(647e) Strong Electrostatic Adsorption and Cryogenic IR Spectroscopy As a General Synthesis and Characterization Approach for Oxide Supported Single Atom Rh Catalysts

Heterogeneous oxide supported Rh catalysts are employed in a range of important applications from NO_x reduction in automotive catalysis to carbonylation and hydrogenation reactions in hydrocarbon upgrading. The ability to synthesize stable Rh catalysts where Rh exists exclusively as atomically dispersed single atoms on the oxide support promote the sustainable usage of this rare element and potentially open unique catalytic pathways. In this work, we will describe how principles of strong electrostatic adsorption and cryogenic temperature CO probe molecule Fourier transform infrared spectroscopy (FTIR) can be combined to synthesize and characterize catalysts containing exclusively single Rh atoms on various oxides. We exploit commercially available oxide supports of ~ 5 nanometer scale (CeO₂, g-Al₂O₃ and TiO₂), low Rh weight loadings (< 0.25 wt%), and utilize pH control during synthesis to generate optimized interactions between charged the oxide support and Rh ionic species in solution to promote the production of atomically dispersed Rh. The resulting catalysts are primarily characterized by cryogenic temperature CO FTIR. CO is an excellent probe molecule for characterizing supported Rh catalysts, as unique CO vibrational fingerprints are associated with adsorption single Rh atoms versus small Rh clusters. However, it is known that CO can induce fragmentation of small Rh clusters to form single atoms with very low kinetic barriers. We will show how correlations between low temperature CO IR and aberration corrected scanning transmission electron microscopy can be used to unequivocally characterize the state of Rh in synthesized catalysts. Further it will be shown that performing the same probe molecule IR experiments at room temperature could lead to incorrect conclusions regarding the state of Rh, as the probe molecule becomes an active player in dictating Rh structure. The influence of pretreatment (oxidative versus reduction at varying temperature) conditions on the produced Rh structure will also be highlighted. Finally, characterization of the chemical properties of the produced atomically dispersed Rh single atoms via CO temperature programmed desorption will be used to demonstrate the strong influence of the oxide support on the chemical reactivity of supported single Rh atoms.