Oxidation of Supported Nickel Nanoparticles: Effects of Lattice Strain and Vibrational Excitations of Active Sites

This work investigated the oxidation in an atmosphere of N₂O of different surface areas of single nickel nanoparticles deposited on highly oriented pyrolytic graphite (HOPG). Using scanning tunneling microscopy and spectroscopy, it was shown that oxide formation begins at the top of the nanoparticle, while the periphery is resistant to oxidation. The active site of oxygen incorporation is a vibrationally excited group of nickel atoms, and the gap between them is the place where an oxygen adatom penetrates. The characteristic time of vibrational relaxation of the active site is 10^-9-10^-7 s. The reason for the oxidation resistance is the deformation of the nanoparticle atomic lattice near the Ni-HOPG interface. A relative compression of the nanoparticle atomic lattice <i>ξ</i> = 0.4-0.8% was shown to be enough for such an effect to manifest. Such compression increases the activation energy for oxygen incorporation by 6-12 kJ/mol, resulting in inhibition of oxide growth at the periphery of the nanoparticle. In fact, in this work, oxygen adatoms served as probes, and their incorporation between nickel atoms allowed the measurement of the nanoparticle's lattice parameters at different distances from the Ni-HOPG interface. The developed theoretical framework not only accounts for the observed oxidation behavior but also offers a potential pathway to estimate charge transfer and local work functions for deposited nickel catalysts.