Dispersed Calcium Oxide as a Reversible and Efficient CO2−Sorbent at Intermediate Temperatures

Dispersion of calcium oxide on high surface area γ-Al2O3 creates a stable and reversible CO2−sorbent that overcomes the problems associated with bulk CaO, such as limited long-term stability, slow uptake kinetics, and energy-intensive regeneration. This sorbent is a candidate for the sorption-enhanced hydrogen production via steam reforming and/or water-gas shift reactions. CO2 uptake tests were performed in a 15% CO2/N2 atmosphere to evaluate the efficacy at typical hydrocarbon reformer gas partial pressure. CO2 uptake kinetics and capacities are investigated in TGA studies, while the long-term stability is shown in multicycle experiments. The dispersed CaO is an active sorbent at low temperatures and binds CO2 at 300 °C up to 1.7 times as efficiently as compared to bulk CaO powder. Furthermore, the sorbent can be regenerated in a CO2-free atmosphere at intermediate temperatures between 300 and 650 °C. Multicycle CO2 uptake and release has been tested for 84 cycles at a constant temperature of 650 °C and shows the superior long-term stability of dispersed CaO as compared to bulk CaO. The attempt to increase the uptake capacity from 0.16 to 0.22 mmol CO2 per gram of sorbent occurred with a commensurate loss in BET area from 115 to 41 m2, leading to a decline in overall uptake efficiency from 15% to 6%. Infrared spectroscopy is used to characterize the CO2−sorbent binding interaction on a molecular level and to distinguish between CO2 adsorbing on the bare support, on bulk CaO, and on dispersed CaO/Al2O3.