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

Abstract

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), Fe 3 O 4 (111) and α-Fe 2 O 3 (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 adsorptiondesorption 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 α-Fe 2 O 3 (0001) containing surface defects is catalytically active, whereas Fe 3 O 4 (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 sensors.-2 -"2kyEi9FYSQjBVrZxIPQ.FHIAC_WRa02_review.doc", Datum: 19.02.03

Morphology and CO adsorption on platinum supported on thin Fe3O4(111) films

“…In other words, the Pt lattice constant is expanded from 0.277 to 0.297 nm [17]. In principle, this assumption would be consistent with the high adhesion energy between Pt and iron oxide as discussed above.…”

“…The termination of the close-packed surfaces of iron oxides has been addressed by Weiss and Ranke in their review [17]. In particular, it has been suggested that the Fe 3 O 4 (111) surface is terminated by ¼ monolayer of Fe cations.…”

“…The Fe 3 O 4 (111) films were grown on a Pt (111) substrate as described in details in [16,17,20,22]. Briefly, about one monolayer (1 ML = 1.3×10 15 Iron and Pt (both 99.95%, Goodfellow) were deposited using commercial evaporators (Focus EFM3, Omicron).…”

Surface Structure of Protonated R-Sapphire (11̅02) Studied by Sum-Frequency Vibrational Spectroscopy

“…All spectra can be fitted using Eqs. (1)(2)(3) with three discrete resonant modes having resonant frequencies ( q  ) at 3365, 3520, and 3670 cm - We consider first the three models of -Al 2 O 3 (1102 ) described in the introduction section. The ideal bulkterminated model (Fig.…”

“…[35][36][37] Here, we present only the key points needed for data analysis. The SF signal generated by overlapping incoming beams of frequencies  vis and  ir at a surface in the reflected direction is given by 3  at the interface. The surface nonlinear susceptibility (2)   can be approximated by (2) …”