Looking at heterogeneous catalysis at atmospheric pressure using tunnel vision

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.