As in our previous works [18], precipitation method using nitrate type precursors was adopted to prepare CaO alone absorbent, CaO–Ca12Al14O33 absorbent, and CaO–Ca12Al14O33–Ni catalytic absorbent. The preparation procedure is as follows. First, the designated amounts of calcium nitrate tetrahydrate (Ca(NO3)2∙4H2O, reagent grade, Aldrich) and aluminum nitrate enneahydrate (Al(NO3)3∙9H2O, reagent grade, Aldrich) were dissolved, either separately or together, in a mixture of isopropyl alcohol (IPA, 130 mL) and distilled water (760 mL). The prepared solution was stirred for 1 h at 348 K and then evaporated at 393 K in a drying oven overnight. The powder was collected from the oven and calcined at 773 K for 3 h in air. Then, the prepared powder was hydrated in a mixture of IPA (65 mL) and distilled water (380 mL) and the resultant slurry was stirred for 3 h at 348 K. Then, the solution was evaporated at 393 K overnight and the residue was calcined at 1173 K for 1.5 h. The collected powder was crushed and sieved to designated particle size range (diameter: 150–300 μm). The samples were named according to their composition: CA for CaO alone, CA–AL for the sample with 75 wt% CaO and 25 wt% Ca12Al14O33, and CA–AL–Ni for CaO–Ca12Al14O33–Ni composite. Also, for the CaO–Ca12Al14O33–Ni composites their Ni weight percentage is appended with ‘-weight percentage’. For instance, CA–AL–Ni-7 indicates CaO–Ca12Al14O33–Ni composite containing 7 wt% of Ni.