The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.