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.

Iron(II) oxalate dihydrate has been used as a readily decomposable substance for the controlled synthesis of nanosized iron(III) oxides. The polymorphous composition, particle size and surface area of these iron oxide nanoparticles were controlled by varying the reaction temperature between 185 and 500°C. As-prepared samples were characterized by XRD, low-temperature and in-field Mössbauer spectroscopy, BET surface area and the TEM technique. They were also tested as heterogeneous catalysts in hydrogen peroxide decomposition. At the selected temperatures, the formed nanomaterials did not contain any traces of amorphous phase, which is known to considerably reduce the catalytic efficiency of iron(III) oxide catalysts. As the thickness of the sample (≈ 2 mm) was above the critical value, a temporary temperature increase ("exo effect") was observed during all quasiisothermal decompositions studied, irrespective of the reaction temperature. Increasing the reaction temperature resulted in a shift of the exo effect towards shorter times and an increased content of maghemite phase. The maghemite content decreases above 350°C as a result of a thermally in-

Thermal decomposition of the FeC 2 O 4 .2H 2 O powder has been studied isothermally in static air conditions at minimum decomposition temperature of 180 °C using 57 Fe Mössbauer spectroscopy, XRD, HRTEM, AFM and BET surface area measurements. Dehydration and liberation of carbon oxides from powdered sample is accompanied by direct oxidation of Fe(II) to nanocrystalline Fe 2 O 3 without any indications of the stabilization of the magnetite (Fe 3 O 4 ) phase. Decomposition process is completed after two hours and as-prepared nanopowder (3-5 nm), with a large surface area of about 250 m 2 /g, is comprised of ultrafine superparamagnetic particles of γ-Fe 2 O 3 (maghemite) and α-Fe 2 O 3 (hematite) as proved by low temperature Mössbauer spectroscopy, XRD and HRTEM analysis. The simultaneous formation of both polymorphs is probably related with the non-equivalent diffusion conditions on the surface and in the bulk of oxalate particles. With increasing time, the particle size induced phase transformation of maghemite to hematite has been observed by AFM and XRD. Mössbauer spectra demonstrate that maghemite particles are formed only in superparamagnetic state (doublet), while magnetic components (sextet) in room temperature spectra can be assigned exclusively to hematite particles with size distribution produced by gradual crystallization of primary superparamagnetic nanoparticles of the same structure as well as by the phase transition of maghemite phase.

The thermal decomposition of ferric oxalate tetrahydrate Fe2(C2O4)3·4H2O was studied in dynamic oxidative and inert atmospheres by using a simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) analytical device equipped with an evolved gas analyzer (EGA). Solid‐state decomposition products formed during the decomposition were analyzed by 57Fe Mössbauer spectroscopy, in situ and ex situ X‐ray powder diffraction, and magnetic measurements. In the dynamic inert atmosphere, we observed the formation of a tiny amount of superparamagnetic iron oxide (most likely Fe3O4) together with a majority of ferrous oxalate (FeC2O4) and remains of undecomposed Fe2(C2O4)3 after the first decomposition step, which finished at 210 °C. The astonishing presence of the oxidic phase at such low temperatures is a highly probable side effect of the main reduction action of the electrons on the FeIII cations in the ferric oxalate structure, thus resulting in the creation of intermediate FeC2O4. The final product of decomposition of the FeC2O4 intermediate in a dynamic inert atmosphere is a mixture of wüstite (FexO), α‐iron (α‐Fe), and magnetite (Fe3O4). Their proportion accurately reflects actual disproportionation/synproportionation/redisproportionation processes likely encouraged by the preserved size and morphology of the initial ferric oxalate crystals and that are dependent on temperature. In the oxidative atmosphere, the decomposition proceeds in the three overlapped stages that include dehydration, the astonishing reductive formation of FeC2O4 as an intermediate, and final decarboxylation to hematite (α‐Fe2O3). The principal effect of the experimental conditions on the amount of intermediate formation of FeC2O4 in the oxidative atmosphere is also discussed and evaluated from the isothermal experiments carried out at 180 °C.