Ferrates Synthesis, Properties, and Applications in Water and

Chapter 7

Higher Oxidation States of Iron in Solid State: Synthesis and Their Mössbauer Characterization 1

Downloaded by STANFORD UNIV GREEN LIBR on April 30, 2012 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch007

Y u r i i D . Perfiliev and Virender K. Sharma

2

1

Faculty of Chemistry, Moscow Lomonosov State University, Leninskii gory, Moscow 119992, Russia (email: [email protected]) Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, F L 32901 2

The solid state synthesis of iron ion in high oxidation states (≥+4) is briefly reviewed. Reactions in solid state are mainly considered through some unusual cases of obtaining iron ions with oxidation states of Fe(V), Fe(VI), Fe(VII), and Fe(VIII). The significance of the Mössbauer spectroscopic technique for identifying valence forms of elements in complex oxygencontaining compounds is emphasized. Sodium ferrate(IV), Na FeO , is highly hygroscopic and decomposes to Fe(VI) and Fe(III) upon contact with water. The basic Mössbauer parameter, isomer shift, 8, for alkali and alkaline earth metal ferrate(VI) changes in a very narrow range of -0.87 to -0.91 mm s (with respect to α-Fe). This suggests a weak influence of the outer ions on iron bound in an oxygen tetrahedron. The Mössbauer spectra of unknown oxoferrate ions with oxidation states of Fe and Fe are reported. The isomer shift, which is a fundamental parameter decreases with increasing valence state. This tendency persists for all possible high oxidation states of iron. 4

4

-1

5+

8+

Introduction There has been increasing interest in higher oxidation states of iron (Fe(IV), Fe(V), and Fe(VI)) because they are involved as an alternate for battery cathodes, green oxidants for organic synthesis, environmental-friendly oxidants 112

© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


113 in pollution remediation processes, and as intermediates in Fenton-type reactions and biological transfer processes (7-7). Investigations on the synthesis conditions, stability, reactivity, and physical chemical properties of iron compounds in higher oxidation states (over +3) are critical to understanding their properties and how they can be used in various applications. Of the higher oxidation states of iron, Fe(VI) has special interest because of its high oxidation power in removing pollutants in wastewater with the formation of nonhazardous products (7,2). Other higher oxidation states, Fe(V) and Fe(IV) are stronger oxidants in solutions than Fe(VI) (£-10). The oxidation states higher than +6, Fe(VII) and Fe(VIII), are also of interest and such iron compounds have been suggested (77). It is often difficult to identify the oxidation state of iron in compounds using chemical methods. However, the nuclear y-resonance method based on the Mossbauer effect can easily identify different valence states of iron in multiphase systems and complex compounds concurrently containing several differently charged ions (72). This is particularly true for iron oxo derivatives containing iron in high oxidation states. The basic parameter that suffices to identify the oxidation states of iron is the isomer shift, which tends to decrease with an increase in oxidation states below six (13). This trend may be extended to the possible oxidation states +7 and +8 (72). This is supported by analogy with the isomer shifts of ruthenium (14), the nearest iron homologue in the group (Figure 1).

o Solution

2.0

n

• MgO

1.5

o Fe(V)

1.0 -J x: c/>

E o

A Ru

0>

0.5 0.0 -0.5

o

-1.0 -1.5 -

O

-2.0 3

4

5

6

8

Oxidation state 57

Figure 1. Correlation between Fe isomer shift (relative to standard substance, a-Fe) and oxidation state of iron in oxo derivatives at 77 K (12). The isomer shift of Fe(V) in K Mn0 is taken from (27). The isomer shifts (relative to Ru in Rh) for ionic (with oxygen andfluorine) ruthenium compounds at 4 K (14). 3

4


114 Ruthenium shows the opposite trend in which the isomer shift increases with an increase in oxidation state (Figure 1). The opposite change is due to the positive AR/R (AR - the change of the nuclear radius upon excitation) ratio for the Ru nucleus. This ratio for Fe is negative. For simplicity, Figure 1 shows data for systems in which several forms of iron can coexist and the isomer shift of the iron line is little affected by the crystal lattice or by the character of chemical bonding. This can happen in emission Mossbauer experiments where the isomer shift of a "nucleogenic" Fe is measured. Thus the emission spectra of Co introduced into MgO (75) and frozen solutions containing different oxidation states of iron (16) were used. Other details are provided elsewhere (72). Different approaches have been applied to synthesize higher oxidation states of iron in the solid state. These approaches attempt to create physicochemical conditions favorable to the formation of a new species with higher-valent iron states than found in the starting material. Variable parameters are usually temperature, pressure, concentration, and the reactivity of the components. This paper briefly summarizes synthesis of oxo compounds of iron in oxidation states >+4. The Mossbauer parameters of these compounds are also discussed. 99

57

57


57

Iron(IV) Compounds Solid sodium metaferrate(IV) (Na Fe0 ) has been obtained by heating Na 0 and Fe 0 (the molar ratio Na : Fe = 2 : 1) in oxygen at 370 °C (17). Importantly, the trituration of mixture components with carbon tetrachloride was mandatory for preparing it. Its Mflssbauer spectrum gave a single line with the isomer shift -0.08 mm/s at 294 K, which was consistent with the shifts of Fe oxidation state (75). The effective magnetic moment of the iron ion at 293 K was determined to be 4.65 uB, which corresponds to the 3d configuration of the tetravalent high-spin iron ion. The black powder of sodium orthoferrate(IV)has been synthesized using solid-state reaction of sodium peroxide Na 0 and Fei_ O in a 4:1 molar ratio of Na to Fe at 400 °C (exposition 15 h) (18). Powder X-ray and neutron diffraction studies suggest that Na Fe0 is in the triclinic system P-l with the following cell parameters: 2

2

2

2

3

3

4+

4

2

4

a = 8.48(1) A b = 5.76(1) A c = 6.56(1) A

2

x

4

a = 124.7(1)°' p = 98.9 (3)° y = 101.8 (3)°

Na Fe0 is isotypic with other known solid phases of the form Na M0 (where M = Ti, Cr, Mn, Co, Ge, Sn, and Pb). The structure of Na Fe0 is characterized as a three-dimensional network of isolated Fe0 tetrahedra connected by Na 4

4

4

4

4

4


4

115 4

atoms. This compound exhibits Jahn-Teller distortions due to their high spin d configurations in the tetrahedral Fe0 coordination (18). Crystals of Na Fe0 are highly hygroscopic and dissociate to Fe and Fe 0 " in water (as shown in Eq. 1). 4

4

3+

VI

4

2

4

+

vl

2

3+

3 Na Fe0 + 8 H 0 -> 12 Na + Fe 0 " + 2 Fe + 16 OH 4

4

2

-

(1)

4

3+

The hydrolysis of Fe then gives amorphous Fe(OH) or FeO(OH). The Mossbauer spectrum of Na Fe0 gave one doublet (5 = -0.218(5) mm/s, A = 0.407(5) mm/s at 295 K (Figure 2). A sextet at 6 K was observed, which characterizes Fe(IV) in a high spin tetrahedral Fe0 coordination. Figure 2 shows an increase in isomer shift with decreasing temperature. The reaction of iron(III) oxide with potassium peroxide did not yield potassium ferrate(IV), presumably, due to the extremely favorable kinetics for the formation of ferrate(VI). However, potassium ferrate(IV) could be prepared upon thermal decomposition of K Fe0 at 500°C in dry oxygen, and its spectrum is a singlet with 8 = -0.16(2) mm/s and r e x p = 0.35(4) mm/s (19). The same procedure resulted in cesium ferrate(IV) with an indefinite chemical composition; this ferrate has the isomerl shift -0.11(2) mm/s, which is typical of Fe(IV) ions in an octahedral environment of oxygen atoms (20). Crystals of barium iron(IV) oxides, Ba Fe0 and Ba Fe0 were first prepared by dehydrating the barium iron(III) hydroxides with the appropriate Ba/Fe ratios at 700 °C under a dynamic atmospheric pressure of oxygen (21). A molten KOH-Ba(OH) flux synthesis has also been reported (22). The lowtemperature isomer shifts obtained were -0.152 and -0.142 mm/s (relative to o c iron) for Ba Fe0 and Ba Fe0 , respectively. These isomer shifts, together with magnetic susceptibility measurements, confirmed the valence state of +4 for iron in these compounds (22). Finally, efficient monitoring of iron oxidation states allowed improving their preparation procedures by a solid-phase synthesis and many iron salts, oxides, and different oxidants were tested. The reaction of Fe 0 with M O (M = Na, K, Cs) as the oxidant gave the best results. In all other cases, the total ferrate(IV) content did not exceed 25 mol % (17). 3

4

4


4

2

4

2

4>

3

5

2

2

4

3

5

2

3

x

Iron(V) Compounds Though, iron(V) is an uncommon oxidation state, the preparation of its compounds has been reported (23,24). The oxidation states of iron in these compounds were determined by chemical analysis and magnetic susceptibility measurements. Some workers thus do not rule out the possibility that these compounds are represented by a mixture of equal amounts of ferrate(IV) and ferrate(VI) (25). The structure of K Fe0 crystals has been recently studied (26), but Mossbauer spectroscopic characterization of this compound has not been carried out. 3

4



116

1 I I I I I I I I I I J I •6-5 -4 -3 -2 -1 0 1 2 3 4 5 6 1

1

1

1

1

1

1

1

1

1

1

1

Velocity mm/s Figure 2. Mossbauer spectra of Na Fe0 at different temperatures. (Reproducedfrom reference 18. Copyright 2002.) 4

4


117 The +5 oxidation state of iron was stabilized by introducing iron ion into potassium manganate(V) (K Mn0 ) (27). The K M n 0 compound was separately synthesized by the reaction of M n 0 with K 0 at 800 °C. The synthesized compound was amorphous to X-ray diffraction. Figure 3 shows the Mossbauer spectra of iron embedded in K M n 0 at room temperature (27). 3

4

3

2


3

• •1.6

i

4

3

2

4

i -0.8

i

i 0

i

i 0.8

i

Velocity, mm/s Figure 5. Mossbauer spectrum of K MnOj doped with iron at 294 K. 3

The room-temperature spectrum shows a single broadened line (5 =0.549(1) mm/s). Using Figure 1, this line was assigned to the pentavalent iron ion. As expected, this 5 value is intermediate between the 5 values for Fe (0.92 to -0.88 mm/s for K Fe0 (24)) and Fe (-0.13 to -0.08 mm/s for metal ferrates (IV) (13) and is close to the isomer shift -0.45 mm/s; assigned to the +5 iron ion in an octahedral position (28). However, 5 = -0.549(1) mm/s in Figure 2 is 0.14 mm/s lower than the isomer shift for octahedral +5 iron, which suggests tetrahedral coordination of the iron ion in K Mn0 . The isomer shifts of Fe or Fe ions occupying tetrahedral or octahedral positions in oxygen iron compounds differ by the same value (75). Recently, Fe iron in manganates was obtained by a similar procedure by using the initial molar composition [K0 ]:[Mn0 ]=8:l at 800 °C (29). The M5ssbauer parameters obtained were 5 = -0.538(2) mm/s and A = 1,067(10) mm/s, which are consistent with the +5 oxidation state of iron. The introduction of +5 valent iron has also been demonstrated in V 0 and K Ru0 (27). 6+

4+

2

4

3

2+

4

3+

5+

2

2

2

5

2

4

Iron(VI) Compounds A number of alkali and alkaline earth ferrates of iron(VI) have been synthesized using both dry and wet techniques. These techniques have been recently summarized (30). Dry techniques are generally carried out using a thermal technique whereas chemical and electrochemical procedures are applied


118 in wet techniques (30 ). The wet techniques are given in other chapters, hence are not reviewed here. Ferrate(VI) of the M Fe0 composition (where M = Na, K, Rb, Cs, and Ag), two alkali earth metal ferrates(VI) (SrFe0 and BaFe0 ), and two mixed cation ferrates(VI) (K Na(Fe0 ) and K Sr(Fe0 ) ) have been prepared successfully (30). When thermal techniques are used, a substantial yield of sodium ferrate(VI) (Na Fe0 ) can be reached only by using the multistage temperature program and a special prior treatment of the mixture of oxides (30). A 4:1 molar ratio of Na to Fe (370 °C, exposition >12 h), results in a nearly 100% yield of ferrate(VI). This compound was formulated as Na Fe0 based on the chemical analysis data (31). The main advantages of dry methods are high yields and a one-step process. The characteristics of ferrates(VI) using Mossbauer spectroscopy technique are given in Table 1 (32). In wet techniques, iron(VI) was generally produced by oxidizing a basic solution of Fe(III) salt by hypochlorite ion (38). However, the use of chlorine creates chlorinated by-products; hence it is not an environmentally friendly procedure. Moreover, there is also an emphasis on green chemistry; therefore, chlorine alternates are being sought. Recently (39), the formation of iron(VI) in ozonalysis of iron(III) in alkaline solution was demonstrated (eq 2). 2

4

4

3

2

4

2

4

4

2

4

4


2

5

2Fe(OHV + 30 + 20H' -> 2Fe0 3

2_ 4

+ 30 + 5H 0 2

(2)

2

The UV-visible spectrum of iron(VI) was compatible with a tetrahedral geometry of other high-valent metal oxoanions such as Cr0 " and Mn0 ". Furthermore, iron(VI) in the synthesis was confirmed by Mossbauer spectroscopic techniques (Figure 4). The 6 in Figure 4 is similar to values of 8 for various salts of the iron(VI) ion (see Table 1). Oxoferrates(VI) with molecular formula M F e 0 with M= Li, Na, (CH ) , N(CH ) BzI, and N(CH ) Ph were also synthesized using cation exchange reactions with K Fe0 , followed by freeze-drying of the resulting aqueous solution (40). Crystals of lithium ferrate(VI) (Li Fe0 ), which decompose at -10 ± 3 °C, were monohydrated. Recently, an Fe(VI)-nitrido complex was prepared photochemically (41). Interestingly, the 8 values of Fe0 " ion are considerably lower than those of the iron(VI)-nitrido complex ([(Me cy-as)FeN](PF ) ) (8 = 0.40 mm s") . This is not surprising because both have different geometries and electronic structures. The coordination number influences the isomer shift, as has been demonstrated clearly by comparing salts of the [FeCl ]' anion with [FeCl ] " trianion (42), in which the isomer shift of the latter is larger by 0.23 mm s". In case of +6 oxidation states of iron, the iron(VI)-nitrido has an octahedral coordination, while the iron(VI) ion has a coordination number of four. Another significant difference is that the iron(VI)-nitrido complex has one strong iron-nitrogen multiple bond, whereas the iron(VI) ion has four strong and covalent Fe=0 3

4

4

v,

2

3

3

3

2

4

3

3

4

2

4

2

4

1

3

6

2

3

4

6

1


4

119

Table 1. Mossbauer characteristics of ferrate(VI).


Formula

8 (mm s ) 298 K

K Na(Fe0 ) 3

4

K Fe0 2

4

Rb Fe0 2

Cs Fe0 2

4

4

K Sr(Fe0 ) 2

4

BaFe0

2

1

H(T)

-0.89

0.21

no magnetic ordering down to 4.2 K

-0.88 -0.90

0

14.4±2 at 2.8 K 14.7 at 0.15 K

-0.89

0

14.9±2at2.8K

-0.87

0

15.1+2 at 2.8K

-0.91° -0.90

0.14

8.7 at 2 K unresolved sextet

d

0.16

11.8±2at2.8K

a

2

N

e

e f

-0.90

4

T (K)

A (mm s") 298 K

e

4.2 3.6

f

e

e

2.8-4.2

e

4.2-6.0

6

C

~3

e

e

7.0-8.0'

-.90 (a)

The results from (32) are printed in bold; errors in these data do not exceed ±0.01 mm-s" ; Shinjo et al. (33); Ogasawara et al (34)\ ±0.02 mm-s" , Ladriere et al. (35); Herber and Johnson (36)\ Corson and Hoy (37). Table 1 demonstrates that isomer shifts, 5, of different iron compounds are for +6 valent state, which change very little (-0.87 to -0.91 mm/s). This indicates a weak influence of the outer ions on iron bound in an oxygen tetrahedron, which is the main structural unit ofallferrates(VI). 1

(e)

(b)

( c )

(d)

1

(f)


120


A, ° o

V, 111111/s Figure 4. Mossbauer spectrum of frozen solution of Fe(VI) in 5 MNaOH (77 K). (Reproducedfrom reference 39. Copyright 2007.)

double bonds. This may also influence the variation in the isomer shifts of two species.

Iron(VII) Compounds The existence of iron(VII) compounds is still not verified though the possibility of iron(VII) in iron-doped compounds of heptavalent elements has been suggested. Iron-doped sodium and cesium ruthenates were studied by Mossbauer spectroscopy (43). The possibility of substituting iron for ruthenium was proven by synthesizing iron-containing sodium ruthenate(VI). The valence state of iron in this compound was +6, which was expected from simple isomorphous miscibility considerations. The synthesis of cesium ruthenate(VII) gave a mixture of two phases, CsRu0 and Cs (Ru0 ) . In this composition, doped iron ions occurred in two forms. Spectrally, one of them was easily identified as the Fe(VI) state (8 = -0.76(1) mm/sat 77 K). The other form, manifested in the spectrum as a weaker line at the left slope of the major line of Fe , was characterized by the isomer shift -1.03(2) mm/s . This value identified this form to a higher oxidation state. 4

3

4

2

6+


121

Iron(VIII) Compounds


An alkaline solution containing octavalent iron, Fe(VIII), was prepared by anodic dissolution of iron metal in an alkaline medium (12,16). The Mossbauer spectrum of afrozensolution showed five lines of various intensities (Figure 5).

G O

& 0

1

- 2 - 1

0

1

2

Velocity, mm/s Figure 5. Mossbauer spectrum of a sample containing iron in different oxidation states. (Reproducedfrom reference 12. Copyright 2002 American Chemical Society.)

The line positions corresponded to velocities \ \ = -1.62, v = -0.82, v = -0.10, v = 0.18, and v = 0.92 mm/s. Peaks 4 and 5 had equal intensities in all experiments, whereas their contributions to the total resonance absorption could change considerably. Thus it was deduced that these peaks arise from the same oxidation state. That, along with the corresponding Mossbauer parameters, (5 = 0.56(4) mm/s, A = 0.71(4) mm/s), made it possible to assign peaks 4 and 5 to the doublet from Fe(III) ions. The positions of peaks 2 and 3 corresponded to Fe(VI) and Fe(IV). The remaining peak, whose intensity did not correlate with other absorption lines, was assigned to octavalent iron. The isomer shift of this peak, in contrast to the isomer shift of Fe(VII), fits the general trend of a decrease in the isomer shift with an increase in the iron oxidation state. The Fe(VIII) state is unstable. It should be pointed out that separate spectra recorded within the first hours after electrolysis were combined in the spectrum shown in Figure 5. 2

4

3

5


122 Acknowledgment We wish to thank Professor Zoltan Homonnay for useful comments.

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21.

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Ferrates Synthesis, Properties, and Applications in Water and

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