Foam, fleece and honeycomb: catalytically active coatings from colloidally prepared nanoparticles

Abstract

Although monolithic catalysts offer distinct advantages such as low pressure drops and easy catalyst separation compared to tube bundle reactors filled with pellets and are for instance widely used for automobile exhaust treatment, the coating of the monolithic structures with the catalytically active phase is often complex and recipes need to be established empirically. In contrast to traditional methods, such as dip-coating or impregnation of the monolith, where the active nanoparticles are formed during the preparation on an oxidic washcoat usually used and deposited simultaneously or in a previous step, we demonstrate in this study that alternatively preformed colloidally synthesized nanoparticles can be employed to obtain homogeneous coatings with or without a washcoat. In this way, one can take advantage of the far-reaching possibilities of colloidal methods to control the structure and size of the nanoparticles and also to tune and optimize their binding to the monolithic surface. For cases where beneficial metal–support interactions between the nanoparticles and a washcoat improve the catalytic properties we demonstrate that colloidally prepared nanoparticles can be directly mixed with a washcoat slurry and successfully deposited on monolithic supports. Turnover frequencies comparable to the corresponding powder catalysts could be reached. In a second approach, we present here a facile method to directly coat three different monolithic supports (cordierite honeycomb, Al₂O₃ foam and Nickel fleece) with preformed Pt nanoparticles in the presence and absence of organic ligands. In order to realize high metal loadings, the beneficial influence of a ligand “double-layer” (coating of nanoparticles and the support by organic ligands) enhancing the adhesion between the Pt nanoparticles and the underlying monolithic support will be discussed. In the case of the metallic Ni substrate, this approach furthermore allows to circumvent alloy formation and nanoparticle diffusion into the metallic substrate. This can greatly increase long-term stability of systems coated directly onto metallic substrates without an additional oxidic washcoat.