(252b) Promoting Alcohol Oxidation over Au Catalysts Via Dilute-Limit Alloying with Rh

Alcohol oxidation enables the conversion of biomass-derived oxygenates into valuable platform chemicals. Au is well-known to be a selective metal for such oxidations, though its behaviors can be sensitive to alloying with highly-active dopant metals such as Pd, Pt, Ni, and Rh. A subclass of these alloys termed single-atom alloys (SAAs) possesses dopant atoms which are fully isolated from one another, creating unique electronic and geometric environments that break traditional scaling relationships and limit over-binding. This study focuses on doping with Rh—a particularly strong-binding promotor—to better understand how C–H, O–H, and C–O bond activations can be manipulated in selective oxidations over Au and how sensitive these behaviors are to bulk and local Rh compositions.

Rh₁Au_x (x = 5–50) nanoparticles were synthesized via sequential reduction. TEM and EDS confirmed uniform particles (7–9 nm) with Rh enriched locally in Au, and CO-DRIFTS showed Rh remains mostly isolated within Au. Catalytic studies reveal that Rh₁Au_x promotes ethanol oxidation but with different selectivity than previously observed Pd₁Au_x systems. While Pd favors oxidative coupling to yield ethyl acetate, increasing Rh content shifts selectivity toward acetaldehyde. This is attributed to differences in ethoxy stabilization versus decomposition: Pd promotes coupling by stabilizing ethoxy species, allowing for facile coupling with acetaldehyde, while Rh stabilizes ethoxy species to a greater extent and triggers comparatively rapid C-H activation, leading to acetaldehyde formation and low ethoxy surface coverages. Acetaldehyde selectivity and oxidation activity increased with Rh content up to a 1:10 Rh:Au ratio, beyond which phase segregation occurred, consistent with Rh–Au immiscibility in bulk. These findings show that Rh doping in SAAs enables tunable control over product distributions, offering design strategies for optimizing activity and selectivity in partial oxidation reactions.