ИСТИНА

Pd supported catalysts are widely accepted as the most active ones for combustion of methane. Owing to the easy Pd oxidation and inertness of Pd(O*), the PdO phase is the most active oxidant that should be regenerated in the redox process. The processes of PdO formation from oxidized Pd(O*) are poorly understood. In this work, we consider the oxygen diffusion process from the surface to the deeper Pd layers of a Pd(100)/γ-Al2O3(100) slab model. First, we propose a method to reduce the size of the slab model taking into account the charge distribution and the geometry of the Pd layers as in ref. [1]. Using a small Pd slab, a series of oxidized Pd surfaces covered by both atomic O and peroxo-species is obtained. Respective coverages span an O concentration interval around the c(2×2) Pd(O*) oxidized model studied herein. The climbing image nudged elastic band (ciNEB) method was used for oxygen diffusion modeling at the DFT level considering dispersive corrections. The energy cut-off was set to 500 eV. Spin polarized solution was considered, when O2 was involved. The Brillouin zone k-sampling was restricted to the Γ–point for the geometry optimization and transition state (TS) search via ciNEB calculations, and was made (442) for the PDOS calculation. The atomic charge density distribution was analyzed using Bader analysis. The full Pd(100) model was arranged as a slab containing 4-6 metal Pd layers over γ-Al2O3(100) support. It was demonstrated that average atomic charge in the metallic slabs quickly converges to nearly zero values with the exception of the 1st atomic layer contacting with the oxide surface or chemisorbed O atoms [1]. Pd charges in the internal metallic layers are very small and weakly respond relative to the reaction with the oxide surface but the geometry of the Pd slab “transfers” the influence of the oxide to the external layer. Thus, the nearly neutral 5-layer (5L) Pd slab models without the contact layer were used for the modeling of O2 dissociation as well as less costly model for O diffusion throughout the Pd(100)O* slab which reflects the structural perturbation produced by the surface oxide. The lowest Pd layer of the 5L slab was frozen. The diffusion barriers were compared with those in the full model and then were calculated at different stages of O diffusion. A reasonable agreement was achieved with experimental O diffusion barriers of 0.621 eV [2], 1.45 eV from denser Pd(111) surface [2]. The authors thank the Bulgarian Science Fund for financial support through contracts DNTS/ Russia /01/1 and Russian Foundation of Basic Researches within the grant 17-53-18026-Bolg_а. The research is carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University [3]. [1] A.A. Rybakov, I.A. Bryukhanov, A. V. Larin, S. Todorova, G.M. Zhidomirov, Struct. Chem. 30 (2019) 489–500. [2] G. Ketteler, D.F. Ogletree, H. Bluhm, H. Liu, E.L.D. Hebenstreit, M. Salmeron, J. Am. Chem. Soc. 127 (2005) 18269–18273. [3] V. Sadovnichy, A. Tikhonravov, V. Voevodin, V. Opanasenko, «Lomonosov»: Supercomputing at Moscow State University, в: J.S. Vetter (Ред.), Contemp. High Perform. Comput. From Petascale Towar. Exascale, CRC Press, Boca Raton, USA, 2013: 283–307.