Surface Structure and Reactivity of Rhodium Oxide

Density functional theory calculations have been performed in conjunction with ab initio thermodynamics and density of states analysis to investigate the stability and reactivity of the (0001) and (1 (1) over bar 02) Rh2O3 surfaces. A total of seven surfaces have been investigated using denisty functional theory (DFT) and the DFT+U extension. DFT and DFT+U (U = 3.5 eV) predicted nearly identical lattice parameters and similar trends in surface stability and reactivity. Using ab initio thermodynamics, the c-cut and r-cut surfaces were determined to be close in stability and the most thermodynamically stable surfaces were predicted to be among the most reactive. An oxygen-terminated c-cut surface and an oxygen-terminated r-cut surface were found to exhibit high Lewis acidity and to be close in stability at larger oxygen chemical potentials, possibly explaining the high experimentally observed catalytic activity of Rh2O3 at low temperatures. As compared to DFT, DFT+U predicted a crossover point in surface stability to occur at a larger oxygen chemical potential and predicted surface stability to occur over a wider range of oxygen chemical potential. In this work, Rh2O3 exhibited similar trends in surface stability as compared to alpha-Fe2O3 and alpha-Al2O3.