Single transition metal atoms supported on oxides can provide tunable and emergent catalytic properties that benefit from complete dispersion of the active metal atom. In this talk, I will discuss our work on two single-atom catalyst systems, late transition metals on CeO2 for CO oxidation catalysis and Ag-anatase TiO2 for selective hydrogenation. We prepared a series of single atom catalysts (SAC) with late transition metals supported on ceria nanocubes. Characterization, experimental kinetic measurements, density functional theory (DFT) calculations, and microkinetic modeling (MKM) using Bayesian inference approaches are combined to determine the elementary catalytic mechanism of catalytic CO oxidation. A combination of experimental kinetics, DRIFTS of CO adsorption, and DFT/MKM are used to illustrate that Pd single atoms uniquely expose multiple oxidation states under CO oxidation conditions. DFT calculations suggest Pd more easily accesses a range of oxidation states across relatively narrow redox conditions relative to Pt and Ni atoms. We construct elementary reaction mechanisms that include supported metal atoms varying among oxidation states, and use MKM with Bayesian inference to reconcile elementary kinetic parameters with experimentally measured activation barriers and reaction orders. For supported Pd atoms, a reduced 8 step mechanism, with Pd oscillating among two formal oxidation states, cannot explain the experimental CO reaction order greater than 1. A 19 step mechanism that allows Pd atoms to access 4+, 2+, 0, and reduced 0 oxidation states is needed to match experimentally observed kinetics. Collectively, these results reveal a unique synergy between Pd and CeO2 nanocubes to facilitate emergent redox properties in which Pd can experience a wider range of oxidation states. The Ag-anatase TiO2 single atom system also demonstrates emergent properties that enable selective hydrogenation catalysis. DFT studies demonstrate that Ag single atoms can activate H2 and spillover H atoms, with hydridic H atoms forming on the reducible oxide that are proposed to enable hydrogenation catalysis. The important joint roles of computation (DFT, microkinetics) and experimental synthesis, characterization, and testing will be highlighted for single atom catalytic systems.

Combined experimental, density functional theory, and microkinetic modeling for mechanism determination on single atom catalysts

ACS Spring 2022

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