Various iron(III) oxide catalysts were prepared by controlled decomposition of a narrow layer (ca. 1 mm) of iron(II) oxalate dihydrate, FeC(2)O(4).2H(2)O, in air at the minimum conversion temperature of 175 degrees C. This thermally induced solid-state process allows for simple synthesis of amorphous Fe(2)O(3) nanoparticles and their controlled one-step crystallization to hematite (alpha-Fe(2)O(3)). Thus, nanopowders differing in surface area and particle crystallinity can be produced depending on the reaction time. The phase composition of iron(III) oxides was monitored by XRD and (57)Fe Mössbauer spectroscopy including in-field measurements, providing information on the relative contents of amorphous and crystalline phases. The gradual changes in particle size and surface area accompanying crystallization were evaluated by HRTEM and BET analysis, respectively. The catalytic efficiency of the synthesized nanoparticles was tested by tracking the decomposition of hydrogen peroxide. The obtained kinetic data gave an unconventional nonmonotone dependence of the rate constant on the surface area of the samples. The amorphous nanopowder with the largest surface area of 401 m(2) g(-1) revealed the lowest catalytic efficiency, while the highest efficiency was achieved with the sample having a significantly lower surface area, 337 m(2) g(-1), exhibiting a prevailing content of crystalline alpha-Fe(2)O(3) phase. The obtained rate constant, 26.4 x 10(-3) min(-1) (g/L)(-1), is currently the highest value published. The observed rare catalytic phenomenon, where the particle crystallinity prevails over the surface area effects, is discussed with respect to other processes of heterogeneous catalysis.

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Controlled decomposition of FeC2O4·2H2O in air at 175°C allows the simple synthesis of amorphous Fe2O3 nanoparticles and their controlled one-step crystallization to α-Fe2O3 (hematite). Depending on the reaction time, nanopowders differing in surface area and particle crystallinity are prepared. The samples are characterized by XRD, HRTEM, SAED, and 57 Fe Moessbauer spectroscopy, and their catalytic activity in the decomposition of H2O2 is studied. The amorphous nanopowder with the largest surface area shows the lowest catalytic efficiency, while the highest efficiency is achieved with a sample having a significantly lower surface area, exhibiting a prevailing content of crystalline α-Fe2O3 phase. -(HERMANEK, M.; ZBORIL*, R.; MEDRIK, I.; PECHOUSEK, J.; GREGOR, C.; J.

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Thermal decomposition of iron(II) oxalate dihydrate, FeC2O4·2H2O, was studied under isothermal conditions in air by using of X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic measurements, and Mössbauer spectroscopy. Direct in situ monitoring of sample temperature was found to be a breaking point in understanding the decomposition mechanism resulting in different polymorphous product compositions which are simultaneously dependent on the powder layer thickness and reaction temperature. At a certain temperature, there exists a critical sample layer thickness (weight, m c), below which the optimal oxygen access into the whole powder volume is enabled and the reaction proceeds under truly isothermal conditions. In such a case hematite (α-Fe2O3) is the only crystalline decomposition product. Above m c, at worsened diffusion conditions, a dramatic time-limited increase in the sample temperature has been observed. The appearance of such an “exoeffect” is a consequence of the violent diffusion of air oxygen inside the sample volume which depends precisely on the layer thickness and set temperature. During this exothermal stage of the reaction process the changes in kinetics and mechanism of decomposition are evidenced by an abrupt fall in the oxalate content and by the formation of the vacant structure of maghemite (γ-Fe2O3). The generalization of these phenomena has been demonstrated by several examples of oxidative decompositions of other metal salts, where the analogous exoeffect has been detected and various polymorphous mixtures containing α-, β-, γ-, and/or amorphous Fe2O3 have been identified among the reaction products. Our findings explain unambiguously the literature discrepancies concerning the different phase compositions of samples prepared by the “isothermal” solid-state decomposition processes. What is more, when the key reaction parameters are precisely controlled, single-phase nanocrystalline products possessing superior properties can be obtained.