Insights into the Mechanism of In Situ CO₂ Conversion during CaCO₃ Hydrogenation

Carbonate hydrogenation is promising approach to mitigate the CO₂ emissions in hard-to-decarbonize industries, such as cement and refractory production, which involve the thermal decomposition of inorganic carbonates. Compared to traditional air calcination, the introduction of H₂ during carbonate decomposition offers two key advantages: (1) enhanced decomposition rates, and (2) in situ CO₂ conversion. In this work, we aim to elucidate the underlying mechanism behind the promotional effect of H₂ in CaCO₃ hydrogenation through both experimental studies and theoretical calculations. CaCO₃ hydrogenation tests and in situ DRIFTS results indicate that, in addition to the reaction equilibrium (CaCO₃ ↔ CaO + CO₂) shift driven by in situ CO₂ conversion, H₂ promotes CO₂ evolution by forming HCO₃^– species upon interacting with CaCO₃. Density Functional Theory (DFT) calculations further show that the formation of HCO₃^– species can reduce (compared to CO₃^2– species) the CO₂ dissociation energy barrier by 0.84 eV. Regarding the origin of CO production in CaCO₃ hydrogenation, experiments under controlled reaction atmospheres and in situ DRIFT spectra clearly demonstrate that CO is produced via the reverse water–gas shift (RWGS) reaction, with CaO acting as the self-catalyst, rather than through the direct reduction of CaCO₃ by H₂. Based on these results, the CaCO₃ hydrogenation process follows a tandem reaction mechanism: H₂ promotes CaCO₃ decomposition to emit CO₂ through the formation of bicarbonate species, the released CO₂ then reacts with H₂ over CaO to produce CO via the formate mechanism. This study provides valuable insights into the promotional effect of H₂ and the origin of CO in CaCO₃ hydrogenation, paving the way for the development of more efficient technologies for CO₂ emission reduction.