Molecular or dissociative adsorption of water on clean and oxygen pre-covered Ni(111) surfaces

Abstract

Water adsorption and dissociation on clean and oxygen pre-covered Ni(111) surfaces have been computed systematically by using density functional theory and ab initio atomistic thermodynamics. The adsorption of H, O and OH prefers 3-fold faced centered cubic hollow sites, and that of H2O prefers the top site. For (H2O)n aggregation, direct O–Ni interaction and H-bonding synergistically determine the adsorption energy, which is structure insensitive for large adsorbed clusters. At low coverage (θ ≤ 0.25 ML), OH adsorbs perpendicularly and prefers remote distribution without H-bonding, and the OH saturation coverage should be 0.625 ML on the basis of H2O dissociative adsorption. The adsorption configuration for 4O (0.25 ML) prefers a p(2 × 2) structure, in agreement with the experiment, and the O saturation coverage should be 0.25 ML based on H2O dissociative adsorption. On the 0.25 ML O pre-covered Ni(111), H2O greatly prefers molecular adsorption [4O + 4H2O(s)] over dissociative adsorption [8OH] thermodynamically, and this is in agreement with the photoelectron spectroscopy results and in disagreement with previously and recently proposed results from single crystal adsorption calorimetry of D2O adsorption. It is noted that the computed bond energy and formation enthalpy of the supposed surface hydroxyls on the basis of the molecular adsorbed state [4O + 4H2O(s)] are much closer to the experimentally estimated results than those computed on the basis of the dissociatively adsorbed state [8OH]. Furthermore, the computed H2O desorption temperature on the basis of the molecular adsorbed state [4O + 4H2O(s)] is in excellent agreement with experimental results (284 vs. 275–300 K), while that from the dissociatively adsorbed state [8OH] differs strongly (197 K). All these support H2O molecular adsorption instead of dissociative adsorption, and this needs further experimental investigations and confirmations. The vibrational frequencies of 4O, 4O + 4H2O and 8OH adsorption configurations have been computed to aid experimental studies.