In
situ time-resolved diffuse reflectance spectroscopy provided
the redox dynamics of Pd nanoparticles supported on an oxygen storage
material CeO2–ZrO2 (CZ) under lean/rich
perturbation conditions. Because the reflectance at 450 nm is sensitive
to the Pd oxidation state but is not affected by the redox of Ce3+/Ce4+ species of CZ, the real-time Pd redox can
be monitored every second during oxygen storage/release in simulated
engine combustion exhaust gas (CO–C3H6–NO–O2) corresponding to gasoline air-to-fuel
ratios of 14.1 (rich) and 15.0 (lean). Although a large amount of
O2 was stored by CZ upon the rich-to-lean switch, the rate
of Pd oxidation during this event was found to be much more moderate
compared to that with a reference catalyst, Pd/Al2O3. Because rapid oxygen uptake by CZ reduces the local O2 partial pressure near the surface, the oxidation of Pd should
be retarded. This can preserve active metallic Pd and thus contribute
to longer retention of high NO reduction efficiency even under the
lean condition. However, the reduction of Pd oxide (PdO) upon reverse
(lean-to-rich) switching occurred at a similar rate irrespective of
the support material. The metallic Pd deposits near the interface
with CZ promote the catalytic activation of reducing gases (CO and
C3H6), resulting in significant oxygen release
from CZ. The temperature dependence of the redox rate demonstrates
that oxidation of metallic Pd to PdO is much slower than reduction
over Pd/CZ, whereas oxidation is faster than reduction over Pd/Al2O3. The preservation of active metallic Pd under
lean/rich perturbation conditions is another key role of the oxygen
storage CZ cocatalyst.
This
study investigated the thermal decomposition behaviors of
platinum oxide (PtO2) nanoparticles deposited on polycrystalline
TiO2 in different crystal phases. The dissociation of PtO2 to metallic platinum in air occurred at 400 °C on anatase
TiO2 (Pt/TiO2-A), but required 650 °C or
higher on rutile TiO2 (Pt/TiO2-R). The higher
thermal stability of PtO2 on rutile TiO2 is
caused by thermodynamic effect and rather than kinetic effect. In
contrast to the thermodynamic prediction, metallic Pt (Pt0) on TiO2-R was reversibly oxidized to PtO2 (Pt4+) at 650 °C. This behavior was attributed to
the coherent interface structure formed by strong interactions between
PtO2 and rutile TiO2, as revealed by combined
extended X-ray adsorption spectroscopy (EXAFS) and density functional
theory (DFT) studies. At the optimized interface structure, between
the (100) planes of α-PtO2 and rutile TiO2, the interface formation energy was −17.04 kJ mol–1 Å–2 versus −9.84 kJ mol–1 Å–2 in the anatase TiO2 model.
The larger interface formation energy provides a stabilizing effect
against PtO2 dissociation. Therefore, the widely used Pt-loaded
rutile TiO2 typifies the interfacial interactions under
an oxidizing atmosphere, which differ from the strong metal–support
interactions prevailing under a reducing atmosphere.
In situ time-resolved diffuse reflectance spectroscopy was first applied to supported Rh catalysts (0.4 wt % Rh/ZrO 2 and Rh/ZrP 2 O 7 ) under dynamic three-way catalysis conditions fluctuating between fuel-lean and fuel-rich gas atmospheres. The optical absorption at 650 nm was found to decrease upon lean-to-rich switching of the gas feed, which led to the reduction of Rh oxide (Rh 3+ ) to metallic Rh (Rh 0 ), followed by a reversible increase upon back switching rich-to-lean. The kinetic analysis suggested that the reduction of Rh 3+ to Rh 0 was faster than the reoxidation over Rh/ ZrP 2 O 7 , whereas the reduction was comparable with or slower than the reoxidation over Rh/ZrO 2 . The activation energy of Rh/ZrP 2 O 7 for the reduction, 13.6 kJ mol −1 , was smaller than that for the oxidation, 48.7 kJ mol −1 , which contrasted with those of Rh/ZrO 2 (21.4 and 34.1 kJ mol −1 , respectively). These results were closely associated with the higher NO reduction activity of Rh/ZrP 2 O 7 than Rh/ZrO 2 under a lean-gas atmosphere because Rh was more active in the metallic state than in the oxide state. Applying fast lean−rich perturbation of the gas feed with 1 s intervals led to an immediate and significant drop of the optical absorption intensity, suggesting that the reduction of Rh substantially penetrated to deeper layers under the surface. This study provided the first in situ evidence for the formation of active metallic Rh species under high-frequency lean−rich oscillations.
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