Effect Of Preparation And Surface Hydroxyl Tuning Of Au/MO_x-Al₂O₃ Catalyst On Their Catalytic Performance For Co Oxidation

Nano gold catalyst has very high catalytic activity in the CO oxidation reaction and shown promising prospects in actual practice such as exhaust purification catalyst of automobile.However,it is difficult to prepare supported gold nanoparticles with higher dispersion and higher activity in the field of gold catalysis.The reaction mechanism,especially the role of absorbed H₂O,was not known exactly.Here,a series of Au/MO_x-Al₂O₃ catalysts were prepared by adding metal oxides,changing the preparation method or pretreatment atmosphere.Various physicochemical techniques,including XRD,HADDF-TEM,Dew Point Meter,XPS,TG/DSC and in-situ DRIFT,were used to study the properties of the catalysts.The effects of surface composition of Au/MO_x-Al₂O₃,especially the amount of absorbed H₂O,on the reaction activity and mechanism were studied in detail.1)Firstly,a novel Au/MgAl₂O₄-Al₂O₃ catalyst was synthesized by deposition-method and pretreated with different atmospheres,in relation to their activities and mechanisms on CO oxidation were investigated.The higher initial activity was obtained for Au/MgAl₂O₄-Al₂O₃-H₂ catalyst,which,however,suffered the faster deactivation with time on stream.While the activity of sample pretreated with air increased slowly rather than decreased with time adding in the initial stage.The change of water content on the surface of sample was in-situ monitored by Dew Point Meter and DRIFT,it was also verified by TG/DSC and XPS that H₂O on the surface of catalyst has a decisive effect on CO oxidation activity.In addition,the results of in-situ DRIFT show that the amount of weakly adsorbed water on the surface of catalyst can modulate reaction paths.What is more,by activating the catalyst with air,more absorbed water and OH^-were fixed on the surface of the sample,and which shift the reaction paths from HCO₃^-decomposed to COOH decomposed.2)In addition to the inert support,a novel Au/ZnAl₂O₄-Al₂O₃ catalyst was also synthesized by deposition-precipitation?DP?method and the effect of the activation under different atmospheres?air and H₂?on their activities and reaction paths on CO oxidation were investigated.The higher initial activity was obtained over Au/ZnAl₂O₄-Al₂O₃-H₂catalyst,which,however,suffered the faster deactivation with time on stream.While the activity of sample activated with air increased slowly with time increasing in the initial stage.HADDF-STEM results showed that the activation atmosphere has a negligible effect on the gold particle size.Dew-Point Meter revealed that the amount of surface water on the catalyst showed crucial impact on tuning the intrinsic activity for CO oxidation.Furthermore,in situ DRIFT,TG/DSC and XPS also demonstrated that the superior activity of Au/ZnAl₂O₄-Al₂O₃-air has been attributed to its higher fraction of Au⁺and Zn²⁺so that can fix more adsorbed water rather than suppressed accumulation of carbonate-like surface species.In addition,the presence of adsorbed water on the surface of the sample is conducive to the formation of COOH,while HCO₃^-is easier to form,when there is little water.3)According to the first part,the content of-OH group is very important to the activity of the sample.Here,Au/MgO_x-Al₂O₃ and Au/ZnO_x-Al₂O₃ catalyst with more hydroxyl groups was prepared by one-step precipitation method.The activity of Au/MgO_x-Al₂O₃ and Au/ZnO_x-Al₂O₃ catalysts were much higher than that of Au/Al₂O₃,and the reason is that the addition of Mg and Zn greatly increased the content of-OH group on the catalyst surface.The pretreatment atmospheres also have important effect on the catalyst activity.Au/MgO_x-Al₂O₃ catalyst treated with H₂ contained more surface oxygen species and it also has better stability.This work has confirmed our conjecture of the mechanism of CO oxidation over gold catalyst prepared with Al₂O₃ as the basic carrier,which provides theoretical support for the subsequent development of more efficient nano-gold catalysts.