An element-selective study of the occupied and unoccupied density of electronic states in Pt nanoparticles was performed using hard X-ray resonant inelastic X-ray scattering (RIXS). An opening In the valence d band is observed when CO is adsorbed. The gap originates from bonding and antibonding orbitals between Pt and CO. The adsorption of CO blocks sites and changes the electronic structure, thus Further passivating the catalytic activity of Pt. The experimental results are Supported by full Multiple scattering calculations.