PUB - Publikationen an der Universität Bielefeld

Carbon monoxide and volatile organic compounds (VOCs) are serious air pollutants that may give rise to deleterious health and environmental effects. Such compounds are commonly found in the atmosphere at ground level in all urban and industrial centers. Total catalytic combustion has been considered as an effective and viable approach in controlling environmental emission; however some problems such as the non-availability of catalysts with low cost, high activity and stability in prevailing conditions remain. Hence, this thesis aims at developing economic combustion catalysts such as transition metals oxides (TMOs), using an elaborated Pulsed Spray Evaporation Chemical Vapor Deposition (PSE-CVD) approach and improved understanding of their behavior. Catalysts based on TMOs were successfully synthesized by PSE-CVD. Comprehensive characterization techniques, e.g. X-ray diffraction (XRD), Raman spectroscopy, Fourier transformed infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Helium ion microscopy (HIM), Energy dispersive spectroscopy (EDS), X-Ray photoelectron spectroscopy (XPS) and Ultra-Violet Visible spectroscopy (UV-Vis) as well as temperature-programmed reduction/re-oxidation (TPR/TPO) techniques were used to characterize the obtained films. To assess their functionality, the catalytic performance of the prepared catalysts was investigated for the conversion of some representative volatile organic compounds which may be contained in exhaust stream of industrial processes. One important conclusion that can be drawn based on the results presented is that TMOs are active catalysts with real potential mainly in processes relevant to air pollution control. Aiming for eventual commercial applications of some very active TMOs catalysts, fundamental studies concerning various aspects of TMOs catalysis have been carried out throughout this thesis. The investigations have been performed using TMO-based catalysts deposited on supports, with grid mesh of stainless steel as the common used substrates. Five main types of TMO-based catalysts were prepared, including single oxides such as Co3O4, Mn3O4, α-Fe2O3, CuO and mixed oxides (Co-Fe oxides). Several reactions with direct relevance to air-pollution control were studied, i.e. oxidation of carbon monoxide (CO), propene (C3H6), n-butene (n-C4H8), dimethyl ether (C2H6O) and n-butanol (n-C4H8O) in the presence of argon and oxygen. The catalytic performance of the chosen oxides was compared to reference results from the literature. Different aspects of these reactions were studied: (i) the effect of solvent and deposition temperature on the catalyst morphology, (ii) the effect of the deposition condition (substrate temperature), the thermal properties, the morphology and the doping on the catalyst performance. In our preliminary results, we briefly describe the controlled synthesis of Co3O4 spinel using Co(acac)3 as precursor; in this investigation, special attention was focused on the role played by solvents, deposition temperature and pressure on the thin film growth and morphology. As application, the performance of the deposited Co3O4 samples was tested towards CO and C3H6 conversion (Publication 1). As interesting materials, α-Fe2O3, CuO and Mn3O4 were also prepared. Thin films of α-Fe2O3 were selectively synthesized, and the effect of the deposition temperature and lattice oxygen on the catalytic combustion of C3H6 and CO was studied (Publications 2 and 3). In the same logic, copper oxide was prepared and found to be catalytically active towards CO and C3H6 oxidation (Publication 4). Besides the activity, the thermal properties of the catalyst were investigated, and a detailed study of the synthesis of the catalytically active Mn3O4 spinel and its thermal stability were performed. The obtained Mn3O4 exhibited high thermal stability and good catalytic performance in the combustion of CO and C3H6 (Publication 5). Based on the results obtained with Co3O4 and α-Fe2O3, we have prepared a mixed oxide of cobalt and iron (Co-Fe-O). Cobalt ferrite with spinel structure has presented technologically interesting solid-solution phases. In fact the combination of Fe and Co at different ratios has improved the physico-chemical properties and the catalytic performance of the materials. It was noticed that the composition played a significant role concerning the film morphology, band gap energy and the redox properties with consequence for the catalytic behavior of the material. For example, just small amounts of cobalt in cobalt ferrite mixed oxides were sufficient to enhance the performance in the CO oxidation (Manuscript 1) and the deep oxidation of olefins (such as for n-butene and propene) and DME (Manuscript 2). In the framework of a cooperation project with Moroccan partners with the topic “Catalytic application of natural clay”, we have compared the catalytic performance of Moroccan natural clays with Co3O4 and α-Fe2O3 towards the conversion of n-butanol (Publication 6). Synthesis and Catalytic Application of Functional Transition Metal Oxides