THE ADSORBATE-INDUCED REMOVAL OF THE PT(100) SURFACE RECONSTRUCTION .1. NO

The molecular adsorption of NO on both the reconstructed (hex) and unreconstructed (1 x 1) surfaces of Pt(100) has been studied using a combination of infrared reflection-absorption spectroscopy (IRAS) and low energy electron diffraction (LEED) at temperatures between 90 and 300 K. On the (1 x 1) surface at 300 K adsorbed NO gives rise to an N-O stretching band at initially 1596 cm-1 shifting to 1641 cm-1 at a coverage of theta = 0.5. The LEED pattern at this coverage is interpreted in terms of a c(4 x 2) structure in which all the molecules occupy a single type of adsorption site between the on-top and bridge positions. At temperatures below 300 K, a higher coverage disordered phase is observed, giving rise to an N-O stretching band at 1680 cm-1 associated with an on-top NO species. On the (hex) phase surface above 210 K, NO adsorption gives rise to bands characteristic of adsorption on the (1 x 1) phase indicating that the reconstruction is immediately lifted. Below 200 K initial adsorption actually occurs directly on the (hex) phase, resulting in a band at 1680 cm-1, which is assigned to on-top NO. This band increases in intensity until, at a critical coverage dependent on temperature, the (hex) --> (1 x 1) surface phase transition is induced. This is indicated by the disappearance of the band at 1680 cm-1 and its replacement by bands characteristic of adsorption on islands of the (1 x 1) structure.