Analysis of H2 Release from Organic Polycyclics over Pd Catalysts Using DFT

Density functional theory (DFT) was used to study H 2 release from dodecahydro- N -ethylcarbazole over Pd. Dehydrogenation of the molecule starts from the five-membered ring by weakening the axial C–H bonds adjacent to the N atom. In the adsorption mode identified for dodecahydro- N -ethylcarbazole, the five-membered ring was located on a hollow site and the axial H atoms adjacent to N were positioned atop Pd atoms with an adsorption energy of −95 kJ/mol. The two axial C–H bonds of the five-membered ring showed a large increase in bond distance upon adsorption, from 1.10 to 1.16 Å, that was responsible for the weakening of these bonds. The adsorption mode for dodecahydro- N -ethylcarbazole explained the product distribution obtained from the dehydrogenation reaction with octahydro- N -ethylcarbazole and tetrahydro- N -ethylcarbazole identified as primary and secondary products, respectively. DFT was also used to investigate the structure sensitivity of dodecahydro- N -ethylcarbazole dehydrogenation observed on Pd catalysts. The effect of the ethyl group and N atom on the rate of dodecahydro- N -ethylcarbazole dehydrogenation was also investigated by comparing the adsorption energies of dodecahydro- N -ethylcarbazole with two other H 2 storage candidates, dodecahydrocarbazole and dodecahydrofluorene. The latter compounds had much slower H 2 recovery rates, a consequence of the strong adsorption of these compounds relative to dodecahydro- N -ethylcarbazole.