Effect of the Nickel Precursor on the Impregnation and Drying of γ-Al₂O₃ Catalyst Bodies:  A UV−vis and IR Microspectroscopic Study

The elemental preparation steps of impregnation and drying of Ni/γ-Al 2 O 3 catalyst bodies have been studied by combining UV−vis and IR microspectroscopy. The influence of the number of chelating ligands in [Ni(en) x (H 2 O) 6-2 x ] 2+ precursor complexes (with en = ethylenediamine and x = 0−3) has been investigated. UV−vis measurements after impregnation showed that, regardless of the en:Ni 2+ ratio in the precursor solution, Ni 2+ was already detected in the core of the pellets 5 min after impregnation. The alumina support did bring about, however, gradients in the nature of the Ni 2+ species upon first contact with the lowest en:Ni 2+ ratio solutions, but these gradients disappeared after 30−60 min of impregnation. After 60 min, UV−vis seemed to indicate Ni−O−Al interactions between Ni 2+ and the support, when water was initially part of the first coordination sphere of Ni 2+, which was supported with the results obtained after drying. For dried samples, UV−vis showed a gradient of [Ni(en) x (H 2 O) 6-2 x ] 2+ inside the pellets, with an en-poor region in the core and an en-rich region in the edges of the catalyst bodies, when [Ni(en) 3 ] 2+ or [Ni(en) 2 (H 2 O) 4 ] 2+ were the starting complexes. IR microspectroscopy confirmed these en radial profiles, while EDX measurements showed that an Ni 2+ egg-shell distribution goes hand-in-hand with an egg-shell distribution of en. The number of en ligands determined the interactions of Ni 2+ with the support after impregnation and controlled the redistribution of metal-ion species during drying. Moreover, when redistribution of Ni 2+ occurred during drying, its transport toward the outer rim of the extrudates came about together with changes of the local en:Ni 2+ ratio.