ABSTRACT

Researches and developments of materials are required minimization of the influence on environment and &e securable supply nowadays. Hydrotalcite-like compounds, which are simply referred to as hydrotalcites hereafter, are a family of anionic clays. They consist of hydroxides of common and abundant metals on the earth, such as magnesium and aluminum, and can be synthesized at ambient temperature and pressure. Because of these properties, hydrotalcite can be a compound that satisfies the above requirements. In the present study, we aim to prepare novel functional materials with hydrotalcites.

Hydrotalcites are a layered double hydroxide, with the general formula [M²⁺_1-xM³⁺_x (OH)₂] [A^n-_x/n·zH₂O]. The host layers are charged positively by replacement of divalent metal cations with trivalent ones. The positive charge is compensated by the interlayer anions. The interlayer anions are exchangeable, and inserting various anions into interlayers (intercalation) can give hydrotalcites new functions. Hydrotalcite whose interlayer anions are carbonate (CO₃-hydtotalcite) decomposes to oxide at 500°C, releasing CO₂ and H₂O. When the obtained oxide is put into an aqueous solution, it can be reconstructed to the original hydrotalcite structure, taking anions and water. Intercalation method using this property is recognized as a unique method that is available to only hydrotalcites and is called as reconstructed method.

Up to date, using hydrotalcites as precursor of oxide catalysts has been main application of hydrotalcites. Application of hydrotalcite intercalation compounds is still in the experimental stage. In many researches, reconstruction method is frequently adopted as a facile method, because there is no selectivity of anions in the method and the method can be applied for various anions. However, thermal decomposition behavior of CO₃-HTlcs that is starting material for reconstruction method is not studied thoroughly to date. This is the first problem to resolve in the study. On application of hydrotalcite intercalation compounds, intercalation of polyoxometalates is currently attracting interests, because this is a technique that can provide microporous materials at room temperature. Space between large polyoxometalete anions intercalated in interlayers can be micropores in the structure. Most polyoxometalate intercalates of hydrotalcite that have been reported, however, have poor crystallinity. Both hydrotalcite host layers and polyoxometalate anions are decomposed when they are mixed in aqueous solution, because hydrotalcites are basic whereas polyoxometalate are acidic. This is the second problem.

First of all, we studied the thermal decomposition behavior of CO₃-HTlcs, in particular, sequence of thermal decarbonation behavior in wide temperature range, which has never been reported. This study is important for reconstruction method. Historically, it has been thought that decarbonation reaction occurred at 400°C and carbonate anions scarcely remained in the materials calined above 500°C. However, the present study demonstrated that decarbonation still occured above 500°C, and 20-30% of carbonate anions remained in the material calined at 500°C when HTlc with x=0.33, which contained carbonate anions most, was calcined. This result meant that HTlcs reconstructed even with anions other than carbonate involved some amount of carbonate anions. [Chapter 2]

To resolve the above problem, a couple of methods were conducted. The novel method that carbon clusters were intercalated in the interlayers together with carbonate anions accelerated the decarbonation around 400°C. However, quantity of remaining carbonate anions above 500°C did not decreased by the method. Finally, we found that HTlc with x=0.25, which contained less carbonate anions, could be the most suitable precursor for reconstruction, because its carbonate anions scarcely remained above 500°C. [Chapter 3]

On reconstruction of calcined HTlcs, there was a new finding. Spinel that was obtained above 900°C by one-step calcination could be obtained by repetition of calcination at 400°C and reconstruction in an aqueous solution. Spinel is a useful oxide that can be fine ceramics having high heat-resistance and good optical transmission after sintering. The method using repetition of calcination and reconstruction can lower the temperature of synthesizing spinel remarkably. The reaction was concluded to be the consequence of the reaction between edges of crystallites, because the reaction occurred more easily when the crystallites were smaller. [Chapter 4]

On the base of above results, we applied calcined HTlcs to catalyst for decomposition of N₂O, which recently received considerable attention as strong green house effect gas. As a result, calcination material that was obtained from HTlcs containing small amount of rhodium at 500°C showed high activity for the decomposition of N₂O. [Chapter 5]

Concerning intercalation of polyoxometalates, we chose paranolybdate anion (Mo₇O^6-₂₄) out of polyoxometalte anions. Instead of aqueous solution, ethanol/water solution that contained the minimum amount of water to solve paramolybdate salt was used as medium to prevent the decomposition of both of HTlc and paramolybdate. The ethano1/water solution prevented the mutual decomposition of host and guest, and then highly ordered HTlc containing paramolybdate anions could be obtained. [Chapter 6]

The present study has provided several important results about intercalation to obtain novel functional material using HTlcs, and paved the way to useful applications.

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