Enhanced transport in Gas-Liquid-Solid catalytic reaction by structured wetting properties: nitrite hydrogenation

This work presents a new approach to improve mass transfer in and around catalyst particles in three-phase operation with micro-structured catalysts, containing hydrophilic and hydrophobic domains. Partially hydrophilic catalysts were prepared via physical mixing of hydrophobic perfluorinated octyltrichloro silane (FOTS)/γ-Al2O3 domains and hydrophilic Pd/γ-Al2O3 domains, resulting in manipulation of water wetting, both at the external surface and the pores inside the support particles. The modified catalysts were characterized with elemental analysis, XRF, N2 physisorption and light microscopy after selective dyeing hydrophobic and hydrophilic domains. The catalysts are tested for hydrogenation of nitrite in water, which is an extremely fast reaction whereas the product distribution (N2 versus NH4+) is also easily influenced by internal concentration gradients. Noticeably, the partially hydrophilic catalyst is more active and produces more ammonium compared to hydrophilic catalyst. This work demonstrates that this way of structuring the catalyst enables influencing the internal concentration gradients for aqueous systems. For the case of nitrite hydrogenation, we show that structured catalysts achieve the same rate per gram Pd at lower hydrogen pressure compared to classical hydrophilic catalysts. This results in formation of less ammonia, which is of practical importance for cleaning of drinking water.