Surface Modification As A Post-treatment Method Of Supported Platinum Catalysts

Supported metal catalysts often suffer from some shortages like poor stability at high temperatures.This thesis took efforts on introducing modification layer of inorganic oxides to supported metal catalysts in order to improve the stability of active metal components at high temperatures.On one hand,the modification layer of inorganic metal oxides may help improve the stability of active metal components at high temperatures through steric hindrance.On the other hand,the modification layer may change the catalytic behavior of the catalyst by synergic interaction with the active metal components.Several methods were developed in this thesis for introducing a SiO₂ modification layer on the Pt/Al₂O₃ catalyst.Among these methods,the―one-step‖method included using tetraethylorthosilicate（TEOS）as the Si source,using acetic acid as the catalyst for hydrolysis of TEOS,and introducing these reagents to the surface of the Pt/Al₂O₃ catalyst simultaneously.The characterization techniques like transmission electron microscopy（TEM）,diffuse-reflectance infrared Fourier transformation spectroscopy（DRIFTS,using CO as the probe molecule）were utilized to characterize the modified catalyst（SiO₂/Pt/Al₂O₃）.The results of characterizations showed that the―one-step‖method can dramatically improve the stability of the Pt component at high temperatures.The more amount of SiO₂ was introduced,the more obvious of the sintering-resisting effect is for the Pt particles.In a―two-step‖method for modification of the catalyst surface of Pt/Al₂O₃ by a SiO₂ layer,the hydrolysis step was preceded by that of introducing the Si source.However,this―two-step‖method did not result in improving the stability of Pt particles at high temperatures,and on the contrary,did result in more severe aggregation of the Pt particles.For the SiO₂/Pt/Al₂O₃ catalyst obtained through the above-mentioned―one-step‖method,the DRIFTS characterization with the probe molecules of Py and CO were performed.The results showed the Bronsted acid sites were introduced by the SiO₂ layer,and the SiO₂ layer tended to selectively cover the edge and side sites on the Pt aggregate.This selective covered lead to the increase of the percentage of the face sites on the Pt aggregate.The SiO₂/Pt/Al₂O₃catalyst was also used to catalyze the oxidative dehydrogenation of ethane（ODHE）.The results of catalytic test showed this catalyst is benefit for improving the ethane conversion,ethylene selectivity,decreasing the CO₂ selectivity,and reducing the formation of coke compared to the unmodified Pt/Al₂O₃ catalyst.The Pt/Al₂O₃ catalyst was also modified by a TiO₂ layer by introducing tetraethyltitanate as the Ti source.The preliminary results showed the TiO₂ layer tended to selectively cover the Pt atoms at the face sites,and such selectivity resulted in the improvement of percentage of the Pt atoms at the side/edge sites.The SiO₂/Pt/Al₂O₃ catalyst was used for the catalytic hydrogenation of cinnamaldehyde.This catalyst can afford higher selectivity for cinnamic alcohol compared to the Pt/Al₂O₃ catalyst,although the conversion of cinnamaldehyde was somewhat lower for the former than the latter.