Fischer-Tropsch synthesis reaction to produce sulphur free hydrocarbons has enjoyed a resurgent in interests due to increases in world oil prices. In this work, the suitability of Mo and Mo-W carbides has been investigated as a possible cost-effective alternative to noble metals in Fischer-Tropsch synthesis. The molybdenum and tungsten monometallic and bimetallic carbides were prepared through precipitation from homogeneous solution to the sulphide followed by carburization with a mixture of propane and hydrogen to produce the resulting metal carbide. A 23 factorial design strategy was employed to investigate the effect of three carburizing variables, namely, time, temperature and gas ratio on the resulting catalyst. In particular, the effect of supports was also examined through four common semiconductor oxide supports, namely: Al2O3, SiO2, TiO2 and ZrO2. Thermogravimetric analysis of the carburization reactions showed that the conversion from metal sulphide to the metal carbides is a multistep process producing different phases of carbides, namely ??-MoC1-x, ??-Mo2C, ?? -WC1-x and ??-W2C, depending on heating rate and temperature. The rate determining step of the carburising reaction is the diffusion of carbon atoms into the metal matrix, hence giving relatively low activation energy values. Statistical analysis of the factorial design revealed that all three carburizing variables affect the final physiochemical makeup of the catalyst. SEM analysis showed that the carbides are well dispersed on the surface of the support and catalyst particles produced are nanoparticles in the range of 25 to 220 nm depending on the support. Fischer-Tropsch activity test showed that monometallic molybdenum carbide is active under Fischer-Tropsch conditions while tungsten carbide is inactive for the conditions studied in this project. However, bimetallic carbide catalyst, consisting of the two mentioned metals gave overall higher reaction rates and decreased methane selectivity. Steady state analysis revealed that there are two active sites on the surface of molybdenum carbide catalyst resulting in two chain growth propagation values when analysed via the Anderson-Schulz-Flory kinetics. Overall, ZrO2 support appeared to be the most suitable support followed by SiO2, TiO2 and Al2O3. Finally, kinetic modelling of data showed that methanation and higher hydrocarbons formation path occurs via combination of the oxygenated intermediate and Eley-Rideal mechanism.
| Identifer | oai:union.ndltd.org:ADTP/257219 |
| Date | January 2006 |
| Creators | Nguyen, Tuan Huy, Chemical Sciences & Engineering, Faculty of Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Chemical Sciences and Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Tuan Huy Nguyen, http://unsworks.unsw.edu.au/copyright |
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