Using a simple Ni/CaO catalyst, the integration of CO2 capture and utilistion was carried out to produce a maximum of 10.4 mmol g-1 syngas (DOI: 10.1126/sciadv.aav5077). CO2 conversion was around 60% and methane conversion was over 80%. The conversion of methane was reduced with the increase of the number of cycles (as shown in the following figure). The increase of Ni/(Ca+Ni) ratio resulted in the increase of CH4 conversion, as Ni is the active site for the dissociation of CH4. At the stage of CO2 utilisation (sorbent regeneration), the authors proposed that CO2 was desorbed from CaCO3 and reacted with carbon (derived from CH4 dissociation) to produce CO. This should be further investigated. Could CH4 react directly with CaCO3?
It is noted that carbon deposition was obtained during the ICCU-DRM process. This is a potential challenge. Because the produced carbon can react with CO2 during the carbon capture stage, releasing CO, which can be a new pollutant in the CO2 sources. The production of CO during the carbon capture stage (Figure 3b in the original paper) was clearly demonstrated in ICCU-DRM by S.Molina-Ramírez et al. (https://doi.org/10.1016/j.jcou.2020.101201).
The authors also investigated the kinetics of ICCU-DRM (DOI: 10.1126/sciadv.aav5077). The decomposition of CaCO3 under N2 or CH4 atmosphere was very slow, as shown below. In the presence of Ni, slow reaction kinetic was also observed under N2 atmosphere. However, the carbonated Ni/CaO could be quickly decomposed under CH4. This clearly demonstrates the reaction between CaCO3 and CH4 is largely promoted in the presence of Ni. Therefore, catalyst, sorbent and reducing agent (CH4 in this case) are essential for ICCU.