The
oxidized palladium nanoparticles comprising Pd4+ species
were prepared by radio frequency (RF) discharge in an O2 atmosphere and analyzed with X-ray photoelectron spectroscopy.
PdO
x
particles were deposited on CeO2 or the reference support (Ta2O5) with
variation of the RF sputtering time. Regardless of the used support,
small PdO
x
particles (d < 1 nm) contained only Pd2+ species, while an increase
of the particle size led to the appearance of the additional oxidized
Pd statePd4+. The stabilization of Pd4+ on the surface of defect PdO particles was proposed. The Pd4+ species in the PdO
x
/CeO2 system was stable during heating in ultra-high vacuum conditions
up to 250 °C. Pd4+ species demonstrated a high reaction
probability toward CO oxidation at room temperature. However, a transition
from the relatively inert support (Ta2O5) to
the reducible oxide (CeO2) did not lead to a significant
improvement of the Pd4+ reaction probability. Pd4+ species could not be recovered by the exposure of the reduced systems
to molecular oxygen at room temperature. The obtained results bring
new insights into consideration of Pd4+ species as active
sites for oxidation processes at low temperatures.
The nano oxides (TeO)x(Mo,V,Nb)5O14 with so-called M1 structure are highly attractive objects due to unsurpassed catalytic properties. In situ X-ray powder diffraction method with high time resolution (0.4 seconds per diffraction pattern) was applied to study the structure stability and the mechanism of phase transformation under heating in the reductive conditions. It was shown that M1 structure gradually transforms into the rutile-type (Mo,V,Nb)O2 phase without intermediate phases and/or amorphisation at temperatures >480°C in a reductive media. The process is initiated by tellurium reduction and its sublimation to the gas phase.
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