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 H-2 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 H-2 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, H-2 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 CO32- 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 H-2. Based on these results, the CaCO3 hydrogenation process follows a tandem reaction mechanism: H-2 promotes CaCO3 decomposition to emit CO2 through the formation of bicarbonate species, the released CO2 then reacts with H-2 over CaO to produce CO via the formate mechanism. This study provides valuable insights into the promotional effect of H-2 and the origin of CO in CaCO3 hydrogenation, paving the way for the development of more efficient technologies for CO2 emission reduction.