Abstract. CO oxidation on a clean Pt(111) single crystal and thin iron oxide films grown on Pt(111) was studied at different CO:O 2 ratios (between 1:5 and 5:1) and partial pressures up to 60 mbar at 400 -450 K. Structural characterization of the model catalysts was performed by scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. It is found that monolayer FeO (111) films grown on Pt(111) are much more active than clean Pt(111) and nm-thick Fe 3 O 4 (111) films at all reaction conditions studied. Post-characterization of the catalysts revealed that at CO:O 2 >1 the FeO(111) film dewets the Pt surface with time, ultimately resulting in highly dispersed iron oxide particles on Pt(111). The film dewetting was monitored in situ by polarisation-modulated infrared reflection absorption spectroscopy. The reaction rate at 450 K exhibited first order for O 2 and non-monotonously depended on CO pressure. In O 2 -rich ambient the films were enriched with oxygen while maintaining the long range ordering.Based on the structure-reactivity relationships observed for the FeO/Pt films, we propose that the reaction proceeds through the formation of a well-ordered, oxygen-rich FeO x (1 < x < 2) film that reacts with CO through the redox mechanism. The reaction induced dewetting in fact deactivates the catalyst. The results may aid in our deeper understanding of reactivity of metal particles encapsulated by thin oxide films as a result of strong metal support interaction.

Thickness matters: Ultrathin oxide films on metals can greatly enhance catalytic activity, for example, in CO oxidation on an FeO(111) film grown on a Pt(111) substrate. Under the reaction conditions, the bilayer FeO film restructures to form a trilayer OFeO film (see picture). Experimental evidence for the structure/morphology of the film and theoretical modeling of the mechanism of its formation and CO oxidation on its surface are presented.

The morphology and thermal stability of Pt particles deposited on Fe 3 O 4 (111) films were studied by scanning tunneling microscopy (STM) and temperature programmed desorption of CO. Vacuum annealing at temperatures above 800 K led to significant Pt sintering that reduced CO uptake to a much higher extent than the Pt surface area. A similar effect on CO adsorption was observed after mild oxidation-reduction treatment at 500 K. The results are rationalized in terms of the strong metal-support interaction between Pt and Fe 3 O 4 , whereby the Pt particles were encapsulated by a FeO (111) monolayer film as shown by STM. The high adhesion energy between Pt and iron oxides derived from STM data is suggested to be the key factor for encapsulation.

International audienceThe structural stability of an FeO(111) film supported on Pt(111) was studied by density functional theory (DFT) as a function of oxygen pressure. The results showed formation of O-rich phases at elevated O-2 pressures and revealed a site specificity of the oxidation process within the coincidence (Moire) structure between FeO(111) and Pt(111), ultimately resulting in an ordered pattern of O-Fe-O trilayer islands, as observed by scanning tunneling microscopy (STM). In addition, high resolution STM images revealed a (root 3 x root 3)R30 degrees superstructure of the FeO2 islands with respect to pristine FeO(111). This structure is rationalized by DFT in terms of strong relaxations within the Fe sublayer and can be considered as an intermediate state of the FeO(111) transformation into an Fe2O3(0001) film

Günstige Lage: Ultradünne Oxidfilme auf Metallen können die katalytische Aktivität stark erhöhen, z. B. bei der CO‐Oxidation an einem FeO(111)‐Film auf Pt(111). Unter den Reaktionsbedingungen lagert sich der zweilagige FeO‐Film zu einem dreilagigen OFeO‐Film um (siehe Bild). Neben experimentellen Befunden zur Struktur und Morphologie des Films wird eine theoretische Modellierung des Filmbildungsmechanismus und der CO‐Oxidation auf der Oberfläche vorgestellt.

Nucleation, growth and thermal stability of Pt particles supported on well-ordered Fe 3 O 4 (111) thin films grown on Pt(111) were studied by scanning tunneling microscopy (STM) and temperature programmed desorption (TPD) of CO. STM studies showed that Pt grows through the formation of single layer islands that coalesce at high coverage. Vacuum annealing at 600 K caused Pt sintering and the formation of extended two-dimensional (2D) islands of one and two layers in thickness at sub-monolayer coverage. Well-faceted, three-dimensional (3D) Pt nanoparticles formed by annealing to temperatures above 800 K were encapsulated by a FeO (111) monolayer. These results were rationalized in terms of the high adhesion energy for Pt on iron oxide surfaces. CO TPD studies showed that 2D-structures, formed at 600 K, exhibit much lower CO adsorption capacity as compared to the Pt(111) single crystal surface. The effect has been tentatively assigned to lattice expansion in the Pt 2D-islands leading to weakening of the Pt-CO bond due to reduction of the Pt-> CO π * back donation.

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Abstract:We studied reactivity of well-defined Pt model catalysts, supported on crystalline iron oxide Fe 3 O 4 (111) films, in low temperature CO oxidation. It is shown that pre-annealing in vacuum at ~850 K suppresses CO adsorption but increases CO 2 production rate. This finding is rationalised in terms of Strong Metal-Support Interaction between Pt and iron oxide resulting in particles encapsulation by a thin FeO(111) film that catalyses CO oxidation similarly to the extended FeO(111)/Pt(111) surfaces previously studied (Sun et al. J. Catal, 266 (2009) 359). The results show that the stability and the atomic structure of the encapsulated layer under reaction conditions play a critical role in oxidation reactions over Pt catalysts supported on reducible oxides.