Trimetallic Single-Atom Alloys: Active Site Synergy or Most Reactive Atom Dominance? The Case of RhPtCu(111)

Elucidating how different atomically dispersed dopant atoms interact and control reaction pathways is central to the rational design of trimetallic single-atom alloys (SAAs). Here, we investigate CH_{3 r}I decomposition on Cu(111), PtCu(111), RhCu(111), and RhPtCu(111) trimetallic SAAs in order to elucidate how coexisting dopant sites influence C-H activation, hydrogenation, and C-C coupling pathways. Temperature-programmed desorption measurements show distinct reactivity of the two bimetallic systems: PtCu(111) promotes C-C coupling to yield ethene and the methane produced originates mostly from C-H activation and subsequent hydrogenation of methyl groups. In contrast, RhCu(111) strongly suppresses C-C coupling chemistry reducing the ethene yield and produces methane mostly via hydrogenation by existing surface H. When Rh and Pt are codeposited to form a RhPtCu(111) SAA, the resulting reactivity and selectivity are not the sum of the two bimetallic SAAs. Instead, methane formation kinetics and product selectivity closely resemble bimetallic RhCu(111). Systematic variation of the Rh/(total Rh and Pt dopant) ratio shows that increasing Rh content progressively biases reactivity toward hydrogenation-dominated pathways while suppressing the C-C coupling pathway that leads to ethene formation. CO coadsorption experiments provide site-specific evidence for this behavior via selective blocking of Rh sites, which leads to a return to PtCu-like chemistry. Together, these results elucidate the reaction pathways on the RhPtCu(111) trimetallic SAA, showing that isolated Rh atoms act as dominant active sites that govern low-temperature reactivity and selectivity, thereby suppressing the C-C coupling facilitated by Pt sites at higher temperatures. This work demonstrates that RhPtCu(111) chemistry cannot be understood as linear combinations of their bimetallic counterparts, highlighting how one dopant atom can dominate the reaction pathways.