Light Alkanes Catalytic Combustion Over Precious Metal (Pd,Pt) Catalysts

There are many kinds of volatile organic compounds（VOCs）,thus the comprehensive treatment of VOCs emission is relatively difficult.It is very meaningful to use light alkanes as model reactant to evaluate the catalytic performance of precious metal catalysts,because the light alkanes with stable bonds are very difficult to be oxidized.This thesis mainly described the catalytic combustion of light alkanes（such as methane and propane）over precious metal Pd,Pt catalysts and discussed the influence factor and reaction mechanism of combustion reaction.A series of catalysts(Ba O modified Pd/Al₂O₃and Pt/Al₂O₃ catalysts;MoO₃and Nb₂O₅modified Pt/ZrO₂ catalysts;Pt-SO₄^2-/Al₂O₃ catalysts calcined under different temperatures（T=400,500,600 and 700℃）were prepared for light alkanes combustion.The detail characterizations of XRD,H₂-TPR,XPS,Raman,TEM,NH₃-TPD and in-situ FTIR were conducted to investigate the combustion mechanism of light alkanes over catalysts.The specific contents of this thesis are shown in following:1.The Pd and Pt catalysts supported by Al₂O₃ were prepared by impregnation method.The catalytic combustion activities of catalysts for methane,ethane and propane,and the effect of Ba additive on the catalytic performance were investigated.For Pd/Al₂O₃ catalyst,both the size of Pd O active species and its reduction temperature increased when Ba additive was added,which resulted in the formation of more stable Pd O active species.This should be the main reason for the improved activity of the Pd-Ba/Al₂O₃ catalyst compared with Pd/Al₂O₃ catalyst.The amount of active species Pt⁰ reduced and PtO_x species became difficult to be reduced to Pt⁰ due to the enhanced interaction between PtO_x and Al₂O₃ support after introduction of Ba for Pt/Al₂O₃ catalyst,which was responsibility to the decrease of the catalytic activity of Pt-Ba/Al₂O₃.The same activity regular pattern of Pd and Pt catalysts was obtained for alkane oxidation:propane>ethane>methane.The Pd/Al₂O₃ catalyst might be favorable for C–H bond activation,while Pt/Al₂O₃ catalyst should be beneficial for C–C bond break.The Pt/Al₂O₃ catalyst gave significant difference of catalytic performance for C1 to C3 alkane deep oxidation compared with of Pd/Al₂O₃ catalyst.The Pt/Al₂O₃ catalyst showed a higher activity for alkanes with high carbon content.2.Pt/ZrO₂ catalysts promoted with MoO₃ and Nb₂O₅ were tested for short-chain alkanes（methane,ethane,propane and n-hexane）combustion.For short-chain alkanes combustion,the inhibition of MoO₃（for methane reaction）would dramatically transform to promotion（for ethane,propane and n-hexane reaction）as carbon chain increases,while the remarkable promotion of Nb₂O₅ is gradually weakened with the increase of the carbon chain.Based on the detailed study on oxidation reactions of methane and propane over catalysts,the different roles of promoters in the reactions were related to the differences in the structural properties and the oxidation states of the Pt species.The MoO₃promoter could be decorated on the surface of Pt species for Pt-Mo/ZrO₂ catalyst,while Nb₂O₅promoter on the support may be partially covered by Pt particles for Pt-Nb/ZrO₂ catalyst.The generation of accessible Pt-MoO₃interfacial sites,high concentration of metallic Pt species and high surface acidity in the Pt-Mo/ZrO₂ were responsible for the enhanced activity for propane combustion.The lack of enough accessible Pt-Nb₂O₅ interfacial sites in the Pt-Nb/ZrO₂ but enhanced surface acidity explained the slight improvement in the activity for propane combustion.However,the stabilized Ptⁿ⁺species in the Pt-Nb/ZrO₂ was responsible for the much improved activity for methane combustion,while the Ptⁿ⁺in the Pt-Mo/ZrO₂ could be reduced during the reaction,and the less exposed surface Pt species by MoO₃-decoration accounted for the inhibited activity for methane combustion.In addition,it can be concluded that MoO₃promotion is favorable for the activation of C–C bonds,while the Nb₂O₅ promotion should be more beneficial for the activation of C–H bonds with high energy such as methane.3.Pt catalysts supported by solid acid(SO₄^2-/Al₂O₃)are used to catalyze the complete oxidation of methane and propane,and the effect of calcination temperature on the performance of the catalyst is investigated in detail.It is found that the particles of Pt species on the catalyst gradually increase when the calcination temperature of Pt-SO₄^2-/Al₂O₃catalyst increases（from 400℃to 700℃）,in addition,the metallic Pt species with big size can be observed for Pt-SO₄^2-/Al₂O₃-600 and Pt-SO₄^2-/Al₂O₃-700catalysts.Compared with the Pt/Al₂O₃ catalyst,the Pt-SO₄^2-/Al₂O₃ catalyst calcined at different temperatures can significantly promote the complete oxidation of propane.However,the promotion effect gradually decreased when the calcining temperature was increased.For the complete oxidation of methane,the Pt-SO₄^2-/Al₂O₃ catalyst calcined under different temperatures exhibited an inhibition effect on the complete oxidation of methane compared with the Pt/Al₂O₃ catalyst.The Pt-SO₄^2-/Al₂O₃catalysts show completely different effects（promoting or inhibiting）for the complete oxidation of propane and methane,indicating that the active sites required for the complete oxidation of propane and methane are different.The promotion effect of Pt-SO₄^2-/Al₂O₃ catalyst for the propane oxidation is related to the acidic sites on the surface of the catalyst,while the surface acid sites do not significantly promote the methane oxidation.The addition of acidic sites may be beneficial to the C–C bond activation of long-chain alkanes.The activation of the C–H bond by acidic sites is not obvious.The acid sites may affect the exposure of Pt species,resulting in a decrease of oxidation activity for methane over the catalyst.NH₃-TPD results show that the loss of SO₄^2-will cause the decrease of surface acid sites when the calcination temperature of Pt-SO₄^2-/Al₂O₃catalyst is increased,which may be responsible for the gradual decrease in the promotion of propane oxidation.In addition,the gradual increase of Pt species particles will also reduce the promotion effect.The change in particle size of Pt species is also the main reason for the decrease of methane oxidation activity with the increase of temperature.