Design Of Pd-based Catalysts And Their Catalytic Performance In Dehydrogenation Of Liquid Organic Hydrogen Carriers

One effective method to achieve the goal of"carbon peak and carbon neutrality"is to use hydrogen cycle instead of carbon cycle in nature.However,how to realize safe,efficient,long-term,and large-scale storage and transportation of hydrogen has become the main"bottleneck"problem that restricts the development of hydrogen energy.Organic liquid hydrogen storage technology is considered to be one of the effective ways to solve the problem of safe and efficient storage and transportation of hydrogen gas.By catalyzing the reversible hydrogenation and dehydrogenation of organic liquid hydrogen storage materials under mild conditions,hydrogen storage and release can be achieved.Developing efficient hydrogenation and dehydrogenation catalysts is the key to the commercial application of organic liquid hydrogen storage technology.Currently,most organic liquid dehydrogenation catalysts use heterogeneous catalysts,and most of the research is based on high-loaded Pd-based catalysts,which are expensive when scaled up for application.Therefore,developing efficient Pd-based dehydrogenation catalysts has become the key to solving the problem of scaling up organic liquid dehydrogenation catalysts.This thesis focuses on the study of Pd-based dehydrogenation catalysts for organic liquid hydrogen storage materials.Using Al₂O₃ as a carrier,the inherent mechanism of the stepwise dehydrogenation of 12H-N-Propylcarbazole（12H-NPCZ）by Pd nanoparticles with different particle sizes was analyzed through theoretical calculations and experimental characterization.Furthermore,small-sized Pd nanoparticles were selected as the research object,and WO₃ was introduced as a co-catalyst to regulate the effective contact between WO₃ and Pd nanoparticles,improve the electronic structure of metallic Pd,and promote complete dehydrogenation of 12H-NPCZ catalyzed by small-sized Pd nanoparticles.Based on the research foundation of the interaction between Pd and WO₃,a one-pot synthesis process coupled with cold plasma technology was designed and developed to construct highly dispersed Pd nanoparticles interacting with WO₃particles,finally to obtain Pd-based dehydrogenation catalysts with high catalytic activity and stability.The main research contents and results are as follows:（1）Different sizes of Pd nanoparticles（2.8-8.6 nm）were synthesized through liquid-phase reduction and the relationship between their structure and dehydrogenation performance was studied by characterizing and conducting dehydrogenation experiments.It was observed that compared to the larger Pd particles,Pd_2.8 had a stronger interaction with the Al₂O₃ support,which led to the presence of a large number of micropositive Pd^δ+species and super acidic sites on the catalyst surface.Moreover,the Pd^δ+species facilitated the s Tableadsorption of hydrogen species,thus inhibiting the dehydrogenation reaction.Density functional theory was used to investigate the surface adsorption energies of Pd clusters with different sizes on the 12H-NPCZ dehydrogenation stages,which showed that smaller Pd particles had lower adsorption energies on 12H-NPCZ,thereby promoting its conversion.However,the whole process of 12H-NPCZ→NPCZ had higher reaction energies and slower dehydrogenation rates,confirming that Pd particle sizes affected the stepwise dehydrogenation of 12H-NPCZ.The apparent activation energies of the Pd_2.8 catalysts were 70.72 k J/mol,106.66 k J/mol,and 145.04 k J/mol,respectively,and compared to commercial catalysts,the first stage of apparent activation energy for Pd_2.8 was significantly reduced.The stability of the small particle size Pd_2.8 catalyst was tested for 10 cycles,but it showed significant deactivation after six cycles with its dehydrogenation performance reduced to 2.8 wt%.（2）Based on the small particle size Pd_2.8,highly dispersed WO₃ nanoparticles were added to the catalyst surface by stepwise impregnation to investigate the effect of WO₃content on the structure of Pd_2.8 and the stepwise dehydrogenation of 12H-NPCZ.The addition of WO₃ not only reduced the refractory palladium oxide species in the catalyst but also promoted the reduction of WO₃.However,an excessive amount of WO₃decreased the dispersion of WO₃ on the Al₂O₃ surface,and the degree of contact with Pd decreased.The addition of WO₃ increased the total acid amount on the catalyst surface and inhibited the generation of strong and super-strong acids,while significantly modulating the electronic structure of Pd by increasing the electron cloud density of the micropositively charged Pd^δ+species and inhibiting the generation of strongly adsorbed hydrogen species in the catalyst.The Pd-Wx catalyst significantly increased the conversion rates of 8H-NPCZ→4H-NPCZ and 4H-NPCZ→NPCZ,and Pd-W₂₀ had the highest dehydrogenation rate,achieving complete dehydrogenation of 12H-NPCZ.Density functional theory showed that the addition of WO₃ drove the Pd₉ clusters to receive a large number of electrons,thereby increasing the adsorption energy of the catalyst for adsorbates such as 12H-NPCZ and reducing the reaction energy for the conversion of 8H-NPCZ to 4H-NPCZ.The apparent activation energies of the three stages of the Pd-W₂₀ catalyst were 65.89 k J/mol,85.99 k J/mol,and 102.97 k J/mol,respectively,and compared to the Pd_2.8 catalyst,the apparent activation energies of the second and third stages of the Pd-W₂₀ catalyst were significantly reduced.The cycling stability of the Pd-W₂₀ catalyst was significantly improved and the dehydrogenation was only 4.47 wt%until the 10th cycle.The structural characterization of the catalysts before and after the reaction showed that the micropositive Pd^δ+species were partially reduced to Pd⁰,while the agglomeration of WO₃ and Pd nanoparticle size growth occurred,resulting in a decrease in the effective contact between Pd nanoparticles and WO₃nanoparticles,thus leading to Pd nanoparticles strongly interacting with Al₂O₃ again.（3）The effective contact degree between WO₃ and Pd and the beneficial effects produced were deeply investigated.WO₃@Al₂O₃ composite carriers were prepared by the sol-gel method,the effects of catalyst structure with high content of highly dispersed WO₃,and the effect of Pd nanoparticle structure on the stepwise dehydrogenation of 12H-NPCZ were studied.The highly dispersed WO₃ nanoparticles and Pd nanoparticles enhanced the interaction between WO₃ and Pd.After the addition of WO₃,not only reduce the refractory Pd species in the catalyst,the percentage of Pd⁰ and the electron cloud density of Pd nanoparticles both increased significantly,while the strong acid,super acid and strongly adsorbed hydrogen species almost completely disappeared from the catalyst.The highly dispersed WO₃ nanoparticles and Pd nanoparticles enhanced the interaction between WO₃ and Pd.After the addition of WO₃,not only did the hard-to-reduce Pd species not appear in the catalyst,the percentage of Pd⁰ and the electron cloud density of Pd nanoparticles both increased significantly,while the strong acid,super acid and strongly adsorbed hydrogen species almost completely disappeared from the catalyst.Comparing with the Pd-Wx catalysts,Pd/x W@A modified by different contents of WO₃promoted the stepwise dehydrogenation activity of 12H-NPCZ,with Pd/20W@A having the highest rate of dehydrogenation,and fully dehydrogenation in only 4 h.Using density functional theory,the addition of WO₃ has an electron transfer effect on Pd₉ clusters and Al₂O₃,which facilitates the reduction of the reaction energy of 12H-NPCZ stepwise dehydrogenation and improves the catalytic activity of the catalyst.The apparent activation energies of the three stages of the Pd/20W@A catalyst were 45.66 k J/mol,69.23 k J/mol and 82.63 k J/mol,respectively.Compared to the Pd_2.8 and Pd-W₂₀ catalysts,the apparent activation energies of the Pd/20W@A catalyst were significantly reduced.After cycling of 10th,the dehydrogenation activity of Pd/20W@A catalyst did not decrease significantly.The content of Pd⁰,W⁵⁺and Pd^δ+species in the catalyst had no significant change,demonstrating good dehydrogenation stability.（4）To simplify the catalyst preparation process and control the experimental variables,Pd-x W-A catalysts with high specific surface area and high metal dispersion were prepared by one-pot method,and the effects of WO₃ content and cold plasma treatment atmosphere to the catalyst structure and Pd nanoparticle structure were studied in detail.The addition of WO₃ not only increased the specific surface area of the catalyst and promoted the dispersion and reduction of Pd nanoparticles,but also increased the total amount of acid in the catalyst and inhibited the formation of super acid.However,the metal Pd particles were encapsulated inside the oxide causing the Pd particles mainly existed as a large number of micropositron Pd^δ+,finally,the Pd-x W-A could not achieve the complete dehydrogenation of 12H-NPCZ,which Pd-20W-A having the best dehydrogenation performance.The cold plasma treatment of the Pd-20W-A catalyst with excellent dehydrogenation using three different atmospheres exposed the metal Pd encapsulated inside the oxide,increasing the Pd⁰ content and facilitating the dehydrogenation reaction.Compared to the H₂ and Ar atmosphere treatments,Pd-20W-A-PO₂ catalyst treated by O₂,has a minimum active metal size of 1.2 nm and was able to achieve complete dehydrogenation of 12H-NPCZ.The apparent activation energies of the three stages of the Pd-20W-A-PO₂ catalyst were 49.39 k J/mol,73.11 k J/mol and 81.44k J/mol,respectively.The results of the 10 cycle stability test showed that the dehydrogenation amount started to decrease slightly to 5.07 wt%after the 8th cycle,which was significantly better than the cycle stability of catalysts Pd_2.8 and Pd-W₂₀.