engleski

Formation of crystalline phases by thermal treatment of amorphous rhodium hydrous oxide

Two methods were used to prepare amorphous rhodium hydrous oxide precursors: (a) precipitation from Rh(NO3)3 solution containing urotropin at 90°C and (b) precipitation from Rh(NO3)3 solution at a very high pH, as adjusted with NaOH. Nanocrystalline metallic rhodium was produced by heating at 300°C in air of the precipitate obtained by method (a) and additional ageing for 1 day and 3 days at 90°C. The crystallite sizes of metallic rhodium of 4 and 7 nm were measured. With a further increase in temperature the crystalline oxide phases such as a-Rh2O3, b-Rh2O3 and RhO2 appeared, and the phase composition depended on the time of hydrothermal ageing of amorphous precursor and the temperature of its thermal treatment. The precursors produced between 7 and 90 days of ageing of aqueous suspension at 90°C were amorphous after thermal treatment up to 500°C. The crystallite size of a-Rh2O3 and RhO2 produced at 600°C decreased from 6 to 3 nm with a prolonged ageing time in the suspension. An increased chemical reactivity was observed with a decreased content of water in the particles of amorphous precursor and with a decreased crystallite size of oxide phases observed at higher temperatures. The amorphous precursor produced by method (b) was amorphous up to 550°C, as found with XRD. After heating at 650°C, RhO2 and b-Rh2O3 were produced, whereas at 900°C a mixture of a-Rh2O3 and b-Rh2O3 was found. The crystallite sizes of the dominant oxide phases RhO2 and a-Rh2O3 were 21 and 45 nm, respectively. The presence of urotropin in the method (a) played a key role in the formation of nanocrystalline metallic rhodium.

rhodium hydrous oxide ; urotropin ; a-Rh2O3 ; b-Rh2O3 ; RhO2 ; Rh ; X-ray diffraction ; FT-IR ; TEM

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