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<strong>Journal</strong> <strong>of</strong><br />
<strong>Materials</strong> <strong>Chemistry</strong><br />
Cite this: J. Mater. Chem., 2011, 21, 3838<br />
www.rsc.org/materials<br />
A citrate sol–gel method to synthesize Li 2 ZrO 3 nanocrystals with improved<br />
CO 2 capture properties<br />
Qiang Xiao, Yefeng Liu, Yijun Zhong and Weidong Zhu*<br />
Received 28th September 2010, Accepted 17th December 2010<br />
DOI: 10.1039/c0jm03243c<br />
<strong>PAPER</strong><br />
Li 2 ZrO 3 nanocrystals with improved CO 2 capture properties were synthesized by a citrate sol–gel<br />
method. The morphology and structure <strong>of</strong> the synthesized Li 2 ZrO 3 samples were characterized in detail<br />
by SEM, TEM, and XRD techniques. The CO 2 capture–regeneration properties <strong>of</strong> these nanocrystals<br />
were investigated by thermogravimetric analysis (TGA) over a wide range <strong>of</strong> temperatures and <strong>of</strong> CO 2<br />
partial pressures. The nanosized Li 2 ZrO 3 crystallites with a tetragonal phase exhibit a better CO 2<br />
capture performance than reported sorbents, exhibiting a faster uptake and a higher, nearly<br />
stoichiometric adsorption capacity (26 wt.%). Furthermore, the adsorbent shows a good stability,<br />
confirmed by capture–regeneration cycles. Based on these excellent properties, application in sorptionenhanced<br />
reaction process (SERP) is anticipated.<br />
Introduction<br />
Recently, hydrogen has gained considerable interest as a potential<br />
alternative fuel for zero-emission vehicles, and by 2005,<br />
about 55% <strong>of</strong> all hydrogen was produced globally from natural<br />
gas, i.e. via steam–methane reforming (SMR) and water–gas shift<br />
(WGS) reaction. 1 The endothermic SMR reaction is generally<br />
carried out in a catalytic (Ni/Al 2 O 3 ) reactor at temperatures<br />
ranging from 1023 to 1173 K, the effluent gas is then cooled and<br />
fed to another catalytic (Fe or Cu/Al 2 O 3 ) reactor for the<br />
exothermic WGS reaction, and finally a multicolumn pressure<br />
swing adsorption (PSA) process containing 4–12 adsorption beds<br />
is usually applied to produce pure hydrogen. 2 The aforementioned<br />
process has been the dominant technology utilized for H 2<br />
production for many years. However, the high-temperature<br />
operation <strong>of</strong> the SMR reactor, the catalyst deactivation due to<br />
coking, the use <strong>of</strong> special high-temperature steel for the<br />
construction <strong>of</strong> the SMR reactor, the requirement <strong>of</strong> large<br />
interstage heat recovery systems, and the complex design <strong>of</strong> the<br />
multicolumn H 2 -PSA system significantly raise the capital cost <strong>of</strong><br />
H 2 production. 3<br />
A process configuration based on a hybrid reaction/adsorption<br />
system, the so-called sorption-enhanced reaction process<br />
(SERP), has been developed by Air Products in the 1990s as an<br />
alternative technology for the industrial production <strong>of</strong> pure<br />
hydrogen. 3,4 When the SERP concept is applied to SMR and<br />
WGS reactions for the production <strong>of</strong> pure hydrogen, an adsorbent<br />
(admixture with the catalyst in the reactor) selectively<br />
Key Laboratory <strong>of</strong> the Ministry <strong>of</strong> Education for Advanced Catalysis<br />
<strong>Materials</strong>, Institute <strong>of</strong> Physical <strong>Chemistry</strong>, Zhejiang Normal University,<br />
321004 Jinhua, P. R. <strong>of</strong> China. E-mail: weidongzhu@zjnu.cn; Fax: +86<br />
579 82282932; Tel: +86 579 82282932<br />
removes in situ CO 2 produced from the reactions and the<br />
adsorbent is periodically regenerated using the principle <strong>of</strong><br />
temperature swing adsorption (TSA), resulting in a higher<br />
conversion <strong>of</strong> the reactant methane at relatively lower temperatures.<br />
Inherently, the purification requirements for the desired<br />
product H 2 can be dramatically reduced or even eliminated. In<br />
order to achieve this objective, it is crucial to find an efficient<br />
adsorbent that can adsorb CO 2 rapidly with high adsorption<br />
capacity at the reaction temperatures.<br />
Many solid adsorbents, mainly CaO- and MgO-based materials,<br />
have been used for the in situ removal <strong>of</strong> CO 2 produced<br />
from SMR and WGS reactions in order to enhance H 2 production.<br />
1,5,6 However, the use <strong>of</strong> this type <strong>of</strong> adsorbent encounters<br />
some obstacles such as very slow CO 2 capture–regeneration rate,<br />
requirement <strong>of</strong> high temperature for regeneration, poor<br />
mechanical stability due to a large volume expansion after CO 2<br />
adsorption, and dramatic reduction <strong>of</strong> CO 2 adsorption efficiency<br />
in terms <strong>of</strong> uptake rate and capacity in the presence <strong>of</strong> steam.<br />
Recently, Li-containing materials, such as lithium zirconate<br />
(Li 2 ZrO 3 ), have been reported to be promising candidates with<br />
high CO 2 capture capacity, high stability, and ease <strong>of</strong> regeneration.<br />
7,8 Nakagawa and Ohashi 7,9 have reported that Li 2 ZrO 3 can<br />
react with CO 2 in a temperature range from 723 to 873 K, based<br />
on the following reaction:<br />
Li 2 ZrO 3(s) +CO 2(g) % Li 2 CO 3(s) + ZrO 2(s) (1)<br />
From eqn (1), the stoichiometric adsorption capacity <strong>of</strong> CO 2 is<br />
up to 28.7 wt.%. Furthermore, the volume increase <strong>of</strong> a Li 2 ZrO 3<br />
particle after the saturated adsorption <strong>of</strong> CO 2 is only 34%, much<br />
less than when CaO or MgO is used as adsorbent, 7 implying<br />
a comparably higher mechanical stability when the adsorbent is<br />
admixed with the catalyst in the reactor. Additionally, the<br />
3838 | J. Mater. Chem., 2011, 21, 3838–3842 This journal is ª The Royal Society <strong>of</strong> <strong>Chemistry</strong> 2011
temperature for the regeneration <strong>of</strong> CO 2 -adsorbed Li 2 ZrO 3 is<br />
much lower, and the presence <strong>of</strong> steam can improve the CO 2<br />
capture properties <strong>of</strong> Li 2 ZrO 3 . 10 These merits <strong>of</strong> Li 2 ZrO 3 lead to<br />
its applicability as an adsorbent for the in situ removal <strong>of</strong> CO 2<br />
produced from SMR and WGS reactions to enhance H 2<br />
productivity. 9<br />
Various solid-state synthetic methods have been used to<br />
prepare Li 2 ZrO 3 powder. 7,11 In a solid-state process, starting<br />
materials, usually ZrO 2 and Li 2 CO 3 , are mechanically mixed and<br />
heated at high temperatures for a long time. Consequently, sintering<br />
at high temperatures results in large particle sizes <strong>of</strong> the<br />
resulting Li 2 ZrO 3 . Meanwhile, the sublimation <strong>of</strong> Li 2 O cannot<br />
be avoided due to high-temperature calcination. These aspects<br />
lead to poor CO 2 adsorption properties <strong>of</strong> the Li 2 ZrO 3 powder<br />
prepared by the solid-state method. Nair et al. 11 pointed out that<br />
the smaller the size <strong>of</strong> Li 2 ZrO 3 particles, the faster their CO 2<br />
uptake. In addition, Lin’s group 12,13 proposed a ‘double-shell’<br />
model for CO 2 adsorption in Li 2 ZrO 3 , which indicates that<br />
Li 2 ZrO 3 <strong>of</strong> a smaller particle size shows a faster CO 2 uptake.<br />
In order to prepare Li 2 ZrO 3 with a fine particle size, a liquidphase<br />
method was proposed by Yi and Eriksen. 14 A dramatic<br />
improvement <strong>of</strong> CO 2 adsorption efficiency in the Li 2 ZrO 3<br />
particles prepared by the liquid-phase method was observed,<br />
yielding a higher capacity and about ten times faster uptake.<br />
Later, Chen and coworkers 15,16 reported a modified liquid-phase<br />
method to prepare nanocrystalline Li 2 ZrO 3 , in which soluble Zr<br />
and Li compounds were used as raw materials to obtain<br />
a complex solution containing Zr and Li precursors with<br />
appropriate amounts, and then the solution was spray dried to<br />
form Li 2 ZrO 3 powder. After calcination, the obtained nanocrystalline<br />
Li 2 ZrO 3 showed a better uptake for CO 2 under the<br />
applied conditions.<br />
Although Li 2 ZrO 3 prepared by the liquid-phase method exhibits<br />
a faster uptake, compared to those synthesized by the solid-state<br />
method, the uptake <strong>of</strong> CO 2 is still slow, especially for the case at<br />
CO 2 partial pressures below 0.5 bar. Furthermore, the adsorption<br />
<strong>of</strong> CO 2 hardly reaches its stoichiometric adsorption capacity. It<br />
should be noted that in the liquid-phase procedure soluble Li and<br />
Zr salts are dissolved together, followed by evaporation <strong>of</strong> the<br />
solvent to form a solid mixture containing Li and Zr species.<br />
Differences in the solubility <strong>of</strong> each component would result in<br />
a heterogeneous distribution <strong>of</strong> Li and Zr species. Consequently,<br />
the heterogeneity <strong>of</strong> their spatial distribution would bring forth an<br />
incomplete formation <strong>of</strong> the Li 2 ZrO 3 phase after calcination.<br />
Therefore, the Li 2 ZrO 3 adsorbents prepared by this liquid-phase<br />
method could not possess the desired CO 2 capture properties.<br />
The citrate sol–gel method was first proposed by Pechini 17 for<br />
preparing ABO 3 -perovskite type powders. The typical synthesis<br />
procedure is described as follows: metallic ions and citrate form<br />
complex anions at first and then a sol forms by the polymerization<br />
<strong>of</strong> citrate in glycol. After the evaporation <strong>of</strong> solvent, a gel<br />
comes into being, during which multi-metallic components are<br />
confined in the gel net. After the removal <strong>of</strong> the organic gel by<br />
calcination, a fine powder <strong>of</strong> desired stoichiometric composition<br />
can be obtained. Subsequently, Choy and Han 18 developed<br />
a modified citrate sol–gel method, in which water was used<br />
instead <strong>of</strong> glycol. The citrate sol–gel method is now a common<br />
approach for the synthesis <strong>of</strong> ABO 3 -type nanocrystals. Although<br />
the synthesis <strong>of</strong> ABO 3 -type powders such as Pb(Zr,Ti)O 3 oxides<br />
etc. via the citrate (or modified) sol–gel method has been intensively<br />
investigated, the synthesis <strong>of</strong> Li 2 ZrO 3 nanocrystals via this<br />
method is rarely reported.<br />
In the present work, a citrate sol–gel method has been used to<br />
synthesize Li 2 ZrO 3 nanocrystals. The CO 2 capture–regeneration<br />
properties <strong>of</strong> the synthesized nanocrystals have been investigated<br />
over a wide range <strong>of</strong> CO 2 pressures at different temperatures by<br />
thermogravimetric analysis (TGA).<br />
Experimental<br />
Chemicals<br />
Zirconium oxynitrate [ZrO(NO 3 ) 2 $2H 2 O] was purchased from<br />
Tianjin Guangfu Fine Chemical Research Institute. Lithium<br />
nitrate (LiNO 3 ) was purchased from Shanghai Shanpu Chemical<br />
Co., Ltd. Citric acid (CA, C 6 H 8 O 7 $H 2 O) and urea [CO(NH 2 ) 2 ]<br />
were purchased from Shantou Xilong Chemical Factory. All the<br />
reagents were analytical grade and used as received without<br />
further purification. The deionized water was obtained from<br />
Millipore Milli-QÒ ultrapure water purification systems with<br />
a resistivity greater than 18.2 MU.<br />
Syntheses<br />
In a typical citrate sol–gel synthesis, 2.67 g ZrO(NO 3 ) 2 $2H 2 O,<br />
1.38 g LiNO 3 , 6.30 g CA, and 3.00 g urea were dissolved in 46 ml<br />
deionized water. The solution was heated to 343 K and aged for 6<br />
h with stirring and then was exposed to air to evaporate the<br />
solvent until a gel was formed. The resulting gel was dried at 393<br />
K in an oven overnight, and the formed solid is referred to as the<br />
as-obtained sample. The as-obtained powder was put into<br />
a dense alumina crucible in a furnace box where the temperature<br />
was first increased to 673 K at a rate <strong>of</strong> 1 K min :1 and then<br />
maintained at 673 K in stagnant air for 2 h. Afterwards the<br />
temperature was further increased to 923 K at a rate <strong>of</strong> 1 K min :1<br />
and maintained for 6 h. After being cooled down to room<br />
temperature, the final white powder was obtained.<br />
For comparison, Li 2 ZrO 3 particles were synthesized by<br />
coprecipitation and liquid-phase methods. In a coprecipitation<br />
synthesis, the dose and addition order <strong>of</strong> the raw materials and<br />
the heat treatment that followed were the same as those in the<br />
sol–gel synthesis, except that no citric acid was used. In a liquidphase<br />
method, neither citric acid nor urea was used. The samples<br />
prepared by the citrate sol–gel, coprecipitation, and liquid-phase<br />
methods are referred to as LZ-sg, LZ-co, and LZ-lp, respectively.<br />
Characterization<br />
The X-ray diffraction patterns were recorded on a Philips<br />
PW3040/60 diffractometer using CuKa radiation (l ¼ 1.54 A).<br />
The scans were recorded in the 2q range between 10 and 90 with<br />
a step width <strong>of</strong> 0.03 . The average crystallite size <strong>of</strong> powder was<br />
calculated using Scherrer’s equation from the width <strong>of</strong> the<br />
reflection peak located at 2q ¼ 42.3 . Silicon standard, Si, was<br />
used as reference material to determine the instrumental line<br />
broadening.<br />
The scanning electron microscope (SEM) observations were<br />
performed on a Hitachi S-4800 apparatus equipped with a field<br />
emission gun. The acceleration voltage was set to 5 kV. The<br />
This journal is ª The Royal Society <strong>of</strong> <strong>Chemistry</strong> 2011 J. Mater. Chem., 2011, 21, 3838–3842 | 3839
sample was stuck on the observation platform and sprayed with<br />
gold vapor under high vacuum for about 20 s. The transmission<br />
electron microscopy (TEM) observations were carried out on<br />
a Philips Tecnai F20 transmission electron microscope working<br />
at 200 kV. The sample was diluted in ethanol to give a 1 : 5<br />
volume ratio, and sonicated for 10 min. The ethanol slurry was<br />
then dripped onto a Cu grid covered with a thin film <strong>of</strong> carbon.<br />
The TGA-DTA experiments with ca. 15 mg <strong>of</strong> the as-obtained<br />
samples were carried out on a NETZSCH STA449 thermogravimetric<br />
analyzer with a heating rate <strong>of</strong> 20 K min :1 in an air flow<br />
<strong>of</strong> 40 ml min :1 .<br />
CO 2 adsorption<br />
The adsorption <strong>of</strong> CO 2 in the synthesized Li 2 ZrO 3 nanocrystals<br />
was studied on a thermogravimetric analyzer (NETZSCH<br />
STA449). The sample cell (a dense alumina crucible pot) was<br />
loaded with ca. 15 mg <strong>of</strong> adsorbent. Prior to the CO 2 adsorption<br />
experiments, the adsorbent was heated up to the desired<br />
temperature (823 K or 773 K) at a rate <strong>of</strong> 10 K min :1 in<br />
a nitrogen flow <strong>of</strong> 40 ml min :1 , during which the adsorbed<br />
impurities in the sample were removed. Then, the N 2 flow was<br />
switched to a mixture flow containing CO 2 and N 2 or to a pure<br />
CO 2 flow for adsorption, all at 1 bar. The partial pressure <strong>of</strong> CO 2<br />
was determined by its fraction <strong>of</strong> the total feed flow rate and the<br />
total pressure. The total feed flow was always set at 40 ml min :1 .<br />
The adsorption <strong>of</strong> CO 2 was carried out for 60 min, referred to as<br />
period I. Afterwards, the feed gas stream was switched to a pure<br />
N 2 flow <strong>of</strong> 40 ml min :1 and the temperature was raised at a rate <strong>of</strong><br />
10 K min :1 to 923 K, maintaining this temperature for 50 min.<br />
This desorption (regeneration) process lasted 60 min, referred to<br />
as period II. The combined period I and period II is defined as<br />
one adsorption–desorption (capture–regeneration) cycle. To<br />
correct for the weight change caused by the change in the density<br />
<strong>of</strong> the gas phase, blank experiments were performed. In a blank<br />
experiment the TGA response was measured with the empty<br />
dense alumina crucible pot used.<br />
Results and discussion<br />
The as-obtained sample prepared via the citrate sol–gel method<br />
contains a large amount <strong>of</strong> organic species. As shown in the TGA<br />
curve (Fig. 1), a decrease <strong>of</strong> 85% by weight can be seen in the<br />
range from 298 to 1123 K. The whole weight loss process can be<br />
divided into two stages. The first with a weight loss <strong>of</strong> 55%<br />
accompanied with a broad endothermic peak at temperatures<br />
between 298 and 533 K should be attributed to the desorption or<br />
decomposition <strong>of</strong> the organic species, partially from the citrate<br />
precursor. 19 The second stage with a weight loss <strong>of</strong> ca. 30% starts<br />
at 673 K and an exothermic peak around 847 K can be observed<br />
in the DTA curve, corresponding to the drastic combustion <strong>of</strong><br />
the organic species in the sample. The weight stays constant at<br />
temperatures over 923 K, which is much lower than those (ca.<br />
1273 K) required for preparing Li 2 ZrO 3 powder via the solidstate<br />
reaction method. 7<br />
The adsorption and desorption pr<strong>of</strong>ile <strong>of</strong> CO 2 in the Li 2 ZrO 3<br />
adsorbent prepared via the citrate sol–gel method is shown as<br />
curve a in Fig. 2. At 823 K and a CO 2 partial pressure <strong>of</strong> 0.5 bar,<br />
the uptake <strong>of</strong> CO 2 reaches 26 wt.% within 20 min, which is about<br />
90% <strong>of</strong> the stoichiometric adsorption capacity <strong>of</strong> CO 2 in Li 2 ZrO 3<br />
(28.7 wt.%). Additionally, the desorption in flowing N 2 is also<br />
fast and the complete desorption takes less than 20 min. To<br />
illustrate the role <strong>of</strong> citric acid in the synthesis <strong>of</strong> Li 2 ZrO 3 , two<br />
samples without the use <strong>of</strong> citric acid have been synthesized for<br />
comparison. The adsorption and desorption pr<strong>of</strong>iles <strong>of</strong> CO 2 in<br />
these two samples are also shown in Fig. 2. The Li 2 ZrO 3 sample<br />
prepared by the liquid-phase method, i.e. LZ-lp, displays the<br />
lowest CO 2 uptake rate under the investigated conditions, see<br />
curve c in Fig. 2. Although the Li 2 ZrO 3 sample prepared by the<br />
coprecipitation method, i.e. LZ-co, shows some improved CO 2<br />
capture properties in comparison with LZ-lp, its CO 2 uptake is<br />
still slower, compared to that in the Li 2 ZrO 3 sample prepared by<br />
the sol–gel method, i.e. LZ-sg. These results clearly show that<br />
LZ-sg has the best CO 2 capture properties.<br />
It has been reported that the crystallite phase <strong>of</strong> Li 2 ZrO 3 can<br />
have a great influence on CO 2 capture properties. 11 Fig. 3 shows the<br />
XRD patterns <strong>of</strong> the Li 2 ZrO 3 samples prepared by the aforementioned<br />
three methods, indicating that both tetragonal and<br />
monoclinic crystallite phases are present in all samples. However,<br />
as shown in Fig. 3, the crystallite phase <strong>of</strong> LZ-sg is mainly<br />
tetragonal. Nair et al. 11 reported that Li 2 ZrO 3 crystallites with<br />
a tetragonal phase showed better CO 2 -capture properties in terms<br />
<strong>of</strong> uptake rate than those with a monoclinic phase. The current<br />
results are in agreement with this observation from the literature.<br />
Fig. 1 TGA and DTA curves <strong>of</strong> the as-obtained sample synthesized via<br />
the citrate sol–gel method with a heating rate <strong>of</strong> 20 K min :1 in an air flow<br />
<strong>of</strong> 40 ml min :1 .<br />
Fig. 2 Adsorption and desorption pr<strong>of</strong>iles <strong>of</strong> CO 2 for three different<br />
Li 2 ZrO 3 samples. (a) LZ-sg, (b) LZ-co, (c) LZ-lp; (I) adsorption at 823 K<br />
and a CO 2 partial pressure <strong>of</strong> 0.5 bar, (II) desorption by heating (10 K<br />
min :1 ) to 923 K in a pure N 2 flow <strong>of</strong> 40 ml min :1 .<br />
3840 | J. Mater. Chem., 2011, 21, 3838–3842 This journal is ª The Royal Society <strong>of</strong> <strong>Chemistry</strong> 2011
Fig. 3 XRD patterns <strong>of</strong> three different Li 2 ZrO 3 samples. (a) LZ-sg, (b)<br />
LZ-co, (c) LZ-lp.<br />
The uptake rate <strong>of</strong> CO 2 should be also related to the crystallite<br />
size <strong>of</strong> Li 2 ZrO 3 . The crystallite sizes <strong>of</strong> the three different<br />
samples, estimated from the XRD data using Scherrer’s equation,<br />
are 17, 26, and 34 nm for LZ-sg, LZ-co, and LZ-lp,<br />
respectively. It is logical from an engineering point <strong>of</strong> view that<br />
the smaller the size <strong>of</strong> Li 2 ZrO 3 crystals, the faster the CO 2 uptake<br />
in case <strong>of</strong> a diffusion limited process.<br />
The stability <strong>of</strong> an adsorbent is crucial when applied to the in<br />
situ removal <strong>of</strong> CO 2 produced in some reactions such as steam–<br />
methane reforming (SMR) and water–gas shift (WGS) reaction<br />
to enhance hydrogen production as well as in the separation <strong>of</strong><br />
CO 2 from fuel-fired power stations at high temperatures.<br />
Therefore, the stability <strong>of</strong> LZ-sg was investigated by TGA. Fig. 4<br />
shows three CO 2 capture–regeneration cycles, indicating an<br />
excellent stability <strong>of</strong> the Li 2 ZrO 3 crystals prepared by the citrate<br />
sol–gel method. This indicates the potential applicability <strong>of</strong> the<br />
synthesized nanosized Li 2 ZrO 3 crystallites with a tetragonal<br />
phase in the removal <strong>of</strong> CO 2 at high temperatures.<br />
Fig. 5A, B, and C show the morphologies <strong>of</strong> the unused, onecycle<br />
used, and three-cycles used LZ-sg samples by SEM images,<br />
indicating that all the samples consist <strong>of</strong> crystallite agglomerates,<br />
confirmed by the TEM observation, as shown in Fig. 5D.<br />
Additionally, these SEM images give evidence that the particles<br />
do not become larger after CO 2 capture–regeneration cycles,<br />
unlike CaO- or MgO-based adsorbents, in which the particles are<br />
easily enlarged after regeneration at high temperatures, 20 resulting<br />
in the rapidly reduced uptake rate <strong>of</strong> CO 2 . The results from<br />
Fig. 5 SEM images <strong>of</strong> the unused (A), one-cycle used (B), and threecycles<br />
used (C) LZ-sg samples and the TEM image (D) <strong>of</strong> the unused<br />
LZ-sg sample.<br />
Fig. 6 Uptake pr<strong>of</strong>iles <strong>of</strong> CO 2 in LZ-sg at different partial pressures <strong>of</strong><br />
CO 2 and 823 K.<br />
Fig. 5 give an explanation for the stability <strong>of</strong> the Li 2 ZrO 3<br />
prepared by the citrate sol–gel method for CO 2 capture.<br />
The uptake pr<strong>of</strong>iles <strong>of</strong> CO 2 in LZ-sg at different partial pressures<br />
<strong>of</strong> CO 2 are shown in Fig. 6. Obviously, the uptake rate<br />
decreases with decreasing CO 2 pressure at the same temperature.<br />
When the partial pressure <strong>of</strong> CO 2 is below 0.25 bar, the uptake<br />
becomes rather slow and the capacity is not fully reached within<br />
60 min. In addition, almost no uptake takes place at CO 2 pressures<br />
below 0.15 bar, probably due to thermodynamic limitations.<br />
Thus, the uptake rate <strong>of</strong> CO 2 in Li 2 ZrO 3 strongly depends<br />
on CO 2 pressure.<br />
Conclusions<br />
Fig. 4 Three cycles <strong>of</strong> CO 2 capture–regeneration in LZ-sg. (I) adsorption<br />
at 773 K and a CO 2 partial pressure <strong>of</strong> 0.5 bar, (II) regeneration by<br />
heating (10 K min :1 ) to 923 K in a pure N 2 flow <strong>of</strong> 40 mL min :1 .<br />
The citrate sol–gel method has been developed to prepare<br />
Li 2 ZrO 3 nanocrystals that can adsorb 26 wt.% CO 2 within 20<br />
min, which is about 90% <strong>of</strong> the stoichiometric capacity <strong>of</strong> CO 2 in<br />
Li 2 ZrO 3 (28.7 wt.%), at 823 K and CO 2 partial pressures above<br />
0.5 bar. The adsorbent shows a good stability, confirmed by<br />
repeated CO 2 capture–regeneration cycles. The small sol–gel<br />
This journal is ª The Royal Society <strong>of</strong> <strong>Chemistry</strong> 2011 J. Mater. Chem., 2011, 21, 3838–3842 | 3841
synthesized Li 2 ZrO 3 crystallites with a tetragonal phase exhibit<br />
better CO 2 capture–regeneration properties than those prepared<br />
by other synthesis methods. The uptake rate <strong>of</strong> CO 2 decreases<br />
with decreasing CO 2 pressure, reaching a thermodynamic limit at<br />
around 0.15 bar. The synthesized Li 2 ZrO 3 nanocrystals with<br />
excellent properties for CO 2 capture–regeneration display a good<br />
prospect for application in the in situ removal <strong>of</strong> CO 2 produced<br />
in some reactions such as steam–methane reforming (SMR) and<br />
water–gas shift (WGS) reaction to enhance hydrogen production<br />
as well as in the separation <strong>of</strong> CO 2 from fuel-fired power stations<br />
at high temperatures.<br />
Acknowledgements<br />
The financial supports by the National Basic Research Program<br />
<strong>of</strong> China (2009CB626607) and the Natural Science Foundation<br />
<strong>of</strong> Zhejiang Province, China (R4080084 and Y4090473) are<br />
gratefully acknowledged.<br />
Notes and references<br />
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3842 | J. Mater. Chem., 2011, 21, 3838–3842 This journal is ª The Royal Society <strong>of</strong> <strong>Chemistry</strong> 2011