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

Abstract

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.Key words: Low energy electron diffraction, X-ray photoelectron spectroscopy, temperature programmed desorption, catalysis, surface chemical reaction, hydrogen atom, iron oxide IntroductionIron oxides, widespread in nature, are of interest to many scientific disciplines from geology to biology [1]. The reduction of iron oxides is inevitable in many processes, such as metallurgy, corrosion, and heterogeneous catalysis. It was long found that the very rapid dissolution of the passive iron oxides film on iron involves a reductive mechanism [2]. The surface reduction, together with coke formation, is responsible for the deactivation of the iron oxide-based catalyst for the dehydrogenation of ethylbenzene to styrene [3,4]. Because of their relevance for corrosion processes, the reduction of iron oxides in solution has been extensively studied and a clear understanding of the mechanism and kinetics is acquired [5]. However, fundamental understanding of of iron oxide reduction by gas phase hydrogen is poor.Most fundamental understanding of the gas-solid interactions comes from surface science studies of relevant model systems under ultra-high-vacuum (UHV) conditions. Very little is known about the interaction of hydrogen with well-characterized single-crystal iron oxides. The nearly stoichiometric, well ordered a-Fe 2 O 3 (0001) surface is inert towards molecular hydrogen at room temperature and interaction occurs only at defects [6]. Thus it is difficult to approach the reduction of iron oxides by employing molecular hydrogen. The reason is, of course, the high dissociation energy of H 2 (432 kJ/mol). Atomically adsorbed hydrogen, OH groups or protons are formed in catalytic dehydrogenation reactions or during electrochemical reduction. Therefore it makes sense to study the interaction of surfaces...

Hydrogen Spillover Enhanced Hydroxyl Formation and Catalytic Activity Toward CO Oxidation at the Metal/Oxide Interface

“…In the corresponding D 2 desorption spectra, a peak appears at 353 K and can be attributed to the recombinative desorption of D(a) on the Pt(111) surface formed by the dissociative adsorption of D 2 in the residual gas of our UHV chamber, and the other peak appears at 489 K and can be assigned to the surface reaction of hydroxyl groups formed by the dissociative adsorption of D 2 O at the Pt(111)/FeO(111) interface 18. The D 2 desorption feature at 489 K grows with the D 2 O exposure and reaches to the maximum at a D 2 O exposure of 0.05 L, but then weakens with the further increase of D 2 O exposure to 0.1 L. The formation of hydroxyl groups at the Pt(111)/FeO(111) interface affects the local oxygen vacancy concentration on the involving FeO(111) monolayer islands and thus the reactivity of water and hydroxyl groups according to previously established oxygen‐vacancy‐controlled reactivity of water and hydroxyl groups on FeO(111) monolayer structures 19–21. The O 1s X‐ray photoelectron spectroscopy (XPS) results agree with the thermal desorption spectroscopy (TDS) results.…”

Oxidation of Ultrathin FeO(111) Grown on Pt(111): Spectroscopic Evidence for Hydroxylation

“…Numerous interesting properties of the FeO(111) films have been discovered in various studies that focused on: (i) the reduction of the film by atomic hydrogen [11][12][13]; (ii) reduction of the film by CO [14]; (iii) nanopattering using the films moiré structure [15][16][17][18]; (iv) adsorption of molecules on the film [5,[19][20][21][22]; and (v) catalytic activity of the film [23,24].…”

Termination and Verwey transition of the (111) surface of magnetite studied by scanning tunneling microscopy and first-principles calculations

“…Adsorption and reaction of molecules on Fe 3 O 4 ͑111͒ have also been studied, but with the assumption of a certain type of termination. [17][18][19][20][21][22][23] These results may have to be revisited when the stable surface termination is reliably identified.…”