Supporting information (Environmental Science and Technology) for
Tailor-Made Core－Shell CaO/TiO2－Al2O3 Architecture as a
High-Capacity and Long-Life CO2 Sorbent
Weiwei Peng†,‡, Zuwei Xu†, Cong Luo†, Haibo Zhao*,†,‡
† State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,
1037 Luoyu Road, Wuhan 430074, China
‡ China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and
Technology, 1037 Luoyu Road, Wuhan 430074, China
*
E-mail: [email protected], [email protected]; phone: +86 27 8754 4779-8208; fax: +86 27
8754 5526
Environmental Science and Technology
Date: 20th March 2015
20Pages, 12Figures
Text S1–S13
Pages 2,5,6-7,8,9,10,12,14,16-17,18,19
Reference
Page 20
Figures S1–S12
Pages 3-4,5,9,10-11,12-13,15,17,18,19
S1
Text S1-S3
The nano-TiO2, which was used for manufacturing CaO/TiO2－Al2O3 sorbent in our study, was
made by flame synthesis technology – a single step, continuous and scalable process. The gas
flame synthesis facility was designed, as shown in Figure S1, in which a four-concentric-tube
burner was fed with TiCl4 (vapor) and N2 (as dilute gas and carrier gas), CH4 and Ar (fuel), O2
(oxidizer), and N2 (sheath gas), from the center to the outside. The co-flow diffusion CH4 flame
established a controllable and stable high temperature field that provided the energy needed for the
nanoparticle synthesis, including the oxidation of the TiCl4, the crystal transition and morphology
evolution of particles. Some parameters were listed here: pressure of vaporizer was 1 atm, and the
temperature of vaporizer was constant with 65 °C. The flow rate of carrier gas (N2), oxidizing
agent (O2) and shielding gas (N2) was 0.6 SLPM, 2.4 SLPM and 20 SLPM, respectively. The flow
rate of fuel gas, i.e. mixture of CH4 and Ar, was 1.8 SLPM (CH4 accounted for 1.2 SLPM and Ar
accounted for 0.6 SLPM).
Figure S2 shows XRD pattern of flame-synthesized nano-TiO2, which illustrated the complete
rutile phase of TiO2 particles. The crystal structure and morphology of TiO2 particles is showed in
Figure S3 through FSEM and TEM images. The FSEM image presented the uniform distribution
of nano-TiO2. The TEM image exhibited the nano-TiO2 had good crystalline nature within size
range 10~50 nm. The nano-TiO2, with complete rutile phase and good crystalline nature, could be
obtained by regulating gas flow rates, and these characteristics were contributed to the superior
performance of CaO/TiO2－Al2O3.
Figure S1. Schematic of the flame-synthesized rutile nano-TiO2 (product nano-TiO2 as an additive
to self-assembly template synthesis derived sorbents).
S2
Figure S2. XRD pattern of flame-synthesized rutile nano-TiO2.
Figure S3. Images of flame-synthesized rutile nano-TiO2: (a) FSEM photo; (b) TEM photo.
S3
(a)
(b)
S4
Text S4
Figure S4 (a) presents the morphology of CaO-based precursor, which corresponds to step 4 in
the preparation process of self-assembly template synthesis (SATS) method. It can be found that
the sorbent showed fluffy and snow flake-like appearance after drying in an electrically heated
drying oven at 80 °C for 24 h.
Figure S4 (b) shows the photos of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 after 30 cyclic tests
under the severe calcination condition of 900 °C in pure CO2. Shrinkage phenomenon was clearly
observed in calcined CaO and CaO/Al2O3 sorbents. On the contrary, the novel CaO/TiO2－Al2O3
sorbent still remained fluffy appearance, meaning CaO/TiO2 － Al2O3 sorbent had better
sintering-resistant capacity than other sorbents.
Figure S4. Photos of sorbents during preparation processes: (a) CaO-based sorbent precursor; (b)
Sorbents after 30 cycles under the severe calcination condition of 900 °C in pure CO2 for 5 min.
(a)
(b)
Sample A
Sample B
Sample C
S5
Text S5
It is noted that the mass ratio of CaO, Al2O3 and TiO2 was 80 % : 15 % : 5 % in the
preparation process of self-assembly template synthesis method. Liu et al.1, 2 reported that the
maximum CO2 capture capacity of the CaO-based sorbents was directly proportional to the CaO
loading, and the maximum theoretical CaO content at which a continuum framework of inert
support was maintained to separate CaO was 82.7%, as predicted by the three-dimensional
percolation theory. Therefore, the CaO loading in CaO/TiO2－Al2O3 sorbent was chosen to be 80
wt%. In addition, the principle of selecting the mass ratio of Al2O3 and TiO2 is to realize
completely coverage of core-structured Al2O3 by shell-structured TiO2，The theoretical calculation
processes were listed below:
4 × π ×（ D ）2
2
N=
π ×（d 2）2
(1)
d
4
VTiO2 = N × × π × ( )3
3
2
(2)
VAl2 O3 =
x
=
4
D
× π × ( )3
3
2
mAl2 O3 ρ Al2 O3 × VAl2 O3
ρ Al2O3 × D
=
=
mTiO2
ρTiO2 × VTiO2
ρTiO2 × 4 × d
(3)
(4)
where N is the number of TiO2 in core-shell structure, VTiO2 , VAl2 O3 , ρTiO2 and ρ Al2 O3 are the
volume of TiO2, volume of Al2O3, density of TiO2 and density of Al2O3, respectively. D and d is
the radius of Al2O3 and TiO2, respectively, and x is the mass ratio of Al2O3 and TiO2.
According to the material presentation about particle size (i.e., commercial Al2O3 power with
less than 200 meshes, and flame-synthesized nano-TiO2 within size range 10~50 nm), D was
assigned a value of 1000 nm and d was assigned a value of 50 nm. The true density of TiO2 and
Al2O3, which was measured by AccuPyc 1330 automatic true density analyzer (Micromeritics
S6
Instrument Corp), were 3850 kg/m3 and 3970 kg/m3, respectively. Therefore, the mass ratio of
Al2O3 and TiO2 in theory is about 5:1. Taking the safety factor into account, the final mass ratio of
Al2O3 and TiO2 used in preparation processes was determined as 3:1.
To make an accurate comparison between three CaO-based sorbents, the CaO loading in
CaO/Al2O3 was also chosen to be 80 wt%.
S7
Text S6
The calculation formulas of carbonation conversion and carbonation conversion rate in the
study were listed below.
The carbonation conversion of active CaO, XN, was calculated as:
XN
=
mN − mcal M CaO
×
m0 × A
M CO2
(5)
where XN is the carbonation conversion of active CaO in the form of molars of CO2/molar of
active CaO, mN is the mass of sorbent during reaction at cycle N, mcal is the mass of sorbent after
the first calcination, m0 is the initial mass of sorbent, A is the mass fraction of active CaO in initial
sorbent, MCaO and MCO2 are molar mass of CaO and CO2, respectively.
The CO2 uptake quantity per unit sorbent, YN, was given by:
YN =
mN − mcal
mcal
(6)
where YN is the CO2 uptake capacity in the form of grams of CO2/gram of sorbent
The carbonation conversion rate, R, was calculated as:
R=
dX N
dt
(7)
where R is the carbonation conversion rate, XN is the carbonation conversion of active CaO, t is
the reaction time.
S8
Text S7
Figure S5 and S6 presents the CO2 capture performance of CaO, CaO/Al2O3 and CaO/TiO2－
Al2O3 with reaction time under the mild and severe calcination conditions, which proved the
high-capacity and long-life of the novel CaO/TiO2－Al2O3 sorbent directly.
Figure S5. CO2 capture performance of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 with reaction
time under the mild calcination condition; carbonated at 700 °C in 10 vol% CO2 for 20 min and
calcined at 700 °C in pure N2 for 20 min.
Figure S6. CO2 capture performance of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 with reaction
time under the severe calcination condition; carbonated at 700 °C in 10 vol% CO2 for 20 min and
calcined at 900 °C in pure CO2 for 5 min
S9
Text S8
As shown in Figure S7, carbonation conversion in the first (a), tenth (b), twentieth(c) cycles,
which was conducted in the mild calcination condition of 700 °C in pure N2, exhibited the
evolution of CO2 capture capacity of unit of CaO in sorbents. All of three sorbents showed a
typical CO2 capture profile characterized by a fast chemical reaction controlled phase which
transits abruptly to a slow diffusion controlled phase. The chemical reaction controlled phase
accounted for a large part of the ultimate absorption ratio measured after 20 min of carbonation
process. In the first cycle, the carbonation conversion of three sorbents was between 0.69 mol
CO2/ mol CaO (mol/mol for logogram in the following descriptions) and 0.75 mol/mol. However,
a big difference occurred with increasing of cycles, and the SATS-derived CaO/TiO2－Al2O3
sorbent achieved the highest carbonation conversion of around 0.89 mol/mol in the 10th cycle
compared with 0.80 mol/mol of CaO/Al2O3 and 0.49 mol/mol of pure CaO. Even after 20
carbonation/calcination cycles, the carbonation conversion of CaO/TiO2 －Al2O3 reached the
considerable value of 0.8 mol/mol.
FigureS7. Evolution of the carbonation conversion of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3
under the mild calcination condition, Carbonated at 700 °C in 10 vol% CO2 for 20 min and
calcined at 700 °C in pure N2 for 20 min. (a) carbonation conversion at the 1st cycle; (b)
carbonation conversion at the 10th cycle; (c) carbonation conversion at the 20th cycle
S10
S11
Text S9
Evolution of the carbonation conversion of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 under the
severe calcination condition of 900 °C in pure CO2 is shown in Figure S8. In the first carbonation
process, CaO presented superior carbonation conversion of 0.91 mol/mol, compared with 0.78
mol/mol of CaO/Al2O3 and 0.77 mol/mol of CaO/TiO2－Al2O3. Degraded carbonation conversion
was observed in the tenth cycle for all of three sorbents, CaO/TiO2－Al2O3 suffered from a 12%
decline compared to about 28% for CaO/Al2O3 and about 39% for CaO. The drop-off in the first
10 cycles suggested that the degraded reactivity was attributable to a loss of reactive surface area
and pore volume due to sintering. After 30 calcination/carbonation cycles, the carbonation
conversion of CaO/TiO2－Al2O3 was stabilized at 0.68 mol/mol, compared with 0.48 mol/mol for
CaO/Al2O3 and 0.4 mol/mol for CaO. It was concluded that CaO/TiO2 － Al2O3 was less
susceptible to sintering under the severe calcination condition, and exhibited higher reactivity and
satisfactory mechanical strength.
Figure S8. Evolution of the carbonation conversion of CaO, CaO/Al2O3 and CaO/TiO2－Al2O3
under the severe calcination condition, carbonated at 700 °C in 10 vol% CO2 for 20 min and
calcined at 900 °C in pure CO2 for 5 min. (a) carbonation conversion at the 1st cycle; (b)
carbonation conversion at the 10th cycle; (c) carbonation conversion at the 20th cycle; (d)
carbonation conversion at the 30th cycle
S12
S13
Text S10
Figure S9 is the carbonation conversion and carbonation reaction rate of CaO/TiO2－Al2O3 at
the 1st, 10th, 20th, 30th and 104th cycles under the severe calcination condition of 900 °C in pure
CO2. According to the carbonation conversion curve in Figure S9 (a), the interaction between
CaO/TiO2－Al2O3 sorbent and CO2 could be divided into two distinct different stages. The first
stage (chemically controlled initial reaction period) is followed by a much slower product
diffusion stage. This shift in the reaction mechanism occurred when small pores were blocked by
the formation of a 50-nm-thick non-porous carbonate product layer.3 As indicated in Figure S9 (a),
CaO/TiO2－Al2O3 nearly finished the chemically controlled phase after 3min, regardless of the
number of cycle. The carbonation conversion of CaO/TiO2－Al2O3 in cycle 1, cycle 10, cycle 20,
cycle 30 and cycle 104 was 0.80 mol/mol, 0.72 mol/mol, 0.69 mol/mol, 0.67 mol/mol and 0.63
mol/mol, respectively. The decay ratio during the first 30 cycles was only 7%, and finally obtained
a satisfactory decay rate of 12.5%. According to Figure S9 (b), CaO/TiO2－Al2O3 suffered from a
decrease of the carbonation reaction rate during the first 10 cycles, which was attributed to the
decrease of surface area and pore volume caused by particle sintering. However, CaO/TiO2－
Al2O3 remained stable and fast carbonation reaction rate even after 104 cycles, which proved its
high reactivity.
Figure S9. Carbonation reaction performance of CaO/TiO2－Al2O3 at the 1st, 10th, 20th, 30th and
104th cycles under the severe calcination condition; Carbonated at 700 °C in 10 vol% CO2 for 20
min and calcined at 900 °C in pure CO2 for 5 min. (a) evolution of carbonation conversion; (b)
evolution of carbonation reaction rate
S14
S15
Text S11
The Figure 3(b) in manuscript shows the result of pore size distribution, the surface of pores
in diameter of 30-100 nm of the CaO/TiO2－Al2O3 sorbent, which was important for CO2
adsorption, was smaller than that of CaO. However, the surface of pores in diameter of 30-100 nm
for the used CaO/TiO2－Al2O3 sorbent after 30 cycles is much larger than that of used CaO.
In details, Figure S10(a) exhibits the pore size distribution within range from 1 nm to 180 nm
for fresh and used CaO and CaO/TiO2 －Al2O3, and Figure S10(b) exhibits the pore size
distribution within range from 30 nm to 100 nm for fresh and used CaO and CaO/TiO2－Al2O3.
The used CaO sorbent and CaO/TiO2－Al2O3 sorbent were conducted in the severe calcination
condition of 900 °C in pure CO2 and experienced 30 cyclic tests. The horizontal ordinate and
vertical coordinate in Figures are set as the form of common logarithm to clearly present the
change of pore size distribution in diameter of 30-100 nm. It can be found from Figure 10(a) that
the pore distribution was mainly on the diameter of 2-10 nm and the pore diameter of 30-100 nm
accounted for a small proportion in total amount. Figure 10(b) presents clearly that the fresh CaO
showed a larger surface of pores in diameter of 30-100 nm than that of CaO/TiO2－Al2O3, which
agrees with the results of CO2 capture capacity at the first cycle under severe calcination condition
(shown in Figure 2(a) of manuscript), however, after 30 cyclic tests, the pore diameter of 30-100
nm for CaO decreased obviously while CaO/TiO2－Al2O3 remained a stable value. This may be
attributed to competition between positive “self-activation” function and negative sintering effect
of CaO/TiO2－Al2O3 sorbent. At the first cycle, CaO/TiO2－Al2O3 sorbent and CaO suffered from
high-temperature sintering; what is more, CaO/TiO2－Al2O3 sorbent lost a few of active CaO due
to the chemical reaction between CaO and TiO2. However, the “self-activation” function of
S16
CaO/TiO2－Al2O3 sorbent in cyclic tests contributed to the development of fissures, which led to a
rise in porosity and reaction surface (or volume), and CaO/TiO2－Al2O3 sorbent performed better
sintering-resistance than CaO during successive cycles due to its superior core-shell structure
(which was proved by FSEM results in this study). Therefore, the CO2 capture capacity of CaO
was better than CaO/TiO2－Al2O3 at the first cycle under the severe calcination condition, and
CaO/TiO2－Al2O3 showed a rather high-performance after 30 cycles.
Figure S10(a). Pore size distribution within range from 1 nm to 180 nm for fresh and used CaO
and CaO/TiO2－Al2O3 (used CaO and CaO/TiO2－Al2O3 were conducted in the severe calcination
condition of 900 °C in pure CO2); horizontal ordinate and vertical coordinate are set as the form of
common logarithm.
Figure S10(b). Pore size distribution within range from 30 nm to 100 nm for fresh and used CaO
and CaO/TiO2－Al2O3 (used CaO and CaO/TiO2－Al2O3 were conducted in the severe calcination
condition of 900 °C in pure CO2); horizontal ordinate and vertical coordinate are set as the form of
common logarithm.
S17
Text S12
Figure S11 shows the zeta potentials of micron-Al2O3 and flame-synthesized nano-TiO2
dispersions under different pH values, which were tested by POWEREACH JS94H
micro-electrophoresis instrument. The zeta potentials of micron-Al2O3 and nano-TiO2 decreased
with the increasing of pH value according to Figure S11. The isoelectric points of micron-Al2O3
and nano-TiO2 were 9.0 and 3.4 respectively, and the surface potential of micron-Al2O3 was
maximally differentiated from that of flame-synthesized nano-TiO2 when pH value was equal to 6.
Thus, the pH value of solution was adjusted by titrating small amount of dilute acetic acid or
ammonia solution during the SATS preparation process to make sure maximal differentiated
surface potential. Through this way, the core-shell structure composite was obtained.
Figure S11. Zeta potential of micron-Al2O3 and nano-TiO2 dispersions under different pH value
S18
Text S13
The force, which was required to fracture a single particle sized between 0.2mm and 0.3mm,
was regarded to the crushing strength of sorbents in this study. As shown in FigureS12, the
crushing strengths of fresh CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 were 1.46 N, 1.88 N, 1.86 N
respectively, compared to 1.08 N, 1.67 N, 1.74 N for CaO，CaO/Al2O3 and CaO/TiO2－Al2O3
which were experienced 30 cyclic calcination/carbonation tests. Each of these results was obtained
by taking the average value of 40 measurements. The standard deviations of crushing strength for
fresh CaO, CaO/Al2O3 and CaO/TiO2－Al2O3 were 0.57, 0.28 and 0.23, but reduced to 0.1, 0.26
and 0.17 after 30 cycles. All of these deviation values were considered within the range of
acceptance. In addition, it was considered that materials with crushing strength over 1 N were
suitable for large-scale industrial processes,4 thus all of three samples satisfied strength demand to
conduct next FBC experiments.
Figure S12. Crushing strength results for CaO, CaO/Al2O3, CaO/TiO2－Al2O3 before and after 30
cyclic tests under severe calcination condition of 900 °C in pure CO2
S19
Reference
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2.Liu, W.; An, H.; Qin, C.; Yin, J.; Wang, G.; Feng, B.; Xu, M., Performance enhancement of calcium
oxide sorbents for cyclic CO2 capture——sA review. Energ Fuels 2012, 26, (5), 2751-2767.
3.Bhatia, S. K.; Perlmutter, D. D., Effect of the product layer on the kinetics of the CO2-lime reaction.
AIChE J. 1983, 29, (1), 79-86.
4.Shulman, A.; Cleverstam, E.; Mattisson, T.; Lyngfel, A., Manganese/iron, manganese/nickel, and
manganese/silicon oxides used in chemical-looping with oxygen uncoupling (CLOU) for combustion
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