Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A high-temperature polymorph of IrSn4 (BETA-IrSn4) was prepared from the elements by annealing tin-rich melts at temperatures between 800°C and 900°C and subsequent rapid quenching. The crystal structure of BETA-IrSn4 was determined from single-crystal X-ray data (tetragonal, space group I41/acd, Z=8, a=6.3096(6) ?, c=22.770(7) ?) and corresponds to that of MoSn4. In BETA-IrSn4, Ir atoms are arranged in layers centering Sn8 square antiprisms. This is in contrast to the trigonal low-temperature modification (ALPHA-IrSn4) where homogeneously distributed Ir atoms are coordinated in a bisdisphenoidal fashion by eight Sn atoms. The structure of BETA-IrSn4, however, is very similar to that of the highpressure modification of IrSn4 (HP-IrSn4), which adopts the PtSn4 structure. Structural stability of the IrSn4 polymorphs was investigated by first-principles calculations. It was found that ALPHA-IrSn4 represents the ground state of the system. At ambient pressure BETA-IrSn4 is 0.027 eV per formula unit (2.61 kJ/mol) higher in energy than ALPHA-IrSn4. At a pressure of about 5 GPa, our calculations reveal a phase transition ALPHA-IrSn4ARROWBETA-IrSn4; the latter remains the most stable polymorph up to pressures of at least 10 GPa. Thus, reported HP-IrSn4 would correspond to a high-pressure-high-temperature form of IrSn4. However, it is suggested that the structures of HP-IrSn4 and BETA-IrSn4 are identical.