Surface Chemistry of a [C2C1Im][OTf] (Sub)Wetting Layer on Pt(111): A Combined XPS, IRAS, and STM Study

Hanna Bühlmeyer; Timo Talwar; Roman Eschenbacher; Jade Barreto; Jonas Hauner; Lukas Knörr; Hans-Peter Steinrück; Florian Maier; Jörg Libuda

doi:10.1021/acsami.4c02239

Surface Chemistry of a [C₂C₁Im][OTf] (Sub)Wetting Layer on Pt(111): A Combined XPS, IRAS, and STM Study

ACS Appl Mater Interfaces. 2024 Apr 23. doi: 10.1021/acsami.4c02239. Online ahead of print.

Authors

Hanna Bühlmeyer¹, Timo Talwar², Roman Eschenbacher¹, Jade Barreto², Jonas Hauner¹, Lukas Knörr¹, Hans-Peter Steinrück², Florian Maier², Jörg Libuda¹

Affiliations

¹ Interface Research and Catalysis, ECRC, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany.
² Chair of Physical Chemistry II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany.

PMID: 38652177
DOI: 10.1021/acsami.4c02239

Abstract

The concept of a solid catalyst with an ionic liquid layer (SCILL) is a promising approach to improve the selectivity of noble metal catalysts in heterogeneous reactions. In order to understand the origins of this selectivity control, we investigated the growth and thermal stability of ultrathin 1-ethyl-3-methylimidazolium trifluormethanesulfonate [C₂C₁Im][OTf] films on Pt(111) by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) in time-resolved and temperature-programmed experiments. We combined these spectroscopy experiments with scanning tunneling microscopy (STM) to obtain detailed insights into the orientation and adsorption geometry of the ions in the first IL layer. Furthermore, we propose a mechanism for the thermal evolution of [C₂C₁Im][OTf] on Pt(111). We observe an intact IL layer on the surface at temperatures below 200 K. Adsorbed [C₂C₁Im][OTf] forms islands, which are evenly distributed over the surface. The [OTf]^- anion adsorbs via the SO₃ group, with the molecular axis perpendicular to the surface. Anions and cations are arranged next to each other, alternating on the Pt(111) surface. Upon heating to 250 K, we observe changes in geometry and structural distribution. Whereas at low temperature, the ions are arranged alternately for electrostatic reasons, this driving force is no longer decisive at 250 K. Here, a phase separation of two different species is discernible in STM. We propose that this effect is due to a surface reaction, which changes the charge of the adsorbates. We assume that the IL starts to decompose at around 250 K, and thus, pristine IL and decomposition products coexist on the surface. Also, IRAS and XPS show indication of IL decomposition. Further heating leads to increased IL decomposition. The reaction products associated with the anions are volatile and leave the surface. In contrast, the cation fragments remain on the surface up to temperatures above 420 K.

Keywords: Pt(111); SCILL; X-ray photoelectron spectroscopy; infrared reflection absorption spectroscopy; ionic liquids; scanning tunneling microscopy.