Elsevier

Solid State Ionics

Volume 128, Issues 1–4, February 2000, Pages 161-175
Solid State Ionics

Preparation of proton–β/β″-aluminas by the ion-exchange under hydrothermal conditions and their characterisation

https://doi.org/10.1016/S0167-2738(99)00341-0Get rights and content

Abstract

Preparation of proton–β/β″-aluminas is realised by an ion-exchange process under hydrothermal conditions using dilute acetic acid or sulphuric acid as the medium, and the corresponding alkali β/β″-aluminas as the starting materials. The optimum conditions involved in this process is around 425–450 K under the autogenerated pressure below 50 MPa, and a duration of 2–10 h. At higher temperatures and pressures or extended duration, proton–β-alumina converts to boehmite, γ-AlOOH, in the same medium. This method yields products with a variable range of composition with [H2O]:[Al2O3]=1:5 to 1:17. The resulting products convert to α-alumina when heated to temperatures above 1273 K under ambient pressures. High resolution transmission electron microscopy (HRTEM) indicates the variable contents of microsyntactic defects and stacking faults with the changing composition. This study also shows the retention of the β-alumina structure during the displacement of alkali by hydroxonium ions under optimum hydrothermal conditions.

Introduction

Proton and ammonium ion β″/β-aluminas find potential use in water electrolysis and fuel cells using hydrogen as the fuel. For these applications, protonic electrolytes should have high conductivity at the operating temperatures of these cells and the electrolyte material must be stable between 373 and 673 K. At these temperatures, the electrokinetics of the hydrogen and oxygen reactions are fast but not so high that corrosion reactions are severe. Proton–β″-alumina exhibits a conductivity value of 5 ×10−3 S cm−1 at 573 K [1]. Nicholson et al. [1] demonstrated the use of proton–β″-alumina in a water electrolysis cell that operated between 373 and 573 K. The temperature of working of the fuel cells based on this solid electrolyte material is much lower than that used in solid oxide fuel cells (SOFC). The working temperature of this doped/stabilised zirconia based fuel cell is between 1223 and 1273 K [2] because of the low conductivity of these electrolytes at lower temperatures. Thus, the efficiency of the fuel cells based on β″-alumina is expected to be better than that of SOFC based on other electrolytes.

Proton–β″/β-aluminas are prepared conventionally by an ion-exchange reaction using sodium or potassium–β″/β-aluminas with concentrated sulphuric acid around 510 K for several days [3]. The shortcomings of this process are long durations of reaction and handling of concentrated sulphuric acid at high temperatures (see also Ref. [16]). A field assisted ion-exchange procedure using dilute acetic acid was proposed by Nicholson [1], [4] for the preparation of proton–β″/β-aluminas. This process also needs prolonged durations for completion of the ion-exchange process. In this paper, a novel ion-exchange procedure carried out under hydrothermal conditions using dilute acetic acid or dilute sulphuric acid is presented for the preparation of proton–β-aluminas. The temperatures involved in this process are 423–438 K and the durations involved are 2–10 h normally. As lower temperatures are involved in this process when compared to the conventional methods of exchange, the decomposition of proton–β-alumina (a thermally unstable compound) is avoided. Using this procedure the conditions for the preparation of proton–β″/β-aluminas using acetic acid and sulphuric acid are optimised and the results are presented here. Also, the simplicity of this procedure is demonstrated by exchanging sodium ions in Na–β-alumina with proton which is difficult to achieve otherwise. Finally, the thermal stability of proton–β-alumina under ambient pressures and its stability under hydrothermal conditions were studied and the results are presented in this paper.

Section snippets

Experimental

An ion-exchange process under hydrothermal conditions is basically a soft chemical technique for the preparation of novel compounds,a process of this kind was developed by Kutty [5]. In principle, this method utilises the advantages of an ion-exchange process and hydrothermal method of preparation of materials. For the preparation of proton–β″/β-alumina, the precursor compound namely, K–β″/β-alumina was obtained using a gel-to-crystallite (G–C) conversion method [6] and was reacted with dilute

Effect of temperature on the ion-exchange properties of acetic acid under hydrothermal conditions

Fig. 1 shows the XRD patterns of the reactant, K–β-alumina, obtained from gel-to-crystallite (G–C) conversion process [6] and the products isolated from ion-exchange process under hydrothermal conditions in dilute acetic acid medium as a function of duration at 423 K. Analysis of the product after 7 h of reaction showed near absence of K+ in the ion-exchanged samples, indicating that this duration is sufficient for the formation proton–β-alumina. The XRD patterns (Fig. 1) of the exchanged and

Discussion

Hydrothermal synthesis of ceramic powders is a chemical process for the preparation of high purity crystalline or anhydrous ceramic powders [7], [8], [9], [10]. This process differs from other processes by the temperatures and pressures used in the synthesis compared to other processes, such as the sol–gel and co-precipitation processes. In hydrothermal preparations, the supercritical water has different solvation characteristics than those of water at ambient conditions. The diminishing ionic

Conclusions

The present studies show the preparation of proton–β/β″-aluminas by a novel ion-exchange procedure under hydrothermal conditions. This method offers the products with variable range of composition with [H2O]:[Al2O3]=1:5 to 1:17 by properly choosing the initial precursor and the fluid medium for the ion-exchange reaction. The stability of proton–β-alumina with composition [H2O]:[Al2O3]=1:17 is explained based on the variable widths of spinel blocks (revealed from HRTEM studies) arising from the

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