The particle size effect of Pd nanoparticles supported on alumina with various crystalline phases on methane combustion was investigated. Pd/θ, α-Al O with weak metal-support interaction showed a volcano-shaped dependence of the catalytic activity on the size of Pd particles, and the catalytic activity of the strongly interacted Pd/γ-Al O increased with the particle size. Based on a structural analysis of Pd nanoparticles using CO adsorption IR spectroscopy and spherical aberration-corrected scanning/transmission electron microscopy, the dependence of catalytic activity on Pd particle size and the alumina crystalline phase was due to the fraction of step sites on Pd particle surface. The difference in fraction of the step site is derived from the particle shape, which varies not only with Pd particle size but also with the strength of metal-support interaction. Therefore, this interaction perturbs the particle size effect of Pd/Al O for methane combustion.
The
redox properties of supported Pd catalysts can directly affect
their methane combustion activity. Here, the effect of the support
on methane combustion was elucidated using Pd nanoparticles supported
on various metal oxides (θ-Al2O3, γ-Al2O3, ZrO2, CeO2, MgO, La2O3, TiO2, SnO2, and Nb2O5). To eliminate the effect of Pd particle size
and morphology, uniform Pd particles were synthesized in the liquid
phase. Interestingly, the methane combustion activity formed a volcano
plot when plotted against the oxide formation enthalpy (Δf
H
M‑O°) of the support.
Combining X-ray photoelectron spectroscopy and scanning transmission
electron microscopy–electron energy loss spectroscopy, the
structure of the Pd nanoparticles on the support in an oxidizing atmosphere
was identified. Pd particles on θ-Al2O3, γ-Al2O3, ZrO2, and CeO2 with moderate Δf
H
M‑O° adopted a metal-core–oxide-shell structure and showed
the highest activities for methane combustion. In contrast, completely
oxidized Pd particles were present on MgO and La2O3, which have lower Δf
H
M‑O° and were not very active. Pd metal particles
on TiO2, SnO2, and Nb2O5 with higher Δf
H
M‑O° were not effective catalysts for methane combustion.
What prompted you to investigate this topic/problem? Supported Pd catalystsa re used for combustion of methane exhausted by natural gas vehicles. However,f urtheri mprovement of Pd catalystsi snecessary not only to lower combustion temperature but also to reduce the amount of preciousP d metal required. With the aim of designing ac atalyst with enhanced combustion activity,w ea nalyzed the key dynamics of PdO-catalyzed methane combustion.
What inspired you for the cover image?Dispersive X-ray absorption fine structure (DXAFS) spectroscopy allows us to observe the dynamics of local structure during chemicalr eactions in millisecond time resolution. In the image, white X-rays are dispersed by ap rism (a bentc rystal polychromator). The dispersed X-raysi rradiate aP dO nanoparticle, while methane reacts with it to form CO 2 and Pd metal species. The DXAFS analysisr eveals reduction kinetics of PdO nanoparticles by methane to yield highly active Pd species for methaneo xidation.
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