Effect of the nickel precursor on the impregnation and drying of γ-Al2O3 catalyst bodies: a UV-vis and IR micro-spectroscopic study

Abstract

The elemental preparation steps of impregnation and drying of Ni/g-Al2O3 catalyst bodies have been studied by combining UV-vis and IR microspectroscopy. The influence of the number of chelating ligands in [Ni(en)x(H2O)6-2x]2+ precursor complexes (with en ) ethylenediamine and x ) 0-3) has been investigated. UV-vis measurements after impregnation showed that, regardless of the en:Ni2+ ratio in the precursor solution, Ni2+ 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 Ni2+ species upon first contact with the lowest en:Ni2+ 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 Ni2+ and the support, when water was initially part of the first coordination sphere of Ni2+, which was supported with the results obtained after drying. For dried samples, UV-vis showed a gradient of [Ni(en)x(H2O)6-2x]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(H2O)4]2+ were the starting complexes. IR microspectroscopy confirmed these en radial profiles, while EDX measurements showed that an Ni2+ egg-shell distribution goes hand-in-hand with an egg-shell distribution of en. The number of en ligands determined the interactions of Ni2+ with the support after impregnation and controlled the redistribution of metal-ion species during drying. Moreover, when redistribution of Ni2+ occurred during drying, its transport toward the outer rim of the extrudates came about together with changes of the local en:Ni2+ ratio.