(41f) Adsorption of Transition Metal Precursors on Reducible Metal Oxide Supports: Toward Rational Synthesis of Single-Site Catalysts

Authors: 
Mukhopadhyay, A., Pennsylvania State University
Rioux, R. M., Pennsylvania State University
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 oxide supports. However, fundamental molecular level of metal–ceria interface interactions lack substantial information on the energetics of binding and their influence on particle size and growth.

In this work, we study the adsorption of Pt and Pd based precursors, chloroplatinic acid, tetraammineplatinum (II) nitrate and ammonium tetrachloropalladate (II) on ceria and alumina under strong electrostatic solution conditions and by incipient wetness impregnation. Equilibrium adsorption isotherms aid in quantifying the amount of metal adsorbed on the support surface and by varying choice and weight loading of the precursors, we could identify that chloride ligand speciation chemistry around main metal center strongly influenced metal uptake. The binding energy of adsorption of metal precursors on the ceria surface is determined through the use of isothermal titration calorimetry (ITC) to experimentally evaluate the equilibrium binding constants, binding stoichiometry, and enthalpies of adsorption. Experimentally measured free energy of adsorption is found to be higher than theoretical columbic free energy under strong electrostatic conditions, thus substantiating the importance of solvation and ligand chemistry effects. The formation of atomically dispersed metal nanoparticles through formation of an M-O-Ce bond under strong oxidative conditions is examined using XPS. Support of high dispersion of ceria is provided via TEM imaging that shows Pt nanoparticles on ceria are under 2 nm, even at high metal weight loadings whereas they sinter to form clusters up to 10 nm in size on alumina. Based on these observations, we synthesized faceted nano-crystals of cerium oxide and subsequently deposited Pt precursors over a range of metal weight loading to optimize the support vacancy usage to form single atom catalysts. These catalysts are tested on methane oxidation reaction to study the effect of the transition from single atoms to nano-clusters on the activity of the reaction.