickel, cobalt, iron, and group VIII metals, are known to catalyse steam reforming (SR).
[4] More recently, it has been reported that gadolinia-doped ceria [8] and nickel-containing
perovskite [9] are active as SR catalysts. CDR is catalysed by iron and cobalt, [3,4] as well
as nickel and noble metals. [3,4,10]
While many different substrates can be used as catalyst support, potential applications in
the field of oxygen transport membranes and internal reforming in SOFCs would clearly
benefit from the direct use of oxygen permeable support materials as the catalyst support.
[6,8,11-14] Moreover, oxide-conducting supports have been associated with both low
carbon formation under typical process conditions [6,8,13,15-17] and high activity
attributed to strong metal-support interactions. [1]
The present paper aims at evaluating the influence of support materials like ceria and
perovskite materials on the catalytic behaviour of rhodium catalysts for the reforming of
methane.
Table 1. Composition, supplier and BET-surface of the selected support materials
Support material
BET-surface area
Supplier
2
[m /gr]
Perovskites
SrFe0.67Co0.33O3+• (SFC)
<1
Praxair Special Ceramics
La0.6Sr0.4Co0.2Fe0.8O3+• (LSCF)
6.4
Praxair Special Ceramics
La0.80Ca0.13Cr0.97O3+• (LCC)
not measured Praxair Special Ceramics
La0.77Sr0.13MnO3+• (LSM)
6.9
Praxair Special Ceramics
Fluorites
Ce0.8Gd0.2O1.9+• (CGO)
39.1
Praxair Special Ceramics
CeO2/ZrO2 (30%wt./70%wt.) (CZ)
61
ECN
Alumina
•-Al2O3 (AL)
4–5
Nabaltek
2. Experimental
2.1 Catalyst Preparation
CO - thermodyn.
100%
carbon selectivity (%)
CO2
80%
CO (thermo)
carbon monoxide
CO2 (thermo)
carbon dioxide
60%
40%
20%
CO
0%
300
CO2 - thermodyn.
400
500
600
700
800
900
temperature (°C)
Figure 2. Carbon selectivity (moles of CO or CO2 produced / moles of CH4 converted)
towards CO and CO2 for CPO over bare LSCF
The different support materials have been summarised in Table 1. Rhodium catalysts were
prepared by incipient wetness impregnation of the support material with Rh(NO3)3 solution
(Alfa Aesar), resulting in a final content of 1%wt Rh. After drying, the catalysts were
calcined at 400°C. In addition a series of LSCF-supported metal catalyst was prepared
using the same procedure as above with metal salts, i.e. Pt (Pt(NO3)2, Merck), Ru
(Ru(NO)(NO3)x(OH)y, Aldrich), Ni (Ni(NO3)2, Merck), Pd(Pd(NO3)2, Aldrich), each
yielding a metal loading of 1 %wt. All catalysts were tested in a sieve fraction of 0.2120.425 mm.
2.2 Reforming experiments
Experiments were carried out in a microflow reactor setup containing a quartz reactor tube
of 4 mm internal diameter. Nitrogen-diluted gas feeds were applied at a total flow of 100
ml/min. The reactor was filled with 100 mg of the respective catalyst to a height of several
millimetres. The outlet gas composition was measured online with an Advance Optima
Multi-gas analyser set detecting CH4, CO2, CO (URAS14 module) and H2 (Caldos17
module) and an online Micro-GC system equipped Poraplot and Molsieve 5A columns
detecting CO2, C2H4, C2H6, C3H6, C3H8, CH4, and CO.
HSC chemistry software package version 5.11 [18] was used for thermodynamic
calculations. For these calculations the formation of coke was suppressed, assuming coke
free operation during the experiments.
Catalytic partial oxidation of methane
CPO experiments were conducted in 2 ml/min of methane, 5 ml/min of air, and balance
nitrogen (to 100 ml/min). The reaction was started at 300°C after which the temperature
was raised up to 900°C and again lowered down to 300°C, in steps of 40°C. During every
step, the reaction system was allowed to stabilise for about 25 minutes before the outlet gas
composition was sampled.
Steam reforming of methane
SR experiments were conducted in 5 ml/min of methane, 5 ml/min of steam, and balance
nitrogen (to 100 ml/min). The reaction was started at 400°C after which the temperature
was raised up to 925°C and again lowered down to 400°C, in steps of 35°C. During every
step, the reaction system was allowed to stabilise for about 25 minutes before the outlet gas
conversion CH4 (%)
100%
80%
CGO, CZ, and AL with 1%wt. Rh
60%
40%
20%
pure carriers
thermodynamics
CGO
C-Z
Rh/CGO
Rh/CZ
Rh/AL
0%
300
400
500
600
700
800
900
temperature (°C)
Conversion
thermodyn.
CGO
CZ
Rh/CGO
Rh/CZ
Rh/AL
300°C (up)
0%
0%
29%
31%
5%
300°C (dn)
31%
0%
0%
3%
11%
1%
625°C (up)
30%
27%
96%
95%
95%
625°C (dn)
94%
23%
26%
93%
95%
95%
900°C (up)
38%
42%
100%
100%
100%
900°C (dn)
100%
38%
42%
100%
100%
100%
Figure 3. Conversion of methane for CPO over bare carriers CGO and CZ and over Rh/CGO,
Rh/CZ, and Rh/AL
composition was sampled.
3. Results
3.1 Activity of metal oxides for the reforming of methane (CPO and SR)
Activity plots of the various bare oxide supports (not impregnated with rhodium) are
shown in Figure 1. For CPO conditions, a methane conversion level of approximately 25%
is reached for temperatures below 750°C, with only total combustion products carbon
dioxide and steam appearing. Above approximately 750°C, reforming starts to take place,
converting the remaining unconverted methane with carbon dioxide and steam to carbon
monoxide and hydrogen. The selectivity plot of CPO over bare LSCF in Figure 2, which is
also typical for other the carrier oxides examined, confirms this hypothesis. The selectivity
towards carbon dioxide decreases in favour of the selectivity towards carbon monoxide
only when the temperature is further increased above 750°C.
As expected from these CPO experiments, the results for SR (also shown in Figure 1) show
that bare oxide supports are only able to catalyse the steam reforming reaction (SR) above
approximately 750°C.
3.2 Effect of the support on Rh catalyst performance for methane reforming (CPO)
Activity plots of the conversion of methane under CPO conditions using oxide supports
impregnated with rhodium as catalyst are shown in Figure 3 for the fluorite and alumina
support materials and in Figure 4 for the perovskite support materials.
Rhodium supported on either CGO, CZ, or AL is an active catalyst for the reforming of
methane, reaching thermodynamic equilibrium already at temperatures as low as 400°C.
For the perovskites, the situation is more complex. Rh/SFC showed to be a poor catalyst.
Its behaviour resembles the behaviour of bare carriers and the addition of rhodium only
results in a small improvement in methane conversion. Rh/LSCF performs slightly better
during heating. However, even for temperatures as high as 900°C thermodynamic
conversion of methane is not reached and upon the subsequent decrease in temperature,
conversion CH4 (%)
100%
← Rh/LCC
→ Rh/LSM
80%
thermodynamics
Rh/LSCF
Rh/SFC
Rh/LCC
Rh/LSM
60%
40%
20%
Rh/LCC →
0%
300
400
500
600
700
800
900
temperature (°C)
Conversion
thermodyn.
Rh/LSCF
Rh/SFC
Rh/LCC
Rh/LSM
300°C (up)
0%
0%
0%
5%
300°C (dn)
31%
0%
0%
0%
0%
625°C (up)
47%
12%
0%
93%
625°C (dn)
94%
25%
10%
94%
41%
900°C (up)
91%
42%
100%
94%
900°C (dn)
100%
90%
42%
100%
93%
Figure 4. Conversion of methane for CPO for Rh/perovskite catalysts. All catalysts except for
Rh/LCC are more active (i.e., a higher conversion) during the initial increase in temperature
than during the subsequent decrease in temperature.
Rh/LSCF is only slightly more active than bare LSCF. The only perovskites-supported
rhodium catalysts that reach thermodynamic equilibrium are Rh/LSM and Rh/LCC.
Rh/LSM is readily activated from 400°C upwards; the conversion of methane follows the
thermodynamic equilibrium conversion. However, deactivation is observed from 650°C
upwards and during the decrease in temperature. The activity of Rh/LCC remains poor
until apparent activation occurs around 900°C. During cooling the methane conversion
over Rh/LCC matches thermodynamic equilibrium in the temperature region of 600°C till
900°C.
3.3 Effect of the supported metal on LSCF catalyst performance for the reforming of
methane
Since the poor performance of Rh/LSCF might be caused by a specific interaction between
rhodium and the perovskite support, the interaction between several alternative metals and
the LSCF perovskite was examined. The methane conversion for CPO over Rh/LSCF,
Pt/LSCF, Ru/LSCF, Ni/LSCF, and Pd/LSCF is shown in Figure 5. No improvement is
made upon replacement of rhodium by other group VIII metals, as Rh/LSCF is most active
during the increase in temperature and all catalysts perform equally during the decrease in
temperature.
4. Discussion
From CPO experiments it is clear that all of the bare oxide supports are active in the
combustion of methane between approximately 500°C and 800°C. In this range full
oxygen conversion is achieved with CO2 and H2O being the main reaction products.
However, their activity in reforming is low. CPO and SR experiments over these bare
oxides show some reforming activity (SR and/or CDR) only at temperatures above 750°C,
conversion CH4 (%)
100%
80%
thermodynamics
Rh/LSCF
Pt/LSCF
Ru/LSCF
Ni/LSCF
Pd/LSCF
60%
40%
20%
0%
300
400
500
600
700
800
900
temperature (°C)
Conversion
thermodyn.
Rh/LSCF
Pt/LSCF
Ru/LSCF
Ni/LSCF
Pd/LSCF
300°C (up)
0%
0%
0%
0%
0%
300°C (dn)
31%
0%
0%
0%
0%
0%
625°C (up)
47%
36%
27%
25%
40%
625°C (dn)
94%
25%
24%
24%
21%
28%
900°C (up)
91%
67%
84%
73%
80%
900°C (dn)
100%
90%
66%
83%
72%
78%
Figure 5. Conversion of methane for CPO over Rh, Pt, Ru, Ni, and Pd supported on LSCF.
All catalysts are more active (i.e., a higher conversion) during the initial increase in
temperature than during the subsequent decrease in temperature.
however never reaching thermodynamic equilibrium. The activity for total combustion of
methane of the oxide supports manifested in this study is well known in literature. [19-21]
Whereas the bare oxide supports display similar behaviour as catalyst for CPO and SR
reactions, remarkable differences in catalyst performance appear when rhodium is applied
onto these supports. Impregnation of AL, CGO, and CZ with rhodium yields highly active
CPO and SR catalysts. The conversion of methane reaches the thermodynamic equilibrium
conversion at temperatures as low as 350°C, which is in accordance with literature data of
Rossignol et al. [1] who have proven both Rh/CGO and Rh/AL to be active catalysts for
CPO of methane. Rhodium supported on LSM provides a catalyst that rapidly deactivates
and rhodium on LCC apparently needs activation at elevated temperatures before catalytic
activity is observed. Finally, the activity of LSCF and SFC is only slightly improved by the
addition of rhodium.
In literature, it is known that rhodium supported on irreducible oxides provides successful
reforming catalysts. Ruckenstein and Wang [2] showed that rhodium supported on
reducible oxides is a poor catalyst for reforming methane, especially when compared to
rhodium supported on irreducible oxides. This might be due to the formation of suboxides
that migrate over the exposed metal surface, thereby decreasing the number of active metal
sites. Also, the catalytic activity for methane reforming of nickel and ruthenium supported
on irreducible perovskites has been demonstrated. [6,12,13]
Rhodium supported on reducible mixed valence perovskite oxides (B = Fe, Co in ABO3) is
a poor catalyst for methane reforming, so apparently the reducibility of the support has a
strong influence on the activity for methane reforming. Figure 6 shows the reducibility of
the carrier materials under investigation (or materials that are chemically similar)
expressed as the oxygen nonstoichiometry as a function of the oxygen partial pressure at
800°C. (La,Ca)CrO3+•, (La,Sr)MnO3+•, and (Ce,Gd)O1.9+• become oxygen deficient only at
very low oxygen partial pressures (p(O2) below 10-14 bar). On the other hand,
(La,Sr)(Fe,Co)O3+• perovskites are oxygen deficient already at atmospheric oxygen partial
pressure and their oxygen deficiency increases rapidly when the oxygen partial pressure is
lowered. The former, less easily reducible oxides, seem to show better activity for methane
reforming compared to the latter. Further research is presently carried out to elucidate the
nature of the relation between reducibility and catalytic activity.
These findings may have strong implications for the intended application of the different
materials. (La,Sr)(Co,Fe)O3+•, often used as oxygen transport membrane, does not seem to
be attractive as catalyst support for the production of synthesis gas. Rhodium supported on
LCC may prove to be a reforming catalyst in the anode compartment of SOFCs. Finally,
ceria based materials are of interest for internal reforming in SOFCs and for fuel
processing applications. It might also perform well in combination with oxygen transport
membranes.
5. Conclusion
Catalytic activity of rhodium based catalysts for catalytic partial oxidation and steam
reforming of methane depends strongly on the nature of the support. Bare oxidic support
materials SrFe0.67Co0.33O3+•, La0.6Sr0.4Co0.2Fe0.8O3+•, Ce0.8Gd0.2O1.9+•, and CeO2-ZrO2 have
been tested and shown to catalyse the total combustion reaction of methane to carbon
dioxide and steam.
Rhodium supported on •-Al2O3, CeO2-ZrO2, and Ce0.8Gd0.2O1.9+• yields active catalysts for
the reforming of methane. Both Rh/La0.77Sr0.13MnO3+• and Rh/La0.80Ca0.13Cr0.97O3+• show
significant potential for catalysis of the reforming of methane. When rhodium is supported
on SrFe0.67Co0.33O3+• or La0.6Sr0.4Co0.2Fe0.8O3+•, the catalytic activity is only slightly
improved when compared to the bare oxides. The poor catalytic activity for methane
reforming over La0.6Sr0.4Co0.2Fe0.8O3+•-supported rhodium catalysts seems not to be
specifically related to the rhodium-perovskite interaction since application of platinum,
ruthenium, nickel, or palladium onto La0.6Sr0.4Co0.2Fe0.8O3+• leads to a similarly poor
increase of the reforming activity as well.
The reforming activity of rhodium catalysts, and possibly of other (noble) metal catalysts
as well, seems to be inhibited by reducible support materials, a phenomenon on which
further research is presently taking place.
La0.9Ca0.1CrO3+δ (X)
0
δ [-]
Ce0.8Gd0.2O1.9+δ (o)
La0.6Sr0.4Co0.2Fe0.8O3+δ (n)
-0.2
La0.8Sr0.2MnO3+δ ( )
-0.4
La0.2Sr0.8Co0.8Fe0.2O3+δ (¡)
-0.6
-25
-20
-15
-10
-5
0
log( p(O2 ) / p0 ) [-]
Figure 6. Oxygen nonstoichiometry, expressed as the value of • for (La,Ca)CrO3+•, (Ce,
Gd)O1.9+•, (La,Sr)MnO3+•, and (La,Sr)(Co,Fe)O3+• as a function of the oxygen pressure (p0 =
105 Pa) at 800°C [22-25]
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