Density functional theory (DFT) calculations by Mayernik and Janik demonstrated CeO2 surfaces substituted with Pd in Ce lattice positions are more reducible (i.e., more exothermic oxygen vacancy formation) and favor the activation of methane relative to the undoped CeO2(hkl) surfaces1. Subsequently, PdxCe1-xO2(hkl) surfaces possess higher activity for methane oxidation due to decreased methane activation barrier compared to PdO or Pd surfaces2. Substitution of Pd into ceria can provide an isolated Pd site, and the ceria substrate helps facilitate 4+/3+/2+ oxidation state changes of the Pd atom to allow for low activation barriers for methane dissociative adsorption, oxidation and vacancy refilling. In current collaborative work, we aim to examine the reactivity of single Pd atoms in various local environments (Pd as an adatom on a low index surface, occupation at a surface defect or doping into the oxide surface) on well-defined CeO2(hkl) nanostructures. We have synthesized a series of low weight loading (0.005 – 0.05%) Pd/CeO2 catalysts on CeO2 nanostructures – cubes and octahedron. Pd-containing CeO2 catalysts with Pd adatoms and Pd occupying defects were prepared by strong electrostatic adsorption (SEA). CeO2 supports containing ‘doped’ Pd atoms were prepared by solution combustion synthesis. Catalysts were characterized by a battery of physical and chemical techniques to characterize the local environment of the Pd associated with CeO2. High angle annular dark field (HAADF) scanning transmission microscopy, x-ray absorption spectroscopy and infrared absorption spectroscopy of adsorbed CO were employed to describe the local coordination environment of Pd. The influence of Pd incorporation on the reducibility of CeO2 was examined with 16O2/18O2 isotope exchange and electron spin resonance (ESR) spectroscopy. Rigorous evaluation of the reaction mechanism for lean methane oxidation was explored under differential conditions for each of the ‘different’ single-atom type catalysts. In addition to single-atom Pd/CeO2, we included CeO2 supported Pd nanoparticle catalysts for comparison of reaction kinetics and mechanism, including the kinetic influence of water will be examined on both catalyst types.

Methane combustion over Pd/CeO2 catalysts: Mechanistic distinctions between supported single atoms and nanoparticles

2019 Spring National Meeting

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