Stereoselectivity in catalytic reactions: CO oxidation on Pd(100) by rotationally aligned O2 molecules

We report on stereodynamical effects in heterogeneous catalytic reactions as measured by molecular beam-surface experiments. Specifically for CO oxidation on Pd(100) we find that the rotational alignment of the incoming O-2 at low (I similar to = 0.04 ML) and at intermediate (I similar to(CO) = 0.17 ML) CO pre-coverage, causes a higher reactivity of molecules in high and in low helicity states, respectively (corresponding to helicoptering and cartwheeling motion of O-2). In first approximation, at low CO pre-coverage the difference in reactivity is determined by the different location of the O atoms generated in the dissociation process by the different parent molecules, while at intermediate CO pre-coverage the reactivity is influenced also by the different ability of cartwheeling and helicoptering O-2 to penetrate through the CO adlayer. In accord with this the total amount of CO2 produced is always largest for helicopters which generate supersurface O atoms at least in the low CO pre-coverage limit. A deeper inspection of the data indicates, however, that the dynamics is more complex, two different pathways being present for the reaction with O generated by helicopters and one for O generated by cartwheels. Moreover, cartwheels generated oxygen influences the reactivity of subsequently arriving helicopters.