Synergistic alloying behaviour of Pd50Cu50 single crystals upon adsorption and co-adsorption of CO and NO

We have studied the surface reactivity of the Pd50Cu50 (111) and (110) single crystals in the adsorption and the co-adsorption of CO and NO at low temperature. The bond strengths of CO and No on Pd and Cu in the alloy are significantly different from those observed on pure metals. Carbon monoxide and nitric oxide are more tightly bound to Cu and less tightly bound to Pd than to the pure components. The calculated NO coverages on PdCu samples are approximately four times lower than for CO due to the dissociation of a fraction of NO on the surfaces. However, the CO or NO adsorption did not suggest an important difference of reactivity between the two surfaces studied. In the co-adsorption experiments, the total surface coverage of CO and NO on Pd50Cu50 (111) and (110) appears quite dependent on the order in which the gases are adsorbed in the successive adsorptions. The pre-saturation with NO leads to molecular adsorbed states and irreversible dissociated species which are not removed upon an exposure to CO. The subsequent CO co-adsorption induced only a slight replacement of molecular NO by CO and the amount of adsorbed CO is quite low (θCO=0.06 ML) as compared to the coverage measured with CO alone (θCO=0.40 ML). The pre-saturation with CO gives a different behaviour, the subsequent co-adsorption of NO leads to the replacement of CO by NO up to the saturation coverage found for NO alone (θNO=0.10 ML). Finally, when NO is adsorbed on a partially CO covered surface, NO displaces CO up to around 0.15 ML but the total amount of surface species (0.30 ML) is lower than the amount (0.55 ML) of pre-adsorbed CO, indicating a strong displacement of CO by NO. The competitive adsorption of CO and NO on PdCu surfaces is discussed in terms of dissociation of NO at low temperature which partially inhibits the further adsorption of CO on the PdCu alloy surfaces.