Influence of Alkali Metals (Na, K, and Cs) on CO2 Adsorption by

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Influence of Alkali Metals (Na, K and Cs) on CO2 Adsorption by Layered Double Oxides Supported on Graphene Oxide Diana Iruretagoyena, Xiaowen Huang, Milo Sebastian Peter Shaffer, and David Chadwick Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02762 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 28, 2015

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Industrial & Engineering Chemistry Research

Influence of Alkali Metals (Na, K and Cs) on CO2 Adsorption by Layered Double Oxides Supported on Graphene Oxide Diana Iruretagoyenaa, Xiaowen Huangb, Milo S.P. Shafferb and David Chadwicka* a

Department of Chemical Engineering, bDepartment of Chemistry, Imperial College London,

South Kensington Campus, London, SW7 2AZ, UK

KEYWORDS Layered double oxides, layered double hydroxides, hydrotalcites, graphene oxide, CO2 adsorption, alkali metals

ACS Paragon Plus Environment

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ABSTRACT The CO2 adsorption capacities, isotherms, and thermal stability of sodium, potassium and cesium impregnated layered double oxides (LDOs) supported on graphene oxide (GO) are reported. Alkali cations were found to modify the distribution and density of chemisorption sites resulting in higher CO2 adsorption capacities at 573 K (0.56-0.69 molCO2/kg) compared to the unpromoted adsorbents (0.29 molCO2/kg) (LDO and LDO/GO hybrids). The improved thermal stability imparted by incorporation of GO in the LDO was not compromised by the addition of any of the alkali species. Potassium produced the most marked enhancement in CO2 capacity (0.47 molCO2/mol alkali) due to its relatively strong Lewis basicity and even distribution on the surface of the adsorbents. Cesium was found to have a tendency to agglomerate that reduced the effectiveness of promotion. The CO2 adsorption equilibrium of the fresh calcined materials is described by the Freundlich model. After thermal adsorption-desorption cycles the isotherms fit closer to the Langmuir isotherm.

1. INTRODUCTION Sorption enhancement is considered an attractive technology to improve the efficiency of hydrogen production by the water gas shift reaction (WGS), which is an important stage in the steam reforming of methane or the gasification of coal or biomass. Sorption enhancement is typically conducted between 573 K and 773 K and involves the in situ sequestration of CO2 as it is formed so that the thermodynamic constraints on WGS are lessened and the production of H2 is increased. This technology requires the development of CO2 adsorbents with adequate capacities and that are sufficiently stable to tolerate cyclic adsorption-desorption operation.


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Adsorbents based on layered double hydroxides (LDHs), also known as hydrotalcites, are promising candidates for sorption enhanced applications. LDHs are two-dimensional structures composed by positively charged brucite-like layers, i.e. Mg(OH)2, in which divalent cations are partially substituted by trivalent cations (typically Al3+). When LDHs are thermally treated, they decompose into a mixture of oxides that partially maintain the original structure. The highly defective nature of the resulting layered double oxides (LDOs) provides sites with sufficient basicity to adsorb CO2 under the operation conditions required. Activated LDHs usually show outstanding adsorption-desorption kinetics compared to other adsorbents and their regeneration is not very energy demanding. Additionally, they perform well in the presence of competing species such as H2O and H2S.1 However, an improvement in the multicycle stability and in the CO2 adsorption capacity of LDOs is desirable to facilitate application. The use of a support material such as zeolites,2 carbon nanofibers,3 alumina,4,5 and multiwalled carbon nanotubes,5 has been demonstrated to be an effective approach to improve the multicycle stability and CO2 intrinsic capacity (i.e., per mass of LDO) of layered double oxides. However, the improvement in performance is observed only with high loadings of support, which may result in large sorption units. In recent studies, graphene oxide (GO) was found to increase the thermal stability of LDOs significantly by the addition of only modest amounts as support (below ∼ 20 wt.%).6,7,8 As a consequence of the low GO loading, the volumetric capacity of these LDO/GO hybrids, i.e. their CO2 capacity per total volume, is significantly higher than those obtained using other supports8 so that in principle, a higher throughput can be attained in an adsorption column of a given volume. The relatively low amount of GO required to improve the CO2 adsorption performance of LDOs is related to its obvious compatibility with the LDH platelets in terms of geometry and charge. 7


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While the presence of GO was found previously to mediate the loss of adsorption capacity of LDO over repeated adsorption-desorption cycles, the CO2 uptake of these hybrids is still relatively low. Bulky monovalent alkali cations are stronger Lewis bases than the divalent (Mg2+) and trivalent (Al3+) cations of the LDO surface and therefore many researchers have demonstrated a promoting effect of sodium and potassium on the CO2 capacity.9,10,11,12,13 Some studies have suggested that the interaction between the alkali metals and the aluminium oxide centers in the LDO plays a key role in the formation of strong basic sites which are more active for CO2 adsorption.11 Alternatively, the gain in capacity observed has been attributed to a higher concentration of surface defects created by the presence of the monovalent ions.13 The majority of the available literature on alkali promotion has focused on the performance of unsupported layered double oxides and only a few works have dealt with promotion of supported LDOs. Meis et al.13 studied the influence that Na and K have on the adsorption capacity of LDOs supported on carbon nanofibers (CNF). The authors prepared LDO/CNF composites with large CNF contents (around 90 wt%) and the promoters were introduced either by leaving alkali residues in the washing step or were added by impregnation. An increase in the CO2 uptake was observed in all the promoted LDO/CNF samples although the capacities were slightly higher for those with alkali retained from the synthesis. Recently, we reported that the presence of small amounts of sodium residues from the precipitation process (2 wt%) deliberately left by minimum washing almost doubled the CO2 adsorption capacity of unsupported LDOs and LDO/GO hybrids containing 5 wt% GO.6,8


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In this contribution we report the influence of GO on the CO2 adsorption and stability under thermal cycling of sodium, potassium and cesium impregnated LDO/GO hybrids. This contrasts with our previous study where residual Na was incorporated in LDO/GO via the precipitation process8. A relatively low loading of GO (5 wt.%) was used for the present study since it was shown previously that a modest content leads to greatly improved stability of unpromoted LDO.8 The influence of the impregnated alkali metals on the nature of the adsorption sites has been investigated by temperature programmed desorption of CO2 (CO2-TPD), and on the CO2 adsorption isotherms at 573 K. The stability of the materials has been examined over continuous adsorption-desorption temperature swing cycles (TSA). We show that the addition of Na, K or Cs to increase the CO2 adsorption capacity of the LDO/GO hybrids does not compromise the improvement in thermal stability imparted to the hybrid materials by the presence of GO.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Mg-Al LDHs Unsupported LDHs, Mg0.6Al0.3(OH)2(CO3)0.15·nH2O, were prepared via co-precipitation under high supersaturation conditions. To prepare 1 g of LDH, an aqueous solution (4.5 mL) of 2 M Mg(NO3)2·6H2O and 1 M Al(NO3)3·9H2O was added dropwise to an aqueous solution (6.7 mL) of 1.2 M Na2CO3 and 4.7 M NaOH under vigorous stirring. The inorganic salts used in the synthesis were purchased from Sigma-Aldrich. The measured pH of the solution was 10. The resulting white suspension was aged at 333 K for 12 hours under vigorous stirring. Subsequently, the precipitate was filtered using 0.2 µm nylon membranes and thoroughly washed with DI water (0.5 L per 1 g of solid) at 333 K. The samples were dried for 24 hours at 393 K in a recirculating oven.


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2.2 Synthesis of LDH/GO Hybrids Graphene oxide (Nanoinnova Technologies) was dispersed in a solution containing 1.2 M Na2CO3 and 4.7 M NaOH from which the LDH was co-precipitated. LDO/GO hybrids were prepared with nominal weight ratios 20/1 (LDH20) using 50 mg of graphene oxide. The resulting suspension was aged, filtered, thoroughly washed and dried as described above for the unsupported LDH.

2.3 Alkali Promoted LDH and LDH/GO Unsupported LDO and LDO/GO hybrids were thoroughly-washed after the synthesis to be essentially alkali-free. Subsequently, the materials were promoted by incipient wetness impregnation using aqueous solutions of Na2CO3, K2CO3 or Cs2CO3, aiming to achieve equimolar amounts of the alkali metal. The materials were then dried for 24 hours at 393 K in a recirculating oven. The unsupported samples are denoted as LDH-Na, LDH-K and LDH-Cs, and the hybrids are denoted as LDH20-Na, LDH20-K and LDH20-Cs.

2.4 Activation of LDHs and LDH/GO Hybrids Prior to the CO2 adsorption measurements, the materials were calcined in situ flowing 20 mL min-1 of N2 at 673 K for 4 hours to produce the corresponding unsupported LDOs (LDO, LDONa,

LDO-K and LDO-Cs) and the LDO/GO hybrids (LDO20, LDO20-Na, LDO20-K and LDO20-Cs).

The activation was performed in situ in the TGA to avoid irreversible loss in capacity due to CO2 captured from exposure to the atmosphere. For characterization purposes, the samples were


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activated ex situ flowing 100 mL min-1 of N2 at 673 K for 4 hours using a quartz cylinder (ID = 4 cm, L = 60 cm) placed in a horizontal furnace.

2.5 Characterization N2 physisorption measurements at 77 K were conducted using a Micrometrics Tristar 3000 instrument. The samples were dried at 393 K under N2 for at least 12 hours. The surface area was determined by the BET method and the pore volume was measured at P/Po = 0.991. The average pore diameter was calculated from the desorption isotherm using the BJH method. Powder X-ray diffraction (XRD) was carried out using a PAnalytical X’Pert Pro diffractometer (Cu Kα radiation) in reflection mode at room temperature. The elemental composition of the adsorbents was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in a Perkin Elmer Optima 2000DV apparatus. Thermo-gravimetric analysis (TGA), performed in a TAQ500 instrument, was used to determine the actual loading of GO in the hybrids. To this end, approximately 5 mg sample were dried at 393 K under N2 for 20 minutes and then heated from 393 K to 1273 K at 10 K min-1 in 60 ml min-1 of air. Transmission electron microscopy (TEM) images were obtained using a JEOL 2000FX microscope. STEM-EDS images were collected in a JEM-2100F instrument.

Temperature programmed desorption (TPD) of CO2 was carried out using a quartz micro-flowcolumn system operated at atmospheric pressure. The pre-calcined sample (20 mg) was preconditioned by heating from room temperature to 673 K at 10 K min-1 flowing 45 mL min-1 of Ar and held for 1 hour. After cooling to 313 K, the sample was exposed to a 20% CO2/Ar premixed gas (BOC) for 1 hour. The system was then purged in flowing Ar for 2 hours to


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remove physisorbed CO2, and the temperature was increased to 1023 K at 50 K min-1. The argon used in the experiments was purified using moisture and oxygen traps. A mass spectrometer (ESS GeneSys II with heated capillary sample line) was used to monitor the CO2 gas concentration at the exit of the quartz tube. Prior to each TPD experiment the response was calibrated by injecting 2 mL of the premixed CO2 gas.

2.6 CO2 Adsorption Measurements A thermogravimetric analyzer (Perkin Elmer, TGA4000) was used to determine the adsorption capacity of the samples. An amount (∼ 15 mg) of adsorbent was calcined in situ as described earlier. After activation, the temperature was decreased from 673 K to 573 K at 10 K min-1 and held for 10 min. The feed was then switched to a 20% CO2/Ar premixed gas (BOC), and held usually for 1 hour. The adsorption capacity of the materials was determined from the change in mass during the adsorption step. Effects due to changes in gas viscosity and gas density were accounted by measuring the TGA response of a silicon carbide sample and then subtracting it from the adsorbent response. Under the operating conditions used this blank was very small. In multicycle tests, the adsorption step was carried out at 573 K for 1 hour flowing the premixed CO2 gas and the desorption step was performed at 673 K during 30 minutes flowing nitrogen. The flow rate was kept constant at 20 mL min-1 during the experiment.

Adsorption isotherms of the materials were obtained using the TGA4000 instrument. The samples were activated (calcined) in situ flowing 20 mL min-1 of N2 at 673 K for 4 hours. The adsorption isotherms were determined using premixed gases: 2%, 10%, 15%, 20%, 50%, 80%, 90% (v/v) CO2 in Ar. For first-contact isotherms (in the first exposure to CO2/Ar) a fresh sample was activated and then subsequently exposed to a specific CO2 mixture at 573 K until the sample


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weight was essentially constant. In multicycle experiments a mixture of 20% CO2/Ar was used during the adsorption cycle and the desorption step was performed by increasing the temperature to 673 K under flowing nitrogen for 30 min. After near-constant capacities were obtained in consecutive measurements, the adsorption isotherm was determined by continuing the adsorption-desorption cycles but changing successively the concentration of CO2 in the adsorption step using the premixed gases.

The adsorption capacities reported in the first-contact isotherms were taken as the average of at least three measurements under the same operating conditions. For the multicycle adsorption isotherms, the standard deviation in the mean was calculated from the adsorption capacity after twenty cycles obtained in at least three different experiments. The error was assumed to be the same for the adsorption capacities obtained at the different CO2 partial pressures. The experimental adsorption isotherms were fitted to the Freundlich and Langmuir models applying a nonlinear least square method using OriginPro8.6.

3. RESULTS AND DISCUSSION 3.1 Characterization of the Adsorbents Chemical Composition The compositions of the as-synthesized adsorbents are given in Table 1. The Mg/Al ratio of all the samples determined by ICP-OES was close to the intended ratio of 2. In the thoroughlywashed adsorbents residual sodium was detected only at trace levels indicating that the NaOH and Na2CO3 used during the synthesis were successfully removed such that the samples can be considered to be essentially sodium-free. For the LDH/GO hybrids, slightly higher amounts of


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Na remained after thorough washing due to strong interactions between Na ions and GO. However, these sodium traces do not influence the performance of the hybrids.8 The alkali molar loadings of the samples impregnated with sodium, potassium and cesium are between 0.9 and 1.3 mol alkali/kg LDH and are close to the intended contents. All the values are slightly above the monolayer capacity of the unsupported LDH which is 1.16, 0.79 and 0.51 mol alkali/kg LDH for Na, K and Cs respectively according to the respective van der Waals ionic radii. In situ STEMEDS analysis confirmed that magnesium and aluminum were distributed evenly in the materials. (Figure 1). The alkali ions are also evenly distributed but Cs has a tendency to form small aggregates. The TGA patterns of all the LDH containing samples exhibited three distinctive regions of weight loss that can be attributed mainly to dehydration (below ∼ 483 K), partial dehydroxylation and decarbonation (483 K – 733 K) and further decarbonation (above ∼ 733 K), (Figure S1). The TGA of the pure GO presents two regions of weight loss, one around 500 K due to the decomposition of functional groups and oxidation debris, and other around 750 K attributed to the combustion of damaged graphitic domains (Figure S1).14 These stages of weight loss are not clearly distinguished in the LDH/GO hybrids due to the low content of GO in the samples. However, it is possible to estimate the actual GO loading in the supported adsorbents from the residue of each sample as described elsewhere.7 Table 1 shows that the actual weight percentages of GO in the hybrids were close to the nominal GO values.


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Morphological Characterization The textural properties of the as-synthesized materials and the activated adsorbents were investigated by N2 physisorption. All the samples exhibit a combination of type IV isotherms with H1 hysteresis loops that indicate a wide distribution of pore sizes (Figure S2).15 An increase in the BET surface areas and the mesopore volumes of the adsorbents is observed after thermal treatment at 673 K, Table 2; the values for the as-synthesised adsorbents are given in Table S1. The surface areas and pore volumes of the alkali promoted adsorbents are significantly lower than those of the corresponding unpromoted materials, possibly due to blockage or filling of the pores with alkali species. The decrease in surface area becomes more marked as the size of the alkali atoms increases, parallel to the increasing tendency to form alkali agglomerates revealed by STEM-EDS (Figure 1).

Crystallographic Characterization The crystallinity of the as-synthesized adsorbents was investigated by X-ray diffraction, Figure. 2. All the as prepared samples show the typical reflections of hydrotalcite-like compounds. After activation at 673 K, the layered structure of the LDH collapses partly and results in amorphous LDO phases (Figure S3). For comparison purposes Figure S4 shows the XRD patterns of GO and activated GO. No diffractions peaks associated with graphene oxide were observed in XRD of the LDH/GO and LDO/GO hybrids or GO in the TEM images due to the low contents of GO support (Figures S3 and S5). The XRD patterns of the uncalcined and calcined alkali promoted samples, either unsupported or supported, did not exhibit diffraction lines associated with Na, K or Cs carbonates or nitrates. This contrasts with the diffractograms of minimum-washed samples containing residual sodium reported previously which showed the characteristic reflections of NaNO3 crystallites, possibly occluded in the materials.8 Therefore, it is likely that the promoting


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species are distributed evenly over the surface of the adsorbents and any small clusters (noted above for Cs) are non-crystalline. For the as-synthesized and activated samples that were promoted with alkali ions, the crystallite sizes are similar to those of the corresponding thoroughly-washed materials. However, there is a decrease in the intensity of the diffraction patterns in Figure 2 (and Figure S3) in the order Na > K > Cs which is caused by the dilution of the LDH and LDO phases with the different masses of promoter used in order to achieve similar alkali contents on a molar basis.

The LDH and LDO crystallite sizes reported in Table S1 and Table 2 respectively for the sodium impregnated samples are significantly smaller than the values reported previously for minimallywashed adsorbents containing a similar loading of residual sodium.8 This suggests that extensive washing during the filtration step, as used here, decreases the final crystallite size of the adsorbents, probably due to erosion.

Temperature Programmed Desorption of CO2 To assess the effect of Na, K and Cs on the basic properties of the LDO phase, CO2-TPD measurements were conducted using unsupported samples (LDO-Na, LDO-K and LDO-Cs), Figure 3. All the profiles can be deconvoluted into four Gaussian peaks (Figure S6) corresponding to the desorption of bicarbonates (α, 423 K), bidentate carbonates (β, 543 K) and monodentate carbonates (γ, 813 K and δ, 963 K).16,17 The addition of alkali ions by impregnation alters significantly the basicity of the materials (Figures 3, Tables 3 and Table S2). The amount of CO2 released from the bicarbonates formed on Brønsted OH-groups (low temperature peak) is maintained with respect to the unpromoted LDO, whereas the evolution in the intermediate temperature state decreases. Additionally, the number of monodentate carbonates adsorbed on


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low-coordinated oxygen anions (high temperature peak) is significantly higher for the alkali promoted samples. The very strong basic sites that desorb CO2 at 1115 K are possibly related to the decomposition of bulk alkali carbonates at the surface of the mixed oxides.11 As shown in Table 3, the presence of alkali metals results in a 50% to 70% increase in the number of active sites available for CO2 chemisorption compared to the pure LDO. Deconvolution of the CO2 desorption profiles of the impregnated materials reveals a similar distribution of basic strength for the different alkali species, Table S2, whereas the total density of adsorption sites per mass of LDO increases in the order Cs > K > Na, i.e. following the trend of basicity of the promoter oxides, Table 3. As the ionic radii of the alkali metals increase, the partial charge of the associated oxygen anions becomes more negative inducing the formation of sites with higher electron donor capacity.18 Increasing basicity of the surface results in an increment in the CO2 capacity (molCO2/g LDO) of the promoted adsorbents. However, when the CO2 capacities per mole of alkali (molCO2/mol alkali) are compared, there is marked benefit in using potassium instead of sodium but a further increase in the basicity (and size) of the promoter to cesium is counterproductive, Table 3. This indicates that the tendency of Cs to form bulky agglomerates, which is evidenced by STEM-EDS (Figure 1) and N2 physisorption (Table 2), reduces the effectiveness of its interaction with the LDO surface and results in a decrease in the number of sites accessible for adsorption. A comparison between the desorption profiles of the pure LDO and LDO20, and of the analogous sodium impregnated adsorbents (LDO-Na and LDO20-Na) revealed that contrary to the alkali metals, the presence of GO in the loadings used does not influence markedly the density nor the distribution of basic sites of the pure LDO (Figure 4), as reported previouly.8


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3.2 CO2 Adsorption Capacity Measurements The CO2 adsorption capacities of the activated materials were determined by single point measurements using a 20% CO2/Ar premixed gas at atmospheric pressure in a TGA analyzer. All the adsorbents showed similar adsorption kinetics with a fast initial stage followed by a slower CO2 uptake (Figure S7). The adsorption capacities of the alkali free and the alkali promoted LDO and LDO/GO samples in their first exposure to the adsorptive gas after activation (firstcontact adsorption) are given in Table 4, and correspond to ca. 95% of the actual equilibrium capacities. These working capacities were found to be practical for the purpose of comparison. The CO2 capacities per mass of LDO for the alkali free LDO and LDO20 are the same within the experimental error, which is consistent with the TPD measurements and with previous studies that indicate that the presence of graphene oxide in the range of loadings used does not modify the nature nor the number of basic sites.8 On the other hand, as expected, the unsupported LDO and the hybrids show a significant increase in their first-contact adsorption capacities when alkali metals are present, Table 4. Promotion results in more than a 40% increase in the CO2 uptake compared to the alkali free materials. It is worth noting that the enhancement in adsorption capacity achieved by alkali promotion is higher for the LDO than for the LDO20. It is likely that the impregnated alkali carbonates do not interact with the magnesium and aluminum oxides as efficiently as in the unsupported LDO. This effect can be ascribed to strong interaction between the alkali cations and the oxygen groups of the support that occur during impregnation preventing the optimal incorporation of the promoter on the LDO.13 In fact, a blank experiment using a GO sample loaded with 6 wt% Na (added as Na2CO3) showed an enhancement of capacity of 0.06 molCO2 kg-1 GO. This low promotion of CO2 adsorption by sodium on the


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support demonstrated the necessity of having a good interaction between the alkali promoter and the LDO. The intrinsic capacities (mol CO2/ kg LDO) of the promoted LDO and LDO/GO adsorbents were found to increase in the order Cs > K > Na, which coincides with the chemisorption trend observed by CO2-TPD. However, due to the differences in the atomic masses of the alkali promoters, it is of interest to assess the efficiency of promotion of the different metals by making the comparison of adsorption capacities on the molar basis (molCO2/mol alkali). In that case, potassium is observed to promote the CO2 capacity of the adsorbents better than sodium, but there is no additional benefit in further increasing the alkalinity to cesium, Table 4. As discussed above, this is probably caused by the tendency of Cs to form agglomerates which reduce the surface area available for adsorption and decrease the amount of Cs available to interact with the active centers of the LDO. Therefore, the beneficial effect of increasing the basicity of the LDO is opposed by the tendency of bulky alkaline cations to form clusters, which renders potassium an optimal promoter for LDOs. This conclusion is in agreement with a previous study focused on unsupported LDOs that compared the influence of K and Cs on a weight percent basis.12 The CO2 adsorption capacities obtained in the present study for the sodium impregnated samples (LDO-Na and LDO20-Na) can be directly compared with values previously reported for unsupported and GO supported LDOs promoted with sodium residues that were deliberately left in the materials by minimum washing (LDOMW and LDO20-MW).8 The CO2 uptakes of the unsupported LDOs are the same (0.56 mol CO2 kgads-1) regardless of the method employed to incorporate sodium. The increment in capacity achieved for the unsupported LDO-Na and LDOMW samples (∼0.25 molCO2 kgGO-1) are similar to enhancements reported by other authors for significantly higher sodium contents (12 wt% Na).13 This suggests the existence of an


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optimum amount of sodium which stems from the compromise between the creation of adsorption sites promoted by the alkali species and the occlusion of pores by Na2CO3 agglomerates at high loadings.10 The intrinsic capacity of the LDO/GO hybrid impregnated with sodium (i.e. LDO20-Na) is slightly lower (0.49 mol CO2/kgads-1) than our previously reported minimum washed sample where the same amount of sodium was deliberately left (0.54 molCO2 kgads-1).8 The cations introduced by impregnation are strongly attracted to the support as discussed above, while sodium residues remaining from the synthesis interact more effectively with the LDO phase leading to higher capacities. A strong interaction between sodium cations and negatively charged sites of GO has been previously inferred from the redispersion of NaNO3 crystallites in LDH/GO samples that occurs during thermal activation.8 Meis et al.13 also found that the promotion of LDOs supported on carbon nanofibers is more effective when alkali ions are left from the synthesis than when added by impregnation and ascribed this to a detrimental interaction between the promoter and the support. It is worth mentioning that in their work the authors observed higher intrinsic capacities compared to the values reported in Table 4 for the present study (0.08 molCO2 kgLDO-1 for LDO/CNF impregnated with K and 0.22 molCO2 kgLDO-1 for LDO/CNF with residual Na). Their measurements were carried out at lower temperatures and in the presence of water, which is known to increase the adsorption capacity of LDOs.19,20 Moreover, the CO2 capacities per total mass of adsorbent that they obtained are very low (0.08 molCO2 kgads-1 for K-LDO/CNF and 0.22 molCO2 kgads-1 for Na-LDO/CNF) since very large support loadings were used, i.e. 90 wt.% CNF. Regarding previous studies using unsupported LDOs promoted with impregnated potassium, Ding et al.21 and Hufton et al.22 reported dry CO2 capacities of comparable magnitude to those obtained in this work (0.40 and 0.52 molCO2 kgads-1 respectively), if differences in temperature and CO2 partial pressure are


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taken into account. Oliveira et al.12 compared the promoting effect of potassium and cesium on the CO2 capacity LDOs when the alkali ions were added in a loading 20 wt.%. Under wet conditions, the adsorption capacity of the sample promoted with potassium was found to be almost 70% higher than with cesium, 0.72 and 0.44 molCO2 kgads-1 respectively. However, due to the difference in the atomic mass of potassium and cesium, it is not possible to make a direct comparison of the effectiveness of both promoters. 3.3 Adsorption-Desorption Thermal Cycles The regeneration and stability of the adsorbents under dry conditions were assessed through repeated cycles of adsorption at 573 K and 20% CO2/Ar, and desorption at 673 K with a N2 purge. The CO2 adsorption capacities of the unsupported and the GO supported LDOs –both thoroughly washed and alkali promoted- measured over 20 adsorption-desorption cycles and normalized with respect to the first-contact adsorption capacities, are shown in Figure 5. The CO2 capacities of the adsorbents decrease during the first cycles but become stable gradually. This decrease comes alongside a reduction in the surface area. The surface area of the pure LDO falls 10% in the first cycle whereas the surface area of the LDO/GO hybrid is reduced by only 2%. This suggests that the marked drop observed after the first exposure to the adsorptive gas may be associated with some structural annealing. Some CO2 irreversibly chemisorbed on the fresh material may also contribute to the observed profiles as suggested by previous works dealing with dry CO2 adsorption on LDO based adsorbents at high temperatures (573 K – 753 K).22,21 In the presence of modest amounts of graphene oxide, LDOs exhibit a superior thermal stability as revealed by continuous adsorption-desorption temperature swing cycles, Figure 5. Further enhancement in the stability of the LDO samples is expected by adding higher contents of GO,8


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although this compromises the capacity per total mass of adsorbent. The improvement in stability of LDOs achieved in the presence of GO has been ascribed to the compatibility between the carbon nanosheets and the LDO platelets which allows GO to act as an effective spacer hence preventing the thermally driven growth of the LDO particles.7 Additionally, CO2-TPD tests revealed that GO helps to preserve the initial distribution of basic sites over cycling.8 Remarkably, the temperature swing adsorption profiles of the alkali promoted adsorbents are very close to those of the corresponding thoroughly-washed materials, i.e. LDO and LDO20. Therefore, the contents of alkali used in the present study have no significant influence on the stability of the unsupported LDOs and of the GO hybrids but greatly increase their CO2 adsorption capacities. 3.4 CO2 Adsorption Isotherms The adsorption isotherms of the unsupported and GO supported materials were determined using a thermogravimetric analyzer as described above. The impact of the presence of alkali species on the adsorption isotherms in the first exposure to the adsorptive gas was assessed using the unpromoted and the potassium promoted materials, namely LDO, LDO20, LDO-K and LDO20-K. The first-contact adsorption capacities at different CO2 partial pressures increase with the adsorptive concentration, Figure 6 (and Figure S8). Such profile is characteristic of a wide distribution of active sites present in the fresh adsorbents and can be fitted to the Freundlich model (1).23,24,25,8 ଵ/௡

‫݇ = ݍ‬ൣܲ஼ைమ ൧

(1)

The Freundlich parameters for the first-contact adsorption data are given in Table 5. The value of the n parameter is similar for all the samples, although it increases slightly in the presence of potassium. This may be related to the influence of the alkali promoter on the distribution of the


18

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strength of basic sites, revealed by the CO2-TPD measurements (see Figure 4). The interaction parameter, k, is larger for the alkali promoted adsorbents, as expected for materials with significantly higher CO2 adsorption capacities.

The CO2 adsorption isotherms after 20 consecutive temperature swing adsorption-desorption cycles were obtained for all the adsorbent samples (i.e. unsupported and GO-supported LDO unpromoted and promoted with Na, K or Cs). Contrary to the first-contact adsorption data, the isotherms of the thermally cycled adsorbents become flat above CO2 partial pressures of 500 mbar, Figure 6 and Figure S9 and are better described by the Langmuir equation (2).

8,23

CO2-

TPDs of the unpromoted LDO and LDO/GO hybrids reported previously indicate that after thermal cycling there is a reduction in the number of low and intermediate temperature desorption sites8 suggesting the remaining adsorption sites have a more uniform energy. Hence, the materials tend toward a more ideal adsorption isotherm.8 The detailed structural changes responsible for this transition are unclear. ௠௕௣಴ೀమ

‫ = ݍ‬ଵା௕௣

಴ೀమ

(2)

The Langmuir parameters fitted to the multicycle adsorption data are given in Table 5 for the samples impregnated with K, and in the Table S3 (SI) for Na- and Cs- promoted adsorbents. The monolayer capacity m and the parameter b are larger for the alkali promoted samples, which is consistent with the observed increase in single point first contact adsorption capacities given in Table 4 and with the stronger basicity of the promoted sites. In our previous studies of LDO/GO hybrids we have shown that the adsorption model of Toth is able to describe the transition from a heterogeneous fresh surface toward a uniform thermally cycled adsorbent, as it captures features


19


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Page 20 of 29

of both Langmuir and Freundlich equations.8 The experimental data was also fitted to the Toth isotherm and the parameters are included in Table S4 and Table S5 (SI). 4. CONCLUSIONS Addition of sodium, potassium or cesium carbonates by incipient wetness impregnation to unsupported LDH and to LDH/GO hybrids at loadings ca. 1.1 mol alkali per kg of adsorbent was found to increase the CO2 adsorption capacity of the calcined materials by more than 40%. Crucially, the markedly enhanced stability of the LDO imparted by the addition of graphene oxide was not compromised by the inclusion of the impregnated alkali promoters. The impregnated cations modified the distribution of basic strength in a similar manner regardless of the type of promoter, but the densities of chemisorption sites and the total CO2 adsorption capacities at 573 K depended on the alkali metal added. Potassium showed a higher promoting effect than sodium because of its stronger Lewis basic character. The addition of cesium, which is an even stronger base, resulted in lower surface areas and working capacities due to its tendency to form bulkier agglomerates. The enhancement in CO2 adsorption capacity achieved by alkali promotion is slightly higher for the unsupported LDO compared to the corresponding LDO/GO hybrid suggesting that the impregnated alkali carbonates do not interact with the LDO as efficiently in the hybrids as in the unsupported materials, possibly due to interactions between the promoters and the oxygen groups of the GO. The absolute adsorption capacities of the alkali promoted LDO/GO hybrids are low compared to other alkali promoted chemisorbents (e.g. K-CaO26 or Li2ZrO327) and zeolites.28 Despite this, they offer advantages for application in sorption-enhanced hydrogen production considering their


20

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tolerance to water at the required adsorption temperature of around 573K, stability, and relatively small temperature swing needed for regeneration. The first-contact adsorption isotherms for the unpromoted and alkali promoted adsorbents (LDO and LDO/GO hybrids) are better described by the Freundlich model, but after multiple TSA cycling tend to fit better to the Langmuir equation indicating that the surfaces become more uniform. The underlying structural modifications responsible for the change in isotherm behavior are unclear and further investigation would be desirable. FIGURES

Figure 1. STEM-EDS images of (a) LDH-Na, (b) LDH-K and (c) LDH-Cs


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Figure 2. XRD diffraction patterns of as-synthesised (a) LDH and (b) LDH20 adsorbents. Characteristic reflections of LDH (♦).

Figure 3. CO2-TPD of pure LDO, LDO-Na, LDO-K, and LDO-Cs following pretreatment at 673 K and adsorption at 313 K (dark lines). The blanks were subjected to preatment only (high lines).

Figure 4. CO2-TPD of pure LDO, LDO-Na, LDO20 and LDO20-Na following pretreatment at 673 K and adsorption at 313 K (dark lines). The blanks were subjected to preatment only (high lines).


22

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Figure 5. Normalized CO2 adsorption capacity for LDO/GO samples after 20 adsorption-desorption cycles (adsorption 573 K; desorption 673 K).

Figure 6. First-contact and multicycle adsorption isotherms at 573 K obtained from TGA for (a) LDO-K and (b) LDO20-K.

TABLES Table 1. Composition of as-synthesized adsorbents Sample namea Pure LDH

wt% GO Nom.

wt% GO Actual

Mg/Al (mol/mol)

mol alkali/kg LDH Nom.

mol alkali/kg LDH Actual

wt% alkali Actual

0

0

2.3

0

0

0.01

LDH-Na

0

0

2.3

1.3

1.1

2.40

LDH-K

0

0

2.3

1.3

0.9

3.50

LDH-Cs

0

0

2.3

1.3

1.3

14.50

LDH20

5

4.6

2.2

0

0

0.02

LDH20-Na

5

4.6

2.2

1.4

1.1

2.50

LDH20-K

5

4.6

2.2

1.4

0.9

3.30

LDH20-Cs

5

4.6

2.2

1.4

1.3

14.40

[a] LDHx, where x is the nominal LDH to GO weight ratio of the sample


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Table 2. Physicochemical properties of the activated adsorbents Activated adsorbents SBET (m2/g)

Vpore (cm3/g)

µmol alkali/m2

Crystallite size (003) (nm)

Pure LDO

226

0.54

0

2.1

LDO-Na

116

0.45

9.1

2.5

LDO-K

98

0.41

9.0

2.7

LDO-Cs

76

0.30

16.3

2.7

Sample namea

LDO20

215

0.55

0

2.3

LDO20-Na

116

0.47

9.2

2.7

LDO20-K

100

0.43

8.4

2.4

LDO20-Cs GO

84 81

0.36 0.15

12.9 0

2.7 ----

[a] LDHx, where x is the nominal LDO to GO weight ratio of the sample

Table 3. Density of basic sites: CO2-TPD wt% GO Actual

Total CO2 evolved (µmol/g)

CO2 evolvedb (minus blank) (µmol/g)

CO2 evolvedb (minus blank) (µmol/g LDO)

CO2 evolvedb (minus blank) (µmol/m2)

Pure LDO

0

599

437

437

1.93

-----

LDO-Na

0

913

453

464

3.91

0.25

LDO-K

0

1023

616

639

6.29

0.43

LDO-Cs

0

919

554

648

7.28

0.30

LDO20

4.6

690

410

430

1.91

-----

LDO20-Na

4.6

1230

470

498

4.05

0.27

Sample namea

molCO2/mol alkalic

[a] LDOx, where x is the nominal LDO to GO weight ratio of the sample [b] Values obtained by subtracting the corresponding blank from the total CO2 evolved [c] The value of mol alkali/ kg LDO was calculated using the value of mol alkali/ kg LDH (Table 1) and considering that 60% of the mass of the adsorbent is lost during activation

Table 4. First-contact CO2 adsorption capacities of the adsorbents at 573 K and PCO2 = 200 mbar Sample namea

wt% GO

mol alkali / kg LDHb

molCO2/ kg LDO

molCO2/ kg ads

µmolCO2/ m2

molCO2/ mol alkali

Pure LDH

0

0

0.29 ± 0.01

0.29 ± 0.01

1.3

0

LDH-Na

0

1.1

0.57 ± 0.01

0.56 ± 0.01

4.8

0.31

LDH-K

0

0.9

0.71 ± 0.02

0.69 ± 0.02

7.0

0.47

LDH-Cs

0

1.3

0.81 ± 0.02

0.69 ± 0.02

10.4

0.37

LDH20

4.6

0

0.29 ± 0.03

0.28 ± 0.03

1.3

0

LDH20-Na

4.6

1.1

0.52 ± 0.02

0.49 ± 0.02

4.2

0.28

LDH20-K

4.6

0.9

0.66 ± 0.02

0.61 ± 0.02

6.1

0.44


24

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LDH20-Cs

1.3

4.6

0.79 ± 0.02

0.64 ± 0.02

7.6

0.36

[a] LDOx, where x is the nominal LDO to GO weight ratio of the sample [b] Converted to mol alkali/kg LDO considering that 60% of the mass of the adsorbent is lost during activation

Table 5 Parameters for CO2 adsorption isotherms at 573 K from TGA under dry conditions Adsorbenta

Freundlich isotherm (First-contact) -1

Langmuir isotherm (multicycle)

k (mol kg )

n (-)

m (mol kg-1)

b (bar-1)

Pure LDO

0.40 ± 0.01

4.77 ± 0.38

0.19 ± 0.01

27.2 ± 4.3

LDO-K

0.88 ± 0.02

5.41 ± 0.37

0.48 ± 0.01

42.6 ± 6.7

LDO20

0.39 ± 0.01

4.87 ± 0.25

0.26 ± 0.01

22.2 ± 1.5

LDO20-K

0.85 ± 0.01

5.02 ± 0.21

0.54 ± 0.01

42.6 ± 6.7

[a] LDOx, where x is the nominal LDO to GO weight ratio of the sample

ASSOCIATED CONTENT Supplementary Information (SI) available: Figure S1 Representative thermograms of assynthesized samples; Figure S2 Nitrogen adsorption-desorption isotherms; Figure S3 XRD diffraction patterns of activated LDO and LDO20 adsorbents; Figure S4 XRD diffraction patterns of GO and activated GO; Figure S5 Representative TEM images of as-synthesized adsorbents; Figure S6 Deconvolution of the CO2-TPD of pure LDO; Figure S7 Representative CO2 adsorption profiles of LDO20 hybrids; Figure S8 First-contact and multicycle adsorption isotherms for pure LDO and LDO20; Figure S9 Multicycle adsorption isotherms for LDONa and LDOCs LDO20-Na and LDO20-Cs. Table S1: Physicochemical properties of the as-synthesised adsorbents. Table S2. Percentage contribution of each desorption peak: CO2-TPD. Table S3: Parameters for multicycle isotherms (Na and Cs samples). Table S4. Toth parameters for first-contact isotherms. Table S5. Toth parameters for multicycle isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding author *E-mail: [email protected] (D.C.) Author contributions Concepts for the research program were conceived by all authors. Experiments were devised by D. C., M. S., together with D. I. and X. H. who carried them out and performed data analysis. All authors have given approval to the final version of the manuscript. ACKNOWDLEGEMENTS The authors are grateful to Raul Montesano and Ainara Garcia-Gallastegui for discussions and assistance. D.I. thanks CONACyT and SEP for the scholarships awarded. ABBREVIATIONS LDH, layered double hydroxide; LDO, layered double oxide; GO, graphene oxide; CNF, carbon nanofiber; XRD, X-ray diffraction; TGA, thermogravimetric analysis; TEM, transmission electron microscopy; SEM, scanning electron microscopy; ICP-OES, inductively coupled plasma optical emission spectroscopy; BET, Brunauer Emmett and Teller; CO2-TPD, temperature programmed desorption of CO2; m, monolayer capacity; b, gas-solid interaction parameter in Langmuir isotherm; k, prefactor in Freundlich isotherm; n, fitting parameter in Freundlich isotherm; pCO2, partial pressure of CO2 in the feed; q, adsorption capacity; qo, adsorption capacity in the first cycle.


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15. Rouquerol, F.; Rouquerol, J.; Sing, K., Adsorption by powders & porous solids. Academic Press: London, UK, 1999. 16. Di Cosimo, J. I.; Dı́ez, V. K.; Xu, M.; Iglesia, E.; Apesteguı́a, C. R., Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides. Journal of Catalysis 1998, 178 (2), 499-510. 17. Debecker, D. P.; Gaigneaux, E. M.; Busca, G., Exploring, Tuning, and Exploiting the Basicity of Hydrotalcites for Applications in Heterogeneous Catalysis. Chemistry – A European Journal 2009, 15 (16), 3920-3935. 18. Di Cosimo, J. I.; Díez, V. K.; Apesteguía, C. R., Base catalysis for the synthesis of α,βunsaturated ketones from the vapor-phase aldol condensation of acetone. Applied Catalysis A: General 1996, 137 (1), 149-166. 19. Ding, Y.; Alpay, E., High Temperature Recovery of CO2 from Flue Gases Using Hydrotalcite Adsorbent. Process Safety and Environmental Protection 2001, 79 (1), 45-51. 20. Ram Reddy, M. K.; Xu, Z. P.; Diniz da Costa, J. C., Influence of Water on HighTemperature CO2 Capture Using Layered Double Hydroxide Derivatives. Industrial & Engineering Chemistry Research 2008, 47 (8), 2630-2635. 21. Ding, Y.; Alpay, E., Equilibria and kinetics of CO2 adsorption on hydrotalcite adsorbent. Chemical Engineering Science 2000, 55 (17), 3461-3474. 22. Hufton, J. R.; Mayorga, S.; Sircar, S., Sorption-enhanced reaction process for hydrogen production. AIChE Journal 1999, 45 (2), 248-256. 23. Do, D. D., Adsorption analysis: Equilibria and Kinetics. Imperial College Press: London, UK, 1998. 24. Soares, J.; Casarin, G.; José, H.; Moreira, R. P. M.; Rodrigues, A., Experimental and Theoretical Analysis for the CO2 Adsorption on Hydrotalcite. Adsorption 2005, 11 (1), 237-241. 25. Cobden, P. D.; van Beurden, P.; Reijers, H. T. J.; Elzinga, G. D.; Kluiters, S. C. A.; Dijkstra, J. W.; Jansen, D.; van den Brink, R. W., Sorption-enhanced hydrogen production for pre-combustion CO2 capture: Thermodynamic analysis and experimental results. International Journal of Greenhouse Gas Control 2007, 1 (2), 170-179. 26. Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Losch, J., Adsorption of CO2 on Zeolites at Moderate Temperatures. Energy & Fuels 2005, 19 (3), 1153-1159. 27. Xiong, R.; Ida, J.; Lin, Y. S., Kinetics of carbon dioxide sorption on potassium-doped lithium zirconate. Chemical Engineering Science 2003, 58 (19), 4377-4385. 28. Choi, S.; Drese, J. H.; Jones, C. W., Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2009, 2 (9), 796-854.


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254x190mm (96 x 96 DPI)


Influence of Alkali Metals (Na, K, and Cs) on CO2 Adsorption by

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