Preparation, Structure, and Surface Chemistry of Ni–Au Single Atom Alloys

Ni/Au is an alloy combination that while, immiscible in the bulk, exhibits a rich array of surface geometries that may offer improved catalytic properties. It has been demonstrated that the addition of small amounts of Au to Ni tempers its reactivity and reduces coking during the steam reforming of methane. Herein, we report the first successful preparation of dilute Ni–Au alloys (up to 0.04 ML) in which small amounts of Ni are deposited on, and alloyed into, Au(111) using physical vapor deposition. We find that the surface structure can be tuned during deposition via control of the substrate temperature. By adjusting the surface temperature in the 300–650 K range, we are able to produce first Ni islands, then mixtures of Ni islands and Ni–Au surface alloys, and finally, when above 550 K, predominantly island-free Ni–Au single atom alloys (SAAs). Low-temperature scanning tunneling microscopy (STM) combined with density functional theory calculations confirm that the Ni–Au SAAs formed at high temperature correspond to Ni atoms exchanged with surface Au atoms. Ni–Au SAAs form preferentially at the elbow regions of the Au(111) herringbone reconstruction, but at high coverage also appear over the whole surface. To investigate the adsorption properties of Ni–Au SAAs, we studied the adsorption and desorption of CO using STM which allowed us to determine at which atomic sites the CO adsorbs on these heterogeneous alloys. We find that small amounts of Ni in the form of single atoms increases the reactivity of the substrate by creating single Ni sites in the Au surface to which CO binds significantly more strongly than Au. These results serve as a guide in the design of surface architectures that combine Au’s weak binding and selective chemistry with localized, strong binding Ni atom sites that serve to increase reactivity.