Study On The Catalytic Dehydrogenation Performance Of Mesoporous SiO₂ Supported Palladium-based Nano-catalysts

Liquid organic hydrogen carrier（LOHC）hydrogen storage utilizes highly reversible reactions between unsaturated double-bonded liquid organic molecules and hydrogen gas for hydrogen storage and transportation,hydrogenation for storage,and dehydrogenation for release.The dehydrogenation reaction is still subject to high reaction temperature and slow hydrogen release rate due to thermodynamic and kinetic constraints.The development of high-performance dehydrogenation catalysts is crucial to LOHC hydrogen storage technology.In this thesis,Pd catalysts are prepared by inorganometallic chemical adsorption（ICA）combined with glow discharge plasma and ultrasound-assisted reduction using mesoporous Si O₂ as a support,and Ni was introduced to prepare Pd-Ni bimetallic catalysts with different structures.The prepared catalysts are evaluated using dodecahydro-N-ethylcarbazole（H12-NEC）dehydrogenation as the probe reaction.The geometric effect,coordination effect,strain effect,and degree of interaction with the support of the supported Pd-based catalysts on the performance of H12-NEC dehydrogenation reaction are studied.The Pd/SBA-15 catalyst（Pd-IP/S15）was prepared by the ICA-glow discharge plasma reduction method.The Na OH-treated SBA-15 surface Si-OH formed Si-O^--Na⁺,and then the inorganic metal chemical reaction with Pd（NH₃）₄²⁺to obtain（Si-O^-）₂-Pd（NH₃）₄²⁺.Finally,the Pd-IP/S15 catalyst was prepared by reducing Pd（NH₃）₄²⁺using a glow discharge plasma.The ICA makes Pd（NH₃）₄²⁺precisely adsorbed on the surface of SBA-15,and the plasma in situ reduction effectively prevents the aggregation and uncontrolled growth of Pd nanoparticles（NPs）.Ultrafine Pd NPs with a particle size of 2.5±0.4 nm were prepared with high catalytic H12-NEC dehydrogenation activity.The first use of the Pd-IP/S15 catalyst at 180℃resulted in100%dehydrogenation of the reaction at 4 h.The hydrogen production efficiency of Pd-IP/S15is higher than that of commercial Pd/C catalysts when the dehydrogenation temperature is reduced by 10°C.To improve the stability of the catalyst,KIT-6 was selected to replace SBA-15,and the Pd/KIT-6 catalyst（Pd-EU/K6）was prepared by the ICA-ultrasonic assisted reduction method.Pd（NH₃）₄²⁺was adsorbed directionally on the surface of KIT-6.Pd（NH₃）₄²⁺adsorbed directionally on the KIT-6 surface with the carbonyl group generated during the ultrasound-assisted isopropanol reduction and Si-OH coordination to form a metal complex structure Si-O-Pd with a tetradentate ligand,which improved the dispersion of Pd and the interaction of Pd NPs with the support.The prepared catalysts exhibit excellent catalytic stability.The dehydrogenation rate of the first Pd-EU/K6 at 180°C for 6 h was 97.4%,and it was 95.6%after7 recycling cycles,which was significantly better than the 82.2%of Pd-EU/S15.To enhance the coordination effect and reduce the amount of precious metal Pd,a Pd-Ni bimetallic catalyst was prepared by introducing Ni.Pd（NH₃）₄²⁺and Ni²⁺were simultaneously anchored on the surface of KIT-6,and Pd-Ni NPs with an average particle size of 2.3±0.6 nm were prepared with an alloy structure and uniformly distributed on the surface of KIT-6.The transfer of electrons from Ni to Pd changed the coordination environment of the Pd atom.The Pd-Ni（4:1）/K6 catalyst exhibited higher catalytic activity under the synergistic effects of small particle size Pd-Ni NPs,the coordination effect between Pd and Ni,and Pd lattice defects in the Pd-Ni alloy.The dehydrogenation rate of the catalyst was 1.7 times higher than that of Pd/KIT-6 at 180°C.To improve the utilization efficiency of Pd,Ni@Pd core-shell nano-catalysts（Ni@Pd（x:y）/K6）were prepared by a galvanic replacement reaction under ultrasonic radiation using Ni NPs as a template and reductant without adding any protective agent.The d-band center of Pd was shifted to the negative direction of the Fermi level by the synergistic effect of coordination and strain effects,which effectively weakened the adsorption strength of the catalyst for H12-NEC dehydrogenation intermediates and significantly improved the dehydrogenation rate.The dehydrogenation rate of Ni@Pd（6:1）/K6 at 180°C for 3 h was 100%.The dehydrogenation temperature was reduced by 20°C and the dehydrogenation rate on Ni@Pd（6:1）/K6 was comparable to that of commercial Pd/C catalysts.Ni@Pd（x:y）/K6catalysts provide a reference for the preparation of supported Pd-based catalysts with high performance and low cost,and provide a new idea for the design of LOHC dehydrogenation catalysts.