Controlled Preparation And Catalytic Performance Of Porous Noble Metal Catalysts For The Oxidation Of Organic Compounds

In recent years,atmospheric pollution in our country?especially in Beijing?has become more and more serious.It is highly required to control the emission of atmosphere pollutants.Volatile organic compounds?VOCs?are a kind of complicated organic compounds,which are important sources for PM2.5 and O₃ formation.Catalytic oxidation is regarded as one of the most efficient strategies for VOCs removal,and the key issue is the availability of catalysts with high low-temperature activity,good high-temperature stability,and relative low cost.Supported noble metal catalysts exhibit high low-temperature activities and good SO₂-tolerant ability for VOCs oxidation.However,their low thermal stability and high cost hardly meet the oxidative removal of VOCs from various sources,thus limiting their wide applications.Since the catalysts with three-dimensionally ordered porous structures are beneficial for the adsorption and diffusion of VOCs,and a large surface area can effectively disperse the noble metal nanoparticles?NPs?,thus resulting in the noble metal-based catalysts that are highly active and stable for VOCs oxidation.In addition,modification of noble metal NPs and selection of suitable supports can prevent the sintering of noble metal NPs.Modifying the noble metal NPs with base metals can reduce the use and enhance the utilization efficiency of noble metals.In this dissertation,we herein report the preparation,characterization,and catalytic properties of three-dimensionally ordered macro-or mesoporous Co₃O₄,CoO,Mn₂O₃,and CeO₂,and its supported Au,Pd,Au-Pd,Au-Pd-M?M=Cr,Mn,Fe,and Co?,and Pd@Co NPs.It is found that there was a correlation between physicochemical property and catalytic activity of the catalysts for VOCs or CO oxidation.The strategies for promoting the catalytic activity,thermal stability,and water vapor tolerance ability of the noble metal-based catalysts were proposed.Transition metal ions with low valance together with noble metal ions were efficiency in activating oxygen molecules,and metallic noble metals were beneficial for VOCs adsorption.The main results obtained in the present dissertation are as follows:?1?The polymethyl methacrylate?PMMA?-templating and polyvinyl alcohol?PVA?-protected reduction strategies were adopted to prepare the 3DOM Co₃O₄,3DOM Mn₂O₃,and their supported Au nanocatalysts?xAu/3DOM Co₃O₄ and xAu/3DOM Mn₂O₃?.These catalysts possessed a high quality of 3DOM architecture and Au NPs with a particle size of 2.4-5.0 nm was highly dispersed on the 3DOM support surface.Catalysts?xAu/3DOM Co₃O₄ and xAu/3DOM Mn₂O₃?with a 3DOM structure performed much better than 5.8Au/bulk Co₃O₄ and 5.7Au/Bulk Mn₂O₃ for toluene or CO oxidation.The excellent catalytic performance of these catalysts was attributed to their high oxygen adspecies concentration and good low-temperature reducibility,which were due to their 3DOM structure,highly dispersed Au NPs with small particle sizes,and strong interaction between the Au NPs and the support.?2?The xAuPd/3DOM Co₃O₄ and xAuPd/3DOM Mn₂O₃ catalysts were prepared via PMMA-templating and PVA-protected reduction routes.AuPd alloy NPs with a diameter of 2.6–2.9 nm were well dispersed on the surface of the 3DOM support.The supported AuPd nanocatlaysts with Au/Pd mass ratio of 1.0 performed the best for toluene oxidation.It is found that adsorption and activation abilities of oxygen on Au NPs and strong interaction between the Au NPs and the support were enhanced by Pd modification,thus greatly improving the activity of the supported Au nanocatalysts for toluene oxidation.The T_90%over 1.99AuPd/3DOM Co₃O₄ at a space velocity?SV?of 40,000 mL/?g h?was 168 ^oC.Supported AuPd nanocatalysts also exhibited better water vapor tolerance ability due to the synergistic effect between Au and Pd.Moreover,the strong interaction between the Au–Pd alloy NPs and the 3DOM support?anchor effect?was responsible for the outstanding thermal stability of the supported Au–Pd nanocatalysts.?3?The modified PVA-protected reduction method was applied to synthesize the transition metal?M=Mn,Cr,Fe,and Co?-doped Au-Pd-M NPs.The Au-Pd-M NPs with a diameter of 3.6–4.4 nm were uniformly loaded on the surface of 3DOM Mn₂O₃or 3DOM Co₃O₄ via the gas bubble-assisted adsorption route.Among the Au-Pd-xM/3DOM Mn₂O₃ samples,the supported Au-Pd-0.21Co performed the best for methane oxidation due to its highest Pd²⁺concentration,giving the T_50%of 365 ^oC and reaction rate at 340 ^oC of 339.0×10^-6 mol/(g_Pd s);the supported Au-Pd-0.22Fe catalyst possessed the best ability for oxygen activation,resulting in the highest catalytic activity for o-xylene oxidation(T_90%=213 ^oC).In the serial catalysts of Au–Pd–yCoO/3DOM Co₃O₄,the Co in Au-Pd-Co NPs was present in the form of CoO,and the supported Au–Pd–yCoO nanocatlaysts with a Co/Pd molar ratio of 0.40performed the best for methane oxidation(T_90%=341 ^oC),whereas the Au–Pd–3.61CoO/3DOM Co₃O₄ catalyst showed the highest TOF_Pd(0.0118 s^–1)and methane reaction rate(110.5?mol/(g_Pd s))at 280 ^oC.Furthermore,the supported Au-Pd-CoO catalysts still showed good thermal stability for methane oxidation,which was associated with their strong interaction between the noble metal NPs and the 3DOM support and the Pd-CoO interface stabilized by Au species.The doping of CoO into Au–Pd NPs enhanced the adsorption and activation ability of methane,in which CoO was the methane adsorption site and the Pd-CoO interface was the active center for methane oxidation.It is concluded that doping the transition metal into Au–Pd NPs could modify the microstructure of Au–Pd to generate an interface between the noble metal and the transition oxide,thus enhancing the adsorption and activation ability of reactants?O₂,CH₄,and/or o-xylene?and catalytic performance.Moreover,the doping of the transition metal led to the decrease in the amount of noble metals,significantly improving the utilization efficiency of noble metals.?4?The PVA-protected two-step reduction strategy was adopted to fabricate the CoPd NPs.The CoPd NPs displayed a core-shell?core:Pd;shell:Co?structure with an average size of 3.5-4.5 nm,and were well dispersed on the wall surface of 3DOM CeO₂.The Co_xPd/3DOM CeO₂ samples were highly active and exhibited super thermal stability for methane oxidation,with the Co_3.5Pd/3DOM CeO₂ sample showing the highest catalytic activity(T_90%=480 ^oC at SV=40,000 m L/?g h?)and excellent thermal stability in the range of 400–800 ^oC.Based on the activity data and characterization results,it is concluded that the excellent catalytic performance of Co_3.5Pd/3DOM CeO₂ was associated with its good oxygen and methane adsorption abilities and unique core-shell Pd@Co structure.It is envisioned that fabricating an active catalyst with a unique structure could further reduce the use of noble metals,thus improving the utilization efficiency of noble metals.?5?The meso-CoO_x and meso-CoO catalysts with a three-dimensionally ordered mesoporous structure were prepared via the reduction of meso-Co₃O₄ by glycerol,and these materials exhibited high catalytic activities for o-xylene oxidation.The meso-CoO_x sample with a surface composition similar to the meso-CoO sample and a larger surface area performed the best for o-xylene oxidation:the T_90%was 244 ^oC,and the reaction rate was nine times higher than that over the meso-Co₃O₄ sample.The good catalytic performance of meso-CoO_x was associated with the high Co²⁺species concentration.The Co²⁺species and oxygen vacancies were highly active for the activation of gas-phase oxygen molecules to form active oxygen species(O₂^–and O₂^2–),especially the superoxide species.The Co²⁺ions were identified as the active site of cobalt oxide for o-xylene oxidation.?6?The meso-CoO-supported Pd catalysts with a three-dimensionally ordered mesoporous structure were prepared using the reduction of meso-Co₃O₄ by glycerol and PVA-protected reduction method.The Pd NPs with a mean size of 3.6-3.7 nm were uniformly loaded on the surface of meso-Co₃O₄ and meso-CoO.It is established that the loaded Pd NPs significantly improved the adsorption ability of o-xylene over meso-Co₃O₄ and meso-CoO,thus enhancing the catalytic activity for o-xylene oxidation.The Pd/meso-CoO catalyst performed the best among all of the catalysts and those mostly reported in the literature,giving the T_90%of 174 ^oC at a space velocity of 40,000 m L/?g h?.It is found that the adsorption of o-xylene could be the rate-determining step for o-xylene oxidation over the supported Pd catalysts under excessive oxygen conditions.The stronger oxygen activation ability also played a key role in enhancing the activity of the sample.Hence,the excellent catalytic performance of Pd/meso-CoO was associated with its good oxygen activation ability and o-xylene adsorption ability,especially the ability of o-xylene adsorption.