On the microdistributions of Cr-ion complexes within mm-sized γ-Al2O3 catalyst bodies upon impregnation as studied by UV/Vis and Raman microspectroscopy

The incipient wetness impregnation steps of 3 mm cylindrical gamma-Al2O3 pellets with CrIII-nitrate and CrVI-oxide as precursors was investigated by using UV/Vis and Raman microspectroscopy. The speciation and microdistributions of metal-ion complexes in the pellets were followed in time after impregnation. Measurements on the catalyst bodies were performed by applying a scan-line on the surface of bisected pellets. The microdistributions of the Cr-ion complexes were altered by varying the pH, the ratio of the CrIII-nitrate and CrVI-oxide and by including either citric acid or phosphoric acid to the impregnation solution. Three factors were found to be most influential: i) Hydrolysis of CrIII complexes to yield solid Cr(OH)3 near the exterior of the pellets, ii) electrostatic interaction between the support and the respective CrIII and CrVI complexes, and iii) competitive adsorption of the additives. The opposite charge of the CrIII and CrVI complexes was exploited to obtain differences in the overall Cr-ion complex microdistributions, for example, egg-shell, egg-yolk, egg-white, and uniform type of distributions.