Electronic State of Iron Complexes in the Interlayer of Hydrotalcite-Like Materials

Abstract

Abstract--lnfrared and Mrssbauer spectroscopic studies were made on hydrotalcite-like materials, MgrA12(OH)~6(NO3)2-4H20, anion exchanged in aqueous solution of K3Fe(CN)6, K4Fe(CN)6'3H20, or Na2Fe(CN)~NO-2H20. The material anion-exchanged in aqueous solution of K3Fe(CN)6 gave infrared (IR) absorption bands at 2120 and 2040 cm -~ in the C~-N stretching region, suggesting that part of the ferrate(III) complex was reduced to ferrate(II) complex on the intercalation chiefly because pure K3Fe(III)(CN)6 and KaFe(II)(CN)6-3H20 gives bands at 2120 and 2040 cm i, respectively. On the intercalation of Fe(CN)64-, no change in the oxidation state of iron was observed. These features were confirmed by M/~ssbauer spectroscopy. In the IR spectra of material anion-exchanged in aqueous solution of Na2Fe(CN)sNO' 2H20, the intensity of bands due to N=-O (1940 cm -l) and C-=N stretching was much less than that observed for Na2Fe(CN)sNO. 2H20, indicating that most of N~O ligand was eliminated during the intercalation. Four bands were observed in the C~N stretching region: a band at 2143 cm -I was assigned to C-= N groups in Fe(CN)~NO2-; a band at 2120 cm-~ was tentatively assigned to Fe(CN)~H2OZ-; bands at 2040-2050 cm I were assigned to Fe(CN)~H203-.

Synthesis and CO2 Adsorption Features of a Hydrotalcite-Like Compound of the Mg2+-Al3+-Fe(CN)64- System with High Layer-Charge Density

“…The band at 2040 + 4 cm-~ is the stretching vibration of u(CN) (Idemura et al, 1989), and bands at 1385 cm ~ and 798 cm ~ can be assigned to the stretching vibrations, v3 and v2, of NO3-ion, respectively (Nakamoto, 1986). The former is not shifted to a lower wavenumber.…”

Intercalation of iron hexacyano complexes in Zn,Al hydrotalcite. Part 2. A mid-infrared and Raman spectroscopic study

“…These results suggest that the samples undergo redox processes resulting in a mixture of hexacyanoferrate(II) and (III). Holgado et al [6] provided evidence for the partial reduction-oxidation of the iron in the interlayer hexacyano-complex by Fe-K XANES measurements, while Idemura et al [7] observed the reduction of Fe(III) to Fe(II) in Fe(CN) 6 3-intercalated LDH by Mössbauer spectroscopy. In addition to the shift observed due to oxidation or reduction of the iron, Crespo et al [3] pointed out that the precise position of the C≡N stretching frequency in hexacyanoferrate(II) is probably also influenced by the hydrogen bonding between the interlayer water molecules or the hydroxyl groups of the LDH sheets and the cyano groups.…”

“…This may cause changes in the interactions between the hexaferrate(II) and (III) anions resulting in the observed shifts in the IR and Raman spectra. In a comparable study Idemura et al [7] prepared for 84 % Fe(CN) 6 3-intercalated LDHs by anion exchange of a nitrate-LDH with potassium and sodium salt solutions under careful exclusion of carbonate. The IR spectrum showed two bands around 2120 and 2040 cm -1 indicating that a part of the Fe(III) was reduced to Fe(II) during intercalation.…”

“…In the particular case of hydrotalcites containing iron, it has been reported [2] that the properties of the final oxides depend on the original location of the iron cations, i. e., in the brucite-like layers, or as part of a complex anion in the interlayer. The ironcontaining anions most widely used to intercalate in hydrotalcites are hexacyanoferrates [1][2][3][4][5][6][7][8][9][10][11][12] and many efforts have been made to explain the precise nature of the interlayer hexacyanoferrate species, as oxidation/reduction processes from Fe(II) to Fe(III) and viceversa have been reported [6,12]. In the present paper we report a detailed mid-infrared and Raman study of Zn,Al hydrotalcites containing intercalated hexacyanoferrate (II) and (III), in order to gain insight in the processes taking place in the interlayer.…”

Intercalation of hydrotalcites with hexacyanoferrate(II) and (III)—a thermoRaman spectroscopic study

“…Further Idemura et al observed a reduction of the Fe in hexacyanoferrate(III) complexes by Mössbauer spectroscopy [45]. Crespo pointed out that the shift in position of the CN stretching wavenumber for the hexacyanoferrate(II) interlayered hydrotalcite was also influenced by the hydrogen bonding between the cyano groups and the interlayer water and also the hydroxyl surfaces of the brucite-like surface [12].…”