Proposed General Sol−Gel Method to Prepare Multimetallic Layered Double Hydroxides: Synthesis, Characterization, and Envisaged Application

Abstract

A versatile sol−gel method for the synthesis of multimetallic layered double hydroxides (LDHs) is presented. The success of this method is due to the accurate control over crucial synthesis parameters, which are thoroughly analyzed, and to the complexation of metallic precursors in order to equalize their hydrolysis and condensation rates to obtain transparent gels. XRD patterns show pure LDH phase, with broad peaks indicative of small crystal size. Solids thus obtained show nanocapsular morphology, instead of the platelet-like particles obtained by coprecipitation and other methods. Thermal decomposition was analyzed by TGA, DTA, and FTIR, revealing that a variety of interlayer anions are present in different proportions, mainly carbonates, nitrates, acetates, and alkoxy groups. This observation was corroborated by the expansion in the interlayer region observed by XRD. N2 physisorption confirmed that all calcined sol−gel LDHs have high specific surface areas (up to 290 m2 g−1) and very narrow pore size distributions (ca. 3−4 nm). The sols are successfully cast as thin films over glass substrates; control over film thickness is achieved by varying aging time. Furthermore, catalytic membranes deposited over cordierite monoliths were obtained by immersion on sols and subsequent calcination; these membranes are chemically uniform, thermally stable, and well-adhered.