Novel synthesis process produces catalyst with just the right structure

Scientists at the Tokyo Institute of Technology in Japan have investigated a novel method for synthesizing manganese dioxide with a specific crystalline structure called β-MnO₂. Their research sheds light on how different synthesis conditions can produce manganese dioxide with distinct porous structures, hinting at a strategy for the development of highly tuned MnO₂ nanomaterials that could serve as catalysts for fabricating bioplastics.

Materials engineering has advanced to a point where scientists are not only concerned about the chemical composition of a material, but also about its nanoscale structure. Nanostructured materials have recently drawn the attention of researchers from a variety of fields, as their physical, optical, and electrical characteristics can be tuned and pushed to the limit by tailoring their nanostructures.

MnO₂ can form many different crystalline structures with applications across various engineering fields, including as catalysts for chemical reactions. One particular crystalline structure of MnO₂, called β-MnO₂, is exceptional at catalyzing the oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA), which can be used to produce environmentally friendly bioplastics. Thus, finding ways to tune the nanostructure of β-MnO₂ to maximize its catalytic performance is crucial.

However, producing β-MnO₂ is difficult compared with other MnO₂ crystalline structures. Existing methods are complicated and involve the use of template materials onto which β-MnO₂ ‘grows’, resulting in the desired structure after several steps. Now, scientists from Tokyo Institute of Technology led by Keigo Kamata have developed a template-free approach for the synthesis of different types of porous β-MnO₂ nanoparticles.

Their method, reported in a paper in ACS Applied Materials & Interfaces, is very simple and convenient. First, manganese precursors are obtained by mixing aqueous solutions and letting the solids precipitate. After filtration and drying, the collected solids are subjected to a temperature of 400°C in a normal air atmosphere, a process known as calcination. During this step, the material crystallizes to produce a black powder that is more than 97% porous β-MnO₂.

The scientists found this porous β-MnO₂ to be much more efficient as a catalyst for synthesizing FDCA than the β-MnO₂ produced using a more widespread approach called the ‘hydrothermal method’. To understand why, the scientists analyzed the chemical, microscopic and spectral characteristics of β-MnO₂ nanoparticles produced under different synthesis conditions.

They found that β-MnO₂ can take on markedly different morphologies according to certain parameters. In particular, by adjusting the acidity (pH) of the solution in which the precursors are mixed, β-MnO₂ nanoparticles with large spherical pores can be obtained. This porous structure gives the nanoparticles a higher surface area, thus providing better catalytic performance.

"Our porous β-MnO₂ nanoparticles could efficiently catalyze the oxidation of HMF into FDCA in sharp contrast with β-MnO₂ nanoparticles obtained via the hydrothermal method," Kamata explained. "Further fine control of the crystallinity and/or porous structure of β-MnO₂ could lead to the development of even more efficient oxidative reactions."

This study also provided much insight into how porous and tunnel structures are formed in MnO₂, which could be key to extending its applications. "Our approach, which involves the transformation of Mn precursors into MnO₂ not in the liquid-phase (hydrothermal method) but under an air atmosphere, is a promising strategy for the synthesis of various MnO₂ nanoparticles with tunnel structures," said Kamata. "These could be applicable as versatile functional materials for catalysts, chemical sensors, lithium-ion batteries and supercapacitors."

This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.