Influence of Ni loading on Ni/CeO2-CaO for ICCU-methanation

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

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The ICCU performance of the physical mixture of CaO and sub-nanometer Ni cluster supported on ceria nanorods were evaluated at 550 °C and 1 bar. The results are presented in Fig. 4 and Table 3. When the nickel-free CeO₂-CaO combination is used, a carbon capture capacity of 14.9 mmol g⁻¹ is achieved, and CO is the sole product in the utilisation step. This partial reduction occurs over oxygen vacancies within the ceria, which absorb CO₂ and promote C-O bond cleavage through the oxidation of CeO₂ support as shown in Eq. (5) [8], with hydrogen reactivating the catalytic sites (Eq. (6)). The carbon balance and CO₂ conversion of CeO₂-CaO are only 48.5% and 39.5%, respectively (Fig. 4b). The obtained low carbon balance is accounted for the incomplete desorption of CO₂ from the adsorbent, shown by CaCO₃ content in Fig. S13. This observation agrees with the literature, which identified low activity but high selectivity for CO₂ reduction over ceria [58]. In addition, pristine CeO₂ can activate CO₂ to generate CO, but it is difficult to further hydrogenate the captured CO₂ to CH₄ (Fig. 4a and b), thus proving that CO₂ methanation is based on the synergistic effect of Ni and CeO₂ [48]. The addition of Ni to the system shows an increase in CO₂ conversion, which reaches 50.4%, 62.2% and 67.7% for 0.5%Ni/CeO₂-CaO, 1%Ni/CeO₂-CaO, and 5%Ni/CeO₂-CaO, respectively (Table 3).

(5)CO2+Ce3+↔CO+Ce4+

(6)H2+Ce4+↔H2O+Ce3+

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“Fig. 4. (a, b) ICCU performance, (c, d) cyclic ICCU performance of 0.5%Ni/CeO₂-CaO, (e) the comparison of ICCU performance between this study and reported in the literature, (f) the enlarged view of blue area in Fig. 4e. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)”

“The addition of Ni sites, even at extreme low loadings of only 0.5 wt%, also drastically influences ICCU selectivity, shifting to CH₄ as the primary product as shown in Fig. 4b and Table 3. Furthermore, the yield of CH₄ in the ICCU process is significantly increased to 1540 mmol g⁻¹ Ni over 0.5%Ni/CeO₂-CaO, which represents a significant enhancement compared to the materials as previously reported in the literature depicted in Fig. 4e. It is noted that the conversion of CO₂ using 5%Ni/CeO₂-CaO is only slightly higher than that using 0.5%Ni/CeO₂-CaO, although the difference of Ni loading is 10-fold. Therefore, 0.5%Ni/CeO₂-CaO exhibits a much higher CH₄ yield per gram of Ni active species (1540 mmol g⁻¹ Ni) and TOF (188 h⁻¹) compared with other materials. The exceptional capacity of 0.5%Ni/CeO₂ for methane formation can be attributed to the high level of CO₂ reduction that occurs over oxygen vacancies on the ceria support, leaving Ni sites for CO hydrogenation to methane. When the heating rate of the reduction process is increased from 2 °C min⁻¹ to 10 °C min⁻¹, the yield of CH₄ is decreased from 168 mmol g⁻¹ Ni to 100 mmol g⁻¹ Ni, which is probably due to the bigger Ni particle size of 5%Ni/CeO₂-10 resulting in fewer oxygen vacancies. This is further confirmed that the sub-nanometer catalysts exhibited higher activity in the ICCU process.”