Mass-transfer characterization of metallic foams as supports for structured catalysts

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 degrees 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 measurements 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.