Multiple adsorption of molecular oxygen on small Au/Pd cationic clusters at finite temperature. A van der Waals...

The adsorption of molecular oxygen on cationic bimetallic palladium/gold clusters, AunPdm (+) (n + m ≤ 5), is studied by means of self-consistent density functional calculations including long range van der Waals non-local interactions. A single O2 molecule is adsorbed preferably on top of Pd sites for m = 0, 1, but bridge or hollow locations between Pd atoms are preferred for m ≥ 2. In the later cases, both the O2 electronic charge and the O-O distance increase as compared with the values for free O2, leading to negatively charged O2 superoxo species which facilitates the CO oxidation. Multiple sequential adsorption of several O2 is considered for the n + m ≤ 3 clusters, which occurs with decreasing adsorption energy, except when severe distortion of the bimetallic support appears. The Gibbs free energy of AunPd2-n (+)O2x complexes with n = 1-2 and x = 1-5 is computed at temperatures 0 K, 50 K, 150 K, and 300 K. We obtain that Pd2 (+) (PdAu(+)) can adsorb 5 (4) oxygen molecules at ambient temperature; however, Au2 (+) can adsorb up to three O2 molecules when the temperature is lower than 150 K.