Envisaging the Physicochemical Processes during the Preparation of Supported Catalysts: Raman Microscopy on the Impregnation of Mo onto Al2O3 Extrudates

Abstract

Raman microscopy has been applied to study the preparation of shaped Mo/Al₂O₃ catalysts. The speciation of different Mo complexes over γ-Al₂O₃ support bodies was followed in time after pore volume impregnation with aqueous solutions containing different Mo complexes. The addition of NO₃^- to the impregnation solutions allows for a quantitative Raman analysis of the distribution of different complexes over the catalyst bodies as this ion can be used as an internal standard. After impregnation with an acidic ammonium heptamolybdate (AHM) solution, the strong interaction between Mo₇O₂₄^6- and Al₂O₃ results in slow transport of this complex through the support and extensive formation of Al(OH)₆Mo₆O₁₈^3- near the outer surface of the support bodies. This may be prevented by decreasing the interaction between Mo and Al₂O₃. In this way, transport is facilitated and a homogeneous distribution of Mo is obtained on a reasonable time scale. A decrease in interaction between Mo and Al₂O₃ can be achieved by using alkaline impregnation solutions or by the addition of complexing agents, such as citrate and phosphate, to the impregnation solution. In general, time-resolved in situ Raman microscopy can be a valuable tool to study the physicochemical processes during the preparation of supported catalysts.