Modification Of Pd-based Catalyst For H₂O₂ Direct Synthesis From Hydrogen And Oxygen

Hydrogen peroxide（H₂O₂）is recognized as an important fine chemical product and inorganic chemical material.It is widely used for pulp and fabric bleaching,chemical synthesis,water treatment,electronic industry,spaceflight and so on due to its “green” characteristic.So far,H₂O₂ is produced on an commercial scale by the anthraquinone process.However,this process is a multi-step method with significant energy input requirement and waste generation.The direct synthesis of H₂O₂ from H₂ and O₂ become a truly promising alternative to the anthraquinone process due to its remarkable advantages of atom economy,lower energy consumption,and low operation costs.But the low H₂O₂ selectivity and the security concerns with the mixing of H₂ and O₂ are still the major problems which negatively affect the development of direct synthesis process in industrial scale.Thereby,in the view of selectivity improvement,we incorporated other metals into Pd catalysts to achieve the improvement of H₂O₂ selectivity and productivity.The selectivity of the Pd catalysts could be enhanced by the addition of Ag.It can be observed that the H₂O₂ selectlvity for optimal PdAg catalyst was increased to 70.9% while pure Pd catalyst just showed a H₂O₂ selectlvity of 54.0%.Both the ensemble effect and electronic effect detected in PdAg catalyst resulted in the improvement of selectivity.Ag additive increased the monomeric Pd sites which were the primary active sites for H₂O₂ formation.In addition,the content of Pd2+ was increased via the electronic interaction between Pd and Ag,which prevented the decomposition and hydrogenation of H₂O₂ on some extent.We also evaluated the catalytic performance of alumina-supported PdZn bimetallic samples for the direct synthesis of hydrogen peroxide.Our results showed that the configuration of Zn and Pd had both a significantly positive effect on H₂O₂ productivity and a corresponding increasing on the selectivity.These improvements were due to both the geometric change caused by Zn and the electronic interactions between Pd and Zn.Compared with monometallic Pd,PdZn catalysts showed a better Pd dispersion and an incremental Pd0 content caused by the electronic interactions between Pd and Zn.As a result,these two effects cooperatively led to a striking H₂O₂ productivity as well.At the same time,the addition of Zn increased the isolated Pd sites,the primary active sites for H₂O₂ formation,which promoted the H₂O₂ selectivity.