Why Single-Atom Alloys (SAAs) Outperform Their Individual Metal Components in Catalysis

Much industrial catalysis employs noble metals such as Pt, Pd, and Rh to facilitate the activation of key diatomic reactants such as H2 and O2, a requisite for the promotion of hydrogenation and oxidation reactions, respectively. However, these metals are often unselective for the conversion of organic feedstocks, so the addition of another, moderating metal is usually needed. The resulting bimetallics have gained prominence recently with an emphasis on so-called single-atom alloys (SAAs), in which the active component (the guest; Pt, Pd) is highly diluted within a matrix of a milder second metal (the host; Cu, Au, Ag). Empirically, SAAs have been shown to improve the selectivity of many catalytic reactions, but a full explanation of how this occurs is still lacking. A simple model has been advanced, based on "modern surface-science" studies using model surfaces under ultrahigh vacuum (UHV), where the guest atoms are isolated on the surface of the host and are presumed to not be significantly affected by it. They are proposed to promote the activation of the diatomic molecule (H2, O2), after which the resulting atoms (H, O) spill over onto the host surface where the rest of the catalytic conversion takes place. Unfortunately, this mechanism fails to explain certain observations from studies of SAAs under catalytic conditions, where the metals exist as nanoparticles (NPs) dispersed on high-surface-area supports, typically porous oxides, and where the surfaces are exposed to atmospheric pressures of the reactants. Our studies have focused on bridging these materials and pressure gaps, using the selective hydrogenation of unsaturated aldehydes with Pt-Cu alloys as a prototypical system. We have determined that, in the very dilute limit, the Pt atoms are located not on the exposed NP surface but rather at the interface between the metal NP and the oxide support, where they are inaccessible to the reactants. Furthermore, their effect on catalytic performance was found to be indirect, via the remote modification of the electronic properties of the surface Cu atoms where the reactions take place. Specifically, the activation barrier for molecular hydrogen dissociation at the Cu sites is decreased, thereby facilitating the production of the atomic hydrogen required for the hydrogenation of the organic adsorbates. Moreover, the added Pt modifies both the adsorption mode and the energetics of the reactants and products on Cu sites and the rate constants of the hydrogenation steps; like pure Cu once it is provided with atomic hydrogen, the Cu-Pt SAAs enhance the production of the desired products (unsaturated alcohols from unsaturated aldehydes), likely by favoring a unique reactant adsorption geometry through the oxygen atom. Our findings contradict the proposed independence of the two metals that constitutes the essence of the current working model for SAA and provide a new explanation of how SAAs function that may lead to better formulations of new bimetallic catalysts.