  Understanding the oxidation and reduction mechanisms of catalytically active
transition metal nanoparticles is important to improve their application in a
variety of chemical processes. In nanocatalysis the nanoparticles can undergo
oxidation or reduction in situ, and thus the redox species are not what are
observed before and after reactions. We have used the novel environmental
scanning transmission electron microscope (ESTEM) with 0.1 nm resolution in
systematic studies of complex dynamic oxidation and reduction mechanisms of
copper nanoparticles. The oxidation of copper has previously been reported to
be dependent on its crystallography and its interaction with the substrate. By
following the dynamic oxidation process in situ, in real time, with high-angle
annular dark-field imaging in the ESTEM, we use conditions ideal to track the
oxidation front as it progresses across a copper nanoparticle by following the
changes in the atomic number (z) contrast with time. The oxidation occurs via
the nucleation of the oxide phase (Cu2O) from one area of the nanoparticle
which then progresses unidirectionally across the particle, with the Cu-to-Cu2O
interface having a relationship of Cu{111} parallel to Cu2O{111}. The oxidation
kinetics are related to the temperature and oxygen pressure. When the process
is reversed in hydrogen, the reduction process is observed to be similar to the
oxidation, with the same crystallographic relationship between the two phases.
The dynamic observations provide unique insights into redox mechanisms which
are important to understanding and controlling the oxidation and reduction of
copper-based nanoparticles.
