Progress in Catalysts Catalyzing Fischer-Tropsch Synthesis

Recently, Professor Ouyang Shuxin's group from College of Chemistry of CCNU and Professor Zhang Tierui's group from Technical Institute of Physics and Chemistry of CAS, jointly developed a series of Fe5C2 loading on tunable N-doped carbon as photothermal catalysts which can achieve efficient Fischer–Tropsch synthesis to olefin (FTO) reaction. The relevant findings were published in ACS Catalysis (2022, 12, 5316−5326), a famous academic journal in the field of catalysis. Doctoral candidate Li Ruizhe is the first author of this paper, and Prof. Ouyang Shuxin is the co-corresponding author.

In the chemical industry, light olefins (the olefins contain 2 – 4 carbon atoms) are regarded as fundamental raw materials for the manufacture of various chemicals such as polymers, solvents, and so on. However, traditional FTS requires consuming fossil energy to drive the reaction, which results in the tremendous amount of CO2 emission. In recent years, due to the clean and sustainable sunlight to drive the reaction, photothermocatalytic FTS is regarded as an effective method to solve energy shortage and achieve carbon emission reduction. Fe5C2 is acknowledged as the active phase due to its intrinsic catalytic activity and high selectivity to light olefins, but has the shortcoming of high reaction temperature (T > 300℃) just as most Fe-based catalysts.

In the previous studies, Ouyang’s group has developed the method of coupling photocatalysis and photothermal catalysis to greatly reduce the activation energy of water–gas shift reaction, so as to effectively reduce the reaction temperature (Angew. Chem. Int. Ed., 2019, 58, 7708–7712; Appl. Catal. B-Environ., 2021, 298, 120551). Further but differently, this study simply used a method of introducing the N-doped carbon as support, which can reduce the activation energy of FTS over Fe5C2-based catalysts by 45%. The optimal sample, Fe5C2/NC600, delivered a selectivity up to 55.6% for light olefins, when CO conversion rate was 22.3% at the lower temperature of 250℃ induced by the light-to-heat conversion.

The N doping in carbon will form different configurations such as pyrrolic N, pyridinic N, graphitic N and so on. It is a grand challenge to identify which N-doped configuration has the main contribution to the improvement of catalytic performance. The experimental characterizations of XPS and XAFS indicated that the pyrrolic N of N-doped carbon support can act as electron donors, transferring electrons to the surface of active phase of Fe5C2 to form an electron-rich active site. Meanwhile, the content of pyrrole N in the support depends linearly with the CO conversion of the catalyst, indicating that the enhancement of catalytic activity over Fe5C2 via N-doped carbon support is closely related to the electron-donor property of pyrrolic N. The experimental characterizations including CO-TPD and CO pulse reaction revealed that, compared to the pristine Fe5C2, the N-doped carbon support enhanced the CO adsorption at the active site and thereby promoted the dissociation activation of CO on the Fe5C2 surface. DFT theoretical calculations show that, compared with other N species as well as the pristine Fe5C2, the Fe5C2 (510) / pyrrolic N is beneficial for the hydrogen-assisted dissociation activation of CO on both thermodynamics and dynamics, confirming that the pyrrolic N improves the adsorption and dissociation capacity of Fe5C2 to CO and thus enhances CO conversion rate of the catalyst. This study develops a rational strategy to regulate the active site and provides insights for improving the conversion efficiency of solar-to-chemical energy.

doi.org/10.1021/acscatal.2c00926

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