Theoretical And Experimental Combined Study On The Construction Of Layered Double Hydroxides-based Photocatalysts And Electrocatalysts,Together With Their Catalytic Mechanism

Layered double hydroxides (LDHs) has attracted much attention in recent years in the fields of photocatalyst and electrocatalyst because of its relatively large variety in their structure. The further application of materials is based on their structural design. Although much experimental study has been carried out on LDHs-based catalysts, these works are lack of theoretical guidance resulted from the limitation of experimental conditions. Moreover, the catalytic mechanism is difficult to be determined with experimental method. Theoretical calculations, fromanother study aspect, can provide helpful method and instructive approach for mechanism understanding and even forecast. In this work,the photocatalytic oxygen evolution and electrocatalytic methanol oxidation mechanisms of various kinds of LDHs are calculated by the density functional theory with Hubbard correction. Several kinds of LDHs with high catalytic performance, such as CoAl-, NiCr- and ZnCr-LDH, are screened. At the same time, we shed a light on the structure-property correlation of LDHs-based catalysts through a theoretical view.The major contents and conclusions in this work are listed as follows:(1) The electronic properties (band structure, density of states, work function, band edge placement) and oxygen evolution thermodynamics mechanism of M?nM?-A-LDHs (M? ? Mg, Co, Ni, Zn; M?= Al, Ga; n ?2, 3; A = Cl, OH, CO3) are calculated. It is found that LDHs with both M?and M? of main group metal, only respond to ultravisible light. Moreover,their photocatalytic oxygen evolution driving forces are not able to overcome their overpotentials. However, transition metal-based ConAl-A-LDHs possess photocatalytic oxygen evolution activities under visible light. The photocatalytic oxygen evolution rate of Co2Al-NO3-LDH is experimentally determined to be 973 ?mol·g-1·h-1.(2) The electronic structures and thermodynamics oxygen evolution mechanisms of 14 kinds of transition metal-based M?nM?/?-A-LDHs(M?=Mg, Co, Ni, Cu, Zn; M?=Cr, Fe; M? = Ti;n=2, 3, 4; A = Cl,NO3, CO3) are calculated. By comparing their photocatalytic driving forces and overpotentials, it was found that Co2Fe-Cl-,Ni2Ti-Cl-,Zn2Ti-Cl-, Zn2Cr-Cl-, and NinCr-A-LDHs can overcome their overpotentials with their driving forces, derived from the photogenerated hole. Therefore, these 9 kinds of LDHs can undergo the photocatalytic reaction without the external bias. Furthermore, the photocatalytic activities of NinCr-A-LDHs are confirmed by experiments.(3) The movement and separation of electron-hole pairs are calculated with the deformation potential theory. The orthogonal x- and y- directions are defined on the (003) surfaces of 26 kinds of LDHs. The carrier mobility of electron and hole in x- and y- directions are calculated. Thus,the polarity in each direction for these LDHs are determined by comparing the mobility of corresponding electron and hole. Co2Mn-,Co2Co-Cl-LDH, Ni2Cr-, Ni2Co-, Ni2Ga-, Zn2Ti-, and Zn2Cr-Cl-LDH are found to be polarity reversed in their x- and y- directions. Therefore,electron and hole can be better separated in these 7 LDHs.(4) The electrocatalytic methanol oxidation mechanism of M?nM?-A-LDHs (M?=Co, Ni, Zn; M? = Al, Co, Fe; n = 2, 3; A = NO3,Cl, CO3) are calculated. By screening all the possible reaction pathways,the results show that all these 12 LDHs undertake the following mechanism: *+ CH3OH?CH2OH*?CHOH*?W CHO*?HCOOH*?CHOO* ?* + CO2. And the potential-determining step is the CHO* +H2O ?HCOOH* + H+ + e-. Among these LDHs, Ni2Fe-N03-LDH and ZnnFe-A-LDHs possess the lowest onset potential. Therefore, these five kinds of Fe-based LDHs are the most promising methanol oxidation catalysts among the calculated LDHs here. Then, the electrocatalytic methanol oxidation activities of Ni2Fe-NO3-LDH and Zn2Fe-NO3-LDH are proved by experiments.In general, an efficient method for screening high-performace light or electronic responsed LDHs-based catalysts is proposed in this work by combining the theoretical calculations and experimental observations together. This work provides richful theoretical information and sheds a light on the construction of materials design toward LDHs-based catalysts.