Coverage-dependent adsorption energy of carbon monoxide on a rhodium nanocluster

Metal–adsorbate nanoclusters serve as useful models to study elementary catalytic and gas-sensor processes. However, little is known about their structural, energetic, and spectroscopic properties as a function of adsorbate surface coverage and structure. Here, we perform a systematic study of the adsorption of carbon monoxide (CO) on a tetra-atomic rhodium cluster to understand the coverage- and structure-dependent adsorption energy of CO as a function of CO coverage and to provide deeper insight into the metacarbonyl bond on metal nanoclusters. The coverage-dependent adsorption energy trends are rationalized with a use of a theoretical model, molecular orbital energy diagrams, electron density difference plots, molecular electrostatic potential plots, and simulated infrared spectra. Our model demonstrates that a critical parameter that determines the coverage-dependent energetics of the adsorption of CO at low coverages is the polarization of metal–metal p-bonds during the effective charge transfer, occurring from the metal cluster to the 2p*2py and 2p*2px states of CO, which enhances the adsorption of CO vertical to the metal–metal bond. This configuration specific effect explains the negative coverage-dependent adsorption energy trend observed at low coverages on metal nanoclusters.