Copper(II) oxide nanopowders exhibit
a high catalytic activity
in CO oxidation at low temperatures. The combination of in situ XPS,
XRD, and HRTEM methods was applied to investigate initial steps of
CuO nanoparticles reduction, to identify oxygen and copper species
and to revealed structural features in the dependence on reducing
power of reaction medium. At the oxygen deficient surface of CuO nanopowders
the metastable Cu4O3 oxide was formed under
the mild reducing conditions −10–5 mbar CO
or CO + O2 mixture with oxygen excess. Destruction of Cu4O3 structures in strong reducing medium (P(CO) ≥ 10–2 mbar) or under UHV
conditions resulted in the formation of Cu2O which was
epitaxially bounded with initial CuO particle. The reversible bulk
reduction of CuO nanopowder to Cu2O at temperatures ∼150
°C can be explained by effortless propagation of Cu2O∥CuO epitaxial front inside the nanoparticle. The model of
the surface restructuring along the {−111}CuO → {202}Cu4O3 → {111}Cu2O planes under the
reduction of CuO nanopowders is proposed. The initial surface of CuO
nanopowders is probably distorted and resembles Cu4O3-like structures that facilitates the CuO
x
↔ Cu4O3 transition in mild reducing
conditions. Such restructuring results in a unique electronic Cu4O3 structure with high oxygen deficiency and low-valence
Cu1+ sites stimulating the formation of highly reactive
CO and O2 adsorbed species. It was shown that the most
active oxygen species on the surface of CuO
x
is stabilized as O–, which was previously
reported in papers by Roberts and Madix in their study of the copper–oxygen
systems.
Oxidized palladium nanoparticles, PdO
x
(x ≈ 1.3), measuring approximately
3 nm
in size were prepared by RF-discharge under an oxygen atmosphere.
The Pd3d X-ray photoelectron spectra (XPS) of oxidized palladium nanoparticles
show two main peaks with binding energy E
b(Pd3d5/2) at ∼336.5 and 338.6 eV, which were assigned
to Pd2+ and Pd4+ species, respectively. Attempts
to synthesize pure Pd4+ nanoparticles by plasma treatment
without the presence of Pd2+ were unsuccessful. High-resolution
transmission electron microscopy (HRTEM) data show the defect structure
of the palladium nanoparticles. The particles’ thermal stability
is relatively high, being stable up to ∼425–450 K. The
oxidized palladium species (Pd4+) were found to be highly
reactive toward CO at room temperature. These results demonstrate
the necessity of further investigation of highly oxidized palladium
species as possible active centers in CO oxidation reactions.
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