Insights Into The Propane Complete Oxidation Over Pt/ZrO₂ Catalyst Promoted By MoO₃

Among various volatile organic compounds（VOCs）,alkanes（such as propane）are one of the most stable organic compounds and difficult to be completely oxidized.Therefore,the catalytic deep oxidation of propane is commonly considered as a representative model reaction to evaluate the catalytic performance.Precious metals（Pt,Pd）have high oxidation activity and excellent stability for hydrocarbons combustion,but there are still serious issues such as low utilization rate and obvious agglomeration at high temperature for precious metals.The additives usually could improve the oxidation activity and inhibit the agglomeration of precious metal,which results in the increase of utilization rate of precious metal.In this work,Mo-modified Pt/ZrO₂ catalysts were prepared.Both the influence factors and reaction mechanism of propane combustion over Pt-based catalysts were discussed in detail.The catalysts were characterized by XRD,Raman,TEM,XPS,H₂-TPR,NH₃-TPD and in situ infrared spectroscopy.It is found that the generation of the Pt-MoO₃ interface and the regulation of its amount have a great influence on the catalytic activity for propane combustion.The mechanism of propane complete oxidation over Pt-Mo/ZrO₂ catalyst was studied.The specific work of this thesis is shown as following:1.The addition of MoO₃ in Pt/ZrO₂ catalysts significantly improves the activities for propane combustion.The highest activity(a reaction rate of 845.4 mol s^-1g _Pt^-1and a turnover frequency（TOF）of 0.688 s^-1 at 200 ℃)was obtained on a Pt-1Mo/ZrO₂catalyst（with 1 wt.%Mo content）,which was much higher than that on the pristine Pt/ZrO₂(a reaction rate of 42.4 mol s^-1g _Pt^-1and a TOF of 0.0186 s^-1 at 200 ℃).Such improvement is ascribed to the generation of the Pt-MoO₃ interfacial sites,on which the cleavage of C-H bond and decomposition of the surface carbonates were accelerated.The highest activity is obtained on the Pt-1Mo/ZrO₂ catalyst is attributed to its highest amount of the Pt-MoO₃ interfacial sites,which possesses much higher intrinsic activity than the Pt⁰-Ptⁿ⁺pairs.Moreover,the intrinsic activities of active Pt sites(TOF_Pt)and Pt-MoO₃ interfacial sites(TOF_Pt-MoO3)were clearly differentiated,namely,the interfacial sites possess ca.40-fold higher activity than the Pt surface atoms(with TOF_Pt-MoO3 of 0.731 s^-1 at 200 ℃ versus TOF_Pt of 0.0186 s^-1)for Pt-1Mo/ZrO₂ catalyst.2.The Mo-modified Pt/ZrO₂ catalysts showed significant enhancement of propane combustion activity,and the addition order of metal Mo and Pt（co-impregnation or stepwise impregnation）had obvious effects on the catalytic performance.Among the Mo-modified Pt/ZrO₂ catalysts prepared by three different methods（Pt-Mo/ZrO₂,Pt/Mo/ZrO₂ and Mo/Pt/ZrO₂）,the co-impregnated Pt-Mo/ZrO₂catalyst exhibited the highest activity(a reaction rate of 7.44μmol g^-1_cat s^-1 at 200 ℃).Based on the TEM,CO chemisorption,CO-IR and H₂-TPR characterization results,it is found that the co-impregnated Pt-Mo/ZrO₂ catalysts have the most sufficient contact between MoO₃ and Pt species,（i.e.,the highest number of Pt-MoO₃interfaces）,where the number of Pt-MoO₃ interfaces on the catalysts is in order:Pt-Mo/ZrO₂(7.60μmol g_cat^-1)>Pt/Mo/ZrO₂(3.40μmol g^-1_cat)>Mo/Pt/ZrO₂(1.20μmol g^-1_cat).The TOF_Pt-MoO3 of the interfacial sites on the catalyst is much higher than TOF_Pt（at least 50 fold）.It can be seen that the difference in the activity of the Mo-modified Pt/ZrO₂ catalysts is mainly ascribed to the difference in number of interfacial sites.The results of the kinetic study demonstrated that propane was weakly adsorbed on the catalyst surface,and exhibited a primary reaction behavior on the catalyst.However,the reaction order of oxygen was negative,indicating that oxygen exhibited a significant inhibition effect on the catalytic activity.The addition of Mo significantly weakened the inhibition effect of oxygen,especially for Pt-Mo/ZrO₂ catalyst,of which the activation energy was reduced the most(Ea for the Pt/ZrO₂catalyst:102.8±4.8 kJ mol^-1;Ea for the Pt-Mo/ZrO₂catalyst:only 40.3±1.8kJ mol^-1).The present work provides a deep understanding of the enhancement of the propane oxidation reaction by Mo-promoted Pt/ZrO₂ catalysts.