Alumina- and alumina-zirconia-supported PtSn bimetallics:: Microstructure and performance for the n-butane ODH reaction

In the present work, a number of Al2O3- and Al2O3-ZrO2-supported catalysts were prepared and studied. The microstructure of the particles formed, in these systems was analyzed by high-resolution electron microscopy (HREM). HREM images of Al2O3-supported catalysts revealed a broad particle-size distribution, with a major Pt phase exhibiting diameters in the range 5-15 nm with a shell of PtSn4. Also, PtSn4 and very large particles were detected. In the case of Al2O3-ZrO2-supported catalysts, ZrO2 was present as nanometer-size crystals of the tetrahedral and monoclinic phases and the trimodal distribution of metal particles disappeared, the smaller particles being dominant. Photoelectron spectroscopy showed that in used catalysts tin is oxidized while platinum remains partially reduced. Upon activation (reduction at 623 or oxidation at 773 K/reduction at 873 K), tin became partially reduced; however, no significant changes were seen in the oxidation state of platinum. The catalysts were tested in the oxidative dehydrogenation of n-butane. The major reaction products were carbon oxides and olefins, with much smaller amounts of cracking and isomerization products. The incorporation of tin into the Pt/Al2O3 catalyst led not only to an increase in conversion but also to an inhibition of cracking reactions. Deactivation was observed in all cases. Surface analysis by photoelectron spectroscopy of exhausted catalysts revealed the formation of carbonaceous deposits on the catalyst surface, indicating that coking is responsible for catalyst deactivation. (C) 2002 Elsevier Science (USA).