Washcoating of γ-alumina on stainless steel microchannels
Nageswara Rao Peela‡ and Deepak Kunzru*
Department of Chemical Engineering, Indian Institute of Technology Kanpur
Kanpur -208016, India
Abstract
In recent years, microreactors have emerged as efficient tools for process intensification. The
main advantage of microreactors is their high surface to volume ratios resulting in high heat and
mass transfer coefficients. Moreover, because of the smaller channel dimensions, the reactor
behavior approaches that of an ideal plug flow reactor, which can lead to selective production of
a desired product. Wall-coated microchannel reactors are commonly used for studying
heterogeneous catalytic reactions. However, success of this kind of reactor depends on the
nature of catalyst layer formed on the microchannels of the substrate and an adherent, uniform
washcoat layer of support or catalyst is desirable. Even though the advantages of using
microreactors have been reported for several reactions, the effect of slurry properties on the
characteristics of the washcoat layer has not been reported. The important properties that are
desired for good washcoat include adherence, uniformity, loading and reproducibility. These
features depend on the solid content of the slurry, slurry pH and viscosity, and more
importantly on primer coating. The primer coating, consisting of a sol and/or viscosity modifier
is necessary to enhance the adherence of the alumina coating. Moreover, binders, viscosity
modifiers and dispersants added in the slurry, can have a significant effect on the washcoat
properties.
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, Dispersal with or without polyvinyl alcohol (PVA) was used. Without PVA, the sol
settled in the micro-channels 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, particle size, solids
loading were optimized to obtain well-adhered, uniform washcoat on stainless steel substrate.
PVA was added in the slurry to enhance the viscosity, results in better adherence. An adverse
effect of adding PVA was a reduction in the surface area. To reduce the amount of viscosity
modifier (PVA) and thereby its adverse effect, binders (colloidal alumina and Disperal) were
evaluated. In conclusion, uniform well-adhered washcoats were obtained with particle size less
than 3 µm, and slurry composition: 20% γ-alumina, 2% PVA, 4% colloidal alumina and
remaining water.
Correspondence:
*Deepak Kunzru: email: dkunzru@iitk.ac.in
‡Nageswara
Rao Peela : email: pnrao@iitk.ac.in
Tel: +91-512-2597193
Fax: +91-512-2590104