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The electrochemical polarisation of metal catalyst films on solid electrolyte substrates can basically lead to three different effects: (a) the generation of mobile surface species (spillover) which spread over the catalyst surface and modify the catalytic activity, (b) potential-controlled segregation of impurities in the catalyst and (c) potential-dependent surface energy (electrocapillarity). The generation of spillover species occurs at the three-phase boundary between metal, solid electrolyte and gas phase and is highly localized. The spreading occurs via diffusion and leads to time-dependent and inhomogeneous surface concentrations. The kinetics of the spillover process can only be observed with in situ surface-analytical techniques in combination with electrochemical methods which offer sufficient resolution in space and time. Model experiments with UV and X-ray photoelectron emission microscopy (PEEM and SPEM) are summarized and discussed with respect to their relevance for the better understanding of electrochemical promotion in catalysis.

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The autoxidation of aqueous sulfide solutions by dioxygen is studied at pH 9 and 14 and at initial sulfide concentrations between 1 and 100 mM. Disulfide as a primary intermediate is proposed to enhance the sulfide oxidation by autocatalytically forming polysulfides. The postulate is supported by the observed acceleration of the process at increasing pH favoring the presence of disulfide rather than that of sulfur. The final reaction products thiosulfate and sulfate are proposed to result from the hydrolysis of the highly instable intermediate polythiosulfite in parallel reaction pathways, assuming probabilities for breaking the sulfur chain at the a (sulfate) and / 3 (thiosulfate) positions to be responsible for the selectivity.A reaction scheme is proposed, considering all experimental observations.

Following the pioneering work of Vayenas and co-workers, the feasibility of electrochemical promotion of heterogeneously catalyzed reactions has been demonstrated in the past two decades with porous metal catalysts interfaced to solid electrolytes. [1][2][3] The catalytic activity of electrically polarized porous metal electrodes on solid electrolytes has been investigated in over 100 reaction systems, and up to 200-fold increases in rate have been reported. Mechanistically, the promotion effect has been attributed to a charged mobile species that is (partially) discharged at the triple-phase boundary (tpb) (solid electrolyte/metal/gas) and then diffuses over the metal surface, thereby modifying its catalytic activity. [3][4][5][6] The crucial role of the electrochemically generated spillover species and its chemical identity have been proven in recent years by surface analytical techniques for only two systems: the oxygen-ion-conducting Pt/YSZ system (YSZ = yttria-stabilized zirconia, ZrO 2 + Y 2 O 3 ) [4,[6][7][8][9] and the alkali-ion-conducting Pt/b''-Al 2 O 3 system. [10,11] We report herein the imaging of the spillover process itself. We observe the diffusional spreading of oxygen upon electrochemical pumping on a dense platinum film interfaced with YSZ. The experiments provide a better basis for a detailed mechanistic understanding of the electrochemical promotion of surface reactions.The typical cell arrangement of an electrochemical promotion experiment is shown in Figure 1 a, and the generation of spillover oxygen at the tpb is illustrated. Oxygen ions are transported through the YSZ solid electrolyte during the anodic (positive) polarization of the metal catalyst (working electrode), and the following reaction [Eq. (1)] takes place at the tpb:The atomic oxygen generated at the tpb (O tpb ) spills over the electrode surface forming a layer of chemisorbed oxygen (O ad ). It was demonstrated with planar YSZ/Pt model catalysts studied in an ultrahigh-vacuum (UHV) chamber that the electrochemically generated spillover oxygen is identical to oxygen adsorbed from the gas phase. [8,12] However, instead of the expected diffusion from the tpb, a seemingly uniform increase in the oxygen concentration across the metal surface was observed. This uniform increase in oxygen coverage could be attributed to a high density of pores (grain boundaries) in the metal film, which act as fastdiffusion paths to transport oxygen from the Pt/YSZ interface to the outer metal surface.To obtain a planar model catalyst, a pore-free, 200-to 250-nm thick Pt film was deposited by pulsed laser deposition on the (111) plane of YSZ. Subsequent annealing produced a dense Pt film with (111) surface orientation as shown in Figure 1 b. Two spatially resolving methods were applied to image the spillover process in situ. With photoelectron emission microscopy (PEEM), the photoelectrons ejected during illumination by a D 2 discharge lamp (5.5 eV-6.2 eV) are used to image the local work function (WF) with a spatial resolution of approximately 1 m...