To investigate the effects of divalent metal ions on the photophysical and photocatalytic properties of MII/Cr–CO3 layered double hydroxides (LDHs), we varied the MII metal ions (Co, Ni, Cu, and Zn) at a constant MII/Cr atomic ratio of 2:1. The phase structures and textural and optical properties of these catalysts were characterized by powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET) surface area, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, and diffuse-reflectance UV–vis (DRUV–vis) spectroscopy. The PXRD measurements demonstrated that all of the as-synthesized LDHs had well-defined layered structures except Ni/Cr–CO3 LDH (LDH2). Optical difference spectra revealed that absorption in the visible region can be attributed to metal-to-metal charge transfer (MMCT) excitation of oxo-bridged bimetallic linkages of MII–O–CrIII in the two neighboring MO6 octahedra. The photocatalytic performances of these catalysts were examined in the degradation of methyl orange (MO) dye under solar light irradiation. The LDH containing cobalt (i.e., LDH1) exhibited the highest photoactivity with 90% MO removal in 3 h under solar light irradiation. The excitation of MII–O–CrIII in solid solution in the visible range was demonstrated to be one of the causes of the enhanced degradation of MO. An MO degradation mechanism over MII/Cr–CO3 LDHs is proposed that is due to the formation of intermediate oxidative species such as hydroxyl radicals and superoxide radicals during the reactions. Moreover, the most active photocatalyst (LDH1) was found to be stable under repeated applications of up to four successive cycles with a nearly constant photocatalytic degradation activity.

Layered double hydroxides (LDHs) are an important class of layered materials consisting of hydroxides of some of the most common and abundant metals on Earth. In addition to compositional flexibility, these materials are good anion exchangers and are important sinks for environmental contaminants. In this article, we review the past achievements of LDHs concerning their synthesis, structural development, characterization, and especially the removal of phosphate from aqueous solution. Major factors such as the nature and concentration of LDH precursor metals, layer charge and interlayer anion charge, competitive anions, pH, temperature, etc. for phosphate removal are discussed. Additionally, the possible phosphate removal mechanisms by LDHs are also discussed to know the efficacy of the materials. The phosphate removal capacities of LDHs are compared with other materials for accessing the future prospects of LDHs. The possible improvement of LDHs in terms of stability, regeneration and reusability are also proposed for cost effective applications.

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MnIII(salen)(OH2)2+ undergoes reversible anation by HOX−via Ia mechanism followed by proton controlled electron transfer involving MnIII(salen)(HOX) and H2OX.

The kinetics of oxidation of glyoxylic acid (HGl) by Mn III (salen)(OH 2) + 2 ((H 2 salen = N,Nbis(salicylidene)ethane-1,2-diamine) is investigated at 30.0-45.0 • C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm −3 (NaClO 4). The products are identified as formic acid, CO 2 and Mn II with the reaction stoichiometry, | [Mn III ]/ [HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn III (salen)(OH 2) + 2 with HGl and its conjugate base Gl − forming the corresponding inner sphere complexes (both HGl and Gl − being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addition, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl − are also observed. The rate, equilibrium constants and activation parameters for various steps are presented. Mn III (salen)(OH 2)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for Mn III (salen)(OH 2)(HGl) + (10 5 k et = 2.8 ± 0.3 s −1 at 35.0 • C) reflecting the involvement of proton coupled electron transfer mechanism in the latter case. A computational study of the structure optimization of the complexes, trans-Mn III (salen)(OH 2) + 2 , trans-Mn III (salen)(OH 2)(Gl), and trans-Mn III (salen)(OH 2)(HGl) + (all high spin Mn III (d 4) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to Mn I I I centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H.. .O hydrogen bond distance 1.745 Å) and the gem-diols are not involved in H-bonding in either case. A rate comparison for the second order paths: Mn III (salen)(OH 2)(HGl)/Gl) +/0 + HGl/Gl − → products, shows that HGl for the (aqua)(HGl) complex is a better reducing agent than Gl − for the (aqua)(Gl) complex (k HG ∼ 5 k Gl). The high values of activation enthalpy (H = = 93-119 kJ mol −1) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.

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