Synthesis and mechanistic study of Ni-based catalysts for CO2 methanation
(2023)
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- Authors
- Supervisors
- Debecker, Damien
- Abstract
- Ni-based catalysts, due to its high CH4 selectivity and attractive cost- effectiveness, is the most widely studied catalyst for CO2 methanation. However, achieving a high methanation activity at low-temperature (especially below 250 °C) for Ni-based catalyst is a great challenge, and the methanation mechanism over Ni-based catalyst is still controversial. In this thesis, three kinds of Ni catalysts have been prepared by sol-gel method in chapter 5 and chapter 6 and by C3N4-assited one-pot method in chapter 7. Furthermore, the methanation mechanism focusing on Ni itself has been studied in chapter 4, and the effect of dopant on the catalytic performance has been studied in chapter 5. Chapter 5 has disclosed the sol-gel preparation of high-loading mesoporous Ni/SiO2 catalysts with small Ni particles. Being based on inert silica, these catalysts are ideal model materials to elucidate the reaction mechanism on nickel. Based on CO2-TPD, in-situ CO2- DRIFTS,TPSR, and DFT calculations, CO2 methanation follows mostly the RWGS+CO-hydrogenation and the formate pathways, the former being dominant at low temperature. Chapter 6 has revealed the effect of dopant (Mn, Co, and Cu) on the catalytic performance of Ni-based catalyst. The effect of promoters is not directly related to improvement of dispersion, reducibility, or basicity. Instead, the promoters orient the reaction mechanism and favor the conversion of key intermediates. Mn addition has the highest promoting effect on the hydrogenation of formaldehyde intermediate (*OCH2) to methoxy intermediate (*OCH3), i.e. the rate determining step of the “RWGS+CO hydrogenation” pathway which is shown to predominate at low reaction temperature. Co addition facilitates the formation of formate species, i.e. the rate determining step of the formate pathway which is also active at high reaction temperature. Cu addition has a negative effect on the rate determining step of those two pathways, resulting a lower performance of Ni-Cu/SiO2. In chapter 7, Ni/CeO2 catalyst is prepared by one-pot method assisted with the C3N4. The activity of the obtained catalyst shows a significantly high CO2 conversion and CH4 selectivity even at a temperature as low as 250 °C. The assistance of C3N4 and the one-pot preparation method increase the number of surface oxygen vacancies and medium basic sites. CO2 methanation on Ni/CeO2 catalysts followed RWGS+CO hydrogenation pathway and the formate pathway, with the former being dominant at low temperatures. The copromotion of those two pathways by the higher amount of oxygen vacancy and medium basic site of Ni/CeO2-CN-OP is responsible for its outstanding low temperature methanation performance.
- Affiliations
Citations
Zhao, Y. (2023). Synthesis and mechanistic study of Ni-based catalysts for CO2 methanation. https://hdl.handle.net/2078.5/234417