Synthesis and CO2 adsorption characteristics of lithium zirconates with high lithia content

Article abstract of DOI:10.1111/j.1551-2916.2010.03769.x

Pure monoclinic phase Li₆Zr₂O₇ and rhombohedral phase Li₈ZrO₆ that coexisted with a fraction of Li₆Zr₂O₇, are synthesized by a liquid phase coprecipitation method and characterized by X-ray diffraction, scanning electron microscopy, and thermo-gravimetric analysis. The high-temperature CO ₂ uptake properties of both samples are systematically investigated. The temperature effect tests indicate that, in the case of low temperature (<973 K), both of the adsorbents exhibit slow CO₂ uptake rates because of the inhibited diffusion of CO₂ in the solid carbonate shell; while at the temperature above the melting point of Li₂CO ₃ (about 983 K), the CO₂ uptake rates are enhanced dramatically for both Li₆Zr₂O₇ and Li ₈ZrO₆, and achieved about 12.3% weight gain and 35.0% weight gain within 15 min at 1073 K, respectively. The thermal stability tests indicate that both samples exhibit gradually reduced capacities during the multicycle processes. The analysis of the crystalline structure reveals that the reduced capacities are resulted from the loss of lithia under high temperatures. Finally, the possible adsorption pathways for both monoclinic phase Li₆Zr₂O₇ and rhombohedral phase Li ₈ZrO₆ are suggested as well.

Full text of DOI:10.1111/j.1551-2916.2010.03769.x

J. Am. Ceram. Soc., 93 [9] 2837–2842 (2010)
DOI: 10.1111/j.1551-2916.2010.03769.x
r 2010 The American Ceramic Society
ournal
J
Synthesis and CO Adsorption Characteristics of Lithium Zirconates
2
with High Lithia Content
w
Xian-Sheng Yin, Shao-Peng Li, Qin-Hui Zhang, and Jian-Guo Yu
State Key Lab of Chemical Engineering, College of Chemical Engineering, East China University of Science
and Technology, Shanghai, 200237, China
Pure monoclinic phase Li Zr O and rhombohedral phase
6
1.5 and 4.0 times higher than that of Li ZrO , respectively, and
2 3
2
7
Li ZrO that coexisted with a fraction of Li Zr O , are syn-
2
it is possible that they might adsorb more CO
Li ZrO under suitable conditions.
2 3
2
than that of
In this paper, the high-
temperature CO adsorption–desorption properties of Li Zr O
8
6
6
7
1
6,17
thesized by a liquid phase coprecipitation method and charac-
terized by X-ray diffraction, scanning electron microscopy, and
thermo-gravimetric analysis. The high-temperature CO uptake
2
6
2
7
and Li ZrO samples are investigated systematically and the
8 6
2
properties of both samples are systematically investigated. The
temperature effect tests indicate that, in the case of low tem-
possible adsorption pathways are proposed.
perature (o973 K), both of the adsorbents exhibit slow CO
Li ZrO þ CO 2Li CO þ ZrO
(1)
2
uptake rates because of the inhibited diffusion of CO in the
2
3
2
2
3
2
2
solid carbonate shell; while at the temperature above the melting
point of Li CO (about 983 K), the CO uptake rates are en-
hanced dramatically for both Li Zr O and Li ZrO , and
2
3
2
6
2
7
8
6
achieved about 12.3% weight gain and 35.0% weight gain
within 15 min at 1073 K, respectively. The thermal stability
tests indicate that both samples exhibit gradually reduced ca-
pacities during the multicycle processes. The analysis of the
crystalline structure reveals that the reduced capacities are re-
sulted from the loss of lithia under high temperatures. Finally,
the possible adsorption pathways for both monoclinic phase
Li Zr O and rhombohedral phase Li ZrO are suggested as
II. Experimental Section
1) Synthesis of Li Zr O Compounds
(
x
y
z
x y z
The Li Zr O compounds were synthesized using a liquid-phase
coprecipitation method, and were designated as LZO-Li6 and
LZO-Li8 corresponding to the target products of Li Zr O and
6
2
7
8 6
Li ZrO , respectively. Starting materials including LiOH.H
2
O,
NH
3
(25% in water) and Zr(NO ) .H O (Shanghai Chemical
3 4 2
6
2
7
8
6
Co., Shanghai, China) were used as received without further
purification. Considering the volatility of lithium under high
temperature during the calcination process, the initial Li/Zr mo-
lar ratios were higher than the stoichiometric amounts in
well.
I. Introduction
18,19
Li Zr O compounds as described in our previous work.
z
x
y
CO
2
is one of the major greenhouse gases and the largest con-
tributor to the global warming effects (about 60% with regard
The detailed preparation procedures were carried out as follow-
ing: Definite Zr(NO .H O and LiOH.H O were weighed with
appropriate Li/Zr molar ratios (4.5 and 10.0 for LZO-Li6 and
LZO-Li8, respectively) and dissolved with deionized water and
)
3 4
2
2
1,2
to its amount in the atmosphere), and coal-burning power
plant is the major source of CO generation with the typical CO
content about 10%–13% ; thus, CO adsorption from the flue
2
2
3
,4
4 3
NH OH solution (2.5 wt% NH ), respectively; After adding the
2
gas might be an effective approach for CO
2
capture and storage
captors,
including zeolites, carbon-based adsorbent, hydrotalcites,
Zr(NO
3
)
4
.H O solution to the lithium hydroxide solution drop
2
(
CCS). Various materials have been proposed as CO
2
by drop and heating in an oil bath at 363 K for 12 h with vig-
orous stirring, the achieved solid mixture was dried at 393 K for
12 h in the oven, and then ground with mortar and installed in
an alumina crucible to be calcinated at 1223 K for a definite time
(24 h and 72 h for LZO-Li6 and LZO-Li8, respectively).
5
6
7,8
9
,10
and metal oxides.
However, as the temperature of the flue
gas vent is about 573–773 K depending on various follow-up
process of waste heat utilization, it is highly desired to separate
CO directly from flue gas at high temperature and the hot CO
2
2
flow could be used directly for CO
syngas (H
2
reforming of methane for
and CO) with little heat transfer, and the adsorbents
(
2) Characterization Methods
The crystalline phases were identified using X-ray diffraction
XRD, D/max-RB using CuKa Ni-filtered radiation with
8
2
should generally have high selectivity and adsorption capacity
for CO at high temperature, as well as good adsorption–regen-
eration kinetics and high multicycle stability. Li
shown CO selectivity and high adsorption capacity of 28.7
(
2
˚
l 5 1.5406 A, Rigaku, Tokyo, Japan), operating at 40 kV, 100
mA, and scanning in the 2y range of 10–801. The relative per-
centages of different phases presented in the product were esti-
mated from the total area under the most intense diffraction
peak for each phase identified, with the assumption that the
peak area was proportional to the volume fraction of each cor-
1
1
2 3
ZrO has
2
acceptor weight percent by operating at 673–973 K based on the
reaction (1), in which, the activation of the adsorbent is origi-
nated from the chemical reaction between CO and Li derived
2
1
from the lithium-based ceramic; hence, the lithium content in the
ceramic might have a great effect on the CO adsorption capac-
20
responding phase. The surface morphology was analyzed by
2
scanning electron microscopy (SEM, JSM-6360LV, JEOL,
Tokyo, Japan). Samples for SEM were covered with gold to
overcome their poor electric conductivity before analysis.
3
ity.
,4,12–15
6 2 7 8 6
The Li/Zr molar ratios of Li Zr O and Li ZrO are
R. Snyder—contributing editor
(
3) Properties Study of CO Uptake and Thermal Stability
2
2
CO adsorption kinetics and capacity were studied using a the-
Manuscript No. 27202. Received December 5, 2009; approved March 6, 2010.
This work was supported by NSFC (20576031 and 20976047), STCSM (0852nm02100),
and SICP (08160704000).
rmo-gravimetric analyzer (TGA, SDTQ600, TA instruments,
New Castle, DE). About 15 mg samples were placed in the
sample pan and heated to the operating temperature with a rate
wAuthor to whom correspondence should be addressed. e-mail: qhzhang@ecust.edu.cn
2
837

2
838
Journal of the American Ceramic Society—Yin et al.
Vol. 93, No. 9
of 20 K/min under 100 mL/min in N
switched to pure CO flow for the test of isothermal adsorption
properties. Desorption properties were studied by heat treating
the CO -adsorbed sample under various temperatures in N
2
flow. Then, N
2
flow was
(3) CO Adsorption–Desorption Properties and Pathway of
2
LZO-Li6 Products
2
The operating temperature is a key parameter to the CO adsorp-
2
2
2
tion rate. Figure 3 shows the isothermal adsorption and desorp-
tion graphs at different temperatures (773, 923, 973, and 1073 K
for adsorption, 1073, 1123, and 1173 K for desorption, respec-
tively). As shown in Fig. 3(a), the sample LZO-Li6 is able to up-
flow. Thermal stability of samples was studied over consecutive
experiments of adsorption–regeneration at appropriate temper-
atures under CO and N atmosphere, respectively.
2
2
take CO
gradually increased uptake rate with the enhancement of temper-
ature. The cases operating at 773 and 923 K present slow CO
2
in the whole range of temperatures tested, and exhibits a
III. Results and Discussion
2
adsorption rates and poor capacities in the allowed time. It is
suggested that the solid carbonate shell is formed on the surface of
particles as the working temperature below the melting point of
Li CO (about 983 K), and consequently limits CO diffusion to
(
1) The Characterization of Crystalline Structure
XRD results of LZO-Li6 and LZO-Li8 are shown in Fig. 1. The
sample LZO-Li6 is composed of pure monoclinic phase
˚
Li Zr O (JCPDS 34-0312, a 5 10.45 A, b 5 5.99 A, and
2
3
2
˚
6
2
7
the reaction interface during the adsorption process in the lithium-
This fact must be the main reason resulting in
˚
24,25
based ceramics.
c 5 10.21 A). However, the sample LZO-Li8 is a mixture of
monoclinic phase Li Zr and rhombohedral phase Li ZrO
JCPDS 26-0867, a 5 5.48 A, and c 5 15.45 A), and the propor-
tion of Li ZrO and Li Zr O are about 60.8% and 39.2%, re-
6
2
O
7
˚
8
6
the slow uptake rates here. In addition, the large particle size an1d
serious aggregates (as shown in Fig. 2) must also restrict the Li
˚
(
2ꢀ
17,26
8
6
6
2
7
and O migrating through the Li–Zr–O matrix core.
As the
spectively, which are estimated semiquantitatively by calculating
from the total area under the most intense diffraction peak of
temperature increases up to 973 K, a skipped enhancement of the
adsorption rate can be observed. For 973 K is close to the melting
point of Li CO , the submolten state Li CO formed during the
adsorption process may improve the CO diffusion in the carbo-
2
2
nate shell, and enhance the CO uptake rate subsequently com-
each phase, (11-1) crystal face for monoclinic phase Li
6 2 7
Zr O
2
3
2
3
and (101) crystal face for rhombohedral phase Li ZrO . It is
8
6
8 6
reported that pure Li ZrO is difficult to prepare due to the loss
of lithia by volatilization on prolonged heating at temperatures
pared with that occurring in solid carbonate shell. As the
temperature further increases to 1073 K, the CO capture rate is
1
9,21
above 1073 K.
generally were carried out under rigorous conditions such as
In addition, the synthesis methods reported
2
faster and reaches 12.3% weight gain within only about 10 min
and then flattened out. This behavior can be explained by a faster
CO diffusion in the formed molten state carbonate shell, and is
2
2
calcination under ultrahigh vacuum. In our experimental pro-
cedures, a mass of lithia must be sublimated inevitably because
the samples are calcinated directly in static air, subsequently,
2
consistent with the previous suggestion. Figure 3(b) shows the
leading to the production of Li Zr
6
O
2 7
in the final product.
CO
2
desorption property investigated at different temperatures in
can be released effectively at
the operating temperatures, and the CO desorption time could be
N
2
flow. The results indicate that CO
2
2
(
2) The Investigation of Surface Morphology
shorted from 120 to 50 min as the operating temperature increased
from 1073 to 1173 K.
In order to detect the detailed reactions occurring during the
above-mentioned adsorption processes, the CO -adsorbed sam-
2
ples gained by heat treating the sample LZO-Li6 in a tube-
The surface morphology of the Li Zr
x
O
y z
products is shown in
Fig. 2. Both of LZO-Li6 and LZO-Li8 are made up of large
particles with irregularly polyhedron-shaped structures as
shown in Figs. 2(a) and (c), respectively. Moreover, the magni-
fied picture shown in Fig. 2(b) indicates that the large particles
of LZO-Li6 are built up by amount of small dense particles with
size ranging between 1.0 and 5.0 mm, while the sample LZO-Li8
exhibits a bulk covered with many small particles on the surface
as shown in Fig. 2(d). It is clear that the aggregation that
occurred in the sample LZO-Li8 is more serious than that of
LZO-Li6. This may result from the prolonged calcination time
and increased calcination temperature conducted for LZO-Li8;
in addition, the higher content of lithium in the process of syn-
2
furnace at different temperatures for 30 min under CO atmo-
sphere and then directly cooling to room temperature out of the
furnace, are further analyzed using XRD analysis as shown in
Fig. 4. In pattern (b), the diffraction peaks of the sample ad-
sorbed at 873 K are mainly made up of monoclinic phase
Li Zr O and a fraction of monoclinic phase Li CO (at 21.34,
6
2
7
2
3
˚
˚
˚
2
3.061, JCPDS 22-1141, a 5 8.36 A, b 5 4.98 A, and c 5 6.19 A).
This result indicates that only a small amount of CO has re-
acted with Li Zr within the allowed time at 873 K, and is
2
6
2 7
O
1
9
thesis may further aggravate the occurrence of agglomeration.
Anyway, the results obtained suggest that it may be difficult for
consistent with the slow rate of CO adsorption at low temper-
2
atures illustrated in Fig. 3(a). However, in pattern (c), the
diffraction peaks of the sample adsorbed at 973 K are changed
significantly compared with that of pattern (a), and mainly com-
2
3
CO
2
to penetrate the particles, and as a consequence, the re-
is expected to be slow for both of the ad-
sorbents produced here.
action rate with CO
2
˚
posed of monoclinic phase Li ZrO (JCPDS 33-0843, a 55.43 A,
2
3
˚
˚
2 3
b 5 9.03 A, and c 5 5.42 A), Li CO and some unidentified
phase (at 18.401, 23.101, and 30.481), while only a trace of peaks
of monoclinic phase Li Zr O (2y 5 17.241) are detected. The
6
2
7
diffraction peaks unidentified may be an interphase of Li
because similar patterns are observed during the synthesis pro-
cesses of Li ZrO using LiOH and ZrO in our experiment. In
addition, similar XRD patterns of Li ZrO were also reported
2 3
ZrO
2
3
2
2
3
2
7
by Iwan et al. In pattern (d) (adsorbed at 1073 K), there are no
peaks of monoclinic phase Li Zr detected, and the diffrac-
tion peaks are attributed to only Li CO and Li ZrO , indicat-
6
2 7
O
2
3
2
3
ing that the reaction between CO and LZO-Li6 is complete.
2
Combining the above illustrations, the possible reaction path-
way of high-temperature CO
Li Zr O could be inferred as in Eq. (2). The theoretical CO
2
2
adsorption for monoclinic phase
6
2
7
adsorption capacity for Li Zr O is 13.1% weight gain
6
2
7
according to Eq. (2), close to the experimental result (12.3%
weight gain) analyzed by TG previously:
Fig. 1. X-ray diffraction patterns of LZO-Li6 and LZO-Li8, r(hkl):
rhombohedral phase Li
Li Zr O crystal face.
6 2 7
8 6
ZrO crystal face, m(hkl): monoclinic phase
Li6Zr2O7ðmonoclinicÞ þ CO2 ! Li2CO3 þ 2Li2ZrO3 (2)

September 2010
CO Adsorption on Lithium Zirconates
2
2839
Fig. 2. Scanning electron micrographs of LZO-Li6 (top) and LZO-Li8 (bottom).
The thermal stability of sample LZO-Li6 is tested with the
multicycle method. According to the above analyzed results, the
manipulating conditions are set as following: the sample is
ꢀ
1
heated to 1023 K at a rate of 20 K min in N
atmosphere, following to a switch in the atmosphere to CO2
100 mL/min) to uptake CO for 60 min; after this, the atmo-
sphere is switched to N and the temperature is up to 1123 K at a
rate of 10 K/min to release CO for 60 min; finally, the temper-
ature is down to 1023 K at a rate of 10 K/min and the atmo-
sphere is switched to CO for the next cycle. As shown in Fig. 5,
the adsorbent exhibits fast CO uptake rate in the three cycles,
2
(100 mL/min)
(
2
2
2
2
2
yet the CO adsorption capacities are reduced gradually for the
2
second and third cycles. These results indicate that the sample
LZO-Li6 holds weak cycle stability and a low CO
capacity.
2
adsorption
To understand the gradually reduced capacity of CO2
adsorption, the crystalline structure of the samples used for
different cycle times are investigated with XRD analysis. The
CO -adsorbed samples that underwent different cycle times are
2
gained by heat treating the sample LZO-Li6 in a tube furnace
Fig. 4. The X-ray diffraction patterns of samples derived from LZO-
Li6 before and after CO
tion at 873 K, (c) adsorption at 973 K, and (d) adsorption at 1073 K (&,
monoclinic phase Li Zr , Li CO ; J, monoclinic phase Li ZrO
& , unknown phase).
2
adsorption. (a) Before adsorption, (b) adsorp-
Fig. 3. Adsorption kinetics of CO
LZO-Li6 at different temperatures in CO
00 mL/min, respectively.
2
(a) and desorption kinetics (b) on
2
flow and N flow at a rate of
2
6
2 7
O ;
ꢁ
2
3
2
3
;
1

2
840
Journal of the American Ceramic Society—Yin et al.
Vol. 93, No. 9
Fig. 5. Thermal stability of LZO-Li6 during consecutive adsorption–
regeneration cycles.
with the same conditions of adsorption and desorption as set in
Fig. 5. During the process of multicycle, the samples used for 1,
2, and 3 cycles (corresponding to the points of A, B, and C as
shown in Fig. 5) are taken out from the tube-furnace directly,
and after cooling to room temperature, are characterized with
XRD respectively. As shown in Fig. 6, patterns of (a), (b), and
(
c) correspond to the samples used for 1, 2, and 3 cycles, re-
spectively. In pattern (a), the diffraction peaks of sample mostly
match triclinic phase Li Zr (JCPDS 36-0122). However, in
patterns of (b) and (c), the peaks are indexed to triclinic phase
Li Zr and monoclinic phase Li ZrO3, which are identified
6
2 7
O
6
O
2 7
2
according to the specific order of peaks intensity, indicating that
a mass of Li O must be sublimated under high temperature and
causes the deficiency of lithium during cycle processes. Thus, the
2
CO
as Eqs. (3) and (4). Firstly, Li
Li O, which sublimated partly under high temperature; subse-
2
desorption processes of sample LZO-Li6 can be suggested
2
CO is decomposed to CO and
3
2
2
quently, part of Li
phase Li Zr , keeping the superfluous part remaining. Hence,
the reduced capacity of CO adsorption during the multicycle
processes must result from the loss of lithia under high temper-
ature:
2 3 2
ZrO reacts with Li O to produce triclinic
6
2 7
O
2
Fig. 7. Adsorption kinetics of CO
2
(a) and desorption kinetics (b) on
LZO-Li8 at different temperatures in CO
1
2 2
flow and N flow at a rate of
00 mL/min, respectively.
Li2CO3 ! CO2 " þð1 ꢀ xÞLi2O þ xLi2O "
Li2O þ 2Li2ZrO3 ! Li6Zr2O7 ðtriclinicÞ
(3)
(4)
to the limitation of CO diffusion in the formed solid carbonate
2
1
2ꢀ
shell and the restriction of ion (Li and O ) migration in the
aggregated particle at low temperature as described in LZO-Li6.
As the temperature increases up to 973 K, a skipped weight gain
(
about 11.0%) is detected within the initial 5 min; after this, the
(
LZO-Li8
4) CO Adsorption–Desorption Properties and Pathway of
2
uptake rate is reduced gradually and reaches another 20.0%
weight gain within the following 2 h. This behavior may be due
to the thick carbonate shell formed progressively that prolongs
Figure 7 shows the CO capture and regeneration properties of
sample LZO-Li8 under different temperatures. As shown in
2
the time of CO diffusing to the reaction interface, which will be
2
Fig. 7(a), the CO uptake rate is slow at 923 K. This may be due
2
discussed further in the following section. As the temperature
2
increases up to 1023 K, the CO uptake rate increases signifi-
cantly and achieves about 35.0% weight gain within 20 min and
then flattens out. Furthermore, the uptake rate is much faster
for the case at 1073 K and reaches 35.0% weight gain within
only 15 min. The fast kinetics of CO
023 and 1073 K must be due to the formed molten carbonate
shell that improves the CO diffusion as described earlier. Figure
2
adsorption for the cases at
1
2
7
(b) shows the test of CO
under N atmosphere. The results indicate that the adsorbed
CO could be released effectively under appropriate operating
2
desorption between 1073 and 1173 K
2
2
conditions, and the CO
within about 60 min as desorbed at 1173 K. To the best of our
knowledge, this is the first time the Li ZrO as a high-temper-
ature CO adsorbent is reported up to date.
2
desorption process could be completed
8
6
2
Figure 8 compares the X-ray diffraction pattern of LZO-Li8
with that of the CO -adsorbed and CO -desorbed samples
gained by the same procedure as that of LZO-Li6. Compared
with CO -free sample, the sample that adsorbed CO at
2
2
Fig. 6. The XRD patterns of samples derived from LZO-Li6 used for
different cycles. (a) One cycle, (b) two cycles, and (c) three cycles. (.,
2
2
923 K (corresponding to pattern (b)) is mainly composed of
Li CO and monoclinic phase Li Zr , as well as a trace of
triclinic phase Li Zr
6
2
O
7
; J, monoclinic phase Li ZrO
2
3
).
2 7
O
2
3
6

September 2010
CO Adsorption on Lithium Zirconates
2
2841
and (5) , which provides with a high CO
5
about 60.8% proportion of Li ZrO and 39.2% proportion of
2
adsorption capacity of
4.4% weight gain. Considering the previous XRD analysis that
8
6
Li
addition of LZO-Li8 after CO
culated by Eqs. (2) and (5), close to the experimental result of
5.0%, further confirming the speculation suggested here. In
succession, pattern (e) shows the crystalline phases of the CO
released sample gained by heating the CO -adsorbed sample
adsorbed CO at 1023 K for 30 min) at 1173 K for 120 min
Zr O
6 2 7
are present in the sample LZO-Li8, the total weight
2
adsorption is about 38.2% cal-
3
2
-
2
(
2
under N
most pure triclinic phase Li
2
atmosphere, and the diffraction peaks comprise of al-
Zr except some unknown peaks
6
2 7
O
at 16.701, 24.141, and 28.161, which may be the impurity phase
introduced during the CO desorption process. It is reported
2
that the synthesis of Li
8
ZrO
Hence, the short desorption time in this
study may lead to only Li Zr being produced with no
Li ZrO in the desorbed sample, and the Eqs. (3) and (4) can
6
needs a long duration above
Fig. 8. The X-ray diffraction patterns of samples derived from LZO-
21,22,28
1073 K.
Li8 before CO
2
adsorption, after CO
a) Before adsorption, (b) adsorption at 923 K, (c) adsorption at 973 K,
d) adsorption at 1023 K, and (e) desorption at 1173 K (m, rho-
ZrO ; &, monoclinic phase Li Zr , Li CO
J, monoclinic phase Li ZrO ; &, triclinic phase Li ; & , un-
known phase).
2 2
adsorption, and CO desorption.
(
(
6
2 7
O
8
6
mbohedral phase Li
8
6
6
2
O
Zr O
6 2 7
7
;
ꢁ
2
3
;
also be considered as the possible reaction during the desorption
process.
Figure 9 shows the thermal stability of sample LZO-Li8
tested under the conditions adsorbed at 1023 K for 30 min
and desorbed at 1173 K for 60 min, respectively. The result in-
2
3
2
dicates that the CO uptake capacity reaches about 35.0%
weight gain for the first cycle, and subsequently reduced grad-
ually for the following cycles. For the fourth cycle, the maxi-
mum CO uptake capacity is reduced to about 23.5% of weight
2
gain. Combining the XRD analysis in Fig. 8 and the previous
explanation in the part of LZO-Li6, the loss of lithia under high
rhombohedral phase Li ZrO , indicating that the adsorption
6
8
reaction mainly occurred between Li ZrO and CO at low tem-
8
6
2
peratures. In the point of 973 K, the diffraction peaks are made
up of only Li CO and monoclinic phase Li Zr while no
Li ZrO can be detected. Hence, it can be inferred that CO may
2
3
6
2 7
O
8
6
2
temperature must be the main reason for the reduced CO
take capacity during the cycles.
2
up-
react with Li ZrO
8
6
as Eq. (5) during the adsorption processes:
2Li8ZrO6 þ 5CO2 ! 5Li2CO3 þ Li6Zr2O7
(5)
IV. Conclusions
Pure monoclinic phase Li Zr O (sample LZO-Li6) and rho-
However, in the case of LZO-Li6, the monoclinic phase
Zr has almost completely disappeared in the sample ad-
sorbed at 973 K. This difference may be due to more Li CO
being produced in LZO-Li8 than that of LZO-Li6 during the
CO adsorption process, as revealed in Figs. 3(a) and 7(a); the
isothermal CO uptake test at 973 K exhibits that more than
0.0% weight gain was achieved within the initial 30 min for
6
2
7
mbohedral phase Li
Li Zr (sample LZO-Li8) are synthesized by liquid-phase
coprecipitation method and applied as high-temperature CO
adsorbents. At the temperatures o973 K, both adsorbents ex-
hibit slow CO uptake rate because of the limitation of CO
diffusion in the solid carbonate shell and ion (Li and O
migration in the large particles. At temperatures higher than the
melting point of Li CO (about 983 K), both Li Zr O and
8 6
ZrO that coexisted with a fraction of
Li
6
2 7
O
6
2 7
O
2
3
2
2
2
2
2
1
2ꢀ
)
2
LZO-Li8, while only about 11.0% weight gain for LZO-Li6 was
observed. The thicker carbonate shell formed in sample LZO-
2
3
6
2
7
8 6 2
Li ZrO exhibit a skipped increase of CO uptake rate with
2
Li8 may prolong the time of CO diffusing to the interface, and
consequently the monoclinic phase Li Zr O in the sample
6
LZO-Li8 could not react with CO effectively in the allowed
2
about 12.3% and 35.0% weight gain within 15 min at 1073 K,
respectively. The adsorbed samples of monoclinic phase
2
7
Li
and the regenerated sample comprises of triclinic phase
Li Zr , while the rhombohedral phase Li ZrO firstly pro-
duces monoclinic phase Li Zr O during the CO adsorption
6 2 7 2 3 2 3
Zr O comprises of monoclinic phase Li ZrO and Li CO ,
operating time.
As the temperature increases up to 1023 K, which is above the
melting point of Li CO , the diffraction peaks of the CO -
6
O
2 7
8
6
2
3
2
adsorbed sample (corresponding to pattern (d)) mainly com-
prise of monoclinic phase Li ZrO and Li CO , similar to the
case of Li Zr illustrated in Fig. 4(c). Thus, the CO adsorp-
tion process of sample Li ZrO may comprise of reactions (2)
6
2
7
2
process, and then proceeds to the same reaction pathways as
that of monoclinic phase Li Zr . The thermal stability tests
2
3
2
3
6
O
2 7
2
6
2 7
O
indicate that both the adsorbents exhibit a gradually reduced
adsorption capacity during the multicycle processes because of
8
6
2 2 3
the volatilization of Li O decomposed from Li CO under high
temperature.
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&