Synthesis of Atomically Dispersed Rh Catalysts on Oxide Supports via Strong Electrostatic Adsorption and Characterization by Cryogenic Infrared Spectroscopy

Oxide-supported Rh catalysts are utilized in applications ranging from NOx reduction reactions in automotive catalysis to hydrogenation reactions in hydrocarbon upgrading. Interest in synthesizing catalysts in which Rh exists as atomically dispersed species on oxide supports rather than Rh nanoparticles derives from the potential to promote the efficient use of this rare element and uncover alternative catalytic reactivity. Synthesizing atomically dispersed Rh catalysts is challenging because the metal species often thermodynamically prefer to form clusters. Characterizing the structure of oxide-supported Rh species can also be challenging because Rh is known to be mobile on oxide supports, leading to sintering or redispersion during characterization. Here, we establish the influence of pH, Rh precursor speciation, and support composition on the synthesis of atomically dispersed Rh species on commercially relevant oxide supports (CeO2 and gamma-Al2O3) via strong electrostatic adsorption. CO probe molecule infrared spectroscopy (CO-FTIR) is used to distinguish atomically dispersed Rh species from Rh clusters, and focus is placed on measurements at cryogenic temperature that prevent Rh mobility. The structural analysis of Rh species by CO-FTIR is substantiated by aberration-corrected scanning transmission electron microscopy imaging. It is observed that synthesis conditions that are optimum for strong electrostatic adsorption of Rh precursors onto the oxide supports are also optimum for producing atomically dispersed Rh species. The synthesis and characterization approach presented here provides a methodology that could facilitate the development of structure-function relationships for catalytic reactions.