Preparation And Properties Of ZIF-67-derived Porous Carbon-supported Pd-based Catalysts

Liquid organic hydrogen carriers（LOHCs）are generally recognized as an important form of hydrogen storage.The hydrogen storage capacity of NEC was 5.79wt%,H12-NEC obtained from complete hydrogenation had a low dehydrogenation enthalpy withΔH_NEC=50.6 k J/mol.NEC had also been achieved the first implementation of LOHCs for hydrogenation/dehydrogenation cycles under 473K,which considered as one of the most promising LOHCs.However,the dehydrogenation reactions require the strict conditions of low pressure and high temperature.Therefore,the development of high-activity catalysts to lower the temperature of dehydrogenation,and improving the rate of reaction are the keys to realize the cycle of hydrogen storage.Supported noble-metal nanoparticle（NPs）catalysts play a crucial role in heterogeneous catalysts.As an ideal class of heterogeneous catalysts supports,many heteroatom-doped carbon nanostructures derived from MOF have abundant pore channel structure,the advantages of high specific surface area,high density of active sites after doping and uniformity distribution,which received extensive attention from researchers.In this paper,a series of NGC-supported Pd was designed and prepared on the basis of chemical reduction method and nitrogen glow discharge plasma reduction method using nitrogen-doped partially graphitized carbon material（NGC）prepared by pyrolysis of ZIF-67 as carrier,which was based on nano-catalyst for the experimental study of H12-NEC dehydrogenation reaction.The size and morphology of the nanoparticles,crystal structure,graphitization degree before and after loading,element type and valence state,saturation magnetic intensity were analyzed and characterized by XRD,XPS,TEM,Raman,SEM,VSM and N₂ physical adsorption and desorption methods.The effects of calcination temperature,Pd loading,plasma treatment time,pressure,and chemical reduction time of carrier NGC on its catalytic performance of H12-NEC dehydrogenation were studied.Using acid-washed NpGC as carrier and Pd（NO₃）₂ as palladium source,NpGC-supported Pd catalyst Pd/NpGC-S was prepared by wet impregnation-chemical reduction method.When the loading of palladium was 5 wt.%and the chemical reduction treatment time was 30 min,the Pd NPs particle size distribution of the prepared catalyst was 6.95-12.85 nm,and the average particle size was 9.39 nm.Due to the strong reducibility of chemical reduction,the catalytic activity and stability of Pd/NpGC-S catalysts are low.At atmospheric pressure of 180°C,the hydrogen release amount was 2.94 wt%in the catalytic H12-NEC dehydrogenation reaction for 1 h and 5.66 wt%in 10 h,and the dehydrogenation rate could reach 97.92%.Using acid-washed NpGC as the carrier and Pd（NO₃）₂ as the palladium source,the supported Pd catalyst Pd/NpGC-P with defect sites was prepared in situ by wet impregnation-nitrogen glow discharge plasma.Nitrogen glow discharge plasma can simultaneously achieve Pd²⁺reduction and nitrogen secondary doping.The successful doping of nitrogen element can anchor the Pd NPs through complexation,which improves the stability of the catalyst.The Pd NPs prepared by the plasma reduction method have an octahedral structure dominated by the Pd（111）crystal plane,which makes the Pd NPs have higher activity.Meanwhile,the synergistic effect of N-doped highly active Pd NPs with partially graphitized supports reduces the electron binding energy of Pd,thereby enhancing its catalytic performance.When the loading of palladium was 5 wt.%,the discharge power was 500 W,and the vacuum degree was 75 Pa,the plasma treatment time was 60 min,the size of Pd NPs on the prepared catalysts was between 1.25 and 5.75 nm,and the average particle size was between 1.25 and 5.75 nm.is 3.20 nm and is uniformly dispersed on the surface of NpGC.The amount of hydrogen released in the dehydrogenation reaction at atmospheric pressure of 180°C for 1 h was 4.06 wt%,the dehydrogenation rate was 70%,and the amount of hydrogen released for 10 h was 5.76wt%,and the dehydrogenation rate reached 99.20%.After the catalyst was continuously operated for four times,the dehydrogenation rate remained above 90%.Using unpickled magnetic Co/NGC as carrier and Pd（NO₃）₂ as palladium source,a magnetic recyclable supported Pd catalyst PdCo/NGC-P was prepared in situ by wet impregnation-nitrogen glow discharge plasma.The plasma reduction method not only prepares Pd NPs,but also achieves the synergistic effect of the catalytically active component and the carrier by the secondary nitrogen doping.In addition,the uniform dispersion effect and charge generated by the alloy structure of the PdCo bimetallic nanoparticles The transfer effect makes the catalyst have excellent catalytic activity.When the loading of palladium was 5 wt.%,the discharge power was 500 W,and the vacuum degree was 75 Pa,the plasma treatment time was 60 min,the average particle size of PdCo NPs on the prepared catalyst was 4.02 nm,and the Pd NPs were uniformly dispersed in the NGC.surface.At normal pressure of 180°C,the amount of hydrogen released in the dehydrogenation reaction for 1 h was 4.39 wt%,the dehydrogenation rate was 75.95%,and the amount of hydrogen released for 6 h was 5.71 wt.%,and the dehydrogenation rate was 98.79%.The PdCo/NGC catalyst with a saturation magnetization of 25emug-1 at room temperature can be effectively recovered under the action of an external magnetic field,and the dehydrogenation rate can still reach 93.60%after being recycled for six times,showing good stability.The kinetic study of dehydrogenation reaction showed that its apparent activation energy was only 67.175KJ/mol.The results show that Pd NPs can be synthesized on ZIF-67-derived nitrogen-doped partially graphitized porous carbon by nitrogen glow discharge plasma method,and the positive charge density in the nitrogen-doped carbon material increases,resulting in more catalytic activity.It can effectively improve the activity of nano metal catalysts.The method provides a green,concise and efficient way for the preparation of supported nano-palladium catalysts.