Desenvolvimentos de catalisadores de espinélios de cobalto-ferro para oxidação da água

Abstract

Nature efficiently converts solar energy into chemical energy through photosynthesis. Photosynthesis presents a viable solution to the global energy crisis, as it utilizes highly efficient catalysts for this conversion. In this way, it is possible to synthesize catalysts that perform the same function as those in photosynthesis for the oxidative splitting of water. In this work, four cobalt ferrite-based catalysts were synthesized by the modified chemical coprecipitation method and characterized by the following techniques: atomic absorption spectroscopy (AAS) , X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), thermogravimetric analysis (TG/DTA), and electrochemical analysis. Analyses regarding the SBET surface area and particle porosity were related using the Langmuir isotherm. The samples exhibited a surface area ranging from 21 to 148 m2 g -1 and pore sizes varying from 2.6 to 22 nm. The diffractograms of the samples were indexed as the ferrite (CoFe2O4 and FeCo2O4). The sample with the highest cobalt content showed (511) and (440) peaks corresponding to the Co3O4 oxide. The crystallite size ranged from 9 to 55 nm for all synthesized samples. The IR data showed characteristic bands of FeCo2O4 ferrites in the range of 500 to 900 cm-1 . The band at 650 cm-1 was attributed to the vibrations of the tetrahedral sites, corresponding to the MO bonds (M = Co2+), and the band at 518 cm-1 was attributed to the vibrations of the octahedral sites, corresponding to the MO bonds (M = Fe3+ or Co3+). The cobalt and iron ferrites, with diferente Fe3+ and Co2 ion ratios, were catalytically tested for water oxidation. The results obtained through chemical assays and with the Clark-type electrode showed that the ferrite with a molar ratio of [Co] / [Fe] = 2.0 has the highest turnover number (TOF = 346) and turnover frequency (TOF = 7.4 s-1 ). This material was heated at different temperatures to study its structural morphology. It was observed that the heating made the materials more crystalline and less active for water oxidation.