Washcoating of γ-alumina on stainless steel microchannels

In the present study, the effect of primer and slurry composition on the washcoat characteristics of γ-alumina on stainless steel substrate has been investigated. Washcoating was done following a two-step procedure: primer coating followed by slurry coating. Washcoat was characterized by SEM, adherence test and BET surface area measurement. For coating of the primer, Disperal with or without polyvinyl alcohol (PVA) was used. Without PVA, the sol settled in the microchannels whereas at high concentrations of PVA, the sol was too viscous. The optimum primer composition was found to be 2% Disperal and 4% PVA. In the subsequent slurry coating step, various properties of the slurry such as pH, viscosity, solid loading were optimized to obtain well-adhered, uniform washcoat on stainless steel substrate. PVA was added to the slurry to increase the binding and also enhance the viscosity, resulting in better adherence. An adverse effect of adding PVA was a reduction in the surface area. To reduce the amount of PVA and thereby its adverse effect, other binders (colloidal alumina and Disperal) were evaluated. Uniform well-adhered washcoats were obtained with particle size of γ-alumina less than 3 μm, and slurry composition: 14 wt.% γ-alumina, 2 wt.% PVA, 6 wt.% colloidal alumina and remaining water. Finally, the best washcoat obtained in this study was impregnated with 2% Rh–5% Ni and tested in the steam reforming of ethanol. For comparison, runs were also taken on powdered catalysts in a packed bed reactor at identical conditions and the conversions obtained in both the reactors were similar.