Understanding the catalytic acceleration effect of steam on CaCO3 decomposition by density function theory

•Fast CaCO3 decomposition with low starting temperature in the presence of steam.•Understanding action mechanism of steam catalyzing the decomposition of CaCO3.•Detailed reaction paths of CaCO3 decomposition in the presence of steam.•Comparison of the activation energy barriers of various H2O-adsorbing sites. Calcium looping is a fast-developing 2nd-generation CO2 capture technology that has emerged in recent decades. In the practical looping of Ca-based sorbents, steam (6–12 vol%) is naturally present in the calcination atmosphere, due to the oxygen-rich combustion of coal in the regenerator; this results in CO2 separation with a high concentration. Until now, the presence of steam has been reported to have significant effects on the sintering of Ca-based sorbents and the decomposition of CaCO3. However, there is no consensus yet in the literature on the mechanism of the steam-accelerated decomposition of CaCO3. There remains a particular lack of understanding on the catalytic effect of steam on CaCO3 decomposition. Therefore, this work has been designed to first determine the effects of H2O on the regeneration characteristics of Ca-based sorbents. It was found experimentally that the presence of H2O can not only advance the starting point of CaCO3 decomposition, but can also increase its reaction rate significantly. More importantly, the detailed reaction paths of CaCO3 decomposition with the presence of steam were investigated at the molecular level by density functional theory. It was found that on the CaCO3 (1 0 –1 4) surface, the hydrogenation of CO3 and the dissociation of HCO3 are the rate-determining steps and lead to a 0.81 eV reduction in the activation energy barrier of CaCO3 decomposition in the presence of H2O, thus revealing the catalytic mechanism of steam on CaCO3 decomposition.