CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst

The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh₁ adatoms and minority Rh₂ dimers on Fe₃O₄(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh₁ sites results exclusively in stable Rh₁CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh₁(CO)₂ gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh₂ dimers via an unstable Rh₂(CO)₃ intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis.