Mixed oxides memory effect

The conversion of the mixed metal oxides into LDHs has been variously referred to as regeneration, reconstruction, restoration, rehydration or the calcination-rehydration process , structural memory effect or simply memory effect . This method is usually employed when large guests are intercalated. It also avoids the competitive intercalation of inorganic anions arising from the metal salts. The procedure is more complicated than coprecipitation or ion-exchange methods, however, and amorphous phases are often produced simultaneously. 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

The COj species in the HT interlayer could be exchanged with OH ions by calcination at 723 K and hydration at room temperature. A spinel phase of Mg-Al mixed oxide obtained after the calcination transforms into the original layered structure during the hydration. This reconstruction is known as the memory effect of HT materials. The reconstructed HT catalyzed the Knoevenagel condensation of various aldehydes with nitriles in the presence of water [119]. The reconstracted HT also showed an aqueous Michael reaction of nitriles with a,p-unsaturated compounds. The layered double-hydroxide-supported diisopropylamine catalyzed the Knoevenagel condensation of aromatic carbonyl compounds with malononitrile or ethyl cyanoacetate [120]. This solid base could be recycled at least four times, and exhibited activity for aldol, Henry, Michael, transesterification, and epoxidation of alkenes. 

Application of LDHs is mostly based on their use after thermal treatment and mixed oxide formation. If the calcination is performed at temperatures below 550 °C, mixed oxides also have, besides the aforementioned properties, the memory effect. This very specific property of mixed oxides derived from thermal degradation of LDHs allows the reconstruction of the layered structure in mild conditions when mixed oxides are in contact with aqueous solution or air. If calcination is carried out at temperatures above 827 °C, irreversible mixed spinels are formed and the memory effect is disabled [48]. The main application of the memory effect is for the synthesis of LDHs with different interlayer anions than CO ". Taking to consideration that carbonate anions have the highest affinity toward the incorporation in the LDH interlayer, during the classical synthesis methods the contamination with carbon dioxide from the air always occurs. If, for example, the synthesis LDH with OH" ions in the interlayer is required, the reconstruction of mixed oxides can be performed by steam or contact with decarbonized water. Similarly, if synthesis of LDHs with other anions in the interlayer is anticipated, reconstruction is carried out in an aqueous solution containing the desired anions. Catalytic properties of mixed oxides obtained by reconstruc-tion/recrystallization procedure depend mainly on the conditions of each activation step. 

There are two mechanisms by which recrystallization of LDH is achieved. One mechanism follows the topotactical conversion that occurs without the dissolution of the sample, whereas the second one follows the LDH recrystallization initiating the memory effect by the dissolution of the mixed oxide [2,32,37,49]. 

In Figure 20.13, the occurrence of the memory effect for HS-MgAl mixed oxide sample (HS coprecipitation synthesis, x=0.3) is presented by XRD spectra. It can be observed that after thermal activation at 500 °C the characteristic LDH XRD peaks disappeared (designated as 0) and peaks of mixed oxides (designated as are formed. After only two weeks of contact with air, partial reconstruction of the layered structure is achieved, whereas after 10 months the layered structure is completely reconstructed, as can be seen from the characteristic XRD peaks. 

Synthesis ofTiOJZnLDH using the reconstruction synthesis method. Based on the memory effect of mixed oxides derived from thermal decomposition of LDH TiO /ZnLDH were obtained. LDHs Zn-LDH were synthesized by coprecipitation method, and then calcined at 550 C and 800 °C afterward, they were added to TiOSO solution in a nitrogen atmosphere. 

The method is based on the so-called memory effect, i. e., the ability of mixed oxides formed by a gentle calcination (usually not exceeding 500 -550°C) of carbonate-LDHs to recover the original layered structure, inserting anions from the medium [56, 57]. The mixed oxides are able to... 

Memory effect The partial dehydroxylation of LDHs under moderate calcination leads to amorphous mixed oxides usually named layered double oxides (LDOs), which can be regenerated back to the original structure by contacting with solutions containing various anions. 

The ferroelectric effect is an electrical phenomenon. Parhcular materials, including the ternary oxides (Ba,Sr)Ti03, Pb(Zr,Ti)03 and (Bi,La)Ti03, exhibit a spontaneous dipole moment which can be switched between equivalent states by an external electric held. Ferroelectric thin hlms are of importance for the production of nonvolahle ferroelectric random access memory devices (FeRAM) °. Two possibilities to synthesize such mixed metal oxides are given by the CVD and ALD methods. Table 10 shows the preparation methods of such materials synthesized from metal enolates recently. 

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