The thermal behavior of amorphous rhodium hydrous oxide

Abstract

Amorphous rhodium hydrous oxide precursors, precipitated from an Rh(NO 3) 3 solution with the addition of NH 3, NaOH or urotropin, were thermally treated in the range 300–1000°C. The obtained amorphous rhodium hydrous oxide precursors and their thermal decomposition products were studied by X-ray powder diffraction, Fourier transform infrared spectroscopy, differential thermal analysis and thermogravimetric analysis. Generally, the thermal behavior of amorphous rhodium hydrous oxide precursors depended on several parameters, such as pH, aging time, temperature, type of atmosphere and the presence of urotropin or products of its chemical decomposition. The thermal behavior of the amorphous rhodium hydrous oxides obtained by the addition of NH 3·aq at pH∼10.4 differed in air or argon atmosphere. Heating in an argon atmosphere led to the partial autoreduction of the rhodium metal. After the calcination of the starting material at 500°C, two crystalline phases, α-Rh 2O 3 and RhO 2, were obtained. With a further increase in the temperature, RhO 2 was reduced. α-Rh 2O 3 was present as a single phase at 650°C in air, whereas β-Rh 2O 3 was present as a single phase at 1000°C. Although all starting rhodium hydrous oxide precursors obtained by the abrupt addition of an NaOH solution at pH∼9 were amorphous, the phase composition of their thermal decomposition products varied depending on the time of the hydrothermal reaction. The crystallization products at 400°C, produced from amorphous rhodium hydrous oxides aged at 90°C between 3 and 21 days were amorphous, whereas the precursor aged for 1 day at 90°C yielded α-Rh 2O 3 and RhO 2 at 400°C. At higher temperatures, RhO 2 decomposed irreversibly to α-Rh 2O 3, which was the single phase at 500 and 600°C. After calcination at 900°C, β-Rh 2O 3 additionally crystallized to α-Rh 2O 3. The appearance of crystalline oxide phases was shifted to higher temperatures with an increase in the time of the hydrothermal reaction at 90°C. During the thermal treatment of amorphous rhodium hydrous oxide prepared in the presence of urotropin, the explosion effect was observed. The formation of nanocrystalline rhodium metal was observed at 300°C after heating amorphous rhodium hydrous oxide precipitated for 1 day in the presence of urotropine. The crystallite size of the thus produced rhodium was 6(2) nm. After additional thermal treatment at up to 825°C, a mixture of β-Rh 2O 3 and α-Rh 2O 3 was obtained.