Highly Efficient Hydrogen Production from Dehydrogenation Reaction of Nitrogen Heterocycles via Pd0–Pdδ+ Synergistic Catalysis

Hydrogen shows great potential as a clean energy source, but its safe and efficient storage/transportation is a crucial issue for the development of hydrogen economy; therefore, the use of liquid organic hydrogen carriers for reversible hydrogen storage has attracted considerable attention. Herein, we report palladium catalysts supported on a series of rod-like Al2O3 with different crystal phases (amorphous, γ, δ, and α), which are employed in hydrogen production from dehydrogenation reaction of dodecahydro-N-ethylcarbazole (12H-NECZ). The optimized 3Pd/Al2O3-γ catalyst, which is featured by a high distribution of Pd nanoparticles (∼3 nm) on the Al2O3-γ support with unique Pd0–Pdδ+ interfacial sites, exhibits good catalytic performance with a complete conversion of 12H-NECZ, a hydrogen selectivity of 99% within 2.0 h. The turnover frequency value reaches up to 281.2 min–1, and H2 production rate attains 0.66 molH2 gPd–1 min–1 at 180 °C, which are preponderant to the state-of-the-art catalysts. An in-depth investigation based on in situ characterizations (Fourier transform infrared and temperature-programmed surface reaction with mass spectrometry pulse measurements), kinetic studies, H/D isotope exchange, and density functional theory calculations demonstrates that Pd0–Pdδ+ synergistic catalysis plays a crucial role in the C–H bond cleavage of 4H-NECZ (the rate-determining step): the surface Pd0 site facilitates the activation adsorption of 4H-NECZ, which reduces the reaction energy barrier of C–H bond rupture, while the interface Pdδ+ site boosts the desorption of final products (H2 and NECZ). The high performance dehydrogenation catalyst developed in this work shows potential applications in chemical hydrogen storage.