Recently, ProfessorOuyang Shuxin’s research group from College of Chemistry of CCNU and the team led by Zhang Tierui, researcher of Technical Institute of Physics and Chemistry CAS, jointly developed a series of Fe5C2 loading on tunable N-doped carbon as photothermal catalysts which can achieve an efficient Fischer–Tropsch synthesis to olefin (FTO) reaction.The relevant research 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 Pro. Ouyang Shuxin is the co-corresponding author.
In the chemical industry, light olefins (C2−4=) 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 use of the clean and sustainable sunlight as the driving force of the reaction, photothermocatalytic FTS is regarded as an effective method to solve energy shortage and achieve carbon emission reduction. Fe5C2 is regarded as the active phase due to its intrinsic catalytic activity and high selectivity to light olefins, but has the shortcoming of high reaction temperature for most Fe-based catalysts (k>300℃）.
In the previous research, Ouyang’s research group has developed the method of coupling photocatalysis and photothermal catalysis to greatly reduce the activation energy of water–gas shift reaction, so as toeffectively reduce the reaction temperature(Angew. Chem. Int. Ed., 2019, 58,7708–7712; Appl. Catal. B-Environ., 2021, 298, 120551). This study simply used the method of the N-doped carbon support, but it canreduce the activation energy of Fe5C2 catalysis FTS by 45%. The most optimized sample, Fe5C2/NC600, delivers a selectivity up to 55.6% for light olefins, when CO conversion rate is 22.3% at the lower temperature of the photothermal 250℃.
The N-doping of C will form different configurations such as pyrrolic N,pyridinic N,graphitic N and so on. It is difficult to identify which N-doped configuration has the main contribution to the improvement of catalytic performance. The experimental characterizationsof XPS and XAFS indicated thatthepyrrolic N ofN-doped carbon carrierscan act as electron donors,transferring electrons to the active phase Fe5C2 surface to form an electron-rich active site. Meanwhile, the content of pyrrole N in the carrier depends linearly with the CO conversion of the catalyst, which indicates that N-doped carbon carrier’s enhancement on the catalytic activity of Fe5C2 is closely related to the electron donor properties of pyrrolic N. The experimental characterizations including CO-TPD and CO pulse reaction revealed that N-doped C enhanced the CO adsorption at the active site compared to the pure phase of Fe5C2, thereby enhancing the CO adsorption and dissociation ability of the Fe5C2-based catalyst. DFT theoretical calculations show that compared with other N species as well as Fe5C2 (510) / pyrrolic, Fe5C2 (510) / pyrrolic N favored on both thermodynamics and dynamics hydrogen-assisted dissociation activation of CO, confirming that the presence of pyrrolic N improved the adsorption and dissociation capacity of Fe5C2 to CO, thus enhancing the catalyst CO conversion rate. This study provides a reasonable strategy to regulate the active site and provides insights for improving the conversion efficiency of solar-chemical energy.
Editor: Tian Weihan
Revisor: Liu Luyu