Controlled Preparation Of Noble Metals Or Supported Noble Metal Catalysts And Their Catalytic Performance For Methane And Benzene Oxidation

In recent years,air pollution in China have become serious.Methane is an important greenhouse gas,whereas benzene is one of the widely utilized aromatic volatile organic compounds?VOCs?.Emissions of methane and benzene are harmful to the environmental and human health.Catalytic oxidation is one of the effective methods to eliminate organic compounds.The key issue of such a technology is to design and prepare high-performance and good-stability catalytic materials.Considering that porous materials?especially ordered mesoporous and ordered macroporous materials?have ordered three-dimensional porous structures and high specific surface areas,they are not only conducive to diffusion,adsorption and activation of the reactant molecules and desorption and diffusion of the product molecules,but also favorable for dispersion of the noble metal nanoparticles and enhancement of the catalytic activity and thermal stability.Therefore,the development of catalysts with ordered porous structures has become one of the hot research topics.In the past years,the Pd-based catalysts have received much attention due to their good catalytic performance at low temperatures.The loading of Pd nanoparticles on the surface of porous transition-metal oxides or mixed metal oxides with high surface areas is expected to increase generation of the active noble metal-metal oxide interfaces,thus effectively inhibiting possible growth and sintering of the nanoparticles during the reaction processes.In addition,addition of a certain amount of platinum to palladium can also promote the activity and improve stability of catalysts for methane oxidation.It has been generally accepted that the metal-support interaction plays an important role in improving activity of the supported noble metal catalysts.In order to exclude the effect of the metal-support interaction,only noble metals can be used as catalyst to investigate their catalytic performance and hence clarify the nature of the active sites.In this dissertation,we report the preparation,characterization,and catalytic performance of three-dimensionally ordered macroporous?3DOM?La_0.6Sr_0.4MnO₃-and three-dimensionally ordered mesoporous cobalt oxide?meso-Co₃O₄?-supported Pd catalysts and mesoporous Pd_xPt alloy catalysts.Physicochemical properties of the materials were characterized by means of various analytical techniques,and their catalytic activities were evaluated for the oxidation of methane or benzene.The catalytic mechanisms have been proposed.The relationship between physicochemical property and catalytic performance for methane or benzene oxidation has been established.The strategy for improving the thermal stability of catalysts were developed.The active sites for methane or benzene oxidation were identified.The main investigation results obtained in the present dissertation are as follows:?1?The rhombohedrally crystallized 3DOM La_0.6Sr_0.4Mn_1-xPd_x O₃ and yPd/3DOM La_0.6Sr_0.4MnO₃ samples were prepared using the polymethyl methylacrylate?PMMA?-templating and in-situ reduction methods,respectively.Among the 3DOM La_0.6Sr_0.4MnO₃ and 3DOM La_0.6Sr_0.4Mn_1-xPd_xO₃ samples,La_0.6Sr_0.4Mn_0.96Pd_0.04O₃exhibited the best performance for methane combustion(the terperatures(T_50%and T_90%)required for achieving methane conversions of 50 and 90%were 458 and 550^oC at a space velocity?SV?of 40,000 mL/?g h?,respectively).The in-situ reduction of the Pd-doped samples could generate the yPd/3DOM La_0.6Sr_0.4MnO₃ catalysts.The slight decrease in catalytic activity of yPd/3DOM La_0.6Sr_0.4MnO₃ was due to the partial destroy of the perovskite structure in La_0.6Sr_0.4MnO₃ after reduction at 500 ^oC.Among the yPd/3DOM La_0.6Sr_0.4MnO₃ samples,1.18Pd/3DOM La_0.6Sr_0.4MnO₃possessed the best thermal stability and CO₂-,H₂O-,and SO₂-tolerant ability,a result possibly due to the strong interaction between Pd nanoparticles and 3DOM La_0.6Sr_0.4MnO₃.Deactivation of the Pd-based catalysts due to aggregation of the nanosized Pd particles at high temperatures is a serious issue in methane combustion.We believe that our in-situ reduction strategy could generate the Pd-based perovskite catalysts that improve the thermal stability and enhance the anti-tolerant ability in methane combustion.?2?The cubic Pd/meso-Co₃O₄ sample was first synthesized via a one-step nanocasting method,and the Pd/meso-CoO and Pd/meso-Co-CoO samples were then prepared via reduction of Pd/meso-Co₃O₄ by H₂ at different temperatures.The reduction treatment led to partial destroy of the mesoporous structure,drop of specific surface area and increase of pore size.Bigger Pd nanoparticles were formed in the Pd/meso-Co-CoO sample.The Pd/meso-CoO sample with the main presence of the Pd⁰ species on the surface performed the best for benzene oxidation(T_10%=142 ^oC,T_50%=167 ^oC,and T_90%=189 ^oC at a SV of 40,000 mL/?g h?).A higher temperature was required for CO₂ yield than for benzene conversion.It is demonstrated that oxygenate species?carbonate bidentate and partially oxidized species?such as acetate and maleate??were the main intermediate species during the benzene oxidation process,and the Pd/meso-CoO sample possessed a stronger oxygen activation ability.Therefore,it can be concluded that the outstanding catalytic activity of Pd/meso-CoO was related to the well dispersed Pd⁰ species,high surface area,and strong oxygen activation ability.?3?The mesoporous Pd,Pt,and Pd_xPt alloys were prepared via the KIT-6-templating route.The mesoporous samples displayed a surface area of25.5-31.9 m²/g and an average pore size of 13.7-19.8 nm.A Pd-Pt alloy was formed in each of the Pd_xPt samples.The addition of Pt to Pd exerted a significant effect on the redox property of the Pd sample.The PdO-PtO₂ were more active than the Pd⁰-Pt⁰.Among all of the samples,the Pd_2.41Pt sample showed the highest catalytic activity for methane combustion(at SV=100,000 mL/?g h?,T_10%=272 ^oC,T_50%=303^oC,and T_90%=322 ^oC;at 280 ^oC,TOF_Pd=0.85×10^-3 s^-1,TOF_Pt=1.98×10^-3 s^-1,TOF_Pd+Pt=0.59×10^-3 s^-1,and specific reaction rate=4.46?mol/(g_cat s)).Deactivation of the Pd_2.41Pt sample induced by 2.5-5.0 vol%CO₂ or H₂O addition was reversible,but its deactivation due to 100 ppm SO₂ introduction was irreversible.It is concluded that the good mesoporous structure,highly dispersed Pd-Pt alloys,PdO-PtO₂ species formation,and good methane and oxygen activation ability were responsible for the excellent catalytic performance of the Pd_2.41Pt sample.