How the anisotropy of surface oxide formation influences the transient activity of a surface reaction

Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) allow local surface analysis and visualising ongoing reactions on a mu m-scale. These two spatio-temporal imaging methods are applied to polycrystalline Rh, representing a library of well-defined high-Miller-index surface structures. The combination of these techniques enables revealing the anisotropy of surface oxidation, as well as its effect on catalytic hydrogen oxidation. In the present work we observe, using locally-resolved SPEM, structure-sensitive surface oxide formation, which is summarised in an oxidation map and quantitatively explained by the novel step density (SDP) and step edge (SEP) parameters. In situ PEEM imaging of ongoing H-2 oxidation allows a direct comparison of the local reactivity of metallic and oxidised Rh surfaces for the very same different stepped surface structures, demonstrating the effect of Rh surface oxides. Employing the velocity of propagating reaction fronts as indicator of surface reactivity, we observe a high transient activity of Rh surface oxide in H-2 oxidation. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty of well-defined surface structures within one sample.

Automated summary

Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) techniques were used to study the high-Miller-index polycrystalline Rh surface structures, revealing the anisotropy of surface oxidation and its effect on catalytic hydrogen oxidation. The locally-resolved structure-sensitive surface oxide formation and comparison of reaction rates showed that Rh surface oxides exhibit high transient activity in hydrogen oxidation.