Preparation Of Pd-based Catalysts With Controllable Particle Size And Hydrophilicity And Their Application In Direct Synthesis Of H₂O₂

The direct synthesis of H₂O₂（DSHP）from hydrogen and oxygen has attracted extensive attention for its advantages of simple process and environmentally friendly.Study shows that Pd-based catalyst is the most effective catalyst for H₂O₂ synthesis.Carbon material were used as Pd catalysts support to synthesis H₂O₂for its stable chemical properties.However,traditionally supported catalysts active components were easy fall off during reaction or migrate and aggregate in the roasting process,then particle size increases and the specific surface area of active components reduced.Furthermore,hydrophilicity of carbon will affect the dispersibility of catalyst in the reaction medium and the mass transfer of reactants.Therefore,it is extremely important to explore a method that can regulate the diameter of Pd nanoparticles and stabilize Pd nanoparticles,but also enhance the dispersion of catalyst in the reaction medium,so as to strengthen the mass transfer of reactants.Many methods have been developed to regulate the particle size of Pd or increase the hydrophilicity of support,but there are still great challenges to obtain Pd based catalysts with controllable particle size and hydrophilicity.In this work,we introduced N and O elements into carbon materials,and modified Pd with imidazole ionic liquids.thereby adjusting the capacity of the catalyst of directly synthesise H₂O₂.Analysis catalysts performance by FTIR、TEM、XPS、XRD、H₂-TPD and O₂-TPD.The effects of morphology,particle size and composition of active components on the direct synthesis of H₂O₂ were also discussed.Specific research content are as follows:1.Pd/CN catalyst was prepared by nitrogen doping into biomass carbon,then used to the direct synthesis of hydrogen peroxide（H₂O₂）.The effects of the loading of metal Pd precursor and stabilizer（citric acid）on the catalytic activity of direct synthesis of hydrogen peroxide（H₂O₂）were investigated.The results showed that N doping of biomass carbon improved the hydrophilicity of Pd/CN catalyst,reduced the nanoparticle size of Pd and improved its performance.The H₂O₂ productivity,selectivity and H₂ conversion are 7283.35 mmol·g _Pd^-1·h^-1,72%and 46.44%,respectively.Compared with Pd/C,it increased by 32.47%,11.54%,34.22%respectively.2.The direct synthesis of hydrogen peroxide（H₂O₂）was studied by using a serials of catalysts Pd/CNT-X（X=N,O）prepared by doped N and O into CNT.The doping of both N and O into a carbon carrier could anchor Pd nanoparticles and reduce the nanosize of Pd,increase catalysts hydrophilicity,then significantly enhanced H₂O₂production and stability.For Pd/CNT-OB-N catalyst,which have appropriate nanoparticle size（4.5±0.2 nm）and contact angle（21.390°）,the ratio of Pd⁰/Pd²⁺is2.648.This catalyst has the best catalytic performance.The productivity and selectivity of H₂O₂ are 9943 mmol·g _Pd^-1·h^-1 and 98%,the conversion of H₂ is 68%were achieved.Compared with Pd/CNT,it increased by 34.57%,21.98%and 43.68%respectively.In addition,after five cycles,the H₂O₂ productivity of the catalyst decreased by only14.4%,39.6%and 50.99%lower than Pd/CNT and Pd/C respectively,indicating that the catalyst also has high cycle stability.3.Pd@IL/CNT-COOH catalyst were prepared by supported carbon nanotube nodified by imidazole ionic liquid（IL）.Compared with Pd/CNT-COOH catalyst,dispersion and the ratio of Pd⁰/Pd²⁺（2.337）were increased in Pd@IL/CNT-COOH catalyst.The productivity and selectivity of H₂O₂ are 11028.57 mmol·g _Pd^-1·h^-1,98.8%respectively,the conversion of H₂ is 73.17%.After added IL,the structure and Pd particle size of the Pd@IL/CNT-COOH catalys did almost not change.Pd@IL₂₀₀/CNT-COOH catalyst has the best catalytic performance.After 5 recycle,the H₂O₂productivity of the catalyst decreased by 10.19%.lower than Pd/CNT-COOH by19.67%,it states that the stability of Pd@IL/CNT-COOH catalyst increased.