Bimetallic alloys play a vital role in heterogeneous catalysis, and it is now fairly well established that the local environment of an alloy can have a profound influence on its activity and selectivity. However, these effects can be difficult to decouple in nanoparticle studies given the inhomogeneity of the active sites, nanoparticle shape and different predominantly exposed surface facets. On the other hand, model studies using well defined single crystal surfaces alloyed with dopants can achieve fundamental understanding of the structure-activity relationship. The first step in this approach involves understanding the alloying mechanism. Herein, a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study was applied to understand the alloying behavior of Rh atoms in Cu(111) and Cu(100) surfaces. The STM results show a striking difference between Rh atoms alloying in Cu(111) versus the more open Cu(100) surface facets. Unlike RhCu(111) where Rh atoms tend to form dense brim above the step edges, homogeneously dispersed Rh atoms can be observed in the terrace of Cu(100). The stark different Rh distribution can be attributed to the competing alloying mechanism between the Cu(111) and Cu(100). DFT calculations show that direct atomic place exchange alloying mechanism prevails in Cu(100) while Rh atoms tend to hop to a preferred sites (like step edges) before alloying in the Cu(111) surface. These model systems will serve as useful platforms for examining structure sensitive chemistry on single-atom alloys.

Investigating the alloying mechanism of RhCu(100) and RhCu(110) for single atom alloy structure sensitivity study

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