A TGA–FTIR perspective of fatty acid adsorbed on magnetite nanoparticles–Decomposition steps and magnetite reduction

The fatty acid stabilization of magnetite nanoparticles is important for a broad field of studies and applications. In numerous previous studies TGA analyses are applied to investigate these compounds and draw conclusions such as magnetite concentration and surface grafting densities of the chemisorbed molecules. There are however deviations in interpretation of the analysis results. In the presented work we contribute to the discussion on the inert gas decomposition of the fatty acid ricinoleic acid adsorbed on the surface of magnetite nanoparticles with a priori knowledge of magnetite concentration. We report on impacts of autoxidation of the fatty acid as well as significant reduction of magnetite from carbonaceous residues. The findings are based on subsequent gas analysis with FTIR coupled to the TG device. We show how stoichiometric calculations on the reduction in the temperature range of 600–900 °C let conclude that the residues are most probably from the chemisorbed fatty acid molecules. Only the physically adsorbed fatty acid molecules have decomposed or detached before 600 °C. In context to the investigations on chemically adsorbed fatty acid on magnetite we compare the decomposition of pristine fatty acid and fatty acid physically adsorbed on a high surface area SiO2 nanopowder. Three distinct steps of decomposition which have often been reported before are found and accounted.

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► Fatty acid adsorbed on Fe3O4 nanoparticles decomposes in three steps in inert gas up to 900 °C.
► Fe3O4 nanoparticles are reduced to iron α-Fe and wüstite FeO by chemisorbed fatty acid residues.
► Three step decomposition and no reduction for physically adsorbed fatty acids on high surface SiO2.
► Autoxidation of fatty acids is an issue leading to volatiles such as aldehydes as products.
► Reduction of Fe3O4 by carbonaceous species is used to calculate chemical adsorbate concentration.