Adatom bond energies and lateral interaction energies from calorimetry: NO, O2, and N2 adsorption on Ni{100}

Abstract

Heats of adsorption and sticking probabilities have been measured for NO and O2 on clean and oxygen precovered Ni{100} at room temperature by single crystal adsorption calorimetry (SCAC). Adsorption is initially dissociative for both species and the initial heat of adsorption is 536 kJ/mol for O2 and 426 kJ/mol for NO. From these data the initial heat of dissociative adsorption of N2 on Ni{100} is estimated to be 136 kJ/mol. With increasing coverage the adsorption heat for both NO and O2 decreases due to repulsive interactions between adsorbed adatoms; the magnitude of the pairwise repulsive interactions between oxygen adatoms in nnn sites is estimated to be about 40 kJ/mol by fitting the experimental data for O2 dissociative adsorption with a Monte Carlo simulation. The pairwise repulsive interaction between nitrogen and oxygen and between nitrogen adatoms in nnn sites is obtained as about 100 kJ/mol. Due to the high magnitude of lateral repulsive interactions between adatoms, a critical coverage of NO is reached for which molecular adsorption is energetically favored with respect to dissociative adsorption. This coverage is ∼0.16 ML for a 1:1 O:N adatom coverage decreasing to ∼0.07 ML for 3:1 O:N adatom ratio. For O2 adsorption, the lower magnitude of repulsive interactions allows dissociative adsorption to take place up to a coverage of about 0.3–0.4 ML; above this coverage the bulk oxide is more energetically favorable and an oxide layer is formed. NO dissociative adsorption is precursor mediated, whereas for O2 the precursor plays only a limited role.