Reconstruction and catalytic activity of hybrid Pd(100)/(111) monolayer on γ-Al2O3(100) in CH4, H2O, and O2 dissociation

Abstract

The importance of the “heterogeneity” of a Pd monolayer induced by interaction with a semi-ionic support in catalysis was evaluated. The geometry of the Pd monolayer was optimized on the (100) plane of γ-Al₂O₃ at fixed unit cell parameters defined by the oxide. Simulation of the deposition of a whole Pd monolayer in the flat Pd(100) form cut from the bulk led to the formation of a slightly distorted Pd(111) monolayer. The subsequent chemisorption or dissociation of CH₄ or H₂O on the Pd(111) layer resulted in a new hybrid Pd(100)/(111) structure containing alternating elements of (100) and (111) planes (the parallel bands of squares and triangles), which are similar for both CH₄ and H₂O reactions, and two isolated Pd mono-vacancies, respectively. The hybrid Pd(100)/(111) layer without chemisorbed species was found to be more stable than the initial distorted Pd(111) layer. The catalytic capabilities of these monolayer structures were investigated for the dissociation of methane and the water–gas shift reaction (WGSR) due to the lower predicted activation barriers for CH₄, H₂O, and O₂ dissociation on the hybrid Pd(100)/(111) layer compared to that on the pure (bulk) Pd(100) surface. Moreover, the exothermic heats of these reactions were calculated to be moderate instead of endothermic heats on the Pd(100) or Pd(111) surfaces. The heats of H₂O and NH₃ adsorption on various monolayers were tested, revealing their dependence on Pd atomic charges. The relevance of the model of the heterogeneous Pd monolayer for explaining the maximum reaction rate experimentally observed at different Pd coverages was discussed. The transferability of the geometry and the extent of charge inhomogeneity of the hybrid monolayer without vacancies were also tested on the same γ-Al₂O₃(100) support for Pt, Rh, and Ag.