chemrxiv.org VS.

sciencedirect.com

Single atom catalysts have gained significant attention due to their promise of maximum atom utilization and uniform active sites. However, significant knowledge gaps remain regarding how coordination environments evolve under realistic operating conditions and how that affects their ability to participate in chemical and physical processes. This study used infrared spectroscopy to investigate the effect of thermal chemical preprocessing on the 13CO adsorption capacity of 0.034 wt.% Pd/CeO2 to yield monomeric palladium carbonyl complexes. While the (13CO) band frequency of the monomer was only slightly responsive to thermal chemical preprocessing, the monomer density spanned an order-of-magnitude. The breadth of the monomer density suggests that single-atom coordination environments are better described as a distribution that can be systematically manipulated to control chemical processes as opposed to static, uniform active sites. Changes in the Pd coordination ensemble, manifested as changes in the monomer density, represented distinct metastable states that were accessed through specific thermal chemical histories. The dependence of the coordination ensemble on thermal chemical history led to different monomer densities under identical experimental conditions. This observation revealed that thermal chemical preprocessing is non-deterministic and that the Pd coordination ensemble is kinetically controlled rather than thermodynamically determined. This work establishes a methodology that uses molecules as both sensitive diagnostics and synthetic tools to direct material properties. The ability to reversibly control coordination distributions opens new pathways for catalyst design and provides fundamental insights into the dynamic nature of atomically dispersed metals.