Liquid
organic hydrogen carriers (LOHC) are compounds that enable
chemical energy storage through reversible hydrogenation. They are
considered a promising technology to decouple energy production and
consumption by combining high-energy densities with easy handling.
A prominent LOHC is N-ethylcarbazole (NEC), which
is reversibly hydrogenated to dodecahydro-N-ethylcarbazole
(H12-NEC). We studied the reaction of H12-NEC
on Pt(111) under ultrahigh vacuum (UHV) conditions by applying infrared
reflection–absorption spectroscopy, synchrotron radiation-based
high resolution X-ray photoelectron spectroscopy, and temperature-programmed
molecular beam methods. We show that molecular adsorption of H12-NEC on Pt(111) occurs at temperatures between 173 and 223
K, followed by initial C–H bond activation in direct proximity
to the N atom. As the first stable dehydrogenation product, we identify
octahydro-N-ethylcarbazole (H8-NEC). Dehydrogenation
to H8-NEC occurs slowly between 223 and 273 K and much
faster above 273 K. Stepwise dehydrogenation to NEC proceeds while
heating to 380 K. An undesired side reaction, C–N bond scission,
was observed above 390 K. H8-NEC and H8-carbazole
are the dominant products desorbing from the surface. Desorption occurs
at higher temperatures than H8-NEC formation. We show that
desorption and dehydrogenation activity are directly linked to the
number of adsorption sites being blocked by reaction intermediates.
Hydrogen can be stored conveniently using so-called liquid organic hydrogen carriers (LOHCs), for example, N-ethylcarbazole (NEC), which can be reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H12-NEC). In this study, we focus on the dealkylation of H12-NEC, an undesired side reaction, which competes with dehydrogenation. The structural sensivity of dealkylation was studied by high-resolution X-ray photoelectron spectroscopy (HR-XPS) on Al2O3-supported Pt model catalysts and Pt(111) single crystals. We show that the morphology of the Pt deposit strongly influences LOHC degradation via C-N bond breakage. On smaller, defect-rich Pt particles, the onset of dealkylation is shifted by 90 K to lower temperatures as compared to large, well-shaped particles and well-ordered Pt(111). We attribute these effects to a reduced activation barrier for C-N bond breakage at low-coordinated Pt sites, which are abundant on small Pt aggregates but are rare on large particles and single crystal surfaces.
Liquid organic hydrogen carriers (LOHC) are potential compounds that can facilitate chemical energy storage and hydrogen logistics using reversible hydrogenation. For the process development, the physical solubility of hydrogen in potential LOHCs is required. In this work, solubility of hydrogen in the potential LOHC systems toluene/methylcyclohexane, N-ethylcarbazole/perhydro-Nethylcarbazole, and dibenzyltoluene/perhydrodibenzyltoluene was measured using the static isochoric saturation method. The data were measured at low pressures up to 10 bar within the temperature range of (293 to 373) K. Hydrogen solubility in hydrogenated forms of the LOHCs was found to be higher compared to the dehydrogenated forms. Solubility in all substances increased with increasing temperature within the whole temperature range under consideration. The temperature dependency of the Henry coefficient of hydrogen in the solvents was correlated using the Benson and Krause correlation.
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