Surface Properties of Al2O3 for Understanding Metal-Support Interaction and Catalytic Properties of Al2O3-based Catalysts

Department of Chemical Engineering??-Al2O3 is one of the most important heterogeneous catalytic materials that has been used as a support for a wide range of applications in petroleum chemistry and automobile emission control. Thanks to its intrinsic acidity, it can also participate in acid-catalyzed reactions such as alcohol dehydration. For decades, numerous studies have been devoted to understanding the fundamental nature of Al2O3 owing to its practical importance in catalysis. The catalytic properties of oxides, such as activity, selectivity, and distribution of active phases on the support, are significantly affected by their surface characteristics because chemical processes mainly occur on the surface. However, understanding the surface properties of aluminas have been challenging due to unresolved structures, mixed effects of various factors (morphology, phases, impurities, and additives), and difficulties of characterization. Therefore, comprehensive approaches should include the preparation of well-defined model aluminas and sensitive characterization tools. This thesis focuses on understanding the surface properties of aluminas and their role in the interaction with the active phases (Pt) and the corresponding catalytic behavior of Al2O3-based catalysts. By preparing the model aluminas for morphology, phase, additive and using ethanol TPD as a surface characterization tool, we studied the surface properties of Al2O3 with various modification and further investigated the interaction with Pt and its reduction behavior on alumina surfaces. First, we synthesized a series of well-defined platelet ??-Al2O3 with the systematic change of (100) facets and investigated the catalytic role of (100) facets by ethanol dehydration. By controlling pH during the preparation of boehmite (??-AlOOH), precursor for ??-Al2O3, (001) facet of boehmite increased under acidic condition. Topotactic transformation of boehmite into ??-Al2O3 maintained their morphologies, leading to platelet ??-Al2O3 with increased (100) facets of ??-Al2O3. Ethylene formation increased with increasing (100) facets, clearly demonstrating the critical role of (100) facets as active sites for ethanol dehydration on ??-Al2O3. In chapter 3, We investigated the effect of morphology, phases, and additives on the surface properties of Al2O3 and which factors are the most important for surface properties of Al2O3 by preparing model aluminas with various modifications and using ethanol TPD. Ethanol TPD showed the desorption temperature (at a maximum rate of ethylene desorption, Td) of dissociative ethanol was significantly dependent on morphology, crystalline phase, and additives. Here, the additives affected the desorption temperature of ethylene most significantly. Ethanol dehydration tests showed that ethylene formation rates, normalized with respect to the amount of dissociative ethanol (quantified by ethanol TPD), exhibited an inverse correlation with Td on Al2O3 with various morphologies, crystalline phases, and additives, which suggests that Td can be used as a descriptor for surface properties of Al2O3, irrespective of modification origins. The activities and activation barriers of various commercial Al2O3 on ethanol dehydration were also consistent with our empirical model. Based on understanding for surface properties of Al2O3, we study how these surface properties of aluminas affect the active phases (Pt) and the catalytic behavior of Pt/Al2O3. We chose two sets of model alumina having different number of sites with the identical properties and different properties of sites with the same number based on ethanol TPD. With two model aluminas, how the number and properties of specific sites on alumina surfaces affect the specific interaction between Pt and alumina was investigated. Pt showed higher dispersion with increasing number of sites and interaction strength because the Pt atoms can interact with specific sites on alumina in greater numbers and more strongly. However, these Pt dispersion changes do not represent the gradual size change, but the relative population change of small (10 nm). The number of highly dispersed Pt clusters increased with increasing number of sites and interaction strength. When Pt showed higher dispersion from more number of sites and stronger interaction on alumina, Pt/Al2O3 showed higher activity for benzene hydrogenation due to more available highly dispersed Pt. Finally, specific interaction between Pt and alumina strongly affected the reduction behavior of oxidized Pt on alumina. We studied the reduction of three-dimensional (3D) PtO2, two-dimensional (2D) PtO2, and atomically dispersed Pt on Pt/Al2O3. Under oxidizing atmosphere, morphologies and sizes of PtO2 on Pt/Al2O3 are determined by the specific interaction between Pt and Al2O3 through Pt-O-Al bond, leading to highly dispersed Pt as ~1 nm 2D-raft PtO2 and atomically dispersed Pt on Al2O3. When all the Pt atoms can???t interact with anchoring sites on support under high Pt loading or SiO2 which has very weak metal-support interaction, 3D PtO2 was formed. Due to weak interaction with support, 3D PtO2 was reduced earlier (-20~-60 oC) than 2D PtO2 (~110 oC) and atomically dispersed Pt (>300 oC). Furthermore, when Pt/Al2O3 was calcined at 500-700 oC, PtO2 was reduced without a reducing agent (auto-reduction) with Pt sintering. Interaction strength also influences auto-reduction when calcination temperature increases. 3D PtO2 was auto-reduced after 550-600 oC calcination, but 2D PtO2 was more difficult to be reduced (50 oC higher). So, the interaction strength with the support determines how long Pt oxide can maintain as Pt oxide rather Pt metal. These morphologies of Pt oxide also affect their sintering behavior. Because 3D PtO2 is more easily auto-reduced than 2D PtO2 and atomically dispersed Pt, 3D metallic Pt clusters become less mobile than 2D PtO2 and atomically dispersed Pt (still, oxidized Pt). So, 3D PtO2 showed more sinter-resistant behavior than 2D PtO2 and atomically dispersed Pt. The metal-support interaction between Pt and alumina is important for the reduction of oxidized Pt on Pt/Al2O3, suggesting the guideline about a careful activation for the efficient utilization of metallic Pt for catalytic reactions.clos