Adsorption, Growth, Stability and Thermal Evolution of Protic and Aprotic Ionic Liquid Films on Non-Reactive and Reactive Metal Surfaces
Creators: name: Massicot, Stephen
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2023

description: In the following, the results and conclusions of four publications [P1-P4] contributing to this this cumulative thesis are summarized. Thin films of ionic liquids (ILs) on high surface area solid supports are a key technology in a novel catalytic concept, the so-called SCILL (Solid Catalyst with Ionic Liquid Layer) approach.29-32 Hereby, the performance of the catalyst system is strongly determined by the properties of the interface formed by the ionic liquid and the solid catalyst. In this thesis, the properties of this interface were studied on the molecular level in ultra-high vacuum (UHV) for several model systems based on ultra-thin IL films on model metal surfaces. In particular, the cleanliness, film growth mode, wetting behaviour, arrangement of the ions at the interface and thermal stability of thin IL films on defined metal surfaces were studied by angle-resolved and temperature-programmed X-ray Photoelectron Spectroscopy (XPS). Intact Deposition by PVD of a protic IL on Au(111) [P1]: For the first time, a protic IL was deposited by PVD in vacuo on a (metal) substrate: a multilayer (2.4 nm) thick film of [dema][TfO] was deposited on Au(111) at 200 K and subsequently analysed quantitatively by angle-resolved XPS. The stoichiometry as derived from all IL core level signals in 0° corresponds to the expected nominal values. This observation indicates that [dema][TfO] stays chemically intact during the PVD process and that intact IL films can be prepared in vacuum. The about six times more surface-sensitive 80° measurements indicate preferential orientation of the [TfO]- anions at the outer surface with the CF3 groups pointing towards the vacuum and the SO3 groups away from the vacuum towards the bulk. Growth of ILs on Metal Surfaces [P1, P3, P4]: Wetting of ILs on solid catalysts is an important parameter for thin film applications like SCILL catalysis. In this context, the growth behavior of [dema][TfO] on Au(111) and Ag(111), and of [C1C1Im][Tf2N] on Cu(111) and Pt(111) was investigated at different temperatures. All investigated systems show a similar behaviour for low coverages. In the wetting layer regime, the cations and anions initially adsorb next to each other in direct contact with the metal surface. The behavior changes for higher coverages to a more or less pronounced 3D island growth for all systems. Notably for [dema][TfO], the coverage does not increase beyond one wetting layer upon IL deposition at RT due to the onset of multilayer desorption that starts already below RT. Thermal Stability of neat IL Films on reactive Metal Surfaces [P1, P3, P4]: Fundamental knowledge on the thermal stability of ILs in direct contact with metal catalysts is important to thoroughly understand SCILL catalysis on the molecular scale. In order to determine possible decomposition processes at the IL/metal interface, thin films of [dema][TfO] on the moderately reactive Ag(111) surface, and [C1C1Im][Tf2N] on the more reactive Cu(111) and Pt(111) surfaces were investigated. The cation of [dema][TfO] on Ag(111) deprotonates starting already below 200 K. The formed proton is then transferred to the [TfO]- anion to form neutral triflic acid (H[TfO]) that partly desorbs and partly decomposes to form a fluorine-containing decomposition product that is stable up to much higher temperatures. For [C1C1Im][Tf2N] on both more reactive surfaces, the anion decomposes to NHx, SOx, and CF3 species starting above 200 K. On Cu(111), the decomposition products remain on the surface until higher temperatures, while they are volatile on Pt(111) and desorb from the surface. In the same temperature range, the cation likely decomposes to non-volatile and mainly carbon containing species on both surfaces. On-Surface Metathesis of a new IL [P2]: A novel synthesis approach of ILs on surfaces is presented. Mixed thin films of the protic [dema][TfO] and the aprotic [C8C1Im][PF6] were sequentially deposited on Ag(111) at 90 K, and monitored by temperature-programmed XPS. Upon heating, thermally induced phenomena of ion exchange and preferential enrichment at the interfaces are observed. Furthermore, selective desorption of specific ion pairs leads to the formation of the new IL [C8C1Im][TfO] by on-surface metathesis at the IL/metal interface.