Quantitative Characteristics and Microscopic Mechanism of Direct Reduction of CaCO₃ with CH₄ for Syngas Production

The direct reduction of CaCO₃ with CH₄ emerges as a pivotal strategy to mitigate intensive carbon emissions with higher energy efficiency than conventional pyrolysis. Despite the promising industrial potential, its advancement is fundamentally hindered by the lack of quantitative metrics for key reaction behaviors and the limited mechanistic understanding of the facilitating role of CH_4. To address these gaps, we quantify the onset decomposition temperature of CaCO₃ and its reduction kinetics under the synergy of CH₄ and the catalyst, and elucidate the underlying microscopic mechanism. Experimental results demonstrate a 29% reduction in the decomposition temperature of Ca₉₅Ni_2.5Fe_2.5 in CH₄ compared to Ca₁₀₀ in Ar, with an empirical correlation derived for quantitative prediction, and the activation energy also decreases drastically by 35%. In situ DRIFTs and DFT calculations confirm that the promotional effect originates from the dissociation of CH₄ at catalyst active sites to generate H^*, which migrates to the CaCO₃ surface, induces the formation of key intermediates, weakens the C─O bond strength, and lowers the decomposition energy barrier. This study quantifies core properties of the direct CaCO₃ reduction with CH₄ and proposes a formate-involved reaction pathway, offering vital guidance for the advanced low-carbon and high-value utilization of carbonates.