Characterization of NiW and CoW Hydrotreating Catalysts

While hydrodesulfurization (HDS) is a process that has already been used for over 50 years and has received large attention in the literature, the world-wide trend towards ultra clean fuels has renewed the interest in HDS catalysis research. EU regulations with respect to fuel specifications involve reduction of the sulfur content in diesel from 350 ppm today to 50 ppm in 2005. These regulations are set up because of environmental concern, since sulfur in fossil fuels produces SO2 after burning the fuel and SO2 is one of the main causes of acid rain. Therefore, sulfur that is present as a natural organic component in crude oil is removed in the oil refinery in the presence of H2 by a process called hydrodesulfurization.
Even more stringent specifications are being discussed in Europe, where Germany is considering a 10-ppm sulfur limit on both gasoline and diesel. Meeting these severe limits in the most cost-effective way represents a significant challenge to the industry. Since HDS at low sulfur levels is very different from conventional HDS (in which CoMo catalysts are commonly applied), one of the solutions is development of new HDS catalysts. NiW catalysts were found to be capable of desulfurizing the most refractory aromatic sulfur compounds and thus suitable for HDS at low sulfur levels.Therefore, NiW catalysts were extensively studied in this Ph.D. project with Mössbauer spectroscopy, extended X-ray absorption fine structure, high-resolution transmission electron microscopy and HDS activity tests. This resulted in a description of structure-activity relationships for NiW catalysts on different support materials. Furthermore, the study of CoW catalysts revealed why these catalysts have an inferior HDS activity. This fundamental knowledge can be used to develop "design rules" for better performing HDS catalysts in the future. The results are described in the thesis entitled "Characterization of NiW and CoW hydrotreating catalysts".