Monolayer bimetallic surfaces: Experimental and theoretical studies of trends in electronic and chemical properties

The current review focuses on the general trends in the electronic and chemical properties of monolayer bimetallic surfaces (MBS). The MBS is defined in this review as a monolayer of admetal deposited on a single crystal metal substrate. Depending on the deposition temperature and reaction environment, the admetal in MBS can be in one of the three structures: occupying the topmost surface sites to produce the surface monolayer, diffusing into the subsurface region to form the subsurface monolayer, and alloying with surface to produce intermixed bimetallic surfaces. The review will summarize recent experimental and theoretical studies of the surface monolayer, subsurface monolayer, and inter-mixed bimetallic structures to demonstrate their unique electronic and chemical properties. In the current review we will use one the most thoroughly studied bimetallic systems, surface and subsurface Ni monolayer on Pt(111), as a model system to introduce the synthesis, characterization, and DFT modeling of MBS structures. We will then utilize the adsorption of atomic hydrogen, atomic oxygen, and CO to demonstrate the correlation between the surface d-band center and the binding energies of both atomic and molecular adsorbates for various bimetallic surfaces. We will also address the issue of stability of the subsurface and surface MBS structures in vacuum, and in the presence of adsorbed hydrogen and oxygen. We will then use the hydrogenation of alkenes on subsurface MBS structures, and the reforming of oxygenates on surface MBS structures, to illustrate the trend between binding energies of adsorbates and chemical activities.