Mass-Transfer Characterization of Metallic Foams as Supports for Structured Catalysts Leonardo Giani, Gianpiero Groppi, and Enrico Tronconi* Centro di Eccellenza per l’Ingegneria dei Materiali e delle Superfici Nanostrutturate, Dipartimento di Chimica, Materiali ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy Open-celled metal foams have been characterized as supports for structured catalysts, considering their utilization in gas-solid catalytic processes with short contact times and high reaction rates, typically controlled by gas-solid diffusional mass transport. Examples of such processes are found in the field of environmental catalysis, including, e.g., catalytic combustion, selective catalytic reduction (SCR-DeNOx), and automotive exhaust gas after treatment. In this work, foams with different cell sizes were coated with a thin layer of palladium-alumina and tested in a 9-mm inner diameter tubular reactor by performing the catalytic oxidation of CO at empty tube velocities in the range of 0.8-2.6 m/s. The coated foams exhibited sufficient catalytic activity to achieve mass-transfer-limited operation in the temperature range of 300-450 °C. Under such conditions, mass-transfer coefficients were determined according to a simple one-dimensional model of the test reactor. Adopting the average diameter of the foam struts as the characteristic length, we obtained a dimensionless correlation for the estimation of mass-transfer coefficients, which correlates all the data: it closely resembles semitheoretical literature correlations for heat transfer in flow across banks of tubes at low Reynolds numbers. Pressure drop measure- ments across foam samples were also collected for air velocities in the range of 1-16 m/s. The performances of foams, packed beds of pellets, and honeycomb monoliths as catalyst supports were compared on the basis of a dimensionless merit index, which accounts for the tradeoff between pressure drop and mass-transfer properties. Foams are largely superior to packed beds, because of their high voidage, but perform slightly worse than honeycomb monoliths. On the other hand, foams can afford marked reductions of reactor volume and weight, with respect to honeycombs, in fast, diffusion-controlled processes where pressure drop is of minor concern. 1. Introduction Open-celled foams are three-dimensional (3D) cellular materials made of interconnected solid struts, forming a network. 1 The unit cell in a foam resembles a polyhedron with pentagonal or hexagonal faces that limit a spherical-like inner space. Each cell, defined by the hollow volume of the polyhedron, constitutes a pore. The cell size is commonly expressed in terms of pores per linear inch (PPI): the overall range of variation in cell size goes from 5 PPI to 100 PPI. Typical porosity values range from 80% to 97%. 2 Many properties make foams attractive for use as catalyst supports, because their low density and high mechanical strength permit the design of light, stiff components. When loading a fixed-bed reactor with a foam cartridge rather than with packed particles, the high foam porosity would result in much lower pressure drops, 3 whereas the use of preformed structures can simplify loading and unloading operations. Further- more, tortuous flow paths through the porous matrix are expected to enhance gas/solid heat- and mass- transfer rates, and high surface-to-volume ratios would yield high activity per unit reactor volume. Examples of applications where metallic or ceramic foams have been used or proposed for use in heteroge- neous catalysis include the catalytic combustion of methane (CH 4 ), 4 the production of hydrogen gas (H 2 ) via water-gas shift reaction, 5 the purification of exhaust gases from automotive engines, 6 the oxidation of am- monia for the production of nitric acid, 7 the partial oxidation of methanol, 8 the deep oxidation of hydrocar- bons, 9 carbon dioxide reforming, 10 and the Fischer- Tropsch synthesis. 11 With respect to more-common ceramic reticulated materials, the use of metal foams is expected to mini- mize the occurrence of hot spots in the catalyst when highly exothermic reactions are performed, while avoid- ing mechanical-strength and thermal-shock limitations. Moreover, the spongelike properties of these supports allow convenient sealing in a reaction chamber via mechanical contact, while a closely matched thermal expansion between the metal foam and the housing chamber can be used to minimize gas channelling around the porous support at higher reaction temper- atures. 12 In this work, we focus on the application of metal foams to gas-solid catalytic processes with short contact times and high reaction rates, which are typically controlled by diffusional limitations. Many examples of such processes can be found in the field of environmen- tal catalysis, including, e.g., catalytic combustion, selec- tive catalytic reduction of NOx by NH 3 (the SCR-DeNOx process), and automotive exhaust gas after-treatment. Our goal is to estimate mass-transfer coefficients in foams of different cell sizes, and to derive a generalized engineering correlation for their prediction. * To whom correspondence should be addressed. Tel.: ++39- 02-2399 3264. Fax: ++39-02-7063 8173. E-mail: enrico.tronconi@polimi.it. 4993 Ind. Eng. Chem. Res. 2005, 44, 4993-5002 10.1021/ie0490886 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/09/2005