The critical factors that determine hydrogen adsorption and desorption are the surface area of the adsorbents and the interaction between hydrogen and the adsorbents. At a low temperature of 77 K, hydrogen uptake on MOFs mainly depends on their total surface areas, particularly at high pressure. So far, various inorganic metal cations and organic linkers have been explored to tune the structure, pore size, and surface area of MOFs for hydrogen adsorption (Table 1).
2.1. Zn-Based MOFs
In 2003, Yaghi et al. reported an interesting MOF material (MOF-5, Fig. 1a) with hydrogen-sorption capacities.24 MOF-5 has a crystal structure where inorganic [Zn4O]6+ groups are joined to an octahedral array of benzene-1,4-dicarboxylate (BDC) groups to form a porous cubic Zn4O(BDC)3 framework. Such a special structure is ideal for gas absorption because of its isolated linkers, which are accessible from all sides to the sorbate gas molecules. The scaffolding-like nature of MOF-5 and its derivatives led to extraordinarily high apparent surface areas (above 2000 m2 g−1). At 77 K and 0.7 bar, 4.5 wt% hydrogen absorption was obtained by using MOF-5.24 This work prompted numerous investigations into storing hydrogen in MOF materials.25, 31–42 Although the 4.5 wt% hydrogen capacity of MOF-5 at 0.7 bar and 77 K was revised and attributed to the adsorption of some impurity gases,37 its maximum hydrogen capacity of 4.5–5.2 wt% has been confirmed at 77 K and about 50 bar by three independent groups.35, 39, 40 Furthermore, by evaluating MOF-5 derivatives composed of the same inorganic [Zn4O]6+ groups and different organic linkers,35 Yaghi et al. found that the maximum H2 uptakes in MOFs correlate well with surface areas. Among these MOFs, MOF-177 (Fig. 1c) with the highest apparent surface area (Brunauer–Emmett–Teller (BET) surface area: 4746 m2 g−1) had the highest hydrogen uptake of 7.5 wt% at 77 K and 70 bar (Fig. 2).35
The preparation approaches for MOFs can have effects on their hydrogen adsorption capacities. Yaghi et al. reported that the hydrogen-storage capacity of MOF-5 was variable with synthesis and handling conditions. The maximum H2 uptakes of MOF-5 samples prepared with and without exposure to air were 5.1 and 7.1 wt%, respectively.36 The N2 adsorption measurements showed that the exposure in air led to a reduction in its BET surface area from 3800 to 3100 m2 g−1, confirming the deleterious effects of air exposure. This discrepancy was attributed to the decomposition of Zn4O(BDC)3 in humid air. They found that exposure of a pulverized and desolvated sample of Zn4O(BDC)3 to air for 10 min resulted in the appearance of a new peak at 2θ = 8.9° in the powder X-ray diffraction (XRD) pattern, suggesting the partial conversion in a second phase (Fig. 3).36 Furthermore, when the sample was further exposed to air, they observed an increase in the relative intensity of this XRD peak and the appearance of two additional peaks at 2θ = 15.8° and 17.8°, indicating the formation of a compound isostructural to Zn3(OH)2(BDC)2 · 2DEF (DEF = N,N-diethylformamide) (MOF-69C). After exposure to air for 24 h, Zn4O(BDC)3 was converted into a solid of formula C24H22O18Zn4. In contrast, the structure of Zn4O(BDC)3 was not affected by exposing to dry O2 or anhydrous organic solvents such as methanol, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Very recently, Hafizovic et al. studied the structural difference between low and high surface area MOF-5 samples, which are dependent on preparation approach.43 The low surface area MOF-5 had two types of crystals. In the dominant phase, the Zn(OH)2 species, which partly occupied the cavities, makes the hosting cavity and adjacent cavities inaccessible, which leads to a reduction in the pore volume and the effective surface area of the material. Furthermore, the minor phase consisted of doubly interpenetrated MOF-5 networks, which lowers the adsorption capacity. Thus, the hydrogen adsorption capacity of Zn4O(BDC)3, which is determined by surface area, strongly depends on preparation conditions. This can explain the difference in hydrogen-adsorption capacities reported for Zn4O(BDC)3 from various groups (Table 1).19, 36–42
The effect of Pd on hydrogen storage in MOF-5 was examined by Sabo et al.44 Although the surface area of MOF-5 decreased from 2885 to 958 m2 g−1 by supporting Pd on it, its hydrogen-adsorption capacity increased from 1.15 to 1.86 wt% at 77 K and 1 bar. This happened probably because Pd can increase the hydrogen adsorption energy, which determines the hydrogen capacity at a low pressure.
The intergrowth of two or more frameworks can also affect their properties for hydrogen storage. As shown in Figure 4, one can see that, compared with non-interpenetrating MOFs, the interwoven IRMOF-11 material showed the greatest hydrogen uptake at 77 K and pressure below 800 torr (1.07 bar).37 This happened because catenation can reduce the free diameter of pores.25, 45, 46
A series of dinuclear paddlewheel-structured MOFs were explored.47–49 Dybtsev et al.47 synthesized a paddlewheel-structured [Zn2(BDC)2(DABCO)] · 4DMF · 0.5H2O (DABCO = 1,4-diazabicyclco[2.2.2]octane). The framework is composed of dinuclear Zn2 units with a paddlewheel structure, which are bridged by BDC dianions to form a distorted 2D square-grid [Zn2(BDC)2]. The axial sites of the Zn2 paddlewheels are occupied by DABCO acting as pillars to extend the 2D layers into a 3D structure (Fig. 5). Although BDC is generally considered to be a linear and rigid linker, the linker in this structure is bent, leading to severe twisting of the Zn2 paddlewheel from an ideal square grid. Interestingly, after evacuation of guest molecules, the BDC ligands linking the Zn2 paddlewheel units became linear, which resulted in a perfect 2D square grid of [Zn2(BDC)2]. Such a paddlewheel-structured MOF, which has a high BET surface area of 1450 m2 g−1, exhibited an adsorption capacity of 2.0 wt% at 77 K and 1 bar. Furthermore, Chun et al. combined various aromatic dicarboxylates,50 including BDC, tetramethylterephthalate (TMBDC), 1,4-naphthalenedicarboxylate (1,4-NDC), tetrafluoroterephthalate (TFBDC), 2,6-naphthalenedicarboxylate (2,6-NDC), DABCO, and 4,4′-dipyridyl (BPY), to form paddlewheel-structured frameworks [Zn2(BDC)2(DABCO)], [Zn2(BDC)(TMBDC)(DABCO)], [Zn2(TMBDC)2(DABCO)], [Zn2(1,4-NDC)2(DABCO)], Zn2(TFBDC)2(DABCO)], and [Zn2(TMBDC)2(BPY)]. These frameworks possess surface areas in the range of 1450–2090 m2 g−1 and hydrogen-adsorption capacities of 1.7–2.1 wt% at 77 K and 1 bar. In addition, using BDC, triethylenediamine (TED), and DMF, Lee et al.48 synthesized the framework [Zn(BDC)(TED)0.5] · 2DMF · 0.2H2O. This framework, which also possesses a paddlewheel structure, adsorbed 2.1 wt% of hydrogen at 77 K and 1 bar. These results indicate that paddlewheel-structured dinuclear Zn-based MOFs have almost the same hydrogen capacity, which indicates that the type of organic linker does not have an obvious effect on the hydrogen uptake of a paddlewheel-structured Zn-based MOF.
Lee et al. examined hydrogen adsorption on a trinuclear framework [Zn3(BPDC)3BPY] · 4DMF · H2O (BPDC = biphenyldicarboxylate).51 The structure of the framework possesses two crystallographically independent zinc centers (Zn1 and Zn2). Two Zn1 atoms and one Zn2 atom form a trinuclear metal cluster [Zn3(BPDC)6(BPY)2], in which one octahedral metal (Zn2) is located at the center and two tetrahedral metals (Zn1) are situated at two ends. The metal nodes are connected to adjacent nodes by carboxylate groups from six BPDC ligands located in the equatorial plane to form 2D double layers. The remaining two apical positions of the Zn1 are bound to the nitrogen atoms of BPY to form a 3D pillared framework. As a result, its BET surface area is 792 m2 g−1. Furthermore, the hydrogen-adsorption measurements showed that it could adsorb 1.74 wt% of H2 at 77 K and 1 bar. The high density of adsorbed H2 falls in the range of liquid H2, which suggests relatively strong sorbent–sorbate interactions in the material.
2.2. Cu-Based MOFs
The synthesis of copper(II)-based MOFs also attracted much attention.38, 39, 52–61 The first Cu–organic framework is HKUST-1 (also called CuBTC, where BTC = 1,3,5-benzene tricarboxylate), which was invented by Williams et al.52 and subsequently studied by numerous research groups.38, 39, 53–57 HKUST-1, which is composed of CuII paddlewheel clusters linked by trigonal benzene-1,3,5-tricarboxylate, possesses a face-centered-cubic crystal that contains an intersecting 3D system of a bimodal pore size distribution with a BET surface area of about 1500 m2 g−1. The advantage of using copper(II) is that its property toward John–Teller distortion weakens the bonding of nucleophiles (such as solvent molecules) at the axial sites. The removal of these species can create the open metal sites and the Cuδ+–Oδ− dipoles on the surface, which leads to an increase in the local interaction energy for hydrogen or other adsorptives. Indeed, the isosteric heat of hydrogen adsorption at low coverage is 1–2.0 kJ mol−1 larger for HKUST-1 than for MOF-5.38, 39 As a result, the amount of H2 adsorbed by HKUST-1 at 1 bar and 77 K was approximately double that of MOF-5.39, 46 However, at higher pressures, MOF-5 had a much higher H2 adsorbed amount than HKUST-1. This occurred because the adsorbed hydrogen amount at low pressure strongly depends on the binding strength of H2 to the frameworks, whereas the amount adsorbed at higher pressure is mainly determined by the surface area.62 Furthermore, the preparation and activation processes can have a significant impact on hydrogen adsorption capacity, surface area, and pore volume of HKUST-1.63 The H2 uptakes on HKUST-1, which were measured by different groups under nominally the same conditions, vary considerably for two possible reasons: crystal defects and the presence of guest molecules (contaminants) in the HKUST-1 samples.63 For example, Liu et al. found the removal of all solvent from HKUST-1 by an improved activation process can increase its maximum hydrogen uptake up to 4.1 wt% at 26 bar and 77 K.63
Yaghi et al. reported a new Cu–organic framework, [Cu(L1)(H2O)2] (L1 = biphenyl-3,3′,5,5′-tetracarboxylic).53 The framework (referred to as MOF-505) has a crystal structure in which CuII is coordinated by five O atoms in a square pyramidal geometry. Pairs of CuII centers are bridged by four carboxylate groups to form [Cu2(O2CR)4] paddlewheel units. A H2O molecule binds to each Cu center along the paddlewheel axis. Each [Cu2(O2CR)4] paddlewheel is linked to four biphenyl liagnds and vice versa. After activation at 393 K, MOF-505 with a surface area of 1830 m2 g−1 can reversibly adsorb 2.47 wt% of H2 at 77 K and 1 bar.53 This adsorbed amount is comparable to that of HKUST-1. When the H2 pressure increased to 20 bar, the uptake of hydrogen on MOF-505 reached 4.02 wt%.54 To examine the effects of organic linkers, Lin et al. replaced the biphenyl-3,3′,5,5′-tetracarboxylic (L1) of MOF-505 with the terphenyl-3,3″,5,5″-tetracarboxylic (L2) and the quaterphenyl-3,3′′′,5,5′′′-tetracarboxylic (L3), to form two new frameworks [Cu2(L2)(H2O)2] (1) and [Cu2(L3)(H2O)2] (2).54 Frameworks 1 and 2 have higher surface areas of 2247 and 2932 m2 g−1, respectively. The pore sizes are narrowly distributed around 6.5, 7.3, and 8.3 Å for MOF-505, framework 1, and framework 2, respectively. At 1 bar (or below) and 77 K, MOF-505 had the highest hydrogen uptake, framework 2 the lowest, and framework 1 in between. However, when the pressure was above 2.5 bar, the H2 uptake of the later two exceeded that of MOF-505. Furthermore, at 20 bar and 77 K, the H2 uptakes of frameworks 1 and 2 reached 6.06 and 6.07 wt%, respectively. Fitting the high pressure region of their H2 isotherms to the Langmuir equation gave a maximum adsorption of 4.2, 6.7, and 7.01 wt% of H2 for MOF-505, framework 1, and framework 2, respectively.54 This indicates that the hydrogen adsorption is dependent on the pore size (related to affinity for H2) at a low pressure, whereas the surface area is a key factor in controlling the H2 adsorption at a high pressure.
Zhou et al. reported the synthesis and structure of framework Cu3(TATB)2(H2O)3 (TATB = 4,4′,4″-s-triazine-2,4,6-triyltribenzoate).58 Its structure possesses dicopper tetracarboxylate paddlewheel secondary building units (SBUs). With axial aqua ligands, which are linked by TATB bridges. This framework adsorbed about 1.9 wt% of hydrogen at 1 bar and 77 K.
Chen et al. successfully synthesized a mixed zinc/copper MOF Zn3(BDC)3[Cu(PYEN)] · (DMF)5(H2O)5 (PYENH2 = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) (Fig. 6).64 Its desolvation generated a bimodal porous structure with narrow porosity (<0.56 nm) and an array of pores in the bc-crystallographic plane. In such a structure, the adsorbate–adsorbent interactions were maximized by both the presence of open copper centers and the overlap of the potential energy fields from pore walls.64 Its heat of hydrogen adsorption is about 12 kJ mol−1 (at zero surface coverage), which is the highest value so far observed for hydrogen adsorption on MOFs. However, this Cu/Zn hybrid MOF had a low hydrogen uptake of 0.2 wt% at 77 K and 10 bar because of its small surface area.
2.3. Mn-Based MOFs
Moon et al.65 reported [Mn(NDC)(DEF)]n, which is a 3D porous MOF generating 1D channels. [Mn(NDC)(DEF)]n has no free space, because the DEF molecules that coordinate the MnII ions occupy the channels. However, the DEF can be removed to obtain desolvated [Mn(NDC)]n that contains accessible coordination sites on MnII sites, which results in a surface area of 191 m2 g−1 and a H2 uptake of 0.57 wt% at 77 K and 1 bar.65
Long et al. synthesized a new Mn-based framework, [Mn(DMF)6]3[(Mn4Cl)3(BTT)8(H2O)12]2 · 42DMF · 11H2O · 20CH3OH, by using the tritopic bridging ligand 1,3,5-benzenetristetrazolate (BTT3−).66 This porous MOF possesses a cubic topology.66 Crystals of the compound with a high surface area up to 2100 m2 g−1 showed a H2 uptake of 6.9 wt% at 77 K and 90 bar. Furthermore, the H2 uptake can further increase with pressure, because H2 adsorption did not reach its saturation at 90 bar. Such a high hydrogen uptake was attributed to its high adsorption heat (10.1 kJ mol−1 at zero surface coverage), which was directly related to H2 binding at coordinatively unsaturated Mn2+ centers within the framework.66
2.4. Cr or Al-Based MOFs
Several Cr- or Al-based MOFs have been evaluated for hydrogen storage (Table 1).67–70 Férey et al.67 investigated the hydrogen adsorption properties on the metal–benzenedicarboxylate M(OH)(O2C-C6H4-CO2) [M = Al3+ or Cr3+] denoted as MIL-53 [MIL: material from Institute Lavoisier]. The frameworks exhibited 1D channels with large free diameters of about 8.5 Å and a large BET surface area of 1100 m2 g−1. The chromium compound showed a maximal hydrogen capacity of 3.1 wt% at 77 K and 16 bar, whereas the aluminum one exhibited the capacity of 3.8 wt%. Furthermore, they evaluated the hydrogen adsorption on the giant-pore Cr-based MIL-100 and MIL-101 (Fig. 7).71 MIL-100 and MIL-101 were built up from carboxylate moieties (BTC for MIL-100 and BDC for MIL-101) and trimeric chromium(III) octahedral clusters that had removable terminal water molecules and, therefore, provided potential unsaturated metal sites in the structure. The smaller of the two types of cages in their architectures was delimited by 12 pentagonal faces and the larger by 16 faces (12 pentagonal and 4 hexagonal). Without guest molecules, the accessible diameters of the cages were 25 and 29 Å for MIL-100 and 29 and 34 Å for MIL-101. The MIL-100, which was previously outgassed at 493 K, had a Langmuir surface area of 2700 m2 g−1 and a maximum hydrogen uptake of 3.28 wt% at 77 K and 26.5 bar. In the MIL-101a that was obtained by outgassing MIL-101 with the same approach as for MIL-100, a significant amount of BDCH2 molecules was still within the pores, which implied that most of metal sites were still poisoned by BDCH2 molecules. Its Langmuir surface was 4000 m2 g−1. At 77 K, the isotherm of hydrogen adsorption exhibited a maximum capacity at 4.5 wt% with a saturation plateau above 40 bar. Furthermore, if MIL-101a was further subjected to additional treatment to evacuate most of the BDCH2 in the pores, the resultant sample (denoted as MIL-101b) had only half of the metal sites that were poisoned by coordinated BDCH2 molecules. Its Langmuir surface area was as large as 5500 m2 g−1. The maximum hydrogen adsorption capacity of MIL-101b reached 6.1 wt% at 80 bar and 77 K. The high adsorption capacity of MIL-101b was attributed to its high adsorption heat (9.3 to 10.0 kJ mol−1 at low coverage), which was larger than that of MIL-100 (5.6 to 6.3 kJ mol−1 at low coverage). The nature of the interaction between hydrogen molecules and the framework that gave rise to such high values for MIL-101b is due to the presence of strong adsorption sites within the microporous supertetrahedra (ST), probably at each corner close to the trimers of chromium octahedra.71
2.5. Ni-Based MOFs
Lee and Suh reported a robust Ni-based metal–organic open framework, [Ni(cyclam)(BPYDC)] · 5H2O, which is constructed of linear coordination polymer chains made of the nickel–macrocyclic complex [Ni(cyclam)](ClO4)2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) and 2,2′-bipyridyl-5,5′-dicarboxylate (BPYDC2−).72, 73 This framework exhibits permanent microporosity with a Langmuir surface area of 817 m2 g−1 and pore volume of 0.37 cm3 cm−3.72 It adsorbed 1.1 wt% of hydrogen at 77 K and 1 bar. Forster et al. synthesized a Na/Ni-based framework NaNi3(OH)(SIP)2 (SIP = 5-sulfoisophthalate), consisting of Na2Ni6O34 clusters bridged by SIP to form a 3D network.74a The dehydration of this framework at temperatures between 573 and 623 K generated a porous material with a BET surface area above 700 m2 g−1. Furthermore, the dehydrated framework possesses accessible, coordinatively unsaturated NiII sites, which leads to a high adsorption heat of 9.4–10.4 kJ mol−1 for the hydrogen molecule. However, the H2 capacity of this Ni-based framework at 1 bar and 77 K is only 0.94 wt% because of its medium surface area.
Dietzel et al. synthesized a Ni-based coordination polymer, Ni(DHTP)(H2O)2 · 8H2O (DHTP = 2,5-dihydroxyterephthalic), which is a 3D honeycomb-like network with channels of ≈11 Å diameter and a Langmuir surface area of 1083 m2 g−1.74 H2 adsorption on the network at 77 K exhibited a type I profile, with which the Langmuir equation yields a saturation value of 1.8 wt% hydrogen capacity.
Zhao et al. evaluated three Ni-based MOFs (denoted as M, E, and C) for hydrogen adsorption and desorption. M and E have the composition Ni2(BPY)3(NO3)4 with linear chains of BPY bridging metal centers, which are connected by T-shaped BPY coordination at the metal into pairs. These pairs were aligned parallel to each other in M and perpendicular in E to form maximum pore cavity dimensions of 8.3 Å.33 The cavities of MOFs M and E are connected by narrower windows, but the dynamics of the bridging BPY molecules confer sufficient flexibility on the framework to allow adsorptives (that appear oversized from a static view of the structure) to pass through the windows and access the pores.75, 76 C, with formula Ni3(BTC)2(3-PIC)6(PD)3 (where PIC = 3-picoline and PD = propane-1,2-diol), has considerably larger windows and cavities of up to 14 Å in size.77 The adsorption–desorption isotherms for E and M showed a marked hysteresis, in which the former showed virtually no desorption even when the pressure was reduced from 1 to 0.01 bar.33 In contrast, the adsorption–desorption isotherms of C did not show any substantial hysteresis up to 14 bar.33 The maximum H2 uptakes (at 77 K and 1 bar) on E, M, and C were 0.8, 0.7, and 2.1 wt%, respectively.33 Notably, the hysteresis in H2 uptake in porous MOF materials, in which the pore window dimensions are similar to the kinetic diameter of H2, differs qualitatively from more rigid classical sorbents. Hydrogen can be loaded at high pressure and stored at low pressure, if the cavities are larger than the windows, which in turn are both close in size to H2 and have sufficient flexibility due to framework dynamics to allow kinetic trapping of the guest molecule.33 Therefore, the design of MOF materials with thermally activated windows in the open channel structure provides a new possibility to improve their hydrogen-storage characteristics by modifying the desorption kinetics.
2.6. Other MOFs
Perles et al. synthesized a 3D polymeric terephthalate of scandium [Sc2(C8H4O4)3] under hydrothermal conditions by the reaction of Sc3+ with a mixture of terephthalic acid and disodium terephthalate. The structure consists of a polymeric 3D framework, in which each scandium atom is octahedrically coordinated to six carboxylic oxygen atoms of six different terephthalate anions.78 Such a structured [Sc2(C8H4O4)3] has a BET surface area of 721 m2 g−1 and a micropore volume of 0.293 cm3 g−1.78 It exhibited a hydrogen uptake of 1.5 wt% at 77 K and 0.8 bar.78
Kaye and Long examined hydrogen adsorption on dehydrated Prussian blue analogues of the type M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, Zn), wherein interactions with bridging cyanide ligands and/or coordinatively unsaturated metal centers lead to higher adsorption heat.79 They reported that the BET surface areas range from 560 m2 g−1 for Ni3[Co(CN)6]2 to 870 m2 g−1 for Mn3[Co(CN)6]2. The hydrogen uptake for the cyano-bridged frameworks varied from 1.4 wt% in Zn3[Co(CN)6]2 to a maximum of 1.8 wt% in Cu3[Co(CN)6]2 at 77 K and 1 bar. The heat of hydrogen adsorption is in a range of 5.9 kJ mol−1 for Mn3[Co(CN)6]2 to 7.4 kJ mol−1 for Ni3[Co(CN)6]2.79
Lee et al. synthesized a trinuclear [Co3(BPDC)3BPY] · 4DMF · H2O framework, which has a similar structure to [Zn3(BPDC)3BPY] · 4DMF · H2O.51 The hydrogen uptake of the [Co3(BPDC)3BPY] · 4DMF · H2O framework is 1.98 wt% at 77 K and 1 bar, which is higher than the 1.74 wt% of [Zn3(BPDC)3BPY] · 4DMF · H2O. The difference in the H2 uptake between the two frameworks can be attributed to their different surface areas (922 m2 g−1 for the Co-based framework and 792 m2 g−1 for the Zn-based framework), pore volumes (0.38 cm3 g−1 for the former and 0.33 cm3 g−1 for the later), and strengths of gas–solid interactions.51
Dincă and Long reported the synthesis of Mg3(NDC)3(DEF)4 (NDC = 2,6-naphthalenedicarboxylate), which is the first porous MOF incorporating Mg2+.80 Its structure consists of linear Mg3 units linked by NDC bridges to form a 3D framework, featuring 1D channels filled with DEF molecules. Such a 3D framework structure is fully analogous to that of Zn3(NDC)3(CH3OH)2 · 2DMF · H2O, which indicates that Mg2+ can directly substitute for the heavier Zn2+.80 The desolvation of Mg3(NDC)3(DEF)4 by heating at 190 °C generated the microporous solid Mg3(NDC)3. This microporous framework has a high H2 adsorption heat (7.0–9.5 kJ mol−1). However, it exhibited a small hydrogen uptake of 0.48 wt% at 77 K and 1 bar because of its small BET surface area (190 m2 g−1).80
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), desorption (▴), and second adsorption (◊). b) Pure IRMOF-8 (□), Pt/AC and IRMOF-8 physical mixture (1:9 weight ratio) (♦), and for a bridged sample of Pt/AC-bridges–IRMOF-8: first adsorption (
), desorption (▴), and second adsorption (◊). Reproduced with permission from Ref. 
