Study of catalytic properties of metallic nanoparticles on iron oxide applied to the methane and ethanol reforming

Abstract

Catalysis is an important technological segment responsible for a huge fraction of the economy in the developed and emerging countries. In heterogeneous catalysis, the catalyst is often composed by noble and transition metal nanoparticles, such as Pd, Pt, Ag, Rh, among others, that are supported on oxides with large surface areas, for instance the zeolites. A particular reaction of great technological importance is the reforming of methane for production of hydrocarbons and hydrogen for fuel cell (water gas shift reaction). It might have also an important impact to reduce carbon emissions at long term, since methane is potentially more harmful to global worming than carbon oxide. Many catalysts have been studied for this reaction. However, there is not enough acknowledges of the physical and chemical mechanisms involved in the reaction to provide information about what are the active sites and how particle size and temperature of activation control the activity and selectivity of the catalyst. In some cases not only the metal particle is active in the catalytic process, but also the oxide surface plays an important rule in the reaction. Due the complexity and number of parameters involved in such a system, those informations are traditionally obtained in an empirical way for real catalysts. With this methodology is difficult to identify and isolate different phenomena involved in the reactions and it could result in wrong or ambiguous conclusions. Another strategy adopted by many groups in the world to study potential catalytic reactions is the so called model catalysts. In this case, the catalyst is projected in controlled conditions. First an ordered oxide surface is prepared and characterized. Metal nanoparticles are then grown on it and the whole system is characterized regarding its electronic, structural and catalytic properties. In this project we will study the catalytic activity of Pd, Pt and Rh metal nanoparticles in the methane reforming. The model catalyst will be prepared as described above under UHV conditions over Iron oxide ultrathin films supported on Ag(111) and Ag(100) single crystals. It is possible to obtain different phases of the oxide depending on the growth conditions. The electronic and structural structures of the sample will be obtained with electron spectroscopy and diffraction (XPS and XPD). The surface morphology, particles size and its distribution will be measured with scanning tunneling microscopy (STM). Finally a comparative characterization of chemical activity for different surfaces will be investigated using Thermal Programmed Desorption (TPD) technique.