ADSORPTION OF NITRIC-OXIDE ON IR(100) - INFLUENCE OF SUBSTRATE RECONSTRUCTION

We present a detailed study of the interaction of nitric oxide (NO) with the reconstructed (5 x 1) and metastable (1 x 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 is much more complex. in particular, dissociation plays an important role. On the (1 x 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 1800-7870 cm(-1). At 300 K, however, molecular adsorption only occurs after formation of a p(2 x 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 x 1) pattern due to atomic oxygen with a coverage of Theta(o) = 0.5. In contrast to the (1 x 1) surface the initial adsorption of NO on the (5 x 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.