A STM study of Ni-Rh bimetallic particles on reducible CeO2(111)

Abstract

Ni-Rh particles were prepared on well-ordered CeOx(111) thin films by physical vapor deposition method under ultrahigh vacuum conditions. CeOx(111) (1.5 < x < 2) thin films with controlled oxidation states were grown on Ru(0001) to examine the effect of support structures on the growth and sintering of Ni-Rh particles. Scanning tunneling microscopy studies reveal that pure Ni and Rh grow small particles with 0.3 ML coverage at room temperature on CeO2(111), suggesting a strong metal-ceria interaction. Smaller metal particles are formed on a reduced CeO1.75(111) surface. The degree of Ce reduction in ceria can also affect the electronic properties of deposited metals. Charge transfer from the metal to ceria was observed upon deposition of both Rh and Ni on CeO2. However, Ni and Rh remain a metallic state on CeO1.75. Bimetallic Ni-Rh particles were prepared by deposition of 0.3 ML Rh followed by 0.3 ML Ni on CeO2 at 300 K. A large fraction of Ni deposits on the pre-dosed Rh particles on CeO2 to produce bimetallic Ni-Rh particles. However, a significant amount Ni was observed to form additional pure Ni particles. The formation of Ni-Rh bimetallic particles can be promoted by increasing the particle density upon deposition of the first metal, which can act as the nucleation sites for the second metal. Ni-Rh bimetallic particles exhibit a significantly smaller size compared to that of pure Ni and Rh upon heating. Addition of Rh can inhibit the sintering of Ni and enhance its thermal stability on ceria. Our study has provided morphological and size information for the understanding of the activity of ceria-supported Ni-Rh catalysts.