Applied Catalysis B: Environmental, Vol.174, 293-307, 2015
Active, selective and robust Pd and/or Cr catalysts supported on Ti-, Zr- or [Ti,Zr]-pillared montmorillonites for destruction of chlorinated volatile organic compounds
Pd and/or Cr catalysts supported on Ti-, Zr- or mixed [Ti,Zr]-pillared montomorillonite clays (PILC), for use in combustion of dichloromethane (DCM) and trichloroethylene (TCE), were synthesized, and characterized with XRD, SEM, TEM/HRTEM, XPS, ESR and Raman spectroscopies, N-2 adsorption/desorption at -196 degrees C, NH3 TPD-MS, and FTIR of pyridine adsorption. The catalysts are porous solids with high surface area (240-390 m(2)/g). Pd exists as fine PdOx nanoparticles, predominantly of <= 10 nm size, while Cr is present as monomeric CrOx species. The highest (Pd4+ and Cr6+), or next to highest (Cr5+), oxidation states dominate at the catalysts surface. The active phases are most reducible when supported on Ti-PILC and most difficult to reduce on [Ti,Zr]-PILC. Catalysts supported on [Ti,Zr]-PILC show highest total acidity, of predominantly Lewis character. The excellent performance of the catalysts in combustion of DCM and TCE is attributed to the combination of highly porous structure of PILC supports, good dispersion of the active phases, and appropriate blend of redox and acid-base functions. In both reactions, for a given type of support, the catalysts with Cr-only active phase show best performance in terms of activity, which is assigned to the presence of well dispersed, highly oxidized Cr6+/Cr5+ species. In DCM combustion, catalysts supported on Ti-PILC show superior activity, attributed to much better reducibility of active phases deposited on this carrier. In TCE oxidation, best activity is obtained for [Ti,Zr]-PILC supported catalysts, which is correlated with the highest Lewis acidity of these samples. Pd-only catalysts are most selective, showing 100% selectivity to HCl and CO2 at T-90 for both studied reactions. Cr-only catalysts are least selective, and emit substantial amounts of Cl-2 and CO. Mixed Pd,Cr-catalysts represent an attractive, cheaper alternative to purely Pd systems. At T-90 their selectivity to CO2 is 100% in both reactions, with the selectivity to HCl ranging from 87 to 100%. PILC-supported catalyst proved very stable in the reaction environment, both in terms of the catalytic performance and the structural identity. (C) 2015 Elsevier B.V. All rights reserved.