  Cerium oxide nanoparticles are known to catalyze the decomposition of
reactive oxygen species such as superoxide radical and hydrogen peroxide.
Herein, we examine the superoxide dismutase (SOD) and catalase (CAT) mimetic
catalytic activities of nanoceria and demonstrate the existence of generic
behaviors. For particles of size 4.5, 7.8, 23 and 28 nm, the SOD and CAT
catalytic activities exhibit the characteristic shape of a Langmuir isotherm as
a function of cerium concentration. Results show that the catalytic effects are
enhanced for smaller particles and for the particles with the largest Ce3+
fraction. The SOD-like activity obtained from the different samples is found to
superimpose on a single master curve using the Ce3+ surface area concentration
as a new variable, indicating the existence of particle independent redox
mechanisms. For the CAT assays the adsorption of H2O2 molecules at the particle
surface modulates the efficacy of the decomposition process and must be taken
into account. We design an amperometry-based experiment to evaluate the H2O2
adsorption at nanoceria surfaces, leading to the renormalization of the
particle specific area. Depending on the particle type the amount of adsorbed
H2O2 molecules varies from 2 to 20 nm-2. The proposed scalings are predictive
and allow determining SOD and CAT catalytic properties of cerium oxide solely
from physico-chemical features.
