NATURE OF THE ACTIVE-SITE FOR PROPENE HYDROGENATION ON CROX/ZRO2 CATALYSTS

The catalytic activity of CrOx/ZrO2 for propene hydrogenation has been investigated at 195 K in a flow apparatus. Samples containing 0.05 to 0.8% chromium by weight (0.1 to 1.9 Cr atoms nm-2) were prepared by the equilibrium adsorption method. After heating in O2 at 773 K (average oxidation number of Cr, nBAR = 5.5), samples were reduced to a controlled extent with CO at 393 to 623 K. The activity at 195 K of partially reduced samples (nBAR = 4.4) is low, but it increases by nearly two orders of magnitude when nBAR is further decreased by 0.2 units. Thereafter the activity changes only slightly with increasing reduction (down to nBAR = 2.5). The concentration of the Ce species observed by ESR after heating the samples with O2 was monitored after each reduction treatment, and it was found to decrease by 17% in the same reduction region in which the catalytic activity increases sharply. The average turnover frequencies per total chromium (N(Cr)/molecules s-1 atom-1) of CrOx/ZrO2 catalysts reduced to the same extent (nBAR = 2.5) at 623 K differ by a factor 2.6, in spite of the large variation (factor of 20) of Cr content. The Cr(III)(NO)2 complex formed on the surface of reduced CrOx/ZrO2 by adsorption of NO at room temperature was reacted with CO at room temperature. By IR spectroscopy, it was found that a fraction of the Cr(III)(NO)2 species leads to mixed ligand complexes, Cr(III)(NO)2CO. Relying on IR, ESR and catalytic results, it is inferred that active sites for the low temperature hydrogenation of propene on reduced' CrOx/ZrO2 catalysts are surface mononuclear Cr(III) species with three coordinative vacancies.