Atomic-Scale Structural Evolution of Rh(110) during Catalysis

We report direct observation at the atomic scale of the pressure- and temperature-dependent evolution of a model Rh(110) catalyst surface during transient and steady-state CO oxidation, using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photo-electron spectroscopy (AP-XPS) correlated against density functional theory (DFT) calculations. Rh(110) is susceptible to the well-known missing row (MR) reconstruction. O-2 dosing produces a MR structure and an O-2 coverage of 1/2 monolayer (ML), the latter limited by the kinetics of O-2 dissociation. In contrast, CO dosing retains the (1 x 1) structure and a CO coverage of 1 ML. We show that CO dosing titrates O from the (2 x 1) structure and that the final surface state is a strong function of temperature. Adsorbed CO accelerates and O inhibits the (2 x 1) to (1 x 1) transition, an effect that can be traced to the influence of the adsorbates on the energy landscape for moving metal atoms from filled to empty rows. During simultaneous dosing of CO and O-2, we observed steady-state CO oxidation as well as a transition to the (1 x 1) structure at temperatures more modest than in the titration experiments. This difference may reflect surface heating generated during CO oxidation. At more elevated temperatures the metallic surface transforms to a surface oxide, also active for CO oxidation. Being one of the first examples, these results demonstrate how operando experiment exploration in terms of correlation between surface structure dominated by reaction conditions and activity of a catalytic material and first-principles models can be integrated to disentangle the underlying thermodynamic and kinetic factors that influence the dependence of catalytic activity on surface structure at nano and atomic scales.