Preparation of Pd-Pt/HZSM-5 catalysts and their catalytic performance for low-concentration methane combustion
Pd-Pt/HZSM-5 catalysts were prepared by impregnation method using Pt(NH3)4Cl2 and Pd(NO3)2·2H2O as precursor salts of Pt as well as Pd,and HZSM-5 molecular sieves as carriers.The Pd-Pt/HZSM-5 catalysts with different Si/Al ratios[n(SiO2)∶n(Al2O3)=38∶1,200∶1,800∶1],named as Pd-Pt/HZSM-5(38),Pd-Pt/HZSM-5(200)and Pd-Pt/HZSM-5(800),respectively,were characterized by XRD,SEM,TEM,Raman,XPS,CO2-TPD and NH3-TPD,and evaluated on their catalytic activity for low-concentration(0.5%volume fraction)methane combustion by a fixed bed reactor.The results showed that Pd-Pt/HZSM-5(38)exhibited the best catalytic performance with smaller Pd/Pt particle size(6.3 nm),the largest number of PdO crystals,active Pd/Pt species and acidic sites,the highest n(Pd0)∶n(Pd2+)and the highest Pt mass fraction(0.19%).Compared with those of Pd-Pt/HZSM-5(800)and Pd-Pt/HZSM-5(200),the ignition temperature(T10)of low-concentration methane combustion catalyzed by Pd-Pt/HZSM-5(38)decreased by 27.54 and 30.92℃,respectively,while the total conversion temperature(T90)was reduced by 3.38 and 17.51℃,respectively.Moreover,Pd-Pt/HZSM-5(38)could run stably at 360℃for 150 h with no significant decrease in the activity,and the methane conversion rate was maintained above 99.6%.
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