268831 Ir-M Alloy Catalysts for Direct Ethanol Fuel Cells

Wednesday, October 31, 2012: 10:10 AM
317 (Convention Center )
N. A. Deskins, Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA, Julien Courtois, Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA, Xiaowei Teng, Dept. of Chem. Eng., University of New Hampshire , Durham, NH, Anatoly Frenkel, Department of Physics, Yeshiva University, New York, NY and Dong Su, Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY

   Ethanol is a promising fuel for direct ethanol fuel cells (DEFCs) and can be produced from a variety of sources, including biomass. A limitation of DEFCs is the anode catalyst for ethanol oxidation. A common anode catalyst is Pt, which is expensive and suffers from low efficiency towards complete oxidation. Complete oxidation produces 12 electrons per ethanol molecule, while partial oxidation may produce as few as 4 electrons per ethanol molecule. Replacement of Pt with new catalyst materials is therefore desirable and the goal of this work. During the oxidation process, a large number of product intermediates may form, which further complicates mechanistic analysis.

    Through the combined efforts of theory and experiment, we have synthesized and characterized several Ir-M alloy catalysts. We previously showed Ir-Sn nanoparticles to be active DEFC catalysts[1], as well as recently Ir-Ru. We determined the structure and composition of these alloys using a variety of experimental techniques such as high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), x-ray diffraction (XRD), and x-ray absorption spectroscopy (XAS). Our results show that the activity of Ir-Sn exceeds traditional Pt catalysts. Density functional theory (DFT) simulations were used to determine the stability of different core-shell particle structures for Ir-Sn. The DFT simulations indicate that Sn prefers shell, or surface, locations, in agreement with experiment. Calculations comparing the reactivity (both C-H and C-C bond scission reactions) of Ir, Ir-Sn, and Pt indicate the Ir-based materials to be more reactive towards ethanol. Similar theoretical and experimental results were found for Ir-Ru.

    Based on our success with Ir-Sn and Ir-Ru, we have also extended our DFT efforts to determine the surface structures of several other alloys involving Ir and other late-transition metals, such as late transition metals Pt, Pd, Rh, etc. We identified whether the alloyed metals prefer surface or bulk locations, in order to identify possible surface alloy structures.  The rate-determining step of ethanol oxidation typically involves C-C bond scission, for example of CHCO[2,3]. Using linear-scaling relationships[3,4] we determined activation energies over the Ir-M catalysts for C-C bond scission of CHCO. The DFT results provide data to screen a large number of potential alloy materials and thus guide experimental synthesis strategies. Our approach thus accelerates the development of new potential DEFC anode catalysts.

References

[1] W. Du, Q. Wang, D. Saxner, N. A. Deskins, S. Dong, J. E. Krzanowski, A. I. Frenkel, and X. Teng. “Highly active iridium/iridium-tin/tin oxide heterogeneous nanoparticles as alternative electrocatalysts for the ethanol oxidation reaction.” Journal of the American Chemical Society 133, 2011, 15172-83.

[2] R. Alcala, M. Mavrikakis, and J. A. Dumesic. “DFT studies for cleavage of C-C and C-O bonds in surface species derived from ethanol on Pt(111).” Journal of Catalysis 218, 2003, 178-190.

[3] P. Ferrin, D. Simonetti, S. Kandoi, E. Kunkes, J. A. Dumesic, J. K. Nørskov, and M. Mavrikakis. “Modeling ethanol decomposition on transition metals: a combined application of scaling and Brønsted-Evans-Polanyi relations.” Journal of the American Chemical Society 131, 2009, 5809-15.

[4] S. Wang, T. Burcin, J. Shen, G. Jones, L. C. Grabow, F. Studt, T. Bligaard, F. Abild-Pedersen, C. H. Christensen, and J. K. Nørskov. “Universal Brønsted-Evans-Polanyi Relations for C–C, C–O, C–N, N–O, N–N, and O–O Dissociation Reactions.” Catalysis Letters 141, 2010, 370-373.


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