The research team of Shanxi Institute of coal and chemical technology has made a breakthrough in the research on the stability of Ni nanoparticles in reaction atmosphere

In the heterogeneous catalytic reaction, the sintering and growth process of metal nanoparticles induced by reaction atmosphere is an important reason for catalyst deactivation Due to the strong interaction between reaction gas molecules and metal atoms, it is easy to form metal reaction molecular complexes of migrating species The transfer of this species between metal particles promotes the occurrence of particle growth However, at present, there is a lack of in-depth understanding of the dynamics of the transfer process, and a perfect theoretical system has not been established Recently, the research team of the State Key Laboratory of coal transformation of Shanxi Institute of coal and chemical technology has made important progress in the growth mechanism of Ni particles in the atmosphere of syngas (CO / H2) Based on the precise analysis of the evolution rule of Ni particles with different sizes in the reaction process and molecular dynamics simulation, the modified Ostwald ripening (MOR) theory was established to explain the growth behavior of Ni nanoparticles in the initial stage of methanation successfully This breakthrough revised the understanding of the classical Ostwald ripening theory on the growth of metal particles in the reaction atmosphere, and provided useful guidance for the design and development of high stability methanation catalyst In the atmosphere of syngas, CO is easy to combine with the atoms on the surface of Ni particles to produce Ni (CO) 4 molecules After the molecules are formed on the surface of smaller Ni particles, they are deposited on the surface of larger Ni particles through the transfer process, making the larger particles grow up and the smaller particles disappear gradually The driving force comes from the difference of equilibrium concentration of Ni (CO) 4 on the surface of particles with different sizes So, in the catalyst system, how small particles will shrink and how large particles will grow? The key is to determine the critical particle size The rate of formation and deposition of Ni (CO) 4 molecules on the surface of Ni particles is the same According to the classical ostwaldripening theory, the critical size can be approximately equal to the average size of the Ni particles in the system, and the particles smaller than the average size will be reduced, otherwise they will be increased However, it is found that the critical size of Ni nanoparticles is significantly higher than the average During the reaction, only a few Ni particles grew up, and the Ni particles in the catalyst after the reaction showed bimodal particle distribution (Fig 1) After in-depth study, the phenomenon that the critical size deviates from the average value is due to the steric effect of CO molecules strongly adsorbed on the surface of Ni particles on the deposition of Ni (CO) 4 molecules Because of this effect, the deposition rate of Ni (CO) 4 was obviously affected, and the concentration of Ni (CO) 4 in the catalyst system accumulated With the increase of the concentration of Ni (CO) 4, the steric effect of CO decreases and the deposition rate of Ni (CO) 4 increases When the deposition rate of Ni (CO) 4 is equal to its formation rate, the molecular concentration of Ni (CO) 4 is called the critical concentration At this time, the critical size of Ni particle growth will be higher than that predicted by classical theory Taking the catalyst in Figure 1A as an example, its average size is 3 nm, while the critical size of particle growth is about 6 nm under reaction conditions Because the fraction of Ni particles above 6 nm is small, it grows rapidly in the initial stage of the reaction and finally forms bimodal particle distribution Fig 1 TEM pictures of three Ni particle sizes (3 nm, 6 nm and 12 nm) before reaction (a, D, g) and after reaction (B, e, H); C, F, I is the size distribution map of Ni particles (based on the area average size) of the corresponding post reaction catalyst (source: ACS catalyst) This research has been funded and supported by Shell Global Solutions International bv Related work was published in ACS Catalysis (DOI: 10.1021 / acscalal 8b00835) The first author is Dr Bai Yunxing, the corresponding author is assistant researcher Liu Xingchen, researcher Han Yizhuo and researcher Tan Yisheng.