Competing adsorption of H and CO on Pd-alloy surfaces: Mechanistic insight into the mitigating effect of Cu on CO poisoning

Journal article, 2026

Multi-component alloys offer broad tunability for materials science applications, but are highly sensitive to operating conditions through adsorption and segregation phenomena. Here, we study Pd-Au-Cu alloy surfaces in H2 and CO environments motivated by their use in H technologies, where alloying can mitigate limitations intrinsic to Pd such as CO poisoning. To this end, we establish a framework combining machine-learned interatomic potentials trained on density-functional theory data with cluster expansions, with effectively no limitations on training set size. By constructing continuous surface phase diagrams for H-CO coadsorption, we find that adsorption under operating conditions is governed by the H coverage during the preceding annealing. Au-rich surfaces, formed under H-poor conditions, suppress both CO and H adsorption, while H-rich conditions yield Pd-rich surfaces that promote CO and H adsorption. Furthermore, Pd-rich alloy surfaces maintain higher H coverages compared to pure Pd at relevant CO partial pressures, indicating improved CO poisoning resistance. The coadsorption behavior of PdAuCu alloys is indistinguishable from that of PdAu alloys, despite experimental observations of Cu, specifically, mitigating CO poisoning. However, kinetic barriers of H sorption in dilute alloys indicate that H pathways near Au are highly unfavorable, while Cu leaves the energetics unchanged compared to pure Pd. Based on these findings and the experimental evidence, we hypothesize that alloying promotes H adsorption while Cu, specifically, preserves viable H absorption pathways in the presence of CO.