Electrocatalysis beyond the metallic state: the key role of dynamic oxide layers in Pt- and Pd-based systems

Understanding the role of surface oxidation states in governing electrocatalytic activity is crucial for the rational design of efficient and durable catalysts based on scarce noble metals. In this work, we present a unified view of electrocatalysis across a series of Pt- and Pd-based systems, including intermetallic compounds (PtSn4 and PdSn4), nanostructured catalysts for alcohol electrooxidation, and oxide-promoted materials for the hydrogen evolution and oxidation reactions. A common feature emerging from these studies is that catalytic performance is governed by the dynamic evolution of the surface oxidation state rather than by the pristine electronic structure. In Pd-based catalysts for ethanol electrooxidation, the formation of PdO surface layers under operating potentials leads to activity losses, whereas their in situ reduction restores metallic active sites and enhances catalytic turnover. Likewise, the introduction of reducing agents or suitably reactive environments enables control over the Pd/PdO equilibrium, directly influencing catalytic activity, selectivity, and stability.[1–3] In parallel, oxide–metal interfaces play a crucial role in modulating catalytic pathways. In Pd–NiO and Pd–CeO2 systems, strong metal–support interactions alter the oxidation state of palladium and promote the formation of catalytically active Pd–OH species, thereby enhancing the kinectics of the hydrogen evolution and hydrogen oxidation reactions in alkaline media. These findings highlights the beneficial role of partially oxidized or hybrid metal–oxide interfaces in facilitating charge transfer and stabilizing key reaction intermediates.[4–8] Consistently with this picture, surface oxidation in PtSn4 and PdSn4 intermetallic compounds generates SnOₓ-rich surface layers that actively participate in catalysis, leading to enhanched hydrogen evolution activity and CO tolerance. Across all the systems, the active phase is therefore more appropriately described as a dynamic metal/oxide interface rather than as a static metallic surface.[9–11] Collectively, these findings establish surface redox chemistry as a unifying descriptor of electrocatalytic activity and provide a general framework for the rational design of advanced electrocatalysts based on controlled surface oxidation states and operando surface transformations.