Washcoating method for Pd/gamma-Al2O3 deposition on metallic foams

A method for coating open celled metal foams with a thin layer of Pd–Al2O3 was developed. The method makes use of a sol–gel of pseudobohemite as a precursor of g-Al2O3 to fill the porous structure by percolation whilst the excess of material is flushed away with an air jet. The influence of solid content, acid content and ageing on the sol–gel rheological behaviour was studied to find a sol–gel dispersion with an appropriate viscosity whose deposition resulted in a 20 mm thick coating layer. Foam samples of different nominal porosities (10, 20 and 40 PPI) were coated with this method and activated with palladium using a wet impregnation procedure with loadings of 3% (w/w) palladium on the alumina. As an alternative approach, dry impregnation of g-Al2O3 with palladium was performed and a slurry was prepared adding water and nitric acid. The slurry was deposited by percolation through the foam structure. The coating quality was evaluated with adhesion tests and the activity of coated foams was measured by performing catalytic oxidation of CO in a 9 mm i.d. tubular reactor. Both methods produced highly active foams, resulting in light off temperatures between 175 and 250 8C depending on CO concentration. Moreover, the mass transfer controlled regime was achieved in all runs independently of the coating method. Nevertheless, conversions reached when the sol–gel dispersion was deposited and activated via wet impregnation were higher than the ones obtained with the coating method based on slurry deposition of active powders, indicating that the sol–gel deposition resulted in more uniform coating.