Journal of Materials Chemistry PAPER

Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 3838
www.rsc.org/materials
A citrate sol–gel method to synthesize Li 2 ZrO 3 nanocrystals with improved
CO 2 capture properties
Qiang Xiao, Yefeng Liu, Yijun Zhong and Weidong Zhu*
Received 28th September 2010, Accepted 17th December 2010
DOI: 10.1039/c0jm03243c
PAPER
Li 2 ZrO 3 nanocrystals with improved CO 2 capture properties were synthesized by a citrate sol–gel
method. The morphology and structure of the synthesized Li 2 ZrO 3 samples were characterized in detail
by SEM, TEM, and XRD techniques. The CO 2 capture–regeneration properties of these nanocrystals
were investigated by thermogravimetric analysis (TGA) over a wide range of temperatures and of CO 2
partial pressures. The nanosized Li 2 ZrO 3 crystallites with a tetragonal phase exhibit a better CO 2
capture performance than reported sorbents, exhibiting a faster uptake and a higher, nearly
stoichiometric adsorption capacity (26 wt.%). Furthermore, the adsorbent shows a good stability,
confirmed by capture–regeneration cycles. Based on these excellent properties, application in sorptionenhanced
reaction process (SERP) is anticipated.
Introduction
Recently, hydrogen has gained considerable interest as a potential
alternative fuel for zero-emission vehicles, and by 2005,
about 55% of all hydrogen was produced globally from natural
gas, i.e. via steam–methane reforming (SMR) and water–gas shift
(WGS) reaction. 1 The endothermic SMR reaction is generally
carried out in a catalytic (Ni/Al 2 O 3 ) reactor at temperatures
ranging from 1023 to 1173 K, the effluent gas is then cooled and
fed to another catalytic (Fe or Cu/Al 2 O 3 ) reactor for the
exothermic WGS reaction, and finally a multicolumn pressure
swing adsorption (PSA) process containing 4–12 adsorption beds
is usually applied to produce pure hydrogen. 2 The aforementioned
process has been the dominant technology utilized for H 2
production for many years. However, the high-temperature
operation of the SMR reactor, the catalyst deactivation due to
coking, the use of special high-temperature steel for the
construction of the SMR reactor, the requirement of large
interstage heat recovery systems, and the complex design of the
multicolumn H 2 -PSA system significantly raise the capital cost of
H 2 production. 3
A process configuration based on a hybrid reaction/adsorption
system, the so-called sorption-enhanced reaction process
(SERP), has been developed by Air Products in the 1990s as an
alternative technology for the industrial production of pure
hydrogen. 3,4 When the SERP concept is applied to SMR and
WGS reactions for the production of pure hydrogen, an adsorbent
(admixture with the catalyst in the reactor) selectively
Key Laboratory of the Ministry of Education for Advanced Catalysis
Materials, Institute of Physical Chemistry, Zhejiang Normal University,
321004 Jinhua, P. R. of China. E-mail: weidongzhu@zjnu.cn; Fax: +86
579 82282932; Tel: +86 579 82282932
removes in situ CO 2 produced from the reactions and the
adsorbent is periodically regenerated using the principle of
temperature swing adsorption (TSA), resulting in a higher
conversion of the reactant methane at relatively lower temperatures.
Inherently, the purification requirements for the desired
product H 2 can be dramatically reduced or even eliminated. In
order to achieve this objective, it is crucial to find an efficient
adsorbent that can adsorb CO 2 rapidly with high adsorption
capacity at the reaction temperatures.
Many solid adsorbents, mainly CaO- and MgO-based materials,
have been used for the in situ removal of CO 2 produced
from SMR and WGS reactions in order to enhance H 2 production.
1,5,6 However, the use of this type of adsorbent encounters
some obstacles such as very slow CO 2 capture–regeneration rate,
requirement of high temperature for regeneration, poor
mechanical stability due to a large volume expansion after CO 2
adsorption, and dramatic reduction of CO 2 adsorption efficiency
in terms of uptake rate and capacity in the presence of steam.
Recently, Li-containing materials, such as lithium zirconate
(Li 2 ZrO 3 ), have been reported to be promising candidates with
high CO 2 capture capacity, high stability, and ease of regeneration.
7,8 Nakagawa and Ohashi 7,9 have reported that Li 2 ZrO 3 can
react with CO 2 in a temperature range from 723 to 873 K, based
on the following reaction:
Li 2 ZrO 3(s) +CO 2(g) % Li 2 CO 3(s) + ZrO 2(s) (1)
From eqn (1), the stoichiometric adsorption capacity of CO 2 is
up to 28.7 wt.%. Furthermore, the volume increase of a Li 2 ZrO 3
particle after the saturated adsorption of CO 2 is only 34%, much
less than when CaO or MgO is used as adsorbent, 7 implying
a comparably higher mechanical stability when the adsorbent is
admixed with the catalyst in the reactor. Additionally, the
3838 | J. Mater. Chem., 2011, 21, 3838–3842 This journal is ª The Royal Society of Chemistry 2011

temperature for the regeneration of CO 2 -adsorbed Li 2 ZrO 3 is
much lower, and the presence of steam can improve the CO 2
capture properties of Li 2 ZrO 3 . 10 These merits of Li 2 ZrO 3 lead to
its applicability as an adsorbent for the in situ removal of CO 2
produced from SMR and WGS reactions to enhance H 2
productivity. 9
Various solid-state synthetic methods have been used to
prepare Li 2 ZrO 3 powder. 7,11 In a solid-state process, starting
materials, usually ZrO 2 and Li 2 CO 3 , are mechanically mixed and
heated at high temperatures for a long time. Consequently, sintering
at high temperatures results in large particle sizes of the
resulting Li 2 ZrO 3 . Meanwhile, the sublimation of Li 2 O cannot
be avoided due to high-temperature calcination. These aspects
lead to poor CO 2 adsorption properties of the Li 2 ZrO 3 powder
prepared by the solid-state method. Nair et al. 11 pointed out that
the smaller the size of Li 2 ZrO 3 particles, the faster their CO 2
uptake. In addition, Lin’s group 12,13 proposed a ‘double-shell’
model for CO 2 adsorption in Li 2 ZrO 3 , which indicates that
Li 2 ZrO 3 of a smaller particle size shows a faster CO 2 uptake.
In order to prepare Li 2 ZrO 3 with a fine particle size, a liquidphase
method was proposed by Yi and Eriksen. 14 A dramatic
improvement of CO 2 adsorption efficiency in the Li 2 ZrO 3
particles prepared by the liquid-phase method was observed,
yielding a higher capacity and about ten times faster uptake.
Later, Chen and coworkers 15,16 reported a modified liquid-phase
method to prepare nanocrystalline Li 2 ZrO 3 , in which soluble Zr
and Li compounds were used as raw materials to obtain
a complex solution containing Zr and Li precursors with
appropriate amounts, and then the solution was spray dried to
form Li 2 ZrO 3 powder. After calcination, the obtained nanocrystalline
Li 2 ZrO 3 showed a better uptake for CO 2 under the
applied conditions.
Although Li 2 ZrO 3 prepared by the liquid-phase method exhibits
a faster uptake, compared to those synthesized by the solid-state
method, the uptake of CO 2 is still slow, especially for the case at
CO 2 partial pressures below 0.5 bar. Furthermore, the adsorption
of CO 2 hardly reaches its stoichiometric adsorption capacity. It
should be noted that in the liquid-phase procedure soluble Li and
Zr salts are dissolved together, followed by evaporation of the
solvent to form a solid mixture containing Li and Zr species.
Differences in the solubility of each component would result in
a heterogeneous distribution of Li and Zr species. Consequently,
the heterogeneity of their spatial distribution would bring forth an
incomplete formation of the Li 2 ZrO 3 phase after calcination.
Therefore, the Li 2 ZrO 3 adsorbents prepared by this liquid-phase
method could not possess the desired CO 2 capture properties.
The citrate sol–gel method was first proposed by Pechini 17 for
preparing ABO 3 -perovskite type powders. The typical synthesis
procedure is described as follows: metallic ions and citrate form
complex anions at first and then a sol forms by the polymerization
of citrate in glycol. After the evaporation of solvent, a gel
comes into being, during which multi-metallic components are
confined in the gel net. After the removal of the organic gel by
calcination, a fine powder of desired stoichiometric composition
can be obtained. Subsequently, Choy and Han 18 developed
a modified citrate sol–gel method, in which water was used
instead of glycol. The citrate sol–gel method is now a common
approach for the synthesis of ABO 3 -type nanocrystals. Although
the synthesis of ABO 3 -type powders such as Pb(Zr,Ti)O 3 oxides
etc. via the citrate (or modified) sol–gel method has been intensively
investigated, the synthesis of Li 2 ZrO 3 nanocrystals via this
method is rarely reported.
In the present work, a citrate sol–gel method has been used to
synthesize Li 2 ZrO 3 nanocrystals. The CO 2 capture–regeneration
properties of the synthesized nanocrystals have been investigated
over a wide range of CO 2 pressures at different temperatures by
thermogravimetric analysis (TGA).
Experimental
Chemicals
Zirconium oxynitrate [ZrO(NO 3 ) 2 $2H 2 O] was purchased from
Tianjin Guangfu Fine Chemical Research Institute. Lithium
nitrate (LiNO 3 ) was purchased from Shanghai Shanpu Chemical
Co., Ltd. Citric acid (CA, C 6 H 8 O 7 $H 2 O) and urea [CO(NH 2 ) 2 ]
were purchased from Shantou Xilong Chemical Factory. All the
reagents were analytical grade and used as received without
further purification. The deionized water was obtained from
Millipore Milli-QÒ ultrapure water purification systems with
a resistivity greater than 18.2 MU.
Syntheses
In a typical citrate sol–gel synthesis, 2.67 g ZrO(NO 3 ) 2 $2H 2 O,
1.38 g LiNO 3 , 6.30 g CA, and 3.00 g urea were dissolved in 46 ml
deionized water. The solution was heated to 343 K and aged for 6
h with stirring and then was exposed to air to evaporate the
solvent until a gel was formed. The resulting gel was dried at 393
K in an oven overnight, and the formed solid is referred to as the
as-obtained sample. The as-obtained powder was put into
a dense alumina crucible in a furnace box where the temperature
was first increased to 673 K at a rate of 1 K min :1 and then
maintained at 673 K in stagnant air for 2 h. Afterwards the
temperature was further increased to 923 K at a rate of 1 K min :1
and maintained for 6 h. After being cooled down to room
temperature, the final white powder was obtained.
For comparison, Li 2 ZrO 3 particles were synthesized by
coprecipitation and liquid-phase methods. In a coprecipitation
synthesis, the dose and addition order of the raw materials and
the heat treatment that followed were the same as those in the
sol–gel synthesis, except that no citric acid was used. In a liquidphase
method, neither citric acid nor urea was used. The samples
prepared by the citrate sol–gel, coprecipitation, and liquid-phase
methods are referred to as LZ-sg, LZ-co, and LZ-lp, respectively.
Characterization
The X-ray diffraction patterns were recorded on a Philips
PW3040/60 diffractometer using CuKa radiation (l ¼ 1.54 A).
The scans were recorded in the 2q range between 10 and 90 with
a step width of 0.03 . The average crystallite size of powder was
calculated using Scherrer’s equation from the width of the
reflection peak located at 2q ¼ 42.3 . Silicon standard, Si, was
used as reference material to determine the instrumental line
broadening.
The scanning electron microscope (SEM) observations were
performed on a Hitachi S-4800 apparatus equipped with a field
emission gun. The acceleration voltage was set to 5 kV. The
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sample was stuck on the observation platform and sprayed with
gold vapor under high vacuum for about 20 s. The transmission
electron microscopy (TEM) observations were carried out on
a Philips Tecnai F20 transmission electron microscope working
at 200 kV. The sample was diluted in ethanol to give a 1 : 5
volume ratio, and sonicated for 10 min. The ethanol slurry was
then dripped onto a Cu grid covered with a thin film of carbon.
The TGA-DTA experiments with ca. 15 mg of the as-obtained
samples were carried out on a NETZSCH STA449 thermogravimetric
analyzer with a heating rate of 20 K min :1 in an air flow
of 40 ml min :1 .
CO 2 adsorption
The adsorption of CO 2 in the synthesized Li 2 ZrO 3 nanocrystals
was studied on a thermogravimetric analyzer (NETZSCH
STA449). The sample cell (a dense alumina crucible pot) was
loaded with ca. 15 mg of adsorbent. Prior to the CO 2 adsorption
experiments, the adsorbent was heated up to the desired
temperature (823 K or 773 K) at a rate of 10 K min :1 in
a nitrogen flow of 40 ml min :1 , during which the adsorbed
impurities in the sample were removed. Then, the N 2 flow was
switched to a mixture flow containing CO 2 and N 2 or to a pure
CO 2 flow for adsorption, all at 1 bar. The partial pressure of CO 2
was determined by its fraction of the total feed flow rate and the
total pressure. The total feed flow was always set at 40 ml min :1 .
The adsorption of CO 2 was carried out for 60 min, referred to as
period I. Afterwards, the feed gas stream was switched to a pure
N 2 flow of 40 ml min :1 and the temperature was raised at a rate of
10 K min :1 to 923 K, maintaining this temperature for 50 min.
This desorption (regeneration) process lasted 60 min, referred to
as period II. The combined period I and period II is defined as
one adsorption–desorption (capture–regeneration) cycle. To
correct for the weight change caused by the change in the density
of the gas phase, blank experiments were performed. In a blank
experiment the TGA response was measured with the empty
dense alumina crucible pot used.
Results and discussion
The as-obtained sample prepared via the citrate sol–gel method
contains a large amount of organic species. As shown in the TGA
curve (Fig. 1), a decrease of 85% by weight can be seen in the
range from 298 to 1123 K. The whole weight loss process can be
divided into two stages. The first with a weight loss of 55%
accompanied with a broad endothermic peak at temperatures
between 298 and 533 K should be attributed to the desorption or
decomposition of the organic species, partially from the citrate
precursor. 19 The second stage with a weight loss of ca. 30% starts
at 673 K and an exothermic peak around 847 K can be observed
in the DTA curve, corresponding to the drastic combustion of
the organic species in the sample. The weight stays constant at
temperatures over 923 K, which is much lower than those (ca.
1273 K) required for preparing Li 2 ZrO 3 powder via the solidstate
reaction method. 7
The adsorption and desorption profile of CO 2 in the Li 2 ZrO 3
adsorbent prepared via the citrate sol–gel method is shown as
curve a in Fig. 2. At 823 K and a CO 2 partial pressure of 0.5 bar,
the uptake of CO 2 reaches 26 wt.% within 20 min, which is about
90% of the stoichiometric adsorption capacity of CO 2 in Li 2 ZrO 3
(28.7 wt.%). Additionally, the desorption in flowing N 2 is also
fast and the complete desorption takes less than 20 min. To
illustrate the role of citric acid in the synthesis of Li 2 ZrO 3 , two
samples without the use of citric acid have been synthesized for
comparison. The adsorption and desorption profiles of CO 2 in
these two samples are also shown in Fig. 2. The Li 2 ZrO 3 sample
prepared by the liquid-phase method, i.e. LZ-lp, displays the
lowest CO 2 uptake rate under the investigated conditions, see
curve c in Fig. 2. Although the Li 2 ZrO 3 sample prepared by the
coprecipitation method, i.e. LZ-co, shows some improved CO 2
capture properties in comparison with LZ-lp, its CO 2 uptake is
still slower, compared to that in the Li 2 ZrO 3 sample prepared by
the sol–gel method, i.e. LZ-sg. These results clearly show that
LZ-sg has the best CO 2 capture properties.
It has been reported that the crystallite phase of Li 2 ZrO 3 can
have a great influence on CO 2 capture properties. 11 Fig. 3 shows the
XRD patterns of the Li 2 ZrO 3 samples prepared by the aforementioned
three methods, indicating that both tetragonal and
monoclinic crystallite phases are present in all samples. However,
as shown in Fig. 3, the crystallite phase of LZ-sg is mainly
tetragonal. Nair et al. 11 reported that Li 2 ZrO 3 crystallites with
a tetragonal phase showed better CO 2 -capture properties in terms
of uptake rate than those with a monoclinic phase. The current
results are in agreement with this observation from the literature.
Fig. 1 TGA and DTA curves of the as-obtained sample synthesized via
the citrate sol–gel method with a heating rate of 20 K min :1 in an air flow
of 40 ml min :1 .
Fig. 2 Adsorption and desorption profiles of CO 2 for three different
Li 2 ZrO 3 samples. (a) LZ-sg, (b) LZ-co, (c) LZ-lp; (I) adsorption at 823 K
and a CO 2 partial pressure of 0.5 bar, (II) desorption by heating (10 K
min :1 ) to 923 K in a pure N 2 flow of 40 ml min :1 .
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Fig. 3 XRD patterns of three different Li 2 ZrO 3 samples. (a) LZ-sg, (b)
LZ-co, (c) LZ-lp.
The uptake rate of CO 2 should be also related to the crystallite
size of Li 2 ZrO 3 . The crystallite sizes of the three different
samples, estimated from the XRD data using Scherrer’s equation,
are 17, 26, and 34 nm for LZ-sg, LZ-co, and LZ-lp,
respectively. It is logical from an engineering point of view that
the smaller the size of Li 2 ZrO 3 crystals, the faster the CO 2 uptake
in case of a diffusion limited process.
The stability of an adsorbent is crucial when applied to the in
situ removal of CO 2 produced in some reactions such as steam–
methane reforming (SMR) and water–gas shift (WGS) reaction
to enhance hydrogen production as well as in the separation of
CO 2 from fuel-fired power stations at high temperatures.
Therefore, the stability of LZ-sg was investigated by TGA. Fig. 4
shows three CO 2 capture–regeneration cycles, indicating an
excellent stability of the Li 2 ZrO 3 crystals prepared by the citrate
sol–gel method. This indicates the potential applicability of the
synthesized nanosized Li 2 ZrO 3 crystallites with a tetragonal
phase in the removal of CO 2 at high temperatures.
Fig. 5A, B, and C show the morphologies of the unused, onecycle
used, and three-cycles used LZ-sg samples by SEM images,
indicating that all the samples consist of crystallite agglomerates,
confirmed by the TEM observation, as shown in Fig. 5D.
Additionally, these SEM images give evidence that the particles
do not become larger after CO 2 capture–regeneration cycles,
unlike CaO- or MgO-based adsorbents, in which the particles are
easily enlarged after regeneration at high temperatures, 20 resulting
in the rapidly reduced uptake rate of CO 2 . The results from
Fig. 5 SEM images of the unused (A), one-cycle used (B), and threecycles
used (C) LZ-sg samples and the TEM image (D) of the unused
LZ-sg sample.
Fig. 6 Uptake profiles of CO 2 in LZ-sg at different partial pressures of
CO 2 and 823 K.
Fig. 5 give an explanation for the stability of the Li 2 ZrO 3
prepared by the citrate sol–gel method for CO 2 capture.
The uptake profiles of CO 2 in LZ-sg at different partial pressures
of CO 2 are shown in Fig. 6. Obviously, the uptake rate
decreases with decreasing CO 2 pressure at the same temperature.
When the partial pressure of CO 2 is below 0.25 bar, the uptake
becomes rather slow and the capacity is not fully reached within
60 min. In addition, almost no uptake takes place at CO 2 pressures
below 0.15 bar, probably due to thermodynamic limitations.
Thus, the uptake rate of CO 2 in Li 2 ZrO 3 strongly depends
on CO 2 pressure.
Conclusions
Fig. 4 Three cycles of CO 2 capture–regeneration in LZ-sg. (I) adsorption
at 773 K and a CO 2 partial pressure of 0.5 bar, (II) regeneration by
heating (10 K min :1 ) to 923 K in a pure N 2 flow of 40 mL min :1 .
The citrate sol–gel method has been developed to prepare
Li 2 ZrO 3 nanocrystals that can adsorb 26 wt.% CO 2 within 20
min, which is about 90% of the stoichiometric capacity of CO 2 in
Li 2 ZrO 3 (28.7 wt.%), at 823 K and CO 2 partial pressures above
0.5 bar. The adsorbent shows a good stability, confirmed by
repeated CO 2 capture–regeneration cycles. The small sol–gel
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synthesized Li 2 ZrO 3 crystallites with a tetragonal phase exhibit
better CO 2 capture–regeneration properties than those prepared
by other synthesis methods. The uptake rate of CO 2 decreases
with decreasing CO 2 pressure, reaching a thermodynamic limit at
around 0.15 bar. The synthesized Li 2 ZrO 3 nanocrystals with
excellent properties for CO 2 capture–regeneration display a good
prospect for application in the in situ removal of CO 2 produced
in some reactions such as steam–methane reforming (SMR) and
water–gas shift (WGS) reaction to enhance hydrogen production
as well as in the separation of CO 2 from fuel-fired power stations
at high temperatures.
Acknowledgements
The financial supports by the National Basic Research Program
of China (2009CB626607) and the Natural Science Foundation
of Zhejiang Province, China (R4080084 and Y4090473) are
gratefully acknowledged.
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