Lithium silicate nanosheets with excellent capture capacity and kinetics with unprecedented stability for high-temperature CO2capture

Article abstract of DOI:10.1039/d0sc06843h

An excessive amount of CO₂is the leading cause of climate change, and hence, its reduction in the Earth's atmosphere is critical to stop further degradation of the environment. Although a large body of work has been carried out for post-combustion low-temperature CO₂capture, there are very few high temperature pre-combustion CO₂capture processes. Lithium silicate (Li₄SiO₄), one of the best known high-temperature CO₂capture sorbents, has two main challenges, moderate capture kinetics and poor sorbent stability. In this work, we have designed and synthesized lithium silicate nanosheets (LSNs), which showed high CO₂capture capacity (35.3 wt% CO₂capture using 60% CO₂feed gas, close to the theoretical value) with ultra-fast kinetics and enhanced stability at 650 °C. Due to the nanosheet morphology of the LSNs, they provided a good external surface for CO₂adsorption at every Li-site, yielding excellent CO₂capture capacity. The nanosheet morphology of the LSNs allowed efficient CO₂diffusion to ensure reaction with the entire sheet as well as providing extremely fast CO₂capture kinetics (0.22 g g^?1min^?1). Conventional lithium silicates are known to rapidly lose their capture capacity and kinetics within the first few cycles due to thick carbonate shell formation and also due to the sintering of sorbent particles; however, the LSNs were stable for at least 200 cycles without any loss in their capture capacity or kinetics. The LSNs neither formed a carbonate shell nor underwent sintering, allowing efficient adsorption-desorption cycling. We also proposed a new mechanism, a mixed-phase model, to explain the unique CO₂capture behavior of the LSNs, using detailed (i) kinetics experiments for both adsorption and desorption steps, (ii)in situdiffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements, (iii) depth-profiling X-ray photoelectron spectroscopy (XPS) of the sorbent after CO₂capture and (iv) theoretical investigation through systematic electronic structure calculations within the framework of density functional theory (DFT) formalism.

Full text of DOI:10.1039/d0sc06843h

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Lithium silicate nanosheets with excellent capture
capacity and kinetics with unprecedented stability
for high-temperature CO2 capture†
Cite this: Chem. Sci., 2021, 12, 4825
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Rajesh Belgamwar,a Ayan Maity,a Tisita Das,d Sudip Chakraborty,b
c
a
*
Chathakudath P. Vinod
and Vivek Polshettiwar
An excessive amount of CO2 is the leading cause of climate change, and hence, its reduction in the Earth's
atmosphere is critical to stop further degradation of the environment. Although a large body of work has
been carried out for post-combustion low-temperature CO2 capture, there are very few high
temperature pre-combustion CO2 capture processes. Lithium silicate (Li4SiO4), one of the best known
high-temperature CO2 capture sorbents, has two main challenges, moderate capture kinetics and poor
sorbent stability. In this work, we have designed and synthesized lithium silicate nanosheets (LSNs),
which showed high CO2 capture capacity (35.3 wt% CO2 capture using 60% CO2 feed gas, close to the
theoretical value) with ultra-fast kinetics and enhanced stability at 650 ꢀC. Due to the nanosheet
morphology of the LSNs, they provided a good external surface for CO2 adsorption at every Li-site,
yielding excellent CO2 capture capacity. The nanosheet morphology of the LSNs allowed efficient CO2
diffusion to ensure reaction with the entire sheet as well as providing extremely fast CO2 capture kinetics
(0.22 g gꢁ1 minꢁ1). Conventional lithium silicates are known to rapidly lose their capture capacity and
kinetics within the first few cycles due to thick carbonate shell formation and also due to the sintering of
sorbent particles; however, the LSNs were stable for at least 200 cycles without any loss in their capture
capacity or kinetics. The LSNs neither formed a carbonate shell nor underwent sintering, allowing
efficient adsorption–desorption cycling. We also proposed a new mechanism, a mixed-phase model, to
explain the unique CO2 capture behavior of the LSNs, using detailed (i) kinetics experiments for both
adsorption and desorption steps, (ii) in situ diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy measurements, (iii) depth-profiling X-ray photoelectron spectroscopy (XPS) of the sorbent
after CO2 capture and (iv) theoretical investigation through systematic electronic structure calculations
within the framework of density functional theory (DFT) formalism.
Received 15th December 2020
Accepted 15th February 2021
DOI: 10.1039/d0sc06843h
rsc.li/chemical-science
combustion.2–4 Low-temperature sorbents (based on amines,
silica, zeolites, carbon, and MOFs) are mainly used for post-
Introduction
combustion CO2 capture from ue gas at low temperatures (25
to 75 ꢀC), while high-temperature sorbents are specically
developed for pre-combustion capture between 500 and 700
ꢀC.2–4 “Pre-combustion capture” is one of the most challenging
but best ways to tackle climate change as CO2 is captured during
the process at high temperatures with ideally zero release into
the environment.5–7 Pre-combustion capture is potentially less
expensive than post-combustion capture, as CO2 gets captured
as soon as it is produced during the process without releasing it
into the environment.5–7
Although a large body of work has been carried out for post-
combustion low-temperature CO2 capture,2 there are very few
high temperature pre-combustion CO2 capture processes.
Lithium silicate (Li4SiO4) is one of the best high-temperature
CO2 capture sorbents due to its relatively high capture capac-
ities as compared to other alkali metal-containing ceramics.5–7
Although lithium silicates display excellent CO2 sorption
Fossil fuels provide greater than 85% of the world's energy
needs.1 An excessive amount of CO2 is the main cause of climate
change. If the use of fossil fuel is to be continued in the coming
years, the CO2 emission needs to be dramatically reduced by
developing sustainable sorbents to capture and separate CO2
from various industrial processes. For CO2 capture, in general,
two protocols can be used, post-combustion and pre-
aDepartment of Chemical Sciences, Tata Institute of Fundamental Research (TIFR),
Mumbai, India. E-mail: vivekpol@tifr.res.in
bMaterials Theory for Energy Scavenging (MATES) Lab, Department of Physics, Indian
Institute of Technology, Simrol, Indore, India
cCatalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory
(NCL), Pune, India
dHarish-Chandra Research Institute, HBNI, Allahabad, Uttar Pradesh, India
† Electronic supplementary information (ESI) available: Detailed experimental,
the effect of mole ratio, the effect of thermal treatment temperature and time,
the effect of CO2 ow, XPS data, and kinetic data. See DOI: 10.1039/d0sc06843h
© 2021 The Author(s). Published by the Royal Society of Chemistry
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capabilities at high temperatures, they face two main limita- desorption process, with a loss of 50% of their capture perfor-
tions, slow capture kinetics and poor stability–recyclability of mance (capacity and kinetics) in the rst three cycles. Thus,
the sorbents.5–7 Formation of a lithium carbonate product layer there was a need for a sorbent that has the following properties:
limits CO2 diffusion, thus limiting kinetic performance; second, (i) high CO2 capture at elevated temperatures (between 600 and
sintering reduces the cycling stability necessary for practical 750 ꢀC), (ii) faster rate of adsorption/desorption, and (iii) cycling
application. Recently, nanowires of lithium silicates and stability for hundreds of CO2 capture–release cycles.
lithium tungstate showed good kinetics of CO2 capture.8,9
In continuation of our work on CO2 capture and conver-
However, these nanowires were not stable against agglomera- sion,10–13 here we report lithium silicate nanosheets (LSNs) as
tion/sintering during the high-temperature CO2 adsorption– a high-temperature CO2 sorbent. The LSNs showed a high CO2
Fig. 1 (a and b) SEM images, (c) PXRD pattern, and (d) N2 sorption isotherm of the LSNs; (e) CO2 adsorption thermograms, (f) CO2 capture
kinetics in the first minute at 650 ꢀC using 60% CO2 for various cycles; (g) CO2 capture by the LSNs (4-cycle activated) at various temperatures; (h)
CO2 capture capacity at 650 ꢀC for feeds with various CO2 concentrations.
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capture capacity of 35.3 wt% CO2 (96.18% efficiency, in at 700 ꢀC. At 500 ꢀC, the diffusion process was kinetically
comparison with the theoretical capacity of 36.7 wt%) and ultra- controlled, and the diffusion barrier is the main reason for the
fast kinetics (0.22 g gꢁ1 minꢁ1). Notably, the LSNs were stable low CO2 sorption capacity. With the increase in temperature to
ꢀ
even aer 200 cycles without any loss in their capture capacity or 650 C, the diffusion and chemisorption processes were accel-
kinetics.
erated and hence the CO2 capture rate was much faster.
However, at 700 C, the desorption kinetics of CO2 was faster
ꢀ
than the adsorption kinetics, reducing the overall capture
capacity and kinetics. Thus, 650 C was found to be the opti-
Results and discussion
ꢀ
mized temperature in terms of CO2 capture capacity. At this
temperature, using these 4-cycle activated LSNs, the effect of
CO2 concentration was also studied, with 20, 30, 40, 50 and 60%
CO2, all of which (except 20% CO2) showed good capture
capacities and kinetics (Fig. 1h and S3†), indicating their
usability in various pre-combustion processes where different
amounts of CO2 are generated. We then studied the effect of gas
ow using 150, 100, and 50 mL minꢁ1 of 100% CO2 (Fig. S4†).
The LSNs showed good capture rates with all the ows, indi-
cating their usability in various processes with different ows.
X-ray photoelectron spectroscopy (XPS) was performed to
study the LSNs and how they change during CO2 adsorption–
desorption steps (Fig. 2b). The XPS spectra contained two peaks
for Li (1s) at binding energies (BEs) of ꢂ54 eV and ꢂ55.9 eV. The
peak at ꢂ54 eV was assigned to Li4SiO4 (major phase)14 as well
as Li2CO3 (minor phase).15 Although their deconvolution was
not possible due to their very close binding energy (BE), based
on PXRD, this peak at ꢂ54 eV was assigned mainly to Li4SiO4.
The signal at ꢂ55.9 eV was assigned to Li2SiO3.14,15 Aer CO2
capture, in the adsorbed 4-cycle activated sample, the area
under the peak of Li2SiO3 at ꢂ55.9 eV increased due to the
conversion of Li4SiO4 to Li2SiO3 and Li2CO3, as also observed in
PXRD (Fig. 2a). The formation of Li2CO3 was observed with the
signal at ꢂ54 eV BE (overlapping with the Li4SiO4 signal), but
clearly differentiable in PXRD. The Si (2p) spectra of the LSNs
showed two Si signals, at BEs of ꢂ101 eV for Li4SiO4 and ꢂ102
eV for Li2SiO3 (Fig. 2a).14,15 Aer CO2 adsorption, ideally, there
should be only one Si site, as PXRD indicates the presence of
Li2SiO3 and Li2CO3 only (Fig. 2a). However, we observed two
signals, at ꢂ102 and ꢂ104 eV. This indicates that the signal at
ꢂ104 eV was for SiO2.16
LSNs were synthesized using dendritic brous nanosilica
(DFNS).12 The DFNS surface area is highly accessible by virtue of
its brous nature instead of tubular pores like in MCM-41 and
SBA-15.12 Aer several synthetic optimizations, in terms of
lithium precursor, heating temperature and time (Tables S1 to
S3†), the desired LSNs were prepared by thermal treatment of
physically mixed DFNS and lithium nitrate with a Li : Si molar
ratio of 4.66 : 1 at 650 ꢀC for 6 h in air. Scanning electron
microscopy (SEM) images indicated that the LSNs are made up
of nanosheets randomly stacked on each other with overall
sheet widths of a few microns (Fig. 1a and b). Powder X-ray
diffraction (PXRD) of the LSNs showed the presence of Li4SiO4
as a major phase, with Li2SiO3 and Li2CO3 as minor phases
(Fig. 1c). These impurities were due to the reaction of the LSNs
with trace amounts of CO2 present (ꢂ400 ppm) in the envi-
ronment during LSN synthesis. Synthesis of the pure phase of
Li4SiO4 was possible by using an inert atmosphere (Fig. S1†);
however, since a small amount of minor phases did not affect
the CO2 performance of the LSNs, we chose sustainable
synthesis conditions in an air environment. The N2 sorption
isotherm (Fig. 1d) displayed type IV characteristics with
hysteresis and a low surface area of 8 m2 gꢁ1
.
The CO2 capture performance of the LSNs was evaluated
using thermogravimetric analysis (TGA). First, the CO2
adsorption/desorption of the LSNs was studied at 650 ꢀC using
60% CO2 (balance nitrogen) with 25 mg of sorbent (Fig. 1e and
f, and S2† for the effect of sorbent weight). Notably, we
observed an increase in capture capacity from 0.277 to 0.353 g
gꢁ1 as well as initial kinetics from 0.06 to 0.22 g gꢁ1 minꢁ1 up
to the 4th cycle. The increase in capture capacity and kinetics
up to the 4th cycle was due to systematic purication of the as-
prepared Li4SiO4, which had Li2SiO3, and Li2CO3 as minor
phases. With every cycle of CO2 adsorption–desorption, these
minor phases were converted to Li4SiO4 and at the 4th cycle,
their concentrations were signicantly reduced (Fig. 2a). The
PXRD pattern indicated the reaction of Li4SiO4 with CO2 to
form Li2SiO3 and Li2CO3 during the adsorption step followed
by the conversion of Li2SiO3 and Li2CO3 phases to Li4SiO4
during the CO2 desorption step (eqn (1)). This process was
reversible (Fig. 2a), which was the reason for the better cycling
stability of the LSNs.
The LSNs' stability and cyclability were then studied and
the results indicated that the LSNs could be used for at least
200 cycles without any loss in their capture capacity or kinetics
(Fig. 3a). We compared the recyclability of the LSNs with that
of the best lithium silicates reported in the literature (Fig. 3b
and Table S4†). The LSNs were found to be stable and did not
lose their capture capacity or kinetics during the adsorption–
desorption cycling. In general, conventional lithium silicates
systematically lose their capture capacity with every cycle with
a signicant loss in the rst few cycles (Fig. 3b);5–7,17–29
however, the LSNs were found to be stable even aer 200
cycles. The LSNs were even better than one of the best recently
reported sorbents, lithium silicate nanowires (NWs)8 (Fig. 3c).
The NW sorbent lost more than 50% of its capture capacity,
Li4SiO4 + CO2 # Li2SiO3 + Li2CO3
(1)
The LSNs' CO2 capture performance was then studied at which fell from 0.35 to 0.15 g gꢁ1, in just four cycles, while the
different temperatures using a 4-cycle activated sample using LSNs capture capacity (0.35 g gꢁ1) did not change even aer
60% CO2 with 25 mg of sorbent (Fig. 1g). The CO2 sorption 200 cycles. The kinetics of the NWs reduced to 0.1 g gꢁ1 minꢁ1
capacity rst increased from 500 to 650 C and then decreased in 4 cycles, while the LSNs kinetics remained at 0.22 g gꢁ1
ꢀ
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Fig. 2 (a) PXRD pattern and (b) XPS spectra of lithium silicates after various CO2 adsorption–desorption cycles.
minꢁ1 even aer 200 cycles. Thus, we found that the LSNs
For most of the reported Li4SiO4, due to its dense
outperform the NWs and other reported sorbents in terms of morphology and large particle size (Table S4†), CO2 capture
capture capacity and kinetics as well as cycling stability. took place mainly at the surface, while the interior surface
Notably, the LSNs were even stable in the presence of a CO2 cannot fully interact with CO2, which limits the capture capacity
feed containing water vapor (Fig. S5†).
of the conventional lithium silicate material. Also, as pore size
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Fig. 3 (a) Stability and cyclability of the LSNs for 200 cycles of CO2 adsorption–desorption, (b) comparison with various best-known lithium
silicate sorbents, and (c) cycle-by-cycle comparison of the LSNs with lithium silicate nanowires (NWs)8 at 650 ꢀC using 60% CO2.
has a critical impact on the CO2 capture performance of sintering even aer 200 cycles, possibly because they were
Li4SiO4,5 due to the formation of Li2SiO3 and Li2CO3, which covalently interlinked with each other between the sheets
have a higher molar volume than Li4SiO4, the pores get blocked (Fig. S6 and S7†), and remained stable for 200 cycles. This was
due to this volumetric expansion, causing further degradation the reason for the excellent CO2 sorption performance of this
in the sorbent's CO2 capture performance (capacity, kinetics, sorbent (with CO2 capture capacity close to the theoretical
and recyclability). Our synthesized sorbent LSNs, due to their value).
thin sheets, provided a good external surface and allowed CO2
According to the double-shell model5 (Fig. 4, le), rst
diffusion to every Li-site without any hindrance. Although the Li4SiO4 reacts with CO2 at the sorbent surface to generate an
LSNs aggregated aer 4 cycles, these sheets did not undergo external shell made up of Li2CO3. Once this shell is formed, the
Fig. 4 Proposed mixed-phase vs. double shell model for CO2 adsorption–desorption.
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diffusion of CO2 and Li+ through this external shell dictates the where y represents the CO2 adsorbed in wt%, x is the time
sorbent performance. The formation of thick carbonate shells required to capture CO2, and k1 and k2 are the exponential
was the main reason for the degradation of capture perfor- parameters for CO2 chemisorption and lithium diffusion
mance for the conventional lithium silicates.5 In the case of the processes, respectively. A and B are constants, and C is the y-axis
LSNs, the sheet thickness is a few nanometers, and hence CO2 intercept.
and Li+ can easily diffuse across the entire sheet, and CO2 reacts
Table 1 summarizes the kinetic parameters obtained aer
with Li+ without the formation of the external shell. The rate of this tting from the LSNs' CO2 adsorption isotherms (Fig. S4†).
the CO2 capture is generally limited by the diffusion of CO2 and At the optimized sorption temperature, 650 ꢀC, k1 was 0.065421
Li+ through the external shell; however, since no such shell was sꢁ1, far better than the reported values for conventional sili-
formed in the LSNs, they showed extremely fast CO2 capture cates,5–7 indicating extremely fast CO2 chemisorption by inter-
kinetics (0.22 g gꢁ1 minꢁ1). Based on these observations, we action with Li-ions on the surface of the LSNs (Fig. 4). This
propose that the LSNs adsorb/desorb CO2 via a mixed-phase conrms the role of nanosheet morphology, which allowed
model and not the double-shell model (Fig. 4).
more external surface and less core, causing efficient CO2
To study this process and understand the effect on CO2 adsorption and indicating the mixed-phase model of CO2
chemisorption and Li+ diffusion, isotherms at different capture. In conventional silicates, in the 2nd step, the Li-ion has
temperatures were tted to the double-exponential equation to diffuse through carbonate shells (as per the double-shell
(eqn (2); Fig. S8†)
model, Fig. 4), and this slows down the CO2 capture kinetics
(k2). In the case of LSNs, k2 was 0.00164 sꢁ1, similar to reported
values. However, in the case of LSNs, more than 90% of the total
CO2 capture happened in the rst 3 minutes, indicating that k1
(CO2 chemisorption) is the rate-limiting step and not k2 (Li+
diffusion). This again indicates that the LSNs' CO2 capture
follows the proposed mixed-phase model.
y ¼ A exp(ꢁk1x) + B exp(ꢁk2x) + C
(2)
Table
1 Kinetic parameters obtained from the LSNs' adsorption
isotherms at different temperatures
T (ꢀC)
k1 (sꢁ1
)
k2 (sꢁ1
)
R2
In order to gain more insight into the mixed-phase model,
we compared the LSN sorbent (prepared from DFNS) with
500
550
600
650
700
0.003510
0.003783
0.006214
0.065421
0.001788
0.00093
0.00028
0.00048
0.00164
0.00015
0.99878
0.99943
0.99771
0.99068
0.99960
¨
a conventional sorbent prepared from Stober silica, named
¨
Stober lithium silicate (SLS), which is known to follow the
double-shell model of CO2 adsorption–desorption. When we
Fig. 5 CO2 capture by the LSNs and SLS at 650 ꢀC using 60% CO2: (a) CO2 adsorption thermograms of the LSNs and SLS; (b) CO2 capture kinetics
in the first five minutes; (c) CO2 desorption kinetics.
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compared the CO2 capture performance of the LSNs and SLS exponential equation (eqn (S1)†) and found that k1 was 0.00285
using feed with various CO2 concentrations, in all cases, the sꢁ1 for the LSNs and 0.00093 sꢁ1 for SLS (Fig. S11 and Table
LSNs showed far better CO2 capture capacity and kinetics as S5†). In addition to adsorption kinetics, desorption kinetics
compared to SLS (Fig. S9†). For further comparison, we chose should also be accelerated by the mixed-phase model. We hence
a feed with a 60 : 40 ratio of CO2 : N2 and observed that in 2 h, tted the desorption (Fig. 5c) to the single exponential equation
the LSNs capture 35.3 wt% CO2 (22.04 wt% in the rst minute) (Fig. S12†) and found that k1 was 0.002027 sꢁ1 for the LSNs as
while SLS captures only 26.2 wt% (16.49 wt% in the rst minute) compared to only 0.001652 sꢁ1 for SLS. This also conrms the
(Fig. 5a). The LSNs showed faster kinetics of 5.8 wt% per presence of a mixed-phase model in the case of LSNs.
minute, compared to the 3.1 wt% per minute for SLS (Fig. 5b),
To gain mechanistic insights into the CO2 capture process of
indicating faster Li diffusion throughout the LSNs, supporting CO2 adsorption on the Li4SiO4 surface and the formation of
the mixed-phase model. This model should also signicantly Li2CO3 and Li2SiO3, we carried out in situ diffuse reectance
affect desorption kinetics, as kinetics should be faster in the infrared Fourier transform (DRIFT) spectroscopy measure-
case of a mixed-phase model compared to the double-shell ments of the LSNs under similar CO2 adsorption and desorp-
model, which is known for slow desorption due to the thick tion conditions. For the adsorption study, in a reactor chamber
shell. The LSNs showed very fast desorption, 6.8 wt% per with a heater and gas ow controller, the LSNs were treated with
ꢀ
minute, while SLS desorbed 5.1 wt% per minute with respect to CO2 (60%) at 650 C, while for the desorption stuꢀdy, the LSNs,
their total capture capacity. A similar trend was observed under aer CO2 treatment, were treated with N2 at 650 C and an IR
all three ow conditions (Fig. S10†). This conrms the presence spectrum was recorded at various time intervals. The band at
of a mixed-phase model in the LSNs but the double-shell model 2342 cmꢁ1 was assigned to the linearly chemisorbed CO2
in SLS.
molecule on the lithium silicate surface (Fig. S13a and b†),30
If the proposed mixed-phase model is correct, CO2 chemi- while the band in the region of 3500 to 3900 cmꢁ1 was assigned
sorption should be a rate-determining step and not lithium-ion to Si–OH10 of lithium silicates (Fig. S13c and d†).
diffusion. The extremely high value of k1 and low value of k2 (in
The intensity of the band in the region of 650–750 cmꢁ1
,
Table 1) clearly indicate that chemisorption is a major path of assigned to the stretching vibration of Si–O–Si of the SiO3 chain
CO2 adsorption, while the lithium-ion diffusion path is insig- present in Li2SiO3 (ref. 31), increased with time (Fig. 6a) as the
nicant. We then tted the adsorption kinetics to a single LSNs on exposure to CO2 (adsorption step) converted to Li2SiO3
Fig. 6 In situ DRIFT study of CO2 adsorption–desorption using the LSNs and SLS at 650 ꢀC. FTIR spectra of LSN (a and b) adsorption and (c and d)
desorption steps. Adsorption is by 60% CO2 while desorption is by 100% N2.
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and Li2CO3. When CO2 was replaced by a N2 ow (desorption peak for Si 2p and C 1s at various depths (Fig. 7b and c) and
step), the intensity of these bands decreased with time (Fig. 6b), notably, Si 2p and C 1s also followed the same trend as Li 1s,
indicating that Li2SiO3 and Li2CO3 converted back into Li4SiO4. conrming the presence of Li2CO3 and Li2SiO3 across the LSNs.
The desorption took place in just 240 s (4 minutes), and such This further supports the proposed mixed-phase model of CO2
fast desorption indicates the mixed-phase model, as the double- adsorption and desorption by the LSNs.
shell model had very slow desorption kinetics. Along similar
lines, the intensity of the band in the region of 1300–1600 cmꢁ1
In order to gain further insight into the adsorption/desorp-
tion kinetics of the LSN sorbent, a theoretical investigation was
,
assigned to the stretching vibration of C–O in the carbonate ion carried out through systematic electronic structure calculations
2ꢁ
(CO3 ) present in Li2CO3 (ref. 32), increased with time (Fig. 6c) within the framework of density functional theory (DFT)
as the LSNs on exposure to CO2 (adsorption step) converted to formalism. In our DFT modeling, we primarily highlighted the
Li2SiO3 and Li2CO3. During the desorption (Fig. 6d), this signal correlation between adsorption characteristics and the corre-
intensity decreased with time. Since complete disappearance sponding charge transfer mechanism for the sorbent and
was not observed, the as-synthesized LSNs have the impurity of adsorbate along with the adsorbate induced work function
lithium carbonate.
change, which are immensely instrumental fundamental
To further study the proposed mixed-phase model of CO2 parameters for any surface investigation. We analyzed the
adsorption–desorption, an XPS depth-proling study of the electronic density of states (DOS) of the composite (CO2
LSNs was carried out by etching at various depths of the LSN adsorbed on the LSNs) as well as the individual systems aer
sample aer the CO2 adsorption step (Fig. 7). If it is a conven- and before the adsorption process to elucidate the salient
tional double-shell model, one should see lithium carbonate features of the electronic structures, as depicted in Fig. 8. The
only at the surface (and not in the core) in XPS, while if it is the middle panel of Fig. 8a illustrates the total DOS of Li4SiO4
proposed mixed-phase model, lithium carbonate should be before CO2 adsorption, exhibiting a wide bandgap semi-
present at the surface as well as inside the core. The XPS depth conducting nature. But as soon as CO2 gets adsorbed on top of
prole of the LSN sample aer CO2 adsorption is shown in the LSNs, nite states are found to emerge near the Fermi level
Fig. S14.† We calculated the area under the peak for Li 1s cor- (EF) due to the interaction between Li4SiO4 and the CO2 mole-
responding to Li2SiO3 and Li2CO3 (Fig. 7a) and observed that at cule, as shown in the top panel of Fig. 8a. Interestingly, we
all depths from 0 to 255 nm, there was a presence of both observed that the density of states corresponding to an isolated
Li2CO3 and Li2SiO3, clearly conrming the mixed-phase model. CO2 molecule (as depicted in the bottom panel of Fig. 8a) has
This was further conrmed by comparing the area under the been shied substantially in the vicinity of EF under the
Fig. 7 Area under the peak of (a) Li 1s for Li2SiO3 and Li2CO3, and (b) Si 2p and (c) C 1s of the LSN sample in the CO2 adsorbed state, from XPS
fitting (Fig. S13†) at different etching depths.
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Fig. 8 (a) Density of states plot for CO2 adsorbed on top of Li4SiO4. The top panel illustrates the total DOS of the composite systems after
adsorption, while the middle and bottom panels represent the DOS of the isolated Li4SiO4 sorbent and CO2 molecule before adsorption. (b)
Charge density distribution between a Li4SiO4 nanosheet and CO2 molecule. The accumulation and depletion of charge are denoted by yellow
and blue color, respectively. The iso-surface value was set to 0.0003 eꢁ A˚ꢁ3. (c) Work function and DFT total energies in the bulk form of
reactants and products.
inuence of the LSN surface, which is also manifested through theoretical analysis focused on the surface adsorption, whereas
the charge transfer mechanism representation in the subse- in the experimental section, the CO2 adsorption occurred within
quent section. It is worth mentioning that we observed the nite the nanosheets of Li4SiO4.
density of states appear near the Fermi region aer adsorption
The charge density distribution is illustrated in Fig. 8b,
in the composite system. This reveals the fact that the adsorbate where it is observed that the charge is mainly accumulated
in the form of CO2 is tuning the bandgap of the surface, which is around the Li–O bond region, indicating a valuable observation
Li4SiO4, which has also been reected in the substantial charge and validation of this work, as reected in the covalent bond
transfer process. As we investigated the interaction of an formation between the LSN surface and CO2. This particular
incoming CO2 with the Li4SiO4 surface, the noticeable change in theoretical nding has shed light not only on the CO2 capture
the electronic structure through the appearance of nite states but also on the subsequent desorption possibility, as found in
aer adsorption signies the fact that CO2 is chemisorbed on the experimental outcome. Bader charge analysis indicates that
the Li4SiO4 surface. The composite system represents the le- 0.85 eꢁ
ꢁ3 (amount of charge) is transferred from a Li atom of
˚
A
hand side of the reaction mechanism of CO2 adsorption by LSNs to an O atom of CO2 during the adsorption process.
Li4SiO4 before converting to the product form, while our During the electron transfer process from the Li atom to the O
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atom of CO2, covalent bond formation occurred, which even- rate of CO2 capture is generally limited by the diffusion of CO2
tually helps in the CO2 adsorption/capture. This particular and Li+ through the external carbonate shell, but since no
covalent bond formation also supports the adsorption/desorp- such shell was formed in the LSNs, they showed extremely fast
tion process, as desorption would be favorable in the absence of CO2 capture kinetics (0.22 g gꢁ1 minꢁ1). Notably, the LSNs
a strong ionic bond between the surface and adsorbate. This is show dramatic stability (even aer 200 cycles without any loss
due to the higher electronegativity of oxygen atoms as compared in their capture capacity or kinetics), as compared to reported
to lithium and carbon, as both of them share their electrons sorbents, which are known to systematically lose their capture
with oxygen, which leads to charge depletion around the capacity signicantly. The LSN sorbent neither formed
lithium and carbon atoms.
a carbonate shell nor underwent sintering, causing efficient
The work function values (Fig. 8c) for Li2SiO3 and Li2CO3 are adsorption–desorption with no loss in capture performance.
higher as compared to those of both LSNs and LSN–CO2 Thus, the LSNs have all three desirable properties (high
composite systems, suggesting that a larger amount of energy is capture capacity, fast kinetics and cycling stability for more
required to extract an electron from Li2SiO3 and Li2CO3 surfaces than 200 cycles) of a CO2 sorbent. Various kinetics and spec-
as compared to the Li4SiO4 system. Therefore, we could infer the troscopic studies (XPD depth proling and in situ DRIFT) were
favorable capture of a CO2 molecule by the LSNs, while the performed to prove the proposed mechanism of the mixed-
subsequent desorption of CO2 had been reected in the previ- phase model, which explained the unique CO2 capture
ously discussed Bader charge analysis. This observation has behavior of lithium silicate nanosheets in terms of good
also been veried theoretically from the reaction energy value of capture kinetics and prolonged stability.
the reaction (Li4SiO4 + CO2 # Li2SiO3 + Li2CO3), which has
been calculated as ꢁ1.36 eV from DFT total energy calculations.
Conflicts of interest
Here, the negative reaction energy signies the exothermic,
spontaneous CO2 capture process. Thus, we have theoretically
validated the experimental observation of not only the CO2
There are no conicts to declare.
capture process but also the CO2 desorption. The adsorption of
CO2 has also been conrmed from the elongated C–O bond Acknowledgements
being closer to the LSN surface as compared to that in the pre-
This work was supported by the Department of Atomic Energy
adsorbed state, while the corresponding adsorption energy is
(DAE), Government of India (under project no. R&D-TIFR-
RTI4003). We acknowledge the use of the EM and XRD facilities
of TIFR, Mumbai.
found to be ꢁ1.54 eV.
The cycling stability of the sorbents is determined by the
efficiency of CO2 desorption, which is known to be difficult from
an interior (core) portion of the double-shell model in conven-
tional lithium silicates, causing a reduction in CO2 capture
performances with every cycle for reported lithium silicates
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carbonate shell nor underwent sintering. This made CO2
adsorption–desorption a completely reversible process and
hence the LSNs were stable and recyclable even aer 200 cycles.
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