Coating of Open Cell Foams Heng Zhang, † Wieslaw J. Suszynski, † Kumar Varoon Agrawal, † Michael Tsapatsis, † Saleh Al Hashimi, ‡ and Lorraine F. Francis †, * † Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States ‡ Department of Chemical Engineering, Petroleum Institute, Abu Dhabi, United Arab Emirates * S Supporting Information ABSTRACT: The interior surfaces of three-dimensional open cell foams were coated by a combination of dip coating and spin coating. Glycerol/water solutions were used as model Newtonian liquids, and the coating processes were studied on open cell carbon foams with 10 or 30 pores per inch (PPI). The amount of liquid retained in the foam structures after dip coating increased with withdrawal speed and coating viscosity, as expected from the conventional understanding of dip coating onto nonporous substrates such as flat plates and rods. However, the liquid retention and hence average coating thickness increased with surface tension, a result counter to the observation with coating onto nonporous substrates. Pockets of liquid were observed after dip coating and results with coatings of alumina suspension showed that after drying, the trapped liquid can block pore windows. Spinning the foams after dip coating resulted in uniform liquid distribution and uniform coatings. Foams were placed in a special apparatus and rotated using a commercial spin coater. The liquid layer thickness decreased with spinning time and rotational speed, and increased with the liquid viscosity, results consistent with spin coating theory. The coating thickness after spinning was not affected by the initial dip coating procedure. The dip and spin process was also used to create γ-alumina and zeolite coatings, which are of interest for catalysis applications. ■ INTRODUCTION Solid foams, manufactured from metal, ceramic, polymer, or carbon have a variety of applications owing to their high surface area, low relative density, and complex interfacial geometries. Solid foams have been used as heat exchangers, energy absorbers, high temperature filters, electron emitters, fuel cell electrodes, and catalytic supports. 1-7 There are two types of solid foams: closed celled foam, in which the cells are isolated, and open celled foam, in which the cells are open and interconnected. 8,9 The properties of open cell foams can be improved and altered by adding a coating onto the internal foam surfaces. 3,7,10-12 Solid foams with interconnected porosity are good candidates for structured catalytic supports. 4,5,13 The foam structure leads to a low pressure drop and turbulent flow, which increases interaction between the catalyst and reactant. For conventional structured catalytic supports, such as honeycombs and monoliths, a mesoporous layer is deposited to further increase the surface area before the active metal catalysts are added. 14-20 This strategy can also be used for solid foam catalytic supports. Chemical vapor deposition (CVD) is one choice to create a coating on the internal foam surfaces. For example, several reports 21,12,2 have detailed the synthesis of carbon nanofibers onto the surfaces of carbon foams. Additionally, Kobashi et al. 3 reported CVD deposition of a diamond coating on carbon foam. In these CVD methods, a two-step process was used. In first step, a catalyst or seed layer was coated on the foam by a solution phase method, and in second step, the carbon nanofiber or diamond coating was deposited onto the foam by chemical vapor deposition. Although the CVD method shows good coating quality, it is relatively expensive and time- consuming. Another disadvantage of CVD is that only a limited number of materials can be deposited due to the gas phase reaction. Another method to coat open celled foam is by liquid phase deposition. Scheffler and co-workers 6 and Bonaccorsi and co- workers 22 reported coating a thin zeolite layer onto foams by direct hydrothermal synthesis. However, this process is complicated, and the final coating thickness is also limited. An easier and faster liquid phase coating alternative is dip coating. This method has been widely used in conventional catalyst industry to create a “washcoat” onto monolith and foam structures. The washcoat is usually made by immersing and removing the foam or monolith from a sol-gel solution or slurry of particles; sometimes vacuum is used to assist filling of the pores or channels. 11 To avoid blockage after drying, the excess liquid trapped is usually removed by blowing com- pressed air 4,5,13,14 or rotating. 23 The development of protocols for these processes has been empirical. In this paper, a method to coat open cell foam by a combination of dip coating and spin coating is developed and studied systematically. The emphasis of the research is on the coating process and understanding the experimental parameters that influence thickness and uniformity. The process is then demonstrated with two coating systems that have catalytic applications: γ-alumina and zeolite. Received: January 30, 2012 Revised: June 13, 2012 Accepted: June 19, 2012 Published: June 19, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 9250 dx.doi.org/10.1021/ie300266p | Ind. Eng. Chem. Res. 2012, 51, 9250-9259