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

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 Pd x Ce 1– x O δ mixed oxides that yield methane oxidation rates unobtainable by the separate systems. Here we demonstrate that rapid C–H activation on Pd x Ce 1– x O δ catalysts can be attributed to emergent behavior of the doped oxide enabling Pd 4+ ↔ Pd 2+ transitions not evident in catalysts featuring a PdO x active phase. Pd x Ce 1– x O δ surfaces activate methane through hydrogen abstraction over Pd 4+ surface states, in contrast to the σ-complex activation route favored over PdO x surfaces. The stability of the active Pd 4+ 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 Pd 4+ state, demonstrating that active Pd 4+ sites are metastable. These states form under the reaction environment and offer lower methane activation barriers in comparison to Pd 2+ states. The Pd 4+ state is stabilized by the incorporation of Pd in the fluorite lattice structure of CeO 2, which in turn provides unique methane activation chemistry from the Pd x Ce x –1 O δ 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.