Enhanced reactivity of NiO/Pd(100) ultrathin films toward H2: Experimental and theoretical evidence for the role of polar borders

NiO layers in the ultrathin regime exhibit an enhanced reactivity toward hydrogen with respect to the typical chemical inertness of bulk-like thicker samples. Such a behavior has been studied by means of photoemission (from both core and valence band levels) and quantum mechanical calculations. It is found that after H2 dosing in mild conditions (from PH2 = 6.5 × 10-7 Pa and T = 330 K) ultrathin films (thickness ≤6MLE, MLE = monolayer equivalent) quickly react forming metal nickel and water. The kinetic of the reaction has been followed in situ recording the intensity of the O 1s and Ni 2p photoemission spectra under different reaction conditions (PH2 ranging from 5 × 10-7 to 2 × 10-5 Pa and T from 330 up to 453 K), and a first-order dependence of the reaction rate on the PH2 and the activation energy of the rate determining step (0.16 ± 0.02 eV) have been determined. A 8 MLE thick film recovers the behavior of bulk-like NiO(100) surfaces, where more drastic reduction conditions are needed, and the kinetic implies an induction period followed by autocatalysis. The enhanced reactivity has been explained assuming the presence of NiO(100) islands exposing polar borders, whose existence was evidenced by previous scanning tunneling microscopy investigations. Such a scenario is confirmed by ab initio quantum mechanical calculations carried out employing polar and nonpolar terminated stepped surface epitaxially strained in order to account for the presence of the metal support which has not been explicitly included. Reported calculations indicate that the polar border can easily dissociate H2 without any activation barrier. The rate determining step of the reaction has been associated to the stage of the reaction where the previously formed hydroxyl groups react with a second hydrogen molecule (an Eley−Rideal-like mechanism) to form metal Ni islands and water, which readily desorbs.