Catalytic Efficiency of Iron(III) Oxides in Decomposition of Hydrogen Peroxide: Competition between the Surface Area and Crystallinity of Nanoparticles
2007
Hermanek, Martin | Zbořil, Radek | Medrik, Ivo | Pechousek, Jiri | Gregor, Cenek
Various iron(III) oxide catalysts were prepared by controlled decomposition of a narrow layer (ca. 1 mm) of iron(II) oxalate dihydrate, FeC₂O₄·2H₂O, in air at the minimum conversion temperature of 175 °C. This thermally induced solid-state process allows for simple synthesis of amorphous Fe₂O₃ nanoparticles and their controlled one-step crystallization to hematite (α-Fe₂O₃). 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 ⁵⁷Fe 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 m² g⁻¹ revealed the lowest catalytic efficiency, while the highest efficiency was achieved with the sample having a significantly lower surface area, 337 m² g⁻¹, exhibiting a prevailing content of crystalline α-Fe₂O₃ phase. The obtained rate constant, 26.4 × 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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