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Figure 2
CO oxidation over Pd(100). Each image (HESXRD and PLIF) is averaged over 2.5 s (shown as a colored area in the graph) for better statistics. The graph in (i) shows how the CO2 PLIF signal, extracted from the area marked with a blue rectangle in (b), (d), (f) and (h), together with the intensity of the surface-oxide rod, changes over time. A linear background is subtracted from the plotted HESXRD surface-oxide rod intensity. The data are plotted with an updating frequency of 0.5 s. (a) HESXRD image of an inactive sample. A surface truncation rod from the (100) substrate is detected (lower left corner), indicating that a metallic surface is present. (b) No CO2 is detected in the gas phase above the sample surface. (c) A metallic surface is detected in the HESXRD data. (d) PLIF image showing the ignition of the reaction as the sample becomes active. (e) After the sample has been active for about 2.5 s, superstructure rods from the diffraction of the ([\sqrt{5}\times\sqrt{5}])-R27° surface oxide (white arrows) appear in the diffraction pattern. (f) As the sample becomes active, a prominent boundary layer of CO2 is detected using PLIF. (g) After additional reaction time, the ([\sqrt{5}\times\sqrt{5}])-R27° surface-oxide diffraction pattern intensifies. (h) PLIF images showing that the CO2 boundary layer is still present over the surface.

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