Abstract

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent-deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100°C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ~0.050 s-1 (equivalent to a CO consumption rate of ~65 mmol gcat-1 h-1), which are 20-30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.In another part of this work a series of Co Fischer-Tropsch synthesis catalysts with the same composition and preparation procedures, but supported on four different aluminas, were synthesized, characterized, and kinetically tested to investigate the effect of the supports on their performance. The results demonstrate that the most active catalyst, with a rate of 49 mmol CO gcat-1h-1 at 220°C and 20 atm, is obtained by using the Al-Si support, which has: high surface area and pore volume, high hydrothermal stability and lower number of hydroxyl groups on the surface.Finally, the effect of Co nanocrystal (NC) size on deactivation of Co FT catalysts by sintering was studied using surface chemical properties of the active metal phase and the support and their interactions. Values of surface energy, adhesion and cohesion energy, chemical potential, diffusivity, and energy required for sintering deactivation versus Co NC diameter were calculated. Effects of the FT reaction environment, such as water concentration and temperature, on sintering was explained and quantified. Co sintering via Ostwald Ripening was found to be a chemically driven phenomena. Also, adhesion energy and appropriate deposition of Co NCs inside support pores are the most important factors affecting the sintering rate via crystal migration and coalescence.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering

Rights

http://lib.byu.edu/about/copyright/

Date Submitted

2019-03-01

Document Type

Dissertation

Handle

http://hdl.lib.byu.edu/1877/etd11081

Keywords

Cobalt, Fischer-Tropsch, Silver, Support, Sintering

Language

English

Download

Share

COinS