(380f) Assessing the Catalytic Applicability of Zirconium and Cerium Oxide Microspheres Prepared By Internal Gelation

While alumina, silica and aluminosilicates are most widely used catalytic materials in the chemical industry, cerium oxide (CeO₂), zirconium oxide (ZrO₂) and their mixtures are increasingly recognized for having unique properties which can enable more efficient or sustainable catalytic processes. For instance, they possess excellent stability in hot liquid water and redox functions desirable for biomass conversion processes. As these less conventional oxides have been primarily used as additives, they are generally not available as engineered bulk catalysts (e.g., pellets, beads) required for practical reactor implementation. Traditional techniques such as extrusion generally employ binders to form engineered catalysts, but the inclusion of binders can compromise the intrinsic properties of active components and make synthesis optimization difficult.

The Oak Ridge National Laboratory has a unique internal gelation technology capable of producing various metal oxide microspheres without using binders. The internal gelation process was initially developed as an advanced production process for spheroid nuclear fuel particles (e.g., UO₂, PuO₂). In the context of these efforts, ZrO₂ and CeO₂ microsphere were also synthesized either as surrogates for more in-depth study of internal gelation processes or as simulants for nuclear fuel development. All these efforts produced microspheres with relatively low porosity using unheated hexamethylenetetramine (HMTA) and urea as a hydrolysis catalyst and a complexing agent, respectively. Recently, ORNL researchers discovered that addition of boiled HMTA to the stock solution leads to very porous CeO₂ microspheres. The ability to produce porous metal oxide microspheres (0.1-1 mm) indicates that this internal gelation approach could be tailored for the synthesis of engineered catalysts as an alternative to conventional forming methods.

In the present work, we assessed the catalytic application potential of microspheres prepared by the internal gelation process by determining physicochemical properties relevant to catalysis such as surface area, porosity, crystallographic structures, crush strength, and surface reactivity. Boiled HMTA-urea and a novel wash technique were used in the preparation of CeO₂, ZrO₂ and their mixtures. After the microspheres were calcined in air at 873 K, they were characterized by N₂ sorption, X-ray diffraction, microscopy, hardness testing, and CO oxidation probe reaction. The results, to be described in detail in this presentation, clearly show the suitability of the internal gelation process for the synthesis of binderless engineered catalysts.