**3.3 Development of combined CaO-CuO materials**

With the objective of improving heat and mass transfer phenomena within the reduction/calcination stage of the process, there is an increasing number of works evaluating the synthesis of combined functional Ca-Cu materials [36–39]. Mechanical mixing of CaO and CuO powders was the selected synthesis route followed by Manovic and Anthony [40] for synthetizing for the first time this combined material. These authors prepared pellets by mixing CaO from calcined natural limestone with commercial CuO particles and Ca-aluminate cement as binder in a proportion that resulted in 45:45:10 mass fraction. Material performance was evaluated in a TGA apparatus along successive reduction/calcination and oxidation cycles. The Cu phase was totally converted during reduction (at 800°C in a CH4 atmosphere) and oxidation in air, indicating that this could be a suitable material for the Ca-Cu process. Trying to explore the possibilities of the synthesis route, the same authors prepared core-in-shell materials with different CaO, CuO and Ca-aluminate cement proportions [41], maintaining the CuO in the inner core of the pellet. The OTC of the pellets indicated that a 25%wt. CaO in the core is sufficient to support the CuO and prevent the decay of its activity as an oxygen carrier. In other works, Quin et al. [39, 42] assessed the performance of materials composed by CaO and CuO supported on to MgO, Al2O3 or cement, prepared following diverse synthesis routes (wet mixing, sol-gel and mechanical mixing). The materials were tested in TGA and showed good reactivity along reduction and oxidation cycles using CH4 and air, respectively. The presence of inert support allowed the combined material to maintain its CO2 carrying capacity along cycles. This was especially clear for materials containing MgO on its structure, as this species greatly reduced the resistance to CO2 diffusion during the carbonation stage. In contrast, the presence of Al2O3 produced Ca12Al14O33 after reaction with CaO reducing in this way the amount of active phase for the carbonation reaction. As it happened for the sorbent and oxygen carriers, co-precipitation has been also a synthesis route explored to produce combined materials. Kierzkowska and Müller [38] prepared through this route combined materials with diverse CaO and CuO contents (CaO:CuO molar ratios of 1:1, 1.3:1 and 3.3:1) to be tested in a TGA along multiple carbonation/reduction/calcination/oxidation cycles. These cycles were performed isothermally at 750°C, carrying out carbonation in 40%vol. CO2 in air, reduction in 10%vol. CH4 in N2 and oxidation 4.2%vol. O2 in N2. According to this study, the best result was obtained for the material with a molar ratio 1:1 that maintained a CO2 carrying capacity of 0.18 gCO2 /g material after 15 reaction cycles. In every case, the Cu phase reacted over 98%. Also these authors explored the effect that inert species might have on the chemical stability of the combined material, in this way they prepared via sol-gel materials supported on to Al2O3, MgO and MgAl2O4 with CuO and CaO molar ratio of 1.3:1 and 3.3:1 [43]. As found by other authors, the presence of Mg in the support stabilized the CO2 uptake and minimized carbon deposition. CuO-MgAl2O4 with a proportion of 1.3:1 was the material with the highest CO2 uptake of 0.13 gCO2 /g material after 15 cycles of repeated carbonation/

calcination-redox reactions. In line with the efforts made to produce effective and economic sorbent materials, Kazi et al. [44] developed combined Ca-Cu materials via the hydrothermal synthesis route. A CO2 carrying capacity of 0.15 gCO2 /g material and an oxygen transport capacity of 0.07 gO2/g material after 50 reaction cycles in a TGA were reported for a material composed of 53%wt. CuO and 22% wt. CaO, being the rest Ca12Al14O33. Conditions used for TGA tests were carbonation using a gas mixture of 15%vol. CO2, 25%vol. steam in N2 at 650°C, oxidation at 870°C with 25%vol. air in CO2 and reduction at 870°C in a 40%vol. CO2, 25% vol. steam in N2.
