Model-based control of iron- and copper oxide particle distributions in porous γ-Al2O3 microspheres through careful tuning of the interactions during impregnation

Up until now, the key phenomena steering metal particle distributions within macroscopically shaped porous supports has not yet been fundamentally understood. In this work, the embedment, upon incipient wetness impregnation, of iron- and copper oxide particles into porous γ-Al2O3 microspheres has been carefully studied. In both cases, specific electrostatic interactions between the dissolved metal ions and accessible support surface were found to be paramount in determining the macroscopic metal oxide distribution in the final material. For copper, these interactions resulted in a homogeneous macroscopic distribution, under the form of a coating directly on the alumina surface. This was contrasted with the more complex iron/γ-Al2O3 system, where competition between dissolved iron species and other dissolved species occurred, which drastically altered the obtained macroscopic and microscopic distribution. Modifying the impregnation procedure, either by altering the pH or ion concentration of the precursor solution, by pretreatment of the support material or by the use of a complexing agent, resulted in particles sizes ranging from 5 nm up to a few μm, and macroscopic distributions varying from homogeneous to egg-white or egg-yolk, all within the context of simple and scalable impregnation-based syntheses. Based on the systematic study of the effects of different impregnation solutions, competitive adsorption of both positively charged iron species and protons on the support surface, and shielding interactions by the negative counter-ions in solution could be identified as main determining factors for the macroscopic distribution. These insights have been validated and quantified within a model, which allows accurate predictions of the metal distribution in porous γ-Al2O3 microspheres for a broad range of synthesis conditions. This model or simulation tool could prove to be a useful tool to guide future catalyst design. •Metal deposition into porous alumina microspheres can lead to various distributions.•This is steered by electrostatic interactions between ionic species and the surface.•Metal distributions can be tuned, e.g. by adjusting the pH or support pretreatment.•A novel simulation tool allows accurate predictions and can guide catalyst design.