In situ and ex situ electron microscopy and X-ray diffraction characterization of the evolution of a catalytic system - from synthesis to deactivation

Abstract

Heterogeneous catalysis represents a research field of undeniable importance for a multitude of technological and industrial processes. Supported catalysts are nowadays at the base of the large-scale production of most chemicals and are used for the removal of air pollutants from automotive engines. In the last years, heterogeneous catalysis has also acquired a major role in the growing field of green chemistry, where minimization of waste products and increased synthesis efficiency are of utmost importance. The catalytic properties of a material system are strongly connected to its overall structure. Factors such as size, shape, chemical composition and crystal structure, define the final activity, selectivity and stability towards a specific chemical reaction. In this perspective, the rational design of novel catalysts requires a strong material characterization effort, in order to maximize the understanding of the structural properties and mechanisms at the origin of catalytic activity. This thesis presents the potential and uniqueness of ex situ and in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques in the characterization of several supported material systems at different stages of their catalytic life. Starting by synthesis, passing by operation and ending through deactivation, the combined use of the two techniques allowed to investigate the evolution of the catalytic activity of these systems in connection with their morphological, crystallographic and chemical properties. Three hydrodeoxygenation (HDO) catalysts, Ni/ZrO2, Mo2C/ZrO2 and Ni-MoS2/ ZrO2 have been investigated in the initial and final stage of their catalytic life: synthesis and deactivation.The combined use of TEM imaging and spectroscopy allowed to investigate theinfluence of the synthesis procedure on the activity of Ni/ZrO2. A relation betweenthe size distribution of supported Ni particles and the catalytic activity profiles could be established using this approach. Furthermore, TEM and XRD allowed to assess the correct formation of the active phases of both Mo2C/ZrO2 and Ni-MoS2/ZrO2 catalysts.The stability of Ni/ZrO2 and Ni-MoS2/ZrO2 catalysts upon exposure to differentpoisoning species was studied by the acquisition of energy dispersive X-ray spectroscopy (EDX) maps. For Ni/ZrO2, sulfur and potassium were found to cause a permanent deactivation whereas chlorine adsorption on the catalyst active site was found to be reversible. The exposure of Ni-MoS2/ZrO2 to different H2S and H2O feed concentrations lead to important modifications of the catalyst active phase. Low H2S concentrations in the feed were found to induce a sulfur depletion of the NiMoS active phase, whereas presence of water in the feed induced an S-O exchange at the edges of MoS2 slabs. Both processes were connected to loss of activity during prolonged catalytic tests. Finally, environmental TEM (ETEM) studies allowed to investigate in situ the effect of water exposure on Mo2C/ZrO2. A combination of high resolution TEM (HRTEM) and electron energy loss spectroscopy (EELS) revealed the degradation of the supported carbide particles probably due to the formation of volatile molybdenum hydroxide species. The activity of silver nanoparticles as catalyst for soot oxidation was studied in operative conditions. The carbon oxidation reaction was investigated in situ in the ETEM and fundamental insights on the mechanisms leading to catalytic activity were obtained. The silver phase exhibited significant mobility during soot oxidation as a result of the presence of attractive forces between Ag nanoparticles and the soot matrix. This mobility was found to be dependent on the silver particle size and it was responsible of lowering the carbon oxidation temperature by a mechanism ensuring the constant presence of a reactive carbon-silver-oxygen interface during reaction. Small silver particles were observed to catalyze carbon oxidation at lower temperature than big ones, due to the lower temperature needed to trigger silver mobility.<br/