Preparation Of Pt/SiO₂ Catalyst And Its Nano-size Effect In The Catalytic Oxidation Of NO In Diesel Vehicle Exhaust

Improving the purification efficiency of diesel vehicle exhaust can effectively solve the pollution problem caused by diesel vehicle exhaust pollutants to the atmospheric environment.The after-treatment systems,including DOC,DPF and NH₃-SCR,are being widely applied.The conversion of NO to NO₂ plays a key role in the DOC at the front of the systems.Platinum catalyst,with good oxidation performance and aging resistibility,has been used as the primary choice in single noble metals for NO oxidation in the purification of diesel vehicle exhaust.In this work,SiO₂,Al₂O₃and TiO₂ were used as support materials and the Pt catalysts were prepared with a loading amount of 1.0 wt.%.The conversion rate of NO was detected under the conditions of simulating diesel vehicle exhaust gas to explore the influence of the catalyst on the catalytic oxidation performance of NO.In order to study the structure-activity relationship between the catalysts and the catalytic oxidation performance of NO,a series of nano-sized（2.78-13.94 nm）Pt/SiO₂ catalysts were prepared by thermal effect.The results are as follows:（1）The（EA）₂Pt（OH）₆ solution was loaded on the SiO₂,Al₂O₃ and TiO₂ carrier,respectively,by the impregnation method;and the Pt/SiO₂,Pt/Al₂O₃ and Pt/TiO₂ were successfully prepared at the calcination temperature of 500℃.The NO catalytic oxidation reaction was carried out under the conditions of NO（1000 ppm）-O₂（10.0vol.%）-N₂ balance atmosphere.The results show that the Pt/SiO₂ catalyst has the best NO catalytic oxidation activity.（2）On the Pt/SiO₂ catalysts,NO catalytic oxidation can occur through the nitrites and bridging nitrates intermediate pathways;On the Pt/Al₂O₃ and Pt/TiO₂ catalysts,NO catalytic oxidation can occur through the bridging nitrates intermediate pathway.And bridged nitrates can transform into interlayer nitrate species that are difficult to decompose and desorb on the Pt/TiO₂ catalyst.The H₂-TPR results show that the reduction temperature of the Pt/Al₂O₃,Pt/SiO₂,and Pt/TiO₂ catalysts gradually shifts to the low temperature region,and the Pt/SiO₂ catalyst obviously has the largest total low temperature（<200℃）H₂ consumption.Therefore,the Pt/SiO₂catalyst shows the best catalytic oxidation activity of NO.（3）The Pt/SiO₂-500℃ with a smaller nanometer size is dispersed on the SiO₂ in an amorphous state;the Pt/SiO₂-600℃ and Pt/SiO₂-700℃ with a larger nanometer size are supported on the SiO₂ in the form of nanocrystals.Bridging nitrates and nitrites intermediates can be decomposed and desorbed faster on platinum catalysts with smaller nanometer sizes,thereby exhibiting better NO catalytic oxidation activity.Nitrites and bridged nitrates are easy to transform into interlayer nitrate species on the platinum catalyst with a large size（～14 nm）.And interlayer nitrate species are difficult to decompose and desorb during the reaction,which shows lower catalytic oxidation activity of NO.（4）The chemical states of Pt play a key factor in NO catalytic oxidation activity,and the Pt⁰ is considered as the active center of NO catalytic oxidation reaction.The high-valence Pt species contained in the catalyst can be reduced to Pt⁰ active species by trace NO under the atmosphere of NO（1000 ppm）-O₂（10.0 vol.%）-N₂.The remaining high-valence Pt species in the vehicle exhaust atmosphere can be further reduced to Pt⁰by the trace CO and C₃H₆.After repeated cycles of reaction under the conditions of simulated diesel vehicle exhaust gas,Pt/SiO₂-500℃（～2.78 nm）shows the best NO catalytic oxidation performance:the maximum NO conversion rate of 67%can be achieved at 310℃.（5）The fresh catalysts were aged under airflow at 800℃ for 3 hours to investigate the thermal stability.After being aged treatment,the Pt/SiO₂-600℃（～7.64 nm）has the best resistance to high temperature aging.