Is gold actor or spectator in the reaction of small AunPdm+ clusters with O2?

The reactivity of free binary gold-palladium clusters (AuPd2+, Au2Pd+, Au2Pd2+, and Au2Pd3+) toward molecular oxygen was investigated in an ion trap experiment under multi-collision conditions and compared to the reactivities of bare Au-n(+) and Pd-m(+) (n, m = 2-5) clusters. Reaction kinetics measurements revealed that the reaction rate is mainly determined by the number of palladium atoms in the clusters and only weakly influenced by additional gold atoms. The same holds true for the observed reaction product distributions. Most interestingly, the most reactive cluster ions Pd-3(+), Au2Pd3+, and Pd-5(+) exhibit a strong preference to form tetroxide products, AunPdmO4+. In addition, employing temperature dependent mass spectrometry, a second adsorption species consisting of several weakly bound oxygen molecules was identified for all investigated palladium containing clusters which is, however, only formed at cryogenic temperatures. All these observations suggest that the gold atoms largely act upon a spectator role in the reaction of the binary clusters. Nevertheless, a rough estimation of the relative O-2 binding energies via statistical rate theory indicates that the addition of gold to the Pd-n(+) clusters decreases the O-2-cluster interaction strength, although the reaction rate stays constant. This effect in the binary clusters may be of importance to a potential activation and dissociation of the adsorbed O-2 molecules.