Effect of Chromate Anion on the Photocatalytic Activity of Mg-Al Layered Double Hydroxide

The aim of this work is to explore the possibility of use of layered double hydroxides (LDH) as a photocatalyst for the selective oxidation of toxic nitrogen oxides, NO and NO₂ (NO_x ), to nitrate. Recent studies suggest that layered double hydroxides could be suitable for this aim.

LDH consists of brucite-like layers, with a partial M^II/M^III substitution, which causes a positive charge in the layer. The excess of charge generated is replaced by the intercalation of anions in the interlayer space. LDHs are layered materials with general formula [M^II_1-xM^III_x(OH)₂]X^n-_x/n·nH₂O where M^II = Mg²⁺, Zn²⁺, etc; M^III = Al³⁺, Fe³⁺, etc; and X^n- an interlayer anion which could be a great variety of inorganic and organic species depending on the preparation method. Calcination at 500 ºC converts the LDHs into mixed oxides, rehydration of which restores the original LDH structure by interaction anions from aqueous solution (reconstruction).

Thus, we synthesized MgAl-CrO₄ LDH by reconstruction method, from the precursor MgAl-CO₃ with Mg/Al ratio of 3, obtained by coprecipitation method, and compared these two LDH as photocatalysts for decomposition of NO_x gases (deNO_x). Techniques such as XRD, FT-IR and UV-Vis spectroscopy have been used to characterize the samples.

Compared to the pristine LDH system, the textural characteristics, optical and electronic properties of chromium containing LDH samples exhibited a better harvesting of sunlight and the highest specific surface area (> 50 m². g^-1). In addition, the photocatalytic results showed high efficiency of LDH (60%) towards the complete oxidation of NO_x.