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The Fischer-Tropsch synthesis (FTS) is a heterogeneously catalysed process whereby synthesis gas (a mixture of carbon monoxide and hydrogen) is converted to liquid fuels (gasoline and diesel) and chemicals. There are two modes of operation for the Fischer-Tropsch synthesis, each with its specific selectivity targets. The high temperature (300–350 ¿C) Fischer-Tropsch process (HTFT) aims at the production of gasoline and linear low molecular mass olefins, whereas the low temperature (200–240 ¿C) Fischer-Tropsch process (LTFT) is used for the production of diesel and high molecular mass linear waxes. The HTFT process comprises a complex network of elementary reaction steps. Apart from the usual linear FT products (olefins and paraffins), these steps include the formation of CO2, carbon, branched products, aromatics and oxygenates (alcohols, acids, aldehydes and ketones). To date, the product distribution of the HTFT process has not been fully described by the mechanisms in the literature. This thesis employs isotopic techniques to elucidate the mechanism of the HTFT process. The Steady-State Isotopic Transient Kinetic Analysis (SSITKA) technique was mainly used in this thesis. This method keeps the catalyst under steady-state conditions and introduces an isotopic transient by abruptly replacing one reactant with its isotope (e.g. H2/12CO/Ar ¿ H2/13CO/He) with minimum disturbance to the system. The inert gas is also switched to determine the gas hold-up in the reactor. Apart from isothermal and isobaric reactor conditions, the surface composition of the catalyst does not change during SSITKA, making this technique ideal for reaction mechanistic studies. The methodology included the solution of ordinary differential equations (or ODE’s) which were mole balances written for the labeled atom (in this thesis, mainly 13C from 13CO SSITKA experiments) and for a plug flow reactor at isobaric and isothermal conditions. Two reaction mechanisms for the methanation reaction were proposed, using the SSITKA technique over an Fe-based catalyst at HTFT conditions (330 ¿C, 1.2 bar, and H2/CO = 15). Both mechanisms have two active pools of carbon (C¿ and C¿) on the catalyst surface with both leading towards the formation of methane and higher hydrocarbons. The C¿ pool was 25 to 50 times less active than the C¿ pool for methanation and occupied 92% of the total CHx coverage (0.25 ML). The C-C coupling reaction was shown to involve both the C¿ and C¿ pools. Another important conclusion from this study was that the concentration of molecularly adsorbed CO on the Fe-based catalyst is extremely low, with an estimated surface coverage of 9 10-4 ML. Deuterium tracing experiments in combination with hydrogenation experiments (both isothermal and temperature programmed) provided information on the nature and reactivity of the surface intermediates on an Fe-based catalyst at HTFT conditions. This was performed on both fresh and carbided catalysts. On both catalysts, carbon deposition occurred to the same extent but water formation and methane formation were faster on the carbided catalyst. More reaction intermediates for C2 hydrocarbon formation were detected at the start of the Fischer-Tropsch reaction on the freshly reduced catalyst. However, the CHs intermediate for methane formation and the CCHs intermediate for C2 hydrocarbon formation were found to be the most stable surface intermediates on both catalysts. Surface carbon (13Cs), deposited via the Boudouard reaction using 13CO, was active and was detected in the C2+ hydrocarbon products as the result of a coupling reaction with 12Cs rather than with 13Cs. Six distinct carbon pools (Ca1, Ca2, Cß1, C¿1, C¿2 and Cd1) were identified during isothermal and temperature programmed surface reactions of which graphitic carbon (Cd1) had the highest coverage on the end of the run sample. The effect of co-fed ethene on the Fischer-Tropsch synthesis was also investigated. The main aim was to identify reaction pathways for readsorbed olefins. Steady state results (at 330°C, 1.2 bar, H2/CO = 15 and 7580 ml.gcat-1.hr-1) showed that the hydrogenation of ethene to ethane is the main reaction pathway but chain growth does occur to a lesser extent. Repeating these co-fed ethene experiments at the same reaction conditions but with 13CO in the feed showed that in terms of chain growth, propene formation was favoured instead of propane. This suggests that the olefins share the same surface intermediate. This result allowed for the development of different mechanistic pathways for olefin and paraffin formation, which were later used for the development of the FT mechanisms. The mechanism of the methanation reaction pathway was extended to account for the formation of C2+ hydrocarbons at the same reaction conditions. Three different mechanistic models were tested whilst considering two cases; in the first case, the initiation and chain growth rates coefficients are equal (kini = kp) and in the second case, these rate coefficients differ (kini ¿ kp). Only one model, in which there are two surface intermediates for the Cn hydrocarbons (n = 2) with direct olefin readsorption towards the surface intermediate for paraffin formation, gave the best fit. The hydrogenation of CO2 was investigated and compared to CO hydrogenation (normal Fischer-Tropsch synthesis) over an iron based catalyst at high temperature (330 ¿C). In comparison to CO hydrogenation, the catalyst activity, deactivation and olefinicity were the same during CO2 hydrogenation at similar reactor operating conditions, especially the H2/CO ratio. However, the transients obtained during 13CO and 13CO2 SSITKA experiments differ in both cases. During CO2 hydrogenation, the reactant and product (CO2 and CO) became kinetically indistinguishable. From some of the data, a two pool model was proposed based on the shape of the 13C decay in 13CO2. A formate mechanism, in which CHOs and COOHs are the surface intermediates during the water-gas-shift reaction, is the most plausible for the water-gas-shift mechanism. The hypothesis in this thesis was that Cs is active and should play a role in the mechanism of the HTFT process. This was shown to be true (see Chapter 3) with the 13CO deposition (Boudouard reaction) experiment in which the 13Cs was active for Fischer-Tropsch. Moreover, in the deuterium tracing experiment, CCHs was identified as an active species for C2 formation. This intermediate most probably forms from the reaction of Cs and CHs. The latter intermediate is most likely the monomer in the HTFT process. |
