Single-atom catalysis is a growing area of research due to the potential to substantially transform the activity and selectivity of a general base metal on support catalytic system. However, the unique properties of single atom transition metal systems in the form of supported adatoms and mixed oxide surfaces, relative to their metal cluster counterparts, have not been widely studied. Here we investigate the reactivity of formic acid on several model Rh catalysts, including single Rh adatoms, Rh incorporated into the support oxide lattice, and metallic Rh clusters on Fe3O4(001) under ultrahigh vacuum conditions. The single Rh adatoms are found to be the most active towards CO2 production per deposited Rh, followed by Rh incorporated into the surface oxide layer and then by Rh nanoclusters. While the reaction of formic acid with Fe3O4(001) primarily results in CO desorption at ~560 K, the addition of small amounts of single adatom Rh from 0.17 Rh per Fe3O4(001) unit cell (u.c.) results in a dramatic shift towards CO2 production starting at ~450 K. The mixed Rh oxide system promotes the simultaneous formation of CO, CO2, and H2O at ~540 K. Rh nanoclusters exhibit reactivity similar to metallic Rh with low temperature CO2 and H2 desorption at ~310 K while still producing substantial amounts of CO2 at 450-500 K not observed on pure Fe3O4(001). These differences demonstrate that different forms of Rh supported on Fe3O4(001) affect the reactivity of the catalytic surface beyond the local stabilized transition metal site. The stabilized adatoms, turn over multiple adsorbates to products demonstrating the importance of spill over from the support oxide. Furthermore, the single Rh adatoms and incorporated Rh oxide exhibit an intermediate activity distinct from pure metallic Rh or the pure Fe3O4(001) substrate, demonstrating the additional degree of reactive tuning provided by single atom catalysts.

Reactivity of formic acid on single atom Rh supported on Fe3O4(001)

ACS Spring 2022

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