O₂ Activation by Metal Surfaces: Implications for Bonding and Reactivity on Heterogeneous Catalysts

The activation of O₂ on metal surfaces is a critical process for heterogeneous catalysis and materials oxidation. Fundamental studies of well-defined metal surfaces using a variety of techniques have given crucial insight into the mechanisms, energetics, and dynamics of O₂ adsorption and dissociation. Here, trends in the activation of O₂ on transition metal surfaces are discussed, and various O₂ adsorption states are described in terms of both electronic structure and geometry. The mechanism and dynamics of O₂ dissociation are also reviewed, including the importance of the spin transition. The reactivity of O₂ and O toward reactant molecules is also briefly discussed in the context of catalysis. The reactivity of a surface toward O₂ generally correlates with the adsorption strength of O, the tendency to oxidize, and the heat of formation of the oxide. Periodic trends can be rationalized in terms of attractive and repulsive interactions with the d-band, such that inert metals tend to feature a full d band that is low energy and has a large spatial overlap with adsorbate states. More open surfaces or undercoordinated defect sites can be much more reactive than close-packed surfaces. Reactions between O and other species tend to be more prevalent than reactions between O₂ and other species, particularly on more reactive surfaces

Hydrophilic Interaction Between Low-Coordinated Au and Water: H₂O/Au(310) Studied with TPD and XPS

In this work, we study the relatively weak H₂O–Au interaction on the highly stepped and anisotropic (310) surface with temperature-programmed desorption and X-ray photoelectron spectroscopy. Compared to Au(111), we report an enhanced adsorption energy of H₂O–Au­(310) as observed from the (sub)­monolayer desorption peak. This peak shows zero-order desorption kinetics, which we do not explain with a typical two-phase coexistence model but rather by desorption from the ends of one-dimensional structures. These could cover both the steps and (part of) the terraces. We do not observe crystallization of ice clusters as observed on Au(111). This leads to the conclusion that this stepped surface forms a hydrophilic template for H₂O adsorption. We also notice that the precise orientation of the steps determines the H₂O binding strength. Despite the surface’s enhanced H₂O interaction, we do not observe any significant H₂O dissociation. This indicates that the presence of low-coordinated Au atoms is not enough to explain the role of H₂O in Au catalysis

Oxygen-Promoted Methane Activation on Copper

The role of oxygen in the activation of C–H bonds in methane on clean and oxygen-precovered Cu(111) and Cu₂O­(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and “moderate pressures” was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C–H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole–dipole attraction of O–H and Cu–CH₃ species. The C–H bond activation barriers on Cu₂O­(111) surfaces are large due to the weak stabilization of H and CH₃ fragments