Influence of Ni and Ce on ICCU-RWGS performance

https://doi.org/10.1016/j.apcatb.2018.11.040

“The integrated CO₂ capture and conversion performances of DFMs are depicted in Fig. 7. The difference between the recorded CO₂ concentrations in the outlet gas stream during CO₂ capture stage and a blank experiment (as shown in Fig. S3, i.e., without any CO₂ sorbent) is equal to the capacity of CO₂ captured. The integrated CO₂ capture and conversion performance, as well as the TOF derived from different DFMs, are summarised in Table 3. The example of TOF calculation is shown in supporting information. The capture capacity of the synthesized CaO is determined as 15.7 mmol g⁻¹ in the first step, which is much higher than that reported in the previous literature due to the porous structure generated by the sol-gel method [64,65]. In the second step, a flow of 100 ml min⁻¹ of 5% H₂ balanced with N₂, ensuring a ratio of H₂ to CO₂ over 3:1, is used. Only a small amount of CO (3.2 mmol g⁻¹) is observed in Fig. S4a for the synthesized CaO due to the absence of Ni active sites, while the majority of captured CO₂ (7.6 mmol g⁻¹)) is released through the regeneration of the DFMs in the H₂ atmosphere. The CO₂ conversion is only 23.4% without the introduction of Ni and Ce species. The unreleased CO₂ is proposed to be contained inside the synthesized CaO as the regeneration temperature is the same as capturing temperature.”

“Fig. 7. Integrated CO₂ capture and conversion reactions at 650 ˚C of (a) Ca₁Ni_0.1; (b) Ca₁Ni_0.1Ce_0.033.”

Table 3. The integrated CO₂ capture and conversion performance, as well as turnover frequency (TOF) of different DFMs at 650 ˚C.

“After the incorporation of Ni, CO₂ capture capacity is slightly decreased due to the reduced mass content of CaO in the DFMs and CO yield is dramatically increased to 6.9 mmol g⁻¹ as shown in Fig. 7a indicating that the addition of Ni can promote the reaction of Eq. (3) in a positive direction through the reaction of CO₂ and H_2. After the incorporation of Ce with a Ca/Ce molar ratio of 1:0.017, the CO yield is slightly increased in Fig. S4b because of the small amount doping of Ce. Further increasing the loading of Ce, Ca₁Ni_0.1Ce_0.033 (Fig. 7b) exhibits the best integrated CO₂ capture and conversion performance with a highest CO yield (7.3 mmol g^-1) and a relatively lower CO₂ yield (6.7 mmol g^-1). The highest TOF of the Ca₁Ni_0.1Ce_0.033 is 0.78 s^-1 indicating a promising catalytic activity of the Ca₁Ni_0.1Ce_0.033 due to the enhanced oxygen storage capacity of Ca-Ce promoted the catalytic performance of Ni species. The Ca₁Ni_0.1Ce_0.033 exhibits the highest CO₂ conversion of 51.8% coupled with a nearly 100% CO selectivity, which is much better than the performance reported by Ranjbar et al. (CO₂ conversion and CO selectivity are 38% and 95%, respectively at 650 ˚C with 10% Ni loading) [66] and Wang et al. (CO₂ conversion and CO selectivity are 38% and 100%, respectively at 600 ˚C with 10% Co loading) [67]. The detailed comparison of integrated CO₂ capture and conversion and reported in the literature are summarised in Table S1. It is found that compared to the conversional RWGS reaction, the performance of integrated CO₂ capture and conversion proposed in this work exhibits a good CO₂ conversion and a remarkable CO selectivity, as well as a good stability.”