Size effect of RhPt bimetallic nanoparticles in catalytic activity of CO oxidation: Role of surface segregation

We show that catalytic activity of bimetallic Rh0.5Pt0.5 nanoparticle arrays under CO oxidation can be tuned by varying the size of nanoparticles. The tuning of size of RhPt nanoparticles was achieved by changing the concentration of rhodium and platinum precursors in one-step polyol synthesis. We obtained two dimensional Rh0.5Pt0.5 bimetallic nanoparticle arrays in size between 5.7 nm and 11 nm. CO oxidation was carried out on these two-dimensional nanoparticle arrays, revealing higher activity on the smaller nanoparticles compared to the bigger nanoparticles. X-ray photoelectron spectroscopy (XPS) results indicate the preferential surface segregation of Rh compared to Pt on the smaller nanoparticles, which is consistent with our thermodynamic analysis. Because the catalytic activity is associated with differences in the rates of O2 dissociative adsorption between Pt and Rh, we suppose that the surface segregation of Rh on the smaller bimetallic nanoparticles is responsible for the higher catalytic activity in CO oxidation. This result suggests a control mechanism of catalytic activity via synthetic approaches for colloid nanoparticles, with possible application in rational design of nanocatalysts.