Ni-based DFMs for ICCU-DRM

Ni is a commonly used catalyst for conventional dry reforming of methane. Due to the very limited amount of work available in ICCU-DRM, at the time of writing, the majority of dual functional materials for ICCU-DRM used Ni-based catalysts. The following content of ICCU-DRM related to Ni-based DRM is from this reference (https://doi.org/10.1016/j.ccst.2022.100052). It is noted that the conversion of CO₂ from the following discussion is quite different. In this reported work (Kim et al., 2018), unbelievable high conversion (>95%) was reported. The same research group didn’t report further work related to this research area. In other literature and our experimental work, the conversion of CO₂ and CH₄ is much lower. This is due to the release of CO₂ from the decomposition of sorbent at the CO₂ utilisation stage.

“The carbon capture and dry reforming of methane (DRM) coupled in a single reactor is demonstrated to be feasible with Ni-CaO as the DFM (Kim et al., 2018). In this study, Ni supported on MgO-Al₂O₃ was prepared via co-precipitation and then mixed with CaO as a DFM composite. It was used for cyclic CO₂ capture and DRM in a fluidized bed reactor (Fig. 10a). The CO₂ capture and conversion process occurred isothermally at the temperature of 720°C, to match the CO₂ release rate with the DRM activity of the Ni-based catalyst. They found negligible CO₂ (less than 0.08%) was released in the off-gas during the conversion process, and almost all of the CH₄ was converted in the pre-breakthrough stage (as shown in Fig. 10b). It should be noted that the ratio of H₂ to CO was about 1.1, exceeding the theoretical value (∼ 0.94 at 720 °C) and probably ascribed to the occurrence of methane decomposition (Eq. 7). In addition, the cyclic stability of the DFM composite was tested, in which the high conversion efficiency of CH₄ (97.1% to 96.4%) and CO₂ (96.3% to 95.3%) were maintained over ten cycles. However, due to the low Tammann temperature of CaCO₃ (533°C), serious sintering and agglomeration of CaO were observed during cycling, causing the loss of CO₂ uptake capacity (Fig. 10c).”

Fig. 10. (a) Schematic of the proposed ICCC-DRM directly convert CO₂ into synthesis gas; (b) ICCC during the 1^st cycle; (c) cyclic CO₂ capture of CaO evaluated using TGA and fluidized bed (Kim et al., 2018).

“Ni was impregnated on the CaO surface for CO₂ capture, followed by DRM to generate syngas dynamically (Cruz-Hernández et al., 2017). The CO₂ capture was conducted by increasing the temperature from 30 to 600°C, then maintained at 600°C for one hour and cooled down to 200°C to complete the whole capture step. In the subsequent DRM stage, 2% CH₄ was introduced while increasing the temperature from 200 to 900°C. During the DRM process, syngas was produced at two different temperature range: between 500 and 650°C, and at temperatures over 800°C, corresponding to the DRM and CH₄ partial oxidation, respectively. The Ni-impregnated CaO was compared with the CaO-NiO physical mixture, and it was found that Ni-impregnated CaO always showed better performance than the latter.”

“The CO₂ capture and DRM were further investigated by Tian et al. (Tian et al., 2019), using Ni-CaO sorbent-catalyst synthesized by a sol-gel method as the DFM. The process was operated in a temperature swing mode, in which the CO₂ capture occurred at 600°C while the DRM occurred at 800°C. About 60% and > 80% conversion efficiency were achieved for CO₂ capture and DRM over ten cycles, respectively, which was competitive with other novel methane reforming. The cyclic CO₂ uptake capacity was also decreased due to the sintering of CaO, which caused less CO₂ available in the subsequent conversion stage for CO production. The H₂/CO ratio (∼ 1.1) was higher than the theoretical DRM (i.e., ∼ 1), indicating the concurrence of CH₄ decomposition and dry reforming in the conversion stage, as also reported by Kim et al. (Kim et al., 2018). The presence of Ni accelerated the decomposition of CaCO₃ due to the continuous consumption of CO₂ by CH₄ reforming, providing an additional driving force for CaCO₃ decarbonation (La Chatellier’s principle). Thus the decarbonation rates were significantly improved compared to the CaCO₃ alone, as shown in Fig. 11. With increasing the Ni content, both the CO₂ and CH₄ conversion increased.”