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Synthesis and properties of novel Ba(II)Fe(III) layered double hydroxides
Applied Clay Science 48 (2010) 214–217
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Synthesis and properties of novel Ba(II)Fe(III) layered double hydroxides Dávid Srankó a, Attila Pallagi a,b, Ernő Kuzmann c, Sophie E. Canton d, Monika Walczak d, András Sápi e, Ákos Kukovecz e, Zoltán Kónya e, Pál Sipos a,⁎, István Pálinkó b,⁎ a
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 Laboratory of Nuclear Chemistry, Eötvös Lóránd University, Pázmány Péter sétány 1/A, Budapest, H-1117, Hungary d Chemical Physics Department, Chemical Centre, Lund University, PO Box 124, Lund, SE-221 00, Sweden e Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, H-6720 Szeged, Hungary b c
a r t i c l e
i n f o
Article history: Accepted 13 November 2009 Available online 26 November 2009 Keywords: BaFe LDHs NaOH solutions with extreme base concentrations TG and DTG XRD XAS Mössbauer spectroscopy
a b s t r a c t Double hydroxides of Ba(II) and Fe(III) were prepared by the co-precipitation method. Co-precipitation was facilitated by applying highly alkaline, carbonate free NaOH solutions with varying base concentrations (2−20 M). The substances, thus obtained, were characterised by thermal methods, XRD spectra of samples treated at various temperatures, Mössbauer and X-ray absorption spectroscopies. It was found that in extremely concentrated base solutions (≥10 M) layered double hydroxides, most probably with intercalated OH− ions, were formed, indeed, while at low base concentration the Fe(III) ions were precipitated as various oxyhydroxides and the Ba(II) ions remained dissolved. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Compounds containing oxygen, barium and iron ions have been known for long. Already in 1953, Scholder gave a recipe for their preparation (Scholder, 1953): the iron constituent of these metaand ortoferrates was in an oxidation state of + 4. More than a decade later a paper was published from the same laboratory (Scholder and Schwochow, 1966) describing the synthesis of some double hydroxides (alkali and alkaline earth hydroxometalates), among them Ba(II)/Fe(III) hydroxides with Ba3[Fe(OH)6]2 stoichiometric composition. For the synthesis highly alkaline aqueous solution was needed containing as high as 33 m/m% NaOH (corresponding to cNaOH ≈ 10 M). It has been stated that the XRD spectrum was satisfactory but the spectrum itself was neither published nor further discussed. At significantly higher base concentrations (cNaOH ≈ 20 M) formation of Ba2[Fe(OH)7].1/2H2O was reported by the same authors (Brauer, 1965). We came across these works during our recent research project concerning the structure of Fe(III) ions in strongly alkaline aqueous solutions and the solid ferric hydroxo complex salts isolated from such solutions (Sipos et al., 2008). Among other iron-containing
compounds we tried to reproduce the synthesis of the double hydroxide described by Scholder and Schwochow and after accomplishing the task successfully, we have taken the XRD spectrum too. To our surprise, the diffractogram resembled the features of layered double hydroxides (LDHs). We were surprised for two reasons. One is that our literature search did not identify any description of LDH with Ba(II) and Fe(III). Although some patents were located (Miyata et al., 1971, Stamires and Jones, 2002; Eisgruber et al., 2002; Vierheilig, 2008) there was neither description nor characterisation of this specific material. The other reason was the accepted knowledge that LDHs are only formed when the ionic radii of the two metallic components are close to each other (Evans and Slade, 2006). Here, this is clearly not the case. The ionic radius of Ba(II) [1.49 Å for octahedral arrangement (Cotton and Wilkinson, 1988a)] is significantly larger than that of Fe(III) [0.69 or 0.79 Å for octahedral arrangement (Cotton and Wilkinson, 1988b). Therefore, we studied in detail the synthetic and structural aspects of Ba(II)Fe(III) double hydroxides. 2. Experimental 2.1. Materials used
⁎ Corresponding authors. Tel.: +36 62 544 288; fax: +36 62 544 200. E-mail addresses: [email protected] (P. Sipos), [email protected] (I. Pálinkó). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.11.028
Concentrated NaOH (∼ 20 M) stock solutions were prepared from Millipore MilliQ water and a.r. grade solid NaOH (Spectrum 3D) and their carbonate content was minimised as described previously (Sipos
D. Srankó et al. / Applied Clay Science 48 (2010) 214–217
et al., 2000). The NaOH solution was stored in an airtight, caustic resistant Pyrex bottle. Ba(ClO4)2, Fe(ClO4)3 and HClO4 solutions were made of solid Ba(ClO4)2·3H2O (Fluka, p.a. grade), Fe(ClO4)3·xH2O (Sigma-Aldrich, p.a. grade) and cc. HClO4 (ca. 30 m/m%, Merck, p.a. grade), respectively. The accurate Fe(III) concentration of the stock solution made of Fe(ClO4)3·xH2O was determined iodometrically. Double hydroxides were prepared via dropwise addition of the Ba(ClO4)2/Fe(ClO4)3/HClO4 solution to hot (ca. 80 °C), vigorously stirred and N2-blanketed NaOH solution, as described in the literature (Brauer, 1965). The relative decrease of the [OH−]T during the synthesis was less than 10%, therefore the pH of the solution can be considered constant in the course of the preparative process. The precipitates formed were rapidly filtered until air dry in a practically CO2-free atmosphere, with the aid of a caustic resistant vacuum filter unit (Nalgene) equipped with an appropriate membrane (Versapor, 0.45 μm). The solid material was washed with small quantities of pure and hot NaOH with concentration identical to that used during the synthesis. The moisture sensitive crystals were kept at room temperature in a desiccator over dry SiO2. 2.2. Characterisation Thermal analytical measurements were performed using a MOM Derivatograph Q-1500D instrument. The heating rate was 2 °C/min. Powder X-ray diffraction (XRD) patterns of the air-dried and heattreated solid samples were registered in the 2Θ = 3–70° range on Philips PW1710 instrument, using CuKα radiation. Reflection positions were determined via fitting a Gaussian function. Reflection positions were found to be reproducible within 0.05° (2Θ), therefore the uncertainty of the basal spacing was estimated as ±0.1 Å. 57 Fe Mössbauer spectra of the samples were recorded with conventional Mössbauer spectrometers (Wissel and Ranger) in transmission geometry at the temperature of liquid nitrogen (78 K). A 57Co/Rh γ-radiation source of 3.109 Bq activity was used. Fe–K-edge (7112 eV) X-ray absorption spectra (XAS) were measured on beamline I811 at the MAXlab facility, Lund, Sweden. Data were measured in the fluorescence mode for the crystalline samples. 3. Results and discussion 3.1. General considerations The XRD diagrams of LDHs (Drits and Bookin, 2001) consist of sharp reflections and the (003), (006) and (009) reflections are easily identified. During heating (Rives, 2001) they lose water in three steps (desorption from the outer surface, then desorption of the looselybound water from the interlayer space without changing the interlayer distance [d(003)], followed by the removal of strongly-bound (structural) water from the interlayer space with the decrease of d(003) culminating in the collapse of the layered structure). During heat treatment, above 140–180 °C, with rising temperature the interlayer distance [d(003)] gradually decreases, and the octahedral environment becomes more and more distorted. In order to prove decisively that our Ba(II)Fe(III) double hydroxides are layered materials indeed, the above-described properties were investigated. The so-called “memory effect” [the ability of extensively heat-treated and thus collapsed LDHs to regain their close to original structures, when they are allowed to stand in water vapour (Béres et al., 1997)] was not tried since our compounds were sensitive to moisture.
215
In the Ba(ClO4)2/Fe(ClO4)3/HClO4 (1 M or 0.75 M or 0.5 M:0.25 M:0.1 M) system, instantaneously a white to pale yellow precipitate appeared at [NaOH]T ≥10 M showing that the otherwise expected formation of Fe(III)-oxyhydroxides (FeOOH) did not take place. (Preliminary EDX measurements indicated that both ions were present in our samples with close to the ratio applied at the start of the synthesis.) At lower concentrations of the base (5 M and 2 M) formation of FeOOH was observed. The XRD patterns of the products (BaFe-LDH-XX, where XX represents [NaOH]T in M) are shown in Fig. 1. The XRD patterns of the white to pale yellow products obtained for BaFe-LDH-10 and BaFe-LDH-20 are characteristic of LDHs. The products obtained at 5 and 2 M base concentrations did not show such patterns. Thus, LDH formation in this system requires a sufficiently high [NaOH]T. At this very high base concentrations the solubility of Ba(OH)2 is relatively low. Fe(III) ions under these conditions are precipitated in the form of Fe(III)-hydroxo complexes consisting of [Fe(OH)6]3− structural units (Sipos et al., 2008). From the XRD patterns, the basal spacings of our LDHs varied between 11.2 and 11.4 Å. After aging (ca. 1 year at room temperature in a desiccator over SiO2) no significant decrease was seen, i.e., dehydration either did not occur or its effect on basal spacing was not significant. The basal spacing of Ba-Fe-LDH-10 was somewhat smaller than that obtained for Ba-Fe-LDH-20; on the basis of the available water for hydration during their formation, an opposite pattern would be expected. Upon changing the Ba(II) to Fe(III) ratio the XRD patterns remained similar (Fig. 2), and no systematic variation was seen in the d(003) basal spacings (11.1–11.4 Å). It is unusual, but sometimes observed (Mandal and Mayadevi, 2008) that the intensity of the (003) reflection was lower than that of the (006). A possible reason may be the substantial difference in the radii of the metal ions indicating that we are at the edge of conditions where LDH formation can be expected. Nevertheless, in spite of low intensities the (003) reflections were clearly visible. 3.3. Thermal behaviour The thermal behaviour of our substances was investigated in two ways: first, by thermogravimetry as usual, and second, via heattreating the freshly prepared air-dried samples at various temperatures for an hour. These temperatures were chosen according to the TG and DTG curves. TG and DTG traces indicated three dehydration steps (Fig. 3), just as it is expected for LDHs. In the 50−120 °C region water physically adsorbed on the outer surface of the particles was removed. In the 140−180 °C
3.2. Synthesis and the XRD patterns Several samples have been prepared by co-precipitation and varying two parameters. The Ba(II) to Fe(III) ratio was changed from 2:1 to 4:1 and the NaOH concentration was varied from 2 M to 20 M.
Fig. 1. XRD patterns of the freshly prepared, aged and air-dried products obtained at various [NaOH]T-s. The colour of the materials and the basal spacings for the LDHs obtained from solutions with [NaOH]T ≥10 M are also shown.
216
D. Srankó et al. / Applied Clay Science 48 (2010) 214–217
Fig. 2. XRD patterns of the freshly prepared and air-dried BaFe-LDH-20 with varying Ba(II) to Fe(III) ratios. The ratios and the basal spacings are indicated.
region part of the interlayer water was desorbed without changes of the basal spacing. Above this temperature structural dehydration took place decreasing the basal spacing, and finally leading to the collapse of the layered structure. In a second series of experiments samples were heated to a given temperature with the same heating rate. In Fig. 4 XRD patterns corresponding to such steps are seen with BaFe-LDH-10 as an example. The XRD patterns showed a close correspondence with the TG and DTG curves. The basal spacing decreased between 30 and 130 °C and between 180 and 200 °C. At 200 °C the (003) reflection almost completely disappeared.
3.4.
57
Fe Mössbauer spectroscopy and XAS measurements
Fig. 4. XRD patterns corresponding to calcination of BaFe-LDH-10 at different temperatures. The basal spacings are also indicated.
Further information on the local structure of Fe(III) in BaFe-LDH-10 and BaFe-LDH-20 was obtained from XAS measurements. Fig. 6 shows that the spectra of the two LDHs were identical revealing once again the same structure of Fe(III). Since XAS is a very sensitive technique to pinpoint differences in coordination, measurements were performed with heated BaFeLDH-10 (Fig. 7). No significant changes were seen in the pre-edge (XANES) part at low temperatures. However, above 130 °C the originally octahedral environment of the Fe(III) became distorted as indicated by the increasing preedge peak at ca. 7125 keV. Reaching 250 °C the coordination environment of Fe(III) turned to tetrahedral. In agreement with thermal and XRD measurements of the heated samples, this also indicated that the layer structure was decomposed at T ≥200 °C.
Mössbauer spectra recorded for BaFe-LDH-10 and BaFe-LDH-20 were practically identical consisting of a singlet at IS = 0.40 mm/s (Fig. 5). Thus, the local environments of Fe(III) were identical in the two samples. Scholder (Scholder and Schwochow, 1966; Brauer, 1965) suggested that Ba3[Fe(OH)6]2 was formed at cNaOH ≈ 10 M, while Ba2[Fe(OH)7].1/2H2O at cNaOH ≈ 20 M. This appears to be unlikely. Very similar singlet was recently published for Na3[Fe(OH)6]. xNaOH (Sipos et al., 2008), in which the Fe(III) central atoms are in a highly symmetrical octahedral coordination environment (i.e., in [Fe(OH)6]3−). Thus, it is highly likely that the Fe(III) ions are in octahedral environment in our BaFe-LDH-10 and BaFe-LDH-20, which is typical to the trivalent cations in LDHs (Evans and Slade, 2006).
Fig. 3. TG and DTG curves for BaFe-LDH-20 indicating the three major dehydration steps.
Fig. 5. 57Fe Mössbauer spectra of BaFe-LDHs obtained from solutions with [NaOH]T= 10 (a) and 20 M (b) measured at liquid N2 temperature.
D. Srankó et al. / Applied Clay Science 48 (2010) 214–217
217
11.4 Å. These novel materials were characterised by several methods. It is expected that this method of preparation is suitable for producing LDHs with divalent metal ions other than Ba(II). Acknowledgements This research was supported by grants from the Hungarian Science Foundation (OTKA K68135 and K67835) and by the TARI Grant from MaxLab (experiment No. I811-072). The supports are highly appreciated. References
Fig. 6. Fe–K-edge XANES spectra of the solid substances obtained from solutions with varying concentrations of NaOH.
Fig. 7. Fe–K-edge X-ray absorption spectra on BaFe-LDH-10 samples after calcination at various temperatures.
4. Conclusion Ba(II)Fe(III) double hydroxides were prepared by the co-precipitation method applying aqueous NaOH as precipitating agent with concentrations varying from 2 M to 20 M. At and above 10 M NaOH Ba(II)Fe(III) layered double hydroxides were formed with basal spacings of 11.2–
Béres, A., Pálinkó, I., Bertrand, J.-C., Nagy, J.B., Kiricsi, I., 1997. Dehydration–rehydration behaviour of layered double hydroxides: a study by X-ray diffractometry and MAS NMR spectroscopy. J. Mol. Struct. 410–411, 13–16. Brauer, G. (Ed.), 1965. 2nd ed. Handbook of Preparative Inorganic Chemistry, vol. 2. Academic Press, New York, p. 1688. London. Cotton, F.A., Wilkinson, G., 1988a. Advanced Inorganic Chemistry, 5th ed. Wiley, New York, p. 1387. Cotton, F.A., Wilkinson, G., 1988b. Advanced Inorganic Chemistry, 5th ed. Wiley, New York, p. 1388. Drits, V.A., Bookin, A.S., 2001. In: Rives, V. (Ed.), Layered Double Hydroxides: Present and Future. Nova Science Publishers, Inc, pp. 38–92. Ch. 2. Eisgruber, M., Ladebeck, J., Koy, J., Schieszling, H., Buckl, W., Ebert, H., 2002. Preparation of synthetic hydrotalcites. WO 2002-EP4352 20020418. Evans, D.G., Slade, R.C.T., 2006. The structural aspects of layered double hydroxides. Struct. Bond. 119, 1–87. Mandal, S., Mayadevi, S., 2008. Adsorption of fluoride ions by Zn–Al layered double hydroxides. Appl. Clay Sci. 40, 54–62. Miyata, S., Kumura, T., Shimada, M., 1971. Complex metal hydroxides. Ger. Offen. CODEN: GWXXBX DE 2061136 19710715 (German). Rives, V., 2001. In: Rives, V. (Ed.), Layered Double Hydroxides: Present and Future. Nova Science Publishers, Inc, pp. 115–137. Ch. 4. Scholder, R., 1953. Uber Verbindungen Anomaler Wertigkeit von Chrom, Manga, Eisen und Kobalt. Angew. Chem. 65 (1953), 240–241. Scholder, R., Schwochow, E.F., 1966. A new method for preparation of alkali and alkaline earth hydroxometallates. Angew. Chem. Int. Ed. Engl. 5, 1047. Sipos, P., Hefter, G.T., May, P.M., 2000. Carbonate removal from concentrated hydroxide solutions. The Analyst 129, 955–958. Sipos, P., Zeller, D., Kuzmann, E., Vértes, A., Homonnay, Z., Canton, S., Walczak, M., 2008. The structure of Fe(III) ions in strongly alkaline aqueous solutions from EXAFS and Mössbauer spectroscopy. Dalton Trans. 5603–5611. Stamires, D., Jones, W., 2002. Process for producing Al-containing non-Mg-anionic clay. EP20000960435 20000811. Vierheilig, A.A., 2008. Process for making, and use of, anionic clay materials. EP19980948188 19980915.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Synthesis and properties of novel Ba(II)Fe(III) layered double hydroxides Dávid Srankó a, Attila Pallagi a,b, Ernő Kuzmann c, Sophie E. Canton d, Monika Walczak d, András Sápi e, Ákos Kukovecz e, Zoltán Kónya e, Pál Sipos a,⁎, István Pálinkó b,⁎ a
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 Laboratory of Nuclear Chemistry, Eötvös Lóránd University, Pázmány Péter sétány 1/A, Budapest, H-1117, Hungary d Chemical Physics Department, Chemical Centre, Lund University, PO Box 124, Lund, SE-221 00, Sweden e Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, H-6720 Szeged, Hungary b c
a r t i c l e
i n f o
Article history: Accepted 13 November 2009 Available online 26 November 2009 Keywords: BaFe LDHs NaOH solutions with extreme base concentrations TG and DTG XRD XAS Mössbauer spectroscopy
a b s t r a c t Double hydroxides of Ba(II) and Fe(III) were prepared by the co-precipitation method. Co-precipitation was facilitated by applying highly alkaline, carbonate free NaOH solutions with varying base concentrations (2−20 M). The substances, thus obtained, were characterised by thermal methods, XRD spectra of samples treated at various temperatures, Mössbauer and X-ray absorption spectroscopies. It was found that in extremely concentrated base solutions (≥10 M) layered double hydroxides, most probably with intercalated OH− ions, were formed, indeed, while at low base concentration the Fe(III) ions were precipitated as various oxyhydroxides and the Ba(II) ions remained dissolved. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Compounds containing oxygen, barium and iron ions have been known for long. Already in 1953, Scholder gave a recipe for their preparation (Scholder, 1953): the iron constituent of these metaand ortoferrates was in an oxidation state of + 4. More than a decade later a paper was published from the same laboratory (Scholder and Schwochow, 1966) describing the synthesis of some double hydroxides (alkali and alkaline earth hydroxometalates), among them Ba(II)/Fe(III) hydroxides with Ba3[Fe(OH)6]2 stoichiometric composition. For the synthesis highly alkaline aqueous solution was needed containing as high as 33 m/m% NaOH (corresponding to cNaOH ≈ 10 M). It has been stated that the XRD spectrum was satisfactory but the spectrum itself was neither published nor further discussed. At significantly higher base concentrations (cNaOH ≈ 20 M) formation of Ba2[Fe(OH)7].1/2H2O was reported by the same authors (Brauer, 1965). We came across these works during our recent research project concerning the structure of Fe(III) ions in strongly alkaline aqueous solutions and the solid ferric hydroxo complex salts isolated from such solutions (Sipos et al., 2008). Among other iron-containing
compounds we tried to reproduce the synthesis of the double hydroxide described by Scholder and Schwochow and after accomplishing the task successfully, we have taken the XRD spectrum too. To our surprise, the diffractogram resembled the features of layered double hydroxides (LDHs). We were surprised for two reasons. One is that our literature search did not identify any description of LDH with Ba(II) and Fe(III). Although some patents were located (Miyata et al., 1971, Stamires and Jones, 2002; Eisgruber et al., 2002; Vierheilig, 2008) there was neither description nor characterisation of this specific material. The other reason was the accepted knowledge that LDHs are only formed when the ionic radii of the two metallic components are close to each other (Evans and Slade, 2006). Here, this is clearly not the case. The ionic radius of Ba(II) [1.49 Å for octahedral arrangement (Cotton and Wilkinson, 1988a)] is significantly larger than that of Fe(III) [0.69 or 0.79 Å for octahedral arrangement (Cotton and Wilkinson, 1988b). Therefore, we studied in detail the synthetic and structural aspects of Ba(II)Fe(III) double hydroxides. 2. Experimental 2.1. Materials used
⁎ Corresponding authors. Tel.: +36 62 544 288; fax: +36 62 544 200. E-mail addresses: [email protected] (P. Sipos), [email protected] (I. Pálinkó). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.11.028
Concentrated NaOH (∼ 20 M) stock solutions were prepared from Millipore MilliQ water and a.r. grade solid NaOH (Spectrum 3D) and their carbonate content was minimised as described previously (Sipos
D. Srankó et al. / Applied Clay Science 48 (2010) 214–217
et al., 2000). The NaOH solution was stored in an airtight, caustic resistant Pyrex bottle. Ba(ClO4)2, Fe(ClO4)3 and HClO4 solutions were made of solid Ba(ClO4)2·3H2O (Fluka, p.a. grade), Fe(ClO4)3·xH2O (Sigma-Aldrich, p.a. grade) and cc. HClO4 (ca. 30 m/m%, Merck, p.a. grade), respectively. The accurate Fe(III) concentration of the stock solution made of Fe(ClO4)3·xH2O was determined iodometrically. Double hydroxides were prepared via dropwise addition of the Ba(ClO4)2/Fe(ClO4)3/HClO4 solution to hot (ca. 80 °C), vigorously stirred and N2-blanketed NaOH solution, as described in the literature (Brauer, 1965). The relative decrease of the [OH−]T during the synthesis was less than 10%, therefore the pH of the solution can be considered constant in the course of the preparative process. The precipitates formed were rapidly filtered until air dry in a practically CO2-free atmosphere, with the aid of a caustic resistant vacuum filter unit (Nalgene) equipped with an appropriate membrane (Versapor, 0.45 μm). The solid material was washed with small quantities of pure and hot NaOH with concentration identical to that used during the synthesis. The moisture sensitive crystals were kept at room temperature in a desiccator over dry SiO2. 2.2. Characterisation Thermal analytical measurements were performed using a MOM Derivatograph Q-1500D instrument. The heating rate was 2 °C/min. Powder X-ray diffraction (XRD) patterns of the air-dried and heattreated solid samples were registered in the 2Θ = 3–70° range on Philips PW1710 instrument, using CuKα radiation. Reflection positions were determined via fitting a Gaussian function. Reflection positions were found to be reproducible within 0.05° (2Θ), therefore the uncertainty of the basal spacing was estimated as ±0.1 Å. 57 Fe Mössbauer spectra of the samples were recorded with conventional Mössbauer spectrometers (Wissel and Ranger) in transmission geometry at the temperature of liquid nitrogen (78 K). A 57Co/Rh γ-radiation source of 3.109 Bq activity was used. Fe–K-edge (7112 eV) X-ray absorption spectra (XAS) were measured on beamline I811 at the MAXlab facility, Lund, Sweden. Data were measured in the fluorescence mode for the crystalline samples. 3. Results and discussion 3.1. General considerations The XRD diagrams of LDHs (Drits and Bookin, 2001) consist of sharp reflections and the (003), (006) and (009) reflections are easily identified. During heating (Rives, 2001) they lose water in three steps (desorption from the outer surface, then desorption of the looselybound water from the interlayer space without changing the interlayer distance [d(003)], followed by the removal of strongly-bound (structural) water from the interlayer space with the decrease of d(003) culminating in the collapse of the layered structure). During heat treatment, above 140–180 °C, with rising temperature the interlayer distance [d(003)] gradually decreases, and the octahedral environment becomes more and more distorted. In order to prove decisively that our Ba(II)Fe(III) double hydroxides are layered materials indeed, the above-described properties were investigated. The so-called “memory effect” [the ability of extensively heat-treated and thus collapsed LDHs to regain their close to original structures, when they are allowed to stand in water vapour (Béres et al., 1997)] was not tried since our compounds were sensitive to moisture.
215
In the Ba(ClO4)2/Fe(ClO4)3/HClO4 (1 M or 0.75 M or 0.5 M:0.25 M:0.1 M) system, instantaneously a white to pale yellow precipitate appeared at [NaOH]T ≥10 M showing that the otherwise expected formation of Fe(III)-oxyhydroxides (FeOOH) did not take place. (Preliminary EDX measurements indicated that both ions were present in our samples with close to the ratio applied at the start of the synthesis.) At lower concentrations of the base (5 M and 2 M) formation of FeOOH was observed. The XRD patterns of the products (BaFe-LDH-XX, where XX represents [NaOH]T in M) are shown in Fig. 1. The XRD patterns of the white to pale yellow products obtained for BaFe-LDH-10 and BaFe-LDH-20 are characteristic of LDHs. The products obtained at 5 and 2 M base concentrations did not show such patterns. Thus, LDH formation in this system requires a sufficiently high [NaOH]T. At this very high base concentrations the solubility of Ba(OH)2 is relatively low. Fe(III) ions under these conditions are precipitated in the form of Fe(III)-hydroxo complexes consisting of [Fe(OH)6]3− structural units (Sipos et al., 2008). From the XRD patterns, the basal spacings of our LDHs varied between 11.2 and 11.4 Å. After aging (ca. 1 year at room temperature in a desiccator over SiO2) no significant decrease was seen, i.e., dehydration either did not occur or its effect on basal spacing was not significant. The basal spacing of Ba-Fe-LDH-10 was somewhat smaller than that obtained for Ba-Fe-LDH-20; on the basis of the available water for hydration during their formation, an opposite pattern would be expected. Upon changing the Ba(II) to Fe(III) ratio the XRD patterns remained similar (Fig. 2), and no systematic variation was seen in the d(003) basal spacings (11.1–11.4 Å). It is unusual, but sometimes observed (Mandal and Mayadevi, 2008) that the intensity of the (003) reflection was lower than that of the (006). A possible reason may be the substantial difference in the radii of the metal ions indicating that we are at the edge of conditions where LDH formation can be expected. Nevertheless, in spite of low intensities the (003) reflections were clearly visible. 3.3. Thermal behaviour The thermal behaviour of our substances was investigated in two ways: first, by thermogravimetry as usual, and second, via heattreating the freshly prepared air-dried samples at various temperatures for an hour. These temperatures were chosen according to the TG and DTG curves. TG and DTG traces indicated three dehydration steps (Fig. 3), just as it is expected for LDHs. In the 50−120 °C region water physically adsorbed on the outer surface of the particles was removed. In the 140−180 °C
3.2. Synthesis and the XRD patterns Several samples have been prepared by co-precipitation and varying two parameters. The Ba(II) to Fe(III) ratio was changed from 2:1 to 4:1 and the NaOH concentration was varied from 2 M to 20 M.
Fig. 1. XRD patterns of the freshly prepared, aged and air-dried products obtained at various [NaOH]T-s. The colour of the materials and the basal spacings for the LDHs obtained from solutions with [NaOH]T ≥10 M are also shown.
216
D. Srankó et al. / Applied Clay Science 48 (2010) 214–217
Fig. 2. XRD patterns of the freshly prepared and air-dried BaFe-LDH-20 with varying Ba(II) to Fe(III) ratios. The ratios and the basal spacings are indicated.
region part of the interlayer water was desorbed without changes of the basal spacing. Above this temperature structural dehydration took place decreasing the basal spacing, and finally leading to the collapse of the layered structure. In a second series of experiments samples were heated to a given temperature with the same heating rate. In Fig. 4 XRD patterns corresponding to such steps are seen with BaFe-LDH-10 as an example. The XRD patterns showed a close correspondence with the TG and DTG curves. The basal spacing decreased between 30 and 130 °C and between 180 and 200 °C. At 200 °C the (003) reflection almost completely disappeared.
3.4.
57
Fe Mössbauer spectroscopy and XAS measurements
Fig. 4. XRD patterns corresponding to calcination of BaFe-LDH-10 at different temperatures. The basal spacings are also indicated.
Further information on the local structure of Fe(III) in BaFe-LDH-10 and BaFe-LDH-20 was obtained from XAS measurements. Fig. 6 shows that the spectra of the two LDHs were identical revealing once again the same structure of Fe(III). Since XAS is a very sensitive technique to pinpoint differences in coordination, measurements were performed with heated BaFeLDH-10 (Fig. 7). No significant changes were seen in the pre-edge (XANES) part at low temperatures. However, above 130 °C the originally octahedral environment of the Fe(III) became distorted as indicated by the increasing preedge peak at ca. 7125 keV. Reaching 250 °C the coordination environment of Fe(III) turned to tetrahedral. In agreement with thermal and XRD measurements of the heated samples, this also indicated that the layer structure was decomposed at T ≥200 °C.
Mössbauer spectra recorded for BaFe-LDH-10 and BaFe-LDH-20 were practically identical consisting of a singlet at IS = 0.40 mm/s (Fig. 5). Thus, the local environments of Fe(III) were identical in the two samples. Scholder (Scholder and Schwochow, 1966; Brauer, 1965) suggested that Ba3[Fe(OH)6]2 was formed at cNaOH ≈ 10 M, while Ba2[Fe(OH)7].1/2H2O at cNaOH ≈ 20 M. This appears to be unlikely. Very similar singlet was recently published for Na3[Fe(OH)6]. xNaOH (Sipos et al., 2008), in which the Fe(III) central atoms are in a highly symmetrical octahedral coordination environment (i.e., in [Fe(OH)6]3−). Thus, it is highly likely that the Fe(III) ions are in octahedral environment in our BaFe-LDH-10 and BaFe-LDH-20, which is typical to the trivalent cations in LDHs (Evans and Slade, 2006).
Fig. 3. TG and DTG curves for BaFe-LDH-20 indicating the three major dehydration steps.
Fig. 5. 57Fe Mössbauer spectra of BaFe-LDHs obtained from solutions with [NaOH]T= 10 (a) and 20 M (b) measured at liquid N2 temperature.
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11.4 Å. These novel materials were characterised by several methods. It is expected that this method of preparation is suitable for producing LDHs with divalent metal ions other than Ba(II). Acknowledgements This research was supported by grants from the Hungarian Science Foundation (OTKA K68135 and K67835) and by the TARI Grant from MaxLab (experiment No. I811-072). The supports are highly appreciated. References
Fig. 6. Fe–K-edge XANES spectra of the solid substances obtained from solutions with varying concentrations of NaOH.
Fig. 7. Fe–K-edge X-ray absorption spectra on BaFe-LDH-10 samples after calcination at various temperatures.
4. Conclusion Ba(II)Fe(III) double hydroxides were prepared by the co-precipitation method applying aqueous NaOH as precipitating agent with concentrations varying from 2 M to 20 M. At and above 10 M NaOH Ba(II)Fe(III) layered double hydroxides were formed with basal spacings of 11.2–
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