China-US team develops new platinum-cobalt nanocatalysts for low-temperature aqueous phase Fischer-Tropsch synthesis
7 March 2013
Researchers from China and the US have developed Pt−Co nanoparticles (NPs) which proved to be effective and efficient catalysts for aqueous-phase Fischer-Tropsch synthesis (FTS) at 433 K (160 °C)—a lower operational temperature than can be achieved with conventional catalysts. A report on their work is published in the Journal of the American Chemical Society.
Fischer−Tropsch synthesis is a well-established catalytic process that converts syngas derived from fossil fuels or biomass to liquid fuel products. As the process is highly exothermic and thermodynamically favored at low temperature, it is desirable to develop a catalyst system that could facilitate working at low reaction temperature while maintaining excellent catalytic performance, they note.
Research has shown that Ru, Fe, and Co catalysts working in the temperature range of 200−350 °C have the best performance. Among these, less expensive Co-based catalysts are optimal choice for industrial synthesis of long-chain hydrocarbons due to their higher hydrocarbon productivity, good stability, and commercial availability.
The researchers recently showed that stabilized Ru nanoparticles in aqueous- phase FTS show a 35-fold increase in activity over traditional supported Ru catalyst at an operating temperature of 150 °C and a 16-fold increase at only 373 K (99 °C).
Because hydrocarbons are insoluble in water, they can be easily separated from the reaction mixture. The team attributed that significant increase in catalytic activity at relatively lower temperatures to the reduction of the catalyst particle size; the high dispersion of the NP catalysts; and maintaining the three-dimensional freedom of the particles in water.
For low-temperature aqueous-phase FTS to be commercially attractive, however, would require non-noble metal alternatives for the Ru NP catalyst. The team began working with the conventional FTS element iron; they found, however, that Fe is not stable in aqueous phase.
Cobalt is another attractive commercial catalyst material; however, traditional Co-based catalysts usually operate at a temperature 200 °C to achieve acceptable activity and selectivity, especially for those working under liquid-phase or slurry-phase reaction conditions.
Moreover, it is rather challenging to realize FTS in liquid, especially in aqueous phase. Indeed, there have been tremendous endeavors to realize low-temperature FTS over Co-based catalysts in the past decade...Until now, however, there are no reports on the successful construction of a Co-based FTS catalyst system operating at relatively low reaction temperature. Therefore, it is important to improve the activity of Co-based aqueous-phase FTS catalysts at lower temperatures.
It is well documented that the addition of a small amount of noble metals into the transition-metal catalysts may greatly promote the catalytic activity of the latter...For the bimetallic catalysts, depending on the preparation and reaction conditions, the two components may form distinct structural motifs, such as an alloy or core−shell structure (both in metallic states) or metal/metal oxide interfaces. How the two metal components interact with each other may affect significantly the reaction performance. In this work, we report a new hydrogen-reduction route, a Pt seed method, for the aqueous one-step synthesis of Pt−Co bimetallic NP catalysts.—Wang et al.
Among the catalysts they tested at 433 K, they found that using a 10% Pt loading delivered the best results, with 70% selectivity toward C5+. The catalyst was also very stable.
The team, which included scientists from Peking University; Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Oak Ridge National Laboratory; and Brookhaven National Laboratory, attributed the “outstanding activity” of the nanocatalysts to the formation of strained Co layers on Pt or Pt−Co alloy particles, which improve the overall energetics and kinetics by forming favorable transition states (TSs) due to the lattice expansion of the supported Co layers.
They suggested that this can used as the basis for the rational design of bimetallic catalysts with low-temperature activities.
Hang Wang, Wu Zhou, Jin-Xun Liu, Rui Si, Geng Sun, Meng-Qi Zhong, Hai-Yan Su, Hua-Bo Zhao, Jose A. Rodriguez, Stephen J. Pennycook, Juan-Carlos Idrobo, Wei-Xue Li, Yuan Kou, and Ding Ma (2013) Platinum-Modulated Cobalt Nanocatalysts for Low-Temperature Aqueous-Phase Fischer–Tropsch Synthesis. Journal of the American Chemical Society doi: 10.1021/ja400771a
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