Zn,Al hydrotalcites calcined at different temperatures: Preparation, characterization and photocatalytic activity in gas–solid regime

Abstract

Layered double hydroxides (LDH) containing zinc and aluminium within the brucite-like layers and carbonate as the interlayer anion have been used as precursors for the preparation of mixed metal oxides by calcination. Zinc oxide and the ZnAl₂O₄ spinel were formed with different degree of crystallinity depending of two parameters, i.e., the Zn²⁺/Al³⁺ molar ratio and the calcination temperature (500–800 °C) of the LDH precursor. A pure spinel phase was obtained to remove ZnO upon a basic post-treatment in NaOH solution of the samples calcined at 800 °C. All the samples were tested for the photodegradation of 2-propanol in gas–solid regime. All the samples resulted active as heterogeneous photocatalysts. The photocatalytic activity increased by increasing the Zn²⁺/Al³⁺ molar ratio and the calcination temperature of the solids.

Introduction

There is an increasing demand for catalysts to eliminate the pollution associated to toxic compounds. Semiconductors such as TiO₂ [1], [2], ZnO [3], [4], SnO₂ or CdS [5] have been used in photocatalytic degradation of organic pollutants in liquid–solid [6] and in gas–solid regimes [7]. In addition to these oxides, it is worth studying the photoactivity of metal spinels, such as ZnAl₂O_4, which band gap, 3.8 eV [8], makes it suitable for photocatalytic processes [9]. The classic route for preparation of semiconductor metal oxides photocatalysts, commonly obtained by calcination of salts, leads to solids with low specific surface area which is a disadvantage in heterogeneous catalysis. To avoid this problem various preparation methods such as sol–gel processes have been proposed [10]. The sol–gel methodology allows to obtain materials with high specific surface area, but the nature of the precursors, generally metal alkoxides, makes it very expensive. An alternative preparation method is the calcination of layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays [11], [12], [13], [14]. These materials are layered solids with positively charged sheets, due to isomorphic substitution of divalent cations by trivalent ones in brucite layers, and charge-balancing anions in the interlayer space. These solids can be described by the general formula [M³⁺_xM²⁺_1−x(OH)₂]^x+ [Aⁿ⁻]_x/n mH₂O where M²⁺ and M³⁺ are the divalent and the trivalent cation, respectively, and Aⁿ⁻ the exchangeable anion in the interlayer, where water molecules are also found. LDHs easy preparation, low cost and physico-chemical properties have allowed many applications for these materials as catalysts, catalyst supports and precursors, adsorbents, drug matrices, etc. [11], [12], [13]. The LDH compounds as prepared or calcined by a controlled decomposition are often used as catalysts or catalyst precursors. LDHs calcination have resulted to be an excellent methodology to prepare mixed oxides with better catalytic performance than conventional preparations, such as ceramic or wet chemical routes. For instance, Qi et al. have recently conducted studies on a mixed oxide catalyst prepared by calcination of LDHs which exhibited a comparable activity, but better stability compared with a commercial Cu-based catalyst [15]. Also Li et al. have reported that pure spinel ferrites can be obtained by calcination of tailored hydrotalcite-like layered double hydroxides [16]. These authors report that the main advantage of this preparation methodology is that the uniform distribution of metal cations in the LDH precursors drives to the formation of spinel ferrites in shorter time and requiring a much lower temperature.

In this work, Zn,Al hydrotalcites with different Zn²⁺/Al³⁺ molar ratios and carbonate as the interlayer anion have been prepared. After their calcination at different temperatures (500–800 °C) the photocatalytic activity of the materials was tested for 2-propanol oxidation in gas–solid regime. Photocatalytic oxidation of 2-propanol by using various types of solids is a widely studied reaction and different degradation pathways are reported depending upon the physico-chemical features of the solid [17]. Moreover, pure ZnAl₂O₄ spinel was obtained by selective leaching of ZnO from the hydrotalcites calcined at high temperature and their photocatalytic activity was also tested.