Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis.

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Nanosized 2D ceria islands randomly distributed over the Pt(111) surface have been prepared by oxidation of the nucleating Ce submonolayer and characterized using XPS and STM for coverages of 0.3 and 0.7 ML. Catalytic CO oxidation over the resulting, well-defined CeO x /Pt(111) model catalytic system of the "inverse supported catalyst" type has been studied using the UHV chamber as a flow reactor. The CO 2 production rate was monitored mass spectrometrically in the temperature range of 413 to 553 K at variable CO/O 2 compositions in the 10 -5 mbar pressure range. The behavior of the present CeO x /Pt(111) system in the CO oxidation reaction is summarized in kinetic phase diagrams separating regions of high and low reactivity (both monostable) and that of bistability. A significantly enhanced reactivity and a remarkable shift of the bistable region of the reaction toward higher CO pressures were observed when compared to a clean Pt(111) surface. An "active border" concept is proposed to explain the strong local enhancement of catalytic activity.

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It is well documented that different surface structures of catalytically active metals may exhibit different catalytic properties. This is typically examined by comparing the catalytic activities and/or selectivities of various well-defined smooth and stepped/kinked single crystal surfaces. Here we report the direct observation of the heterogeneity of active polycrystalline surfaces under reaction conditions, which is manifested by multifrequential oscillations during hydrogen oxidation over rhodium, imaged in situ by photoemission electron microscopy. Each specific surface structure, i.e. the crystallographically different µm-sized domains of rhodium, exhibits an individual spiral pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction. This reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as feedback mechanism of the observed oscillations. The current experimental findings, backed by microkinetic modeling, may open an alternative approach towards addressing the structure-sensitivity of heterogeneous surfaces.