In this work, we investigate the adsorption of carbon dioxide on rhodium (Rh) nanocrystals as well as its catalytic reaction with hydrogen, at the nanoscale, using field ion microscopy (FIM), video-field emission microscopy (FEM), and one-dimensional atom probe (1DAP). A FEM pattern-and-brightness analysis during the ongoing dissociation process at 700 K provides information on various facet reactivities and how these facets communicate with each other. Our results show CO 2 dissociative adsorption to be fastest on {012} facets. Initially dark {113} facets transiently appear bright, and we suggest this behavior is due to subsurface oxygen states occupied via spillover from {012} facets. Although local surface reconstructions of individual Rh facets may likewise be encountered, they fail to explain the sequence and time dependence of the observed FEM pattern-andbrightness changes. CO 2 /H 2 coadsorption studies suggest surface and subsurface oxygen can be reacted off as water. The observations are discussed within the context of the reverse water gas shift reaction. Comparative FEM studies are performed with other O-containing molecules. While the adsorption of N 2 O and O 2 leads to similar FEM pattern-and-brightness changes on an otherwise different time scale than those of CO 2 , nondissociative CO adsorption does not produce any noticeable such changes. We conclude that the mechanism of interfacet communication involving subsurface oxygen states is of general importance in reaction studies with oxygen-containing molecules undergoing surface dissociation.

The oxygen adsorption and its catalytic reaction with hydrogen on Pt–Rh single crystals were studied at the nanoscale by Field Emission Microscopy (FEM) and Field Ion Microscopy (FIM) techniques at 700 K. Both FEM and FIM use samples prepared as sharp tips, apexes of which mimic a single nanoparticle of catalyst considering their similar size and morphology. Oxygen adsorption on Pt-17.4 at.%Rh samples leads to the formation of subsurface oxygen, which is manifested in the field emission (FE) patterns: for O2 exposure of ~3 Langmuir (L), {113} planes appear bright in the emission pattern, while for higher oxygen doses, i.e. 84 L, the bright regions correspond to the high index planes between the {012} and {011} planes. Formation of subsurface oxygen is probably accompanied by a surface reconstruction of the nanocrystal. The subsurface oxygen can be effectively reacted off by subsequent exposure of the sample to hydrogen gas at 700 K. The hydrogenation reaction was observed as a sudden, eruptive change of the brightness seen on the FE pattern. This reaction resulted in the recovery of the initial field emission pattern characteristic of a clean tip, with {012} facets being the most visible. It was shown that the oxygen accumulation-reduction process is completely reversible. The obtained results indicate that the presence of subsurface species must be considered in the description of reactive processes on Pt–Rh catalysts.

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