Influence Of Acid-basic Properties Of Promoter And Support On Catalytic Performance Of Noble Metal Catalysts For Oxidation Of VOCs

The removal of volatile organic compounds?VOCs?is very important to control the air pollution.Catalytic combustion technology is one of the most promising treatment technologies.Obviously,the key to the technology is how to design a catalyst which costs low and has higher activity simultaneously.Noble metal catalysts have excellent catalytic performance,but limited resources and expensive prices strongly restrict their wide application.To meet the challenge,two strategies were proposed including decrease the dosage of noble metal and develop a new noble metal catalyst.In this thesis,part I adopts Pd/Al₂O₃ as model catalyst,which introduces basic promoter to achieve the goal of decreasing the dosage of Pd.Part II develops a new noble metal catalyst,and the catalytic oxidation performance is adjusted by acid-basic properties of support for the development of new precious metal-based catalyst.Combined with characteristic technologies such as H₂-TPR,XPS and In-situ DRIFT,the relationship between catalysts structural properties and catalytic performance was investigated.The conclusions are as follows:?1?ZrO₂,MgO and BaO modified 0.5 wt.% Pd/Al₂O₃ catalysts were prepared by stepwise wet impregnation method and used for complete oxidation of toluene/isopropyl.The characterization results indicated that basic promoters could significantly improve the dispersibility of Pd species,the rate of oxygen migration and the efficiency of unit Pd active site,thereby improving the catalytic oxidation performance of toluene.Among them,the medium-strength MgO modified catalyst had a highest turnover frequency value of 17.9 h^-1.On this basis,the reaction mechanism of toluene on Pd/Al₂O₃ and Pd/MgO-Al₂O₃ catalyst was carried out by In-situ DRIFT.The results showed that Pd^?+was responsible for providing lattice oxygen to react with toluene,and Pd⁰ was responsible for activating oxygen molecules to fill the oxygen vacancies of Pd^?+.However,most of the Pd existed in oxidation state due to the strong interaction between Pd and MgO.As a result,a large number of Pd⁰/Pd^?+pairs formed,keeping the rate of oxygen activation in consistent with lattice oxygen supply.?2?Supported Ir catalysts on three supports?i.e.,SiO₂,Al₂O₃ and ZSM-5?with different acid-basic properties were prepared by impregnation method and used for complete oxidation of toluene.The results showed that the strength of metal-support interaction was the main factor affecting the catalytic activity of toluene compared with the surface area and Ir particle size.The acid/basic sites of different supports would selectively interact with Ir,and further affected the strength of metal-support interaction.Among them,the Ir/ZSM-5 catalyst with minimum alkali content and medium acid content exhibited best performance of toluene oxidation and its 100%conversion temperature was 280 ^oC.This was attributed to its suitable interaction between Ir and ZSM-5,which was conducive to exerting the best effect of size and valence state.At the same time,Ir⁰ was the active site of toluene oxidation at low temperature,making the Ir/SiO₂ catalyst with highest Ir⁰ content exhibit the most excellent low temperature activity.