Water Types and Their Relaxation Behavior in Partially Rehydrated

Article pubs.acs.org/Langmuir

Water Types and Their Relaxation Behavior in Partially Rehydrated CaFe-Mixed Binary Oxide Obtained from CaFe-Layered Double Hydroxide in the 155−298 K Temperature Range Valéria Bugris,†,# Henrik Haspel,† Á kos Kukovecz,†,‡ Zoltán Kónya,†,§ Mónika Sipiczki,∥,# Pál Sipos,∥,# and István Pálinkó⊥,#,* †

Department of Applied and Environmental Chemistry, University of Szeged, Rerrich tér 1, Szeged, H-6720 Hungary MTA-SZTE “Lendület” Porous Nanocomposites Research Group, University of Szeged, Rerrich tér 1, H-6720 Szeged, Hungary § MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich tér 1, Szeged, H-6720 Hungary ∥ Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720 Hungary ⊥ Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720 Hungary # Material and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Dóm tér 7-8, H-6720 Szeged, Hungary ‡

ABSTRACT: Heat-treated CaFe-layered double hydroxide samples were equilibrated under conditions of various relative humidities (11%, 43% and 75%). Measurements by FT-IR and dielectric relaxation spectroscopies revealed that partial to full reconstruction of the layered structure took place. Water types taking part in the reconstruction process were identified via dielectric relaxation measurements either at 298 K or on the flash-cooled (to 155 K) samples. The dynamics of water molecules at the various positions was also studied by this method, allowing the flash-cooled samples to warm up to 298 K.

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like main layers contain an ordered arrangement of Ca2+ and Al3+ or other trivalent ions, seven- and six-coordinated, respectively, in a nonvariable ratio of 2:1.3 A peculiar property of LDHs is the so-called memory effect.4 It is known that LDHs lose their layered structure on heat treatment. The procedure may be followed by thermogravimetry, and the dehydrated structure is often found to be only partially crystalline. It is also known that in an environment saturated with water vapor, if the temperature of heat treatment was not too high (generally lower than ∼873 K, but the accurate value depends on the particular LDH), rehydration occurs and the layered structure is more or less regained as verified by, e.g., X-ray diffractometry (XRD).5,6 The steps of dehydration, especially for MgAl-LDH, are well documented;7−12 however, the rehydration procedure is not so well described. There are studies, when a controlled amount of water or water vapor was used either alone over calcined LDH13−16 or during the aqueous intercalation of the terephthalate ion of known concentrations (and thus known amounts of water).15 The reconstruction was followed by

INTRODUCTION The natural mineral containing Mg2+, Al3+, CO32− ions and having [Mg6Al2(OH)16]CO3·4H2O chemical formula is called hydrotalcite. This mineral was discovered in Sweden in the first half of the nineteenth century.1 Soon after this, other minerals have also been found, in which other than Mg2+ or Al3+ were the di- and trivalent ions, but they were isostructural with hydrotalcite. This group of double hydroxides was named “hydrotalcite-like” or “hydrotalcite-type” compounds. In a recent publication,2 commissioned by the International Mineralogical Association, this group of substances is called the hydrotalcite supergroup, consisting of eight subgroups of naturally occurring minerals. It also turns out that these materials have layered structure, resembling that of brucite, a layered Mg(OH)2 consisting of edge-sharing Mg(OH)6 octahedra; therefore, they are also called layered double hydroxides (LDHs). In hydrotalcites, the Mg2+ ions are partially substituted by trivalent ions of, generally, similar radii. The positive charge thus generated on the layers is compensated by anions that are located, together with water molecules, in the interlayer region. Although the octahedral arrangement is typical around the cations in the supergroup, in a subgroup called hydrocalumites (the name giving mineral has the formula of [Ca2Al(OH)6]A·nH2O), the corrugated brucite© XXXX American Chemical Society

Received: September 12, 2013 Revised: September 27, 2013

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XRD13,15,16 or even better, with time-resolved XRD using synchrotron radiation,14 but even with the latter, the various forms of water taking part in the reconstruction process were not distinguished. In the work leading to one of our recently published papers,17 the partial rehydration of CaFe-LDH was followed at several preset relative humidity (RH%) values by XRD, thermogravimetry (TG; measuring the weight losses occurring during controlled heat treatment of the partially dehydrated samples), and dielectric relaxation spectroscopy (DRS). Water types and their role in rehydration have been identified mainly through studying the weight losses of the fully as well as the partially rehydrated samples. Dielectric spectra registered at room temperature allowed the identification of structural OH groups, also gained from rehydration, and interlayer water, but only at relatively low RH% values (up to 38%). Physically adsorbed water was always out of the frequency range of our instrument, and the relaxations of the other water types slipped out as well at higher RH% values. It was thought that the value of dielectric measurements could be enhanced significantly, if after partial rehydration, the sample was flash-cooled, i.e., the actual state was frozen. Then, all water types should be seen even at high RH% values, and even closely related ones might be distinguishable (peaks in the derivative TG (DTG) curves were always wide17 indicating, that more than one type of water molecules constitute the moiety called interlayer water). As an added bonus, the possibility of studying the relaxation behavior of the various water types in the samples during warm-up to near room temperature is given too. Results obtained are communicated in this contribution.

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Hz to 107 Hz by a Novocontrol Alpha-A frequency response analyzer (FRA). The dielectric measurements were carried out under isothermal conditions; the temperature was controlled by a homemade cryosystem, with stability better than 0.5 K. The temperature was increased stepwise from 155 to 283 K in 2.5 K steps. The dielectric properties were measured19 by inserting the sample powder into a concentric cylindrical capacitor. To avoid density-dependent conductivity variation,20,21 the sample was measured in powder form without pressing it into a pellet. The RH% dependence of the dielectric properties was measured in a closed, grounded metal vessel containing saturated salt solutions, which maintained the desired RH% levels22 of 11%, 43% and 75%. The samples were allowed to equilibrate to constant electrical response.

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RESULTS AND DISCUSSION General Considerations. The coprecipitation of CaCl2 and FeCl3 under alkaline condition provided with a CaFe-LDH indeed, as the X-ray diffractogram attests (Figure 1).

EXPERIMENTAL SECTION

Materials and the Method of Synthesis. For the synthesis of the CaFe-LDH the coprecipitation18 of Ca and Fe salts was used. The aqueous solution of the 2:1 M mixture of CaCl2 (Molar Chemicals, puriss) and FeCl3·6H2O (Molar Chemicals, puriss special) were transformed to double hydroxide with carbonate-free 3 M aqueous NaOH (Spektrum 3D, a. r.) via slowly raising the pH of the solution to 13 under N2 protecting gas, to avoid CO2 interference. Under these conditions, the material was found to be precipitated with a layered structure. Method of Dehydration and Rehydration. The as-prepared airdry LDH was heat-treated in dry air at 773 K for 5 h. After this, portions of this material were kept under preset and controlled RH% values of 11% (LiCl·H2O), 43% (K2CO3), and 75% (NaCl) for two weeks at 298 K. For producing the atmosphere of preset humidity, oversaturated aqueous solutions of the above listed salts were prepared and placed in a closed flask of 200 cm3 filled with dry air and kept for two weeks closed at 298 K before the experiments with the dehydrated LDH samples. These samples were placed above these solutions. Characterization Methods. For checking whether we have LDH in our hands, the X-ray diffractogram of our sample was registered in the 2Θ = 3−60° range on a Rigaku Miniflex II instrument, using Cu Kα (λ = 1.5418 Å) radiation. The Fourier-transform infrared (FT-IR) spectra of the partially rehydrated samples were recorded on a BIORAD FTS-65A/896 spectrometer equipped with a DTGS detector in diffuse reflectance. Spectra were taken immediately after removing the samples from the humidity-controlled conditions. Spectral resolution was 4 cm−1 and 256 scans were collected for a given spectrum. The spectra were baseline corrected and smoothed using the WIN-IR software package. These measurements were performed at 298 K (room temperature). Dielectric Relaxation Measurements. The complex dielectric function ε*(ω) = ε′(ω) − iε″(ω) (where ε′(ω) is the real and ε″(ω) is the imaginary part) was measured in the frequency range from 10−1

Figure 1. The X-ray diffractogram of the freshly prepared CaFe-LDH.

We have already learnt that the memory effect works with CaFe-LDH as well (see Figure 1 in ref 17). After the synthesis, the air-dry product was heat treated in dry air, at 773 K for 5 h. The layered structure collapsed and partially crystalline CaFe mixed binary oxide was formed. However, after stirring it in water, the original layered structure was regained. It has also been learnt previously from thermogravimetric mesurements17,23 that three major forms of water is typical for CaFe-LDH (just as for LDHs, in general). Physisorbed, interlayer, and structural water (this latter is accurately called “structural OH”; however, it is removed in the form H2O, therefore, it will be denoted “structural water” in the followings) were removed in the 100−150 °C, 175−325 °C and 350−475 °C temperature ranges, respectively. The FT-IR spectra of the partially rehydrated samples (Figure 2), especially their comparison, give hints on the sequence of the rehydration steps. That of the sample rehydrated at RH% = 11% displays a peak (Figure 2, black trace) at 3643 cm−1, and it can be assigned to isolated OH groups. This is structural OH, and it is seen, because the relative humidity is low, and the amount of water is not sufficient or the rehydration of the layers is not fast enough to form hydrogen-bonded OH network exclusively. At higher RH% values (Figure 2, red and green traces) the broad bands above 3000 cm−1 are only observed, i.e., hydrogen-bonded OH network was formed. This corresponds to the structural OH B

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It is important to note that although ε*(ω) is commonly associated with the dielectric relaxation, in fact it describes the true dielectric retardation in the framework of the linear response theory.25 Recently, there has been a renewed interest in its use for interpreting dielectric spectroscopy data.26,27 The isothermal dielectric spectra measured as a function of frequency between 10−1 Hz and 107 Hz were detected at low (RH% = 11%), medium (RH% = 43%), and high (RH = 75%) RH% values. For the spectra the Havriliak−Negami (HN) function was fitted considering the conductivity and the electrode polarization terms:28,29 4

ε*(ω) = ε∞ +

∑ i=1

⎛ σ ⎞ A − ia⎜ dc S ⎟ + S (1 + (iωτi) ) ⎝ ε0ω ⎠ ω Δεi

α β

where ω = 2πf, ε* is the complex dielectric function, Δε = εS − ε∞ is the dielectric strength, εS and ε∞ are the dielectric constant at very low and at very high frequencies, respectively, ε0 = 8.8542 × 10−12 F/m is the permittivity of vacuum, τ is the relaxation time, α and β are fitting parameters characterizing peak broadening, and σdc is the specific conductivity. S ≤ 1 determines the slope of the conductivity tail in double logarithm formalism. Factor A has the dimension of (rad·Hz) S/Hz, and 0 ≤ A is the magnitude of electrode polarization. During the fitting procedure, the β parameter was always found to be close to 1. Thus, the relaxation equation used is reduced to the simpler Cole−Cole function.30 In Figure 3 the real and imaginary permittivities and the imaginary modulus spectra, recorded at RH% = 11% and 298

Figure 2. FT-IR spectra of the heat-treated CaFe-LDH samples rehydrated at 11%, 43%, and 75% relative humidity (RH%) values.

groups; however, physisorbed water also contributes, probably even at the lowest RH% value. Also at the lowest RH% value, one finds a peak around 1620 cm−1 gaining intensity as the RH% values grow. This band is assigned to the deformation vibration of the interlayer water molecules. The bands under 1000 cm−1 may be assigned to the O−metal ion−O units of the layers.24 The Formalism and Dielectric Relaxation Measurements at 298 K. Broadband DRS is a technique based on the interaction of an external field with the electric dipole moment of the sample.19 This technique can be used for studying the molecular dynamics of water molecules. The LDH powder may be regarded as a heterogeneous system in which various water types are present in a confined environment at different positions and interfaces. The dielectric properties of water molecules at these different positions and interfaces should be reflected in the DRS spectrum, thus there is hope that the gradual rehydration of the dehydrated LDH can be monitored. In order to meet this expectation, the samples equilibrated at the three RH% values were studied by DRS measurements. First, let us describe the formalism used in evaluating the observed data. The complex dielectric function, ε*(ω), is ε*(ω) = ε′(ω) − iε″(ω)

where ε′(ω) and ε″(ω) are the real and imaginary parts, respectively, and ω = 2πf, where f is the frequency. If one has a highly conductive system like LDHs, it is more convenient to evaluate the observed data in terms of the electric modulus formalism. Then, one can quantitatively describe the water-gaining procedures and can give as accurate relaxation times (τ) related to the positions and interfaces involving the various water types as it is possible. The complex dielectric modulus (M*(ω)) is given by the inverse of the complex permittivity: 1 M *(ω) = M′(ω) + iM″(ω) = ε*(ω) ε′(ω) ε″(ω) = +i 2 2 ε′(ω) + ε″(ω) ε′(ω)2 + ε″(ω)2

Figure 3. Permittivity vs frequency data (top two graphs; upper: real part, lower: imaginary part) measured at 298 K, and the derived dielectric modulus vs frequency function (the bottom graph) for CaFe-LDH equilibrated at relative humidity value of 11%.

K, are depicted. The dielectric spectra of the partially rehydrated CaFe-LDH sample are similar in appearance to other water-containing materials, such as oxide-hydroxides,31 (e.g., silica32), zeolites,33 clay minerals,34,35 and so forth. At this temperature, three relaxations can be identified. C

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Visually, at this high RH% value, the sample actually looks wet, thus, it is not surprising that multilayers of physisorbed water molecules sit on the outer surface. At this RH% value, the relaxation attributable to interlayer water molecules also appears in our measuring range. However, the other two relaxation processes are still out of it. On the bases of the information described here and in the previous section, the following picture on the rehydration process may be suggested. Even at the RH% value of 11%, when the availability of water molecules is relatively low, there is physisorbed water (DRS on the flash-cooled sample) on the outer surface. This physisorbed water is probably not more than a monolayer. In the equilibrated sample, however, the layers are partially rehydroxylated (DRS at 298 K and the isolated OH band in the IR spectrum), and there is interlayer water as well (DRS at 298 K and H−O−H bending vibration in the IR spectrum). In the sample equilibrated at the medium RH% value, the rehydroxylation of the layers are probably completed (the isolated OH band disappeared from the IR spectrum), the quantity of the interlayer water content increased (the intensity of the H−O−H bending vibration increased), and there is physisorbed water, probably a monolayer (DRS on the flash-cooled sample). The reconstruction of the layered structure is complete when the rehydrated sample is equilibrated at the highest RH% value and the physisorbed water forms multilayer (DRS on the flashcooled sample). Dielectric Measurements on the Samples Warmed up from 155 K. When the samples are gradually warmed up from 155 K, the equilibria are broken, and the water molecules are allowed to seek for energetically more favorable positions than they had at the lower temperature. Therefore, it may be possible to see some dynamics of the processes. Unfortunately, it is not going to be the full picture, because at increasing temperatures, the high-frequency processes are shifting out of the frequency window available, and the low-frequency processes are moving into it. This means, for instance, that at increasing temperatures, relaxation(s) to be assigned to physically adsorbed water are going to slip out of the available frequency range, but of course, it does not necessarily mean that there is no physisorbed water at the higher temperature. Moreover, it is certain that there is at the medium and high RH% samples, and is very probable for the lowest RH% sample. In spite of the above-detailed limitations, it may be instructive to see the details on representative spectra at each RH% (Figures 5−7). For the sample equilibrated at RH% of 11% (Figure 5), then flash-cooled to 155 K and allowed to warm up gradually, at 165 K beside the relaxation of physisorbed water (P1) that of the interlayer water (P2) is also seen. Relaxation attributed to structural water is within the frequency window first at 248 K, while the relaxation belonging to P1 is gradually disappearing. The spectrum registered at 278 K closely resembles that of the equilibrated sample taken at 298 K, i.e., we have returned to the starting point (as it had to happen). In Figure 6, the imaginary part of the dielectric modulus (M″(ω)) vs frequency functions are displayed for the sample equilibrated at RH% of 43%, then flash-cooled to 155 K and allowed to warm up gradually. At 178 K, beside the relaxation of physisorbed water (P1), that of interlayer water also appears in the frequency window (recall that at 155 K the relaxation of physisorbed water was only seen, and compare to the RH% of 11% sample, where this happened already at 165 K). At 201 K,

The one at the lowest frequency may be assigned to interfacial polarization process (P4; Maxwell−Wagner−Sillars or MWS process) caused by the accumulation of charges at the interfaces.32,33,36 The maximum at ∼125 Hz may be attributed to rehydroxylation of the layers (P3), i.e., to the appearance of structural OH groups.37 Concomitantly, interlayer water also appeared (P2), giving rise to a peak at ∼63 kHz.37 The one attributable to physisorbed water has already, even at this low RH% value, slipped out of the frequency window. Consequently, if we want to identify and characterize each relaxation process even at higher RH% values, the sample must be flash-cooled in order to freeze the actual hydration state of the LDH, and the DRS spectrum must be registered at that temperature. We have done exactly this: the samples were flashcooled to 155 K (in a confined environment like our case, this can be done without the crystallization of water35), and the DRS spectrum was registered. However, a further step was also taken, i.e., the samples were warmed up to 298 K in steps of 2.5 K and at each temperature the DRS spectra were registered. This means quite a large amount of plots; if they are placed in stack, evaluation would be nearly impossible. Therefore, four to five representative spectra were selected for display. The imaginary parts of the complex modulus versus frequency functions are depicted, since changes are the most spectacular there. Dielectric Measurements at 155 K. Figure 4 displays the imaginary part of the dielectric modulus (M″(ω)) as a function of frequency at 155 K for each sample.

Figure 4. The imaginary parts of the dielectric modulus vs frequency functions for the partially hydrated (298 K), then flash-cooled (to 155 K) CaFe-LDH samples from measurements at 155 K.

Now, for the samples prepared at the two low RH% values, the relaxation of the physisorbed water (P1) appeared in the frequency window, indeed. However, those of the other water forms slipped out of it at the low-frequency end. For the sample equilibrated under RH% of 75%, three relaxations could be observed. Two of them (P1 and P1*) are related; they both belong to physisorbed water. P1 possibly correspond to monolayer coverage and P1* to multilayers. D

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Figure 6. The dielectric modulus vs frequency functions registered at various temperatures warmed up from the flash-cooled (155 K) sample, which was obtained from the dehydrated CaFe-LDH allowed to equilibrate under RH% = 43% at 298 K.

Figure 5. The dielectric modulus vs frequency functions registered at various temperatures warmed up from the flash-cooled (155 K) sample, which was obtained from the dehydrated CaFe-LDH allowed to equilibrate under RH% = 11% at 298 K.

From the fitted functions, relaxation times for the various processes could be calculated, and their temperature dependence for the samples at different RH% values and processes are shown in Figure 8. All relaxation times follow Arrhenius behavior in the whole temperature range studied, thus the activation energies (Ea in kJ/mol) could be calculated from the slope (−0.434Ea/R) of the log10 τ vs 1/T fitted straight line. They are listed in Table 1. The activation energies allow some insight into the dynamics of water molecules in the variously hydrated samples with the rising temperature. In the sample equilibrated at RH% of 11%, physisorbed and interlayer water molecules have about the same mobility, and they may change place with each other. However, those water molecules that participated in the partial rehydroxylation of the layers are not mobile any more. The high activation energy indicates that the water molecules at the other two positions do not form an easily accessible reservoir for further hydroxylating the layers. When the equilibration occurred at RH% of 43%, the mobilities of the physisorbed and the interlayer water molecules are not the same, and the rearrangement of water molecules between these two positions are not an easy procedure any more. The activation energies for the interlayer water molecules and for the layer associated water molecules are similar, which may mean that interlayer water molecules may take part in

the relaxation of structural water comes into range, and at 255 K, the MWS polarization (P4) also appears. By this temperature, the relaxation of physisorbed water (P1) gets out of the measurement range completely. On reaching 282 K, even the relaxation of interlayer water (P2) has largely disappeared from the frequency window. At room temperature, one only gets a hint that this type of water actually exists.17 For the sample equilibrated at RH% of 75% (Figure 7), then flash-cooled to 155 K and allowed to warm up gradually, multilayers of physisorbed water and more than one type of interlayer water give 2−2 relaxations at 155 and 173 K, respectively. In this particular sample, there is plenty of water, therefore it is not surprising that more than one type of water molecule is found in these two positions. Thermal measurements have already indicated this feature, since the DTG signals assigned to physisorbed as well as to interlayer water molecules were always wide. At this high RH% value, these relaxations get out of our measurement range fast with the rise of temperature. By 240 K, only the signal of MWS polarization (P4), the relaxation of structural water (P3) and the slight indication for one type of interlayer water remain observable. At room temperature, the spectrum practically does not contain any useful structural information. E

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Figure 8. The temperature dependence of relaxation times for the various samples.

Table 1. Activation Energies (kJ/mol) of the Various Processes for Heat-Treated CaFe-LDH Samples Equilibrated at Various Relative Humidities (RH%), then Flash-Cooled to 155 K and Allowed to Warm up to Room Temperature Figure 7. Double logarithmic representation of the imaginary part of the complex electric modulus vs frequency functions registered at various temperatures warmed up from the flash-cooled (155 K) sample, which was obtained from the dehydrated CaFe-LDH allowed to equilibrate under RH% = 75% at 298 K.

RH%

P1*

P1

P2

P2*

P3

11% 43% 75%

− − 23

24 26 36

26 65 78

− − 64

89 58 42

was possible, except for rehydroxylation, which was a one-way process. Under high-humidity rehydrating conditions, DRS measurements, in accordance with earlier TG results, indicated multilayers of physisorbed and more than one type of interlayer water molecules.

completing the rehydroxylation of the layers and/or the interlayer water molecules and those closely associated with the layers form the same pool. In the sample equilibrated at RH% of 75%, the easy process is forming multilayers of physisorbed water molecules. Since under these circumstances water molecules are plentiful, various kinds are found in the interlayer space closely associated with the hydroxide ions of the layers and interacting with each other. As the temperature rises, some rearrangement in the interlayer region and the immediate vicinity of the rehydroxylated layers may be expected, but there is no need for the participation of physisorbed water molecules to take part in this procedure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financed by the OTKA NK106234 and the TÁ MOP 4.2.2.A-11/1/KONV-2012-0047 grants. V.B. gratefully acknowledges the support of the TÁ MOP 4.2.4.A/2-11-12012-0001 National Excellence Program. All these supports are highly appreciated.

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CONCLUSIONS Positions and role of water molecules were identified in the rehydration of heat-treated CaFe-LDH by FT-IR and dielectric relaxation spectroscopies. Three major procedures could be observed. They were the rehydroxylation of the layers, water accumulation in the interlayer space, and physisorption on the outer surface of the reforming LDH. They were not sequential but proceeded parallel, and rearrangement of the water types

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dx.doi.org/10.1021/la4035276 | Langmuir XXXX, XXX, XXX−XXX