Tailoring the Near-Surface Environment of Rh Single-Atom Catalysts for Selective CO₂ Hydrogenation

We used a combination of experimental spectroscopies, density functional theory calculations, and CO2 hydrogenation studies to investigate the effects of modifying single-atom Rh-1/TiO2 catalysts with functionalized phosphonic acid monolayers. We found that the deposition of specific amine-functionalized ligands resulted in an similar to 8x increase in site-specific CO2 reduction turnover frequency at 150 degrees C and a similar to 2x increase at 250 degrees C. On-stream stability also improved following ligand deposition. The effect of the modifier on reactivity was highly sensitive to the proximity of the amine functional group to the surface, which was controlled by adjusting the length of the phosphonic acid tail. Furthermore, deposition of alkyl phosphonic acids without an amine functional group resulted in blocked CO2 adsorption and a near-complete loss of catalytic activity. Infrared spectroscopy studies suggested that the amine group provided binding sites for CO2 that enabled hydrogenation when the amine was positioned near a Rh-1 site. Phosphonic acid-modified catalysts also exhibited high selectivity to CO over the series product methane; the selectivity effect was traced to modification of the Rh-1 sites to favor CO desorption. Phosphonic acid deposition resulted in 80-90% loss of accessible Rh-1 sites, likely due to blocking by tail groups. However, even with the loss of sites, under low-temperature reaction conditions, the rates of CO2 hydrogenation were improved with the coatings, indicating that the remaining sites are highly efficient. Organic functionalization of the supports for atomically dispersed catalysts offers the opportunity to precisely control the positioning of functional groups in the vicinity of a well-defined active site, potentially enabling an additional level of control over active site design.