RESUMO
Oxygen atoms on transition metal surfaces are highly mobile under the demanding pressures and temperatures typically employed for heterogeneously catalyzed oxidation reactions. This mobility allows for rapid surface diffusion of oxygen atoms, as well as absorption into the subsurface and reemergence to the surface, resulting in variable reactivity. Subsurface oxygen atoms play a unique role in the chemistry of oxidized metal catalysts, yet little is known about how subsurface oxygen is formed or returns to the surface. Furthermore, if oxygen diffusion between the surface and subsurface is mediated by defects, there will be localized changes in the surface chemistry due to the elevated oxygen concentration near the emergence sites. We observed that oxygen atoms emerge preferentially along the boundary between surface phases and that subsurface oxygen is depleted before the surface oxide decomposes.
RESUMO
Niobium superconducting radio frequency (SRF) cavities enable the operation of modern superconducting accelerator facilities. These cavities do not approach the theoretical performance limits of Nb due to the deleterious effects of surface defects and chemical inhomogeneities such as Nb hydrides. Nitrogen doping is known to consistently increase the cavity performance and inhibit Nb hydride growth, but a comprehensive understanding of Nb hydride growth and suppression is not yet realized. Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations presented herein elucidate the real-time, nanoscale structural and electronic evolution of undoped, hydrogen doped, and hydrogen and nitrogen doped Nb(100) due to the growth and suppression of Nb nano-hydrides. DFT calculations in agreement with the experimental data found unique near-surface phases stabilized upon dopant incorporation. The experimental STM and STS results and DFT calculations reported herein provide the first in situ and real-time nanoscale visualization and characterization of the effects of nitrogen doping on Nb hydride suppression and growth. Such information allows for further optimization of nitrogen doping procedures and advances in the performance of SRF materials for next-generation SRF-based accelerators and free electron lasers.
RESUMO
Subsurface oxygen is known to form in transition metals, and is thought to be an important aspect of their ability to catalyze chemical reactions. The formation of subsurface oxygen is not, however, equivalent across all catalytically relevant metals. As a result, it is difficult to predict the stability and ease of the formation of subsurface oxygen in metals, as well as how the absorbed oxygen affects the chemical and physical properties of the metal. In comparing how a stepped platinum surface, Pt(5 5 3), responds to exposure to gas-phase oxygen atoms under ultra-high vacuum conditions to planar Rh(1 1 1), we are able to determine what role, if any, steps have on the capacity of a metal for subsurface oxygen formation. Despite the presence of regular defects, we found that only surface-bound oxygen formed on Pt(5 5 3). Alternatively, on the Rh(1 1 1) surface, oxygen readily absorbed into the selvedge of the metal. These results suggest that defects alone are insufficient for the formation of subsurface oxygen, and the ability of the metal to absorb oxygen is the primary factor in the formation and stabilization of subsurface oxygen.
RESUMO
The interaction of platinum with water plays a key role in (electro)catalysis. Herein, we describe a combined theoretical and experimental study that resolves the preferred adsorption structure of water wetting the Pt(111)-step type with adjacent (111) terraces. Double stranded lines wet the step edge forming water tetragons with dissimilar hydrogen bonds within and between the lines. Our results qualitatively explain experimental observations of water desorption and impact our thinking of solvation at the Pt electrochemical interface.
