The thermal deactivation
of engine-aged Pd/CeO2–ZrO2 three-way
catalysts was studied by chassis-dynamometer driving
test cycles with cold start and in situ diffuse reflectance
spectroscopy (DRS). The extent of the catalyst deactivation after
engine-aging at 800–1000 °C was correlated with the microstructural
evolution, which was analyzed by X-ray diffraction, X-ray absorption
spectroscopy, electron microscopy, and a chemisorption technique.
This suggests that deactivation is caused by degradation of the catalytically
active sites in the three-phase boundary (TPB) region, where Pd, CeO2–ZrO2, and the gas phase meet. The time-resolved in situ DRS revealed that the reoxidation of Pd metal under
fluctuating air-to-fuel ratios was retarded relative to the reduction
of Pd oxide. The retardation is attributable to the oxygen storage
in CeO2–ZrO2. In the fresh catalyst with
a high dispersion, most Pd was close to the TPB. Conversely, after
engine-aging at elevated temperatures, the retardation effect was
less pronounced with respect to Pd particle growth. Grown into large
Pd particles, the Pd at sufficient distances from the TPB was no longer
affected by the oxygen storage. Consequently, from the ratios of the
initial rate constants of the Pd oxidation and reduction under fluctuating
air-to-fuel ratio conditions, we can understand the quality and/or
quantity of the TPB site in engine-aged catalysts. This measure provides
a useful index of the extent of catalyst deactivation.
Aerobic oxidation: in a biomimetic approach, a mixture of redox catalysts forms couples that effect the aerobic oxidation of a mixture of benzylamine and 2-aminophenol derivatives to give the corresponding benzoxazoles. This biomimetic oxidation proceeds smoothly under mild conditions and the protocol can also be used for preparing benzimidazoles and benzothiazoles.
The influence of
high-temperature H2 reduction treatment
on Rh and Pd catalysts supported on Al2O3 was
studied in relation to thermal aging in air. After air-aging at ≥900
°C, the Rh/Al2O3 catalyst was more strongly
deactivated compared with the Pd/Al2O3 catalyst.
As has been widely recognized, the solid-state reactions between Rh
oxide and Al2O3 decreased the active surface
area and stabilized inactive Rh3+ species. The activity
was restored by the postreduction treatment with 20% H2/He at 200 °C, whereas a striking enhancement of activity was
achieved by the reduction at 800–1100 °C, where substantial
deactivation occurred for Pd/Al2O3. A mechanistic
interpretation is proposed based on local structural characterization,
which explains these contrasting thermal behaviors. The high-temperature
reduction treatment produced active and thermostable Rh metal nanoparticles,
which were highly dispersed on Al2O3. The observed
dispersion (as high as ∼20% after reduction at 1000 °C)
is among the highest for supported Rh catalysts reported in the literature.
This is in complete contrast to the rapid sintering of Pd and other
precious metals (Ru and Pt) into large metal agglomerates greater
than 50 nm. Because the thermal behavior observed for Rh/ZrO2 under both oxidizing and reducing atmospheres was similar to that
of Pd/Al2O3, the stability of metal nanoparticles
depended not only on metal species but also on the interactions with
support materials. An important implication of this study is that
Al2O3 is a very efficient support for anchoring
Rh metal nanoparticles via interfacial Rh–O–Al bonding
under strong reducing conditions, in contrast to the well-known incompatibility
with Rh oxide under oxidizing conditions.
Well-defined triflylamide-tethered arene−Ru(Tsdpen) complexes have been developed as highly efficient catalysts for the asymmetric hydrogenation of ketones, in which the suitable carbon chain length of the tether is responsible for the activation of H2 as well as the stereochemical outcome of the reaction. The asymmetric hydrogenation of aromatic ketones with the tethered complex with a C4 side chain gave the corresponding secondary alcohols with 91−98% ee, while the shorter congeners with a C2 or C3 side chain provided unsatisfactory results in terms of reactivity and selectivity.
A series of triflylamide-tethered Cp 0 Rh and Cp 0 Ir complexes, [η 5 :η 1 -(CH 3 ) 4 C 5 (CH 2 ) n NTf]M (M = Rh, Ir, n = 2-4), have been newly prepared and their three-legged pianostool structure has been characterized. The tethered complexes having the (R,R)-MsDPEN ligand have been found to promote asymmetric hydrogenation of acetophenone, which indicates that the introduction of a suitable triflylamide tether unit changes their catalytic function from transfer hydrogenation to hydrogenation.
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