Structure, microstructure, and size dependent catalytic properties of nanostructured ruthenium dioxide

Nanostructured powders of ruthenium dioxide RuO2 were synthesized via a sol gel route involving acidic solutions with pH varying between 0.4 and 4.5. The RuO2 nanopowders were characterized by X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Rietveld refinement of mean crystal structure was performed on RuO2 nanopowders and crystallized standard RuO2 sample. Crystallite sizes measured from X-ray diffraction profiles and TEM analysis varied in the range of 4–10 nm, with a minimum of crystallite dimension for pH=1.5. A good agreement between crystallite sizes calculated from Williamson Hall approach of X-ray data and from direct TEM observations was obtained. The tetragonal crystal cell parameter (a) and cell volumes of nanostructured samples were characterized by values greater than the values of standard RuO2 sample. In addition, the [Ru–O6] oxygen octahedrons of rutile structure also depended on crystal size. Catalytic conversion of methane by these RuO2 nanostructured catalysts was studied as a function of pH, catalytic interaction time, air methane composition, and catalysis temperature, by the way of Fourier transform infrared (FTIR) spectroscopy coupled to homemade catalytic cell. The catalytic efficiency defined as FTIR absorption band intensities I(CO2) was maximum for sample prepared at pH=1.5, and mainly correlated to crystallite dimensions. No significant catalytic effect was observed from sintered RuO2 samples.