Adsorption of nitric oxide on Ir{100}: influence of substrate reconstruction

Abstract

We present a detailed study of the interaction of nitric oxide (NO) with the reconstructed (5 × 1) and metastable (1 × 1) surfaces of Ir{100} using vibrational spectroscopy and low-energy electron diffraction (LEED). Despite certain similarities with the corresponding Pt system, for example in the adsorbate-induced lifting of the reconstruction, the temperature-dependent adsorption behaviour in the present case in much more complex. In particular, dissociation plays an important role. On the (1 × 1) phase molecular adsorption takes place at low temperature, giving rise to characteristic N—O stretching bands in the regions 1570–1700 cm–1 and 1800–1870 cm–1. At 300 K, however, molecular adsorption only occurs after formation of a p(2 × 2) structure due to atomic oxygen (and perhaps atomic nitrogen). The resulting overlayer is structurally rather complicated, showing both half-order and sixth-order features in LEED. Desorption and/or further dissociation of the molecular NO produces a sharp p(2 × 1) pattern due to atomic oxygen with a coverage of Θo= 0.5. In contrast to the (1 × 1) surface the initial adsorption of NO on the (5 × 1) surface is molecular at all temperatures, indicating the lower reactivity of the more closely packed pseudo-hexagonal surface towards dissociation. The reconstruction is not lifted immediately at 300 K, but only after a significant coverage of molecular NO has been reached; the adsorbate-induced phase change still occurs at 90 K, but somewhat less readily. This represents another interesting difference compared with Pt{100} where the lifting of the (hex) phase reconstruction is immediate above 200 K.