and compositionally tuned lithium silicate nanorods as high-performance carbon dioxide sorbents †

Materials Science and Technology Div Interdisciplinary Science and Technology Research (CSIR-NIIST), Pappanamcode, India. E-mail: hareesh@niist.res.in R&D Centre, Noritake Company LTD, 300 Japan. E-mail: bnair@n.noritake.co.jp Nanochemistry Research Institute, Departm Box U1987, Perth, Western Australia 6845, Chemical Resources Laboratory, Tokyo In Midori-ku, Yokohama 226-8503, Japan Academy of Scientic and Innovative Resea 025, India † Electronic supplementary information supplementary gures and tables include Cite this: J. Mater. Chem. A, 2016, 4, 16928


Introduction
Global warming and its effect on climate destabilization have grown into an alarming environmental concern in recent times.The massive consumption of fossil fuels leading to vast greenhouse gas emissions since the industrial revolution is believed to be the major reason for the current rise in atmospheric temperature.Green technological alternatives based on wind, solar and hydropower plants are therefore favored for minimizing carbon in the atmosphere.However, considering the cost and time frame required for the widespread implementation of such renewable energy alternatives, an immediate solution appears remote.To reduce carbon footprints globally, a reduction in CO 2 emission by maintaining a balanced carbon cycle in the atmosphere is imperative.8][9] In several hightemperature chemical and petrochemical processes, CO 2 is a major product and its removal therefore at the temperature and pressure of the reaction offers a less energy intensive method of separation culminating in smaller carbon footprints.In reactions such as methane reforming, the removal of CO 2 from the reaction vessel could also enhance the rate of reaction (sorption enhanced steam reforming) thereby providing additional benets of increased productivity. 10,11However, selective and rate enhanced sorption of CO 2 at elevated temperatures is a challenging task, especially in the temperature range of 573-1073 K where most such reactions occur.Lithium based ceramic absorbents are considered as candidate materials for selective CO 2 removal at high temperature, [12][13][14][15] although their application is thus far limited by the slow kinetics in the lower temperature range (573-823 K) and poor material durability in the higher temperature range (823-1073 K).
7][18][19] The absorption properties of some of the recently published lithium-based ceramic absorbents are shown in Table S1 in section S1 of the ESI † [20][21][22][23][24][25] for a quick evaluation of the status.We also have reported on the sol-gel based synthesis of Li 4 SiO 4 particles (powders termed as sol-gel in this manuscript) with CO 2 absorption performance comparable to such published reports. 26In the work reported herein, we have adapted a facile microwave assisted sol-gel synthesis route 27,28 to successfully realize Li 4 SiO 4 nano-rods with signicantly superior CO 2 absorption performance.A detailed synthesis protocol starting from LiNO 3 and colloidal silica is described under the experimental section.The powders thus obtained exhibited excellent CO 2 absorption capacity, cyclic stability and superior absorption/desorption rates (powders termed as microwave sol-gel in this manuscript).We believe that the enhanced kinetics for CO 2 absorption arise from the very small thickness (20 to 30 nm) of rod-shaped Li 4 SiO 4 particles.Irrespective of their nano-size, the large aspect ratio of the particles provided them with better stability against aggregation at high working temperatures thereby providing improved durability for a large number of absorption/desorption cycles.Besides this, the compositional control of these morphologically tuned materials helped to realize novel eutectic compositions containing Na and K as new generation CO 2 absorbents with exceptionally high CO 2 absorption rates even at low and moderate temperatures (<723 K).

Experimental
2.1 Synthesis of Li 4 SiO 4 through a microwave sol-gel process Li 4 SiO 4 was synthesised from LiNO 3 (Alfa Aesar, UK) and colloidal silica (Aldrich Chemicals, USA) as the starting precursors.Initially 2.39 M LiNO 3 solution was prepared by dissolving it in distilled water.Hydrolysis was carried out by the addition of NH 4 OH to the LiNO 3 solution while stirring at room temperature.Colloidal silica was added dropwise under stirring for 1 hour to obtain a sol.The precursor sol was subjected to 700 W microwave radiation at 2.45 GHz for 10 min.A domestic microwave oven (Panasonic-NNGT231M) was used to carry out this experiment.The precursor was then dried at 423 K and heated to 1073 K at a ramp rate of 1 C min À1 in air atmosphere.The powder calcined at 1073 K for 3 hours was used for CO 2 absorption studies unless specied otherwise.

Characterization techniques
Phase changes of the powder samples during heat treatment were traced using in situ HT-XRD (Rigaku RINT-TTR III).The morphology and crystallinity of the powders on CO 2 absorption/ desorption were further characterised using TEM analysis (HRTEM, FEI Tecnai 30 G2 S-TWIN operated at an accelerating voltage of 300 kV).CO 2 absorption properties were measured using a TGA apparatus (Perkin Elmer STA 6000, Singapore), in the temperature range of 100-750 C. N 2 adsorption based surface area analysis was performed at 77 K using a Micromeritics Tristar 111 surface area analyser aer degassing the sample at 200 C for 2 h.

Results and discussion
The morphological features of the synthesized particles at 673 K, 773 K and 1073 K are observed by TEM and are shown in Fig. 1a-f.Remarkably, the TEM images show that the particles have a nanorod morphology and are entirely different from the powders synthesized by conventional processing techniques. 29,30The XRD patterns of the microwave sol-gel Li 4 SiO 4 samples aer heat treatment (calcination) at 673 K, 773 K and 1073 K are further presented in Fig. 1g.From the XRD results, it is clear that the particles exhibited an amorphous nature even aer calcination at 673 K. Crystalline phases rich in the Li 2 SiO 3 phase emerged only on increasing the temperature to 773 K. Heat treatment at 1073 K was essential to realize Li 4 SiO 4 phase (JCPDS le no.37-1472) rich powders.The temperature dependent phase formation of the powders during the heat treatment of the synthesized particles has also been traced using in situ HT-XRD analysis and the results are detailed in section S2 of the ESI.† The results presented in Fig. 1g and further in ESI S2 † clearly revealed that the formation of the Li 4 SiO 4 phase is achieved at the expense of Li 2 SiO 3 nanobers that are initially nucleated from the amorphous silica particles coated with the lithium species.From the XRD analysis (section S2, Fig. S1 of the ESI †), it was inferred that microwave treatment has inuenced the hydrolysis of LiNO 3 to LiOH.We believe that the formation of the LiOH phase during hydrolysis and further, the intermediate formation of Li 2 SiO 3 bers are critical steps in the successful formation of the Li 4 SiO 4 nanorods (please see section S2 of the ESI † and the gures and video in the section for a detailed explanation of the phase and morphology formation mechanism).Small peaks corresponding to the Li 2 SiO 3 phase are visible in Fig. 1g as well as in Fig. S2 (ESI †).Usually, such presence of the Li 2 SiO 3 phase in small quantities is considered to be due to the reaction between Li 4 SiO 4 and CO 2 in air 1,26 while cooling aer heat treatment or handling during XRD measurements.However, the presence of Li 2 SiO 3 peaks in Fig. S2 † (in situ XRD spectra) indicates the existence of some amount of the lithium meta-silicate phase in our samples.
Based on the XRD and TEM results, 1073 K was used as the calcination temperature for obtaining samples with Li 4 SiO 4 as the predominant phase in our study.Unless otherwise mentioned, the samples named as "microwave sol-gel" in this manuscript are calcined at 1073 K.The BET surface area analysis indicated a value of 7.3 m 2 g À1 (N 2 adsorption isotherm is shown in section S3 of the ESI †) for the particles.
We have initially performed a dynamic thermogravimetric analysis of microwave sol-gel Li 4 SiO 4 particles under a 100% CO 2 ow to determine the temperature range of gas absorption (Fig. 1h).For comparison, similar curves of powder samples prepared by the sol-gel method as reported elsewhere 26 are also shown in Fig. 1h.As is clear from the dynamic absorption curves, the microwave sol-gel sample exhibited signicant CO 2 absorption capacity in the temperature range of 673-973 K.The absorption is initiated at around 673 K leading possibly to the formation of an external shell of lithium carbonate (Li 2 CO 3 ) on the particle surface.This shell formation remained the main mechanism of absorption up to a temperature of about 823 K.It is also evident from the gure that the rate of absorption increases at around 823 K and remains high till the reversible reaction occurs ($993 K).The faster rate of absorption from 823 K is presumed to be due to the soening of the Li 2 CO 3 shell, thereby decreasing the kinetic limitations imposed by the solid shell at lower temperatures. 7As mentioned, at T > 993 K the Li 4 SiO 4 phase becomes stable, leading to the complete desorption of CO 2 .The absorption studies have thus conrmed that the microwave samples have superior CO 2 absorption performances compared to sol-gel samples in the entire range of temperatures measured.
The morphological features of the microwave sol-gel based particles aer CO 2 absorption are shown in Fig. 2a and b.Compared to the original particles that exhibited rods of 20 to 30 nm width [Fig.1e and f], the particles aer CO 2 absorption appeared entirely different.As shown in Fig. 2a and b, the carbonate formation leads to the aggregation of the particles during the absorption stage.It can be inferred that the carbonate-rich particles, easily coalesce together forming a shell around a number of particles in the near vicinity, developing into an aggregate structure.However, desorption of CO 2 leads to the reversible formation of individual Li 4 SiO 4 particles, morphologically similar to the original samples (Fig. 2c and d).The morphological integrity of the particles aer CO 2 desorption process clearly evidences the stability of the nano-structures in the CO 2 absorption/desorption cycle.The XRD patterns of the samples before and aer CO 2 absorption as well as aer desorption are included in section S4 of the ESI † for further analysis of the CO 2 absorption/desorption process.
The CO 2 absorption capacity values of the particles in the temperature range of 673-973 K are compared in Fig. 2e.For collecting the absorption data, the samples were heated to the absorption temperature at 10 C min À1 in 100% N 2 gas and kept for 2 h under a 100% CO 2 gas ow.As expected, the amount of CO 2 absorbed was found to increase with increase in the absorption temperature.This relationship between temperature and absorption capacity as well as the mechanism of absorption are better expressed using kinetic constants and activation energy values calculated from the Arrhenius plots of kinetic constants as shown in Fig. 2f.Further details on the calculation of kinetic constants are included in section S5 of the ESI.† It should be noted that, contrary to the general trend, Fig. 2f indicates that the K 2 values are approximately 10 times larger than those of K 1 .The larger K 2 values obtained here signies faster lithium ion diffusion to the reaction interface compared to the chemisorption reaction. 29The activation energy values calculated from the plots were 22.70 kJ mol À1 for the chemisorption process (corresponding to K 1 ) and 61.10 kJ mol À1 for the diffusion process (corresponding to K 2 ).The higher activation energy value for the diffusion process substantiated the extremely fast diffusion of lithium ions from the core of the material to the reactive interface at absorption temperatures.
Absorption rates were also calculated from the rst two minutes of absorption curves at different temperatures.The particles synthesised through a microwave sol-gel method displayed an enhanced absorption rate of 0.093 wt% s À1 at 973 K.This value is much higher than the values reported in the recent literature (see table in ESI S1 † for a comparison of some of the reported values).This enhanced absorption rate should mainly be attributed to the nano-rod morphology characterized by a very small thickness/width of the particle, facilitating a rapid surface carbonate layer formation over the entire length during the rst stage of absorption.Moreover, the rod morphology should also have enabled easier surface reaction providing shorter diffusion pathways for lithium from the bulk to the surface of the particle.
The cyclic stability and regenerability of the powder samples were evaluated through cyclic absorption-desorption measurements and the results are shown in Fig. 2g [absorption with 100% CO 2 and desorption with 100% N 2 gases].The primary aim of this cyclic loading experiment was to examine whether the sintering of the particles or the segregation of the carbonate phase due to continuous use of the absorbent at high temperatures induced any decay in absorption performance.The initial absorption run was done at 973 K; thereaer the temperature of absorption was switched to 873 K and cyclic absorption-desorption performance for 9 consecutive cycles was recorded.The initial run at high temperature leading to more or less full conversion of the material to Li 2 CO 3 and further to the Li 4 SiO 4 phase helped to realize higher absorption values at 873 K (Fig. 2e and g).As shown in Fig. 2g the samples displayed consistent absorption-desorption performance for all the 10 cycles measured indicating high durability and cyclic stability of the materials.Furthermore, it should be noted that the desorption rate was better than the absorption rate through all the measurements.The large desorption rate obtained without thermal cycling would allow the application of the materials for CO 2 capture in pressure swing mode.Although, further cycling studies of thousands of cycles may be necessary before considering the material for real life applications, initial results as reported in Fig. 2g indicated that the microwave solgel powders may be considered as promising materials for industrial absorption applications.Schematic illustrations comparing the carbon dioxide sorption and desorption mechanisms of Li 4 SiO 4 nanoparticles and the newly developed nano rods, as well the inuence of particle morphology on their durability are provided in Fig. 3a and b.The small thickness of the nanorods enables fast absorption kinetics just as in the case of nanoparticles.The better durability of the nanorod sample is highlighted based on ceramic sintering and particle coalescence mechanisms.It is well known that in liquid phase assisted sintering, the densication is achieved by the rearrangement and shape changes of the particles.Rearrangement is strongly affected by the size and morphology of the particles.Spherical and mono-dispersed particles are advantageous for particle rearrangement and sintering while particles like nanorods, having a high aspect ratio, are difficult to sinter.The Ostwald ripening/particle coalescence process invariably results in a reduction of the total surface area of the particles. 31,32Hence the particle coarsening process as shown in Fig. 3b of the schematic always leads to a reduction in absorption kinetics owing to the reduction of the absorbentgas interface available for chemisorption.In contrast, the use of higher aspect ratio Li 4 SiO 4 nanorods impedes coalescence and agglomerate formation due to which it is possible to maintain high absorption kinetics even aer several absorption/desorption cycles.The effects of morphological tuning on the kinetics of the carbon dioxide absorption process as well as the durability of the resulting particles are clearly depicted in the illustration.
Further, we tried to enhance the rate of absorption as well as the absorption capacity of the materials in the lowtemperature range by modifying the materials with a eutectic composition of mixed alkali carbonates.Carbonates of sodium, potassium and lithium were mixed in the weight ratio of 10 : 60 : 30 [eutectic-1], 32 : 37 : 31 [eutectic-2] and 24 : 45 : 31 [eutectic-3] and further blended with the microwave sol-gel samples in the weight ratio of 20 : 80.The mixtures were then heated to 1073 K before absorption studies.Fig. 4a and b present the TEM images of the eutectic-3 sample obtained aer the heat treatment process.The rod morphology of the sample is well maintained with the thickness varying from 40 nm to 100 nm.The XRD pattern presented in Fig. 4c conrmed the presence of ortho-and metasilicates of lithium in addition to the alkali silicate phases of Na and K.A comparative evaluation of the dynamic absorption isotherms of the sol-gel, 26 unmodied microwave sol-gel and microwave sol-gel samples modied with eutectic powder mixtures are presented in Fig. 4d.It is clear that the addition of eutectic mixtures increased the absorption capacity in the low-temperature range of 623-823 K signicantly.In the unmodied samples, the rate of absorption was very low in the initial absorption step (<823 K) compared to the second absorption step.However, in the samples containing the eutectic mixtures the absorption rate in the initial step was comparable to that of the second absorption step.In the samples eutectic-1 and eutectic-2, two absorption steps are clearly visible as in the case of the unmodied sample.This is  attributed to the carbonate shell formation, although a soer one compared to the unmodied sample, in these two cases.However, in the case of the eutectic-3 sample the two steps have combined to form a more or less single step absorption curve.
Detailed results of the CO 2 absorption performance of the sample (eutectic-3) at temperatures of 623-923 K are shown in Fig. 5a.As shown, absorption capacity as high as 35% could be observed at the temperature of 923 K.It should be noted that the amount of CO 2 adsorbed was higher than the value expected based on the stoichiometry of the reaction of one CO 2 molecule with one orthosilicate available in the eutectic-3 sample (see Fig. 4c and 5a).It is possible that the reaction might have continued till silica is formed (instead of Li 2 SiO 3 ) at least in some of the fractions of the powder mixture.Further studies are required to fully understand the chemisorption mechanism of these samples.
An enhanced absorption rate was observed at temperatures as low as 623 K. Fig. 5b shows a good comparison of the absorption rates of the eutectic-3 sample with the unmodied microwave and sol-gel 26 samples (calculated for the rst 2 minutes of absorption).The rate of absorption observed for the eutectic-3 sample, 0.28 wt% s À1 at 923 K, was signicantly higher than the absorption rates measured with the unmodied microwave solgel and sol-gel samples.A comparison of the reported results (as shown in ESI S1 †) including the most recent literature also shows that the absorption performance of the sample is exceptional.The CO 2 absorption value of the sample, in fact, reached >30 wt% within the rst 42 seconds, based on which the absorption rate could be calculated to be 0.72 wt% s À1 .
In order to nd out the rate determining steps, the absorption curves (Fig. 5a) were tted to the double exponential model and the resulting kinetic parameters were used to derive the Arrhenius plots as shown in Fig. 4c (further details are included in section S5 of the ESI †).It should be noted that the K 1 values are larger in this case compared to the K 2 values and this is contrary to the behaviour of the microwave unmodied samples.However, this is attributed to the presence of eutectic phases in the modied samples, which are supposed to enhance signicantly the chemisorption rates.As a result, the modied and unmodied samples have different reaction limitations in the entire absorption process despite having more or less similar particle sizes.The activation energy values calculated from Fig. 5c were 82.3 kJ mol À1 and 205.18 kJ mol À1 for the chemisorption and diffusion processes, respectively.The activation energy for the chemisorption process was found to be much lower than that for the diffusion process.The sizes of sodium and potassium ions are larger compared to the lithium ions and, therefore, the whole diffusion process could be restricted in this case.
Cyclic absorption-desorption performances for 15 cycles at 948 K using 100% CO 2 and 100% N 2 gases were recorded and the results are shown in Fig. 6a.As shown, the sample retained >95% (33.6 wt%) of its original absorption capacity even aer 15 cycles at this very high temperature.Nevertheless, some structural and morphological changes are suspected as the shape of the absorption curve changed from a near perfect rectangle to smoothed edges as the number of cycles increased.Further improvement in the stability of the powders may be necessary and techniques like the addition of rare-earth second phases as reported by us recently may be required. 30As shown here and in Fig. 5a, the sample showed an absorption capacity of $35% for absorption temperatures >823 K when the partial pressure of CO 2 gas was 1 atm (100% CO 2 ).Fig. 6b shows the comparative absorption curves of eutectic-3 at 873 K for the CO 2 partial pressure values of 1 atm (100% CO 2 ) and 0.15 atm (15% CO 2 / 85% N 2 ).It is clear that the sample is capable of absorbing CO 2 to >25 wt% within the rst few minutes of absorption even at the reduced CO 2 partial pressure of 0.15 atm.
It should be noted that the product streams of several hightemperature chemical reactions well as the ue gases from power plants contain 15% CO 2 .Therefore, the results indicate the possibility of using the developed materials for practical applications of CO 2 capture.This exceptional absorption behavior of eutectic-3 should be due to the morphological as well as compositional features of this sample.With regard to the kinetics and CO 2 absorption capacity, these morphologically and compositionally tailored particles outperform other structured lithium silicate particles reported in the recent literature (data from the literature is shown in the section S1 of the ESI †).

Conclusions
In conclusion, a microwave assisted sol-gel synthetic approach is demonstrated for the synthesis of Li 4 SiO 4 particles with a nanorod morphology.The nanorods exhibited a dramatically enhanced absorption rate for CO 2 along with exceptional durability.The nanorod morphology of the Li 4 SiO 4 particles allowed ultrafast sorption kinetics due primarily to an easier surface reaction with CO 2 by virtue of shorter diffusion pathways for lithium from the bulk to the surface of the rods.In addition, the large aspect ratio of the nanorods helped to enhance the durability of the particles by limiting their Ostwald ripening upon high-temperature cyclic absorption/desorption loading.Further, we have modied the chemical composition of the Li 4 SiO 4 particles by mixing them with a eutectic mixture of K and Na.This compositional control of the materials helped to realize absorbents with extraordinary CO 2 absorption rates of 0.72 wt% s À1 at 100% CO 2 /923 K. Cycling absorption-desorption studies of these powders revealed that the materials remain durable up to 15 cycles without any signicant reduction in CO 2 absorption capacity.Furthermore, the modied samples showed remarkable absorption performance at lower temperatures (573-823 K) as well as lower CO 2 pressures (0.15 atm) demonstrating their potential in practical CO 2 separation applications.

Fig. 2
Fig. 2 TEM images (a and b) of microwave sol-gel Li 4 SiO 4 particles after the CO 2 absorption process; (c and d) particles after desorption process.(e) CO 2 absorption curves at various temperatures.(f) Graph of ln K versus 1/T for the two different processes of chemisorption (K 1 ) and diffusion (K 2 ) observed in the microwave sol-gel sample; (g) absorption-desorption performance of microwave sol-gel Li 4 SiO 4 powders for 10 cycles.The first cycle of the run was done at 973 K and further runs at 873 K.In all cases, desorption was carried out by switching 100% CO 2 gas to 100% N 2 gas.

Fig. 3
Fig. 3 Schematic illustrations comparing the carbon dioxide absorption and desorption mechanisms and the durability of (a) the newly developed Li 4 SiO 4 nanorods and (b) commonly found Li 4 SiO 4 nanoparticles.

Fig. 5
Fig. 5 (a) CO 2 absorption curves of the eutectic mixture (eutectic-3) at various temperatures.(b) Absorption rates of eutectic-3 at different temperatures (rate values calculated from the initial two minutes of the absorption curve) in comparison to those of the microwave sol-gel and sol-gel samples.(c) Graph of ln K versus 1/T for the two different processes of chemisorption (K 1 ) and diffusion (K 2 ) observed for the eutectic-3 sample.

Fig. 6
Fig. 6 (a) Cyclic absorption-desorption performance of eutectic-3 (absorption was carried out at 948 K and desorption at the same temperature by changing 100% CO 2 to 100% N 2 gas).(b) Comparison of the absorption curves of eutectic-3 with 100% CO 2 flow and with 15% CO 2 flow.Absorption was carried out at 873 K (15% CO 2 or 100% CO 2 ).