Towards heterogeneous catalysis and catalyst design :a density functional theory study on Fischer-Tropsch synthesis

Fischer-Tropsch (FT) synthesis, which converts CO and H2 to a large range of hydrocarbons, is of paramount significance in utilizing natural resources, such as natural gas and coal. In this thesis, density functional theory (DFT) calculations together with kinetic analysis are introduced to study this important process. Our work shows that this combination is a good way to investigate chemical reactions occurring on surfaces, which are important in the area of heterogeneous catalysis. First of all, we study chain growth mechanism. With consideration of both coupling barriers and reactant stability, the reaction rates of all possible coupling channels are .evaluated and compared. By this means, the major coupling pathways are identified on the Co surface. We Furthennore extend this approach to the mechanisms on the Rh, Ru, Fe and Re surfaces. We also study a-olefin selectivity. A simple expression of the paraffin/olefin ratio is obtained based on a kinetic model. Combining the expression of the paraffin/olefin ratio and our calculation results, experimental findings are satisfactorily explained. Based on the understanding obtained from previous work, we further examine the a value, namely chain growth probability, one of the most important parameters in FT synthesis. A general expressIOn of chain growth probability is derived, and the relationship between the ex value and chain length observed experimentally IS satisfactorily explained. Attempting to understand the promotion effects of transition metals in Co-based catalysts, we examine two processes pertinent to reactivity and selectivity in FT synthesis, namely CO dissociation and ethylene chemisorption. Some suggestions are raised to modify Co catalysts to improve reactivity and selectivity in FT synthesis. Supplied by The British Library - 'The world's knowledge' Supplied by The British Library - 'The world's knowledge'EThOS - Electronic Theses Online ServiceGBUnited Kingdo