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High Performance Biomass derived Activated Porous Biocarbons for Combined Pre- and Post-Combustion CO Capture 2

Gurwinder Singh, Kripal S. Lakhi, Kavitha Ramadass, Sathish Clastinrusselraj Indirathankam, and Ajayan Vinu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00921 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Sustainable Chemistry & Engineering

High Performance Biomass derived Activated Porous Biocarbons for Combined Pre- and Post-Combustion CO2 Capture

Gurwinder Singh†, Kripal S. Lakhi†, Kavitha Ramadass†, CI Sathish†, and Ajayan Vinu†*

†Global

Innovative Centre for Advanced Nanomaterials, Faculty of Engineering and Built

Environment, The University of Newcastle, University Drive, Callaghan NSW 2308 Australia. *Email: [email protected]

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ABSTRACT This paper demonstrates the synthesis of highly efficient activated porous biocarbons through chemical activation of Lotus seed for pre-and post-combustion CO2 capture. The activation was performed at three different temperatures using four different KOH/biomass impregnation ratios. The synthesized materials are ultra-microporous and display bimodal porosity which can be easily controlled by varying the experimental conditions. The specific surface area can be tuned from 1079 m2 g-1 to 2230 m2 g-1 by adjusting the amount of activating agent and the carbonization temperature. The optimized material with the highest surface area (2230 m2 g-1) and pore volume (0.96 cm3 g-1) showed a simultaneous high promise for both low pressure (6.8 mmol g-1 at 1 bar/ 0 °C) and high pressure (26.4 mmol g-1 at 30 bar/0 °C) CO2 capture, which is extremely difficult to achieve for any CO2 sorbent and has never been reported before. This exceptional performance is attributed to ultra-microporosity, high surface area, and surface oxygenated functional groups. Heat of adsorption (30.4 kJ mol-1) value suggests that adsorption is physical in nature and accounts for easier material regeneration. These multiple merits underline the importance of synthesized materials in the field of CO2 capture and potential for various other environmental applications.


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INTRODUCTION Global warming is becoming a growing issue that needs urgent attention in pursuance of minimising the anthropogenic CO2 emissions which will pave the way for a positive climate change.1 Among all the greenhouse gases (GHGs), CO2 has the most detrimental impact on global warming as it has higher residence time compared to other greenhouse gases and has the highest accumulated concentration in the atmosphere since the beginning of industrial revolution. The primary source of CO2 release in the environment is the combustion of fossil fuels by the energy sector.2 The release of CO2 to the environment could be a result of a pre-combustion or a post combustion process. Although post combustion emissions contribute significantly more to the overall CO2 emissions, pre-combustion emissions cannot be ignored and must be duly addressed. Because of the nature and different types of chemistries involved in pre and post combustion processes, it is difficult to devise a single material that could be used favourably in both the processes. Carbon capture and storage (CCS) is a highly sought after technology for counteracting the CO2 emissions.3 Currently, liquid amine based absorption processes for CO2 capture are widely used in large scale power plants, cement and mining industries etc.4 Aqueous amines can adsorb large amounts of CO2 through chemisorption via the formation of carbamates.5 However, liquid amine based processes cause health hazards because of the toxic nature of the amines, corrosion and eventual degradation of the equipment and the regeneration costs are high.6 The use of solid based adsorption processes has been considered as an alternative as it can offer ease of operation with much less health hazard, and the regeneration of adsorbent is inexpensive and reasonably quick. An array of porous materials has been tried as adsorbents for CO2 capture including zeolites7, metal organic frameworks (MOFs)8-10, amine modified porous organic polymers11, mesoporous 3 ACS Paragon Plus Environment


carbon nitrides12, porous carbons13, and solid amine based adsorbents14. For industrial scale application, the selection of an adsorbent is strongly influenced not only by its ability to capture a large quantity of CO2 but also the cost involved in the synthesis of adsorbent, the ease of availability of the raw materials and the operational costs. Porous carbon based adsorbents such as activated carbons fulfil the above requirements because of their robustness, high surface area, tunable pore volume and size, hydrophobicity, low cost of regeneration, stability and non-toxic nature and most importantly ease of availability and low production costs.15 Conventional carbon based precursors such as petroleum coke and coal have been utilized for the production of activated carbon materials on an industrial scale.16, 17 However, in recent times, attention has turned towards using biomass as a source of activated biocarbons due to their ability to generate materials with high surface areas, high microporosity, variable pore structure, abundant availability and overall low cost.18 Such activated biocarbons are ideally suited for CO2 capture on an industrial scale as the adsorption process is highly reversible and the porosity of the biocarbons can be easily tuned as per the requirement. The reversible nature of the CO2 adsorption process using activated biocarbons facilitates easy regeneration of the adsorbent requiring minimum energy input and thus reduces the operational costs. Although porous activated biocarbons are considered as a good choice for the CO2 adsorption1921,

the material with a high ultra-microporosity and high surface area is considered as the best

choice for the effective capture of the CO2 molecules at different pressure regions. However, it is a challenging task to synthesize a material with such a dual combination. Herein we report for the first time the synthesis of a series of activated porous biocarbons with a high ultra-microporosity and high surface area using the readily available renewable biomass, lotus seed as a source of carbon combined with the KOH activation process. The overall cost of synthesizing activated 4 ACS Paragon Plus Environment

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porous biocarbons from lotus seed will be much lower than some of the popular adsorbents for CO2 such as zeolites, MOFs and aqueous amines. The activated biocarbons exhibited specific BET surface area in the range 1079 – 2230 m2 g-1 with a high ultra-microporosity which helped to achieve exceptional performance for both pre- and post-combustion CO2 capture processes. The sample with the highest BET specific surface area of 2230 m2 g-1 and a microporosity contribution of 74 % exhibited the highest CO2 adsorption capacity of 26.4 mmol g-1 at 0 °C and 30 bar. At 1 bar and 0 °C, the same sample exhibited an exceptionally impressive CO2 adsorption capacity of 6.8 mmol g-1 which is among one of the highest values reported at atmospheric pressure. The CO2 adsorption performance of the materials in conjunction with a low cost of the raw materials demonstrates an immense potential for pre and post combustion CO2 capture. EXPERIMENTAL SECTION Preparation of Lotus seed derived activated porous biocarbons (LSBs) LSBs were synthesized by a conventional two step procedure involving carbonization and activation of the biomass, Lotus seed. The synthesis process is schematically shown in Scheme S1. Briefly, a certain amount of biomass was pyrolyzed at a temperature of 600 °C for 2 hours. The obtained biochar was then impregnated with different amounts of KOH in solution to obtain four composite materials having KOH/biochar ratios of 1 to 4. The excess water from each composite was removed by drying the composite mixture at 100 °C in a hot air oven. Afterwards, each of the individual composite materials was carbonized under a continuous flow of N2 in a furnace at the experimental temperature range of 700 °C - 900 °C for 2 hours using a heating rate of 5 °C min-1. The obtained samples were washed with dilute HCl and water to remove leftover residues. The materials were then dried at 100 °C for 12 hours and the finally obtained black powdered samples were labelled as LSBn-T where LSB stands for Lotus seed derived biocarbons, 5 ACS Paragon Plus Environment


n denotes KOH/biomass impregnation ratio and T indicates the carbonization temperature. For example, LSB3-800 is the optimized sample obtained at a carbonization temperature of 800 °C using a KOH/biomass impregnation ratio of 3. RESULTS AND DISCUSSION X-ray diffraction (XRD) analysis In this work, we optimized the reaction conditions for obtaining the highest specific BET surface area of the LSBs. The carbonization temperature of 800 oC and KOH/biochar ratio of 3 was found to be the optimum conditions to achieve the best textural parameters including a high specific surface area, large pore volume and a high microporosity. Consequently, throughout the manuscript, the samples prepared at 800 oC and different ratios will be discussed in detail while the samples prepared at 700 °C and 900 oC but with different ratios of KOH/biochar will only be discussed in the comparative sense. Figure S1 shows the wide angle powder XRD pattern of the sample LSB-600 prepared without KOH activation. The two broad peaks centered around angle 2 = 23.8° and 43° are attributed to the presence of an amorphous structure associated with partial multilayered graphitic domains corresponding to the reflection planes (002) and (101).22, 23 The extent of graphitization in the optimized activated biocarbons prepared at a fixed carbonization temperature of 800 o C but using different KOH/biochar ratios was also investigated using high angle powder XRD as shown in Figure 1. Figure S2 (A and B) depicts the XRD patterns for all the remaining activated biocarbon samples prepared at carbonization temperatures of 700 oC and 900 oC using different KOH/biochar ratios. Comparing the XRD patterns in Figure 1 and Figure S2 (A and B), the peaks are narrowed as the carbonization temperature is increased from 700 oC through to 900 oC. This confirms an improved degree of graphitization at a high temperature.


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An interesting observation is that the peaks are shifted towards a higher angle region with an increase in carbonization temperature from 700 °C to 900 °C. This indicates a reduction in the interlayer spacing of the graphitic structures caused by the possible loss of linking functional groups due to the strong thermal effect at a higher temperature.24 The position of higher angle peak at 2 = 43° arising from in plane scattering remains almost unchanged. However, a considerable increase in the intensity corroborating with improved graphitic structure is observed with the increase in carbonization temperature.25 A significant change in the shape of the XRD patterns of the samples prepared at different amounts of KOH was observed. For example, Figure 1 shows the XRD patterns of materials prepared at a carbonization temperature of 800 °C using different KOH/biomass impregnation ratios of 1-4. There is a noticeable broadening of the lower angle peak and a slight broadening of the higher angle peak when the mass ratio of KOH/biomass is increased from 1 to 4. This indicates that the severe KOH activation conditions lead to a greater degree of chemical reaction between the base and biomass carbon resulting in much higher defects in the structure which lowers the degree of graphitization. Scanning electron microscope (SEM) analysis The changes in the surface morphology and porosity of the non-activated sample after the KOH activation was monitored using SEM imaging. It is evident from Figure S3 (A) that the nonactivated sample LSB-600 shows a non-porous morphology in the form of scattered smooth lumps of the carbon matter throughout the whole structure. However, upon activation with KOH, these irregular lumps undergo significant changes to their morphology and porous structure. Figure 2 compares the SEM images of the activated biocarbons prepared at a carbonization temperature of 800 °C. The surface morphology changes from the smooth surface into irregular porous sponge like structure. However, the sample that was activated with the KOH/biomass impregnation ratio



of 3 showed graphene sheets like morphology. The high-resolution images reveal the generation of nano to micron sized pores among all the samples. Figure S3 (b and c) also show a similar observation with materials prepared at other two temperatures of 700 °C and 900 °C by using optimized KOH/biomass impregnation ratio of 3. It is evident from all SEM images that both the extremes of temperature and the KOH/biomass impregnation ratio leads rather to a collapse of the carbon structure into a new and unique morphology including graphene sheets like structure. These results show that there is a considerable change in the morphology of the biocarbons upon activation with KOH and they retain very little memory of the structure of the parent non-activated sample. These observations are consistent with other reports on KOH activation of biomass derived biocarbons.26, 27 Textural parameters analysis Figure 3 shows the N2 sorption isotherms for samples prepared at a fixed carbonization temperature of 800 oC. Figure S4 shows the comparison of the N2 sorption isotherms of the samples prepared at 700 oC – 900 oC using a fixed KOH/biochar ratio of 3. The corresponding textural properties are summarized in Table 1 and Table S1. All the isotherms shown in Figure 3 and Figure S4 display type I nature and are classified as microporous materials as per the IUPAC classification.28 The amount of nitrogen adsorbed in the low pressure region increases with an increasing amount of activating agent and reaches a maximum when the KOH/biochar ratio was 3 and then decreases. Similarly, the amount of nitrogen adsorbed increased with increasing the carbonization temperature up to 800 °C and then decreases. The reduction in the amount of nitrogen adsorption at a higher temperature is attributed to the fact that shrinkage of the carbon structure creates large pores which affect the adsorption capacity of the sample at the monolayer region. Interestingly, as shown in Figure 3, the samples prepared at 800 °C using KOH/biochar


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ratio of 1 and 2 exhibit a distinctive H4 type hysteresis loop at relative pressures > 0.4 signifying the presence of narrow slit like pores.29 However, when the impregnation ratio is increased to 3 and 4, the hysteresis loops gradually disappears suggesting widening of the pores resulting from the extensive corrosive action of KOH at 800 oC. A similar trend is observed in samples prepared at 700 °C and 900 oC as shown in Figure S5 and Figure S6. The hysteresis loops for these samples are well pronounced as compared to samples prepared at 800 oC (Figure 3). Figure S4 shows the comparison of the N2 sorption isotherms prepared at the optimal KOH/biochar ratio of 3 and carbonization temperatures 700 °C, 800 °C and 900 oC from which the relative degree of micro porosity could be determined qualitatively. A visual inspection of these isotherms shows that as the carbonization temperature is increased from 700 °C through 900 oC, the steepness of the adsorption arm decreases, and the shape of the knee also becomes wider at 900 oC. The gradual decrease in steepness of the slope also suggests a decrease in the number of micropores with increasing the carbonization temperature. This observation is also substantiated by the values of micropore surface areas as enumerated in Table 1. The percentage micropore surface area and micropore volume of the LSB3-700 sample are 90 % and 87 % which decreases to 32 % and 27 % respectively for the sample LSB3-900. It is also clear that the carbonization temperature of 800 oC results in high surface area values for all KOH/biochar ratios with the highest surface area of 2230 m2 g-1 and pore volume of 0.96 cm3 g-1 corresponding to KOH/biochar impregnation ratio 3. In contrast, the non-activated biochar prepared at 600

o

C shows a low surface area of about 265 m2 g-1. The role of carbonization

temperature in conjunction with KOH activation in generating high specific surface area could be explained in terms of the severity of the corrosive action of KOH on biochar at a given carbonization temperature.



It is assumed that at a lower temperature of 700 °C, not all the KOH is consumed by the biochar whereas extensive reaction occurs between the two at a higher temperature of 900 °C. As a result, a lower temperature of 700 oC is not capable of generating enough pores whereas carbonization at 900 oC causes the destruction of generated micropores into mesopores. The above explanation can be substantiated quantitatively by looking at the contribution of the mesopores to the total surface area and pore volume for the samples LSB3-700 and LSB3-900. For the sample LSB3-700, the contribution of mesopores to the total surface area and pore volume are only 10 % and 13 % respectively whereas, for the sample LSB3-900, the corresponding values are 68 % and 73 % respectively. Further, at any given carbonization temperature, a KOH/biomass impregnation ratio of 3 yields the highest specific surface area. The effect of KOH/biomass impregnation could also be explained in terms of the chemical reactions occurring between KOH and biochar during the carbonization step. For example, a lower surface area of 1614 m2 g-1 is observed for LSB1-800 which was synthesized with the lowest KOH/biochar ratio 1, whereas a progressive increase in surface area with increasing KOH/biochar ratio can be observed for LSB2-800 (2046 m2 g-1) and LSB3-800 (2230 m2 g-1) samples. A further increase in the KOH/biochar ratio to 4 results in a sharp decrease in the specific surface area to 1717 m2 g-1 highlight the severity of the reaction between KOH and biochar. The above trend has also been reported in previous reports involving KOH activation of biochar.30 Pore size distribution (PSD) analysis Figure 4 depicts the PSD of all samples prepared at a fixed carbonization temperature of 800 °C. Figure S7 shows a comparison of the samples prepared using an optimum KOH/biochar ratio of 3 at all three temperatures (700 o C - 900 o C). The PSD of the remaining samples is shown in Figure S8. It is evident from the above figures that all the samples exhibit bimodal pore size


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distribution in the range 0.54 – 1.03 nm. For a given carbonization temperature of 800 oC, using a lower KOH/biochar impregnation ratio of 1 generates a higher volume of pores centred on 0.54 nm, whereas at a higher impregnation ratio of 4, the pore volume decreases significantly with most of the pores centred on 0.75 nm (Figure 4). A similar correlation can be seen for the samples prepared at a fixed carbonization temperature of 700 o C but different KOH/biochar ratios. It is worth noting that pore size less than 0.8 nm are categorized as ultra-micropores and are highly conducive for CO2 adsorption at low pressures.31 As discussed earlier a combination of severe activation ratio of 4 and a high temperature of 900 °C generates a large number of pores with size greater than 1 nm. These observations are in close agreement with the results derived from SEM, XRD and N2 sorption analysis. Similar observations could be drawn for the effect of carbonization temperature on the pore size as shown in Figure S7. The peak maxima in the bimodal pore size distribution shift towards larger pore size as the temperature is increased from 700 °C to 900 °C and result in widening of the pores due to stronger thermal effects as already discussed previously. X-ray photoelectron spectroscopy (XPS) and bulk elemental analysis The surface elemental composition and bulk elemental composition of LSBs obtained from XPS survey spectra measurements is listed in Table S2 and S3 which are an evidence of the highly carbonaceous nature of the synthesized materials. The survey spectra are shown in Figure S9. The survey spectra show that all samples contain carbon as the dominant element and an appreciable amount of oxygen. As per Table S2, the contents of carbon, nitrogen and oxygen in all the samples prepared in this study are in the range 88.9 – 94.5 %, 0.0 – 1.9 % and 5.3 – 8.9 %, respectively. Interestingly, the non-activated biochar has the lowest carbon content, the highest nitrogen content and the second highest oxygen content among all the samples. The highest nitrogen content could be attributed to the retention of some of the basic functional groups present in the original biomass.



The increase in carbon content upon activation could be ascribed to the cleavage of functional groups under the combined effect of carbonization and KOH activation. In case of the sample prepared at 800 °C, at a lower activation ratio of 1, more oxygen (7.9 %) is retained on the surface. However, with increasing activation ratio to 2 and 3, there is a reduction in the amount of oxygen content as some of it is lost due to the chemical action of KOH and heat treatment. LSB4-800 undergoes severe corrosive action by KOH which results in loss of carbon with a corresponding increase in oxygen content (8.9 %). The effect of temperature on surface elemental composition can also be seen from Table S2. For a fixed optimized KOH/biomass impregnation ratio of 3, at a lower carbonization temperature of 700 °C, a relatively higher percentage of the oxygen (8.2 %) is retained and the relative oxygen percentage reduces to 7.1 % and 5.3 % as the temperature is increased to 800 °C and 900 °C, respectively. This signifies the role of temperature in dictating the amount of the surface oxygenated functional groups. It is surmised that these oxygenated groups would impart a negative charge to the surface of the LSBs which, in conjunction with a high specific surface area, could prove highly useful for adsorption of weakly acidic CO2 molecules. The high-resolution core level C1s and O1s XPS spectra of samples prepared at 800 °C were recorded to study the nature and co-ordination of carbon and surface oxygen (Figure 5). In all the samples, C1s is resolved into distinct peaks (283.9 eV – 284.2 eV) representing the presence of sp2 hybridized graphitic carbon structure32 and a very low intensity peak at (285.3 eV – 285.9 eV) which indicates oxygen linkage.33 The O1s high resolution spectra show the presence of two significant peaks occupying the positions at 531.3 eV – 531.6 eV and 532.6 eV – 532.8 eV which collectively correspond to the presence of oxygen in phenolic, carboxylic and ethereal functional groups.34, 35 These results confirm that the surface of


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the LSBs contains plentiful of oxygenated functional groups which play an important role in enhancing their overall CO2 adsorption. Fourier Transform Infra-Red (FTIR) analysis The surface oxygen functional groups in samples synthesized at 800 °C were further confirmed using the FTIR analysis. There are three main bands observed in the FTIR spectra of these samples as shown in Figure 6. The two bands appearing at around 1190 cm-1 and 1570 cm-1 correspond to the C-O stretching vibration and aromatic C=C stretching vibration, respectively, while the presence of a distinct band around 3420 cm-1 is firm evidence of the existence of oxygen atoms involved in O-H stretching vibrations.36 The position of the band at 1190 cm-1 is shifted to a lower position in case of LSB2-800 which might be due to interaction with impurities. Similar observations are encountered in the FTIR spectra of samples synthesized at 900 °C (Figure S10). A collective analysis of the XPS, CHNS and the FTIR studies suggest that the material synthesized are highly carbonaceous and contains oxygen functional groups on the surface. Mechanism of KOH activation The mechanism for the reaction of KOH with a non-porous carbon or biochar is a well-known phenomenon. At low carbonization temperatures, non-porous carbon reacts with KOH to produce carbon oxides or carbonates, metallic potassium and hydrogen gas. The volatile oxides further react with the residual KOH which creates more porosity. With increasing temperatures, carbonates decompose to form potassium oxide which can further react with carbon to produce more of metallic potassium and volatile carbon oxides. It is surmised that the whole of the KOH is consumed at the carbonization temperature of 800 °C employed for the synthesis of porous carbons. All these products formed during the carbonization process are washed with hydrochloric



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acid and water to generate a hierarchical porous structure. The overall mechanistic reaction could be represented as follows: 6KOH + 2C

2K2CO3 + 2K + 3H2↑

(1)

CO2 adsorption analysis The CO2 adsorption performance of the LSBs was investigated for all samples synthesized at 800 °C and the values are listed in Table 2. The isotherms were recorded at 0 oC and within a pressure a range of 0- 30 bar (Figure 7A). As LSB3-800 was found to be the best material in terms of the textural properties, the adsorption isotherms were measured at two more temperatures, 10 and 25 o

C and the results are shown in Figure 7B.

Effect of KOH/biomass activation ratio on CO2 adsorption As shown in Figure 7A, adsorption isotherms obtained at 0 °C show a steep slope in the low pressure region (0-5 bar) which implies high CO2 adsorption in the low pressure region until monolayer formation starts and is represented by the appearance of the round knee. A further increase in CO2 pressure results in multi-layer adsorption as indicated by a low slope region on the isotherms which is followed by saturation of the pores. All materials show some differences in their shapes which is also reflected in the quantity of CO2 adsorbed. The difference in the CO2 capture performance is attributed to the variation in their specific BET surface areas, pore volumes and pore sizes. The CO2 adsorption capacity of LSB1-800 is the highest among all samples at low pressure conditions and it stands at 1.8 mmol g-1 at 0.1 bar and 6.9 mmol g-1 at 1 bar. Such a superior performance is attributed to the presence of a large proportion of micropores. It is worthwhile to mention that this CO2 capture performance is much higher than most of the biomass or nonbiomass based materials reported in the literature. For example, microporous carbon derived from 14 ACS Paragon Plus Environment

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pyrolysis of mesitylene based polymer and NUT-15 polymer adsorbs 6.16 mmoles and 5.4 mmoles of CO2 g-1 at 0 °C and 1 bar which is far less than the CO2 adsorption for LSB1-800 under similar conditions.37, 38 For LSB1-800, the contribution of micropores to the total surface area and total pore volume are 87 % and 81 %. The presence of narrow micropores results in enhanced interactions between the adsorbent and adsorbate at low pressures.39 At the same time, the presence of oxygenated functional groups on the surface provides additional binding sites, thereby enhancing CO2 uptake.40 These values are much higher than most of the other CO2 adsorbents reported in the literature and comparison of CO2 adsorption of LSBs with other materials is tabulated in Table S4. The other three samples LSB2-800, LSB3-800 and LSB4-800 show the CO2 adsorption capacity of 6.3 mmol g-1, 6.8 mmol g-1 and 4.6 mmol g-1 respectively at 0 °C/1 bar. This could again be attributed to the presence of a high proportion of micropores in their carbon structure as explained earlier. It is also clear from Figure 7A and Table 2 that the sample LSB3-800 with the highest surface area of 2230 m2 g-1 shows the highest CO2 adsorption capacity at a high pressure of 30 bar. LSB2-800 (SA = 2046 m2 g-1) and LSB4-800 (S.A = 1717 m2g-1) show CO2 uptake of 23.0 mmol g-1 and 20.1 mmol g-1, respectively at 30 bar pressure. Effect of textural parameters on CO2 adsorption The textural parameters such as surface area, pore volume and pore size have a direct influence on CO2 adsorption and it could be inferred from the values summarized in Tables 1 and 2. Based on the N2 sorption measurements, the surface areas of LSB1-, LSB2-, LSB3 and LSB4-800 are 1614, 2046, 2230 and 1717 m2 g-1, pore volumes are 0.67, 0.90, 0.96 and 0.78 cm3 g-1. All materials display a high volume of pores in the micro size (0.54 nm) along with the dual size of 0.61, 0.69, 0.72 and 0.75 nm, respectively. The CO2 adsorption of these materials recorded at 0 °C and 30 bar is 16.6, 23.0, 26.4 and 20.1 mmol g-1. It can be seen that there is a direct relation between the



surface areas and pore volumes vs CO2 adsorption. The CO2 adsorption also varies in a similar fashion with the pore size, however, the material LSB4-800 shows lower CO2 adsorption than LSB3-800 despite having the larger pore size of 0.75 nm. This is due to the fact that the surface area is the main controlling factor for high-pressure CO2 adsorption. The CO2 adsorption at 0 °C and low pressure of 1 bar could be explained in terms of the microporous content of the materials. The material LSB1-800 with the highest micropore volume of 81 % shows a high CO2 adsorption of 6.9 mmol g-1. The other three materials have lower micropore volume and hence adsorb lesser amounts of CO2 g-1. The only exception here is the material LSB2-800 which shows an surprisingly low CO2 adsorption which could be attributed to its low content of surface oxygen as per Table S2. The overall CO2 adsorption is dependent on the surface area, pore characteristics and the surface oxygenated functional groups. Effect of analysis temperature on CO2 adsorption Adsorption isotherms of the optimized sample LSB3-800 were measured at three temperatures 0 °C, 10 °C and 25 °C (Figure 7B). It shows the highest CO2 uptake of 26.4 mmol g-1 at 0 °C which decreases to 24.7 mmol g-1 at 10 °C and the lowest CO2 adsorption of 20.9 mmol g-1 was recorded at 25 °C. The lower CO2 adsorption at high temperatures is attributed to the physisorption occurring between adsorbent and adsorbate.50 The physical nature of the adsorption process was further confirmed from isosteric heat of adsorption values discussed later. The most fascinating part of our current investigation is the development of porous CO2 adsorbent, LSB3-800, which has both high microporous content and a high surface area that results in high values of CO2 uptake at both high and low pressure as already discussed in earlier sections. Isosteric heat of adsorption (Qst) analysis


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Qst was calculated for the sample LSB3-800 from the Clausius-Clapeyron equation using the isotherms obtained at 0 °C, 10 °C and 25 °C. The strength and type of interactions between the porous carbon and CO2 were deduced from the values of isosteric heat of adsorption (Qst). The software on the instrument uses the Clausius−Clapeyron relation shown as below for calculating Qst. ln p = -Qst/RT + ln C

(2)

p stands for pressure, Qst is the isosteric heat of adsorption, R is ideal gas constant, T is temperature, and C stands for a constant. Adsorption isosteres are obtained by plotting ln p vs 1/T and Qst is read as a slope of this graph using the inbuilt software on the instrument. The variation of Qst with CO2 loading is shown in Figure 8. At low pressures, Qst was determined to be 30.4 kJ mol-1 which suggests physisorption nature of the adsorption process and is significantly higher when compared to Qst for other biomass based CO2 adsorbents reported under similar conditions.51 The plausible explanation for higher Qst, in this case, is the enhanced interactions between the surface-active sites generated by narrow ultra-micro pores and the incoming CO2 molecules. The material shows an almost constant heat of adsorption Qst in the window 2-5 bar and then a slight upward shift is observed as the pressure is increased up to 10 bar. In our previous report, we have observed that the heat of adsorption generally shows a decreasing trend with increasing CO2 pressure or CO2 loading. However, in this case, it is observed that the heat of adsorption shows an increasing trend with increasing CO2 loading. This little increase could be attributed to the stronger hydrogen bonding among the oxygenated functional groups that impart a basic character to the surface of the material and the CO2 molecules.52 Further, the observed Qst values are quite higher than the liquefaction enthalpy of CO2 (17 kJ mol-1), which indicates the absence of intermolecular interactions and as such the CO2 molecules are fully adsorbed onto the surface of the activated



porous biocarbon.53 In summary, we conclude that CO2 adsorption at low pressures is facilitated by the presence of ultra-micro pores and the contribution of the micropores to the total surface area. Higher the micropore surface area, higher would be the CO2 adsorption at low pressures. However, CO2 adsorption at higher pressures is dictated by the wider pores and overall surface area. Conclusions We have demonstrated the synthesis of highly efficient Lotus seed derived activated porous biocarbons (LSBs) for CO2 capture at low and high pressure. It was found that a lower (700 °C) or a higher (900 °C) carbonization temperature resulted in materials with lesser surface areas (1079 m2 g-1 – 1766 m2 g-1) and pore volumes (0.47 cm3 g-1 – 0.90 cm3 g-1) as compared to materials prepared at 800 °C. We found that a combination of KOH/biochar impregnation ratio of 3 and a carbonization temperature of 800 °C to be the optimum condition for producing the LSBs with superior textural parameters. The optimized material LSB3-800 possess a high surface area of 2230 m2 g-1 and a pore volume of 0.96 cm3 g-1. The temperature and pressure swing CO2 capturing behaviour of LSBs reveals their excellent application potential for pre-and post-combustion CO2 capture. LSB1-800, with the highest amount of micro-porosity among all samples, displayed high CO2 adsorption of 1.8 mmol g-1 and 6.9 mmol g-1 at low pressures conditions of 0.1 bar/0 °C and 1 bar/0 °C. However, a combination of both high microporous content and high surface area in the optimized material LSB3-800 allows excellent CO2 adsorption capacities of 6.8 mmol g-1 and 26.4 mmol g-1 at 1 bar/0 °C and 30 bar/0 °C, respectively. Overall, our current investigation contributes to the development of exciting and attractive materials in the field of CO2 capture as LSBs in the ultra-microporous domain. These materials will be suitable for CO2 capture and storage (CCS)


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process operating under different conditions of temperature and pressure with a good potential for CO2 capture from flue gas streams. ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publication website. Details include XRD, SEM images, N2 sorption isotherms, Pore size distribution, XPS survey spectra and textural properties of LSBs. AUTHOR INFORMATION Corresponding author Email: [email protected] ORCID Ajayan Vinu: 0000-0002-7508-251X Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS Authors acknowledge the research support and laboratory facilities provided by The University of Newcastle Australia. A. Vinu is grateful to the University of Newcastle for the start-up funds.



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Table 1. Textural properties of activated porous biocarbons produced from Lotus seed aSA bSA cPV dPV ePW BET micro total micro Sample 3 -1 2 -1 (%) (cm g ) (%) (m g ) (nm) LSB-600 265 91 0.11 91 LSB1-800 1614 87 0.67 81 0.54, 0.61 LSB2-800 2046 75 0.90 67 0.54, 0.69 LSB3-800 2230 74 0.96 67 0.54, 0.72 LSB4-800 1717 64 0.78 58 0.54, 0.75 LSB3-700 1766 90 0.70 87 0.54, 0.68 LSB3-900 1703 32 0.90 27 0.54, 1.03 a Surface

Area (Brunauer Emmett Teller), b Micropore Surface Area using t-plot, c Total Pore Volume at P/Po of 0.99, Volume using t-plot, and e Pore Width using MP method

d Micropore

Table 2. CO2 adsorption capacities of the activated biocarbons derived from Lotus seed Sample CO2 adsorption CO2 adsorption CO2 adsorption -1 -1 (mmol g ) (mmol g ) (mmol g-1) at 0.1 bar/ 0 °C at 1 bar/ 0 °C at 30 bar/ 0 °C LSB1-800 1.8 6.9 16.6 LSB2-800 6.3 23.0 LSB3-800 6.8 26.4 LSB3-800 4.6 (10 °C) 20.9 (10 °C) LSB3-800 3.1 (25 °C) 17.5 (25 °C) LSB4-800 4.6 20.1



Figure 1. XRD patterns of a) LSB1-800, b) LSB2-800, c) LSB3-800 and d) LSB4-800

Figure 2. SEM images of a) LSB1-800, b) LSB2-800, c) LSB3-800 and d) LSB4-800 28 ACS Paragon Plus Environment

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Figure 3. N2 adsorption-desorption isotherms of a) LSB1-800, b) LSB2-800, c) LSB3-800 and d) LSB4-800

Figure 4. Pore size distribution curves of a) LSB1-800, b) LSB2-800, c) LSB3-800 and d) LSB4-800



Figure 5. XPS high resolution C1s and O1s spectra of a) LSB1-800, b) LSB2-800, c) LSB3-800 and d) LSB4-800


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Figure 6. FTIR spectra of a) LSB1-800, b) LSB2-800 and c) LSB3-800 and d) LSB4-800



Figure 7. A) CO2 adsorption isotherms of a) LSB1-800, b) LSB2-800 and c) LSB3-800 and d) LSB4-800 at 0 °C and B) CO2 adsorption isotherms of LSB3-800 at a) 0 °C, b) 10 °C and c) 25 °C

Figure 8. Isosteric heat of adsorption of LSB3-800 calculated at 0 °C, 10 °C and 25 °C 32 ACS Paragon Plus Environment

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This article highlights the synthesis of high surface area biocarbons from lotus seed with excellent performance for CO2 capture

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