A group of researchers from Sichuan University of Science and Engineering and the University of South Africa found the way to improve the performance of catalysts for Fisher-Tropsch synthesis. This reaction is used for receiving synthetic fuel from syngas — a mixture of carbon monoxide (CO) and hydrogen (H₂), which may be produced from natural gas, coal or biomass. Their objective was to increase the yield of valuable liquid hydrocarbons for producing gasoline, diesel fuel or aviation kerosene, and simultaneously to decrease the yield of methane, which in this case is an undesirable side product.
The researchers used hollow carbon spheres fixing particles of cobalt, iron and their combinations with osmium and platinum on the internal or external surfaces. These metals perform as active centers, where syngas is transformed into hydrocarbons. In the course of experiments, the scientists changed the thickness of the carbon spheres skin, the size of metal particles and their positions; they added nitrogen into the carbon structure and varied the temperature of thermal treatment.
The tests showed that even a small amount of osmium adjacent to cobalt noticeably improved the catalyst’s performance. Osmium changes the electronic properties of cobalt improving the capability of the active centers to bound the CO molecules and increasing the yield of long hydrocarbon chains (C5+), which are in demand as fuel. The cobalt particles in the range of 7–8 nanometers turned out optimal: the smaller particles were losing their activity faster, while as the bigger ones were distributed not as evenly.
The thickness of the spheres skins also turned out to be an important parameter. Thin skins contribute to faster recovery of the active centers (metal’s transition to a working form), but they increase the share of light gases. Thick skins slow down the diffusion of molecules, however, they contribute to the formation of heavier liquid fractions.
For iron-based catalysts the key factor was preserving iron in the form of carbide Fe₂C, which assures high activity and stability. This was achieved under the optimal temperature of pyrolysis: too high temperature resulted in formation of graphite blocking the active centers.
The researchers concluded that accurate selection of metal combinations, their size, structure and properties of the carbon carrier allows for controlling not only the speed of the reaction, but also the composition of the end products. Such an approach opens the possibilities for creating catalysts of the new generation providing for higher yield of synthetic fuel with minimum losses.
