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

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 N2 desorption peaks are observed upon heating. The first N2 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 N2 desorption states. For high NO coverages on the (1 × 5) surface, the narrow width of the first N2 desorption peak is indicative of an autocatalytic process for which the parallel formation of N2O appears to be the crucial step. When NO is adsorbed on the metastable unreconstructed (1 × 1) phase of clean Ir{1 0 0} N2 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.