Theoretical Study On Carrier Mobility And Photocatalytic Ammonia Synthesis Over Layered Double Hydroxides

The problems of environmental pollution and energy shortage are becoming more and more prominent,the technology of semiconductor photocatalytic ammonia synthesis driven by solar energy shows great potential.Layered composite metal hydroxides（LDHs）are considered as potential photocatalytic materials for ammonia synthesis due to their special layered structures.The main factors affecting the reaction efficiency of LDHs photocatalytic ammonia synthesis are the separation efficiency of carriers and the surface state of catalyst.Defect engineering is widely used because it can construct surface active sites.At present,there are many experimental methods to design and synthesize LDHs,but the carrier migration behavior has not been explained by experimental means.In addition,the mechanism of photocatalytic synthesis of ammonia over LDHs with vacancies is not clear.Therefore,in this work,the carrier migration behavior in LDHs and the mechanism of photocatalytic synthesis of ammonia over LDHs with vacancies were revealed through theoretical calculation,and the structure-activity relationship between vacancy species together with number and synthetic ammonia performance was explained in this work.The major contents and conclusions are as follows:（1）The transfer integral（V）,recombination energy（λ）,Gibbs free energy change（ΔG）along three paths on the（110）surface in the photocatalytic process of Mg₂Al-Cl-LDH are calculated based on Marcus theory,it can be concluded that path 3（layer to layer）is the most favorable way of electron transfer in the z direction.The carrier mobilities of eight kinds of M₂^ⅡM^Ⅲ-Cl-LDHs(M^Ⅱ=Mg²⁺,Co²⁺,Zn²⁺,Ni²⁺;M^Ⅲ=Al³⁺,Cr³⁺)are 0 cm²·V^-1·s^-1.This result indicates that electron-hole pair migration in the z direction is almost impossible.Combined with the conclusions drawn in the previous work:the carrier mobility of LDHs in the x and y directions（i.e.,along the（003）surface）is larger than 0 cm²·V^-1·s^-1,that is,electron-hole pairs can be transmitted in the x and y directions of LDHs.Moreover,the polarity reversal of Ni₂Cr-,Zn₂Al-,Zn₂Cr-Cl-LDHs in the x and y directions inhibits the electron-hole recombination.Two design strategies are proposed:（1）Reduce the number of layers of LDHs so that the carriers generated by the LDHs can migrate to the reaction site;（2）By adjusting the metal species in LDHs matrix to make the polarity in x and y directions different,which can avoid the rapid recombination of carriers.（2）The electronic properties of six kinds of M₂^ⅡM^Ⅲ-Cl-LDHs(M^Ⅱ=Mg²⁺,Zn²⁺,Ni²⁺;M^Ⅲ=Al³⁺,Cr³⁺)are calculated based on density functional theory with Hubbard correction.The results show that Ni₂Al-,Mg₂Cr-,Ni₂Cr-,Zn₂Cr-Cl-LDHs responded to visible light;Mg₂Al-Cl-LDH and Zn₂Al-Cl-LDH responded to UV light.It is found that all the six LDHs have positive photocatalytic driving force.The photocatalytic mechanism of ammonia synthesis along the distal,alternating and mixed paths on the LDHs（003）surface with the oxygen vacancy was calculated.The results show that the dominant path for Mg₂Al-Cl-LDH and Zn₂Al-Cl-LDH is the mixed path,the dominant path for Ni₂Al-Cl-LDH and Ni₂Cr-Cl-LDH is the distal path.However,Mg₂Cr-Cl-LDH and Zn₂Cr-Cl-LDH could not happen the photocatalytic synthetic ammonia reaction.According to the mechanism of the photocatalytic synthetic ammonia reaction over perfect Ni₂Al-Cl-LDH and Ni₂Al-Cl-LDH with the oxygen vacancy,it can be found that the Gibbs free energy changes(ΔG_PDS)corresponding to the potential determining step（PDS）of the dominant path in the ammonia synthesis reaction were decreased by 1.04 e V with the introduction of oxygen vacancy.The introduction of Ni vacancy further reducesΔG_PDS by 0.17 e V.These results indicate that increasing the type and number of vacancies can further reduce the Gibbs free energy changes,which is favorable for the photocatalytic synthetic ammonia reaction.In this work,the theoretical design approach for LDHs photocatalyst is proposed in view of the electronic level,which provides useful theoretical information and guidance for understanding the mechanism of LDHs for the photocatalytic synthetic ammonia reaction.