Oxidation of the Ru(0001) surface covered by weakly bound, ultrathin silicate films
Graphical abstract
Introduction
Silicate and aluminosilicate thin films grown on metal substrates have recently received considerable attention as well-defined models for studying surface chemistry of silica-based materials, in particular of zeolites, as well as crystal–glass transitions exemplified by silica [1], [2], [3], [4]. It is well established that the thinnest silica film forms a hexagonal layer of corner-sharing [SiO4] tetrahedra (referred to as “silicatene” in analogy to graphene [5]) which is strongly bound to a metal support through Si-O-Metal linkages [6]. On noble metals such as Ru(0001), Pd(100) and Pt(111), double-layer (or bilayer) silicate films may be grown, which are weakly bound to the support via dispersive forces [7], [8], [9], [10]. In this case, a relatively large space between the silicate sheet and the metal support allows, in principle, small molecules to intercalate the interface, diffuse and ultimately react on the metal surface. Despite being potentially interesting, the studies on chemical reactions in such confined spaces remain in its premature state. To date, only a few studies have been reported for metal-supported graphene [11], [12]. In this respect, metal-supported bilayer silicate films, which combine an ultrathin “membrane” and a chemically active metal surface underneath, may become interesting hybrid materials, in particular in catalysis as the silica is a robust material in catalytically relevant atmospheres. (For brevity, we will use “silicate” for the bilayer silicate films studied here).
Obviously, a general scenario for chemical reactions over silicate/metal system exposed to ambient gases must include: (i) penetration of molecules through “pores” in the film; (ii) chemisorption on the metal surface right behind the pore and subsequent diffusion across the metal surface; (iii) surface reactions, and finally (iv) desorption of products (if any) back through the pores in the film. Recently, we have addressed steps (i) and (ii) with respect to CO and D2 [13]. The results showed that a perfect crystalline silicate (which is represented by a honeycomb-like structure of 6-membered rings) is, in essence, impermeable even for these small molecules. Their penetration seems to occur through the pores associated with large N-membered rings present in the amorphous portion of the films.
In continuation of this work, here we address reactions of silicate/Ru(0001) with molecular oxygen. It has been previously shown that the “as prepared” silicates grown on Ru(0001) commonly contain oxygen atoms adsorbed directly on the metal surface [9], [14]. The amount of adsorbed oxygen may be reduced by annealing in ultra-high vacuum (UHV) and recovered by re-oxidation, without changing the structure of the silicate films. As the Ru(0001) surface is prone to form oxidic structures, which are considered as the active phase in catalytic reactions [15], we focus here on oxidation of the Ru(0001) surface underneath the film in more detail, in particular at high O2 pressures and elevated temperatures.
It is well known that chemisorbed oxygen on Ru(0001) forms several ordered overlayers, e.g. (2 × 2)-O, (2 × 1)-O, (2 × 2)-3O, and (1 × 1)-O. Further oxidation of the Ru(0001) surface, resulting in the O uptake of more than a monolayer, has been intensively studied since this so-called “oxygen-rich” Ru(0001) surface was found to be very active in CO oxidation [16]. Böttcher and co-workers discussed this surface in terms of sub-surface oxygen species [17], [18]. However, Over and co-workers clearly demonstrated the formation of a RuO2(110) thin film on Ru(0001) which may coexist with O(1 × 1)-Ru(0001), depending on the preparation conditions [19], [20], [21]. Further studies showed that it is adsorbed oxygen atoms located in terminal position above the coordinatively unsaturated Ru sites on RuO2(110) that is involved in CO oxidation [22], [23], [24], [25]. Nonetheless, it is fair to say that chemisorbed oxygen, surface oxides, buried oxides, and subsurface oxygen may, in principle, all coexist in the near surface region of Ru(0001), thus leading to a rich oxygen-ruthenium surface chemistry [15], [26]. Indeed, a low energy electron microscopy study on the oxidation of Ru(0001) revealed the substantial spatial inhomogeneity in the oxide formation across the surface [27].
Section snippets
Materials and methods
Experiments were carried out in two different UHV setups located in Fritz Haber Institute and Brookhaven National Laboratory. The first chamber (base pressure 2 × 10− 10 mbar) is equipped with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), infrared reflection–absorption spectroscopy (IRAS), and a quadrupole mass-spectrometer for temperature programmed desorption (TPD) experiments.
The Ru(0001) crystal (10 mm in diameter, 1.5 mm thick, from MaTeck) is spot-welded to the two
Pure silicate films
As previously shown, [14] the “as grown” films contain O atoms directly chemisorbed on Ru(0001). Combined TPD and IRAS studies showed that these O ad-atoms suppress CO and D2 adsorption even after UHV annealing at 1275 K [13]. In order to get rid of O species from the Ru(0001) surface underneath the film via chemical reaction, the sample was exposed to high pressures of H2 (1 mbar, 470 K). Subsequent CO and D2 adsorption experiments showed spectral features characteristic of the clean Ru(0001)
Summary and concluding remarks
Bilayer silicate films grown on metal substrates, which are weakly bound to the metal surfaces, allow ambient gas molecules to intercalate the oxide/metal interface. In this study, we investigated the interaction of oxygen with Ru(0001) supported ultrathin silicate and aluminosilicate films at elevated O2 pressures (10− 5–10 mbar) and temperatures (450–923 K). The results show that the silicate films stay essentially intact under these conditions, and oxygen in the film does not scramble with
Acknowledgments
We acknowledge financial support from Deutsche Forschungsgemeinschaft through collaborative research program SFB 1109. E.E. thanks the International Max Planck Research School “Functional interfaces in physics and chemistry” for the fellowship. J.A.B. acknowledges Alexander von Humboldt Foundation for the fellowship while his staying at FHI. We are grateful to Dr. Yu. Martynova for providing us unpublished results for RuOx films on Pt(111). Research was carried out in part at the Center for
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