CO oxidation on Pd(110)

Palladium offers some scope as an alternative precious metal for use in catalytic removal of pollutants from automobile exhaust gases. An understanding of the operation of this metal depends partly on developing a fundamental knowledge of the surface properties for adsorption and reaction. We have used surface science methods to investigate the adsorption and reaction of CO and O-2 on Pd(110) using a molecular beam reactor, temperature programmed desorption and scanning tunnelling microscopy(STM). Both O-2 and CO stick efficiently on the surface (S-O2 = 0.42 +/- 0.02, S-CO = 0.52 +/- 0.02, at room temperature) and react with high probabilities at temperatures as low as 310 K, providing that the CO coverage is low; at high CO pre-coverages the surface reaction is poisoned. There appears to be an initial CO coverage (0.64 +/- 0.1 monolayers) above which the O-2 sticking on the surface is drastically reduced. Experiments with a mixed CO and O-2 beam, run with a step wise increase in temperature, show low steady state rate in CO2 production at low temperature as a result of CO poisoning the surface. The rate goes through a maximum at 396 K and then diminishes at high temperatures due to a low equilibrium coverage of CO. Transient 'Light-off' is observed at 396 K due to self acceleration of the reaction when the CO begins to desorb. STM images of the oxygen c(2x4) structure have been obtained at 520 K. The surface is grossly reconstructed and roughened by oxygen and there is considerable inhomogeneity in the overlayer structure. Both features may be linked with subsurface oxygen formation. CO clean off of the oxygen at 300 K leaves a (1x2) reconstructed Pd surface.