Variations of Alloying Site Density in Pd1Cu Single-Atom Alloy Catalysts Lead to Shifted Product Yields in Electrochemical CO Reduction.

Single-atom alloy (SAA) catalysis research often reports that a SAA catalyst, in the general formulation of a single-atom metal M1 alloyed on the surface of the host metal M2, facilitates a probe reaction. However, for catalytic reactions that present decoupled rate- and selectivity-limiting steps, the alloying site density may significantly manipulate these independent steps, but it has rarely been explicitly examined for any SAA systems. Herein, using the electrocatalytic CO reduction as a probe reaction, we report that the nominal Pd1Cu cube SAA catalysts exhibit distinctive high reactivity toward ethylene or ethanol, respectively, depending on whether the Pd atoms are in dilute or crowded forms. Although the presence of single-atom Pd embedded on Cu uniformly promotes CHO* formation and C─C coupling, the dilute-Pd1Cu favors ethylene formation by enabling low-barrier C─O cleavage from a flat CH2CH2OH* intermediate, whereas the crowded-Pd1Cu promotes ethanol formation by stabilizing an upright hydrogenation transition state of the same intermediate. Furthermore, we present evidence that the catalytic chemistry of crowded Pd1 species differs from that of the Pd2-dimer; the latter, albeit unstable, steers reaction selectivity to acetate instead. These results uncovered the underappreciated importance of controlling SAA catalytic chemistry from the perspective of single-atom site densities.