Identifying the Chemical State of Cerium Hosted by Various CeO2 Facets as Surface Fingerprint and Its Facet-dependent Phosphatase-mimetic Activity
DescriptionAltering the exposed facet of a nanocrystallite and hence the control of surface chemistry at the nano level has been shown to significantly change their catalytic performances. This is particularly true for CeO2-based catalysts with versatile acid-base chemistry and quick switchable Ce oxidation states for oxygen storage, making its applications wider than other transition/lanthanide metal oxides. However, the labile oxygen and corresponding redox of surface cerium at elevated temperature (especially > 180°C) could subsequently change the surface acid-base property, making it extremely difficult for mechanistic study to identify the true catalytically active site. Without a genuine surface tool, there had been different interpretations and even disagreements among researchers in the past decades on CeO2facet activity. Surface tools such as Raman and X-ray photoelectron spectroscopy (XPS) collect data from particle subsurface to bulk except for the top-most surface of a specific facet; while probe-assisted infrared (IR) and temperature-programed desorption (TPD) for the characterization of top-most surface are often unable to distinguish between different Ce cations hosted by various CeO2facets, leading to low resolved spectra with averaged signals. Generally, conventional surface tools are not sensitive enough to detect the change in physiochemical properties on a specific facet and thus severely hinder the study of facet-dependent activities. Probe-assisted nuclear magnetic resonance (NMR) has demonstrated its ability for differentiating various surface micro-environments by corresponding chemical shift values of a given chemical probe. We demonstrated its capability in differentiating metal cations on various ZnO and TiO2facets, thus have enabled us to identify the true active site in facet-dependent reactions. Here, we propose to unambiguously investigate the importance of facet-dependent activity on CeO2morphologies by probe-assisted NMR. We will establish CeO2facet fingerprint with careful attention given to the possible interference from its redox property. Using the as-built up CeO2facet fingerprint, the terminal facets of any given CeO2catalyst can be indexed by the corresponding distribution of Ce chemical state. More importantly, quantitative information such as the percentage of terminal facets and cerium concentration on each of them can be easily obtained. As a potential replacement of phosphatase for dephosphorylation reaction, the role of CeO2facets in the Lewis acid (LA) activation of biomolecules will then be investigated. We believe this exciting approach will not only elucidate the fundamentals of a given CeO2-based reaction, it will also be beneficial for future design of CeO2-based materials with high catalytic activity.
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