NO-CO Reaction Over Metal-supported Ultrathin Oxide Films: Evaluating Novel Catalysts by Density-functional Theory Calculations

Abstract

An ultrathin oxide film supported on a metal substrate (Figure 1) sometimes exhibits extraordinary catalytic activities. To explore the potential of such structures as automotive exhaust catalysts, we have examined NO-CO reaction over oxide/metal slab models by plane-wave density-functional theory calculations. This article summarizes our computational results for TiO₂/Ag and ZrO₂/Cu slabs (Figure 2). For both systems, we found that NO can be adsorbed and activated on the cationic site of the oxide by receiving electronic charge from the metal (Figure 4). The activated NO species then dimerizes to form ONNO (Figure 5). This ONNO decomposes into N₂O and O (Figure 5), and N₂O then decomposes into N₂ and O (Figure 6). The O adatoms left on the surface react with CO to form CO₂, thus completing the catalytic cycle (Figure 7). The calculated energy barriers and adsorption energies indicate the superiority of ZrO₂/Cu compared to TiO₂/Ag. This difference can be attributed to a difference in charge on the cations and a difference in inter-cation distances.