Date of Award
Doctor of Philosophy
Ampofo K. Darko, Bhavya Sharma, Bin Hu
High-entropy materials (HEMs) have emerged as a new class of multi-principal-element materials with great technological prospects. As a unique class of concentrated solid-solution materials, HEMs, formed on the premise of incorporating five or more principal elements into a single crystalline phase, provide an excellent opportunity to access superior catalytic materials ‘hiding’ in the unexplored central regions of a multicomponent phase space of higher orders.
However, the fabrication of HEMs is energy-intensive, typically requiring extreme temperatures and/or pressures under which agglomeration of particles occurs with a commensurate decrease in surface area. For most catalytic applications, non-agglomerated particles with high surface areas are preferred. Accessing nanostructured HEMs with an increased surface area has motivated efforts to explore unconventional synthesis strategies.
On the other hand, ultrasound can be used to drive high-energy chemical reactions via the physical process of acoustic cavitation that provides a unique high-energy environment at such magnitude and time scale that is unattainable with conventional energy sources. Our overarching goal is to exploit this unique high-energy environment toward the synthesis of nanostructured HEM as an emerging new class of catalytic materials. Taking advantage of the acoustic cavitation phenomenon, nanostructured particles of various subclasses of HEMs, including high-entropy fluorite oxides (HEFOs), high-entropy perovskite oxides (HEPOs), and high-entropy alloys (HEAs) were fabricated at seemingly room temperature conditions in the present study. Several characterization techniques were used to understand the crystalline structure, chemical composition, surface chemistry, and textural features of the fabricated nanocatalysts. Their catalytic performances were assessed towards carbon monoxide (CO) oxidation and/or selective phenol hydrogenation. Such a technologically feasible, facile, and scalable synthetic strategy holds great promise towards the synthesis of nanocrystalline materials for different applications.
Okejiri, Francis Uchenna, "Ultrasound-Driven Fabrication of Nanosized High-Entropy Materials for Heterogeneous Catalysis. " PhD diss., University of Tennessee, 2021.