Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface.

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations. Time-lapsed STM imaging on this inverse model catalyst in O2 and CO environments revealed catalytic activity occurring at the FeO-Pt(111) interface and directly showed that the Fe-edges host the catalytically most active sites for the CO oxidation reaction. This is an important result since previous evidence for the catalytic activity of the FeO-Pt(111) interface is essentially based on averaging techniques in conjunction with DFT calculations. The presented STM results are in accord with DFT+U calculations, in which we compare possible CO oxidation pathways on oxidized Fe-edges and O-edges. We found that the CO oxidation reaction is more favorable on the oxidized Fe-edges, both thermodynamically and kinetically.

Unraveling the edge structures of platinum(111)-supported ultrathin FeO islands: the influence of oxidation state.

We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands. Oxidation of the FeO islands by O2 exposure led to the formation of two types of O adatom dislocations and to a restructuring of the FeO islands, creating long O-rich edges and few short Fe-terminated edges. In contrast, reducing the FeO islands led to a dominance of Fe-rich edges and the occurrence of few and short O-rich edges. In addition, for reducing conditions we observed the formation of O vacancy dislocations on the FeO islands. Through the identification of O adatom and O vacancy dislocations known from closed ultrathin FeO films and geometrical considerations we unraveled the atomic structure of the predominant FeO boundaries of pristine, oxidized, and reduced FeO islands. The results indicate an astonishing flexibility of the FeO islands on Pt(111), since the predominant edge termination and the island shape depend strongly on the preparation conditions.

Water-mediated proton hopping on an iron oxide surface.

The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H(3)O(+)-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO(2)(110), where water dissociation is a key step in proton diffusion.

Model studies on CO oxidation catalyst systems: titania and gold nanoparticles.

The peculiar catalytic activity of Au-supported titanium dioxide surfaces in the CO oxidation reaction has been a focus of interest for more than twenty years. Herein, recent data concerning preparation and structural characterisation of planar catalyst model systems consisting of single-crystalline titania and/or gold nanoparticles deposited thereon is presented and reviewed. We first expand on the deposition and growth of TiO(2) films on selected metal host surfaces and then consider the deposition of Au nanoparticles on these surfaces, including information on their geometric and electronic structures. The second issue is the interaction of these materials with carbon monoxide (one of the essential ingredients of the CO oxidation reaction) which serves as a probe molecule and monitor of the chemical activity of the model catalyst samples. Concerted efforts relating the structural and chemical properties of the respective binary materials (titania support plus deposited gold) can help to tackle and finally resolve the still open problems concerning the high activity of Au-TiO(2) catalysts in the CO oxidation reaction.