Abstract
The present work describes the effects of water on Fe-doped nanoparticulate CeO2, produced by flame spray pyrolysis, which is a critical environmental issue because CeO2 is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO2 initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO2·2H2O and ~30 mol % gelled Ce(OH)4. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)4 gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce4+-H2O, Ce3+-H2O, Fe3+-H2O, and Fe2+-H2O systems; and (c) the Pourbaix diagrams for the Ce-H2O and Fe-H2O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO2 component of the hydrated phase CeO2·2H2O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO2 upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present.
Highlights
Ceria (CeO2 ) is well known as a photocatalyst and Fe doping of CeO2 improves its photocatalytic performance [1,2,3,4]
The undoped CeO2 and Fe-doped CeO2 fabricated by flame spray pyrolysis [8] were examined for water absorption by weighing hydrated and desiccated nanopowders using a digital balance with 0.0001 g precision
Hydration was performed by placing ~0.15 g of nanopowder in a plastic bag containing a non-contacting water-saturated sponge and sealing the bag, which subjected the nanopowders to 100% relative humidity (RH) for 48 h
Summary
Ceria (CeO2 ) is well known as a photocatalyst and Fe doping of CeO2 improves its photocatalytic performance [1,2,3,4]. It is less well known that, in common with many rare earth oxides [5,6], CeO2 is hygroscopic and water can be absorbed upon exposure to atmospheric water vapor. A second potential outcome of this water absorption is the conversion from the oxide to a hydrated and/or hydroxylated phase at the surface [7], upon exposure to liquid water. Despite the associated alteration of the CeO2 bulk and surface, where photocatalysis takes place, the presence of a potentially coherent hydrated and/or hydroxylated phase on the surface allows the maintenance of the photocatalytic activity. The present work, which follows previous work on the fabrication, analyses, and photocatalytic performance of Fe-doped CeO2 nanopowders [8], considers the nature of this surface layer when CeO2 is immersed in liquid water and the soluble species associated with it
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