Model car-exhaust catalyst studied by TPD and TP-RAIRS: Surface reactions of NO on clean and O-covered Ir{1 0 0}

Abstract

The adsorption of NO on Ir{1 0 0} has been studied as a function of NO coverage and temperature using temperature programmed reflection absorption infrared spectroscopy (TP-RAIRS), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). After saturating the clean (1 × 5)-reconstructed surface with NO at 95 K, two N 2 desorption peaks are observed upon heating. The first N 2 peak at 346 K results from the decomposition of bridge-bonded NO, and the second at 475 K from the decomposition of atop-bonded NO molecules. NO decomposition is proposed to be the rate limiting step for both N 2 desorption states. For high NO coverages on the (1 × 5) surface, the narrow width of the first N 2 desorption peak is indicative of an autocatalytic process for which the parallel formation of N 2O appears to be the crucial step. When NO is adsorbed on the metastable unreconstructed (1 × 1) phase of clean Ir{1 0 0} N 2 desorption starts at lower temperatures, indicating that this surface modification is more reactive. When a high coverage of oxygen, near 0.5 ML, is pre-adsorbed on the surface, the decomposition of NO is inhibited and mainly desorption of intact NO is observed.