Design And Synthesis Of Defective Semiconductor Photocatalysts For Non-oxidative Coupling Of Methane

Methane is the main component of natural gas,marsh gas and shale gas,which has traditionally been used as fuel.Recently,with the development of detected methods and mined technology the recoverable reserves and supply have increased greatly.Coupled with the depletion of petroleum resources the chemical technology of produce high value-added substance using methane as raw material is getting more and more favor.However,methane molecule has very strongly C-H bonding energy,perfectly symmetric non-polar structure,low thermodynamic equilibrium constant,lack of lowest unoccupied molecular orbital and highest occupied molecular orbitals,which make it very difficult to conversion under mild conditions.At present the industrial conversion of methane is mainly through indirect route.The indirect route means methane should firstly converted to syngas intermediate at high temperature(> 900 K)and then futher reacted to obtained various derivative chemicals with the help of catalysts.The process represents a major expenditure of capital and complex operation.At the same time,the inevitable coking under harsh experimental conditions will cause the deactivation of the catalyst.In addition,the products converted by methane have higher reactivity than that of methane,which brings more difficulty to protect the chemical intermediate from the kinetic and selective separation of target products.In order to simplify the technological process of production,lots of efforts has been made in the directly converted of methane by researchers.But the direct path should overcome high thermodynamic energy barrier.The oxidation agent was introduced to reaction system can reduce the gibbs free energy of reaction and increasing the yield of target products,but inevitably lead to excessive dehydrogenation(carbon deposition)and deep oxidation(carbon oxide)of the product,and reduce the economy of carbon atom.Although the Non-oxidative Coupling of Methane(NOCM)is thermodynamically uphill,this limitation can be overcome by providing photon energy as driving forces in principle,making NOCM occur under mild conditions.However,it is difficult to maintain the stability of the catalyst during the strong reductive reaction system.Lots of experiments have been carried out in order to make the NOCM more competitive,expecially in the selection of catalysts,the construction of active structure,the control of defective states and the regulation of electronic states.The keys that need to be solved are as follows:(1)how to select and prepare stable catalysts under strong reductive system;(2)how to design the active sites to adsorption and activation of methane;(3)how to regulate the surface and interface states of catalyst to constructe the channel which transfer charge carrier unidirectionally.In this paper,we focus on improving the efficiency of NOCM by modifying the structure and the surface states of semiconductor.The details are as follows:(1)design and preparation of defective ionic catalyst rich in polar structural unit to achieve high stability and activity under the strong reducing system;(2)control the microstructural units of the catalyst precisely to promote the strong adsorption and activation of methane molecules,weaken the C-H bond of methane molecules and realize the transformation of methane molecules at room temperature.(3)Introduce structural defects to adjust the surface state of the catalyst and realized the efficient separation and transfer of carriers.Our results are shown as follows based on the above design:1.By using ammonia gas to treat the mixture of Ga2O3 and Zn O,the high stability defect hexagonal wurtzite Ga N-Zn O solid solution was successfully prepared.The existence of strong ionic bonds with high bonding energy in the solid solution caused the stable of the catalyst under the strongly reducing dehydrogenation coupling reaction system.The polarity structure on the surface of the hexagonal wurtzite solid solution can adsorb and polarized methane molecules efficiently and realize the C-H bond of methane easy fracture at room temperature.The nitrided process eliminate the acid sites in the materials,which are widely recognized as key active sites that lead to the further oligomerization of the C2+ products to polycyclic aromatic hydrocarbons and coke.The unique N-Zn-O structure constructed in the nitrided process is considered as the active site to produce methyl radical intermediate and then the coupling of the methyl radical to realize the reconstruction of C-C bond.The existence of defective state increased the electron density of solid solution,greatly promote the transfer of photogenerated electron from the surface of solid solution to adsorbed methane molecules through the interface.Meanwhile,the reduced band gap caused by defects can make full use of light energy achieving efficient and direct conversion of methane under mild conditions.The methane conversion rates of solid solution and the thin film are 57 ?mol g-1 h-1 and 331?mol g-1 h-1 at room temperature.The durability test of Ga N-Zn O solid solution indicated that the conversion rate of the methane can still reach more than 50% after 35 cycles during a continuous 70-hour test2.By selective etching Ce-Zr solid solution(CZ)prepared by gel-combustion method reconstructed to obtain the Ce O2-x islands modified Ce-Zr solid solutions(C@CZ).In this process through selective etching of Zr and O on the surface of Ce-Zr solid solutions,the remaining Ce-rich surface will be reconstructed to form ultrasmall Ce O2-x nano-islands with stepped surfaces and abundant oxygen vacancies,enabling unprecedented adsorption of methane and ideal for the photocatalytic NOCM.Further studies revealed that the resulting photocatalyst can undergo a cooperative photoinduced transition from the initial “resting state” to the “active state” and enhance the photocatalytic performance.The rates of convered methane on C@CZ at 20? was52.3 ?mol g-1 h-1.The durability experiment indicated that the conversion rates of the sample still up to more than 60% that of the first cycle after 20 cycles during a continuous 40-hour test.3.Cerium vanadate solid powder prepared by hydrothermal method was treated in different atmosphere to obtain cerium vanadate with different defect structure and surface state.The results of CO2 temperature-programmed desorption(CO2-TPD)? Xray photoelectron spectroscopy(XPS)? Electron spin resonance(EPR)showed that the cerium vanadate sample treated by hydrogen had the strongest Lewis alkalinity that enhanced closely related to the concentration of oxygen vacancy.Experimental results showed that the sample treated with hydrogen is obviously higher the conversion rate of methane,lower the proportion of carbon oxide significantly,increased the selectivity of ethane obviously.The results show that the oxygen vacancy is the active center for methane adsorption,and the adsorbed methane can be activated on the Ce-O structural unit linked to the vacancy to produce ethane and hydrogen.Reactive oxygen species on the surface of the catalyst are the main sites for the formation of carbon oxides,which can migrate to the oxygen vacancies and occupy the site absorbed methane.The heat treatment process in hydrogen atmosphere eliminated the reactive oxygen species on the surface of catalyst and effectively reduced the generation of carbon oxides.In addition,the hydrogen treatment process will lead to the increase of defect sites in the material and the introduced defect level can widen the spectral response range of the catalyst.The existence of high concentration of oxygen defects can improve the electron density of catalytic,promoted the transfer of electrons between catalyst and reactants efficientiy and enhance the photocatalytic performance.An extremely high methane generation rate is 41.6 ?mol g-1 h-1 on the CV-H2 at ambient temperature.The cycle test indicated that the conversion rates of the cerium vanadate solid powder still up to 20 ?mol g-1 h-1 after 15 cycles during a continuous 30-hour test.