Role of Site Stability in Methane Activation on PdxCe1-xOδ Surfaces

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Abstract

Doped metal oxide catalysts can be optimized by identifying dopant metal/host oxide combinations that exhibit synergistic interactions not present in the parent systems. This is exemplified by PdxCe1-xOδ mixed oxides that yield methane oxidation rates unobtainable by the separate systems. Here we demonstrate that rapid C-H activation on PdxCe1-xOδ catalysts can be attributed to emergent behavior of the doped oxide enabling Pd4+ 虠 Pd2+ transitions not evident in catalysts featuring a PdOx active phase. PdxCe1-xOδ surfaces activate methane through hydrogen abstraction over Pd4+ surface states, in contrast to the σ-complex activation route favored over PdOx surfaces. The stability of the active Pd4+ state is dependent on temperature and oxygen pressure during catalytic operation, and as such we combine reaction kinetics and thermodynamic stability arguments from density functional theory (DFT) calculations to derive the apparent methane activation barrier. This accounts for varying conditions affecting the stability of the Pd4+ state, demonstrating that active Pd4+ sites are metastable. These states form under the reaction environment and offer lower methane activation barriers in comparison to Pd2+ states. The Pd4+ state is stabilized by the incorporation of Pd in the fluorite lattice structure of CeO2, which in turn provides unique methane activation chemistry from the PdxCex-1Oδ mixture. We generalize these results over (T,P) space by deriving phase boundaries demarcating regions where each Pd surface oxidation state is thermodynamically stable or kinetically active. The approach presented here can be readily extended to other systems, providing a method for assessing the interplay between site activity and stability on catalytic surfaces.

Original languageEnglish (US)
Pages (from-to)6187-6199
Number of pages13
JournalACS Catalysis
Volume5
Issue number10
DOIs
StatePublished - Oct 2 2015

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)

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