The Performance Of Titanium Dioxide Supported Palladium-based Catalyst For Direct Synthesis Of Hydrogen Peroxide And In Situ Oxidation Of Methane

The direct synthesis of hydrogen peroxide（H₂O₂）from hydrogen（H₂）and oxygen（O₂）is an efficient and clean strategy.However,it is challenging because of thermodynamically more favorable to the production of H₂O rather than H₂O₂,while the generated H₂O₂ also degrade through further hydrogenation and decomposition.Nanosized palladium（Pd）-based catalysts are widely used in the direct H₂O₂ synthesis,while its selectivity and yield remain inferior because of the O-O bond cleavage from both the reactant O₂ and the produced H₂O₂,which is assumed to have originated from various O₂ adsorption configurations on the Pd nanoparticles such as“Pd-O-O-Pd”,“Pd-O-O”and“bridge”,while the adsorption of O₂ on isolated Pd atom is usually the“Pd-O-O”type and could therefore reduce the possibility of O-O bond breaking.Thus,it would be encouraging to develop a single Pd atom catalyst to improve H₂O₂ yield and selectivity.More recently,some organic molecules,such as methane to methanol and propylene to propylene oxide,have been oxidized under mild conditions using H₂O₂.While H₂O₂ produced in situ by H₂ and O₂ can avoid the direct use of expensive commercial H₂O₂.Among them,methane is selectively oxidized to methanol by H₂O₂produced in situ at low temperature,which avoids the disadvantages of high energy consumption at high temperature and methane is prone to excessive oxidation to carbon dioxide.However,the efficiency of methane oxidation has been very low due to the easy degradation of H₂O₂ at the catalytic active site,which has not been well solved up to now.Classical strong metal-support interaction usually occurs when the active metal is encapsulated in the support,which may reduce the degradation of H₂O₂ and thus effectively oxidize methane.The main research contents are as follows:On the one hand,a series of oxidation state palladium catalysts（O-Pd/Ti O₂,including single atom,cluster and nanoparticle）were prepared by simple hydrothermal method for direct synthesis of H₂O₂.A metallic palladium catalyst（M-Pd/Ti O₂）was also synthesized for comparison.The results show that the performance of O-Pd/Ti O₂catalyst（including single atom and cluster）is better than that of M-Pd/Ti O₂ catalyst.In particular,the H₂O₂ yield of Pd single atom catalyst（0.1%O-Pd/Ti O₂）is 115 mol/g _Pd/h,which is 14 and 135 times that of clusters and nanoparticles,respectively.The selectivity for H₂O₂ is greater than 99%.More interestingly,H₂O₂ degradation was also shut down.The concentration of H₂O₂ reached 1.07 wt.%,outperforming the reported highest values of catalysts.DFT calculations show that the O-O bond breaking is significantly inhibited on the single Pd atom and the O₂ is easier to be activated to form*OOH and H₂O₂;and the energy barrier of H₂O₂ degradation is also higher.As a result,the high yield and selectivity is obtained on single atom catalyst.On the other hand,a common supported catalyst（Pd Au/Ti O₂）was synthesized by solvothermal method,and then calcined Pd Au/Ti O₂ in H₂ atmosphere to successfully prepare the catalyst with Classical SMSI（Pd Au@Ti O_x）.The results showed that the Classical SMSI significantly inhibited the degradation of H₂O₂,allowing Pd Au@Ti O_xto efficiently oxidize methane to methanol at 70℃with a yield of 540 mmol/g _{Pd Au}/h and a selectivity of 96%at a methane conversion rate of 12.5%,which exceeded the highest reported values.In addition,a small amount of dicarbon（C₂）products such as ethanol and acetic acid were produced.C₂ products can be produced at 70℃,which has never been reported in literature.In contrast,the Pd Au/Ti O₂ catalyst has a methane conversion rate of only 2.5%,a methanol yield of 12.1 mmol/g Pd Au/h,and a selectivity of 10%.Most of the products are intermediate CH₃OOH and by-product CO₂ under the same conditions.