ИСТИНА

Pd supported catalysts are widely accepted as the most active catalysts for catalytic combustion of methane and low temperature oxidation of CO. For the methane catalytic combustion applications, the natures of the support and the support-catalyst interactions have a significant effect on the thermal stability and activity of the catalyst. Despite the intensive work on enhancement of thermal stability of palladium-based catalysts, the stability is still quite limited especially at temperatures above 500oC. Therefore, one should apply some additional efforts to the further improvement of the catalytic activity, thus allowing operation of catalytic reactors at lower temperatures. The need of more active catalysts requires a deeper insight and careful monitoring of the interactions between palladium and metal oxides and further experimental and theoretical study. One of the approaches describing interaction between palladium and metal oxides is strong metal–support interaction (SMSI) concept [1], which we analyzed at a theoretical level regarding a series of slab Pd/oxide models whose components (Al2O3 [2-3], ZrO2 [4-5], TiO2 [1, 6], and SiO2 [7]) are the candidates for wide scale applications as a support for full oxidation of lean methane mixtures [2-6]. Often the SMSI effect was assigned either to TiO2 reduction and its inclusion in the metallic species [1], or to silicide formation at Pd/SiO2 boundary [7]. Usually the influence of selected oxide supports on the structure/size of metallic nanoparticles was studied [2] but opposite trend was never traced or shown theoretically. Herein, structural distortions in tetragonal form of ZrO2 were revealed in different layers under the contact zone (shown by ellipses in Figure 1). The O and Zr coordination numbers change from 4 and 8 to 3 and 5, respectively, that can be noted in the Figures 1b, c as the loss of three-dimension structure by respective distorted layers. Similar influence of other metals of 8th group of Mendeleev’ table (Pd, Pt, Rh) was also checked. Variations of ZrO2 structure depend on the number of deposited metallic layers and their orientation but do not depend on what the metal used. This can be considered as a reason of higher O mobility in tetragonal ZrO2 form for redox processes in agreement with experimental observation [4-5]. Comparative influence of PdO layers is studied with the same oxides. It was estimated as a weaker irrespective of PdO plane (Figure 1c). We have constructed several supercell slab models (Pd, oxidized O*/Pd, and PdO layers deposited on supports mentioned above) and performed the reaction modeling at the DFT computational level (PAW-PBE with dispersion correction) with VASP code [8-9], to reproduce and explain the main experimental trends, i.e., moderate activation barrier of CH4 oxidation over PdO and rather high activation energy over chemisorbed O*/Pd both deposited over the oxide supports. A series of defects over Al2O3 surface [10] was admitted and their influence on the deposited Pd species was studied. It was found that the structure of growing Pd slab depends on the type of the γ-Al2O3 defects, but it does not produce significant structural distortion of the support. Such geometric changes in atomic coordination are minimal for γ-Al2O3 slabs possessing defective surface [4] and TiO2 (both rutile or anatase) [2] thus showing other reasons of the SMSI (for example, diffusion of partly reduced TiO2 in the metal [1]). The higher O mobility was also analyzed using ciNEB modeling [11]. The theoretical results for Pd supported on ZrO2, TiO2 and Al2O3 are compared with experimental data from catalytic activity test in CH4 combustion, H2-TPR, CH4-TPR, O2-TPD and reaction kinetics modelling. References. 1. S.J. Tauster, S.C. Fung, R.L. Garten, J. Amer. Chem. Soc. 100 (1978) 170; 2. W.R. Schwartz and L.D. Pfefferle, J. Phys. Chem. C 116 (2012) 8571−857; 3. P. Stefanov, S. Todorova, et al. Chemical Engineering Journal 266 (2015) 329–338; 4. S.C. Su, et al. J. Catal. 176 (1998) 125–135; 5. D. Ciuparu; E. Altman; L. Pfefferle, J. Catal. 203 (2001) 64−74; 6. W. Lin, et al. Applied Catalysis B: Environmental 50 (2004) 59–66; 7. B.K. Min, et al. Catal. Today 85 (2003) 113–124; 8. G. Kresse, J. Hafner, Phys. Rev. B. 47 (1993) 558–561; 9. G. Kresse, J. Furthmüller, Phys. Rev. B. 54 (1996) 11169–11186; 10. A.A. Rybakov, et al. Theoretical Chemical Accounts, 135 (2016) 152; 11. G. Henkelman, B.P. Uberuaga, H. Jónsson, J. Chem. Phys. 113 (2000) 9901–9904