Reaction Pathways in the (De)hydrogenation of N-Heterocyclic Liquid Hydrogen Carriers Over Supported Pd Catalysts

N-heterocyclic aromatics can reversibly store H₂ in chemical bonds through (de)hydrogenation reactions at <473 K, but their reaction pathways remain incompletely understood. Here, we perform (de)hydrogenation reactions of (methyl)indoles over a Pd/Al₂O₃ catalyst, combined with density functional theory (DFT) calculations of adsorbate binding on Pd surfaces. Hydrogenation proceeds through 2,3-dihydroindole intermediates that further hydrogenate to 8H-indoles. Hydrogenation occurs first at the pyrrole ring rather than the benzene ring, likely due to differences in resonance stability. In contrast, dehydrogenation of 8H-indoles proceeds via initial dehydrogenation of the pyrrolidine ring, showing that the pyrrole ring is more reactive than the benzene ring in both reaction directions. (Methyl)indole adsorption free energies on Pd(111) weaken with the degree of hydrogenation but are broadly similar regardless of methyl group position. Dehydrogenation of 8H-indoles forms 6H-imine and/or 4,5,6,7-tetrahydro intermediates which further dehydrogenate to indoles. Initial hydrogenation of indole rings yields predominantly cis-8H-indoles (cis/trans > 10), reflecting kinetically controlled syn-facial H atom addition to planar-bound indoles. At longer times, 8H-indole cis/trans ratios decay toward equilibrium values (cis/trans ∼ 2-3). In 8H-indole dehydrogenation, cis/trans isomerization occurs in parallel with forward dehydrogenation steps. This work provides mechanistic insights into the (de)hydrogenation of N-heterocyclic H₂ carriers over supported Pd catalysts.