Construction Of Low-temperature Dehydrogenation Catalytic System For Dodecahydro-N-ethylcarbazol

In the context of carbon neutrality,hydrogen energy is widely used in the transportation field represented by the emerging hydrogen fuel cell vehicle industry,by virtue of its wide source,high energy density,environmental friendliness and renewability.In the industrial chain of hydrogen energy utilization,the high safety risk and high cost in the process of storage and transportation have become the key factors which limited the large-scale development of hydrogen energy.At present,the commonly used hydrogen storage methods mainly include high-pressure gaseous hydrogen storage,low-temperature liquefaction hydrogen storage,physical or chemical adsorption hydrogen storage and liquid organic hydrogen carrier storage.Among them,the process of using dodecahydro-N-ethylcarbazole(12H-NECZ)as a new organic liquid hydrogen storage and using precious metal Pd-based catalyst to realize dehydrogenation can provide high hydrogen storage density and can realize storage and transportation under normal temperature and pressure,which plays an important role in the process of hydrogen energy application.However,this process still suffers from high dehydrogenation reaction temperature and high catalyst cost,which are not conducive to large-scale commercial applications.To address these problems,this thesis focuses on the development of low-cost metal catalysts with excellent low-temperature dehydrogenation activity.By introducing non-precious metals and using inexpensive modified carbon carriers to reduce the catalyst preparation cost,PdCoOx/Zr-C3N4 and PdCoOx/Sn-C bimetallic oxide catalysts with low Pd loading were prepared.The main research contents and results are as follows:(1)A series of Pd-M catalysts(M=Co,Fe,Ni,Cu)were prepared by NaBH4 reduction method using Zr-C3N4 as supporting to reduce the Pd loading.Comparing the low-temperature dehydrogenation activity of each catalyst before and after oxidation under the reaction condition of 140℃ and 8 h,it was found that the PdCoOx active site had a significant promotion effect on the 12H-NECZ dehydrogenation reaction.Adjusting the metal loading ratio of Pd and Co to 4:1,2:3 and 1:4,and preparing a series of PdCoOx/Zr-C3N4 catalysts,respectively.Combined with ICP-OES,BET,XRD,XPS,EPR,EIS,H2-TPR,CO-DRIFTS and TEM characterization methods,the effects of Co introduction on the catalysts structural properties were further investigated to reveal the structure-activity relationship of catalysts.The results showed that the PdCoO-1 catalyst with the loading of 2.6 wt.%Pd and 17.3 wt.%Co had the best performance,which achieved 100%conversion and 95.6%yield of final dehydrogenated product in the 12H-NECZ dehydrogenation reaction.The introduction of Co contributes to forming PdCo alloy,which enhanced the metal-carrier synergy,promoted the uniform dispersion of the active metal on the catalyst surface,increased the specific surface area and mesopore volume of the catalyst.Meanwhile,the electron transfer between Pd and Co metals changes the overall electronic geometry performance of the catalyst,improving the charge transfer rate on the catalyst surface in the dehydrogenation reaction.(2)Using PdCoOx as the active metal,the dehydrogenation activity of cheap activated carbon supported catalysts prepared by different methods were compared.It was found that the activated carbon prepared by starch-ZnCl2 calcination method had better reactivity(93.3%conversion and 72.0%yield of final dehydrogenated product).The activated carbon was further modified to investigate the effects of different metal auxiliaries M(M=Zn,Mn,W,Zr and Sn)and their ratios on the dehydrogenation performance of the activated carbon carrier catalysts.ICP-OES,BET,XRD,XPS,EPR,EIS,H2-TPR and TEM characterization methods were used to analyze the effect of metal auxiliaries on catalyst structure and reveal the structure-activity relationships of catalysts.The results showed that when the mass ratio of added starch to SnCl2 precursor was 10:2,the obtained PdCoO/Sn2-C catalyst performed the highest catalytic activity for the 12H-NECZ dehydrogenation system,which achieved 99.2%12H-NECZ conversion and 92.5%yield of final dehydrogenated product at 140℃.This catalyst had lower metal loading of Pd and Co,which were 2.2 wt.%and 1.8 wt.%,respectively.The introduction of an appropriate amount of Sn into the activated carbon carrier resulted in a more uniform surface dispersion,smaller metal particle size(5.8 nm),faster charge transfer rate and larger specific surface area(1625.8 m2/g),all of which are beneficial properties for its dehydrogenation performance.