(689c) Periodic Trends in Adsorption Energies of Transition Metal Precursors on Reducible Cerium Oxide: Towards Rational Synthesis of Single-Site Catalysts
In this work, we study the adsorption of five ligated transition metal precursors of Ag, Pd, Pt, Rh and Ir under strong electrostatic solution conditions on shape controlled, faceted ceria. The choice of the five transition metals are based on being able to cover the wide range of metals having a weak affinity for oxygen (Ag) to those with a significantly stronger oxygen affinity (Ir). Isothermal titration calorimetry (ITC) is used to determine adsorption enthalpies for binding of the complexes on the support at the interface inclusive of solution and ligand influences. The measured âH values from ITC corroborate the predicted trends from DFT calculations but differ by an order of magnitude because it take into account the effects of hydration sheath interactions, ligand chemistry, ligated complex charge and steric interferences of the precursors as well as the solvation properties of the oxide support.
Based on the knowledge gained from ITC regarding the intrinsic adsorption interactions between ligated and solvated transition metal precursors and solvated CeO2 nanocubes, very low weight loadings of Pd/CeO2 are synthesized by strong electrostatic adsorption to generate active isolated sites on the support where the local coordination environment of the metal can be controlled. Their catalytic performance is examined for lean methane combustion at low temperatures. The presence of isolated Pd atoms embedded in the ceria lattice or at a step edge of the ceria support is proposed by previous DFT calculations to demonstrate a paradigm shift in the catalytic cycle â a Pd2+/Pd4+ redox cycle rather than a Pd0/Pd2+ redox cycle with enhanced water tolerance instead of the usual inhibitory effect on Pd nanoparticles. Our experimental studies on a series of single atom, reduced Pd atom catalysts seek to confirm these theoretical predictions.