Combining IR Spectroscopy and Monte Carlo Simulations to Identify CO Adsorption Sites on Bimetallic Alloys

The atomic distribution on the surface of alloys dictates the nature of the ensembles available as possible active sites during catalytic reactions. In the present work, an infrared spectroscopic study of carbon monoxide adsorption on the surface of AuPd/Pd(111) alloys, combined with Monte Carlo simulations of the surface and bulk atomic distribution, identifies the correct distribution of available surface adsorption sites. For gold coverages >0.9 monolayers (ML), CO adsorbs weakly on top of Au atoms and with higher adsorption energy on top of Pd atoms (COtop), distributed mostly as monomers on the surface. For theta(Au) = 0.8-0.4 ML, Pd-COtop is the predominant species, even though several other sites with multiple coordination are available. The simulations show no perfect ordering of the surface but a slight tendency to form lines of Pd atoms, thus wfavoring the appearance of bridge but not 3-fold hollow sites. Using (CO)-C-13:(CO)-C-12 isotopic mixtures, the frequency shifts due to chemical and intermolecular coupling effects has been determined for the COtop IR signal. These effects mostly cancel each other out, so that only small frequency shifts are seen, implying the presence of significant electronic/ligand effects. At theta(Au) < 0.5 ML, hollow sites are experimentally observed in agreement to the simulated model surfaces. Their IR absorption bands are tentatively distinguished as fcc and hcp hollow sites by correlating with the simulated distribution of Au and Pd atoms on subsurface sites, where for theta(Au) < 0.5 ML an enrichment by Au atoms is seen in the near-surface region.