The nature of surface chromium species on CrO x/ZrO 2 catalysts

Abstract

The nature and stability of chromium species on the surface of CrO x / ZrO 2 catalysts (Cr content, 0.05 – 5 wt.%) have been investigated by means of ESR, IR and XPS spectroscopies. On samples heated in oxygen at 773 K or at 923 K, chromium is present on the surface as Cr V (ESR) and Cr VI (IR and XPS). Experiments with the 53Cr isotope show that the species observed by ESR spectroscopy is a surface mononuclear chromyl complex in a square-pyramidal configuration. Surface chromates are demonstrated by IR spectroscopy. In more concentrated samples, polynuclear chromium species, and/or Cr VI and Cr V polychromates, are observed in addition to chromates and isolated Cr V Upon reduction with CO or H 2 at increasing temperatures, Cr V is reduced to Cr III (ESR and IR) even at ca. 383 K. On further reduction at higher temperatures (up to 623 K), Cr VI is also reduced to Cr II. Specifically, samples reduced at 623 K contain Cr III and Cr II in nearly equal amounts. The catalytic pattern of the CrO x /ZrO 2 system, examined in the terms of the chromium species, allows us to conclude that isolated Cr III species, arising from the reduction of Cr V, are the active sites for H 2-D 2 equilibration and propene hydrogenation. The role of Cr II, either isolated or as dimers, is ruled out in the case of CrO x /ZrO 2 catalysts. The zirconia matrix stabilizes the Cr III active species in surface sites of high coordinative unsaturation.