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

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 gamma-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 O-2 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 gamma-Al2O3(100) support for Pt, Rh, and Ag.

Automated summary

The study evaluated the importance of heterogeneity of a Pd monolayer induced by interaction with a semi-ionic support in catalysis, finding that the hybrid Pd(100)/(111) layer exhibits lower activation barriers and greater stability, suitable for methane dissociation and water-gas shift reaction. Additionally, the characteristics of chemisorption and exothermic reactions on monolayer structures depend on Pd atomic charges and can explain the maximum reaction rate observed at different Pd coverages.