The Rhodium-Molybdenum Interaction in Rh/Mo/Alumina

Rhodium and molybdenum form mixed metal particles when supported on alumina. Results of chemisorption, IR, and SIMS measurements indicate that the composition of the surface of these particles is sensitive to the environment in which the measurements are made. Molybdenum appears concentrated in the surface when the atmosphere is nonreducing, while rhodium appears concentrated in the surface when hydrogen is adsorbed. Chemisorption and infrared results indicate that molybdenum reduces the surface available for the chemisorption of carbon monoxide and hydrogen. The band for the linear carbonyl is shifted 8-11 cm−1 higher when molybdenum is present, which may indicate oxidation of the rhodium by the molybdenum or increased dipolar interactions between carbon monoxide molecules adsorbed on the smaller surface. Chemisorption and infrared results also suggest that adsorbed hydrogen increases electronic density at the rhodium sites. Bands for both the gem dicarbonyl and the linear carbonyl are shifted 7-10cm−1 lower in either Rh/alumina or Rh/Mo/alumina when hydrogen is adsorbed.