Investigation of the physico-chemical state and aggregation mechanism of surface Cr species on a Phillips CrO x/SiO 2 catalyst by XPS and EPMA

Abstract

X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA) were jointly applied to achieve some basic understandings of the physico-chemical state and aggregation mechanism of surface Cr species on an industrial Phillips CrO x /SiO 2 catalyst calcined in dry air at 800°C for 20 h with 0.4 Cr/nm 2. The XPS results showed the coexistence of surface-stabilized hexavalent chromate species (70.4% expressed as Cr(VI)O x, surf ) and trivalent chromium oxide (29.6%) on the catalyst. The latter derived from calcination-induced reduction of the Cr(VI)O x, surf species is mostly chemically-bonded to the silica surface (expressed as Cr(III)O x, surf ). The EPMA map and line curves of the Cr distribution state on the catalyst revealed a small amount of the trivalent chromium oxide existed as a few aggregates in sizes of 200–300 nm on the surface of each particle, which were supposed to be crystallized aggregates of Cr 2O 3. Consequently, the calcination-induced reduction of Cr(VI)O x, surf to Cr(III)O x, surf species and the formation of Cr 2O 3 microcrystals on the Phillips catalyst with relatively low Cr loading were specifically confirmed. The variation of distribution and oxidation states of surface Cr species for the catalyst after being further calcined at 800°C for 2 h in the presence of moisture had also been studied in terms of the role of moisture and effect of atmosphere (pure air or N 2). It was found that the purposely introduced moisture induced the transformation of all the Cr(III)O x, surf and one-seventh of Cr(VI)O x, surf species into aggregates of Cr 2O 3 at high temperature, whereas oxidizing and inert atmospheres made no obvious difference. Finally, the formation mechanism of aggregates of Cr 2O 3 induced by moisture through cleavage of Cr(III)O x, surf species during the calcination had been speculated considering of the indispensable evolution of traces of moisture from the simultaneous dehydroxylation of residual hydroxyl groups on silica surface. The advantages of the combination of XPS and EPMA techniques for the basic investigation of Phillips catalysts were substantiated by the results obtained in this study.