We present the first structure determination by surface x-ray diffraction during the restructuring of a model catalyst under reaction conditions, i.e., at high pressure and high temperature, and correlate the restructuring with a change in catalytic activity. We have analyzed the Pt(110) surface during CO oxidation at pressures up to 0.5 bar and temperatures up to 625 K. Depending on the O 2 =CO pressure ratio, we find three well-defined structures: namely, (i) the bulk-terminated Pt(110) surface, (ii) a thin, commensurate oxide, and (iii) a thin, incommensurate oxide. The commensurate oxide only appears under reaction conditions, i.e., when both O 2 and CO are present and at sufficiently high temperatures. Density functional theory calculations indicate that the commensurate oxide is stabilized by carbonate ions (CO 2ÿ 3 ). Both oxides have a substantially higher catalytic activity than the bulk-terminated Pt surface.

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This paper addresses the ''pressure gap'' between traditional surface science experiments and catalysis under practical conditions. We review high-pressure, microflow experiments at elevated temperatures during the catalytic oxidation of CO. Using a specially constructed ''Reactor-STM'' we simultaneously determine the surface structure of a model catalyst by scanning tunneling microscopy and the reaction kinetics by online mass spectrometry. For both Pt (110) and Pd(100) we find that under O 2 -rich conditions surface oxides are formed on the otherwise metallic surfaces. The presence of the oxide is correlated with a superior catalytic activity. Whereas the reaction on the metal surfaces shows traditional Langmuir-Hinshelwood kinetics, the reaction on the oxides follows the Mars-Van Krevelen oxidation-reduction mechanism, as we conclude from the reaction kinetics and the reaction-induced roughening of the surface. We emphasize that in addition to a pressure gap there can also be a temperature gap, requiring experiments to be performed not only at high pressures but also at sufficiently high temperatures.

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An experimental investigation of the epitaxial growth of iron-phthalocyanines on Ag(111) has been conducted in the monolayer range by low-energy electron diffraction and scanning tunneling microscopy. During the growth, the molecular overlayer undergoes few structural phase transitions, and various superlattices with oblique or rectangular unit cells have been identified. We show that the morphologies of the most stable molecular superstructures formed for different molecular coverage are adopted by various phthalocyanines with different central metal atom and phthalocyanine derivatives.