4 research outputs found
Unusual Redox Behavior of Rh/AlPO<sub>4</sub> and Its Impact on Three-Way Catalysis
The
influence of the redox behavior of Rh/AlPO<sub>4</sub> on automotive
three-way catalysis (TWC) was studied to correlate catalytic activity
with thermal stability and metal–support interactions. Compared
with a reference Rh/Al<sub>2</sub>O<sub>3</sub> catalyst, Rh/AlPO<sub>4</sub> exhibited a much higher stability against thermal aging under
an oxidizing atmosphere; further deactivation was induced by a high-temperature
reduction treatment. In situ X-ray absorption fine structure experiments
revealed a higher reducibility of Rh oxide (RhO<sub><i>x</i></sub>) to Rh, and the metal showed a higher tolerance to reoxidation
when supported on AlPO<sub>4</sub> compared with Al<sub>2</sub>O<sub>3</sub>. This unusual redox behavior is associated with an Rh–O–P
interfacial linkage, which is preserved under oxidizing and reducing
atmospheres. Another effect of the Rh–O–P interfacial
linkage was observed for the metallic Rh with an electron-deficient
character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed
CO or NO, which is a possible reason for the deactivation by high-temperature
reduction treatments. On the other hand, surface acid sites on AlPO<sub>4</sub> promoted oxidative adsorption of C<sub>3</sub>H<sub>6</sub> as aldehyde, which showed a higher reactivity toward O<sub>2</sub>, as well as NO, compared with carboxylate adsorbed on Al<sub>2</sub>O<sub>3</sub>. A precise control of the acid–base character
of the metal phosphate supports is therefore a key to enhance the
catalytic performance of supported Rh catalysts for TWC applications
Rhodium Nanoparticle Anchoring on AlPO<sub>4</sub> for Efficient Catalyst Sintering Suppression
Rhodium
catalysts exhibited higher dispersion with tridymite-type
AlPO<sub>4</sub> supports than with Al<sub>2</sub>O<sub>3</sub> during
thermal aging at 900 °C under an oxidizing atmosphere. The local
structural analysis via X-ray photoelectron spectroscopy, transmission
electron microscopy, X-ray absorption fine structure, and infrared
spectroscopy suggested that the sintering of AlPO<sub>4</sub>-supported
Rh nanoparticles was significantly suppressed because of anchoring
via a Rh–O–P linkage at the interface between the metal
and support. Most of the AlPO<sub>4</sub> surface was terminated by
phosphate P–OH groups, which were converted into a Rh–O–P
linkage when Rh oxide (RhO<sub><i>x</i></sub>) was loaded.
This interaction enables the thin planar RhO<sub><i>x</i></sub> nanoparticles to establish close and stable contact with the
AlPO<sub>4</sub> surface. It differs from Rh–O–Al bonding
in the oxide-supported catalyst Rh/Al<sub>2</sub>O<sub>3</sub>, which
causes undesired solid reactions that yield deactivated phases. The
Rh–O–P interfacial linkage was preserved under oxidizing
and reducing atmospheres, which contrasts with conventional metal
oxide supports that only present the anchoring effect under an oxidizing
atmosphere. These experimental results agree with a density functional
theory optimized coherent interface RhO<sub><i>x</i></sub>/AlPO<sub>4</sub> model
Tuning the Electron Density of Rh Supported on Metal Phosphates for Three-Way Catalysis
The
automotive three-way catalysis (TWC) performance of Rh supported
on alkaline-earth and rare-earth phosphates was studied in comparison
to that of Rh on aluminum phosphate (AlPO<sub>4</sub>). The anchoring
of Rh via interfacial Rh–O–P bonding in Rh/AlPO<sub>4</sub> leads to efficient Rh sintering suppression. However, the
electron-withdrawing nature of the phosphate affords electron-deficient
Rh, which has a negative impact on its catalytic activity under a
reducing atmosphere due to a decrease in back-donation from the Rh <i>d</i>-orbitals to the antibonding π* orbitals of adsorbed
CO and NO molecules. Notably, the extent of this electron deficiency
could successfully be reduced by replacing AlPO<sub>4</sub> with alkaline-earth
or rare-earth phosphates, and the Rh oxide formed on these phosphate
supports was readily reduced to metallic Rh. This behavior is in complete
contrast to that of corresponding metal oxide supports, because the
higher basicity of these supports yields Rh oxides that are more difficult
to reduce. Among the phosphate-supported catalysts investigated in
the present study, Rh/LaPO<sub>4</sub> demonstrated the highest TWC
performance after thermal aging under both oxidizing and reducing
atmospheres. The effect of the higher basicity of LaPO<sub>4</sub> compared to that of AlPO<sub>4</sub> is most obvious in its improved
catalytic activity for elementary CO–O<sub>2</sub>, CO–H<sub>2</sub>O, and CO–NO reactions. Importantly, this improvement
is achieved while maintaining the activity toward C<sub>3</sub>H<sub>6</sub> as an advanced feature of phosphate supports
Rh/ZrP<sub>2</sub>O<sub>7</sub> as an Efficient Automotive Catalyst for NO<sub><i>x</i></sub> Reduction under Slightly Lean Conditions
The
three-way catalyst performances of honeycomb-coated Rh catalysts
were studied on several metal phosphate supports (AlPO<sub>4</sub>, YPO<sub>4</sub>, ZrP<sub>2</sub>O<sub>7</sub>, and LaPO<sub>4</sub>) using various simulated exhausts with different air-to-fuel ratios
(<i>A</i>/<i>F</i>). These supports significantly
improved the NO<sub><i>x</i></sub> purification (deNO<sub><i>x</i></sub>) efficiency under slightly lean conditions
(14.6 < <i>A</i>/<i>F</i> ≤ 15.3) as
compared with conventional Rh/ZrO<sub>2</sub> catalysts. The deNO<sub><i>x</i></sub> activity exhibited the following sequence
of increasing the mean electronegativity of the supports: ZrO<sub>2</sub> < YPO<sub>4</sub> < LaPO<sub>4</sub> < AlPO<sub>4</sub> < ZrP<sub>2</sub>O<sub>7</sub>. Although both NO–CO
and NO–C<sub>3</sub>H<sub>6</sub> reactions contributed to
the deNO<sub><i>x</i></sub> process, the latter reaction
was more favored on Rh/ZrP<sub>2</sub>O<sub>7</sub> than on Rh/ZrO<sub>2</sub>. Partially oxidized C<sub>3</sub>H<sub>6</sub> was adsorbed
on Rh/ZrP<sub>2</sub>O<sub>7</sub> in the form of reactive aldehyde
species, in contrast to the less-reactive carboxylate species adsorbed
on Rh/ZrO<sub>2</sub>. Furthermore, Rh oxide was more easily reduced
to the active metallic state on ZrP<sub>2</sub>O<sub>7</sub> compared
with Rh/ZrO<sub>2</sub> when the atmosphere was changed from lean
(<i>A</i>/<i>F</i> > 14.6) to rich (<i>A</i>/<i>F</i> < 14.6). Metallic Rh formed on ZrP<sub>2</sub>O<sub>7</sub> was only slowly oxidized on exposure to excess
O<sub>2</sub>, whereas Rh on ZrO<sub>2</sub> was readily oxidized
to less-active
Rh<sub>2</sub>O<sub>3</sub>. The high activity of Rh/ZrP<sub>2</sub>O<sub>7</sub> toward C<sub>3</sub>H<sub>6</sub> oxidation via aldehyde
species as well as the resistance of metallic Rh against reoxidation
are considered to be possible reasons for the enhanced deNO<sub><i>x</i></sub> performance of this catalyst in a slightly lean
region