Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG, Utrecht, The Netherlands.
J Am Chem Soc. 2012 Oct 3;134(39):16207-15. doi: 10.1021/ja304958u. Epub 2012 Sep 19.
The Fischer-Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of key chemical building blocks from non-petroleum-based sources such as natural gas, coal, or biomass. The influence of the iron carbide particle size of promoted and unpromoted carbon nanofiber supported catalysts on the conversion of synthesis gas has been investigated at 340-350 °C, H(2)/CO = 1, and pressures of 1 and 20 bar. The surface-specific activity (apparent TOF) based on the initial activity of unpromoted catalysts at 1 bar increased 6-8-fold when the average iron carbide size decreased from 7 to 2 nm, while methane and lower olefins selectivity were not affected. The same decrease in particle size for catalysts promoted by Na plus S resulted at 20 bar in a 2-fold increase of the apparent TOF based on initial activity which was mainly caused by a higher yield of methane for the smallest particles. Presumably, methane formation takes place at highly active low coordination sites residing at corners and edges, which are more abundant on small iron carbide particles. Lower olefins are produced at promoted (stepped) terrace sites that are available and active, quite independent of size. These results demonstrate that the iron carbide particle size plays a crucial role in the design of active and selective FTO catalysts.
费托合成低碳烯烃（FTO）是一种替代工艺，可从非石油基资源（如天然气、煤或生物质）生产关键的化学基础原料。研究了在 340-350°C、H₂/CO=1 和 1 及 20 巴压力条件下，负载型铁碳化物体积分数对合成气转化率的影响。在 1 巴下，未负载的初始活性基础上，基于表面特异性活性（表观 TOF）的活性提高了 6-8 倍，而甲烷和低碳烯烃的选择性没有受到影响。在 20 巴下，Na 和 S 促进的催化剂粒径减小相同，初始活性基础上的表观 TOF 增加了两倍，这主要是由于最小粒径的甲烷产率更高。推测甲烷的形成发生在位于拐角和边缘的高活性低配位活性位上，而这些活性位在小碳化铁颗粒上更为丰富。低碳烯烃是在促进（阶梯）平台位上生成的，这些位是可用的且具有活性，与粒径大小无关。这些结果表明，碳化铁颗粒尺寸在设计活性和选择性 FTO 催化剂方面起着至关重要的作用。
