The catalytic activity of deposited Pt 7 clusters has been studied as a function of the reduction state of the TiO 2 (110)-(1 × 1) support for the CO oxidation reaction. While a slightly reduced support gives rise to a high catalytic activity of the adparticles, a strongly reduced one quenches the CO oxidation. This quenching is due to thermally activated diffusion of Ti 3+ interstitials from the bulk to the surface where they deplete the oxygen adsorbed onto the clusters by the formation of TiO x (x ≃ 2) structures. This reaction is more rapid than the CO oxidation. The present results are of general relevance to heterogeneous catalysis on TiO 2 -supported metal clusters and for reactions involving oxygen as intermediate.

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We determined the CO oxidation rates for sizeselected Pt n (n ∈ {3,7,10}) clusters deposited onto TiO 2 (110). In addition, we investigated the cluster morphologies and their mean sizes before and after the reaction. While the clusters are fairly stable upon annealing in ultrahigh vacuum up to 600 K, increasing the temperature while adsorbing either one of the two reactants leads to ripening already from 430 K on. This coarsening is even more pronounced when both reactants are dosed simultaneously, i.e., running the CO oxidation reaction. Since the ripening depends on the size initially deposited, there is nevertheless a size effect; the catalytic activity decreases monotonically with increasing initial cluster size. ■ INTRODUCTIONIt is well established that small metal clusters have physical and chemical properties distinctively different from those of the bulk. While the properties change rather smoothly in the socalled scalable size regime, each additional atom can substantially change the properties of interest in the nonscalable size range. 1 Metal clusters of well-defined size, shape, and composition, supporting on metal oxide surfaces, provide model catalysts that offer a fundamental understanding of the processes taking place in real catalysts via structure−reactivity correlation at the molecular level. 2 This has been shown for the smallest clusters in numerous examples, three decades ago in the famous photographic experiment by Fayet et al., 3 and more recently in the work of Heiz et al. for metal clusters on MgO(100) 4−9 and of Anderson et al., 10,11 Watanabe et al.,12 and our group 13 for metal clusters on rutile TiO 2 (110). In all these examples, size-selected cluster deposition was employed in order to prepare monodispersed supported clusters which are difficult to obtain by conventional preparation methods. We note that experiments with a finite cluster size distribution also make it possible to study size effects in the scalable regime by varying the mean cluster size. 14 However, to firmly associate a catalytic property with a certain size, one has to probe the cluster morphology not only before but also after the reaction. This information is essentially lacking in most of the experiments cited above.It is fairly well accepted that metal catalysts deactivate, in particular under reaction conditions, as a consequence of morphological changes of the supported clusters, and that this change depends on the cluster size and cluster−support interaction. 2,15−24 This argument is even more important when the smallest entities, with only a handful of atoms, are under investigation. Accordingly, not only in model catalysts with well-defined initial conditions but also in real catalysts, this stability under the reaction conditions sets a clear limitation to the minimum cluster size that can be reasonably employed.Here we present a systematic study of the initial sizedependent reactivity and of the thermal as well as chemical stability of very small Pt clusters on rutile TiO 2 (110)-(1×1) in ...

Abstract.We report on a home-built detector for catalytic reaction measurements offering good gas isolation from the surrounding ultrahigh vacuum components, high sensitivity for reaction products and a fast response time of 10 ms enabling dynamic studies correlated to reactant gas pulses. The device is mounted in ultrahigh vacuum and combined with a low-temperature scanning tunneling microscope and a source for the deposition of mass-selected clusters. This combination allows for a direct correlation between surface morphology and catalytic properties of model catalysts. The performances of the new detector are illustrated by measurements on two model systems. Thermal desorption spectroscopy of CO carried out on morphologically well characterized Pt on TiO2(110)-(1×1) reveals several desorption features, which can be attributed to different surface sites. Catalytic CO oxidation performed by alternatingly pulsing isotopic CO and O2 on a Pt film on yttria stabilized zirconia reveals the CO or O rich temperature regimes. The CO2 production rate correlated with either one of the reactants can perfectly be reproduced by a kinetic reaction model giving access to the respective adsorption energies.

One of the key steps in nanotechnology is our ability to engineer and fabricate low-dimensional nano-objects, such as quantum dots, nanowires, two-dimensional atomic layers or three-dimensional nano-porous systems. Here we report evidence of nanotunnel opening within the subsurface region of a wide band-gap semiconductor, silicon carbide. Such an effect is induced by selective hydrogen/deuterium interaction at the surface, which possesses intrinsic compressive stress. This finding is established with a combination of ab-initio computations, vibrational spectroscopy and synchrotron-radiation-based photoemission. Hydrogen/deuterium-induced puckering of the subsurface Si atoms marks the critical step in this nanotunnel opening. Depending on hydrogen/deuterium coverages, the nanotunnels are either metallic or semiconducting. Dangling bonds generated inside the nanotunnel offer a promising template to capture atoms or molecules. These features open nano-tailoring capabilities towards advanced applications in electronics, chemistry, storage, sensors or biotechnology. Understanding and controlling such a mechanism open routes towards surface/interface functionalization.