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Quantum size effect on the dissociation of O2 molecules on ultrathin Pb(111) films

Using first-principles calculations, we systematically study the dissociation of O$_2$ molecules on different ultrathin Pb(111) films. Based on our previous work revealing the molecular adsorption precursor states for O$_2$, we further explore that why there are two nearly degenerate adsorption states on Pb(111) ultrathin films, but no precursor adsorption states exist at all on the Mg(0001) and Al(111) surfaces. And the reason is concluded to be the different surface electronic structures. For the O$_2$ dissociation, we consider both the reaction channels from gas-like and molecularly adsorbed O$_2$ molecules. We find that the energy barrier for O$_2$ dissociation from the molecular adsorption precursor states is always smaller than from O$_2$ gases. The most energetically favorable dissociation process is found to be the same on different Pb(111) films, and the energy barriers are found to be modulated by the quantum size effects of Pb(111) films.