Date of Award
2018
Document Type
Open Access Dissertation
Department
Chemical Engineering
Sub-Department
College of Engineering and Computing
First Advisor
John R. Regalbuto
Abstract
Cobalt and iron based catalysts are commercially predominant for Fischer-Tropsch synthesis, Co-based catalysts for their stability, activity, and inactivity for the water gas shift reaction and Fe-based catalysts for their cheap cost, high light-olefins and alcohols selectivity, and tolerance to H2/CO ratio. These catalysts are structure sensitive with lower activity at nanoparticle sizes smaller than 6-8 nm. Co-based catalysts exhibit high activity, low CH4 selectivity, and high C5+ selectivity, while Fe-based catalysts are more selective to low-molecular-weight hydrocarbons such as olefins and alcohols. The reducibility of cobalt and iron is another important factor which may be key for improving their performance, and this might be achieved by adding a noble metal which, via hydrogen spillover, can facilitate Co oxide and Fe oxide reduction to the metal nanoparticles which are more active for CO hydrogenation reaction.
The promotion of transition metals by a noble metal will be most efficacious if the noble metal is in direct contact with transition metal nanoparticles, and none is wasted on the surface of the support. In this study, we demonstrate that such a selective deposition is possible. We first employ incipient wet impregnation to synthesize silica supported cobalt oxide and iron oxide catalysts of the optimum size. Second, Pt is steered onto the transition metal oxide surfaces by controlling the impregnation pH. We compared these samples at various Pt loadings to Pt/Co and Pt/Fe catalysts of the same nominal composition prepared by the common, undirected method of co-impregnation of Pt-Co and Pt-Fe. Cobalt and iron loadings were 10 wt % and Pt loading varied between 0.03 and 1.72 atomic % in each catalyst set. Catalysts were characterized by XRD, TPR, and STEM. The catalyst reactivity was evaluated at conventional Fischer-Tropsch conditions of (220oC for Pt-Co/SiO2 and 250 for Pt-Fe/SiO2) and 20 bar with a H2/CO ratio of 2:1.
The reactivity of the Co catalysts clearly depends on the preparation method, which can be attributed to the intimate coordination of Pt with Co. At higher Pt loadings, the Pt has effects not only on Co oxide reduction but also are extended to Fischer-Tropsch reaction itself. At very low Pt loadings, the promotional role appears to be limited to improving Co reducibility. Methane selectivity is decreased and C5+ selectivity is increased by unprecedented levels. Thus, promotion of Co by Pt for FTS is effective with very small amounts of Pt selectively adsorbed onto the Co phase.
The effect of Pt on Fe-based catalysts is different from the one in the Co-based catalyst. The effect on Fe-based catalysts is limited to the promotional of iron oxide reduction, and no promotional effect was observed for the FT reaction. This difference in Pt effects for Co/SiO2 and Fe/SiO2 reaction performance is attributed to the nature of the active phase, thus, the active phase in Co-based catalyst is the Co0 metal phase, which is strongly promoted by Pt, while Fe carbide is the widely accepted active phase and therefore the Pt has no promotional effect on transformation Fe-oxides species to Fe-carbides in a H2 environment.
Rights
© 2018, Fahad A. Almalki
Recommended Citation
Almalki, F. A.(2018). Selective Deposition of Platinum by Strong Electrostatic Adsorption onto Cobalt- and Iron-based Catalysts for Fischer-Tropsch Synthesis. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4810