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The electrical properties of tin oxide varistors doped with CoO, Nb 2 O 5 , and Cr 2 O 3 , were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400°C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O 2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200°C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of Cr Sn -at the surface, which promotes the adsorption of the OЈ and OЉ species which are in turn proposed as being responsible for the barrier formation.

Pt-modified RuO2 was prepared by a sol−gel procedure on titanium substrates in the form of thin films of ∼2-μm thickness. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that these films actually consist of Pt nanoparticles dispersed in RuO2 and that neither metallic Ru nor Pt−Ru alloy are present on the surface. Electrodes with different Pt:Ru nominal compositions were prepared and their electrocatalytic activity for the oxidation of methanol was investigated by potential sweeps and chronoamperometry. The results obtained show an enhancement effect for methanol oxidation that can be interpreted as associated to the formation of hydrous oxides on the RuO2 surface.

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