Controlled Preparation Of Porous Oxide-supported Noble Metal Catalysts And Their Catalytic Performance For Methane Oxidation

The burning process of many kinds of fossil materials is accompanied with the production of nitrogen oxides.As a green energy,methane combustion requires high temperatures?above 1200 ^oC?.At such high temperatures,the nitrogen in air can react with oxygen,generating NO_x that deteriorates the quality of atmospheric environment.Catalytic combustion of methane,however,has many advantages over the conventional flame combustion in suppressing emissions of NO_x and CO and improving energy utilization efficiency.When a catalyst is employed,methane can be combusted at a much lower temperature,thus reducing the emissions of NO_x,PM,CO,and hydrocarbons.If the temperatures for the combustion of methane in natural gas can be decreased,NO_x generated would be reduced.Although catalytic combustion technology has found applications in electric generation from natural gas,automotive exhaust purification,and organic pollutant removal,the design and preparation of catalytic materials with high surface areas,good thermal stability,and high performance is a key issue to be solved for the development of catalytic combustion technologies of clean fuels.The porous material-supported noble metal nanocatalysts are expected to show excellent catalytic performance for the combustion of methane.Compared to the bulk materials,firstly,the porous materials possess higher surface areas,facilitate the contact with reactants,and hence improving methane conversion efficiency;secondly,noble metal nanoparticles?NPs?are catalytically active for methane combustion,but they are not stable and easily sintered,thus influencing their catalytic performance.The combination of noble metal NPs and porous materials has the advantages:?i?the porous structure can uniformly disperse the noble metal NPs on the surface of the porous support,thus avoiding the decrease induced by the aggregation and sintering of noble metal NPs at high temperatures;and?ii?the formation of noble metal-metal oxide active interfaces is beneficial for the improvement in catalytic activity;?iii?the addition of a certain amount of the second noble metal to palladium,forming a Pd-Pt bimetallic catalyst.It could improve the stability and poison-resistant ability of the catalytsts.In this thesis,the three-dimensionally ordered mesoporous silica?KIT-6?hard-templating,incipient wetness impregnation,and bubble-assisted polyvinyl alcohol?PVA?-protected reduction strategies were adopted to prepare the three-dimensionally ordered mesoporous Mn₂O₃?meso-Mn₂O₃?and its supported noble metal alloy nanocatalysts,respectively.The polymethyl methacrylate?PMMA?colloidal crystal-templating and PVA-protected reduction methods were used to fabricate the three-dimensionally ordered macroporous Mn-doped hexaaluminate(3DOM LaMnAl₁₁O₁₉)and its supported Pd nanocatalysts,respectively.highly order porous structure catalysts,such as meso-Mn₂O₃,3DOM LaMnAl₁₁O₁₉,3DOM Mn₂O₃supporters,and supported noble metal catalysts.Physicochemical properties of the catalysts were characterized by means of various analytical techniques,and their catalytic activities were evaluated for the oxidation of methane.The catalytic mechanisms have been probed.The relationship between physicochemical properties and catalytic performance for methane oxidation has been established.The results obtained are useful in guiding the design and preparation of novel and high-performance catalysts,so that methane could be oxidized at low temperatures,which is beneficial for the saving of energy and reduction of pollutant emissions,hence improving the quality of the atmosphere.Therefore,the results obtained in the present thesis are of academic significance and practical value.The main research outcomes obtained in the thesis are as follows:?1?Ordered mesoporous Mn₂O₃?meso-Mn₂O₃?and its supported Pd,Pt,and Pd-Pt alloy?xPd_yPt/meso-Mn₂O₃;x=0.10-1.50 wt%;Pd/Pt molar ratio?y?=4.9-5.1?nanocatalysts were prepared using the KIT-6-templating and polyvinyl alcohol?PVA?-protected reduction methods,respectively.It is found that the meso-Mn₂O₃with a high surface area of 106 m²/g was cubic in crystal structure and the noble metal nanoparticles?NPs?with a size of 2.1-2.8 nm were uniformly dispersed on the surface of meso-Mn₂O₃.The alloying of Pd with Pt enhanced the catalytic activity for methane combustion,with the 1.41Pd_5.1Pt/meso-Mn₂O₃ sample performing the best:the T_10%,T_50%,and T_90%?the temperatures required for achieving methane conversions of 10,50,and 90%?were 265,345,and 425 ^oC at a space velocity of 20,000 mL/?g h?,respectively.Furthermore,the effects of SO₂,CO₂,H₂O,and NO on methane combustion over 1.41Pd_5.1Pt/meso-Mn₂O₃ were also examined.It is concluded that the good catalytic performance of 1.41Pd_5.1Pt/meso-Mn₂O₃ was associated with its high-quality porous structure,high adsorbed oxygen species concentration,good low-temperature reducibility,and strong interaction between Pd-Pt alloy NPs and meso-Mn₂O₃ support.?2?Three-dimensionally ordered macroporous?3DOM?LaMnAl₁₁O₁₉ and 0.97wt%Pd/3DOM LaMnAl₁₁O₁₉ samples with a good-quality 3DOM structure have been prepared using the poly?methyl methacrylate??PMMA?-templating and PVA-protected reduction methods,respectively.The Pd nanoparticles?NPs?with a size of 2-5 nm were uniformly dispersed on the macropore wall surface of 3DOM LaMnAl₁₁O₁₉.Due to the highest adsorbed oxygen species concentration and the best low-temperature reducibility,the 0.97 wt%Pd/3DOM LaMnAl₁₁O₁₉ sample showed the best catalytic activity for methane combustion,with the reaction temperatures(T_10%,T_50%,and T_90%)required for achieving methane conversions of 10,50,and 90%being 259,308,and 343 ^oC at SV=20,000 mL/?g h?,respectively.The 0.97 wt%Pd/3DOM LaMnAl₁₁O₁₉ catalyst was catalytically stable,whereas the 0.98 wt%Pd/3DOM Mn₂O₃ sample was partially deactivated after 50 h of methane oxidation.The introduction of 3.0 vol%H₂O or 2.0 vol%CO₂ to the reaction system resulted in the reversible deactivation of 0.97 wt%Pd/3DOM LaMnAl₁₁O₁₉,but the addition of100 ppm SO₂ led to the irreversible deactivation of the catalyst.It is concluded that the good-quality 3DOM structure,uniformly dispersed Pd NPs,high adsorbed oxygen species concentration,good low-temperature reducibility,and strong interaction between Pd NPs and 3DOM LaMnAl₁₁O₁₉ were accountable for the good catalytic performance of 0.97 wt%Pd/3DOM LaMnAl₁₁O₁₉.?3?Three-dimensionally ordered macroporus LaMnAl₁₁O₁₉?3DOM LMAO?and its supported Pd,Pd-Pt,and Pt nanoparticles were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods,respectively.The 1100 ^oC-calcined 3DOM LMAO support possessed a hexaaluminate phase,and its supported noble metal samples displayed a high surface area of 26-29m²/g.A Pd-Pt alloy was generated in the PdPt/3DOM LMAO samples,and the particle sizes of the noble metal NPs were 3-5 nm.The 0.97 wt%Pd/3DOM LMAO?0.97Pd/3DOM LMAO?sample possessed the highest surface adsorbed oxygen concentration and the best low-temperature reducibility,showing the highest catalytic activity(T_10%=259 ^oC,T_50%=308 ^oC,and T_90%=343 ^oC at SV=20,000 mL/?g h?)for methane combustion.The 1.14 wt%Pd_2.8Pt/3DOM LMAO(1.14Pd_2.8Pt/3DOM LMAO)sample performed the best(T_10%=284 ^oC,T_50%=372 ^oC,and T_90%=456 ^oC at SV=20,000 mL/?g h?)among the PdPt/3DOM LMAO samples.Doping of Pt to the Pd-based catalyst could improve the H₂O-,CO₂-,and SO₂-resistant ability without significant influence on thermal stability,although 1.14Pd_2.8Pt/3DOM LMAO was less active than 0.97Pd/3DOM LMAO for methane combustion.The ex situ X-ray photoelectron spectroscopy was used to explore the formation of active PdO species during the oxidation processes of Pd in 1.14Pd_2.8Pt/3DOM LMAO and 0.97Pd/3DOM LaMnAl₁₁O₁₉ at different temperatures.