Tuning ceria catalysts in aqueous media at the nanoscale: how do surface charge and surface defects determine peroxidase- and haloperoxidase-like reactivity.

Designing the shape and size of catalyst particles, and their interfacial charge, at the nanometer scale can radically change their performance. We demonstrate this with ceria nanoparticles. In aqueous media, nanoceria is a functional mimic of haloperoxidases, a group of enzymes that oxidize organic substrates, or of peroxidases that can degrade reactive oxygen species (ROS) such as H 2 O 2 by oxidizing an organic substrate. We show that the chemical activity of CeO 2- x nanoparticles in haloperoxidase- and peroxidase-like reactions scales with their active surface area, their surface charge, given by the ζ-potential, and their surface defects ( via the Ce 3+ /Ce 4+ ratio). Haloperoxidase-like reactions are controlled through the ζ-potential as they involve the adsorption of charged halide anions to the CeO 2 surface, whereas peroxidase-like reactions without charged substrates are controlled through the specific surface area S BET . Mesoporous CeO 2- x particles, with large surface areas, were prepared via template-free hydrothermal reactions and characterized by small-angle X-ray scattering. Surface area, ζ-potential and the Ce 3+ /Ce 4+ ratio are controlled in a simple and predictable manner by the synthesis time of the hydrothermal reaction as demonstrated by X-ray photoelectron spectroscopy, sorption and ζ-potential measurements. The surface area increased with synthesis time, whilst the Ce 3+ /Ce 4+ ratio scales inversely with decreasing ζ-potential. In this way the catalytic activity of mesoporous CeO 2- x particles could be tailored selectively for haloperoxidase- and peroxidase-like reactions. The ease of tuning the surface properties of mesoporous CeO 2x particles by varying the synthesis time makes the synthesis a powerful general tool for the preparation of nanocatalysts according to individual needs.