Size-Dependence of the Adsorption Energy of CO on Pt Nanoparticles: Tracing Two Intersecting Trends by DFT Calculations

With density functional calculations, we studied the size dependence of adsorption properties of metal nanoparticles (NPs) via the example of CO as a probe on Pt_n clusters with n = 38-260 atoms. For the largest NPs considered, the calculated adsorption energies lie below the corresponding value for the (ideal) infinite surface Pt(111). For clusters of 38-116 atoms, we calculated a sharp increase of the adsorption energy with decreasing cluster size. These opposite trends meet in an intermediate size range, for clusters of about 200 atoms, yielding the lowest adsorption energies. These computational results suggest that a nanosized transition to a pronounced higher adsorption activity occurs for Pt NPs at notably larger nuclearities than for Pd NPs. We analyze the results by invoking the concept of generalized coordination numbers, adapted to the second-order level.