(689c) Periodic Trends in Adsorption Energies of Transition Metal Precursors on Reducible Cerium Oxide: Towards Rational Synthesis of Single-Site Catalysts

The ability of metal oxide supports to enhance the dispersion of the active metal on their surface and control their morphology and sintering kinetics is fundamentally related to the nature and strength of the metal–support interaction which is determined at the time of adsorption at the solid-liquid interface. Ceria is a reducible oxide support for transition metal catalysts well-known for its high “oxygen storage capacity” that allows it to successfully stabilize noble metals, inhibit sintering and maintain small sized nanoparticles on its surface compared to other refractory oxide supports. However, fundamental molecular level metal–ceria interface interaction studies lack substantial information on the energetics of binding and their influence on particle size and growth. DFT studies of several gas phase single transition metal adatom adsorption on CeO₂ single crystal surfaces have served as predictors of metal stability on support surfaces based on the reducibility of the surface and the oxide formation enthalpy of the metal. However, these predicted trends pose significant experimental limitations because in case of practical catalysts synthesis, an isolated gas phase metal atom and a single crystal plane of oxide surface is replaced by a ligated metal atom in solution phase and bulk oxide surface with a mixture of different crystal planes.

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 ∆H 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 – 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.