Cyclic stability of Ni/CeO2-CaO for ICCU-methanation

https://doi.org/10.1016/j.cej.2022.135394

“The cyclic stability, in particular at elevated temperatures, is crucial for the industrial application of sub-nanometer catalysts in the ICCU process. Fig. 4c and d, as well as Fig. S14 show the stability of 0.5%Ni/CeO₂-CaO and 1%Ni/CeO₂-CaO after 10 cycles of ICCU process at 550 °C, respectively. The initial CH₄ yield of 0.5%Ni/CeO₂-CaO after the 1^st cycle of ICCU process is 7.9 mmol gcat.^-1 and the CO₂ conversion and CH₄ selectivity are 51.0% and 86.2%, respectively. After 10 cycles of the ICCU process, even though the CH₄ yield is decreased to 5.2 mmol gcat.^-1, the CO₂ conversion and CH₄ selectivity are still as high as 53.5% and 78.4%, respectively, which indicate that the sub-nanometer catalysts exhibited a good stability. Through the comparison of ICCU performance after 10 cycles, the CO₂ conversion using 0.5%Ni/CeO₂-CaO is 53.5% which is higher than 1%Ni/CeO₂-CaO (50.4%). This is attributed to the high Ce³⁺/Ce⁴⁺ ratio of 0.5%Ni/CeO₂-CaO (0.30), while 1%Ni/CeO₂-CaO has a Ce³⁺/Ce⁴⁺ ratio of 0.24, as shown in Fig. S15, further implying that the oxygen vacancies are important for the conversion of captured CO₂ during the ICCU process. In addition, XPS of spent materials reveal a nickel state which is hydroxidic in nature (Fig. S16), as evidenced by subtle shifts to a higher binding energy, accompanied by a significant decrease in the satellite structure at ∼+3 and ∼+7 eV compared with the oxidic fresh material [59]. Moreover, both the spent 0.5%Ni/CeO₂-CaO and 1%Ni/CeO₂-CaO displayed an obvious peak centered at 700 °C as shown in Fig. S17, which is attributed to decomposition of CaCO₃, further indicating the incomplete desorption of CO₂ from the adsorbent. However, we do not find any peaks assigned to the coke deposition because the coke always be burned below 600 °C. Therefore, the deactivation of the physical mixing materials during the ICCU process might be attributed to the sintering of sorbents because the capacity of carbon capture is decreased from 18.0 mmol g⁻¹ to 12.4 mmol g⁻¹ after 10 cycles of the ICCU process.”

“Fig. S14. Cyclic ICCU performance of 1%Ni/CeO₂-CaO: CO₂ capture capacity, production distribution (a); carbon balance, CO₂ conversion and CH₄ selectivity (b).”

“Fig. S15. Ce 3d XPS spectra of spent 0.5%Ni/CeO₂-CaO and 1%Ni/CeO₂-CaO after 10 cycles of ICCU process.”

“Fig. S16. Ni 2p XPS spectra of spent 0.5%Ni/CeO₂-CaO and 1%Ni/CeO₂-CaO after 10 cycles of ICCU process.”

“Fig. S17. (a) TGA and (b) TPO with the corresponding derivative weight of the spent 0.5%Ni/CeO₂-CaO and 1%Ni/CeO₂-CaO after 10 cycles ICCU process.”