Nanoparticle Exsolution from Pyrochlore and Defect-Fluorite Oxides—Tailoring Catalyst Properties Through Composition and Process Engineering

In the transition to net zero, advances in heterogeneous catalysis play a critical role in maintaining efficient, sustainable production of the materials, chemicals and fuels required by society. Current research focuses on improving catalytic activity, stability, and precious metal utilisation, with the aim of developing catalysts that minimise the use of precious material while retaining sufficient activity and stability to meet the demands of industrial-scale production, and ideally providing the opportunity for facile recycling and re-use to avoid material waste. One approach to the preparation of heterogeneous catalysts that has emerged in recent decades is nanoparticle ex-solution – the controlled growth of highly stable metal nano-catalysts from oxide supports upon partial reduction [1,2]. The morphology, size and distribution of these metal nano-catalysts have demonstrated a dependence on the conditions imposed to achieve partial reduction, and the composition of the oxide support [3-5]; both could provide the opportunity for precisely tailoring the properties of these ex-solved catalysts, but a better understanding of the processing-structure-property relationship is required for this to be achieved in practise.<br/><br/>In this work, we investigate nanoparticle ex-solution from the relatively unexplored pyrochlore and defect-fluorite systems. Despite defective fluorite-type oxides showing considerable promise as hosts for exsolved nanoparticles in previous work [6], further studies exploring the factors governing exsolution in this system, and the closely related pyrochlore system, have been limited. To address this, we have taken a systematic approach to examining how the properties of the host oxide and the reduction conditions influence nanoparticle ex-solution in both of these systems. The former is investigated by taking advantage of the compositional flexibility of the pyrochlore and defect-fluorite systems to tune their intrinsic structural disorder and vacancy concentration, while the latter is explored by employing near-ambient-pressure XPS to study the ex-solution process in situ under a range of gas compositions. Through building our understanding of these relationships between processing, structure, and properties, we aim to develop principles that can better inform the design of future ex-solved catalyst materials.<br/><br/>[1] Neagu, D., Tsekouras, G., Miller, D. N., Ménard, H., & Irvine, J. T. (2013) <i>Nat. Chem.</i> <b>5</b>(11), 916-923.<br/>[2] Neagu, D. <i>et al.</i> (2019) <i>ACS Nano</i> <b>13</b>, 12996–13005.<br/>[3] Zhang, J., Gao, M. R. & Luo, J. L. (2020) <i>Chem. Mater.</i> <b>32</b>, 5424–5441.<br/>[4] O’Leary, W., Giordano, L. Park, J. Nonnenmann, S. S. Shao-Horn, Y. & Rupp, J. L. M. (2023) J. Am. Chem. Soc. <b>145</b>(25), 13768-13779.<br/>[5] O’Leary, W., Giordano, L. & Rupp, J. L. M. (2023) <i>J. Mater. Chem. A</i> <b>11</b>(39), 21429-21442.<br/>[6] Naeem, M. <i>et al.</i> (2019) <i>ACS Catal.</i> <b>10</b>(3), 1923–1937.