Elsevier

Surface Science

Volume 648, June 2016, Pages 2-9
Surface Science

CO adsorption on a silica bilayer supported on Ru(0001)

https://doi.org/10.1016/j.susc.2015.10.027Get rights and content

Highlights

  • Using DFT with inclusion of dispersion corrections, we investigate the adsorption of CO on well-ordered, ultrathin silica film grown on the Ru(0001) surface.

  • The CO molecules adsorb at the interface between the SiO2 film and the Ru(0001) surface.

  • A phase transition in the structure of the SiO2/CO/Ru(0001) occurs upon increasing the CO coverage.

Abstract

Silica bilayers are built up of two layers of corner sharing SiO4–tetrahedra and constitute an inert ultra-thin membrane supported on the Ru(0001) surface. We have investigated the adsorption of CO on that system using DFT with inclusion of dispersion corrections. The molecules adsorb at the interface between the SiO2 film and Ru(0001) surface. The estimated barrier for diffusion of CO through the silica bilayer is around 0.5 eV. The CO bond length, the C–O stretching frequency and the silica–ruthenium distance depend strongly on the CO coverage. The band observed at 2051 cm 1 in previous experiments can be assigned to a CO coverage of around 0.5 ML on Ru(0001), with the silica bilayer floating above the CO molecules.

Introduction

Silica (SiO2) thin films play a key role in many modern technological applications, such as silicon based micro- and nano-mechanical systems. Silica thin films serve as gate dielectrics in metal-oxide semiconductor transistors [1] and have potential use for protective films against corrosion and as support for metal nanoparticles in sensors and heterogeneous catalysts. The recent progress in preparation methods of silica ultrathin films on diverse metals, such as Ni(111) [2], Pd(100) [3], Mo(112) [4], and Ru(0001) [5], has opened the opportunity to perform experimental studies on these low-dimensional crystalline systems [6], [7]. Depending on the choice of the supporting metal and the preparation method, silica mono- or bilayers can be formed, consisting of layers of corner sharing SiO4 tetrahedra [6]. The monolayer binds to the supporting metal via O–M bonds [8], whereas the silica bilayer, being an electronically saturated structure, interacts with the support only weakly via van der Waals (vdW) forces [9].

Silica bilayers exist in crystalline and amorphous phases, where the perfect crystalline phase consists of 6-membered rings (see e.g. Fig. 2, Fig. 5). The amorphous phase is built up of 4 to 10-membered rings which constitute pores with a diameter around 0.4–1 nm [10]. Being only weakly bound to the support, the silica bilayer represents a chemically inert two-dimensional membrane with potential use as molecular sieve [11]. This hybrid entity formed by an inert silica membrane on a reactive metal support represents a system of interest for catalysis, because after size-selective diffusion of the molecules through the bilayer, reactions may be performed at the silica–metal interface. Considering the small pore size of crystalline silica bilayer, the adsorption of small molecules appears to be the next logical step.

Recently, Emmez, Yang, Shaikhutdinov, and Freund have studied the adsorption of CO and D2 at the interface between silica bilayer and Ru(0001) using infrared adsorption spectroscopy (IRAS) [11] (Fig. 1). At low temperatures (100–125 K), the infrared (IR) spectra taken at 2 × 10 6 mbar CO show two bands at 1930 cm 1 and 2051 cm 1. The second band emerges at temperatures above 125 K, indicating an activated adsorption of CO on the corresponding adsorption site, e.g. because of an activated diffusion through the silica bilayer prior to adsorption on the Ru(0001) surface. Going to higher temperatures, the first band at 1930 cm 1 disappears while the second is red-shifted by about 30 cm 1. The red-shift may indicate a reduction of CO coverage due to desorption [11]. At 250–300 K, a third band emerges around 2045 cm 1. According to Emmez et al., this band falls into the range which is typical for linear CO on metallic Ru but these species have no significant contribution to the total CO uptake.[11] Emmez et al. proposed that CO diffuses through the silica pores to adsorb at the interface between silica and Ru(0001) with a coverage of about 0.5 ML with respect to the exposed Ru–top sites [11]. Since the experimental stretching wavenumber for a CO gas phase molecule is 2143 cm 1 [12], [13], the shift between the second band and the molecular CO vibration is ~ 100 cm 1.

The scope of the present study is to investigate the CO interaction with the SiO2/Ru(0001) films, as well as its capability to penetrate the pores of the structure and to bind into the silica cages or at the silica/ruthenium interface. Density functional theory (DFT) was employed to assign the available experimental information to the bilayer model [5], [9] that had been proposed before as a computational model for hydrophobic silica surfaces [14]. The focus of the study is the C–O stretching frequency for various CO adsorption sites and coverages. Given the role that dispersion forces have in this system, various ways to introduce them have been compared.

The paper is organised as follows. After introducing the computational details, we discuss the structure of SiO2/Ru(0001) (Section 3.1) and the CO adsorption on the clean Ru(0001) surface (Section 3.2) as a pre-requisite for the analysis of the results on CO/SiO2/Ru(0001) (Section 3.3). In Section 3.4, we discuss the role of dispersion and we compare different approaches to take it into account. The role of co-adsorbed oxygen is shortly considered in Section 3.5. The role of surface defects and the coverage effects are treated in 3.6 Role of defects on the SiO, 3.7 Coverage effect in CO/SiO, respectively. In Section 3.8 we analyse the potential energy surface for CO penetration into the pores of the silica film.

Section snippets

Computational details

Density functional theory (DFT) calculations with periodic boundary conditions have been performed using the Vienna Ab initio Simulation Package (VASP 5.2) [15]. Generalised gradient approximations (GGA) for the exchange-correlation functional were applied within the Perdew, Burke and Ernzerhof (PBE) formulation [16]. To describe electron–ion interactions, the projector augmented wave (PAW) method was used [17]. Only the valence electrons were explicitly considered. Wave functions were expanded

Silica bilayer on Ru(0001)

We start by providing a brief summary of the properties of the SiO2/Ru(0001) system. First of all, depending on the preparation conditions, the films can be obtained with or without excess oxygen at the interface [5], [11], [23]. Second, different lateral positioning of the silica film has the highest stability, depending on the presence of the interfacial species [23]. We used three different models of SiO2/Ru(0001) (Fig. 2). Case (a) corresponds to the silica bilayer over the 3O(2 × 2)/Ru(0001)

Conclusions

We have investigated in detail the nature of the interaction of CO with SiO2/Ru(0001) ultrathin films. At low CO coverage, a configuration is preferred in which the CO molecule penetrates the bottom layer of the silica film, due to the relatively strong attraction between the Ru(0001) surface and the silica film. This attraction is dominated by dispersion forces. Comparison of different methods of taking dispersion into account has shown that the short Ru–SiO2 interface distance in the presence

Acknowledgements

Financial support from the European Marie Curie Project CATSENSE, from the Italian MIUR (FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications”), and from the German Research Foundation (DFG) within CRC 1109 “Metal Oxide–Water Interfaces” is gratefully acknowledged. We also thank the COST Action CM1104 “Reducible oxide chemistry, structure and functions”.

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