(25c) Adsorbate-Induced Surface Rearrangements in Au/Ni Near-Surface Alloys: A Density Functional Theory Investigation

Catalytic surfaces of metal and metal alloy particles are often considered as static and unchanging during reaction conditions due to the difficulty in ascertaining surface structures under reaction conditions and for simplifying the model of the catalyst surface. However, it is known that metal nanoparticles may rearrange to expose different Miller index planes under different reaction environments as compared to vacuum conditions. Compositional changes are also known to occur when surface adsorbates induce selective surface segregations of some components of a metal alloy.

Here we will discuss a third type of rearrangement of metal atoms on a catalytic surface – rearrangement of surface ensembles in a binary near-surface alloy (NSA). In the Au/Ni NSA system, alloying is restricted to the top layer of metal atoms. Periodic, self-consistent Density Functional Theory (DFT) calculations were used to calculate the thermodynamic binding energies of several surface intermediates involved in steam reforming, NO reduction, and direct H₂O₂ synthesis adsorbed on different arrangements of Au atoms embedded in the top layer of a Ni(111) surface. The transition states and reaction barriers for the rearrangement of the surfaces in the absence and presence of the adsorbates were calculated. While the clean surface is most stable when Au and Ni atoms are highly dispersed (at 0.33 ML Au coverage), with no three-fold Ni₃ hollow sites, strongly adsorbing intermediates such as O and C prefer surfaces where high coordination between adsorbate and Ni are possible. These same adsorbates also lower the activation energy for rearranging the metal atoms in the surface layer. The consequences of these arrangements for reactivity will be discussed in the context of direct synthesis of H₂O₂.