Reaction of CO oxidation on platinum, rhodium, a platinum-rhodium alloy, and a heterophase bimetallic platinum/rhodium surface

The reaction of CO oxidation on thin metal films of platinum, rhodium, and their alloy and on a heterophase bimetallic Pt/Rh surface that consisted of platinum particles of size 10-20 nm on the surface of rhodium was studied in the region of low reactant pressures (lower than 2 x 10(-5) mbar). At low temperatures (T < 200 degrees C), the activity of samples increased in the order Rh > Pt/Rh > Pt-Rh alloy > Pt. Above 200 degrees C, the rate of reaction on the heterophase Pt/Rh surface was almost twice as high as the sum of the rates of reaction on the individual metals; this fact is indicative of a synergistic effect. The nature of this effect is considered.