Studies of the Hierarchical Structure in UCT Manganese Oxides

Abstract

In a recent report from our group, inverse micelles were used as the soft template in the synthesis of well-ordered mesoporous manganese oxide using a sol-gel-based method. The materials synthesized using this method are known as University of Connecticut (UCT) mesoporous materials. UCT materials are formed in an acidic mixture that consists of a defined ratio of hydrotropic ion precursor, metal precursor, interface modifier, and surfactant. The surfactant molecules form inverse micelles, which encapsulate the reagents during the synthesis process. These features serve as uniformly sized nano-reactors within which hydrolysis and condensation of oxo-clusters occurs. Upon calcination, highly monodisperse nanoparticles will form within the inverse micelles, and the subsequent packing of these nanoparticles during aggregation leads to monodisperse meso-pores defined by intra-particle voids. The surfactant and hydrotropic nitrate ions in this process play key roles in controlling aggregation and condensation of oxo-clusters. Moreover, by modifying the ratio of the components in the mixture, and the process conditions with respect to the chosen metal precursor, this method can be used to synthesize mesoporous oxide materials from across the periodic table, including transition metals and non-metals [1-4]. The focus of the present work is on the synthesis of mesoporous manganese oxide, which is a promising candidate for catalytic and adsorption applications due to the variety of oxidation states and polymorphic structures that can be adopted. In this study, we have synthesized a series of mesoporous UCT manganese oxides by adding different surfactants to the primarily gel. This gel consists of Mn(NO 3) 2 as the metal precursor, 1-butanol as the interface modifier, and HNO 3 as the hydrotropic ion precursor. The ratios of the constituents in the gel and the amount of surfactant required to produce optimized UCT mesoporous manganese oxides were identified in preliminary synthesis trials. These values were then used to synthesize six different UCT manganese oxides with the only variable being the choice of surfactant. The hierarchical structures exhibited by these mesoporous materials were then studied using a combination of N 2 adsorption, XRD, SEM and TEM techniques. Here we present a selection of data from the sample that gave the highest specific surface area (166 m 2