Hybrid Effects in Graphene Oxide / Carbon Nanotube-Supported Layered Double 1 Hydroxides : Enhancing the CO 2 Sorption Properties

Graphene oxide (GO) and multi-walled carbon nanotubes (MWCNT) have been previously used 10 independently as active supports for Layered Double Hydroxides (LDH), and found to enhance the 11 intrinsic CO2 sorption capacity of the adsorbents. However, the long-term stability of the materials 12 subjected to temperature-swing adsorption (TSA) cycles still requires improvement. In this contribution, 13 GO and MWCNT are hybridized to produce mixed substrates with improved surface area and 14 compatibility for the deposition of LDH platelets, compared to either phase alone. The incorporation of a 15 robust and thoroughly hybridized carbon network considerably enhances the thermal stability of activated, 16 promoted LDH over twenty cycles of gas adsorption-desorption (96% of retention of the initial sorption 17 capacity at the 20 cycle), dramatically reducing the sintering previously observed when either GO or 18 MWCNT were added separately. Detailed characterization of the morphology of the supported LDH, at 19 several stages of the multicycle adsorption process, shows that the initial morphology of the adsorbents is 20 more strongly retained when supported on the robust hybrid GO/MWCNT network; the CO2 adsorption 21 performance correlates closely with the specific surface area of the adsorbents, with both maximized at 22 small loadings of a 1:1 ratio of GO:MWCNT substrate. 23


Introduction
etc.), 2 biomedical vectors, 3, 4 flame retardants, 5 adsorbents 6 and nanocomposite reinforcements. 7 In 32 particular, the Mg/Al LDH family (with Mg:Al ratios of 2 and 3) has been extensively studied in the past 33 years, and recognised as an excellent composition for CO2 adsorption applications. 8 The gas adsorption 34 properties are manifested after thermal treatment of the material, that evolves into an active mixed metal 35 oxide (MMO) form with finer particle size and increased surface area compared to the original structure. 36 The associated progressive dehydroxylation and decarbonation of the structure generates the basic sites 37 responsible for the subsquent (acid-base type) interaction with the CO2 molecules. 9 Compared to known 38 low temperature CO2 solid adsorbents (such as zeolites, activated carbons, and metal organic frameworks), 39 thermally-activated LDH typically manifest a lower intrinsic specific CO2 adsorption capacity, but can 40 operate at higher temperatures. 10 On the other hand, they manifest faster adsorption kinetics and much 41 better regenerability than other high temperture solid adsorbents (such as lithium zirconates and calcium 42 oxides). 11 LDH are frequently supported on high surface area substrates in order to improve adsorption 43 capacity and stability over prolonged use, aspects that still hamper practical industrial implementation. and electrical 22 characteristics, as well as high aspect ratios and surface areas, 23 which make them suitable 49 for preparing robust, porous and high surface supporting networks. 24 Activated carbons are a possible 50 alternative high surface area substrate, 25, 26 but tend to have relatively inaccessible slit micropores that are 51 readily blocked by LDH deposition. 27 The use of LDH/ nanocarbons composites has been considered for 52 a wide range of applications, 28, 29 but here we focus on CO2 sorption. Mg/Al LDH platelets loaded dilutely 53 on CNF (up to 90 wt% carbon), were reported to manifest an order of magnitude increase in the CO2 54 adsorption capacity compared to the unsupported counterparts. 12 Oxidized MWCNT were subsequently 55 adopted as supports, 13 where the acidic surface was used to drive nucleation of the positively-charged 56 LDH platelets through electrostatic interactions; once activated, these MWCNT-LDH exhibited 57 significant enhancement of the first-contact gas adsorption capacity compared to the unsupported material. 58 In addition, an improvement in the multicycle stability was observed over twenty continuous cycles of gas 59 adsorption-desorption. 13 The improvements were attributed to a reduction in the LDH particle size and an 60 associated increase in the basic site concentration available for gas adsorption. 30 Subsequently, 2 -30 wt% 61 of a GO substrate was found to be even more effective than the other CN for improving the CO2 adsorption 62 performance of LDH; 16, 31 the improvement was attributed to the two-dimensional geometric compatibility 63 of the GO nanosheets with LDH platelets. Small loadings of GO were found to be most promising, with 64 benefits plateauing at higher concentrations due to apparent restacking of the carbon sheets. However, it 65 is not clear whether the GO only promotes multicycle stability or also first-contact adsorption capacity. 16, 66 32 In addition to the use of a support, the CO2 adsorption capacity of LDH can be improved by doping 67 with alkali metals. 33,34 The benefits of alkali metal promotion have been confirmed with both CNF-LDH 34 68 and GO-LDH 16 materials; in general, residual sodium from the LDH synthesis may remain in the LDH, 69 resulting in a significant enhancement of the first-contact adsorption capacity. 16 Whilst the literature 70 identifies consistent trends for the adsorption capacity of CN-LDH hybrids, the absolute capacities vary 71 due to the specific adsorption conditions adopted, including adsorption temperature and pressure, presence 72 of water, concentration of carbon support and degree of doping.

73
Overall, the multicycle stability of CN-LDH still has considerable scope for improvement; for instance, 74 for the best materials reported so far (GO-LDH), the loss of CO2 adsorption performance over 20 cycles 75 of gas adsorption-desorption under dry conditions remained a significant 15 -40% (though improved 76 compared to the 50 -60% loss for pure LDH). 16,31 Despite their promise, neither MWCNT nor GO are 77 ideal supports: MWCNT, though offering a mechanically strong network for LDH deposition, require 78 higher loadings to match the performance improvement provided by GO, as the surface area is not as 79 intrinsically high, and the geometry less well matched. Increased CN loading negatively impacts the 80 overall cost and weight of the final sorbent. On the other hand, GO alone has poor network forming ability, 81 limiting benefits even at modest carbon loadings due to restacking of the sheets.

82
Recently, hybrid GO/MWCNT systems have been reported to exhibit interesting synergistic effects in 83 certain applications. [35][36][37][38] Most studies focus on improvements in the properties of electrochemical double 84 layer (EDLC) supercapacitors using hybrid GO/CNT electrodes, compared to carbon nanotubes (CNTs) 85 and GO used independently. 39, 40 Hybrid GO/CNT electrodes have also been found to offer superior drug 86 and gas sensing performance. 41 GO/CNT hybrids are compatible chemically due to the π-π attractions 87 between basal planes and the hydrogen bonding between acidic functional groups. 40 Geometrically, CNTs 88 act as spacers between the graphene sheets, preventing restacking and agglomeration. The enhanced 89 overall surface area of the structure facilitates the access of electrolytes (for electrochemical applications) 90 and may improve the transport of gases (in adsorption applications). GO can help to form intimately mixed 91 hybrid structures by acting as a dispersing agent for unoxidized-SWCNT 42 or MWCNT 43 .

92
In principle, hybridized GO/MWCNT constructs are appealing as LDH supports, but have not yet been 93 thoroughly studied because of the complexity of the resulting three-phase mixture (GO/MWCNT/LDH), 94 although some recent reports have indicated promising potential for the use of hybrid graphene/nanotube 95 systems to support LDH for catalytic (NiFe-LDHs) 44 or electrochemical (NiAl-LDH) 45 applications. Our 96 aim was to explore GO/MWCNT hybrids in more detail, and specifically in the context of CO2 sorption, 97 in order to overcome the limitations identified for LDH supported on either GO or oxidized MWCNT, 98 separately. We promoted the material with a small amount (5 wt%) of potassium to improve the absolute 99 performance and avoid variations due to low loadings of adventitious alkali metal. The materials were 100 tested for CO2 adsorption properties, and were fully characterized to understand the deactivation 101 mechanism over several temperature-swing (TS) adsorption-desorption cycles.

111
In a typical oxidation procedure, 17.5 mL of concentrated H2SO4/HNO3 (3:1 mixture) were added for 112 every 500 mg of MWCNT. The mixture was stirred and refluxed for 30 min at 120 °C. After cooling,

113
MWCNTs were vacuum-filtered over a 0.45 µm PC membrane, and base-washed with 1 L of 114 0.01 M NaOH. The base washing procedure assists the removal of molecular oxidation debris (also known 115 as carboxyated carbonaceous fragments, which are thought to be related to humic or fulvic acids), 46 116 produced during the oxidation treatment. 47,48 In this work, only oxidized MWCNT were used, and will be 117 referred simply as to 'MWCNT'. Aqueous solutions (1 mg mL -1 ) of GO and MWCNT were mixed at 300 rpm for 4 hours at GO/MWCNT 127 ratios of 1:0 (pure GO), 10:1, 3:1, 1:1, 1:3, 1:5 and 0:1 (pure MWCNT). The mixed solutions were then 128 filtered on PC membrane until a wet carbon paste was obtained, and used directly for LDH deposition.

138
The same procedure was applied for the preparation of hybrid GO/MWCNT-LDH materials. In order to 139 achieve a homogeneous hybridization of MWCNT and GO, the corresponding aqueous solutions were 140 mixed in the desired amounts (typically for a total volume of 200 mL) for 4 hours at room temperature.

141
The mixed solution was then filtered as described previously, and transferred to a round-bottom flask for  Raman microscope (Renishaw plc, Wotton-under-Edge, UK). The system calibration was performed using 181 an integrated silicon wafer prior to measurement; Raman spectra were obtained using a green laser 182 (wavelength 532 nm, intensity 1 %, 10 seconds) in edge mode (2400 lines mm -1 diffraction grating); for 183 the determination of the ID/IG ratio, Raman mapping was performed in StreamLine acquisition mode at 184 ca. 10 μm intervals on an area of ca. 500 μm 2 for an acquisition of 1600 independent spectra between 185 1220 -2700 cm -1 , using a green laser (wavelength 532 nm, 2.33 eV) 1% laser power, 10 s of exposure.

186
The elemental composition of the adsorbents was measured by inductively coupled plasma-optical   196 Initially, the hybrid substrates were analyzed without depositing LDH platelets; mixed substrates were 197 prepared at different GO/MWCNT ratios of 10:1, 3:1, 1:1, 1:3, 1:5, and compared to the pure GO (1:0) 198 and MWCNT (0:1) forms. In the XRD patterns (Fig.1a), the intensities of GO (11.3 °) and MWCNT restacks on drying to an average 33 layers; however, on hybridizing with MWCNT at an optimum 1:1 204 ratio, the average number reduces to only 3 -4 layers (Table 1). Hybrids with similar degree of GO 205 layering (4 -32 sheets) have been previously prepared by controlled LbL assembly, 51 however, the level 206 of exfoliation reached in the present work is attributed only to the spontaneous interactions between the 207 GO and MWCNT. This improved exfoliation, retained within the dried hybrids, is reflected in the specific 208 surface areas calculated from N2 adsorption-desorption measurements ( Fig.1b and Table 1). The pure 209 MWCNT have a higher surface area than the pure GO, as they are less prone to restacking/repacking; 210 however, their intrinsic potential surface area is lower due to the multiple shells within their structure.  The addition of a proportion of the increasingly exfoliated GO raises the average surface area, reaching a 225 maximum for the 1:1 hybrid (127 m 2 g -1 ). The synergistic improvement in surface area for mixed materials 226 can be attributed to the MWCNT acting as spacers for the GO sheets, 39 forming a network that evenly  Table 2) match the nominal loadings, assuming that the combustion residues of each 240 phase are simply additive. 13 From ICP analysis, the average Mg/Al ratio of the as-synthesized CN50-LDH 241 materials was determined as 2.1 + 0.1, which is very close to the nominal ratio of 2; the mean potassium 242 doping was found to be 5.1 + 0.14 %, (  (110) and (113), clearly distinguished between 60 ° and 63 ° (JCPDS No. 14-191). The basal 248 reflection (003) at 11.8 ° falls approximately at the same 2θ angle as the GO layer spacing (11.4 °),

249
although the GO peak is weak and tends to diminish further after hydrothermal treatment (LDH synthesis 250 conditions), 15 due to elimination of the oxidative debris. The graphitic carbon (002)  intimate association between LDH particles and the carbon substrates, with LDH aggregates covering the 271 surface of the GO (Fig.2a,d), MWCNT (Fig.2b,e) and the hybrid (Fig.2c,f). Importantly, the 272 GO/MWCNT50-LDH hybrid manifests enhanced specific surface area compared to both GO50-LDH and 273 MWCNT50-LDH (Table 2), continuing the trend identified above for the supports in the absence of LDH 274 (Fig.1). Previous work suggests that the 2D geometry of GO sheets may offer a more compatible  The properties of the CN50-LDH materials as CO2 adsorbents were assessed over 20 cycles of gas 280 adsorption-desorption. All the samples manifested fast intrinsic CO2 adsorption kinetics (Fig.3a), 281 achieving more than 80% of their equilibrium capacity within 30 minutes. Critically, the 282 GO/MWCNT50-LDH hybrid exhibited the highest gas adsorption capacity (0.49 mol CO2 kg -1 LDH), more 283 than twice the value of unsupported LDH (0.18 mol CO2 kg -1 LDH), and significantly higher than LDH 284 supported on GO and MWCNT independently (0.27 and 0.24 mol CO2 kg -1 LDH, respectively). The first-285 contact adsorption capacity of the GO50-LDH sample is higher than the MWCNT50-LDH one, in 286 accordance with previous results. 15 Under these operating conditions, the GO/MWCNT substrate alone 287 has a negligible CO2 adsorption (0.02 mol CO2 kg -1 carbon) compared to the LDH component (Fig.3a).

288
Previous studies have also shown very low capacity on either raw or acid treated nanocarbons, and have  Table 2  295   296 The adsorption capacity of all the samples showed good reproducibility among three repeated 297 measurements for each sample (standard deviation of 0.01 -0.03 mol CO2 kg -1 LDH). The improved gas 298 adsorption capacity manifested by GO/MWCNT50-LDH can be attributed to the presence of the high 299 surface area hybrid support and the enhanced effective LDH surface area, due to changes in crystal size 300 and quality (Table 2). Smaller supported-LDH platelets are thought to have a higher concentration of 301 active edge sites, which were previously shown to provide the binding sites for CO2 adsorption. 9 There is 302 a synergy between the MWCNT, which provide a robust open network, and the GO, which is 303 geometrically compatible with LDH. The regeneration and stability of the adsorbents were also assessed by carrying out continuous adsorption-308 desorption cycles under dry conditions (Fig.3b-c). Unsupported LDH materials are known to suffer from 309 irreversible declines in the adsorption capacity over cycling, 16 due to chemisorption phenomena and 310 particle sintering. 56 The present, unsupported LDH sample follows a similar decreasing trend to previous 311 reports (Fig.3b,c), exhibiting a loss of ca. 50 % of adsorption capacity at the 20 th cycle. This trend is 312 associated with a progressive sintering of the LDH platelets (SEM images, Fig.4), which likely contributes 313 to the loss of surface area and basic site availability. However, this poor multicycle stability is mitigated 314 by supporting LDH on either GO or MWCNT, but only partially. In contrast, the To provide an explanation for this retained stability, the morphological evolution of LDH and CN50-LDH 321 samples was studied, via XRD, SEM and Raman spectroscopy (Fig.5,6,7), at different stages of the 322 multicycle adsorption tests, specifically as-synthesized (AS), calcined (C), after 10 cycles and after 20 323 sorption cycles. Cycling adsorption-desorption tests on unsupported LDH caused the structure to 324 progressively lose surface area, likely due to continuous decarbonation, irreversible chemisorption effects, 325 sintering or carbon deposition. 56 The calcined particles were found to sinter into larger structures, as 326 calculated from the width of the (200) peak of periclase in XRD (Fig.5, and Table in Fig.S4), eventually 327 halving the initial adsorption capacity. The GO50-LDH material exhibits significant structural changes following the thermal cycling (Fig.6a). On 332 calcination, GO is thermally 'reduced' leading to restacking and the appearance of the (002) graphitic 333 peak at ~26.4 °; at the same time, peaks associated with free K2CO3 appear, indicating that some of the 334 impregnated salt was associated with the lost GO oxygen moieties, as expected due to acid-base 335 interactions. By the 10 th cycle, the intensity of the graphitic (002) peak is significantly decreased and 336 eventually lost by the 20 th cycle, consistent with the disappearance of the characteristic G and D bands 337 from the Raman spectra, which are normally associated with graphitic structures (Fig.7a). Previous work 338 reported that heat treatments of Ni/Mn LDH/GO hybrids at high temperatures (450 -800 °C) in inert 339 atmosphere caused gasification and removal of the graphitic material. 57 Here, the temperature is modest, 340 but apparently sufficient to cause a similar loss of the graphitic component. The loss of carbon is supported 341 by TGA (Fig.S5), showing that the carbon content in the GO-LDH material drops from 46 wt% before 342 cycling (Table 2) to ca 6 wt% afterwards. Towards the end of the cycling experiment, the potassium 343 redistributes into the periclase phase, 58 indicated by the disappearance of the initial K2CO3 peaks in XRD 344 pattern (Fig.6a). The degradation of the GO substrate and associated loss of support, allows the MMO 345 particles to sinter, increasing their size as estimated from peak width (Fig.5 and S4), though at a slower 346 rate than the pure LDH. These observations account for the 30% capacity loss measured for GO50-LDH 347 after 20 cycles. Although GO appears not to be an ideal substrate for LDH, it slows the deactivation of the 348 adsorbent due to sintering, both in rate and in extent.

349
For MWCNT50-LDH (Fig.6b), the initial calcined structure is better retained over cycling compared to 350 GO50-LDH, as the carbon (002) and periclase (200) reflections are present in all the XRDs. Though 351 reduced in intensity, the XRD carbon peak and the typical Raman features of MWCNT are still detected 352 at the final adsorption cycle (Fig.7b). However, a more defective structure is confirmed by the increased 353 ID/IG ratio of the cycled sample (Table Fig.7d), and the partial loss of G band splitting. There appears to 354 be insufficient K2CO3, associated with the much smaller oxidized MWCNT surface, to nucleate separate 355 salt crystals. Overall, the MWCNT network is more robust than GO, unsurprisingly as only the surface is 356 initially oxidised, leaving a lower surface area and more perfect graphitic core that appears not to gasify 357 as readily as the GO. Thus, the sintering of the sorbent particles is reduced compared to unsupported LDH 358 and GO50-LDH ( Fig.4 and 5). These observations are in turn reflected by the greater retained adsorption 359 capacity (75%) manifested by the MWCNT50-LDH sample in the last cycle.

360
The GO/MWCNT(1:1)50-LDH hybrid benefits from the presence of both types of CN. Whilst the GO is 361 reduced (no layer peak at 11.4°), the XRD pattern of the calcined hybrid is relatively little altered during subsequent cycling (Fig.6c), and the graphitic Raman band is better retained compared to either pure case 363 (Fig.7c). In the GO/MWCNT(1:1)50-LDH hybrid, the MWCNT and the GO appear to act synergistically.

364
One possible explanation is that the large flat GO flakes, coated in LDH, provide a barrier offering a 365 degree of protection to the MWCNT, whilst the MWCNT maintain a network scaffolding that limits 366 sintering of the LDH/periclase that otherwise leads to exposure of the (reduced) GO framework. The 367 microstructure of the hybrid, shown in the SEM images (Fig.6c), indeed exhibits much coarser plate-like 368 features than the MWCNT50-LDH, but more separated into discrete particles than the GO50-LDH. This 369 stable, hybrid support shows very little sintering of the periclase particles ( Fig.5 and S4), consistent with 370 the excellent retention of the capacity (96%) across the 20 cycles.  more appealing industrially speaking, since they reduce the size of the adsorption units and limit the 386 overall costs; in addition, the previous optimums for the pure GO and pure MWCNT supports were at 387 50 wt% carbon, or less. Generally, these three phase hybrids were formed successfully (XRD, Fig.S6).

388
LDH grown on low GO/MWCNT ratio supports display broader XRD (003) peaks, and thus smaller 389 platelet thickness in the c-direction (Fig.8a and S6), which is consistent with the formation of a well 390 hybridised material and the initial trends discussed above.

391
First contact CO2 adsorption capacity and multicycle stability were assessed as previously, and compared 392 to BET surface area data (Fig.S7). The results can be summarized as follows: 1) the adsorption kinetics 393 for all the adsorbents is very fast, with 80% of the equilibrium capacity reached again in the first 10 394 minutes of gas exposure (Fig.S8a,c,e); 2) regardless of the proportion of the support added to the LDH 395 phase, the highest intrinsic CO2 adsorption capacity is always exhibited by LDH supported on a 396 GO/MWCNT substrate of composition 1:1 (Fig.8b-d); 3) LDH deposited on the GO/MWCNT (1:1) 397 substrate also manifest the highest surface areas within each set of carbon loadings; 4) for each set of 398 samples, the CO2 uptake trend is consistent with the BET surface area of the adsorbents (Fig.8b-d).  Table S2). The error bars for the BET are very small (as indicated in the tabulated data, Table S1), and 402 mostly negligible on the scale of the plots. 403

404
The specific surface area of the hybrids is related to the level of GO exfoliation/MWCNT intercalation 405 achieved, and reflects the results for the pure nanocarbon blends. The enhancement in the intrinsic 406 adsorption capacity (i.e. per mass of LDH) is important from a fundamental perspective; however, in 407 practical terms, it is also important to consider the adsorption capacity per mass of total adsorbent (i.e. 408 LDH+GO+MWCNT). The 3d plots of all the sorption data ( Fig.9)  capacity loss in the best cases, was only between 4% and 9%, much lower than previous reports for other 423 supported HTs, under dry conditions. The absolute CO2 adsorption capacity of the CN-LDH hybrids, though greatly improved compared to 427 unsupported LDH, remains on an average 0.4 -0.5 mol CO2 kg -1 adsorbent. This range is lower than other 428 types of commercially-available solid adsorbents, for example 0.1 -5 mol CO2 kg -1 zeolite and 429 0.1 -3.5 mol CO2 kg -1 activated carbons, tested at their active temperature of 5 -100 °C. 10 However, as  Nevertheless, the 96 % retention of sorption capacity is a striking improvement over previous studies 438 (Fig.10), and the absolute amount gas sorption in the last cycle is also significantly higher 439 (0.42 mol CO2 kg -1 ads) than recent reports. It is worth noting that Meis et al. reported a stable cycling 440 behaviour of their CNF-supported LDH, 12 but under wet conditions (wet CO2 gas feed (83% N2/12 % 441 H2O/5 % CO2), which are known to obscure any specific stabilising effect of the support. 48 In the present 442 case, the increased stability in dry-feed conditions can be directly attributed to the presence of the carefully 443 designed high surface area and robust substrates. normally observed for unsupported LDH. Careful processing was required to generate the intimate, 465 uniform, three phase mixture that ensures an effective hybrid response. The best hybrid identified 466 (GO/MWCNT(1:1)20-LDH) had an intrinsic adsorption capacity of 0.58 mol CO2 kg -1 LDH, 467 corresponding to 0.46 mol CO2 kg -1 of total adsorbent. The good performance per overall weight of 468 sorbent is particularly noteworthy, since many previous studies used much higher dilutions of LDH on the 469 support, increasing the weight significantly. Most strikingly, the intrinsic CO2 adsorption capacity of the 470 hybrids was exceptionally consistent over repeated cycles of gas adsorption-desorption, even under dry 471 conditions, retaining up to 96% of the initial sorption capacity after twenty cycles, significantly more than 472 both unsupported LDH (50% retained) and previous reports for supported . This type of 473 hybrid sorbent may be considered, in the long term, for targeted pre-combustion carbon capture 474 applications, for instance in sorption enhanced water gas shift reactions and sorption enhanced methane 475 reforming, but may also be applied to smaller scale sorption problems. The improved performance of the supported LDH is likely to be relevant to other known LDH applications, including as heterogeneous 477 bases for catalysis, as sorbents for the desulphurization of fuel, 59 and in pseudocapacitors. 29 In addition, 478 the concept of a GO/MWCNT hybrid network as substrate can be readily extended to other adsorbent 479 materials, particularly where problems of sorbent degradation over use are still present. 60 Other forms of 480 1d/2d hybrid materials can also be considered, drawing on the growing body of nanomaterial feedstocks 481 considered to be analogous to graphene/nanotubes. 61-63 482 483