Thermal Stability of Solid Ferrates(VI): A Review - ACS Symposium

Chapter 8

Thermal Stability of Solid Ferrates(VI): A Review 1

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Downloaded by UNIV OF MICHIGAN ANN ARBOR on June 14, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch008

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Libor Machala , Radek Zboril , Virender K. Sharma *, Jan Filip , Oldrich Schneeweiss , János Madarász , Zoltan Homonnay , György Pokol , and Ria Yngard 4

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Nanomaterial Research Centre, Palacky University, Svobody 26, 771 46 Olomouc, Czech Republic Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901 Institute of Physics of Materials AS CR,Žižkova22, 61662 Brno, Czech Republic Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1521 Budapest, Hungary Laboratory of Nuclear Chemistry, Eötvös Lorand University, H-1117 Budapest, Pázmány P. s. 1/A, Budapest, Hungary *Corresponding author: [email protected] 2

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This review critically summarizes currently known results concerning the thermal decomposition of the most frequently used ferrate(VI) salts (K FeO , BaFeO , Cs FeO ). Parameters important in the thermal decomposition of solid ferrates(VI) include the initial purity of the sample, a presence of adsorbed and/or crystal water, reaction atmosphere and temperature, crystal linity, phase transitions, and secondary transformation of the decomposition products. The confirmation and identification of metastable phases formed during thermal treatment can be difficult using standard approach. The in-situ experimental approach is necessary in some cases to understand better the decomposition mechanism. Generally, solid ferrates(VI) were found to be unstable at temperatures above 200 °C as one-step reduction accompanied by oxygen evolution usually proceeds. The most known and used ferrate(VI) salt, potassium ferrate(VI) (K FeO ), 2

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

125 decomposes at high temperatures to potassium orthoferrate(III), (KFe0 ), and potassium oxides. The resulting phase composition of the sample heated in air can be affected by accompanying secondary reactions with the participation of C 0 and H 0 in air. However, the thermal decomposition of barium ferrate(VI) (BaFe0 ) is not sensitive to constituents of air and is mostly reduced to non-stoichiometric BaFeO (2.5 < x < 3) perovskite-like phases stable under ordinary conditions. Such phases contain iron atoms with oxidation state +4; exhibiting the main difference in the decomposition mechanisms of K Fe0 and BaFe0 . 2

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Introduction Iron, generally known in the +2 and +3 oxidation states, can also be obtained in higher oxidation states such as +4, +5, and +6 under in a strong oxidizing environment (1-4). In recent years, there has been an increasing interest in the +6 oxidation state of iron, ferrate(VI) (Fe 0 "), due to its potential use in high energy density rechargeable batteries, in cleaner ("greener") technology for organic synthesis, and in treatment of contaminants and toxins in water and wastewater (5-77). Chemical, electrochemical, and thermal techniques are usually applied to prepare solid ferrate(VI) salts (12-21). The synthesis of the ferrate(VI) salts by the chemical technique requires several synthesis steps and large amounts of chemicals (16,17). Electrochemical synthesis applies electrolysis of iron (or iron salt) in concentrated hydroxide solution followed by a separation step in order to obtain the solid K Fe0 product. The formation of passive iron oxide on the electrode reduces the ferrate(VI) yield (18). Possible reduction of ferrate(VI) in water (2Fe0 *+ 5H 0 — 2Fe + 3/20 + 10OH" (22)) lowers the product yield of K Fe0 . A dry technique is thus attractive as it can avoid difficulties associated with the wet techniques. Dry thermal techniques are relatively simple and are generally based on the reaction between iron(III) oxide and MO (M = Na, K, Cs,...; x = 1 or 2) under a stream of dried oxygen (12-15). However, the decomposition of ferrate(VI) occurs simultaneously at the elevated temperatures used in the thermal synthesis technique, which results in a usually less than 60 % yield of ferrate(VI). Prevention of the decomposition of ferrate(VI) might be accomplished by optimization of the temperature conditions which could lead to an increase in the ferrate(VI) yield. This would thus require a profound understanding of the mechanism of the thermal decomposition of ferrate(VI) salts. In the literature, the results obtained by different authors on thermal decomposition of ferrate(VI) salts are in disagreement, particularly regarding observation and identification of hypothetic intermediate oxidation states of iron, vl

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

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126 Fe(V) and Fe(IV), and observed mass loss in the thermal decomposition of ferrate(VI). This review provides a summary of the present knowledge with respect to the thermal decomposition of the mostfrequentlyused ferrates(VI) salts (K Fe0 , BaFe0 and Cs Fe0 ). A critical discussion of reasons for discrepancies in the results obtained by different authors is also given. 2

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Thermal Decomposition of K F e 0 in Air Downloaded by UNIV OF MICHIGAN ANN ARBOR on June 14, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch008

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Scholder (21) was the first who studied the thermal behavior of K Fe0 under an oxygen stream. The decomposition of ferrate resulted in a strong oxygen evolution between 200 and 350 °C. Microscopic images obtained from samples heated between 350 and 550 °C in this study showed a mixture of two crystalline phases of dark and light green particles. The light green phase was assumed to be potassium orthoferrite(III) (KFe0 ) while the darker phase was calculated to be a solid solution of K Fe0 and K F e 0 in a 1:2 molar ratio. The overall mean oxidation number of iron species was measured to be +4.4 and eq 1 was suggested to explain the decomposition process. 2

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5 K Fe0 — K Fe0 -2K Fe0 + 2 KFe0 + 2 0 2

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(1)

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Finally, the pure +3 oxidation state of iron in the form of KFe0 was obtained at 1000 °C after complete evaporation of K 0 . Ichida (23) applied Mossbauer spectroscopic and X-ray diffraction techniques to determine the decomposition products of K Fe0 in air. Heating the ferrate sample for about 90 days below 200 °C resulted in an X-ray amorphous Fe compound. In this process Fe ions were directly reduced to Fe and none of the intermediate valence states of iron, Fe or Fe , were observed during the decomposition process. Above 250 °C, potassium orthoferrite(III) was identified as the only crystalline compound and the decomposition process was described by chemical equation 2, 2

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K Fe0 -> KFeQ + K O + (2-x/2) 0 2

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where x stands for an uncertainty in the chemical form of a poorly crystalline potassium oxide. Fatu and Schiopescu (24) used simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) to investigate the thermal behavior of K Fe0 in air (Figure 1). A continuous 14.3% decrease of sample weight recorded between 50 and 320 °C (with a heating rate of 10 °C/min) was ascribed to the release of 3/4 moles of oxygen per one mole of decomposed K Fe0 (eq 3). 2

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2 K Fe0 — K O F e 0 + K 0 + 3/2 0 . 2

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(3)

127 However, the theoretical mass loss calculated using eq 3 is 12.1 % and is significantly smaller than that observed in TG experiments (Figure 1). A simultaneously obtained DTA curve displayed a narrow endothermic peak with the minimum at 620 °C, interpreted as a phase transition of Fe 0 by the authors. It is known however that polymorphous transformations of Fe 0 (e.g. maghemite to hematite) take place at considerably lower temperatures (< 500 °C) and exhibit an exothermic effect on the DTA (DSC) curve (25). 2

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T e m per s tare Figure 1. TG/DTG/DTA curves of K Fe0 simultaneously measured in air. Reproduced with permission from Fatu and Schiopescu (24). Copyright 1974. 2

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More recently, Machala et al. (26) reinvestigated the mechanism of the thermal decomposition of K Fe0 in static air using in-situ techniques, including thermal analysis (TG/DSC, 5 °C/min rate), high-temperature Mossbauer spectroscopy and variable temperature X-ray powder diffraction (VT XRD). This approach has the advantage that it allows direct monitoring of the phase composition during the thermally induced process including identification of reaction intermediates. In addition, secondary chemical transformations of the decomposition products due to the interaction with air humidity could be prevented. 2

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128 TG and DSC analyses performed under static air showed a thermal stability of K Fe0 up to 230 °C (Figure 2). The slight weight loss of 0.3 % below 230 °C can be ascribed to the release of adsorbed water, which was also observed in the DSC curve by an endothermic minimum at 80 °C. Between 230 and 280 °C, a weight loss of 8.0 % was observed, which suggests a release of oxygen. This weight loss is however significantly lower compared to the value reported by Fatu and Schiopescu (14.3 %) (24). The main decomposition step is related to an endothermic effect in the DSC curve with a minimum at 256 °C (see Figure 2b). This endo-effect is immediately followed by a broadened exo-effect with a maximum at 270 °C. Importantly, previous studies conducted in an inert atmosphere did not report such an exothermic peak (27,28). Between 280 and 750 °C, no significant change in the sample weight was recorded. Above 750 °C, the mass loss progressively proceeded due to melting and evaporation of the decomposition products. In-situ high temperature Mossbauer spectra were collected at four different temperatures (190, 300, 420 and 590 °C) to observe the transformation process


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Figure 2. TG (a) and DSC (b) curves of K Fe0 measured in static air. Reproducedfrom Machala et al. (26). 2

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129 of Fe(VI) and to identify possible iron-bearing conversion intermediates (Figure 2). The spectrum of the sample heated at 190 °C (see Figure 3) consisted of two components including a singlet (S = -1.01 mm/s) and a sextet with 5= 0.06 mm/s, SQ = 0.06 mm/s and B = 45.8 T. The singlet corresponded clearly to nontransformed K Fe0 . The isomer shift value of the latter sub spectrum, despite of the second-order Doppler shift, is much lower than expected for an octahedral high-spin iron(lll) compound (29). Based on the hyperfine parameters values, the sub spectrum was assigned to potassium iron(III) oxide, KFe0 , where Fe ions are tetrahedrally coordinated (26,30). The sextet of KFe0 represents the only one spectral component in the Mossbauer spectra measured at 300, 420, and 590 °C (Figure 3). Overall, Mossbauer spectroscopy revealed potassium iron(III) oxide, KFe0 , to be the only iron-bearing phase formed during the thermal decomposition of K Fe0 in air. Contrary to some earlier postulations (27), intermediates containing Fe(V) or Fe(IV) or other Fe(III) oxides (e.g. Fe 0 , FeO(OH)) were not identified during in-situ measurements. M

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VT XRD measurements on K Fe0 samples were also carried out (Figure 4) to identify the crystalline decomposition products at high temperatures. These measurements provide information on additional besides iron phases. The V T XRD spectra demonstrate that the K Fe0 incompletely transforms to potassium iron oxide (KFe0 ) upon heating at 190 °C (Figure 4). This process is in agreement with the Mossbauer measurements (see Figure 3). The decomposition of the ferrate(VI) into the KFe0 phase was completed at 300 °C, where new additional phases including monoclinic potassium carbonate (K C0 ) and potassium oxide (K 0), clearly appeared in the XRD pattern (Figure 4). However, KFe0 and hexagonal high-temperature K C 0 were the only phases detected in the XRD patterns recorded at 420 and 590 °C, without any indications of K 0 . The hexagonal high-temperature K C 0 structure that appeared at the expense of the monoclinic K C 0 indicates the thermally induced polymorphous transformation of potassium carbonate. Based on the described results from the in-situ measurements, a new model for the decomposition of K Fe0 in static air was suggested by the authors (26). It was postulated that the primary formation of the mixture of potassium oxide and super oxide together with KFe0 is followed by the rapid secondary reaction of carbon dioxide in air with K 0 (eqs 4 and 5): 2

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K Fe0 2

KFe0 + 1/3 K 0 + 1/3 K 0 + 1/2 0

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1/3 K 0 + 1/6 C 0 -> 1/6 K C 0 + 1/4 0 2

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This assumption is in agreement with the high affinity of K 0 to C 0 (31). Additionally, the presence of slightly overlapping endo- and exo-effects appearing in the DSC curve between 250 and 280 °C indicate a two-step formation mechanism of potassium carbonate (see Figure 2). This reflects the principal difference in the K Fe0 decomposition mechanisms performed in 2

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Figure 3. High temperature Mossbauer spectra exhibiting KFeO2 as the only Fe-bearing product of K Fe0 decomposition. Reproduced from Machala et al. (26).

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KFeO,

K,FeO,



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Figure 4. Variable temperature in-situ XRD patterns of decomposition products ofpotassium ferrate(VI) in static air. Unmarked peaks at 190 °C pattern correspond to non-decomposed potassium ferrate(VI). Reproduced from Machala et al (26).

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K C 0 (monoclinic)

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132 static air and inert atmospheres, where the latter displayed no secondary exoeffect in the DSC curve (see the next section). No other chemical reactions were observed at temperatures above 300 °C. However, the primary decomposition products undergo various phase transitions as demonstrated by VT-XRD data. Thus, potassium oxide, clearly identified at 300 °C, is absent in the XRD pattern, recorded at 420 °C, due to its melting (the melting point of K 0 is -350 °C). Similarly, the thermally induced polymorphous transition as observed in VT XRD patterns (30) changed the potassium carbonate structure at the higher temperatures from monoclinic to hexagonal. Above 750 °C, melting and evaporation of decomposition products occur as documented by a drastic decrease in the sample weight (see TG curve in Figure 2).


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Thermal Decomposition of K Fe0 in Inert Atmosphere 2

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Tsapin et al. (27) studied the thermal decomposition of K Fe0 under nitrogen atmosphere by TG/DSC. The thermogravimetric curve exhibited two main steps with an overall mass loss of 16.2 % (see Figure 5). Evidently, only the second step (125 - 230 °C) with 8% mass loss could be ascribed to the release of oxygen from the ferrate structure, while the first one (50 - 125 °C) was related to desorption of water from the sample surface. Such significant content of water in the initial potassium ferrate(VI) (7.2 wt%) could affect its decomposition as it readily reacts with water. As a result, a complex DSC curve was obtained reflecting a multi-step decomposition of the ferrate sample (see Figure 5). In recent work performed by Madarasz et al. (28), the thermal decomposition of solid K FeO 0.088 H 0 in an inert atmosphere (N or He inert purge gas, heating rate of 10 °C/min) was studied using simultaneous TG/DTA, in conjunction with in-situ analysis of the evolved gases by an online coupled mass spectrometer (EGA-MS). Two decomposition steps were observed in the TG curve up to 500 °C (see Figure 6). The first one below 100 °C corresponds to the evolution of water loosely adsorbed on the sample and the second step between 210 and 310 °C is related to the decomposition of K Fe0 accompanied by a release of oxygen gas as confirmed by EGA-MS. Both decomposition steps are reflected by two endothermic heat effects in the DTA curve (Figure 6). MSssbauer spectroscopic characterization performed on the decomposed sample indicated potassium orthoferrite(III), KFe0 , as the only iron containing compound, which is however metastable in air. The TG curve showed a mass loss of 6.8% occurred during the decomposition (Figure 6), which was ascribed to a mixture of potassium oxide, peroxide and super oxide. 2

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Stability of K Fe0 at Room Temperature - Sample Aging 2

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The stability of the K Fe0 was studied in detail by Nowik et al. (32). The phase composition of the samples sealed, exposed to air, or exposed to moist air 2

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-7.214%

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8.756%

295,81 C

DSC m« 10.00

Temp (C) &05.41C

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V i / 184.40C V 227.06C 427.62C

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Temp (C) Figure 5. TG and DSC curves ofpotassium ferrate(VI) measured in nitrogen atmosphere with heating rate of 5 °C/min. Reproduced with permission from Tsapin et al. (27). Copyright 2000.

was determined by Mossbauer spectroscopy as a function of time. Two sub spectra (Figure 7a) appeared in the Mossbauer spectrum of a K Fe0 , sample that was stored for 14 months in a closed, but not well-sealed container at room temperature (RT). The minor singlet component belongs to the original K Fe0 , while the major doublet sub spectrum shows hyperfine parameters typical for octahedrally coordinated high-spin Fe(III) atoms. To elucidate the nature of the trivalent iron component, low temperature Mossbauer spectra of the K Fe0 sample aged for 15 months were also measured (Figure 7b). While the Fe(VI) absorption sub spectrum does not exhibit any magnetic ordering down to 4.2 K, the evolution of the Fe(III) component with temperature shows the typical 2

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Thermal Stability of Solid Ferrates(VI): A Review - ACS Symposium

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