Adsorption and Hydrogenation of CO₂ on Rh Nanosized Crystals: Demonstration of the Role of Interfacet Oxygen Spillover and Comparative Studies with O₂, N₂O, and CO

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-and-brightness 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.