Understanding the effect of H₂O adsorption on reactant activation is of great importance in heterogeneous catalysis, which remains a grand challenge particularly in oxide catalyst systems with structural complexity. Herein, the effect of D₂O adsorption on D₂ activation over MgO nanocatalysts at different temperatures has been investigated by transmission Fourier transform infrared (FT-IR) and temperature-programmed desorption (TPD). Two sets of hydride and hydroxyl species produced from D₂ dissociation at more active and less active Mg-O pairs can be observed by FT-IR, which all desorb via the product of D₂ as confirmed by TPD experiments. We find that the physically adsorbed D₂O overlayer does not affect the dissociation of D₂ since D₂ may pass through the molecular layer and access the surface-active sites. When D₂O is partially dissociated on the MgO surface, D₂ can only dissociate at the remaining active sites until that dissociated -OD_w groups from D₂O occupy all active sites. These findings provide a fundamental understanding of the effect of water adsorption on D₂ activation on oxide catalysts.

Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors, which in turn determine physicochemical properties of the resulting films. Here, vapor-growth of tungsten carbide overlayers on W(110) surface is investigated by real time low energy electron microscopy. The surface growth is strongly confined by surface steps, which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces. Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film. Furthermore, the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures. These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth, which may be active in many film growth systems.