The catalytic properties of bulk alloys by and large interpolate between their constituent metal components. In contrast, single atom alloys composed of a single active promoter atom within a less reactive host have been shown to escape ubiquitous scaling constraints. Between the bulk alloys and the single atom limit lies a vast and largely unexplored space of intermetallic compounds and dilute surface alloys. Their distinguishing feature is the presence of small ensembles of reactive promoters. Depending on the size and arrangement of these ensembles, their catalytic properties vary between the single atom and the bulk case, allowing room for optimization. While this idea is intriguing, the stability of small ensembles with respect to segregation, large cluster or island formation must be taken into consideration.<br/>Herein, the catalytic performance of dilute surface alloys for competitive CO and NO oxidation and the dehydrogenation of formic acid will be discussed using the Pd/Cu system as example. Density functional theory simulations suggest that dilute Pd surface sites are indeed more reactive than bulk Pd, but associated with a loss in selectivity attributed to Pd affinity to carbon. Depending on the Pd ensemble size, the selectivity loss can be mitigated by selective poisoning of these sites with CO. Concurrently, the presence of Pd in the surface elevates the d-band center of the Cu host, thereby increasing its activity but without sacrificing selectivity. To address the stability question, we have built a computational database of 28x28 dilute surface alloys and have proposed an extendable hybrid model combining physical bond counting arguments with a machine learned error correction term. Together, our results show an exciting avenue to take the concept of isolated atom geometries one step further and enable the design of stable ensembles with unique catalytic properties.

Ensemble effects in metal alloys: From segregated bulk systems to dilute surface alloys down to the single atom limit

ACS Fall 2021

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