Clays pillared by zirconia nanoparticles: structure, adsorption and catalytic properties
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Clays pillared by zirconia nanoparticles: structure, adsorption and catalytic properties Vladislav A. Sadykov1, Tatyana G. Kuznetsova1, Rimma V. Bunina1, Natalya V. Mezentseva1, Dmitrii K. Efremov1, Vasilii I. Avdeev1, Vladimir P. Doronin1, Vladimir L. Kuznetsov1, Aleksander M. Volodin1, Eugenii A. Paukshtis1, Galina M. Alikina1, Valerii A. Matyshak2, Valerii V. Lunin3 1
Boreskov Institute of Catalysis SB RAS, pr. Lavrentieva, 5, 630090, Novosibirsk; [email protected] 2 Semenov Institute of Chemical Physics, Moscow, Russia 3 Chemical Department of Moscow State University, Moscow, Russia; ABSTRACT Modeling of high resolution nitrogen adsorption isotherms by the Grand Canonical Monte Carlo (GCMC) method for zirconia-pillared clays containing 20 wt.% of ZrO2 revealed that the pillars are comprised of separate Zr4(ยต-OH)8(OH)m (H2O)n units (Zr4 tetramers) and their dimers (Zr8 complexes) as sheets or loose 3D structures. This agrees with the results of modeling of the condensation process in solution using the Density Functional Theory approach (DFT) and the semiempirical PM3 method. Bridging and terminal hydroxyls strongly bound with Zr cations are involved in anchoring supported Cu cations and Pt. This nanostructure of the active component as well as the developed micro- and mesoporosity appear to be responsible for the high performance of Pt+Cu/ZrPILC catalysts in the NOx selective reduction by propylene and decane in realistic feeds. INTRODUCTION Selective catalytic reduction (SCR) of nitrogen oxides by hydrocarbons under leanburn conditions (NOx HC SCR) could be a promising option for NOx abatement from diesel engine exhausts. Catalysts based upon zirconia-pillared clays (ZrPILC) [1-4] with supported Cu cations promoted by Pt appear to be suitable for this application. Though preservation of the tetrameric Zr4 unit in zirconia nanopillars in pillared clays is reliably established, the mode of their packing in nanoparticles still remains unclear [4-6]. Aging of pillaring solutions with the addition of Ca or Ba chlorides appears to stabilize rod-like species comprised of 2-3 stacked tetramers, while the addition of Sr chloride stabilizes sheet-like structures [1, 3, 4]. This work presents results on the characterization of the structure of zirconia nanopillars in ZrPILC by modeling the high resolution nitrogen adsorption isotherms using the Grand Canonical Monte Carlo (GCMC) method. To elucidate the relation between the structure of nanopillars in clay galleries and that of pillaring species in solution, the DFT and semiempirical PM3 method were applied for analysis of the relative stability of some polynuclear zirconium hydroxocomplexes which could be present in aged ZrOCl2 solutions. The high performance of Pt+Cu/ZrPILC catalysts in the propylene and decane NOx SCR is explained by their developed pore structure and the specificity of the reaction mechanism involving steps with participation of strongly bound intermediates. EXPERIMENTAL AND COMPUTATIONAL DETAILS Zirconia-pillared clays (ZrO2 content
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