Model Catalytic Studies of Liquid Organic Hydrogen Carriers: Dehydrogenation and Decomposition Mechanisms of Dodecahydro-<i>N</i>-ethylcarbazole on Pt(111)

Abstract

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 <i>N</i>-ethylcarbazole (NEC), which is reversibly hydrogenated to dodecahydro-<i>N</i>-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-<i>N</i>-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.