(668b) Rational Design of Hierarchically Structured Porous Catalysts and Its Application to Autothermal Reforming for Fuel Cells

Porous catalysts are widely used in many applications, ranging from petroleum refining to fuel cells and emission control. Nanoporous catalysts like zeolites often have an extremely large internal surface area (e.g., 1000m2/g), which is beneficial because catalytic reactions occur on the surface. However, their small pore size leads to slow molecular transport and pore blocking, limiting the efficient use of the catalytic material. This indicates that, apart from the nanopores where reactions actually occur, a “distribution” network of macropores is needed for molecules to quickly move in and out of the catalyst. Our recent work showed that, for a single, isothermal reaction, this distribution network is the optimum for a given catalytic reaction, when the nanoporous walls (i.e., the nanoporous catalytic material between two neighbouring macropores) are sufficiently thin so that diffusion limitations vanish inside them (Wang et al., 2007; Johannessen et al., 2007; Wang and Coppens, 2008). It was also found that the performance of the optimal catalyst is dictated by the generalized distributor Thiele modulus, which is defined in a way analogous to the generalized Thiele modulus, but using the molecular diffusivity in the macropores, rather than the effective diffusivity in the nanopores.

In this study, we extended these theoretical insights to design a catalyst for autothermal reforming (i.e., non-isothermal reactions) for fuel cells. Fuel cells are important for clean generation of energy. However, the production, transportation and storage of hydrogen are big headaches for the use of fuel cells in commercial applications. One way to circumvent these difficulties is to produce hydrogen on-board through catalytic reforming of natural gas, methanol or other hydrocarbons. Autothermal reforming of natural gas is a promising option, because it combines an endothermic reaction, steam reforming, and an exothermic reaction, total/partial oxidation. Nevertheless, serious diffusion limitations exist for existing reformers because of very fast intrinsic kinetics of these reactions. We show that the performance of the autothermal reforming catalyst could be improved significantly, when macropores are introduced in an optimized way.

References

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2. Johannessen, E.; Wang, G.; Coppens, M.-O. Optimal distributor networks in porous catalyst pellets. I. molecular diffusion. Ind. Eng. Chem. Res. 2007, 46, 4245-4256.

3. Wang, G.; Coppens, M.-O. Calculation of the optimal macropore size in nanoporous catalysts and its application to deNOx catalysis. Ind. Eng. Chem. Res. 2008, in press.