Single metal atoms, stabilized by oxides, are attracting growing interests because of their intrinsic high metal utilization and unique catalytic behaviors. Understanding better the hydrogenation functionality of such kind of catalysts will provide a valuable tool for engineering active catalytic sites. Herein, using CO2 hydrogenation as the research theme, we hypothesized that controlling the environment and nuclearity of Rh would allow controlling catalytic pathways through manipulating C-O cleavage and hydrogenation rates. Thus, a series of Rh catalysts with varying loadings were prepared on Fe3O4 and SiO2 following strategies based on electrostatic interactions and on cationic replacement driven by reduction potentials. According to EXAFS and HAAD-STEM, single Rh atoms are stabilized on Fe3O4 in an Rh-O-Fe environment at low Rh loadings, whereas Rh clusters form at high loadings (in the 0.05-0.5 wt. % Rh range). Particle size and morphology vary with the synthesis method and support. Single Rh atoms on Fe3O4 have an intrinsic activity one- to two- orders of magnitude higher than Rh particles/clusters on Fe3O4 and SiO2 for CO2 hydrogenation. The presence of Rh-O-Fe bonds seems mandatory to achieve such extraordinary increase in rates. Kinetic experiments performed at atmospheric pressure show that the product distribution shifts from CO/CH4 mixtures to CO with nuclearity decreasing from particles to single atoms. Pressures higher than 5 atm favor C-C coupling. Remarkably, single atom catalysts lead to olefins without the generation of the corresponding alkanes. Thus, the ability of Rh to hydrogenate adsorbed CO2 or CO (hypothesized as the key intermediate) decreases when decreasing its metal character. However, the ionic environment of the Rh-O-Fe sites gives them a strong affinity with CO2, which enhances the reduction rates. Quantification of this fundamental understanding will enable precise predictions of catalytic performance in function of rational design.

From single atoms to clusters, manipulating CO2 reduction pathways on Rh catalysts

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.