PREPARATION AND CHARACTERIZATION OF COPPER THORIUM-OXIDE CATALYSTS .2. ESR STUDY OF COPPER(II) ION-PAIRS AND CORRELATION WITH THE SELECTIVE HYDROGENATION OF ISOPRENE

CuThO oxides prepared with an atomic ratio Cu/Th > 0.01 were extensively studied by electron spin resonance (ESR) and X-ray photoelectron spectroscopies (XPS). The XPS analysis of these solids showed that the thoria surface is enriched with copper. The X-ray diffraction patterns for Cu/Th > 0.25 revealed two distinct phase systems, ThO2 and CuO. ESR studies evidenced two signals of monomeric Cu2+ species (M1 and M2) and a well-resolved D signal (Δms = 1) of Cu2+ ion pairs, accompanied by a weak signal at half magnetic field intensity (Δms, = 2). From the Δms = 2/Δms = 1 intensities ratio and from the fine splitting observed for the D signal, the distance between the Cu2+ ions in the pair have been calculated. These copper-thorium oxide catalysts are active for selective hydrogenation of isoprene at 373 K after reduction of the oxide phase at 523 K. A correlation between this activity and the relative intensities of M1, M2, and D signals was established. © 1990, American Chemical Society. All rights reserved.