Preparation Of Au-Ag/SBA-15Catalyst And Investigation Of Its Catalytic Performance For CO Oxidation

Carbon monoxide (CO) is one of the most important gaseous pollutants in air, and trace amount of CO in hydrogen has become a challenging issue to hinder the application of fuel cells. Low-temperature (even room temperature) catalytic oxidation is an effective method for CO removal, so the development of catalysts with high catalytic performance for CO oxidation at room temperature has become a research hotspot at present.In this paper, AuxAg8-x/SBA-15catalysts were synthesized by two step method. In order to obtain high catalytic activity catalyst at room temperature for CO oxidation, we systemically investigated the different factors influencing the catalytic performance, such as the Au/Ag molar ratio, calcination temperature, reduction temperature, particle size and so on. ICP, N2adsorption-desorption, UV-vis, XRD, XPS, TEM techniques were used to study the relationship between the structure of catalysts and the CO catalytic activity. The main research contents and experimental results of this thesis were shown below:(1) AuxAg8-x/SBA-15catalysts with low metal loading was prepared by two step method, and the effect of the Au/Ag molar ratio on the CO catalytic activity was investigated.. It was found that catalytic activity was strongly related with the Au/Ag molar ratio. When the actual Au/Ag molar ratio was5.4/1, Au6.oAg2.0/SBA-15catalyst exhibited the highest catalytic performance and achieved100%of CO conversion at20℃, while CO conversion obtained at20℃over the monometallic Au8.0/SBA-15and Ag8.0/SBA-15catalysts was only40%and2%, respectively. Thus, it can be induced that bimetallic Au-Ag/SBA-15catalyst showed a strongly synergetic effect during the CO oxidation. Although the catalyst had only bimetallic metal loading of0.34-1.32wt.%, but it exhibited the comparable or even better activity when compared with that of Au-Ag/Al-MCM-41(7.83wt.%), Au-Ag/SiO2-Al2O3(2.20wt.%), Au-Ag/TiO2(4.70wt.%) and Au-Ag/SiO2(5.56wt.%) catalysts reported in the literature.(2) Au6.oAg2.0/SBA-15catalyst calcined at different temperatures (100-700℃) and then reduced in hydrogen at600℃was used to study the influence of calcination temperature on the catalytic activity for CO oxidation. It was found that the catalytic activity was sharply enhanced when the calcination temperature was increased from100to500℃. With a further increase of the calcination temperature to600and700℃, the activity started to decrease. Further research showed that the amine groups could be completely removed at500℃and the bimetallic nanoparticles with small size and unfirom distribution were depositied on the support. Calcination treatment at600～700℃would lead a severe aggregation of particles, resulting in great loss in catalytic activity for CO oxidation.(3) Au6.0Ag2.0/SBA-15catalyst calcined at500℃and then reduced at different temperature (400-700℃) in H2was used to study the influence of reduction temperature on the catalytic activity for CO oxidation. No activity was observed on the unreduced sample. After the Au6.oAg2.0/SBA-15catalyst was reduced with H2at400℃, the catalytic activity was sharply enhanced. The best activity was obtained when the reduction temperature was increased to500-600℃. However, when the reduction temperature was further increased to700℃, the catalytic activity began to decrease. Further study showed that the reduction induced the surface redistribution of gold and silver, and the surface Au/Ag molar ratio was closer to the bulk value after reduction at500-600℃, which indicated the formation of a more random alloy and resulted in the improvement in activity. However, the surface enrichment of Ag NPs and the severe aggregation of bimetallic particles after high temperature reduction (>600℃) caused the decrease of catalytic performance.(4) Bimetallic Au-Ag/SBA-15catalysts with different particle sizes were synthesized by selectively functioalizing the silica hydroxyl on the internal and external surfaces of SB A-15, and the relationship between the particle size and the catalytic activity of CO at low temperature was systemically investigated. It has been found that the CO oxidation performance showed the strong dependence on the particle size. CO conversion increased with the decrease of the size of Au-Ag bimetallic particles. When the mean size of bimetallic nanoparticlcs was3.1nm, CO could be completely converted at15℃. When the mean size increased to6.4nm, the complete conversion of temperature was increased to30℃.