Ni/CaO and CaO dual functional materials

For integrated CO₂ capture and utilisation (ICCU) target CO production, reverse water gas shift reaction (RWGS) is required. CaO is a common sorbent for CO₂ capture and Ni-based catalysts are typical for RWGS. Thus, the combination of Ni/CaO is usually used at the early stage of ICCU-RWGS research.

A few research papers about ICCU using Ni/CaO based dual functional materials are:

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

https://doi.org/10.1039/D1SE01136G

https://doi.org/10.1016/j.jcou.2022.101922

https://doi.org/10.1016/j.fuel.2020.119308

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

For example, Ni/CaO was used for ICCU-RWGS, and CeO₂ was investigated as a promotor (https://doi.org/10.1016/j.apcatb.2018.11.040). An outstanding CO yield (7.3 mmol g^-1) was reported using CeO₂ promoted dual functional materials. The addition of CeO₂ to Ni/CaO enhanced the dispersion of Ni and also acted as a physical barrier of Ni particles, preventing the sintering of Ni and resulting in a stable performance over 20 cycles of ICCU. However, there was an optimal addition of CeO₂.

During the carbonation of CaO, the volume increase of the material could encapsulate Ni particles on the surface of CaO. The investigation of the distance between Ni and CaO was done (https://doi.org/10.1016/j.fuel.2020.119308). The dual functional materials (DFMs) with Ni closely attached to CaO (prepared by the one-pot method) have relatively low efficiency, as the Ni active sites in DFMs were covered by the formed CaCO₃ due to the short distance between active sites and sorbents. When the DFMs were prepared by the physical mixing of CaO and Ni/CeO₂, the DFMs exhibited a much better ICCU performance including a much higher CO₂ conversion (62%) and CH₄ selectivity (84%) as well as a higher CH₄ yield (8.0 mmol g⁻¹). Therefore, the encapsulation of catalytic sites during the stage of CO₂ capture should be considered for DFMs development. However, this is related to the process of ICCU (e.g. temperature, reducing agent and the targeted products).

Furthermore, Ni-CeO₂/CaO was prepared with highly dispersed sub-nanometer Ni clusters and used for ICCU (https://doi.org/10.1016/j.cej.2022.135394). The materials have very high TOF compared to other reported results (as shown in the following figure). “The yield and selectivity values of CH₄ over our optimum system 0.5%Ni/CeO₂-CaO of 1540 mmol g⁻¹ Ni and 85.8%, respectively, coupled with high stability over 10 ICCU cycles, can be ascribed to the cooperative catalytic performance of highly dispersed Ni clusters and oxygen vacancies present on Ni-doped ceria nanorods.” However, the cost of the production of the material should be considered.

A simple CaO-only was used for ICCU-RWGS. It is supervising that CaO alone is very active for ICCU-RWGS (https://doi.org/10.1016/j.ccst.2021.100001). Without the presence of transition metals, the material is very cheap (shown in the above figure) and robust. CaO-only showed excellent performance of ICCU, with up to 75% of captured CO₂ being 100% converted into CO at 600-700 °C (as shown in the above figure) and the cycle performance of CaO was significantly improved under ICCU conditions. The advantage of using CaO-alone is that the spent sorbent can be used directly in the cement industry, enabling zero-waste production.