Nanospectroscopy highlights

Island ripening is a fundamental process in physical chemistry in which small particles of a new phase grow in size in order to reduce interfacial energies. In the well- known Ostwald ripening, an evaporation-condensation mechanism causes large islands to grow at the expense of smaller ones. Here we report a new ripening mechanism based on a chemical reaction. This chemistry-based ripening process was seen during catalytic methanol oxidation on ultrathin vanadium oxide layers. We observe that under reaction conditions neighboring vanadium oxide islands move towards each other and coalesce. In order to elucidate the chemical and structural changes behind the island movement, microprobe XPS and microprobe LEED measurements were performed at Nanospectroscopy beamline at Elettra. The mechanism we propose here is based on an equilibrium, which is sensitive to gradients in the adsorbate coverages, between large

VO_x islands on Rh(111) and isolated small V_nO_m clusters. With such a polymerization-depolymerization mechanism, islands do not move as a whole, but are decomposed into small parts, i.e., clusters, that easily diffuse across the surface. Elucidating the mechanism of island ripening is essential for understanding the dynamic restructuring of supported oxide catalysts under operation conditions, and for designing catalysts with a desired morphology.

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