Defect-controlled halogenating properties of lanthanide-doped ceria nanozymes.

Marine organisms combat bacterial colonization by biohalogenation of signaling compounds that interfere with bacterial communication. These reactions are catalyzed by haloperoxidase enzymes, whose activity can be emulated by nanoceria using milli- and micromolar concentrations of Br - and H 2 O 2 . We show that the haloperoxidase-like activity of nanoceria can greatly be enhanced by Ln substitution in Ce 1- x Ln x O 2- x /2 . Non-agglomerated nanosized Ce 1- x Ln x O 2- x /2 (Ln = Pr, Tb, particle size < 10 nm) was prepared mechanochemically from CeCl 3 and Na 2 CO 3 followed by short calcination. Lanthanide metals could be incorporated into the CeO 2 host without solubility limit, as shown for Tb. The distribution of the Ln 3+ defect sites in the CeO 2 host structure was analyzed by electron spin resonance spectroscopy. Ce 3+ and superoxide O 2 - species are present at surface sites. Their formation is promoted by increasing dopant concentration. Ce 1- x Ln x O 2- x /2 was prepared in copious amounts by ball-milling. This energy-saving and residue-free method can be upscaled to industrial scale. The surface defect chemistry of Ce 1- x Ln x O 2- x /2 was unravelled by vibrational spectroscopy. It is associated with the mechanochemical preparation and leads to enhanced catalytic activity. Although Ce 0.9 Pr 0.1 O 1.95 had a lower BET surface area than pure CeO 2 , its catalytic activity, calibrated by oxidative bromination of phenol red, was much higher because the ζ -potential increased from 15 mV (for CeO 2 ) to 30 mV (for Ce 0.9 Pr 0.1 O 1.95 ). This facilitates adsorption of Br - in aqueous conditions and explains the high catalytic activity of the Ln-substituted CeO 2 . Ce 1- x Ln x O 2- x /2 is an effective and "green" nanoparticle haloperoxidase mimic for antifouling applications, as no chemicals other than the ubiquitous Br - and H 2 O 2 (generated in daylight) are required, and only natural metabolites are released into the environment.