?Modification of physicochemical properties by ball-milling: titania and ceria as supports in heterogeneous catalysis?.

SEBASTIAN LARREGOLA; MARIA R. MORALES; MATIAS GASTÓN RINAUDO; LUIS E. CADÚS

Santa Fé

Conferencia; VI San Luis Conference on Surfaces, Interfaces and Catalysis.; 2018

INTEC

Different metal oxides, CeO2, TiO2, Al2O3 among others, are employed as catalysts supports. Characteristics such as specific surface area (SBET), crystalline phases, structural defects and the presence or not of surface hydroxyl groups have an important influence in adsorption capacity of said supports1. High mechanical strength, redox properties, easy availability, low price and durability in hostile environments make titania an excellent candidate to be used as support2. On the other hand, ceria is widely used because of its well-known oxygen storage capacity. Intensification of this property is pursued for its use as support3. High-energy ball milling is a process that allows preparing materials from fracturing and adhesion, in a repeatedly way, of powder particles. The extreme conditions of pressure and temperature reached at local level make of this a powerful technique for synthesis of materials, which leads to crystalline phase transformations and specific surface area increasement4. The aim of the present research is to evaluate the effect of high-energy ball milling over TiO2 and CeO2 crystalline structure, morphology, texture and structural defects generated as a consequence of milling time. Higher cumulative energies were achieved for both supports when increasing this parameter. From X ray diffraction (XRD), transition: anatase/anatase-brookite-rutile/rutile could be observed increasing the milling time of titania. Ceria only showed a broadening of its diffraction lines. A literature and XRD analysis was done to infer micro-deformations effect over brookite structure, which were far superior than the observed for anatase and rutile phases. In line with the idea that brookite only occurs on grain edges of anatase, a density increasement of structural defects due to migration of said defects from anatase to brookite can be considered5. For longer milling times, smaller particle sizes with consequent higher SBET were obtained. In case of ceria, however, milling time is limited by particle agglomeration and SBET reduction. SEM micrographies confirmed particle size reduction with milling time for both metal oxides. Phase transitions in solid state could generate structural defects (crystalline lattice oxygen vacancies) and a great number of micro-deformations. Presence of these kind of defects can be determined by characterization techniques such as temperature programmed reduction (TPR), oxygen temperature programmed desorption (TPD-O2), X ray photoelectron spectroscopy (XPS) among others. It has been reported that titania, in any of its phases, is not reducible even at high temperatures6. Nevertheless, titania sample milled for 45 min showed and incipient reduction signal at 623 °C. This signal can be related to brookite phase presence in addition to structural defects and vacancies imposed by milling, increasing oxygen species availability and promoting titania reduction on surface. On the other hand, ceria showed lower reduction temperatures at longer milling times, due to promotion of oxygen mobility. As a general conclusion, it can be stated that high-energy ball milling is an effective method for synthesis of materials. Using this technique, a rational design of catalysts supports can be carried out, introducing structural defects and changing physicochemical properties of powders.