Dehydrogenation of the liquid organic hydrogen carrier system 2-methylindole/2-methylindoline/2-methyloctahydroindole on Pt(111)

Abstract

Among other N-heterocycles, indole and its substituted derivatives, such as methylindoles, are considered promising Liquid Organic Hydrogen Carriers (LOHCs) for the storage of renewable energy. We used X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and density-functional theory (DFT) to investigate the low temperature adsorption and consecutive dehydrogenation reaction during heating of 2-methylindole, 2-methylindoline, and 2-methyloctahydroindole on Pt(111) and their viability as the LOHC system. In the photoemission experiments, for all Hx-2-methylindoles, we find deprotonation at the NH bond starting between 240 and 300 K, resulting in a 2-methylindolide species. Simultaneously or before this reaction step, the dehydrogenation of 2-methyloctahydroindole via 2-methylindoline and 2-methylindole intermediates is observed. For 2-methyloctahydroindole, we also find π-allyl intermediates above 230 K. Starting at ∼390 K, decomposition of the remaining 2-methylindolide species takes place under the conditions of our surface science experiments. DFT calculations give insight into the relative energies of the various species, reaction intermediates, and their isomers both in the gas phase and on the Pt(111) surface.

Thermal Stability of Ionic Compounds on Nanostructured Metal Catalysts: Conversion of Quaternary Ammonium to Amines on Gold Nanoparticles

“…Given our earlier assignment of that shoulder to carbon located next to the cationic N + , the diminishing of this species lines up well with the disappearance of quaternary ammonium. The broadening of the main feature toward the higher BE side might be a consequence of carbon centers now bound to a nitrogen atom in a molecular amine, which would be expected around 285 eV. , The broadening toward the lower BE side is a bit more unclear but may result from the formation of electron-rich carbon atoms, such as e.g. elemental carbon, frequently observed as a consequence of coking on metal surfaces, or metal carbides. − In principle, alkenes and other unsaturated compounds would result in a shoulder at a similar position.…”

“…The broadening of the main feature toward the higher BE side might be a consequence of carbon centers now bound to a nitrogen atom in a molecular amine, which would be expected around 285 eV. 68,69 The broadening toward the lower BE side is a bit more unclear but may result from the formation of electronrich carbon atoms, such as e.g. elemental carbon, frequently observed as a consequence of coking on metal surfaces, or metal carbides.…”

Catalytic Dehydrogenation of Liquid Organic Hydrogen Carrier Model Compounds by CpM+ (M = Fe, Co, Ni) in the Gas Phase

“…Cyclic alkanes and N-heterocyclic species are of special interest due to their stability and high energy density. Cycloalkane LOHC systems such as cyclohexane/benzene are good models for mechanistic studies. , Others that are viable on the industrial scale include perhydro-dibenzyl toluene/dibenzyl toluene , and methylcyclohexane/toluene. , Although the cyclohexane/benzene system (eq ) is not very useful in the context of scaling up owing to the carcinogenic properties of benzene, it is often used as a model due to its relatively easy access to mechanistic studies and has been investigated in the context of reactant/product separation. , N-Heterocycle couples such as piperidine/pyridine (eq 2, and derivatives), octahydroindole/indole, and perhydrophenazine/phenazine (eq 3) are all viable substrates for stable hydrogen storage. N-Heterocycles have the added benefits of relatively better biodegradability, low volatility, and more efficient hydrogen liberation …”

Metastability of palladium carbide nanoparticles during hydrogen release from liquid organic hydrogen carriers

“…Previously, the detailed mechanism of LOHC dehydrogenation was investigated using model catalysts such as well-ordered Pt(111) and Pd(111) single crystals 6,[11][12][13][14] and well-defined Pt and Pd supported nanoparticles. [15][16][17] It was found that dehydrogenation of both homocyclic and N-heterocyclic LOHCs involves multiply dehydrogenated intermediates.…”