Structure and Reactivity of the Ionic Liquid [C1C1Im][Tf2N] on Cu(111)
Stephen MASSICOT1, Rajan ADHIKARI1, Lukas FROMM2, Timo TALWAR1, Jan BROX1, Gezmis AFRA1, Manuel MEUSEL1, Andreas BAYER1, Simon JAEKEL1, Florian MAIER1, Andreas GÖRLING2, Hans-Peter STEINRÜCK1
1Lehrstuhl für Physicalische Chemie II, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany
2Lehrstuhl für Theoretische Chemie, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany
In the past decades, novel catalytic concepts were developed such as the SCILL (Solid Catalyst with Ionic Liquid Layer) approach. Hereby, the IL layer coating ideally modifies reactive sites of the heterogeneous catalyst making it less vulnerable towards poisoning while simultaneously improving selectivity and reactivity. In this context, several surface science studies of thin IL films deposited under ultrahigh vacuum conditions onto metallic substrates as model catalysts have been carried out in the last 15 years. However, to break down the complexity of the IL/metal model systems and make them more accessible for surface science investigations, many studies in the past were limited to unreactive surfaces, e.g. Au(111) and Ag(111). These noble metals differ considerably from real SCILL applications where more reactive metals are involved. The aim of the present study is to address the more reactive Cu(111) surface using [C1C1Im][Tf2N] as model IL, which already had been investigated on Au(111) and Ag(111). The focus is on the initial adsorption and film growth behaviour, thermal evolution and reaction of the IL in contact with copper. Interestingly, two studies for related systems already had been carried out by Biedron et al. for [C8C1Im][Tf2N] on Cu(100) and Uhl et al. for [BMP][Tf2N] on Cu(111) with apparently contradicting conclusions. Based on our study, a reinterpretation of the data on Cu(111) leads to a consistent picture. We deposited (sub-) wetting layer films of [C1C1Im][Tf2N] on Cu(111) by physical vapour deposition (PVD) and investigated them by Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS), non-contact Atomic Force Microscopy (nc-AFM) and Scanning Tunneling Microscopy (STM) at various temperatures. Around 200 K, [C1C1Im][Tf2N] forms well-ordered islands with anions and cations adsorbed next to each other in a rectangular checkerboard-type structure. After annealing at 300 K over hours, these checkerboard-type islands first change into islands with a hexagonal and then a porous honeycomb structure. Simultaneously, in-between these well-ordered large islands, many small disordered islands appear. ARXPS reveals that they are comprised of non-volatile fragments as a result of slow IL decomposition at room temperature. At around 350 K, only the small islands remain of decomposed IL while more volatile fragments have been desorbed. From density-functional theory (DFT) calculations, we obtain additional information on the structures and the interaction of the IL with the Cu(111) surface.
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