Pd Nanocages (PdNCs) For Direct Synthesis Of Hydrogen Peroxide From Hydrogen And Oxyge

In recent years,the direct synthesis of hydrogen peroxide（H₂O₂）from hydrogen and oxygen is a green and economical method that has been attracting attention.Palladium-based catalysts are considered to be the most effective catalysts for the direct synthesis of H₂O₂（DSHP）from hydrogen and oxygen,it has been studied extensively by previous authors.Supported palladium-based catalysts currently have some problems:the prepared palladium nanoparticles are unstable,the palladium particles easy to fall off from the carrier,causeing the palladium particles be leached out and destroying the morphology by acid,which increases the defects on the surface of the palladium crystals and making O-O bonds are easy to break;in addition,the catalytic reaction only occurs on the surface of the catalyst,which means that only the surface palladium atoms are utilized,resulting in low palladium utilization and causing great waste.To improve the selectivity and yield of H₂O₂,the palladium particles must be highly stable and able to inhibit the O-O bond breakage.The metal nanocage is a hollow nanostructure,which is insensitive to the environment and can maintain great stability,and its hollow structure design gives itself a large surface area,which can expose more active sites,and it can also change its shell thickness by regulating the amount of precursors to improve the metal utilization.For these reasons,this study adopts the template sacrifice method:silver nanoparticles are used as sacrificial templates to prepare hollow and structurally stable palladium nanocage（PdNC）through direct chemical reduction and electrical replacement reactions,and then loaded on activated carbon to prepare palladium nanocage catalysts（PdNC/C）.On the one hand,the specific surface area of palladium is enlarged,the active sites are increased,and the silver atoms enter the crystalline phase of the palladium crystal in the shell layer during the preparation process,which regulates the geometric structure and electronic state of the palladium crystal surface;on the other hand,the thickness of the shell layer is directly controlled by regulating the amount of palladium precursor solution,which reduces the amount of palladium used and saves the cost.The structure and properties of the catalysts were investigated using TEM,XRD,XPS,H₂-TPD and O₂-TPD characterization,and their performance on the direct synthesis of H₂O₂ was tested,and the effects of the particle structure,particle size and the amount of Pd precursor of PdNC on the performance of the DSHP from hydrogen and oxygen were explored.The specific studies are as follows:（1）The study of the effect of a novel catalyst PdNC/C on the direct synthesis of H₂O₂.A series of activated carbon loaded PdNC/C-x（x=0.3,0.5,0.7）catalysts were prepared by controlling the amount of Pd precursor solution.The results showed that the PdNC/C catalysts possess a larger surface area of palladium than the conventional palladium-based catalysts,and their outer and inner cavity surfaces can participate in the catalytic reaction.The activity evaluation results showed that the prepared PdNC/C catalyst facilitated the synthesis of H₂O₂,and the H₂ conversion,H₂O₂selectivity and yield of PdNC/C were significantly improved compared with Pd/C,andtheNumericsizewererankedasfollows:PdNC/C-0.5>PdNC/C-0.7>PdNC/C-0.3>Pd/C.When the Pd precursor solution was added at 0.5 m L,the H₂O₂ selectivity and yield reached 59.29%and 4256.28mmol/g _Pd·h^-1,respectively.This is because the silver atoms entered the lattice of the palladium crystal during the preparation process,which optimized the geometric and electronic structure of the palladium surface,adjusted the Pd²⁺/Pd⁰ species ratio on the surface of the active component and inhibited the O-O bond breaking on the palladium surface,thus improving the selectivity and yield of H₂O₂.（2）The study of the effect of palladium nanocage size in catalyst PdNC/C on the direct synthesis of H₂O₂.The PdNC/C-1,PdNC/C-2 and PdNC/C-3 catalysts were successfully prepared by controlling the size of the silver particles,and the unpredictable deformation of the outer surface when the size of the palladium nanocage is too large and the reduction of the reactivity when the size is too small due to the amorphous morphology.Only when the size of the palladium nanocage is suitable,the morphology is stable and the shell layer is controllable.The activity tests showed that the suitable size of PdNC/C-0.3,PdNC/C-0.5 and PdNC/C-0.7 were more favorable than the conventional Pd/C for the DSHP from hydrogen and oxygen.The optimal H₂O₂ selectivity and yield reached 71.48%and 4842.38 mmol/g _Pd·h^-1,respectively.Hydrogenation and decomposition experiments of H₂O₂ also verified that the presence of silver suppressed the side reactions,and cycling experiments also demonstrated that the palladium nanocage catalyst had higher stability than the conventional single palladium catalyst.（3）The study of the effect of catalyst PdNC/CN on the direct synthesis of H₂O₂with nitrogen modified carbon carriers.The catalyst PdNC/CN was prepared using nitrogen-doped modified carbon material loaded with palladium nanocage.It was shown that nitrogen doping into the activated carbon changed the hydrophilic properties of the catalyst.The activity evaluation results showed that the nitrogen-modified catalyst PdNC/CN possessed better catalytic activity,which was attributed to the newly added pyridine nitrogen（N-6）that could bond well with-OH in the reaction medium.Hydrogenation and decomposition experiments of H₂O₂revealed that the nitrogen-modified PdNC/CN catalyst could inhibit the occurrence of side reactions and improve the stability of H₂O₂.