(175d) Electronic, Ensemble, and Coverage Effects in PdCu Alloys: From Segregated Bulk Systems to Dilute Surface Alloys Down to the Single Atom Limit

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 and clustering must be considered.

Based on density functional theory simulations and using the PdCu alloy system as example, I will discuss the various aspects that contribute to its catalytic performance in the Guerbet reaction, formic acid decomposition, and the competitive oxidation of CO and NO. Electronic effects enhance the activity of alloyed Pd, but at the expense of lower selectivity. Depending on the Pd ensemble size, the selectivity loss can be mitigated by selective poisoning with CO. Concurrently, alloying Cu with Pd elevates the d-band center of surface Cu atoms, thereby increasing its activity but without sacrificing selectivity. To address the stability question, we are extending the PdCu composition rage of our previously published hybrid stability model, which combines physical bond counting arguments with a machine learned error correction term. Collectively, 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.