MgO promoted CaO for CO2 capture

https://doi.org/10.1002/cssc.202002078

“Inert metal oxide stabilizers such as MgO do not form mixed phases with CaO and do not form carbonates at typical CaL conditions (i. e., T>600 °C).^54, 63, 76 However, a similar tendency for agglomeration of the stabilizer phases as observed in reactive stabilizers (e. g., Al₂O₃ or ZrO₂), is observed for inert stabilizers.^12, 23, 76 In an early study, Liu et al.⁵¹ used a wet-mixing method to synthesize MgO-stabilized CaO with an almost constant CO₂ uptake of 0.59 g /g_Sorbent over the course of 24 cycles (25 wt % MgO, carbonation at 650 °C and calcination at 900 °C in N₂). The authors showed via EDX-TEM that MgO forms agglomerates on the surface of CaO particles. It was argued that these agglomerates act as barriers against sintering of CaO. In a more recent study, Kurlov et al.⁷⁶ investigated the distribution of MgO and changes thereof over cycling in MgO-stabilized CaO-based sorbents using EDX-TEM, see Figure 9b. Initially, nanoparticles of MgO of diameters up to 50 nm, were evenly distributed on the surface of CaO grains in the as-prepared sorbent. After 30 cycles, the MgO nanoparticles agglomerated leading to an increase in particle size (100–200 nm). It was argued that larger MgO nanoparticles were less effective in mitigating sintering and hence led to a decrease in the CO₂ uptake. Naeem et al., who synthesized hollow microspheres of CaO stabilized by MgO,²⁴ also observed an increase in the size of the MgO nanoparticles over cycling. The shell of these microspheres consists of CaO and MgO nanoparticles. Over cycling, the size of the MgO nanoparticles increased from 15–35 nm in the as-prepared sorbents to about 80–100 nm after 10 cycles under realistic conditions (i. e., calcination under CO₂ at 900 °C). However, the authors observed a homogeneous distribution of the MgO nanoparticles in the sorbent even after cycling and no significant segregation of larger MgO aggregates from CaO.”