Alloys can have a number of structures, such as ordered, fully mixed, segregated, shell-core, or near surface. The alloy structure in turn determines the electronic and catalytic properties of the alloy. Iridium-based alloys have shown promise in a number of catalytic applications, including ethanol oxidation[1, 2], oxygen reduction[3], and hydrogen generation[4]. Iridium alloys possess high melting point, high corrosion and oxidation tolerance, as well as excellent mechanical properties. Our goal was to provide models and predictions of realistic iridium alloys, including their stability and electronic properties at various temperatures and synthesis conditions. Identifying how structure affects stability and electronic properties is key to predicting catalyst performance. We used density functional theory (DFT) calculations to model these alloys. Statistical methods, such as the cluster expansion approach, were also combined with DFT to model a large number of possible bimetallic configurations. We studied iridium-platinum alloys as a prototypical bimetallic system. We have identified viable structures of these alloys at various composition levels (IrxPt1-x; 0 < x < 1), and shown how these metals prefer to segregate at low temperature. While identification of the most thermodynamically stable alloy 0 K structures can be straight forward, real working catalysts often have not reached equilibrium. Monte Carlo simulations at finite temperature (600 K+) indicate that disordered iridium-platinum structures occur with larger degree of mixing. We have examined how the electronic structure (e.g. density of states) changes at various compositions and structures (e.g. segregated versus mixed), and potential implications of these changes on catalysis. We have also modeled a large number of other iridium alloys with various metals, including Pd, Rh, Sn, Cr, Fe, etc. This systematic study of iridium alloys provides details on which bimetallic systems are stable, their expected electronic properties, and how to realistically model these catalysts from various synthesis conditions. Our results show how theoretical methods can advance alloy development and lead to potential new catalysts.
1. Cao, L.; Sun, G.; Li, H.; Xin, Q., Carbon-supported IrSn catalysts for direct ethanol fuel cell. Fuel Cells Bulletin 2007, 2007 (11), 12-16.
2. Du, W.; Wang, Q.; Saxner, D.; Deskins, N. A.; Su, D.; Krzanowski, J. E.; Frenkel, A. I.; Teng, X., Highly active iridium/iridium–tin/tin oxide heterogeneous nanoparticles as alternative electrocatalysts for the ethanol oxidation reaction. J Am Chem Soc 2011, 133 (38), 15172-15183.
3. Ioroi, T.; Yasuda, K., Platinum-iridium alloys as oxygen reduction electrocatalysts for polymer electrolyte fuel cells. Journal of the Electrochemical Society 2005, 152 (10), A1917-A1924.
4. Singh, S. K.; Xu, Q., Bimetallic nickel-iridium nanocatalysts for hydrogen generation by decomposition of hydrous hydrazine. Chemical Communications 2010, 46 (35), 6545-6547.
