Abstract
Whereas the synthesis principles of supported metal catalysts are well documented in the open literature, impregnation protocols on shaped bodies represent sensitive industrial know-how and are, therefore, rarely found. We investigated various synthesis parameters for both wetness (WI) and dry (DI) impregnations to prepare Pd/γ–Al2O3 alumina beads. Two kinds of catalysts were achieved: homogeneously dispersed catalysts with no metal gradient across the beads and eggshell catalysts. A combination of optical images, Castaing microprobe analysis, elemental analysis, and TEM made it possible to discriminate between catalysts according to their metal loading, location across the bead diameter, and metal dispersion. Regardless of the macropore structure of the alumina beads, we found that volatile solvents (acetone) were preferred for preparing homogeneous catalysts by WI, whereas the use of a viscous aqueous solution (water/glycerol) in DI resulted in an eggshell-type catalyst. The atomic layer deposition (ALD) method was also investigated as a physical vapor phase deposition method for preparing eggshell catalysts. Representative-shaped catalysts were tested for CO oxidation as a model reaction in order to highlight the differences between catalysts with a homogeneous metal distribution (no metal gradient) and eggshell-type.
Highlights
The size and shape of catalyst carriers are a trade-off between the desire to minimize pore diffusion effects in the catalyst bodies while the pressure drop across the reactor
The second support, obtained from Norpro, had interconnected, randomly dispersed macropores, and will be referred to as “RP.” Both supports were impregnated by 0.2 wt.% Pd composing the catalysts, following the impregnation protocols investigated
In many cases, impregnation protocols led to severe heterogeneity in metal loading location and dispersion for shaped catalysts
Summary
The size and shape of catalyst carriers are a trade-off between the desire to minimize pore diffusion effects in the catalyst bodies while the pressure drop across the reactor. It is well known that it is important to control the impregnation profile for pre-shaped supports when preparing commercial catalysts [1]. The location of the metal throughout the support is a key parameter in determining catalyst effectiveness, i.e., the portion of the catalyst that is working under reaction [2]. For reactions with high intrinsic activities, a gradient of metal concentration will be desired, as in an eggshell configuration, in order to reduce the content of unused metal, and reduce the price of the catalyst. For “slow” reactions, a homogeneous location of the metal particles in the support body will be preferred [3]
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