MORPHOLOGY OF PT-RH ALLOY CRYSTALLITES ON AMORPHOUS SIO2

High-resolution scanning transmission electron microscopy is used to examine the structures of 20- to 200-Å-diameter Pt-Rh alloy particles on planar amorphous SiO2 substrates following treatment in N2 and air at one atmosphere and temperatures up to 1000 °C. Heating in N2 produces only alloy particles at all temperatures, while heating particles in air above 400 °C produces a rhodium oxide layer around each particle. As temperature is increased above ~600 °C the oxide appears to migrate onto the SiO2 substrate to form a thin oxide layer surrounding the metal core. The oxide is identified as Rh2O3 by electron diffraction, and dark field imaging confirms the morphology postulated. Pure Rh particles are completely oxidized by 600 °C and remain stable to at least 1000 °C. Continuous 10-Å-thick films of Rh or Pt-Rh cannot be broken up into particles below ~700 °C by heating in air because the oxide film is stable and adheres strongly to the SiO2. This alloy system is quite different than Pt-Pd in that the oxide forms as a thin Rh2O3 layer around each particle for Rh, while for Pd, a PdO crystallite nucleates at the side of the metal crystallites. Both oxides adhere strongly to SiO2, but the nucleation and growth of Rh2O3 is evidently much different than PdO. © 1979.