Molecular design and characterization of catalysts for NO_x selective reduction by hydrocarbons in the oxygen excess based upon ultramicroporous zirconia pillared clays

Approaches to design of zirconia pillared clays via control of the properties of pillaring species in solutions were elaborated. Structural features of pillars and Pt + Cu active components fixed at these nanoparticles determined catalytic properties of pillared clays in NO_x selective reduction by hydrocarbons in the oxygen excess. The optimum pillaring species were nanorods comprised of several Zr₄ tetramers stacked with relative 45° rotation. Basic structural features of the tetramers were preserved in zirconia nanoparticles fixed between alumosilicate layers in pillared clays. In calcined samples, those nanoparticles contained only bridging hydroxyls and/or oxygen anions responsible for bonding within pillars and between pillars and clay sheets. Low Zr-O coordination numbers in zirconia nanoparticles correlated with their enhanced ability to coordinate O₂^- radicals involved in deep oxidation of hydrocarbons. Much stronger interaction of supported Cu cations and Pt atoms with zirconia nanopillars as compared with bulk zirconia supports was reflected in a higher bonding strength and a low reactivity of oxygen bound with those active components. However, for combined Pt/CuO_x active component fixed at pillars, pronounced interaction between the metal and oxide constituents played a positive role ensuring a high activity and N₂ selectivity in the reactions of NO_x selective reduction by hydrocarbons (propane, propylene, and decane) in the excess of oxygen, especially at a high oxygen content and humidity.