(196d) Direct Observation of Macropore Formation in Hierarchically Structured Metal Oxides

Hierarchical materials with pores at two different length scales integrated in the same particle are of interest in the design of novel structured materials for applications such as catalyst supports, adsorbents, chromatographic materials as well as in optical, electronic and biological applications. Starting from liquid metal alkoxides, a spontaneous self-assembly in solution offers a convenient template-free route to unique hierarchical meso?/ macroporous metal oxide powders. The final powder particles contain macropores in form of parallel cylindrical channels, while the mesoporosity is created due to the interstitial spaces in between the aggregating metal oxide nanoparticles. Such structure offers a combined benefit of both high specific surface area due to mesopores (5-10nm) and enhanced transport properties to the surface of the mesopores due to macropores (1-5 μm). Subsequent to our first report of hierarchical meso-/macroporous alumina materials via a spontaneous self-assembly, significant effort has focused on the synthesis of the other hierarchical materials, and now such patterns have also been demonstrated for titania, zirconia and also mixed-metal oxides and carbon materials. However, very few studies are devoted to mechanistic understanding of the formation process. Although, the formation of mesopores is known to take place via a nanoparticles assembly mechanism, the formation mechanism for macropores has defied explanation for almost a decade, resulting into an array of hypotheses and speculations. The rapid metal alkoxide reaction rates that are necessary for pattern formation makes it extremely difficult to follow the process. Presented will be an unprecedented direct observation of the macropore formation made possible by the use of an unusual precursor, which provides a great reactivity compromise; fast enough to yield the macroporous pattern and also slow enough to allow a direct visualization. These observations provide new insight into the development of a mechanistic understanding of macropore formation via a spontaneous self-assembly process.