A novel perovskite based catalyst with high selectivity and activity for partial oxidation of methane for fuel cell applications

Article abstract of DOI:10.1039/c4dt01288g

Solid oxide fuel cells (SOFCs) have the potential to revolutionise the present fuel economy due to their higher fuel conversion efficiency compared with standard heat engines and the possibility of utilizing the heat produced in a combined heat and power system. One of the reasons they have yet to fulfil this potential is that the conventional anode material of choice, a nickel/yttria-stabilised zirconia cermet, requires a high temperature production process and under operating conditions is susceptible to carbon and sulphur poisoning. Perovskite-based materials have been proposed as potential anode materials for SOFCs due to their potentially high electronic conductivity and catalytic properties. One of the problems in realizing this potential has been their low catalytic activity towards methane reforming compared to conventional nickel based cermet materials. A nickel doped strontium zirconate material produced by low temperature hydrothermal synthesis is described which has high activity for methane reforming and high selectivity towards partial oxidation of methane as opposed to total oxidation products. Initial studies show a very low level of carbon formation which does not increase over time.

Full text of DOI:10.1039/c4dt01288g

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A novel perovskite based catalyst with high
selectivity and activity for partial oxidation of
methane for fuel cell applications
Cite this: Dalton Trans., 2014, 43,
15022
J. Staniforth,* S. E. Evans, O. J. Good, R. J. Darton and R. M. Ormerod
Solid oxide fuel cells (SOFCs) have the potential to revolutionise the present fuel economy due to their
higher fuel conversion efficiency compared with standard heat engines and the possibility of utilizing the
heat produced in a combined heat and power system. One of the reasons they have yet to fulfil this
potential is that the conventional anode material of choice, a nickel/yttria-stabilised zirconia cermet,
requires a high temperature production process and under operating conditions is susceptible to carbon
and sulphur poisoning. Perovskite-based materials have been proposed as potential anode materials for
SOFCs due to their potentially high electronic conductivity and catalytic properties. One of the problems
in realizing this potential has been their low catalytic activity towards methane reforming compared to
conventional nickel based cermet materials. A nickel doped strontium zirconate material produced by low
temperature hydrothermal synthesis is described which has high activity for methane reforming and high
selectivity towards partial oxidation of methane as opposed to total oxidation products. Initial studies
show a very low level of carbon formation which does not increase over time.
Received 30th April 2014,
Accepted 18th July 2014
DOI: 10.1039/c4dt01288g
www.rsc.org/dalton
use within solid oxide fuel cells.14–16 Perovskites have the
general formula ABO3, with A and B being cations whose
Introduction
Despite their greater efficiency in converting fuel into electri- charges must sum to +6 to offset the −6 from the three
city1 fuel cells have so far only found niche applications. This oxygens within the structure.17 This gives a wide range of
is due to many factors but for most types of fuel cell a major almost limitless possibilities especially when taking account of
stumbling block to commercialization has been the need to the fact that the A and B sites can be occupied by a variety of
use hydrogen as a fuel.2 Solid oxide fuel cells (SOFCs) over- cations with multiple valencies. The challenge is to find a per-
come this difficulty and are able to utilize a wide variety of ovskite with the right properties for use as an anode material,
carbon based fuels.3–6 Unfortunately the high operating temp- which is catalytically active and selective for methane reform-
eratures of SOFCs, whilst allowing for internal fuel reforming, ing but with limited catalytic activity towards the carbon
means that they have a propensity to suffer from deleterious forming reactions.
carbon deposition.7,8 Carbon deposition can result in either
The initial step in utilizing a carbon based fuel such as
direct poisoning of the catalytic process within the fuel cell methane within a fuel cell is to reform the fuel into a mixture
through blocking of active nickel sites or mechanical blocking of hydrogen and carbon monoxide (syn-gas) by the use of an
of the gas flow in the system. The present anode of choice is a oxidant such as steam (steam reforming)18 (1), carbon dioxide
nickel/yttria-stabilised zirconia (YSZ) cermet,9,10 which shows (dry reforming)19 (2) or limited oxygen (partial oxidation)20 (3).
high conductivity and catalytic activity, but is also catalytically
CH4 þ H2O ! 3H2 þ CO
ð1Þ
active for the carbon forming reactions. The need to reduce
the carbon-forming activity has led to the search for suitable
replacement anode materials.11–13
CH4 þ CO2 ! 2H2 þ 2CO
ð2Þ
Due to their potential for electronic and ionic conduction
as well as their catalytic properties, perovskites have attracted
considerable interest for use as alternative anode materials for
1
2
CH4 þ O2 ! 2H2 þ CO
ð3Þ
Due to its exothermic nature the least energy intensive of
these reforming processes is partial oxidation (3), but a peren-
nial problem with this approach is the tendency to go to com-
plete or total oxidation21 (4) or deposit carbon through
methane decomposition (5).
Catalysis and Sustainable Materials Group, School of Physical and Geographical
Sciences, Lennard-Jones Laboratories, Keele University, Staffordshire, ST5 5BG, UK.
E-mail: j.staniforth@keele.ac.uk
15022 | Dalton Trans., 2014, 43, 15022–15027
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CH4 þ 2O2 ! 2H2O þ CO2
CH4 ! C þ 2H2
ð4Þ
ð5Þ
In order to make partial oxidation a viable process a catalyst
needs to show a significantly higher selectivity towards carbon
monoxide and hydrogen than towards carbon dioxide and
water.
Previous studies have shown that nickel based catalysts can
demonstrate high selectivity to partial oxidation over total oxi-
dation,22,23 although carbon deposition through methane
decomposition (5) and CO disproportionation can be proble-
matic. In order to utilize this ability to catalyse partial oxi-
dation several attempts have been made to incorporate nickel
into a perovskite based catalyst.24,25 Unfortunately it has
proved to be difficult to successfully incorporate nickel into
the perovskite structure, and so in practice examples have
simply been perovskite supported nickel based catalysts,26,27
which have often suffered from the same problems as more
traditional nickel supported catalysts. Hydrothermal synthesis
has been used extensively in the preparation of nanoporous
framework materials such as zeolites, and more recently for
the synthesis of mixed metal oxides. The method has been
shown to offer several benefits over traditional solid state syn-
thesis techniques including lower synthesis temperatures,
single step process, control of oxidation states and isolation of
metastable phases.
Fig. 1 Temperature programmed reaction profile of methane and
limited oxygen (CH4:O2 = 2) over lanthanum strontium iron cobaltite.
Fig. 2 Temperature programmed reaction profile of methane and
limited oxygen (CH4:O2 = 2) over a 90 : 10 Ni/YSZ anode cermet.
In this paper we describe the incorporation of nickel into a
strontium zirconate using
a
hydrothermal synthesis
method28–30 producing a high purity perovskite31 with high
activity and selectivity for the partial oxidation of methane. The
doped nickel is contained within the structure isolating the
nickel and so limiting any ensemble characteristics, which
could lead to carbon formation. The nickel doped strontium zir-
conate perovskite also shows stability under reaction conditions
and is still phase pure after a reaction cycle of four days.
oxidised at the lower temperatures leading to a surface more
suitable for total oxidation. In order to investigate this further
a reverse temperature programme was ran from 900 °C down-
wards. The results of this are shown in Fig. 3.
The reverse programme shows a steady change over from
partial to total oxidation and a falling off in catalyst activity at
lower temperatures. Thus a definite hysteresis is observed
between the forward and reverse reaction over 90 : 10 Ni/YSZ.
This suggests the catalyst surface had undergone partial re-
oxidation prior to reaction creating a surface with a propensity
to total oxidation, once sufficient hydrogen is available the
surface remains reduced and in a state to promote partial
oxidation.
Results and discussion
For comparison purposes a standard commercially available
catalytic perovskite was chosen, in this case lanthanum stron-
tium iron cobaltite. The temperature programmed reaction of
a 2 : 1 methane–oxygen mixture over this perovskite is shown
in Fig. 1.
It can be seen that total oxidation dominates at all tempera-
tures with only limited partial oxidation products, hydrogen
and carbon monoxide, starting to become evident at tempera-
tures in excess of 800 °C. In contrast, the reaction of limited
oxygen with methane for a 90 : 10 Ni/YSZ anode cermet, Fig. 2,
shows some total oxidation as well as partial oxidation at lower
temperatures but with partial oxidation predominating at
temperatures above 650 °C.
The reaction profile shows a sudden change from total oxi-
dation products to partial oxidation, this could be due to con-
Fig. 3 Reverse temperature programmed reaction profile of methane
ditioning of the catalyst or that the catalyst has partially re- and limited oxygen (CH4:O2 = 2) over a 90 : 10 Ni/YSZ anode cermet.
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The hydrothermally synthesized Sr0.8Ni0.2ZrO3, shows a higher temperatures (Fig. 6), which is almost entirely confined
similar forward profile to 90 : 10 Ni/YSZ with initial total oxi- to total oxidation products.
dation products followed by a sudden change to partial oxi-
dation at around 700 °C, as shown in Fig. 4.
Exchanging the zirconium for hafnium or exchanging the
strontium for barium has only minimal effect on the partial
Again the reverse temperature programmed reaction shows oxidation performance of the perovskite (Fig. 7 and 8, both
a steady conversion from partial oxidation to total oxidation showing reverse temperature programme profiles to mitigate
products but in this case partial oxidation is more predomi- the hysteresis effect). Both display good activity at comparably
nant and deactivation is more immediate once partial oxi- low reaction temperatures, and high selectivity towards syn-
dation is effectively finished, see Fig. 5.
thesis gas formation above ∼750 °C, suggesting that the major
In order to confirm that the catalytically active component effect is that of the incorporated nickel.
in this perovskite was related to the nickel incorporation into
Reaction measurements were carried out at 700 °C, 800 °C
the material, pure strontium zirconate, SrZrO3, was also tested. and 900 °C in order to investigate the stability and longevity of
Undoped strontium zirconate shows limited catalytic activity at the Sr0.8Ni0.2ZrO3 perovskite catalyst and to determine the
profile of any carbon deposition with reaction time. The
methane conversions, shown in Fig. 9, show a slow decline in
conversion over time during reaction at 900 °C and 800 °C. No
such decrease in activity over time is observed at 700 °C, which
could be due to slight sintering of the material resulting in a
loss of surface area at higher reaction temperatures.
Carbon deposition during reaction was investigated by post-
reaction temperature programmed oxidation after 20, 42 and
70 hours. The results are summarized in Table 1. It can be
seen that there is little or no correlation between the amount
of carbon deposited and time on stream or temperature. The
amount of carbon deposited is extremely low. Even if there was
some build-up of carbon over time it would take a consider-
able amount of time before this becomes an issue.
Fig. 4 Temperature programmed reaction profile of methane and
limited oxygen (CH4:O2 = 2) over Sr0.8Ni0.2ZrO3.
Fig. 5 Reverse temperature programmed reaction profile of methane
Fig. 7 Reverse temperature programmed reaction profile of methane
and limited oxygen (CH4:O2 = 2) over Sr0.8Ni0.2ZrO3.
and limited oxygen (CH4:O2 = 2) over Sr0.8Ni0.2HfO3.
Fig. 6 Temperature programmed reaction profile of methane and
Fig. 8 Reverse temperature programmed reaction profile of methane
limited oxygen (CH4:O2 = 2) over SrZrO3.
and limited oxygen (CH4:O2 = 2) over Ba0.8Ni0.2ZrO3.
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Fig. 11 XRD pattern of Sr0.8Nio.2ZrO3 post 4 days isothermal reaction of
methane and limited oxygen at 900 °C, compared to pre-reaction
profile.
Fig. 9 Methane conversion during reaction of methane and limited
oxygen (CH4:O2
temperatures.
= 2) over Sr0.8Ni0.2ZrO3 at different reaction
undertaken to see if this loss of activity continues or if a stable
rate of conversion is eventually achieved.
Table 1 Effect of temperature and reaction time on carbon deposition
during reaction of methane and limited oxygen (CH4–O2 = 2) over
Sr0.8Ni0.2ZrO3, given in mg of carbon per gram of catalyst
The phase purity and stability of the nickel doped stron-
tium perovskite was investigated by powder X-ray diffraction
(XRD) before and after a four day isothermal partial oxidation
reaction at 900 °C. The initial XRD pattern, Fig. 11, shows a
phase pure sample with no sign of nickel oxide impurities
suggesting that the available nickel is incorporated in the
structure and not just supported on strontium zirconate.
After reaction no change was observed in the XRD pattern,
Fig. 11, with no discernable nickel oxide peaks at around 37,
43 and 63 degrees 2θ, showing a good stability under reaction
conditions.
Time/h
Temperature/°C
20
42
70
700
800
900
0.5
1.15
0.2
5.95
0.3
8.75
2.45
1.7
0.10
In order to investigate further the stability of the nickel
doped strontium zirconate several reaction cycles were under-
taken at 900 °C the results of which in terms of methane con-
version are shown in Fig. 10. The reaction cycles consisted of a
pre-reduction, followed by isothermal reaction for 20 hours
and a post reaction TPO.
Experimental
These results show a loss of activity of about 2% for each
cycle undertake. Further investigations are presently being
For comparison purposes several catalysts were prepared by
varying methods. A physically mixed 90 wt% nickel/yttria-
stabilised zirconia (YSZ) anode cermet was produced by
mixing nickel oxide and YSZ in a trichloroethene–methanol
based solvent, milled for 3 hours and then sintered at
1300 °C.31 This material was chosen to mimic the outer com-
ponent of a dual layer SOFC anode. A commercially available
lanthanum strontium iron cobaltite (Praxair) was used.
A
hydrothermally synthesised nickel doped perovskite
based around the formula Sr0.8Ni0.2ZrO3 was prepared. The
nickel doped perovskite was prepared by combining
ZrOCl2·8H2O (Alfa Aesar, 98%), Sr(NO3)2 (Alfa Aesar, 99%),
Ni(NO3)2·6H2O (Aldrich, 99%, NaOH (Aldrich) >97%) and
ultra-pure water within the Teflon liner of a stainless steel
autoclave and stirred to form a homogenous gel. The liner was
sealed within the autoclave and heated in an oven at 180 °C
for 72 hours. The product was then removed, washed with
ultra-pure water and recovered using several cycles of centrifu-
ging at 4000 rpm. The sample was then dried and ground to
form a powder used for catalytic testing.32 For the barium
A site perovskite, Ba(NO3)2 (Alfa Aesar, 99%) replaced Sr(NO3)2,
Fig. 10 Methane conversion during reaction of methane and limited
oxygen (CH4:O2 = 2) over Sr0.8Ni0.2ZrO3 for different cycles at a reaction
temperature of 900 °C.
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and for the hafnium B site perovskite HfOCl2·8H2O (Alfa Aesar cations in the A and B sites.34 This will enable the ability to
98+%) replaced ZrOCl2·8H2O. select characteristics depending on the reforming application.
Catalyst testing was performed by placing 20 mg 0.5 mg Supported nickel catalysts possess inherent difficulties, such
of catalytic material between two pieces of quartz wool in a as susceptibility to sintering and ensemble variability, which is
quartz tube of 7 mm external diameter and 4.5 mm internal not the case for a perovskite with the active nickel bound in.
diameter. This was placed in a furnace controlled by a Euro- These factors should result in a greater scope for flexible,
therm 818 controller with a K-type thermocouple. A gas mani- focused catalyst production and application.
fold delivered reactant gases to the furnace all independently
controlled by mass flow controllers (Unit 7300). The exhaust
gases were directly fed into a quadrupole mass spectrometer
Acknowledgements
(MKS minilab), capable of simultaneously following up to
12 mass fragments in essentially real time.33
The UK Engineering and Physical Sciences Research Council
Experiments were essentially of two types, temperature pro- (EPSRC) is gratefully acknowledged for funding (grant EP/
grammed reaction and isothermal. The temperature pro- I037059/1).
grammed experiments could be followed both in the ‘up’ cycle
of increasing temperature and the ‘down’ cycle of decreasing
temperature, to elucidate any hysteresis effects. Isothermal
experiments were carried out to study long term stability of the
Notes and references
catalyst, behaviour over time and to quantify carbon depo-
sition with time. For all experiments reported here the reaction
mixture consisted of a 2 : 1 methane–oxygen mixture, 2 ml
min−1 CH4, and 1 ml min−1 O2, diluted in 18 ml min−1 He.
Prior to reaction the catalyst samples were reduced in
hydrogen, 2 ml min−1 diluted in 18 ml min−1 helium as a
carrier gas. This was done by a temperature programme to
850 °C at a rate of 10 °C min−1 to ensure complete reduction
of the sample.
Post-reaction the sample was analysed for carbon depo-
sition by temperature programmed oxidation. A flow of 2 ml
min−1 oxygen in 18 ml min−1 helium was passed over the
sample, which was then linearly heated from room tempera-
ture to 900 °C at a rate of 10 °C min−1. The carbon dioxide and
carbon monoxide evolved from the oxidation of the carbon de-
posited on the catalyst surface was followed continuously by
mass spectrometry, and the carbon mass determined by inte-
gration and reference to a standard curve.
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