Preparation and characterization of MnIn-layered double hydroxides (LDHs), extension of the synthesis to fabricate MnM(III)-LDHs (M = Al, Sc, Cr, Fe, Ga), and the comparison of their photocatalytic and catalytic activities in the oxidation of hydroquinone

Abstract

The preparation of a new member of the hydrotalcite supergroup, i.e., MnIn-LDHs with Mn:In molar ratios from 5:2 to 8:2 was developed for the first time, by using the co-precipitation technique. The effect of the presence of air, N2 or H2 was systematically tested. The physico-chemical properties of the solids were characterized in detail. The application of reductive atmosphere proved to be vital for achieving phase-pure products. While the thermal behavior of the LDHs with different Mn:In molar ratios were largely similar, the increasing Mn:In ratio resulted in decreasing direct optical band gaps from 4.06 to 3.14 eV. By using the optimized conditions found for MnIn-LDHs, Mn4M(III)-LDHs were also prepared with M = Al, Sc, Cr, Fe, Ga. The catalytic performance of the various Mn(II)-based LDHs were tested under dark and UV–Vis-illuminated conditions for the transformation of hydroquinone in acetonitrile medium. The photocatalytic mechanism over MnIn-LDHs was studied by radical trapping tests, indicating the photogenerated holes play an important role in the decomposition of hydroquinone and even benzoquinone. The incorporation of Sc(III), Cr(III) and Fe(III) resulted in catalysts with outstanding performance both with and without illumination; under these conditions several metal oxides/semiconductors (like BiVO4, Bi2WO6, SrTiO3, Bi2O3, SnO2, WO3, In2O3, ZnO and Degussa P25 TiO2) and even the nonmetallic g-C3N4 proved to be inactive. The largest photocatalytic contributions were measured for the Mn4Cr- and Mn4In-LDHs. The selectivity and yield of the benzoquinone formation was found to be the largest in the case of the Mn4Al-, Mn4Fe- and Mn4In-LDHs using UV–Vis light source.