Green Synthesis of Nanosilica from Coal Fly Ash and Its Stabilizing Effect on CaO Sorbents for CO₂ Capture

High-temperature sorption of CO₂ via calcium looping has wide applications in postcombustion carbon capture, sorption-enhanced hydrogen production, and inherent energy storage. However, fast deactivations of CaO sorbents and low CO₂ uptake in the fast carbonation stage are major drawbacks of this technology. For the first time, we developed a green approach through the reuse of nanosilica derived from coal fly ash (CFA) to enhance both the cyclic CO₂ uptakes and the sorption kinetics of CaO sorbents. The as-synthesized nanosilica-supported CaO sorbent showed superior cyclic stability even under realistic carbonation/calcination conditions, and maintained a final CO₂ uptake of 0.20 g(CO₂) g(sorbent)⁻¹ within short carbonation time, markedly increased by 155% over conventional CaO sorbent. Significantly, it also exhibited very fast sorption rate and could achieve almost 90% of the total CO₂ uptake within ∼20 s after the second cycle, which is critical for practical applications. These positive effects were attributed to the formation of larnite (Ca₂SiO₄) and the physical nanostructure of silica, which could yield and keep abundant reactive small pores directly exposed to CO₂ throughout multiple cycles. The proposed strategy, integrating the on-site recycling of CFA, appears to be promising for CO₂ abatement from coal-fired power plants.