Trace Alcohol Adsorption by Metal Hexacyanocobaltate Nanoparticles

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Trace Alcohol Adsorption by Metal Hexacyanocobaltate Nanoparticles and the Adsorption Mechanism Miyuki Asai, Akira Takahashi, Yong Jiang, Manabu Ishizaki, Masato Kurihara, and Tohru Kawamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03015 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Trace Alcohol Adsorption by Metal Hexacyanocobaltate Nanoparticles and the Adsorption Mechanism Miyuki Asaia,b, Akira Takahashia, Yong Jianga,c, Manabu Ishizakib, Masato Kuriharaa,b, and Tohru Kawamotoa,b* a. Nanomaterials Research Institute, AIST, 1-1-1 Higashi, Tsukuba 305-8565, Japan. b. Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan, c. Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 3058572, Japan KEYWORDS. Prussian blue analogues, metal hexacyanometallate, adsorption, alcohols

ABSTRACT: Adsorption of alkyl chain alcohols, ranging from methanol to n-hexanol, on manganese hexacyanocobaltate (MnHCCo) and copper hexacyanocobaltate (CuHCCo) nanoparticles was evaluated. The equilibrium adsorption capacity at low pressure was found to be larger than previously published results using other kinds of adsorbents, metal organic frameworks, zeolites, and activated carbons. For example, MnHCCo adsorbed 5 mmol/g of methanol at only 8.9 Pa, less than 1/10 of the lowest pressures used in previous studies. The adsorption can be understood using a two-step process: initial adsorption into the crystal (intra-nanoparticle adsorption) followed by that among the nanoparticles (inter-nanoparticle adsorption). The suggested mechanism was supported by analysis of the adsorption isotherm with the dual-site Langmuir equation, and the entropy loss in the adsorption process. The highest adsorption amount at low pressure was caused by a combination of coordination bonding between alcohol molecules at the high-density open metal sites in the adsorbent and by the intermolecular interaction between the framework of the adsorbent and the alkyl chain of alcohols.

1. INTRODUCTION Prussian blue and its analogues (PBA)—porous coordination polymers—attract attention as the former has been used as a pigment for many years.1 They are used as adsorbents for cations and gas molecules,2–8 for electrochromism,9,10 electrodes for secondary batteries,11,12 sensors,13 catalysis,14,15 information storage,16 and photomagnets.17–19 The adsorption property toward small gaseous molecules is fascinating. For example, cobalt hexacyanocobaltate (CoHCCo) shows the highest capacity for NH3 among porous materials,5 and high capacity was also reported for hydrogen.8 The adsorption of gaseous molecules by porous materials strongly depends on their structure such as pore size, vacancies, and lattice constant.20–22 In this paper, we systematically investigated the relation between the adsorption property of PBAs and the structure of the gaseous adsorbate, where we found that alkyl chain alcohols, whose kinetic diameter was larger than the pore size, were also adsorbed. Consideration the gas adsorption properties of PBAs, the structures, at both the nano-meter scale and atomic scale are important. The nanoparticle of PBA is easily obtained by mixing a solution of raw materials,23 e.g., Fe(NO3)3 and K4[Fe(CN)6] in the case of Prussian blue. Therefore, the powders of PBA should be considered as aggregates of nanoparticles, as shown in Figure 1, where porous network exists among nanoparticles. The size of pores is the same scale as the nanoparticle size.

PBAs also have porous networks on the atomic scale. The crystal structure of PBA is shown in Figure 1. By substituting the metallic site, the pore size can be controlled while maintaining the crystal structure. Most PBAs have cubic crystal _ _ structure of Fm3m or F43m with a wide range of lattice constant a = 9.8–10.9 Å depending on the metallic sites. As shown in Figure 1, PBAs have cage-like structures in their crystal. In a perfect lattice without vacancies (Figure 1a), the size of a cage is a/2×a/2×a/2. The pore diameter, rp, is approximately evaluated from the lattice constant and CN bonding distance, dCN ≈ 1.16 Å, as a/2-dCN. For example, rp of manganese hexacyanocobaltate (MnHCCo) and that of copper hexacyanocobaltate (CuHCCo) are 4.08 Å and 3.88 Å, respectively. The vacancies of the hexacyanometallate anion, [M(CN)6]α−, are also important. With the vacancy, the pore size around the vacancy is expanded to a × a × a. The expanded pore is surrounded by six open metal sites that would make coordination bonding with the adsorbate. The PBA introduces vacancies with high densities and thus the expanded pores are essential for adsorption. These structures suggest that the PBA powders have multiscale porous networks, both at meso-pore/macro-pore scale of more than 50 Å and at micro-pore scale of equal to or less than 10 Å. In this paper, the former is called the inter-nanoparticle space and the latter the intra-nanoparticle space. Considering multi-scale pore structure, we elucidated the guiding principle of gaseous molecule adsorption by PBA crystals. For this purpose, alkyl-chain alcohols were chosen as

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adsorbate because the kinetic diameter and other structural parameters such as molecular length can be controlled systematically. In addition, weak coordination bonding is expected between the hydroxy group of the alcohols and open metal sites of the PBAs.

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pressures to define our practical applications. We used the indicator Qe(0.8p0) for comparing the maximum capacity and p5 for equilibrium capacity at low pressure. The meanings of these indicators are shown schematically in Figure 2. These indicators are obtained from the adsorption isotherm. The increase in Qe(0.8p0) is better in terms of maximum capacity, and decrease in p5 better in terms of adsorption at low pressure.

Figure 2. Schematic view of the definition of unified indicators Qe(0.8p0) and p5 with a typical adsorption isotherm.

Figure 1. (a) Schematic view of multi-scale pore-network structure of the aggregate of nanoparticles of Prussian blue analogs, (b) alcohol molecules used as adsorbates.

The kinetic diameter for each alkyl-chain alcohol reported in literature is shown in Table 1 in comparison with the structural diameter and length calculated using quantum chemical calculations. In general, kinetic diameters are used for considering whether adsorbates can penetrate adsorbents. The molecular length becomes larger as the number of carbons increases, while the diameter is almost constant. The kinetic diameter increases as the chain length increases, except at npentanol. Table 1. Molecular sizes of alkyl-chain alcohols, kinetic diameters, 50 and saturated vapor pressure.51

methanol ethanol n-propanol n-butanol n-pentanol n-hexanol

Length (Å)

Diameter (Å)

4.1 5.1 6.3 7.6 8.8 10.1

2.8 3.6 3.6 3.6 3.6 3.6

Kinetic diameter (Å) 3.6 4.5 4.7 5.0 6.7 6.2

Saturated vapor pressure (kPa) 16.9 7.90 2.80 0.89 0.29 0.12

Adsorption behavior of linear alkyl-chain alcohols has been investigated with various adsorbents like metal organic frameworks,24–41 activated carbons,42–44 and zeolites.45–49 There are two important indicators to evaluate the performance of adsorbents for applications. One is the maximum capacity at high concentration of adsorbate, Qm. The latter is the equilibrium capacity, Qe, at low concentration. The former is used for storage and the latter for the removal of vaporized trace hazardous substances from the atmosphere. For example, the methanol adsorption properties of various adsorbents described in previous reports are summarized in Table 2. The evaluation methods differ among the reports and so we adopted unified indicators defined by ourselves based on applied

Qe(0.8p0) represents equilibrium capacity at p = 0.8p0, where p0 represents the vapor pressure. There are a couple of adsorbents that show high maximum capacity. In particular, MIL-101(Cr), a kind of metal organic framework, exhibits Qm∼36 mmol/g.28 To evaluate adsorption performance at low temperature, we used indicator p5, defined by the pressure where the equilibrium capacity reaches 5 mmol/g, as shown in Figure 2. In Table 2, p5 for various adsorbents are shown. It is found that no adsorbent has been reported for p5 < 100 Pa. Known adsorbents exhibit low amounts of adsorption at low concentrations. For example, the performance of adsorbent at low concentration of adsorbate can be compared to the pressure where the equilibrium adsorption capacity reaches a certain level. Table 2. Summary of methanol adsorption properties by various adsorbents. Qe(0.8p0) and p5 represent the equilibrium capacities at 0.8 times of vapor pressure and pressure where equilibrium capacity reached 5 mmol/g, respectively. The values of Qe (0.8p0) and p5 except for MHCCo were estimated from the isotherms given in the references.

metal organic framework

MIL-101(Cr) MIL-100(Cr) CuBTC ZIF-8 Activated carbon zeolite NaZSM5 chabazite SAPO-34 MHCCo MnHCCo (this work) CuHCCo

Qe(0.8p0) (mmol/g) 36 18 20 13 13 4.1 9.2 5 12.1 11.0

p5 (Pa) 800 1,200 150 4,500 1,800 − 1,050 13,000 8.9 19.3

Ref 24 24 25 26 42 46 45 48

Selectivity is also important, but the necessity of selectivity depends of the scope of applications. For example, it is essential for the separation of materials. The concurrent removal of trace hazardous material requires no selectivity, such as in activated carbon. In this paper, we do not discuss selectivity because intrinsic adsorption properties are focused on.


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The Journal of Physical Chemistry First, we investigated the lattice constant of metal hexacyanocobaltates (MHCCo, M[Co(CN)6]x) with various metals to find those materials whose lattice constants were the largest or the smallest. Detailed investigation was done only for the largest and the smallest materials to evaluate the effect of pore size on alcohol adsorption. We chose MHCCo because it has higher stability to avoid chemical reaction during adsorption than metal hexacyanoferrate. Manganese hexacyanocobaltate (MnHCCo) and copper hexacyanocobaltate (CuHCCo) were chosen for investigating adsorption properties because MnHCCo and CuHCCo were found to have the longest and shortest lattice constants. 2. EXPERIMENTAL SECTION 2.1. Synthesis. Manganese hexacyanocobaltate (MnHCCo) was synthesized by mixing aqueous solutions (20 mL) of 0.6 mol L-1 manganese(II) chloride and 0.4 mol L-1 potassium hexacyanocobaltate(III) in a beaker with magnetic stirring for one night for which special-grade reagents were obtained from Wako Pure Chemical Industries Ltd. Powdery MnHCCo was obtained through aqueous rinsing by centrifugation five times, followed by drying at 60 °C under vacuum. MHCCo powders with M2+=Fe, Zn, Ni, and Cu were synthesized in the same way as MnHCCo using FeCl2, ZnCl2, NiCl2, and CuCl2, respectively. CoHCCo powder was synthesized using a flow method with micromixer by mixing aqueous solutions of CoCl2 and potassium hexacyanocobaltate(III) in a micromixer with hole diameter 250 µm. The flow rate was set to 20 mL min-1 for each solution. The concentrations of potassium hexacyanocobaltate(III) and CoCl2 were 200 mmol L-1 and 600 mmol L-1, respectively. 2.2. Characterization. The chemical compositions of MnHCCo and CuHCCo were determined as follows: the compositions of Mn, Cu, Co, and K were evaluated using a microwave plasma–atomic emission spectrometer (MP-AES, Agilent 4100; Agilent Technologies Japan Ltd.) using a standard addition technique with prior decomposition by a microwave sample preparation system (Multi-wave3000; PerkinElmer Corp.). The hydration numbers were determined using thermogravimetry-differential thermal analyses (TG-DTA, Thermo plus evo II; Rigaku Corp.). Sample images were obtained using a field emission scanning electron microscope (FE-SEM, S-4800; Hitachi High Technologies Corp.). All measurements were taken at room temperature. The specific surface area was estimated by Brunauer-Emmett-Teller (BET) analysis using an automatic specific surface area evaluation system (BELSORP-mini and BELSORP-max, Microtrac BEL Inc.). 2.3. Adsorption properties. The alcohol vapor adsorption equilibrium isotherms for MnHCCo and CuHCCo powders at 25 °C were evaluated using a gas-adsorption system (BELSORP-max and BELSORP-maxII; Microtrac BEL Inc.). The adsorption equilibrium was determined by the criterion of pressure difference below 0.3% in 10 min. To remove hydration water, the powders were heated at 150 °C for 24 h under vacuum (using a rotary pump) before the adsorption process. The adsorption properties were analyzed with the dual-site Langmuir model as:

(1),

where qαm and Kα represent the maximum capacity and equilibrium constant for the α-th adsorption site, respectively. The adsorption enthalpy of MHCCo was obtained at 25 °C and 40 °C. The enthalpy was evaluated from the adsorption isotherm using the Clausius–Clapeyron equation52:

∆Habs =

(lnP2- lnP1) ,

(2)

where ∆Habs, R, T, and P represent the adsorption enthalpy, gas constant, temperature, and pressure, respectively. The subscripts indicate two temperatures and pressures with the same amount of alcohol adsorption. The entropy loss ∆S was calculated by ∆G=∆Habs-T∆S. 3. RESULTS AND DISCUSSION 3.1. Characterization. XRD patterns of MHCCo powders are shown in Figure 3a. All of the MHCCo powders had the _ same crystal structure with space group Fm3m. However, the lattice constants of MHCCo powders were different between the samples, e.g., 10.40 Å for MnHCCo, 10.29 Å for FeHCCo, 10.25 Å for ZnHCCo, 10.20 Å for CoHCCo, 10.06 Å for NiHCCo, and 10.04 Å for CuHCCo. The differences were due to the variation of ionic radius of M2+. Hereafter MnHCCo (with the longest lattice constant) and CuHCCo (with the shortest) were investigated in detail.

Figure 3. (a) XRD patterns of various metal hexacyanocobaltates (MHCCo). The peaks with the notation ‘Si’ represent those from silicon for calibration of peak position. (b) SEM images of manganese hexacyanocobaltate (MnHCCo) and copper hexacyanocobaltate (CuHCCo).

The chemical compositions of MnHCCo and CuHCCo were determined as K0.04Mn[Co(CN)6]0.70·4.0H2O, and K0.01Cu[Co(CN)6]0.64·3.18H2O for MnHCCo and CuHCCo, respectively. The K-compositions were almost zero, indicating no occupation of the interstitial site by alkali cations. The smooth migration of adsorbate alcohol molecules in the crystal was anticipated. The MnHCCo and CuHCCo powders were obtained as nanoparticles, whose sizes were around 50–150



nm (500-1500 Å) in the SEM image (shown in Figure 3b). The space among the nanoparticles had micropores. The system had both sub-nm scale micropores in the crystal (intrananoparticle) and macropores of more than 500 Å53 among the nanoparticles (inter-nanoparticles space). From BET analysis, the specific surface area of MnHCCo was 914 m2/g. The N2 adsorption isotherm at 77K is shown in Figure SI 4 in supporting information. That was similar to the previous report of 870 m2/g8 and the highest value among PBAs to our knowledge. The large surface originated from the large lattice constant and exclusion of alkali cations in the interstitial site. 3.2. Adsorption isotherm. Alcohol adsorption isotherms of MnHCCo and CuHCCo are shown in Figure 4 and Figure 5, respectively. The adsorption behaviors of MnHCCo and CuHCCo were similar, indicating the adsorption process was independent of the lattice constant. The adsorption amount of MnHCCo was slightly larger than that of CuHCCo. The difference originated from the difference in intra-nanoparticle pore size. The mass density of the adsorbed alcohol was not dependent on the chain-length of the alcohol. We confirmed via in-situ XRD analysis that the crystal structure retained its original structure even after adsorption with in alcohol atmosphere. The details are shown in the Supporting Information.

highest. MIL-101(Cr) having the highest Qe(0.8p0) = 36 mmol/g had larger pore size than MHCCo, and so the maximum adsorption capacity was enhanced. The adsorption by MHCCo at low pressure was impressive. The adsorption isotherms are re-drawn in Figure 6 as a semilogarithmic graph to clarify the adsorption property at low pressure. The figure indicates that both MnHCCo and CuHCCo started to adsorb alcohols at lower than 100 Pa. In the case of MnHCCo, Qe drastically increased at around 1–10 Pa for all alcohols. For example, at p=8.9 Pa, Qe for methanol reached 5 mmol/g, i.e., p5 = 8.9 Pa. Even with CuHCCo, p5 was estimated at 19.3 Pa. As shown in Table 2, p5 for all reported adsorbents were higher than MnHCCo, 150 to 13,000 Pa. This result implied that MnHCCo was a good adsorbent of methanol, i.e., an alcohol concentration of 10–100 ppmv in air would be recovered by adsorption.

Figure 5. Adsorption isotherms of alcohols on CuHCCo. Red points and lines represent the observed values and fitting curve with the dual-site Langmuir model. Black solid lines and broken lines represent the contribution of each term in the dual-site Langmuir model.

Figure 4. Adsorption isotherm of MnHCCo for alcohols. Red points and lines represent the observed values and fitting curve with dual-site Langmuir model. Black solid lines and broken lines represent the contribution of each term in the dual-site Langmuir model.

From the isotherms, we evaluated Qe(0.8p0) to investigate the maximum capacity. As shown in Table 1, for methanol adsorption, Qe(0.8p0) = 12.1 mmol/g on MnHCCo and 11.0 mmol/g on CuHCCo, respectively. These values were comparable with the adsorbents reported previously, but not the

The dependence on the pressure required for adsorption on the chain length of alcohols was distinctive. As shown in Figure 6, the isotherm shows sigmoidal shape, similar to the result of all-silica chabazite, CHA,26,45 and zeolitic imidazole framework-8, ZIF-8.27,41 However, to focus on the pressure where adsorption increases, the dependence of ps on the chain length of MnHCCo was different from CHA and ZIF-8. With these adsorbents, ps was smaller by around 103-104 times for pentanol than that of methanol. On the other hand, ps changed only a few 10 times between pentanol and methanol. This was due to the strength of coordination bonding between the open metal site of MnHCCo and OH group of alcohols. In general, the coordination bonding with the transition metal, Mn or Cu, was stronger than Zn in ZIF-8 and Si in CHA. The strong interaction between the adsorbate and framework of the adsorbent


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The Journal of Physical Chemistry made the ps to be smaller for all alcohols. The speculation was also supported by the fact that CuBTC (BTC = benzene-1,3,5tricarboxylate), having open Cu sites, showed the largest methanol adsorption at lower concentration, in the previous reports as shown in Table 2. In addition, the appropriate pore size for the size of alcohol led to the enhancement of intermolecular interaction between the adsorbate and adsorbent. The difference of ps among alcohols with different chain lengths is important to discuss selectivity in the use of adsorbent. Large difference in ps allows selective removal of smaller alcohols under appropriate pressure. On the other hand, with a small ps difference, it is worth removing all alcohols such as the role of activated carbon for volatile organic compounds (VOC). Because the pressure required for alcohol adsorption by MnHCCo was smaller than activated carbon, MnHCCo has the potential to be a new trace alcohol remover. CuHCCo also showed similar tendency and so PBA family has potential for removing various alcohols. 3.4 Adsorption mechanism. To clarify the adsorption mechanism, we tried curve fitting of the isotherms. It was difficult to fit the isotherms with the typical Langmuir model because rapid increase in equilibrium capacity at low pressure and gradual increase at high pressure were not reproduced simultaneously. Using the dual-site Langmuir model shown in Eq. (1), most of the isotherms were reproduced, as shown in Figure 4 and Figure 5. This implied that two kinds of adsorption sites existed in the adsorbent. The parameters used for fitting are shown in Table 3 for MnHCCo and Table 4 for CuHCCo. Because K1 was larger than K2, the first site showed faster adsorption than the second one. With the deconvoluted curve for each term in the dual-site Langmuir model shown in Figure 4 and Figure 5, it was found that rapid adsorption at low pressure was dominant from the first site, and gradual adsorption from the second site.

Figure 6. Alcohol adsorption isotherms at 25 °C: (a) MnHCCo, (b) CuHCCo.

Considering the structures of MnHCCo and CuHCCo, the first and second terms were contributions of the intrananoparticle adsorption and inter-nanoparticle ones, respectively. In the crystal of MHCCo, the pore size and the diameter of the alcohol molecules were almost the same, as shown in Table 1. Therefore, in the case of intra-nanoparticle adsorption, the interaction between the adsorbent and adsorbate was strong due to intermolecular forces between the lattice and the long-alkyl chain (due to appropriate relations between their sizes) in addition to coordinating bonding between the open metal sites of MHCCo and hydroxy-groups of alcohol mole-

cules. In the case of inter-nanoparticle adsorption, the alcohol molecules were adsorbed via coordination bonding onto the open metal sites at the nanoparticle surface implying weaker interaction than intra-nanoparticle adsorption. In order to confirm the adsorption and to investigate its mechanism, we observed the infrared (IR) spectrum of MnHCCo in an ethanol atmosphere as shown in Figure SI5 in supporting information. We observed the strong new absorption peaks corresponding to ethanol. The peak height was quite larger than that without MnHCCo, indicating the condensation of the ethanol by the adsorption in MnHCCo. In addition, the peak positions was almost same shown in the previous report for the ethanol adsorption on CeO2 surface with Ce-O bonding.54 The result consists with our explanation with the bonding between the open metal site and the alcohol. For further investigation, molecular simulations would be helpful.55,56 The results showed appropriate pore size of the adsorbent was essential for adsorption at low pressure. In general, for material development of porous coordination polymers including metal organic frameworks, the surface area had to expand as much as possible to increase capacity. However, to enhance adsorption at low pressure, another approach was necessary to fit the pore size to the adsorbate. The case of n-pentanol was not typical, where equilibrium capacity at low pressure was rather small and a single Langmuir model was sufficient for reproduction. The tendency was observed with both MnHCCo and CuHCCo, and was a general feature of MHCCo. The reason for the specialty of n-pentanol was not unclear. It was related to the specifically longer kinetic diameter of n-pentanol, which was out of order in the relation between the alkyl chain length and the kinetic diameter (shown in Table 1). To clarify our hypothesis with two kinds of adsorption sites, entropy loss during the adsorption of MnHCCo and CuHCCo were also evaluated for methanol and n-hexanol from the temperature dependences of the adsorption isotherms with Eq (2). The isotherms of MnHCCo at 25 °C and 40 °C for methanol and n-hexanol adsorption are shown in Figure 7a and b, respectively. The entropy loss of the adsorbate during adsorption into/onto MnHCCo is shown in Figure 7c and d. The data for CuHCCo are shown in Figure SI3 in the Supporting information. Entropy loss, ∆S, as a function of equilibrium capacity had the general tendency that ∆S had a peak and the peak position of Qe was comparable with the value where the adsorption behavior changed from rapid to gradual, as shown in Figure 4 and Figure 5. Table 3. Fitting parameters of the adsorption isotherm with MnHCCo powders for the dual-site Langmuir equation. qm1 methanol ethanol n-propanol n-butanol n-pentanol n-hexanol


10 5.9 4.2 3.6 3.7 2.4

qm2

K1 1

8.4×10 1.9×102 3.8×102 8.7×102 4.9×101 1.2×103

K2 1

7.5×10 5.5×101 2.4×102 3.2×101 0.0 2.3×101

2.3×10-3 1.4×10-2 1.6×10-3 2.1×10-2 7.8×10-2


Table 4. Fitting parameters of the adsorption isotherm with CuHCCo powders for the dual-site Langmuir equation. qm1 methanol ethanol n-propanol n-butanol n-pentanol n-hexanol

8.3 4.2 3.0 2.5 2.3 1.7

K1 1

5.8×10 5.8×101 3.0×102 2.6×104 9.1×101 2.0×103

qm2

K2

3.2 2.1 1.8×102 1.0 0.0 3.8

2.7×10-1 1.5×10-1 1.5×10-3 1.0 3.6×10-1

This behavior supported our hypothesis, intra-nanoparticle rapid adsorption, and inter-nanoparticle gradual adsorption. From a theoretical view, the entropy loss by adsorption was estimated by the decrease in freedom. The decrease in freedom consisted of two factors. One was translational motion of the molecule and the other the intramolecular freedom. The dependence of the former on entropy loss was roughly estimated as the vaporization entropy.57 The latter was calculated from the rotational freedom of C-C bonds with S=kBln3n, where n represents the number of carbon atoms in the adsorbate molecule. The total entropy loss was theoretically estimated as the sum of the effects of vaporization and intramolecular freedom, as shown in Table 5.

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switching from intra-nanoparticle adsorption to internanoparticle was supported by the behavior of entropy loss.

Table 5. Summary of theoretical estimations of entropy loss by the adsorption of methanol and n-hexanol. Entropy loss (J/mol/K) vaporization Intramolecular freedom total

methanol 111 9.2 120

n-hexanol 142 55 197

4. CONCLUSIONS The alcohol adsorption behavior by metal hexacyanocobaltate was investigated. Drastic increase in equilibrium adsorption at low pressure was found in comparison to the adsorbents reported previously. The adsorption behavior fitted the dual site Langmuir equations and the dependence of entropy loss on adsorption amount explained sequential adsorption into the crystal (intra-nanoparticle) and nanoparticle surfaces (internanoparticles). These results implied potential application for the removal of trace vapor alcohols.

ASSOCIATED CONTENT The following contents are supplied in the Supporting Information: XRD patterns of MHCCo before and after alcohol adsorption, the lattice constant of MHCCo before and after alcohol adsorption, temperature dependence of adsorption isotherm for adsorption by CuHCCo, entropy loss by alcohol molecule upon adsorption into CuHCCo, N2 adsorption/desorption isotherms of MnHCCo in 77K, and infrared spectra of the MnHCCo in an ethanol atmosphere.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID: Tohru Kawamoto: 0000-0002-3984-2980

ACKNOWLEDGMENT The authors would like to thank Dr. Hakuta at AIST for cooperation on evaluation of the isotherm, and Ms. Noda at AIST for preparation of metal hexacyanocobaltate. Figure 7. Temperature dependence of adsorption isotherm for adsorption by MnHCCo (a) with methanol and (b) with n-hexanol. Entropy loss by alcohol molecules by adsorption into MnHCCo with (c) methanol and (d) n-hexanol.

Concerning intra-nanoparticle adsorption, the entropy loss increased as the adsorption proceeded because both intermolecular freedom and intra-molecular freedom were suppressed with the increase in filling ratio of the pores in MHCCo. With inter-nanoparticle adsorption, the decrease in entropy loss was reasonable because intra-molecular freedom was similar to the inter-nanoparticle adsorption because of notconfinement in the micropores among nanoparticles. Thus, the

ABBREVIATIONS PB, Prussian blue; PBA, Prussian blue analog; MHCCo, metal hexacyanocobaltate; MnHCCo, manganese hexacyanocobaltate; CuHCCo, copper hexacyanocobaltate; ZIF, zeolitic imidazolate framework; CHA, chabazite; BTC, benzene-1,3,5-tricarboxylate

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Trace Alcohol Adsorption by Metal Hexacyanocobaltate Nanoparticles

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