Doped CeO2 as a Probe of Active Site Speciation for Organophosphate Degradation

Organophosphate degradation is an important reaction for decomposing environmentally harmful compounds and recovering phosphate ions in biological molecules.1,2 CeO2 nanocrystals have been well-studied for dephosphorylation via hydrolysis due to their stable Ce3+ and Ce4+ oxidation states. However, there is ambiguity regarding which defect properties of the CeO2 surface (Ce3+/Ce4+ oxidation state, oxygen vacancies, faceting, and dopants) are responsible for efficient hydrolysis.1,3,4 Trivalent (M3+) dopants have been used as a tool to control the defects, such as the concentration of Ce3+ and oxygen vacancies, and hence the hydrolytic activity of CeO2.5 Herein, we used trivalent transition metals and rare earth metals (M = Y3+, Cr3+, In3+, and Gd3+) to tune the active sites on the CeO2 nanocrystal surface and observed the impacts of the respective metal dopant on the cerium oxide active sites for organophosphate hydrolysis. We synthesized M-doped CeO2 via hydrothermal synthesis to yield colloidally dispersed nanoparticles that we annealed to prime the nanocrystal surface for catalysis. We characterized their properties by powder X-ray Diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Finally, we evaluated the catalytic ability using organophosphate dimethyl-<i>p</i>-nitrophenol phosphate (DMNP) and UV-visible absorption spectroscopy to monitor the degradation to p-nitrophenol over time. While we observed successful doping of CeO2 by all of the dopants, their characteristics have revealed that M3+ dopants can both positively and negatively impact CeO2 in our system. We propose that the CeO2 is extremely sensitive to dopants that cause excessive lattice strain, Ce3+ ions, and oxygen vacancy defects. Thus, CeO2 requires a delicate balance where the active sites are an ensemble of defects in ideal concentration and orientation on the nanocrystal surface to reach high catalytic efficiency.<br/><br/>References:<br/>(1) Tan, Z.; Wu, T.-S.; Soo, Y.-L.; Peng, Y.-K. Unravelling the True Active Site for CeO2-Catalyzed Dephosphorylation. Applied Catalysis B: Environmental 2020, 264, 118508. https://doi.org/10.1016/j.apcatb.2019.118508.<br/>(2) Manto, M. J.; Xie, P.; Wang, C. Catalytic Dephosphorylation Using Ceria Nanocrystals. ACS Catal. 2017, 7 (3), 1931–1938. https://doi.org/10.1021/acscatal.6b03472.<br/>(3) Wang, Y.; Wang, F.; Song, Q.; Xin, Q.; Xu, S.; Xu, J. Heterogeneous Ceria Catalyst with Water-Tolerant Lewis Acidic Sites for One-Pot Synthesis of 1,3-Diols via Prins Condensation and Hydrolysis Reactions. J. Am. Chem. Soc. 2013, 135 (4), 1506–1515. https://doi.org/10.1021/ja310498c.<br/>(4) Vernekar, A. A.; Das, T.; Mugesh, G. Vacancy-Engineered Nanoceria: Enzyme Mimetic Hotspots for the Degradation of Nerve Agents. Angewandte Chemie International Edition 2016, 55 (4), 1412–1416. https://doi.org/10.1002/anie.201510355.<br/>(5) Zhang, Z.; Wang, Y.; Lu, J.; Zhang, J.; Li, M.; Liu, X.; Wang, F. Pr-Doped CeO2 Catalyst in the Prins Condensation–Hydrolysis Reaction: Are All of the Defect Sites Catalytically Active? ACS Catal. 2018, 8 (4), 2635–2644. https://doi.org/10.1021/acscatal.7b04500.