Preparation Of Pd/biomass Carbon Sheet Composite Catalyst And Its Catalytic Performance For Dehydrogenation Of Formic Acid

Hydrogen,as an abundant,high calorific value and environmentally friendly renewable energy source,has attracted attention from researchers in recent decades.The controlled production,safe storage and transportation of hydrogen limit the large-scale application of hydrogen energy.Chemical substances?such as boron hydride,hydrazine hydrate and formic acid?can be used as hydrogen storage media to control the storage and release of hydrogen with the catalyst under suitable conditions,which can overcome problems of the energy density,safety and effectiveness of traditional hydrogen storage technology.Formic acid,the simplest organic acid with hydrogen component of up to 4.4 wt.%,is considered to be the most potential hydrogen storage material with the advatages of liquid at room temperature,non-toxic,easy to transport and non-flammable.The catalyst of formic acid decomposition can be divided into homogeneous catalyst and heterogeneous catalyst.Compared with the homogeneous catalyst which is difficult to separate and has poor stability,it is more practical to study the heterogeneous catalyst which can efficiently catalyze the decomposition of formic acid.The main work of this thesis focuses on the development of a new and efficient formic acid dehydrogenation catalyst.The poplar catkin-derived carbon was used as a catalyst carrier.The activity of the catalyst was improved by the modification of the support and the addition of cocatalyst.The experimental results of formic acid dehydrogenation were used to evaluate the performance of the synthesized catalyst.The main research contents and research results of this paper are as follows:?1?Preparation of Pd/CPC catalyst and the study of their catalytic performance for dehydrogenation of formic acidThe poplar catkin with a tubular structure was directly carbonized in a tube furnace in N₂ flow at 800°C,and then the carbonized poplar-derived carbon was broken by ultrasonication to obtain the carrier?CPC?.Pd nanoparticles?NPs?were deposited on micronsheets by liquid phase reduction to synthesize Pd/CPC catalyst.The characterization results showed that the carrier has porous two-dimensionalmicrosheet structure with thickness of about 200 nm.At the same time,the Pd NPs were highly dispersed on the surface of the CPC,and the average size of them was 3.0nm.The performance of the catalyst was tested by formic acid dehydrogenation.The results showed that the synthesized catalysts exhibited high catalytic performance.The initial turnover frequency TOF is 1613 mol_H2/(mol _Pd·h).The reason can be attributed to the small crystallite size of the catalyst,the high dispersion of Pd particles on the surface of support,large specific surface area and unique two-dimensional microsheet structure of CPC.This result indicated that the Pd/CPC nanocatalyst has a special potential for releasing hydrogen.At the same time,the effects of different Pd loading in catalysts for the catalytic performance and the activation energy of the catalyst were also studied in detail.?2?Preparation of Pd/NPC catalysts and the study of their catalytic performance for dehydrogenation of formic acidIn order to improve the reactivity of the catalyst,the carrier is modified by doping the nitrogen element?N?to obtain a nitrogen-containing carbon material to prepare new catalyst.Using melamine as a nitrogen source,the nitrogen element was successfully doped into the carrier by heat treatment together with the poplar catkin to prepare NPC.Pd NPs were deposited on NPC microsheets by liquid phase reduction to synthesize Pd/NPC catalyst.The characterization results showed that the carrier NPC completely retained the two-dimensional microsheet structure with almost no change in thickness?200 nm?.At the same time,Pd NPs?average particle size was 2.8 nm?were uniformly dispersed on the surface of the porous support.The catalyst had excellent catalytic performance for the dehydrogenation of formic acid,and the TOF value is 2112 mol_H2/(mol_Pd·h).The volum of hydrogen production can still be maintained at 94.2%after 4 cycles,indicated the good recyclability of the catalyst.The reason can be attributed to the uniform distribution of the small size metal particles on the surface of the catalyst,and the electronic interaction between the Pd and N atoms.?3?Preparation of Pd-Co₂P/NPC catalyst and the study of their catalytic performance for dehydrogenation of formic acidIn order to further improve the performance of the catalyst,a novel nanocatalyst was synthesized by adding cocatalyst Co2P.The mixture of CPC microsheets,melamine and cobalt sources were co-pyrolyzed to synthesize Co₂P/NPC,and Pd NPs were further deposited on it to synthesize a novel composite of Pd-Co₂P/NPC.The characterization results showed that the carrier retained a complete porous two-dimensional microsheet structure,with a large specific surface area of 797.6m² g^-1.At the same time,the particles?size was 1.9 nm?were uniformly distributed on the surface of support.The catalyst exhibited excellent catalytic performance for dehydrogenation of formic acid.The TOF value is 2980 mol_H2/(mol_Pd·h),which is about 4 times of Pd/NPC catalyst under the same experimental conditions.The reason for this excellent activity can be attributed to the fact that the addition of Co₂P effectively inhibits the poisoning of the metal palladium surface in the catalyst.And on the other hand,the addition of Co₂P reduced the catalyst resistance to accelerate the electron transport efficiency,which colud effectively improve the catalytic activity.In addition,the uniformly dispersed of particles and the interaction between Co₂P and Pd also contributes to the improvement in catalyst performance.At the same time,the catalyst showed excellent recyclability.The reactivity of the catalyst did not decrease significantly after 6 cycles,and the last volume of hydrogen produced was 97.6%of the initial volume.In summary,three different catalysts were prepared by using carrier modification and co-catalyst in this study:Pd/CPC,Pd/NPC,Pd-Co2P/NPC.The experimental results show that the three catalysts have good catalytic activity and good stability for the hydrogen production system of formic acid.These catalysts have certain application prospects in the reaction system of hydrogen release.