Carbon Nitride With Underlying Nickel:Controllable Synthesis And Application In Catalytic Hydrogenation Reactions

As an unquie metal-free polymer material, carbon nitride has been intensively studied in different areas due to its versatile chemical and physical properties. Especially its unique electronic structure and excellent chemical stability makes it a promising candidate for the application in catalysis. As a metal-free, catalyst, carbon nitride has been widely used in photocatalytic water splitting, oxygen reduction, selective conversion of organic functional groups. As a catalyst support, carbon nitride has been used for supporting noble metals such as Au, Pd, Pt, Ag, etc. Also it can act as an energy storage material for the storage of H2 and CO2. Besides, the property of carbon nitride can be modified by numerous methods in order to make it suitable for different applications. For example, by doping heteroatoms such as S, P, O, B, F and other functional groups into the structure of carbon nitride matrix the band structure of carbon nitride could be modified, also some researches have demonstrated that the property of carbon nitride material would be affected by the material it contacts, which leaves room for the further improvement of carbon nitride. In this dissertation we have designed a new carbon nitride-metal nanocomposite as a hydrogenation catalyst. We used nickel to modify the electronic properties of carbon nitride, meanwhile the carbon nitride could protect the nickel from the reaction environment. We have used different characterization methods to study its properties in order to get more insights of its structure-performance relationship. The contents are summoned below:(1) By depositing nickel on Al2O3 followed by the encapsulation of carbon nitride we have synthesized a nanocomposite material composed of carbon nitride with underlying nickel (CN/Ni/Al2O3). We have applied different characterization methods (XRD, XPS, HRTEM, H/D Exchange, in situ IR, SSNMR) to study its properties and tested its catalytic ability to reduce nitro groups under strong acidic condition. The results shows that the nickel in the catalyst has been reduced to its metal state during the heat treatment of the catalyst. The nickel possesses strong interaction with the carbon nitride, the heterojunction between them enable the nickel to donate its electron to carbon nitride and endow it with the ability to adsorb and activate hydrogen. Also the nickel is physically isolated from the reaction environment to prevent it from loss or poisoning. The catalyst shows very good catalytic performance and stability in the hydrogenation of the nitro groups to amino groups under strong acidic condition while the metal nickel itself is totally nonactive under the same condtions. Moreover, this nanocomposite catalyst shows very high selectivity and activity in the direct hydrogenation of nitrobenzene to 4-aminophenol under 1.5 M H2SO4. The excellent catalytic performance comes from the modification of nickel to carbon nitride and the protection of carbon nitride to nickel.(2) We further use the catalyst in the selective hydrogenation of acetylene under large amount of ethylene. We have tested the catalytic performance of the catalyst under different temeratures and space velocities and investigated the properties of the catalyst after the reaction. The results shows that the catalyst possesses high acetylene conversion (100%) with a fairly high selectivity (65%), on contrary the Ni/Al2O3 catalyst is complete nonselective. The long term stability tests show that the catalyst possesses good stability and we found that the pyridine nitrogen in the carbon nitride matrix could be the active site. The theoretical calculation has studied the electronic interaction between nickel and carbon nitride and the active site for the adsorption and activation of hydrogen. The results demonstrate that the pyridine nitrogen in carbon nitride matrix is the most favourable site to accept the electron from nickel. The nickel could facilitate the hydrogen adsorption and activation on the pyrdinic nitrogen in the plane of the carbon nitride matrix. The revelation of the mechanism of this catalyst could provide new insights for the design of new catalysts.