Preparation And Catalytic Performance Of Porous Composite Metal Oxide-supported Noble Metal Alloy Catalysts

Volatile organic compounds(VOCs) not only induce air pollution but also pose a serious threat to human health.Catalytic oxidation has been considered to be one of the effective pathways to eliminate VOCs,in which the key issue is the development of low cost catalysts with high activity and good thermal stability.Although noble metal catalysts exhibit good performance at low temperatures for VOCs removal,their high-temperature thermal stability is relatively poor and their cost is high.Spinel-type oxides(AB₂O₄)possess good high-temperature stability and anti-poisoning ability,and transition metal oxides(e.g.,Co₃O₄ and MnO₂) are relatively cheap.Macro-or mesoporous materials are conducive to dispersion of the active components on the surface due to their large specific surface areas,which can greatly improve activity and thermal stability of the supported noble metal catalysts for the oxidation of VOCs.Therefore,it is of practical significance to develop the catalysts with high low-temperature catalytic performance,good thermal stability and low cost.In addition,performance of the catalysts can be improved by optimizing their preparation methods.In this dissertation,the xCo₃O₄-MnO₂,three-dimensionally ordered mesoporous Mn₂O₃(meso-Mn₂O₃) and its supported Au,Ru,and Au Ru alloy nanoparticles(NPs),and three-dimensionally ordered macroporous Mn Co₂O₄(3DOM MnCo₂O₄)-supported Co₃O₄ and Au Pd(xAuPd_z/yCo₃O₄/3DOM Mn Co₂O₄)catalysts were prepared.Physicochemical properties of these materials were characterized by means of a number of techniques,and their catalytic activities were evaluated for the oxidation of methane and o-xylene.Through the above investigations,the structure-performance relationship between physicochemical property and activity of the catalysts was clarified.The main research results are as follows:(1)The xCo₃O₄–Mn O₂(x=2.6,8.8,and 13.3 wt%)catalysts were prepared using the polymethyl methacrylate(PMMA)microspheres-templating,incipient wetness impregnation,and nitric acid treatment methods.It is shown that the as-prepared catalysts possessed a cubic crystal structure and a surface area of 51.9–63.9 m²/g.The 8.8Co₃O₄–MnO₂ catalyst showed the highest adsorbed oxygen species concentration and the best low-temperature reducibility,and hence performing the best:the T_10%,T_50%,and T_90%(the temperatures required for achieving o-xylene conversion of 10,50,and90%,respectively)were 231,251,and 273 ^oC at a space velocity of 100,000 mL/(g h).The apparent activation energies obtained over the xCo₃O₄–MnO₂ catalysts for o-xylene oxidation were 72–82 k J/mol.The effects of space velocity,water vapor,and carbon dioxide on activity of the 8.8Co₃O₄–MnO₂catalyst were also examined,in which the partial deactivation due to water vapor or carbon dioxide introduction was reversible.It is concluded that the good catalytic performance of 8.8Co₃O₄–Mn O₂ was associated with its high adsorbed oxygen species concentration,good low-temperature reducibility,and strong interaction between Co₃O₄ and Mn O₂.(2)The meso-Mn₂O₃ and its supported Au,Ru,and Au Ru alloy(0.49 wt%Au/meso-Mn₂O₃,0.48 wt%Ru/meso-Mn₂O₃,and 0.97 wt%Au Ru/meso-Mn₂O₃(Au/Ru molar ratio=0.98))nanocatalysts were prepared using the three-dimensionally ordered mesoporous silica(KIT-6)-templating and polyvinyl alcohol(PVA)-protected reduction methods,respectively.It is found that among all of the catalysts,0.48 wt%Ru/meso-Mn₂O₃ and 0.97 wt%Au Ru/meso-Mn₂O₃ performed the best for methane oxidation(T_90%=530-540 ^oC at space velocity=20,000 mL/(g h)),but the latter showed better thermal stability than the former.The partial deactivation of 0.97 wt%Au Ru/meso-Mn₂O₃ due to H₂O or CO₂ introduction was reversible.We conclude that the good catalytic activity and thermal stability of 0.97 wt%Au Ru/meso-Mn₂O₃ was related to its AuRu alloy NPs(2-5 nm)highly dispersed on the surface of meso-Mn₂O₃,high adsorbed oxygen species concentration,and good low-temperature reducibility.(3)The 3DOM MnCo₂O₄-supported Co₃O₄ and Au Pd(x Au Pd_z/y Co₃O₄/3DOM Mn Co₂O₄;x=0.56-1.98 wt%,z=1.9-2.1,and y=4.80-24.36 wt%)catalysts were prepared using the PMMA-templating,incipient wetness impregnation,and PVA-protected reduction methods,respectively.The results showed that the x Au Pd_z/yCo₃O₄/3DOM MnCo₂O₄ catalysts with a surface area of 26.5–53.1 m²/g possessed a high-quality 3DOM architecture,in which the Au Pd_z NPs with a size of4.6–5.8 nm were highly dispersed on the surface of the macropore framework.Among all of the catalysts,1.98Au Pd_2.1/18.20Co₃O₄/3DOM Mn Co₂O₄ exhibited the highest catalytic activity for methane oxidation:the T_10%,T_50%,and T_90%were 262,340,and408 ^oC at space velocity=40,000 m L/(g h).Partial deactivation of the1.98Au Pd_2.1/18.20Co₃O₄/3DOM Mn Co₂O₄ catalyst due to addition of water vapor or carbon dioxide was reversible.It is concluded that the good catalytic performance of1.98Au Pd_2.1/18.20Co₃O₄/3DOM MnCo₂O₄ was associated with its high adsorbed oxygen species concentration,good low-temperature reducibility,and strong interaction between Co₃O₄ or Au Pd_2.1 NPs and 3DOM MnCo₂O₄.