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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 (ΔfHM-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 ΔfHM-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 ΔfHM-O° and were not very active. Pd metal particles on TiO2, SnO2, and Nb2O5 with higher ΔfHM-O° were not effective catalysts for methane combustion.
Murata et al. (Fri,) studied this question.