Catalytic CO oxidation has been investigated under low pressure conditions (p B 10~6 mbar) employing porous Pt Ðlms on a solid state electrolyte (yttrium stabilized as catalyst. The samples were ZrO 2 \ YSZ) characterized by impedance spectroscopy and linear sweep voltammetry before rate measurements were conducted in an UHV chamber with a di †erentially pumped mass spectrometer, a Kelvin probe for integral and a photoelectron emission microscope (PEEM) for spatially resolved measurements of the work function (WF). Di †erent types of samples prepared by di †erent groups were investigated. An electrocatalytic e †ect was found for all samples but within the experimental uncertainty (up to a factor of two) the e †ect was Faradaic. Surprisingly, despite nearly identical electrochemical characteristics the electrocatalytic behavior of the catalysts with the porous Pt Ðlms varied drastically depending on the preparation. Whereas on the samples provided from a di †erent group the WF of the porous Pt Ðlms followed more or less the variation in the electric potential the samples prepared here exhibited no detectable WF change at all upon variation of (V WR), Within the spatial resolution of PEEM (*x B 1 lm) the observed WF changes occurred spatially V WR . homogeneous.

The catalytic O2+H2 reaction on Rh(111) has been investigated in the 10−6–10−5 mbar range using photoelectron emission microscopy (PEEM) as spatially resolving method. Depending on the pretreatment of the sample in which the sample was exposed to pO2=2×10−4 mbar at T=770 K for varying times (tOX) different dynamic behavior was found. For tOX=12 h we found simple bistable behavior with reaction fronts initiating transitions between an unreactive high oxygen coverage state and a reactive almost bare surface. For tOX=36 h low work function (WF) areas developed in the area where two reaction fronts collided. For very long oxygen pretreatments with tOX=48 h the formation of secondary fronts traveling backwards from the area where two primary fronts collided were seen. The properties of the low WF areas which have been tentatively assigned to subsurface oxygen were studied in adsorption and titration experiments.

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Because of the unsufficient quality of the reproduction of Fig. 2a and 3 in [l], these figures are depicted below in better contrast. 450pm Fig. 2Electrochemically induced surface changes on the YSzlpt microstructure. a) PEEM image of the YSz/pt microstructure showing three circular YSZ domains connected via channels which are surrounded by a Pt film. Inside the windows marked from 1-3 the digitized PEEM intensity has been integrated for the measurements displayed in (b) [ I ] J. Poppe, A. Schaak. J. Janek, and R. Imbihl, Ber. Bunsenges.Phys. Chem. 102, 1019 (1998).

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