Synthesis Of Zeolite Confined Pt/PtMn Catalysts And Catalytic Performance For Oxidation Of VOCs

Volatile organic compounds（VOCs）are important precursors of ozone and PM_2.5,and will be the focus of atmospheric governance at home and abroad for a period of time in the future.Catalytic oxidation has been widely used in the treatment of VOCs due to its advantages of low operating temperature,high purification efficiency,and less secondary pollution.Among many VOCs oxidation catalysts,noble metal Pt-based catalysts exhibit good VOCs catalytic oxidation activity,but they are prone to agglomeration and sintering at high temperatures,resulting in decreased activity.Encapsulation of noble metals by the rigid framework of zeolite can effectively stabilize metal nanoparticles and improve the stability of Pt-based catalysts.Meanwhile,some transition metal species（such as Mn）can synergize with Pt to enhance the catalytic oxidation performance of VOCs.Based on the above analysis,in this paper,Pt@ZSM5 and PtMnx@ZSM5 catalysts were prepared around the strategy of zeolite-confined metal nanoparticles（metal@zeolite）.The catalytic activities of these catalysts for typical VOCs were investigated and the catalytic mechanism was analyzed.The main work is as follows:（1）Using organic ligand-assisted one-pot hydrothermal method,ZSM-5 confined Pt nanoparticle catalyst（Pt@ZSM5）was synthesized,and its catalytic activity on different VOCs（benzene,toluene,acetone）was explored.The results showed that when the Pt loading was 0.66 wt.%,the T₉₀values of Pt@ZSM5 for catalytic oxidation of benzene,toluene and acetone were 146℃,163℃and 200℃,respectively,and the activity of Pt@ZSM5 was better than that of Pt/ZSM5 prepared by impregnation method(The corresponding T₉₀values were 164℃,185℃and207℃,respectively).This was mainly due to the smaller metal particle size and higher surface energy of Pt@ZSM5,which was conducive to improving catalytic activity and metal efficiency.In addition,Pt@ZSM5 exhibited excellent stability and durability,showing promising prospects on practical industrial application.（2）Using the one-pot hydrothermal method,a series of PtMn_x@ZSM5 bimetallic catalysts were synthesized by doping Mn species on the basis of Pt@ZSM5,and the effect of Mn loading content on the catalytic performance was studied.The results showed that the doping of Mn greatly improved the catalytic activity for acetone.When the mass ratio of Pt/Mn was 1/0.2(PtMn_0.2@ZSM5),the T₉₀value was 40℃lower than that of Pt@ZSM5.In addition,it also exhibited excellent stability.However,the catalytic activity of PtMn_x@ZSM5 for benzene and toluene decreased with the increase of Mn loading.When the mass ratio of Pt/Mn was 1/1,the T₉₀values of PtMn₁@ZSM5 for benzene and toluene were 32°C and 16°C higher than those of Pt@ZSM5,respectively.The study found that this was due to the different active sites of the catalysts for different VOCs.Among them,the main active sites for benzene and toluene were Pt sites,while the main active sites for acetone were Pt-Mn interface sites,so the doping of Mn made the catalytic activity for different VOCs vary greatly.（3）The interaction between Pt-Mn in the zeolite-confined PtMn_x@ZSM5bimetallic catalyst and its mechanism of promoting the catalytic oxidation of acetone were investigated.It is found that the doping of Mn promoted the transfer of electrons from Mn to Pt,so that more Pt ions were reduced to active Pt⁰species,thereby promoting the oxidation reaction of acetone.At the same time,the electron transfer generated more acid sites,especially L-acid sites,which were beneficial to the adsorption of acetone molecules,the cleavage of C-C bonds,and the desorption of the acidic product CO₂.These could further accelerate the forward progress of the acetone oxidation reaction.Finally,it is proposed that the reaction of acetone on PtMn_0.2@ZSM5 followed the L-H reaction mechanism,that is,the acetone was first adsorbed on the catalyst surface,and then it was gradually oxidized to acetate,formate,and finally CO₂and water by the adsorbed and activated oxygen species.