Investigation of the physico-chemical state and aggregation mechanism of surface Cr species on a Phillips CrOx/SiO2 catalyst by XPS and EPMA

X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EP MA) were jointly applied to achieve some basic understandings of the physic o-chemical state and aggregation mechanism of surface Cr species on an indu strial Phillips CrOx/SiO2 catalyst calcined in dry air at 800 degreesC for 20h with 0.4 Cr/nm(2). The XPS results showed the coexistence of surface-st abilized hexavalent chromate species (70.4% expressed as Cr(VI)O-x,O-surf) and trivalent chromium oxide (29.6%) on the catalyst. The latter derived fr om 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,O-surf). T he EPMA map and line curves of the Cr distribution state on the catalyst re vealed a small amount of the trivalent chromium oxide existed as a few aggr egates in sizes of 200-300 nm on the surface of each particle, which were s upposed to be crystallized aggregates of Cr2O3. Consequently, the calcinati on-induced reduction of Cr(VI)(x,surf) to Cr(III)O-x,O-surf species and the formation of Cr2O3 microcrystals on the Phillips catalyst with relatively low Cr loading were specifically confirmed. The variation of distribution a nd oxidation states of surface Cr species for the catalyst after being furt her calcined at 800 degreesC for 2 h in the presence of moisture had also b een 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 th e transformation of all the Cr(III)O-x,O-surf and one-seventh of Cr(VI)(x,s urf) species into aggregates of Cr2O3 at high temperature, whereas oxidizin gr and inert atmospheres made no obvious difference. Finally, the formation mechanism of aggregates of Cr2O3 induced by moisture through cleavage of C r(III)O-x,O-surf species during the calcination had been speculated conside ring of the indispensable evolution of traces of moisture from the simultan eous dehydroxylation of residual hydroxyl groups on silica surface. The adv antages of the combination of XPS and EPMA techniques for the basic investi gation of Phillips catalysts were substantiated by the results obtained in this study. (C) 2001 Elsevier Science B.V. All rights reserved.