(598g) Single Atom Catalyst for Oxidation - Understanding the Fundamentals of Synthesis and Reactivity

Understanding synthesis of single atom catalysts (SACs) and the role of underlying metal-support interaction towards reactivity is crucial in tailoring active site chemistry towards desired reactant transformations. Our study combines density functional theory (DFT) calculations and solution phase isothermal titration calorimetry (ITC) adsorption experiments to examine charged and solvated precursor adsorption onto ceria nano-cubes during strong electrostatic adsorption (SEA) synthesis of SACs. DFT calculations bridge from an isolated gas phase metal atom and a single crystal plane of oxide surface to a ligated metal atom and bulk oxide surface (with a mixture of different crystal planes) in solution phase. We demonstrate agreement between DFT and ITC adsorption energy trends, where DFT models the adsorption through partial deligation of the precursors, examining how the ligand chemical potential can alter adsorption thermodynamics.

We used CO oxidation over the synthesized single metal atoms of Ni, Ir, Pd and Pt supported on ceria as a probe for understanding the active site chemistry and the resultant reactivity. DFT calculations in conjugation with experiments examined the variation of catalytic performance across the metals and identified the synergy between individual metal atoms and the support towards CO oxidation. First principles microkinetic modeling (MKM), together with a Bayesian Inference based approach, was used to identify plausible elementary reaction networks for CO oxidation and the dynamics of the active sites. Bayesian inference was used to reconcile elementary mechanisms and energetics to experimentally observed activation barriers and reaction orders. Theory and experiment together provide a conclusive evidence that single Pd atoms supported on CeO₂ nanocubes experience 4 different formal oxidation states during catalytic CO oxidation under the conditions tested experimentally. Together we gather evidence for correlation between active single atom states and the resulting variation in kinetics across the single atoms and those previously reported over extended surfaces/nanoclusters.