Synthesis of bifunctional Ru–Pd catalysts following a double reduction method: hydrogenation/dehydrogenation of liquid organic hydrogen carriers

Abstract

The problem of safe and efficient storage and transportation of hydrogen energy limits the large-scale development of the “hydrogen economy”. The liquid organic hydrogen storage technology is based on reversible catalytic hydrogenation and dehydrogenation reactions of unsaturated compounds to achieve safe and efficient storage and release of hydrogen. Heterogeneous catalysts with excellent catalytic capacity and stability should be developed to drive the reactions. However, it is a challenge to develop these catalysts. We fabricated a series of RuPd-based and supported catalysts following two different methods, and the catalytic hydrogenation and dehydrogenation activities of these catalysts were systematically investigated. The experiment results implied that the Pd_2.5Ru_2.5/H₂ (double reduction method) catalyst exhibited catalytic hydrogenation rates that were similar to those of Pd_2.5Ru_2.5/L (when reduced by NaBH₄). The former exhibited much better catalytic dehydrogenation rates than the latter. The Pd_2.5Ru_2.5/H₂ catalyst also exhibited much better catalytic recyclability than the Pd_2.5Ru_2.5/L catalyst, and the conclusion was arrived at by conducting 10 cycles of hydrogenation–dehydrogenation tests. The relationship between the catalytic performances and the catalyst's properties was further investigated using a combination of H₂-TPR, NH₃-TPD, XPS, and TEM techniques. The characterization results revealed that the double reduction process could be used to enhance the extent of electronic interaction of the RuPd active sites with the support, and more numbers of strong acidic sites and active sites could be generated. Furthermore, DFT calculations also revealed that 8H-NPCZ and 4H-NPCZ formed stable adsorption structures on the Ru₃Pd₃/Al₂O₃(100) surface, facilitating catalytic reactions. Notably, the reaction energies of Ru₃Pd₃/Al₂O₃(100) were 1.16 eV, 1.42 eV, and 1.63 eV, indicating that the dehydrogenation process (8H-NPCZ → 4H-NPCZ and 4H-NPCZ → NPCZ) occurred easily. All the above-mentioned characteristics make the Pd_2.5Ru_2.5/H₂ catalyst an excellent catalyst for hydrogenation and dehydrogenation and endow it with outstanding catalytic capacity and recyclability.