Model catalysts studied by x-rays, visible and infrared light

Sammanfattning: This work is focused on the study of transition metal model catalysts for gas-phase heterogeneous catalysis, studied with light-based in-situ and operando techniques.Metallic model catalysts, such as Palladium (Pd) and Iridium (Ir) single crystals and in minor part deposited Rhodium (Rh), were studied using a combination of x-ray and non x-ray techniques. The aim was to either acquire structural characterization of new materials or pushing the state-of-the-art instrumental boundaries and close the material and pressure gap present in model catalysts studies. In this context were developed new techniques, such as Transmission Surface Diffraction (TSD), or further refined and validated, such as 2D Surface Optical Reflectance (2D-SOR).Pd, used in papers I,II , III , V , IX, was studied with a combination of X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD) techniques as well as 2D-SOR and planar induced fluorescence (PLIF). This model catalyst was used as proof of concept for new approaches and new set-ups developed in collaboration with the combustion physics division, Lund University. One example is the use of the newly developed POLARIS High-Pressure XPS (HPXPS) station at P22 at PETRA III, Desy, Germany, demonstrating the potential and a new state-of-the-art of operando XPS. During this investigation, we have also confirmed Pd metallic islands' formation on PdO during reduction, a potentially critical factor in the modification and/or enhancement of the system catalytic activity. Another important result is obtained combining high energy surface X-ray Diffraction (HESXRD) with 2D-SOR and PLIF, and the characterization of 2D-SOR with an atomically precise diffraction method. During this investigation in particular we characterized the 2D-SOR sensitivity down to 2-3 Å by the observation of 2D-SOR signal from thin PdO (√5x√5)R27°.Similarly to Pd, in paper VI, we used Rh nanoparticles and films to implements a novel approach of Transmission Surface Diffraction (TSD) in which, at normal incidence angle, we performed diffraction in transmission through the sample with the use of high energy x-rays. With this contribution, we show that it is indeed possible to obtain reliable data in this configuration, contributing to mitigating the experimental challenges of experiments otherwise performed at grazing incidence geometry.Ir, used in papers VII, IX, VIII, was studied with XPS and XRD techniques to further understand the behavior of this, previously less investigated, catalyst that has recently gained popularity in both gas-heterogeneous as well as electrocatalysis community. XPS results provide further information on Ir metal and Ir oxide's reactivity and thermal stability while XRD results show the complex behavior of the catalyst under oxidation and reduction conditions. In particular, during the reduction it was observed the formation of Ir metallic islands on to ofIrO₂, in a parallel to the Pd/PdO/Pd case, and suggesting a possible general behavior for oxidized metal catalysts systems. The combination of these, both thermodynamic and structural data, provide new and much-needed information to the catalysis community, and at the same time, encourage a more well-defined approach to the IrO₂/Ir(100) system.