Selective synthesis of α-Fe2O3 thin films and effect of the deposition temperature and lattice oxygen on the catalytic combustion of propene

Abstract

Pulsed spray evaporation chemical vapour deposition (PSE-CVD), an elaborate CVD process, was employed to synthesize thin films of α-Fe₂O₃ for the catalytic combustion of propene. According to X-ray diffraction and Raman spectroscopy, the alpha structure type was presented as the unique phase. α-Fe₂O₃ with fine crystalline structure was revealed by scanning electron microscopy, and probed in depth for the first time by helium ion microscopy. Energy dispersive X-ray microscopy and X-ray photoelectron spectroscopy displayed an overview of the chemical composition of the samples. In situ emission FTIR spectroscopy was used for the accurate determination of the thermal stability of the samples at around 500 °C, and temperature-programmed reduction was performed to correlate the catalytic performance and reduction properties of the obtained α-Fe₂O₃ thin films. The results showed that the increase of the deposition temperature leads to significant changes of film morphology, chemical composition and reduction properties, with a direct consequence on the catalytic performance. α-Fe₂O₃ prepared at a low temperature (350 °C) exhibited high activity towards the deep oxidation of propene, which was attributed to its good reducibility and the plate-like structures. The alternate reduction and oxidation of the oxide surface (favored by the bulk oxygen migration towards the surface) and replenishment of bulk oxygen by gas-phase oxygen suggest that the oxidation of propene may proceed according to the Mars van Krevelen mechanism. The morphology and surface composition of the prepared samples remain the same before and after the catalytic test, demonstrating very good stability and reproducibility. We thus conclude that α-Fe₂O₃ effective in propene conversion can be selectively synthesized with PSE-CVD.