FULL PAPER DOI: 10.1002/ejic.200801068 Transformation of Solid Potassium Ferrate(VI) (K 2 FeO 4 ): Mechanism and Kinetic Effect of Air Humidity Libor Machala, [a] Radek Zboril,* [a] Virender K. Sharma, [b] Jan Filip, [a] Dalibor Jancik, [a] and Zoltan Homonnay [c] Keywords: Solid-state reactions / Iron / Reaction mechanisms / Environmental chemistry / Moessbauer spectroscopy The kinetics of solid-state transformation (aging) of potas- sium ferrate(VI) (K 2 FeO 4 ) under various air-humidity condi- tions (55–95 % relative humidity) at room temperature were studied by in-situ Mössbauer spectroscopy. The kinetic data showed a significant increase in the decomposition rate with increasing air humidity. The decomposition kinetics is very unusual with two almost linear decay steps. The first slow decay was observable at rather lower humidity levels (55– 70 %) probably due to the formation of the narrow compact layer of nanoparticulate Fe(OH) 3 reaction product. This layer limits the access of both H 2 O and CO 2 participating in the reaction as the gaseous reactants. The second decay with much faster rate showed a nearly positive linear relationship with the humidity. The identification and characterization of the final products of aging were conducted by using X-ray Introduction Iron(VI) compounds [ferrates(VI), FeO 4 2– ] represent an advanced class of compounds, which can be used in many promising electrochemical, environmental, and chemical applications such as high-energy-density rechargeable bat- teries [1–5] and cleaner (“greener”) technologies of organic syntheses, [6–8] “environmentally friendly” oxidant useful in innovative technologies for water treatment. [9–24] Among multitudes of other advantageous properties it is worth to emphasize the coagulation and disinfection ef- fects. [11,12,25–28] The wide range of applications induced a great effort to prepare ferrate(VI) of sufficient purity on a large scale. Up to now, several synthetic routes were developed including [a] Centre for Nanomaterial Research and Departments of Experi- mental Physics and Physical Chemistry, Palacky University in Olomouc, Svobody 26, 77146 Olomouc, Czech Republic Fax: +420-58-563-4958 E-mail: zboril@prfnw.upol.cz [b] Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA [c] Department of Nuclear Chemistry, Eötvös Loránd University, Pázmány P. s. 1/A, 1117 Budapest, Hungary Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2009, 1060–1067 1060 diffraction (XRD), variable-temperature and in-field Möss- bauer spectroscopy, magnetic measurements, thermogravim- etry (TG) and differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) techniques. The reac- tion mechanism, assuming formation of KHCO 3 and Fe(OH) 3 in the molar ratio of 2:1 per 1 mol of solid K 2 FeO 4 was sug- gested. The SEM images revealed the formation of large KHCO 3 crystallites whose surface was covered by ultrasmall X-ray amorphous iron(III) hydroxide nanoparticles in a high degree of agglomeration. The obtained results of aging un- der humid conditions are important for the possible storage of K 2 FeO 4 and thus for its environmental and industrial ap- plications. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) chemical, electrochemical, and thermal techniques. [29–35] Regardless of the synthetic route, ferrate(VI) salts are rela- tively stable over a long period of time if stored in a dry atmosphere. However, they become quite unstable if treated at high temperatures [36] and/or if exposed to air humidity even at room temperature. [37] The decomposition mechanism and stability of hexa- valent iron compounds have been investigated in solu- tions, [4,38–42] whereas the solid-state room-temperature transformation has been reported only by Nowik et al. [37] The authors studied the disintegrations of solid K 2 FeO 4 and BaFeO 4 samples sealed and exposed to dry and/or hu- mid air. Mössbauer spectroscopy was applied to identify the possible intermediate oxidation states (V, IV) of iron and to characterize the final iron-bearing products. [37] In the case of K 2 FeO 4 , the final formation of Fe 2 O 3 nanopar- ticles was suggested in both atmospheres, and no intermedi- ates containing iron in higher valence states were detected. The potassium-containing reaction product has not been determined at all. [37] The detailed analytical approach used in this paper pro- vides evidence of the formation of KHCO 3 and amorphous Fe(OH) 3 as the aging products (molar ratio of 2:1) of K 2 FeO 4 in humid air. This newly suggested reaction mecha- nism, assuming participation of the H 2 O and CO 2 gaseous reactants, fills a gap in basic inorganic chemistry of high