Surface Structure and Reactivity of Rhodium Oxide

Rhodium metal catalysts are used in wide a range of applications, including automotive catalytic converters for NO reduction, N2O decomposition, and CO oxidation. 1 7 Theoretical and experimental studies have found that, under realistic environmental conditions, rhodium surfaces can oxidize, forming thin films or thicker bulk oxides. In the case of CO oxidation, it has been shown that the formation of thin film oxides increases the catalytic reactivity of the surface. A variety of phases of rhodium oxide have been found to exist: corundum phase Rh2O3 I (space group R3c), corundum phase Rh2O3 II (space group Pbna), corundum phase Rh2O3 III (space group Pbca), and rutile phase RhO2. 11 Phase I has been shown to be the most stable phase at low to moderate temperatures (below 1350 C) and in the pressure range of 0 3.27 GPa. Although several studies have investigated the formation of the bulk oxide of the phases of rhodium oxide and the structure of the thin film RhO2 oxide phase, to our knowledge there have been no detailed density functional theory (DFT) investigations regarding the structure of the stable corundum surface. Corundum Rh2O3 I is isostructural withR-Al2O3 andR-Fe2O3, differing primarily in the size of the cations and the lattice constant. Previous studies of R-Al2O3 and R-Fe2O3 have found the surfaces to exist in two forms, one in which the surface is cleaved along the (0001) plane, called the c-cut surface, and the other cleaved along the (1102) plane, called the r-cut surface. 18 The configuration of the surface atoms (i.e., whether the structure is more stable with metal cation or oxygen anion termination) is a function of both the temperature and partial pressure of the surrounding environment and has been shown to be a function of catalyst preparation. The surface structure of the oxide influences its catalytic activity through the distribution of charge and is therefore related to the Lewis acidity of the surface sites. The goal of this work is to use DFT to understand the most thermodynamically stable surfaces of the corundum phase I of Rh2O3 under different temperature and pressure environments. However, application of the DFT approach to highly correlated metal oxides can lead to errors in the calculation of the electron self-interaction; unpaired electrons will tend to delocalize over many atoms and reduce the Coulomb repulsion. The selfinteraction error can lead to errors in the calculated band structure, typically resulting in a smaller predicted band gap that can translate into incorrect predictions in the magnetic properties, surface stability, and surface catalytic reactivity. In the case of R-Fe2O3, DFT can even predict the incorrect insulator type. 20,21