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Fischer–Tropsch Synthesis on a Model Co-SiO2 Catalyst Research Paper

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Updated: May 8th, 2022

The aim of the study was to establish optimal levels of pressure and temperature in driving the Fischer-Tropsch Synthesis (FTS), while utilizing the Co/SiO2 catalyst. The need to study the process emerged from earlier findings that showed immature termination of the reaction. Formation of Cobalt silicates, as well as high water partial pressures, quickly led to catalyst deactivation, stopping the reaction. Conducting the reaction using other catalysts yielded different results, promoting the formation of undesirable species of hydrocarbons. As an example, long chain hydrocarbons resulted when FTS occurred in the presence of Cobalt single crystals. In this model study, Yan et al (196) sought to identify optimal conditions that would enable usage of cobalt particles on silicon oxide for industrial application in the manufacture of fuel hydrocarbons. Such application would counter the dependence on crude oil and probably, provide an alternative fuel source. In addition, Cobalt catalysts yield better results when compared against the commonly used iron catalysts.

Methodology

The researchers used a modified commercial PHI 5500 system. The system comprised of chambers for surface analysis, preparation and a high pressure cell. The surface analysis served the function of cleaning and characterization of the samples. The chamber consisted of a differential ion gun, dual Mg/Al anode X-ray source, as well as a hemispherical energy analyzer. Stainless steel material made up the 0.2 Liter high pressure cell. It played the role of a batch reactor and ensured maintenance of ultra-high vacuum conditions during the reaction. A Tantalum foil held the silicon oxide gel, which was formed by evaporating Silicon in the presence of oxygen at a bar pressure of 1.3 x 10-8. Annealing of the film occurred at a temperature of 850 K. A thermocouple attached to the Tantalum film allowed monitoring of temperature. Vaporization of Cobalt occurred by resistively heating a Cobalt wire while wrapped in a Tantalum film. Resulting vapor landed on the SiO2 film. The researchers utilized XPS to calibrate the Si and Cobalt dispensers on a Molybdenum (110) surface. Passing carbon monoxide (CO) gas through an oxygen trap, then a quartz-chip filled quartz tube, allowed elimination of metal carbonyls. Various ratios of CO and Hydrogen were thoroughly mixed in an aluminum cylinder before passing them through a liquid nitrogen trap for purification prior to entry into the reaction chamber. The researchers used a HP 5890 gas chromatography equipment to analyze the reaction products. Set-up for analysis using the chromatograph included an Aluminum Oxide based, HP-PLOT Capillary column coupled with a flame ionization detector. Control for the experiment utilized clean Tantalum foils at different temperatures (Yan et al 197).

Results and discussion

No reaction occurred for the control samples at temperatures equal or below 573 K. Small ratio of Co/Si suggested that Cobalt spread on the silicon oxide film dispersed as nanoparticles. The ratio decreased further during hardening pointing out to possible Cobalt sintering. Selectivity towards methane stayed on an upward trend suggesting possible poisoning of active sites due to carbonaceous deposits. The Arrhenius plot used to interpret the CO conversion rate gave values consistent with previous studies on cobalt-based catalysts in FTS. However, this contrasted with Anderson-Schulz-Flory (ASF) plots commonly used for product characterization. ASF plots exhibited higher values of methane production. This could be explained by the fact that methane formation was evident on sites that did not favor chain growth (Yan et al 199).

Key findings

In FTS reaction, high temperatures favor chain termination. Elevated temperatures favored methane production. In contrast to other findings, oxidation of Cobalt did not occur. However, this does not fully disqualify the observation because partial water pressure in the experiment was lower than what would be observed in an actual FTS application. In addition, higher pressures encouraged chain growth (Yan et al 200).

Reference

Yan, Zhen, Zhoujun Wang, Dragomir B. Bukur, and D. Wayne Goodman. “Fischer–Tropsch Synthesis on a Model Co/SiO2 Catalyst.” Journal of Catalysis 268.1 (2009): 196-200. Print

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