By carefully mixing Pd metal nanoparticles with CeO polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd-O-Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO and creates highly reactive and stable sites constituted by PdO embedded into CeO surface layers. This specific arrangement outperforms conventional Pd/CeO reference catalysts in methane oxidation by lowering light-off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.
Bimetallic Pt–Pd
catalysts supported on ceria have been
prepared by mechanochemical synthesis and tested for lean methane
oxidation in dry and wet atmosphere. Results show that the addition
of platinum has a negative effect on transient light-off activity,
but for Pd/Pt molar ratios between 1:1 and 8:1 an improvement during
time-on-stream experiments in wet conditions is observed. The bimetallic
samples undergo a complex restructuring during operation, starting
from the alloying of Pt and Pd and resulting in the formation of unprecedented
“mushroom-like” structures consisting of PdO bases with
Pt heads as revealed by high-resolution transmission electron microscopy
(HRTEM) analysis. On milled samples, these structures are well-defined
and observed at the interface between palladium and ceria, whereas
those on the impregnated catalyst appear less ordered and are located
randomly on the surface of ceria and of large PdPt clusters. The milled
catalyst prepared by first milling Pd metal and ceria followed by
the addition of Pt shows better performances compared to a conventional
impregnated sample and also to a sample obtained by inverting the
Pd–Pt milling order. This has been ascribed to the intimate
contact between Pd and CeO2 generated at the nanoscale
during the milling process.
By carefully mixing Pd metal nanoparticles with CeO 2 polycrystalline powder under dry conditions,a nu npredicted arrangement of the Pd-O-Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering ac ore of CeO 2 particles.T he robust contact that is generated at the nanoscale,a long with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO 2 and creates highly reactive and stable sites constituted by PdO x embedded into CeO 2 surface layers.T his specific arrangement outperforms conventional Pd/CeO 2 reference catalysts in methane oxidation by lowering light-off temperature by more than 508 8Ca nd boosting the reaction rate.The origin of the outstanding activity is traced to the structural properties of the interface,modified at the nanoscale by mechanochemical interaction.
Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.
The increasing diffusion of alternative
mobility solutions, ranging
from electric technologies to natural gas fueled vehicles (NGVs),
has led to a progressive life-cycle analysis approach of their environmental
impact in terms of greenhouse gases (GHGs) emissions. This new approach
prompted a careful design of the NGVs catalytic aftertreatment system
in order to minimize the catalytic converter carbon footprint as well
as the unburned methane emissions at tailpipe. Here, a series of Pd/CeO2 methane oxidation catalysts were prepared by an environmentally
friendly solvent-free method and compared to the commercial wet-synthesized
state-of-the-art catalysts. Their application in NGVs aftertreatment
systems was evaluated by testing powder catalysts and coated monolith
cores for CH4 oxidation and steam reforming, which are
the main methane abatement reactions occurring in a three-way catalyst
(TWC) under lean and rich conditions, respectively. Pd/CeO2 catalysts prepared by mechanochemical synthesis initially displayed
superior activity compared to their counterpart obtained by conventional
wet impregnation, especially under lean oxidation conditions, but
appeared less resistant to the industrial aging process after core
washcoating. Lambda sweep experiments carried out under full gas composition
proved that, despite needing further optimization in the washcoating
and aging processes, the developed mild milling synthesis procedure
is a viable way for the production of Pd/CeO2 based catalysts
for natural gas TWCs.
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