Role of Site Stability in Methane Activation on PdxCe1鈥搙O未 Surfaces

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• 作者：Thomas P. Senftle ; Adri C. T. van Duin ; Michael J. Janik
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：October 2, 2015
• 年：2015
• 卷：5
• 期：10
• 页码：6187-6199
• 全文大小：809K
• ISSN：2155-5435

文摘

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_xCe_1鈥?i>xO_未 mixed oxides that yield methane oxidation rates unobtainable by the separate systems. Here we demonstrate that rapid C鈥揌 activation on Pd_xCe_1鈥?i>xO_未 catalysts can be attributed to emergent behavior of the doped oxide enabling Pd⁴⁺ 鈫?Pd²⁺ transitions not evident in catalysts featuring a PdO_x active phase. Pd_xCe_1鈥?i>xO_未 surfaces activate methane through hydrogen abstraction over Pd⁴⁺ surface states, in contrast to the 蟽-complex activation route favored over PdO_x 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 Pd_xCe_x鈥?O_未 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.