CO AND O2 ADSORPTION ON RH(110)

The adsorption of CO and oxygen on Rh(110) has been studied using a thermal molecular beam, thermal desorption and LEED. CO adsorbs with an initial sticking coefficient of 0.68 and shows a coverage dependence which is well described by the Kisliuk formalism for precursor kinetics. The desorption shows a main peak at 485 K, which shifts to lower temperature with increasing coverage up to approximately 0.4 monolayers. Above this coverage a shoulder appears at approximately 425 K and a further shoulder at 390 K above about 0.75 monolayers. These effects are due to repulsive lateral interactions in the adlayer, though the only clear ordered pattern seen in the LEED at 320 K is a (2 x 1)p1g1 pattern above 0.9 monolayers. For oxygen the initial sticking coefficient is 0.62 (+/- 0.01) at 310 K which diminishes relatively slowly with increasing oxygen coverage. We believe that this is due to row pairing in the adlayer which produces "stretched" rows of higher reactivity adjacent to them. The row pairing is seen in a wide sequence of LEED patterns with increasing coverage - (1 x 3), (2 x 2)p1g1, (1 x 2)/(1 x 3), (2 x 3)p1g1, c(2 x 6), c(2 x 8). Much higher exposures are required for the latter 3 structures since the sticking coefficient has diminished to a low value by then. The apparent "saturation" point in the sticking curves is 0.65 monolayers, but oxygen can continue to adsorb slowly. We propose that the last three structures are due to completely restructured surfaces, possibly with subsurface oxygen, though this requires further work. Desorption from these high coverage states begins at the relatively low temperature of 750 K.