Preparation Of Polyelectrolyte Membrane Supported Pd-based Catalyst And Its Application In Direct Synthesis Of H₂O₂

The direct synthesis of H₂O₂from hydrogen and oxygen is an atomically economical and environmentally friendly method for the preparation of H₂O₂and has been receiving a lot of attention from researchers.At present,the research on direct H₂O₂synthesis catalysts mainly revolves around loaded Pd-based catalysts and has made some progress.However,the following problems remain:firstly,the Pd particles on the carriers are easily dislodged and the heat treatment,although improving the stability of the catalysts,leads to a reduction in the agglomeration and dispersion of the Pd particles;secondly,the precise control of the Pd⁰/Pd²⁺ratio and the Pd nanoparticle size is still a great challenge.Based on electrostatic interactions,the positively charged N-containing groups can facilitate the dispersion of Pd Cl₄^2–and facilitate the dispersion and size control of Pd particles in the catalyst on the one hand,and the interaction between the N-containing groups and Pd particles on the other hand can also regulate the Pd⁰/Pd²⁺ratio on the catalyst surface and facilitate the stabilisation of Pd particles.Based on this design idea,if a positively charged ammonium group is doped onto the carrier,the Pd nanoparticle size,Pd⁰/Pd²⁺ratio composition and catalyst stability can be regulated by electrostatic interactions between the ammonium group and Pd Cl₄^2–.In contrast,the layer-by-layer self-assembly method is a universal method for obtaining highly dispersed charged group carriers.Based on this,polyelectrolyte membranes containing highly dispersed ammonium groups were designed and prepared as carriers for the preparation of polyelectrolyte membrane-loaded catalysts Pd/SPE_n（n=3,5,7,9）,Pd_xAu/SPE5 and Pd_ySn/SPE₅with Pd,Au-Pd and Sn-Pd as active components by impregnation reduction.Pd⁰/Pd²⁺ratio and hydrophilicity on the performance of the Pd-based catalysts for the direct synthesis of H₂O₂were investigated.The reaction temperature,sulphuric acid dosage and H₂/O₂ratio for the direct synthesis of H₂O₂were also investigated and optimised.The details of the study are as follows.（1）Based on the electrostatic interaction force between ions,the positively charged ammonium group in polydiallyldimethylammonium chloride（PDDA）was self-assembled with the negatively charged oxygen negative ions（surface hydroxyl deprotonation）on the Si O₂surface under alkaline environment;after removing the excess PDDA,polystyrene sodium sulfonate（PSS）was added and the self-assembly was continued by the electrostatic interaction between the negatively charged sulfonic acid group in PSS and the ammonium group in PDDA.The polyelectrolyte membrane was successfully constructed on Si O₂spheres after n times of alternate assembly of PDDA and PSS,and the product was denoted as SPE_n.The Pd/SPE_ncatalyst was prepared by impregnation reduction method.（2）The catalysts Pd/SPE₃,Pd/SPE₅,Pd/SPE₇and Pd/SPE₉were prepared by regulating the number of assemblies of the polyelectrolyte films.The presence of N groups in the polyelectrolyte membrane increases the electron cloud density on the Pd surface,and the Pd⁰/Pd²⁺ratio increases with the increase of the ammonium group content.The hydrophilicity of the catalyst decreased with the increase of the number of assemblies.The results of the activity evaluation show that the selectivity of H₂O₂gradually increases with the number of polyelectrolyte assemblies,while the H₂O₂productivity and H₂conversion are the first to increase and then decrease.When the polyelectrolyte membrane was assembled five times,the catalyst Pd/SPE₅had the best H₂O₂productivity and H₂conversion of 6064.4 mmol?g _Pd^-1?h^-1and 89.8%,respectively,and showed high cycling stability.（3）A series of catalysts Pd₅₀/SPE₅,Pd₁₀₀/SPE₅,Pd₂₀₀/SPE₅and Pd₅₀₀/SPE₅were prepared by varying the dosage of H₂Pd Cl₄using SPE₅as the carrier,and the activity evaluation results showed that the H₂O₂productivity increased first and then decreased with increasing the dosage of H₂Pd Cl₄,and the optimum dosage of Pd was100μL.When the polyelectrolyte membrane is assembled for a certain number of times,the amount of ammonium groups in the membrane layer is constant and the added Pd Cl₄^2–is trapped by an equal amount of ammonium groups.As the amount of H₂Pd Cl₄increased,the amount of Pd Cl₄^2–trapped by each ammonium group also gradually increased,and the particle size of Pd in the catalyst was adjustable in the range of 0.8 nm to 3.3 nm Pd.As the amount of H₂Pd Cl₄increased,the H₂O₂productivity increased first and then decreased,while the H₂O₂selectivity gradually increased and the H₂conversion gradually decreased.（4）Due to the degradation of H₂O₂caused by the high energy sites on the catalyst surface,the H₂O₂productivity and selectivity are reduced,while metal doping can effectively reduce the high energy sites on the Pd surface.In this thesis,the catalysts Pd_xAu/SPE₅（x=1,2,5,10）and Pd_ySn/SPE₅（y=1,2,5,10）were prepared by introducing Au and Sn dopants into the monometallic Pd system and modulating the n _Pd/n _Auand n _Pd/n _Snratios,respectively.The results showed that either Au or Sn doping inhibited the degradation of H₂O₂and increased the H₂O₂productivity.The optimal n _Pd/n _Auratio was 2 and the optimal n _Pd/n _Snratio was 5,while the addition of excessive Au or Sn was not conducive to the increase of H₂O₂productivity.（5）The direct synthesis of H₂O₂by hydrogen and oxygen as a typical G-L-S non-homogeneous reaction,the H₂O₂productivity is not only related to the active component of the catalyst and the mass transfer efficiency of the reactants,but also influenced by the temperature of the catalytic reaction,the amount of acid,H₂/O₂,the reaction pressure and other factors.In this paper,the effect of reaction temperature（A）,sulphuric acid dosage（B）and H₂/O₂ratio（C）on the productivity of H₂O₂was investigated separately using catalyst Pd/SPE₅as a sample,and the synthesis conditions of H₂O₂were optimised using response surface methodology.The optimum reaction conditions were 1.84°C,1.95 m L of sulphuric acid and 2:3 H₂/O₂ratio.The productivity of H₂O₂synthesis directly from the catalyst Pd/SPE₅was6243.0 mmol?g _Pd^-1?h^-1,which was close to the optimised value.