Theoretical And Experiment Study On The Topotactic Transformation And Memory Effect Of Layered Double Hydroxides

Layered double hydroxides(LDHs)are a class of nano layered materials,and its crystal structure has a unique adjustable turning performance.LDHs can occur the topological transformation under calcination conditions;and part of that have memory effect which is that mixed metal oxide from LDHs precursor would reconstruct into the original structure in aqueous solution.The topological transformation and memory effect of LDHs play an important role in the fabrication of high dispersed metal nanocatalysts.But only from the aspects of experiment one can't understand the mechanism of the topological transformation and the memory effect of LDHs.In this paper,the mechanism of the topological transformation and the memory effect of the ZnAl-,NiAl-and MgFe-LDHs are studied under the two key temperatures(ZnAl-LDH:273,800 ?;the leicester-LDHs:365,800 ?;MgFe-LDHs:380,800 ?),respectively,by molecular simulation which based on the density functional theory(DFT).The conclusions are as follows:1,The three LDHs undergo the decomposition of the anion and dehydroxylation of the hydroxide layers at the first key temperature.Both LDHs decompose to CO2 and H2P via the residual O atom generated from CO32-by a monodentate intermediate.2,Through simulating the dehydroxylation of the hydroxide layers and calculating the migration of metal ions and the root mean square displacement,it is founded that the three LDHs is different in the migration of the metal atom and the the memory effect of LDHs.The metal cations in NiAl-and MgFe-LDHs maintain their original distribution within the LDH(001)facet,while along the c-axis direction migrate substantially.However,the distribution of metal cations in ZnAl-LDHs matrix changed heavily along both LDH(001)facet and the c-axis direction perpendicular to the(001)facet.3,The dynamics simulation of the LDHs memory effect,it is founded that MMO from NiAl-,MgFe-LDHs which have memory effect would reconstruct into the original structure,however,the dehydrated products of ZnAl-LDHs cannot reconstruct back to the hydroxide phase,showing no memory effect.4,On the second key temperature,the mass losses of the ZnAl-,NiAl-,MgFe-LDHs are due to the dehydroxylation of the hydroxide layers.A complete collapse of layered structure occurs.During this process,metal ions of ZnAl-,NiAl-,and MgFe-LDHs changed heavily in the LDH(001)and LDH(120),and its coordination number changed from 6 to 4 resulting in a totally disordered cation distribution and plenty of holes in the final product.This work is helpful for a detailed understanding of the topotactic transformation and memory effect of LDHs from micro perspective,which is close related to the design of highly dispersed heterogeneous nanocatalysts.