Metal promoted Ni/CaO for ICCU for methane production

CaO is the most common adsorbent for CO₂ capture as it is cheap and has a very high capacity of CO₂ capture. The key disadvantage of CaO is that high temperature (>600 °C) is required. For Ni/CaO dual functional materials (DFMs), the addition of a second metal could significantly enhance the production of methane. However, using H₂ as the reducing agent to produce methane during ICCU could have economic issues, as CH₄ is a very cheap fuel, although it has a mature infrastructure.

Different metals including Mg, Al, Mn, Y, Zr, La, and Ce are doped into Ni/CaO for ICCU at 600 °C (https://doi.org/10.1002/aic.17520). The general ICCU performance is shown below.

The results showed that Zr promoted Ni/CaO demonstrated the best ICCU performance in terms of CO₂ capture capacity (maintained at 9 mol/kg), methane selectivity (74%) and thermal stability. Mn doped Ni/CaO demonstrated the worst performance due to the instability of Ca₂MnO₄. The authors claimed a two-step mechanism including 1) CO₂ release from CaCO₃ and 2) CO₂ hydrogenation to CH4 over Ni sites. However, this mechanism needs to be further investigated, as we think it is possible to directly hydrogenation of CaCO₃.

Ni/nanorod CeO2 was prepared and mixed with porous CaO to form DFMs for ICCU (https://doi.org/10.1016/j.cej.2022.135394). In the absence of Ni, CaO-CeO₂ showed almost 100% CO selectivity. When Ni was added, the selectity to CH₄ was increased. This is consistent with the above paper. The presence of Ni might enhanced the methantion of CO reactions. A capacity of CO2 capture between 7.9 and 18 mmol/g was reported and 5.2-12.4 mmol/g CH4 production was obtained. The mechanisms were proposed as below:

“Firstly, CO₂ molecules are chemisorbed on the partially reduced NiO-CeO₂ interface, where most of the C = O bonds are broken together with the oxygen transferring to the ceria support to restore the oxygen balance. These oxygens move around in the ceria lattice through the abundant vacancies and provide available sites for further CO₂ chemisorption. Afterwards, different reaction pathways will be created. A fraction of carbon species are converted to CO through the reverse water gas shift reaction and then hydrogenated to CH₄; simultaneously, part of the oxygens removed upon the double bonds breaking are replaced by catalyst oxygen and released as CO or CO₂, leaving reduced sites on the catalyst again. Therefore, the main products of Ni/CeO₂ sub-nanometer catalysts are CO and CH₄.”

It is noted that the yield of CH₄ was reduced with the increase of ICCU cycles (as shown below). This is mainly due to the sintering of Ni particles instead of the deposition of coke on the surface of the materials.