Insights into the Mechanism of In Situ CO<sub>2</sub> Conversion during CaCO<sub>3</sub> Hydrogenation
Jun Shen, Cheng Xu, Wei Wei, Xin Tian, Mingyue Ding
Abstract
Carbonate hydrogenation is promising approach to mitigate the CO 2 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 CO 2 conversion. In this work, we aim to elucidate the underlying mechanism behind the promotional effect of H 2 in CaCO 3 hydrogenation through both experimental studies and theoretical calculations. CaCO 3 hydrogenation tests and in situ DRIFTS results indicate that, in addition to the reaction equilibrium (CaCO 3 ↔ CaO + CO 2 ) shift driven by in situ CO 2 conversion, H 2 promotes CO 2 evolution by forming HCO 3 – species upon interacting with CaCO 3 . Density Functional Theory (DFT) calculations further show that the formation of HCO 3 – species can reduce (compared to CO 3 2– species) the CO 2 dissociation energy barrier by 0.84 eV. Regarding the origin of CO production in CaCO 3 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 CaCO 3 by H 2 . Based on these results, the CaCO 3 hydrogenation process follows a tandem reaction mechanism: H 2 promotes CaCO 3 decomposition to emit CO 2 through the formation of bicarbonate species, the released CO 2 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 CaCO 3 hydrogenation, paving the way for the development of more efficient technologies for CO 2 emission reduction.