Crystal structures and growth mechanism for ultrathin films of ionic compound materials: FeO(111) on Pt(111)

The growth and atomic structures of epitaxial iron-oxide films on Pt(111) were studied with scanning tunneling microscopy and high-resolution low-energy electron diffraction. During the initial layer-by-layer growth of FeO(111) four different structures are formed as the coverage increases to 2.5 monolayers, then a three-dimensional growth of Fe3O4(111) islands begins. The structural transformations demonstrate that the relaxations within the FeO(111) films and the Stranski-Krastanov growth mode are induced by electrostatic surface energies, which dominate the energetics of thin film systems made up of ionic compound materials

Thermal analysis - TDS

Basics 1. Idea, desorption, experimental, example 2. Coverage determination, site occupation and warnings 3. Desorption order Quantitative 4. Redhead analysis 5. Adsorbate-adsorbate interaction: the Elovich-equation 6. “Complete analysis” 7. “Leading edge analysis” 8. Conclusion

Auger electron spectroscopy (AES) and modulation techniques

Content: AES - basics AES - surface sensitivity Electron energy analysis Modulation method, Lock-In Qualitative/quantitative analysis Chemical information e beam influence

Atomic structure of Si and Ge surfaces: Models for (113), (115), and stepped (001) vicinal surfaces

A new class of structure models for the (113) and (115) orientations of Si and Ge is proposed. They are based on dimer and adatom formation as the main building blocks for the reduction of dangling bonds (DB), on relaxation towards more sp2- or s2p3-like configurations, and on the minimization of strain. Four structural alternatives are discussed for (113) which are consistent with the observed 3×1 and 3×2 periodicities. They have either a low DB density and large strain or a higher DB density and low strain. For Si(113), a decision between them on the basis of the available experimental results is not unique. The analysis of the orientation-dependent adsorption of H2S and NO in terms of preferential adsorption by certain structural elements favors the models for Ge(113) which have not the minimum DB density but the minimum of strain. Steps on (001) vicinals induce strain due to bond stretching which is equilibrated over the terraces as long as they are wide enough. The switching of the twofold periodicity of the dimers along [1¯10] on (001) and its vicinals to the threefold periodicity at (115) is explained to occur because the 3×n models resolve bond stretching into bond bending by a meandering arrangement of microterraces which are separated by the energetically most favorable type-SA single-layer steps

T- and p-Measurement

T-measuerement: Resistance T-detectors - Thermocouple - Pyrometer - comparison P-measurement: Direct: Mechanical force - Indirect: Heat conductivity - Indirect: Gas ionization - gauge combinations - QM

Surface science meets catalysis research: epitaxial iron oxide films for in-situ model catalysis

I will review a fairly successful attempt to bridge the gaps between surface science studies and real catalysis for the case of ethylbenzene (EB) dehydrogenation to styrene (St) over unpromoted and K-promoted iron oxide catalysts. Epitaxial films of Fe3O4(111), a-Fe2O3(0001), KxFe22O34 and KFeO2 were prepared and characterized using surface science methods. Their catalytic behavior was studied after vacuum-transfer in a micro flow reactor, followed by post-reaction surface analysis. The results are : (i) Defects are necessary for the dehydrogenation step; (ii) most active is Fe3+ in Fe2O3 or KFexOy; (iii) unpromoted catalysts deactivate by reduction to Fe3O4 and by coking; (iv) both can be prevented by some oxygen in the feed; (v) K is catalytically inactive but suppresses reduction and catalyses carbon removal; (vi) K2Fe22O34 and KFeO2 are K-reservoir phases; (vii) “steaming” (reaction in steam without EB) exhausts the K-reservoir phases; (viii) coke has non-zero catalytic activity and contributes to conversion in real catalysis. In cooperation with the ICVT in Stuttgart, microkinetic modelling was performed aiming at a prediction of the behaviour of technical catalysts. Using physically meaningful parameters, mostly determined in surface science experiments, an excellent fit was achieved which could even be extented to porous samples

Autocatalytic partial reduction of FeO(111) and Fe3O4(111) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(111) and Fe3O4(111) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of ¼ monolayer (ML) of oxygen with a 2´2 pattern appearing in LEED. This FeO0.75 film is passive against further reduction. The first reduction step saturates after removal of ~0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2´2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe3O4 film, irregular desorption bursts of H2O and H2 were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts

Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

Metal-oxide based catalysts are used for many important synthesis reactions in the chemical industry. A better understanding of the catalyst operation can be achieved by studying elemantary reaction steps on well-defined model catalyst systems. For the dehydrogenation of ethylbenzene to styrene in the presence of steam both unpromoted and potassium promoted iron-oxide catalysts are active. Here we review the work done over unpromoted single-crystalline FeO(111), Fe3O4(111) and a-Fe2O3(0001) films grown epitaxially on Pt(111) substrates. Their geometric and electronic surface structures were characterized by STM, LEED, electron microscopy and electron spectroscopic techniques. In an integrative approach, the interaction of water, ethylbenzene and styrene with these films was investigated mainly by thermal desorption and photoelectron emission spectroscopy. The adsorption-desorption energetics and kinetics depend on the oxide surface terminations and are correlated to the electronic structures and acid-base properties of the corresponding oxide phases, which reveal insight into the nature of the active sites and into the catalytic function of semiconducting oxides in general. Catalytic studies, using a batch reactor arrangement at high gas pressures and post reaction surface analysis, showed that only a-Fe2O3(0001) containing surface defects is catalytically active, whereas Fe3O4(111) is always inactive. This can be related to the elementary adsorption and desorption properties observed in ultrahigh vacuum, which indicates that the surface chemical properties of the iron-oxide films do not change significantly across the "pressure-gap". A model is proposed according to which the active site involves a regular acidic surface sites and a defect site next to it. The results on metal-oxide surface chemistry also have implications for other fields, such as environmental science, biophysics and chemical senso