Nature of the active site for propene hydrogenation on CrO x/ZrO 2 catalysts

Abstract

The catalytic activity of CrO x /ZrO 2 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 O 2 at 773 K (average oxidation number of Cr, n̄=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 ( n̄=4.4) is low, but it increases by nearly two orders of magnitude when n̄ is further decreased by 0.2 units. Thereafter the activity changes only slightly with increasing reduction (down to n̄=2.5). The concentration of the Cr v species observed by ESR after heating the samples with O 2 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 CrO x /ZrO 2 catalysts reduced to the same extent ( n̄=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 CrO x /ZrO 2 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 CrO x /ZrO 2 catalysts are surface mononuclear Cr III, species with three coordinative vacancies.