Investigation On The Fabrication Of Nano Silver Catalysts Supported On Mesoporous Silica And Their Low-temperature CO Oxidation Activity

The catalytic oxidation of CO to CO2at low temperature is an important subject for the environmental protection, and has widespread applications in air purification. In addition, in academic fields, CO oxidation reaction has been known as a "model reaction" to study the relationship between the structure of catalysts and their performance. Therefore, the removal of CO is of great significance in basic research and practical application. At present, the catalysts used in this reaction are non-noble metal catalysts and noble metal catalysts. Due to its relatively low cost, supported Ag catalysts have received much attention in CO oxidation. Moreover, the interactions of silver with oxygen have been extensively studied in order to understand the catalytic behavior of silver catalysts. In addition, ordered mesoporous silicas have attracted much attention because of their high surface area, uniform pore size distribution, large pore size and widely potential applications in the fields of catalysis, separation and adsorption. However, the catalytic activity of Ag catalysts for CO oxidation at low temperature (room temperature) is still low. Therefore, it is challenge to prepare a catalyst with high activity at low temperature. In order to solve this problem, Ag/mesoporous silica is used as catalyst in CO oxidation to study catalytic activity. And the effects of preparation methods, supports and pretreatment conditions et al. on the activity and the structure (Ag particle size, Ag species state) of Ag catalysts, as well as the correlation between the surface structure of Ag and catalytic activity, are investigated. The surface resucturing and the diffusion model of silver particles on SBA-15after oxygen pretreatment at high temperatures are proposed. The obtained results are listed below:(1) The active species and particle size of silver catalysts for CO oxidation were discussed in this paper by investigating the effects of Ag loading, different pretreatment conditions and CeO2loading on the crystallite structure of silver catalysts. It was found that8wt%Ag/SiO2catalyst pretreated with H2at200℃following O2treatment at500℃exhibited the high catalytic performance (T98=50℃). Crystallite Ag and Ag2O species were observed after O2pretreatment at500℃. Subsequent H2treatment at low temperatures (200℃) caused the reduction of Ag2O species and the high dispersion of crystallite silver. In addition, the results showed the addition of CeO2(1wt%) favored the dispersion of Ag particles and the increase of catalytic activity. Ag0particles were proposed as the active species for CO low-temperature oxidation and the silver particle size of ca.4.5～5.5nm would be favorable for the low-temperature CO oxidation.(2) Ag nanoparticles supported on mesoporous silica were prepared by in-situ reduction methods (post-assembly and in-situ incorporation methods). The catalysts were characterized by XRD, N2adsorption-desorption, ICP-AES and TEM. It was found that the preparation methods strongly affected the structure of the support and Ag particle size. Meanwhile, HCHO was not only a reducing agent, but also a certain degree of "structural adjusting agent" The catalytic activity of catalysts for low-temperature CO oxidation was studied here. Besides the size effect of Ag nanoparticles, it was believed that the structural order of the samples was also crucial for the high catalytic activity. Moreover, a novel and simple one pot synthesis approach using dodecylamine (DDA) as capping agent and structure director had been employed to synthesize the HMS-supported Ag nanoparticles. Highly dispersed uniformly sized Ag nanoparticles on the mesoporous silica were obtained. The excellent catalytic activity for CO oxidation (T98=20℃) demonstrated that the method for synthesizing the highly dispersed nano-silver catalyst was effective.(3) Ag nanoparticles supported on different mesoporous silica (SiO2, SBA-15and HMS) were prepared. The results showed that BET surface area and well ordered pore structure of silica strongly affected the dispersion of Ag species and the diffusion of reactants, consequently affecting the catalytic activity for CO oxidation. In addition, Ag nanoparticles were incorporated into the channels of mesoporous SBA-15by the "pH-adjusting" method (7.9wt%). The results revealed that the synthesized materials exhibited the highly ordered hexagonal mesoporous structures and highly catalytic activity, and highly dispersed Ag nanoparticles were confined inside the mesopores. It was the first time for us to report such high Ag loading on oxides fabricated by "pH-adjusting" method.(4) The structural and the stability of Ag nanoparticles supported on SBA-15were systematically investigated by changing the silver loading and pretreatment conditions. The good relationship between the structure and activity was obtained, and an excellent Ag/SBA-15catalytst with low silver loading prepared by the conventional incipient wetness impregnation method for CO oxidation at room temperature was obtained. The results showed that the effect of atmosphere on the structure and activity under different Ag loading was different. Ar pretreatment was favorable for the formation of small Ag particles for Ag/SBA-15catalyst with low Ag loading. O2pretreatment was conductive to the formation of subsurface oxygen species on the larger silver particles for Ag/SBA-15catalyst with high Ag loading. It was evident that small metal Ag particles was the key factor for the high catalytic activity for Ag/SBA-15catalyst with low Ag loading, while a large number of subsurface oxygen species played an important effect on the higher catalytic activity for Ag/SBA-15catalyst with high Ag loading. In addition, it was the first time to report that after the oxygen (900℃)-hydrogen (300℃) pretreatment in cycle mode, an excellently catalytic activity (T100=22℃) was obtained on the1.42wt%Ag/SBA-15catalyst prepared by incipient wetness impregnation method. An evaporation-deposition-diffusion mechanism for Ag/SBA-15catalyst was proposed. The oxygen adsorbate induced decrease in the surface free energy at900℃induced the evaporated silver atoms to be re-deposited on the support and meanwhile diffuse into the channels of SBA-15, forming more highly dispersed small silver particles inside the channels.(5) A series of SBA-15supports with different Si/Al ratios had been prepared using mechanically mixed method and "pH-adjusting" method. The results showed that the mechnanicl mixture of SBA-15and Al2O3was favorable for the formation of Ag2O and caused the amount decrease of subsurface oxygen, leading to the decrease of catalytic activity. However, Ag/AI-SBA-15catalyst with an appropriate Si/Al molar ratio of200(MAS200) prepared with "pH-adjusting" method had been found to show highly catalytic activity for CO oxidation (T98=40℃). The results showed that the octahedrally coordinated extra-framework Al that was connected to the framework via an oxygen atom could remove parts of the H-bonded SiOH, which resulted in the formation of highly dispersed metal Ag particles and the increase of the catalytic activity.