title: Dehydrogenation mechanism of liquid organic hydrogen carriers: dodecahydro-N-ethylcarbazole on Pd(111).
abstract: Dodecahydro-N-ethylcarbazole (H12-NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H12-NEC on Pd(111) by using a surface-science approach under ultrahigh vacuum conditions. By combining infrared reflection-absorption spectroscopy, density functional theory calculations and X-ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H12-NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C-H bonds is observed within the five-membered ring. Rapid dehydrogenation occurs to octahydro-N-ethylcarbazole (H8-NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H8-NEC proceeds within the six-membered rings. Starting from clean Pd(111), C-N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature-programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H8-NEC as the main product desorbing from Pd(111). The onset temperature for H8-NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H8-NEC is observed even above the temperature threshold for H8-NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.
authors.affiliation: Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.
authors.forename: Max
authors.initials: M
authors.lastname: Amende
authors.forename: Stefan
authors.initials: S
authors.lastname: Schernich
authors.forename: Marek
authors.initials: M
authors.lastname: Sobota
authors.forename: Ioannis
authors.initials: I
authors.lastname: Nikiforidis
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Hieringer
authors.forename: Daniel
authors.initials: D
authors.lastname: Assenbaum
authors.forename: Christoph
authors.initials: C
authors.lastname: Gleichweit
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Drescher
authors.forename: Christian
authors.initials: C
authors.lastname: Papp
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Steinrück
authors.forename: Andreas
authors.initials: A
authors.lastname: Görling
authors.forename: Peter
authors.initials: P
authors.lastname: Wasserscheid
authors.forename: Mathias
authors.initials: M
authors.lastname: Laurin
authors.forename: Jörg
authors.initials: J
authors.lastname: Libuda
keywords: IR spectroscopy; density functional theory; model catalysis; molecular beams; photoelectron spectroscopy