Thermodynamic assessment of non-catalytic Ceria for syngas production by methane reduction and CO2 + H2O oxidation
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Chemical looping syngas production is a two-step redox cycle with oxygen carriers (metal oxides) circulating between two interconnected reactors. In this paper, the performance of pure CeO2/Ce2O3 redox pair was investigated for low-temperature syngas production via methane reduction together with identification of optimal ideal operating conditions. Comprehensive thermodynamic analysis for methane reduction and water and CO2 splitting was performed through process simulation by Gibbs free energy minimization in ASPEN Plus®. The reduction reactor was studied by varying the CH4/CeO2 molar ratio between 0.4 and 4 along with the temperature from 500 to 1000 °C. In the oxidation reactor, steam and carbon dioxide mixture oxidized the reduced metal back to CeO2, while producing simultaneous streams of CO and H2 respectively. Within the oxidation reactor, the flow and composition of the mixture gas were varied, together with reactor temperature between 500 and 1000 °C. The results indicate that the maximum CH4 conversion in the reduction reactor is achieved between 900 and 950 °C with CH4/CeO2 ratio of 0.7–0.8, while, for the oxidation reactor, the optimal condition can vary between 600 and 900 °C based on the requirement of the final product output (H2/CO). The system efficiency was around 62% for isothermal operations at 900 °C and complete redox reaction of the metal oxide.
KeywordsOxygen carriers Ceria Chemical looping Syngas Thermodynamic analysis
Synthesis of non-fossil fuels through carbon dioxide (CO2) recycling via thermochemical or electrochemical pathways has received significant research interest in recent years as a complementary option to mitigate carbon emissions [2, 6, 13, 30]. Thermochemical redox cycles driven by concentrated solar power (CSP) have been widely studied for simultaneous splitting of H2O and CO2 for syngas (a mixture of H2 and CO) production [43, 44, 50], which can be further processed to valuable compounds via established industrial processes like the Fischer–Tropsch process. Of the numerous multi-step thermochemical redox cycles that have been proposed, the two-step metal-oxide redox pair systems have shown considerable potential for synthetic solar fuel generation [40, 49, 55]. These thermochemical cycles utilize the transition between a higher valence oxidized (MeOoxd) and lower valence reduced (MeOred) form of a metal-oxide usually having multiple oxidation states . At first, MeOoxd undergoes a highly irreversible and endothermic thermal reduction (TR), thereby releasing oxygen and generating MeOred. Subsequently, MeOred is oxidized back to a higher valence state by taking oxygen from water and/or CO2 (often present in industrial effluent gases, viz., oxyfuel power plant exhausts) via an exothermic reaction, in turn, generating H2 and CO in water splitting (WS) and CO2 splitting (CDS) reactions, respectively. This results in the reduction temperature (TRED) to be much higher (often above 1400 °C) than the oxidation temperature (TOXI) . In a solar-driven cycle, this highly endothermic reduction reaction is sustained by concentrated solar power, for which multiple solar reactor types have been investigated. Packed beds reactors , porous structures [8, 19], rotating components [15, 29] and moving particles [41, 56] are some common examples of such reactors.
During reduction (Eq. 1), the metal oxide is reduced by methane, often up to a non-stoichiometric extent, δ. The δ moles of oxygen thus released, form CO and H2 by partial oxidation of CH4. In the subsequent reaction steps, (Eq. 2), MeOx-δ reacts with CO2 and/or H2O to reincorporate oxygen into the metal oxide lattice, while reducing the CO2 and/or H2O into a CO or H2, respectively. Reactions (2a) and (2b) can be intrinsically assumed to result in complete oxidation at thermodynamically favourable temperatures depending on the metal oxide redox pairs.
Cerium (IV) oxide (CeO2) is widely investigated in literature for its structural, chemical and optical properties, making it a promising material in several fields of applications, such as fuel cells, catalysis, CO2 adsorbing materials, nanofluids, etc. . Furthermore, the crystallographic stability of CeO2, even after several runs of thermal processes, is well documented [1, 9]. Ceria has received particular attention in the context of solar-driven thermochemical production of fuels due to large oxygen content capacity and ability to accept and release in response to temperature and oxygen chemical potential change. Higher efficiency can be achieved by structuring ceria in mesoporous or microporous forms to achieve relatively short bulk diffusion lengths, high surface area and increased porosity for easy transport of product and reactant gases. It is observed that the large porosity is desired for higher radiative heat transfer [11, 24]. In recent studies [12, 23] it has been concluded that in the redox cycle, at temperatures above 1000 °C, due to the extremely rapid bulk oxygen diffusion and surface reaction on ceria, the overall rate depends primarily on the reactant gas flow rate. This is usually the limiting phenomenon referred as gas phase limited dynamics (thermo-kinetic controlled) or quasi-equilibrium behaviour, for reduction. For lower temperature (below 1000 °C) or in the oxidation step, the surface reaction on the porous ceria is the rate-limiting step. This rapid kinetics, together with minimal effect of sintering at high temperature (below 1500 °C) [47, 51, 59] with good attrition resistance and mechanical strength makes ceria the state of the art among the non-volatile redox pairs for CO2/H2O splitting application , which can be further exploited for large-scale applications . Even though numerous studies have focused on non-stoichiometric ceria, the complete reduction of CeO2 to Ce2O3 (δ = 0.5) is required for a high H2 and CO yield, primarily constrained by the temperature. The complete thermal reduction to Ce2O3 requires more than 2000 °C, rarely achievable even by CSP systems for continuous operation. In contrast, fuel reduction would assist the metal oxide to be reduced at a much lower temperature allowing the redox cycle to operate isothermally, as well as continuously. With the abundance and low price of natural gas, or the use of biomethane as a renewable fuel, reactive chemical looping partial oxidation of methane together with CO2 and H2O dissociation offers a simple and promising multi-step syngas production process. In effect, such an isothermal redox operation would then lead to a much reduced operation cost, enhanced stability and improved system efficiency due to the elimination of inefficient processes.
Nonetheless, most of the early propositions and experiments of the novel methodology has been performed considering catalytic processes, which, even though would enhance the rate of reaction, often increase the operation costs by using expensive catalysts like Platinum or Rhenium [17, 38]. Hence, the use of non-catalytic ceria is much desired from both technical and commercial significance. In this paper, the thermodynamic limitations of the non-catalytic process, aimed at establishing the theoretical limits of the redox cycle with non-catalytic ceria as the oxygen carrier and methane reduction have been studied and documented.
In the reduction reactor, the methane reduces the metal oxide at a higher oxidation state (CeO2) to a lower oxidation state (Ce2O3), while itself being oxidized to CO and H2 via reaction (3). The reduced ceria oxide is then recycled back to the higher oxidation state through reactions (4a) and (4b). In both the reactors, syngas can be generated, however, with varying H2/CO fractions. While, from thermodynamic and mass conservation conditions, the H2 to CO ratio of the syngas from the reduction reactor is always 2:1, the ratio in the oxidation reactor can be varied based on the inlet gas feed mixture and other thermodynamic parameters.
Multiple studies, mostly based on iron oxide-based redox metal pairs have reported the conversion efficiencies and operating conditions for conversion of methane into syngas over metal oxides [26, 34, 45]. Such studies also include the limiting operation range based on the need to prevent carbon deposition reactions as crucial for the system operation. Thermodynamics of Ceria reduction with hydrogen have been investigated to explore the maximum extent of reaction and reported in the literature . Solar-driven thermal reduction for ceria coupled with either CO2 or H2O splitting has been studied extensively by Welte [41, 56] and other researchers [5, 43]. However, limited literature on the thermodynamic assessment regarding methane reduction of ceria followed by splitting of waste gas (a mixture of CO2 and H2O) is available . Additionally, the need to identify the regimes for carbon formation is crucial to ensure the suitable operation regimes of the reaction system further.
Therefore, the aim of the present study was to perform thermodynamic and process simulation studies of the CeO2/Ce2O3 redox pair chemical looping syngas production via methane reduction to obtain the ideal operating conditions, avoiding carbon deposition. The analysis has been performed by evaluating the thermodynamic equilibrium composition of the reaction system, the impact of the reactant feed molar ratios and temperature on the product compositions for the reduction and oxidation reactors, respectively. Furthermore, the redox cycle performance via system efficiency was also assessed.
The thermodynamic simulation of methane reduction and water and CO2 splitting was performed in ASPEN Plus®. Gibbs free energy minimization principle (GFEM) was used to perform the thermodynamic calculations. For a reaction system, where multiple simultaneous reactions take place, equilibrium calculations are often performed through the GFEM approach, details of which can be found in the literature [10, 20]. For the entire set of reactors and components modelled, the gaseous species include CH4, CO, CO2, H2, and H2O, while the solid species were C, Ce2O3, and CeO2.
The process layout of the simulation system is shown in Fig. 2. The ASPEN Plus® RGIBBS reactor model was used to simulate both the reduction and oxidation reactors, using the Peng Robinson equation of state. Within the RGIBBS reactor, the equilibrium composition of all feasible combination of reactions within the thermodynamic domain was considered. The RGIBBS reactor calculates the most stable phase combination obtained through chemical reactions where the Gibbs free energy of the reaction system reaches its minimum at a fixed mass balance, constant pressure, and temperature. Besides the RGIBBS module, the other components simulated were cyclone units to separate solid and gas streams, and heat exchangers, in which steams are preheated to reach the temperatures of reaction and heat is removed from the reaction products.
For the reduction reactor, the temperature was varied in the range of 500–1000 °C, at a constant pressure of 1 atm. The CH4/CeO2 feed molar ratio was varied from 0.4 (sub-stoichiometric value according to reaction (3)) to 4. The solid product of the reduction reactor was fed to the oxidation reactor after cyclone separator. The oxidation reactor was modelled by a series of two RGIBBS reactors. The oxidation of CO2 and H2O over Ce2O3 is a highly exothermic reaction. Therefore, two rectors with an intercooler were modelled to simulate an ideal isothermal reactor. The first reactor was modelled as an adiabatic reactor, while the second reactor was an isothermal reactor, set at the temperature of the reaction. In the oxidation reactor, the Ce2O3 was reacted with a mixture of steam and carbon dioxide according to reactions (4a) and (4b). Similar to the reduction reactor, the oxidation reactor temperature was varied between 500 and 1000 °C at a constant pressure of 1 atm. The feed flow of the mixture was varied between the range of 0.5 and 2 kmol/h (stoichiometric to excess flow). The study corresponding to the oxidation reactor was performed to obtain the quantitative H2 and CO produced at multiple regimes and hence identify the conditions of operations for different H2/CO ratio requirements for subsequent downstream processes. Additionally, determination of the minimum amount of gas flow and the corresponding composition to regenerate completely the reduced ceria was also aimed for within the regime of favourable reaction thermodynamics. However, it should be noted that, in the present study, all the simulation calculations performed were based on theoretical thermodynamic considerations, since no heat and mass diffusional limitations along with kinetic effects were considered conforming to the present thermodynamic analysis. This corresponds to the theoretical limits that must be considered during further experimental evaluations of the reaction systems.
Results and discussion
The equilibrium composition of H2, CO, CO2, H2O, O2 and CH4 and C, CeO2 and, Ce2O3 obtained from the reduction of methane over CeO2 in a temperature range of 500–1000 °C and CH4/CeO2 feed molar ratios from 0.4 to 4 are discussed in the following section.
Nevertheless, as can be visualized from Fig. 3, an operation with 0.7–0.8 mol of CH4 per mole of CeO2 at around 900–950 °C would provide the ideal operating conditions with respect to methane utilization, without the need to feed a high fraction of methane. A syngas stream of 31% CO and 63% H2 can be obtained (balance 1% H2O, 0.4% CO2 and 4.6% CH4) at around 950 °C and a CH4/CeO2 feed ratio of 0.7 to 0.8. Indeed, for higher methane flows, the excess methane at the outlet of the reduction reactor would decrease the effectiveness of the chemical looping system.
As indicated in Fig. 4c, the carbon deposition initiates at methane to ceria feed ratios above 1.0 and a temperature above 900 °C and subsequently increases with higher molar flows of methane and temperature. This is because the thermodynamics for either the Boudouard and/or the methane decomposition reactions (Eq. 5, 6) are not favourable at other conditions. As discussed before, the production of carbon in the reduction reactor has considerable influence on the system efficiency due to competitive reactions with Ce2O3 in the oxidation reactor. Even though the fraction of carbon content is exceedingly low, this would restrict the working conditions with methane reduction to around 900 °C, and the molar feed ratio, to less than or around 1.0.
Comparison of thermodynamic results with results reported by Warren et al.  for the reduction of ceria with methane
The equilibrium amounts of H2 and CO obtained by splitting CO2 and H2O over reduced Ce2O3 within the oxidation reactor is presented in the following section. The parametric study was carried out within a temperature range of 500–1000 °C, considering completely reduced Ceria (Ce2O3) being fed into the oxidation reactor. A variation of H2O/CO2 mixture composition (from 5% to 95% CO2) and the molar flow rate of the mixture from 0.5 to 2 kmol/h was also performed. In all the cases the flow of Ce2O3 was kept constant at 0.5 kmol/h, considered to be completely reduced from 1 kmol/h of CeO2 in the reduction reactor as per reaction (3).
On the other hand, the CO yield increases at a higher rate till around 650 °C, after which the rate of increase of CO yield drops considerably. The higher the flow of the waste gas, the lower the difference in the rate of yield increase between the two temperature ranges (below and above 650 °C). For molar flows higher than stoichiometry (0.5 kmol/h), the yield becomes stable at about 0.28 kmol/h with a further rise in temperature, irrespective of the increase in molar feed flow.
The yield variations based on the thermodynamic conditions play a critical role in varying H2/CO ratio obtained at the outlet of the oxidation reactor, which, therefore, can be controlled to obtain the H2/CO ratios required for specific processes. Combining the yields of the two gases, for the stoichiometric flow of waste gas (1 kmol/h and equimolar mixture), a syngas stream of 45% H2 and above 40% CO could be obtained. The remaining fraction of the gas is composed of un-reacted species. However, sending above-stoichiometric flows, even though it would result in complete oxidation of Ce2O3 and providing maximum yield, would result in syngas fraction to drop considerably. This would decrease the effectiveness of the process by requiring additional downstream processes to separate CO2 and water for obtaining pure syngas.
The variation of the ratios H2/CO from the oxidation reactor, based on varying compositions of H2O and CO2 at a constant waste gas feed flow of 1 kmol/h of the mixture, is presented in Fig. 7b. The formation of H2 is 18 times more than that of CO for a waste gas containing 80% of water vap+ and 20% of CO2 at a temperature of 500 °C. However, at the same temperature, for a gas containing 80% CO2, the H2/CO ratio is about the same ratio as H2O/CO2. Indeed, as can be followed from the previous discussions, with the formation of H2 peaking at around 600 °C, with the corresponding increase in the CO yield, the ratio of H2/CO decreases to about 2.5 even with 80% H2O at the feed stream. This would result in the outlet gas to contain a significant fraction of unreacted H2O, while all the CO2 would have been converted to CO. At higher fractions of CO2, higher temperatures would yield better result from the conversion perspective of the waste gas feed. It needs to be mentioned that higher flow rates were also studied for variation of composition with similar trends and hence not shown separately in the paper. By such consideration, therefore, the need for determining the operating temperature of the oxidation reactor, depending on the composition of the waste gas, would play a crucial role in determining the most effective conversion, besides ensuring complete oxidation of the reduced metal. Also, the importance of the requirement of the H2/CO ratio for subsequent downstream processes is to be given importance. Nonetheless, it can be concluded that for waste gases, with large fractions of water content, it is preferable to maintain the oxidation reactor at a temperature about 600–700 °C to ensure maximum reactivity of H2O. However, for higher CO2 content, typically occurring for exhaust of power plants, the temperature of the oxidation reactor can be set at higher temperatures of around or above 900 °C, thereby ensuring high conversion of CO2, and also presenting the possibility to operate the redox cycle at isothermal conditions.
On the other hand, the oxidation reaction is exothermic over the entire thermodynamic conditions considered in the paper. As follows from thermodynamic laws, an exothermic reaction is favoured at lower temperatures. This is indeed represented in Fig. 8b, where, at lower temperatures, the heat released from the reaction is much more pronounced, than the heat released at higher temperatures. Additionally, at lower temperatures, the heat released is primarily from the splitting of water, which is much more exothermic than the corresponding CO2 splitting reaction, which gains predominance at higher temperatures. However, the overall reaction continues to be exothermic. Indeed, the drop of exothermicity at higher temperatures impacts on the overall system efficiency and thermodynamics and has been subsequently discussed in the following sections.
As discussed, the advantage of ceria reduction by methane is the lowering of the reduction temperature. Therefore, as can be deduced from the present analysis, an isothermal system with complete reduction and oxidation of ceria in the respective reactors can be obtained via the present layout. This, however, would limit the isothermal operation zone to between 850 and 950 °C, since this would ensure the complete reduction and corresponding oxidation of CeO2 with the selected flow of methane as discussed earlier. In fact, it is interesting to note that even though the oxidation reactor is exothermic, the exothermicity is lower than the endothermicity of the reduction reactor within the defined range of isothermal operations. Hence, external heat would be required for driving the system. This can be achieved either by utilizing concentrated solar power  or by burning additional fuel.
To evaluate the system performance and identify the scope of improvement, the efficiency of the system plays a significant role. As the case, two parallel streams of syngas are produced, of which, however, the syngas from the oxidation reactor is the main aim of such thermo-chemical cycles, as the goal of the system is to produce syngas from waste streams of CO2 and H2O. In the reduction reactor, methane is converted to syngas to drive the redox cycle with the methane content in the syngas varying significantly depending on the operating conditions of the reactor (i.e., temperature and CH4/CeO2 fraction), as discussed previously in the paper. Even with high fractions of unreacted CH4, this syngas can be utilized for multiple purposes as well. Besides being further upgraded to syngas by chemical conversions via steam reforming reactions, it can be utilized directly for combustion. However, efficiencies of such conversions are directly dependant on the downstream conversion process required and hence was left out of scope within the present definitions. In the case that the methane is fully converted, and the reduction syngas composition matches with the one of the syngas obtained in the oxidation reactor, the two syngas flows can be mixed for subsequent use in the same process.
Therefore, considering the diverse opportunities, two efficiencies were defined for the proposed system considering the performance of both the reactors, the preheating requirements of the solids and gas reactants in both the reactors, as well as the heat recuperated from solid. The first efficiency takes into account the syngas produced in both the reactors, while the second efficiency is defined considering only the syngas from the splitting of CO2 and H2O in the oxidation reactor.
Based on the defined efficiency η1 the excess methane plays no significant role in increasing the H2 and CO yield of the system; however, it decreases the system efficiency. Following the discussion, to ensure high system efficiency together with maximum possible system yield, the system should operate with a CH4/CeO2 molar feed ratio between 0.7 and 0.8 at a temperature of 900 °C or higher. In these conditions the efficiency is around 60 to 70%, yielding 1.2 kmol/h of H2 and 0.8 kmol/h of CO from a stream of1 kmol/h (CO2/H2O ratio equal to 1).
As can be seen, the amount of high-temperature heat needed is significant due to the highly endothermic reduction reaction. Such heat, however, can be provided either through concentrated solar energy—even if this option could not allow the continuous operation of the system—or by burning a fuel, for example, additional methane or renewable fuels, thereby enabling the system to run continuously. Even combined solutions can be proposed, by providing heat from burning fuels only to integrate the solar heat flux when it is not sufficient. The analysis of these solutions has not been included in the paper, as it is outside of the scope.
Indeed, as can be seen, due to the considerable amount of heat content from the exit product gas streams from both the reactors, a considerable amount of heat is available at lower temperatures, increasing the system performance as per the defined efficiencies. Integration with larger systems, therefore, would yield benefits through the availability of significant amounts of low-temperature waste heat.
In the present paper, the performance of the CeO2/Ce2O3 redox pair was evaluated for chemical looping syngas production through methane reduction and carbon dioxide and water splitting using thermodynamic analysis. Process simulation was performed to identify the limiting, as well as the most favourable working conditions with corresponding efficiency evaluation. In the fuel reactor, syngas production was studied via reduction of the metal oxide by methane. For the primary aim of the reduction reactor to produce syngas, methane to CeO2 feeding ratio of 0.7–0.8 at a temperature of 900 °C was obtained as the most suitable condition, resulting in a complete reduction of CeO2 to Ce2O3 while avoiding the formation of CO2 and carbon deposition. The temperature and composition of waste gas (a mixture of CO2 and H2O), coupled with the end use of produced syngas, would govern the operating conditions of the oxidation reactor. However, water splitting reaction peaks at temperatures between 600 and 650 °C, while a monotonic increase of CO production with the temperature was obtained for CO2 splitting reaction. A minimum molar flow of 0.75 kmol/h of waste gas at the equimolar composition of CO2 and H2O would be required to oxidize a flow of 0.5 kmol/h Ce2O3 completely to CeO2 to close the redox cycle. This corresponds to a flow of 50% excess than the stoichiometric quantity. Further, the system efficiency was evaluated based on two defined efficiency terms for the chemical looping configuration. It is observed that the variations of the flow of waste gas (a mixture of CO2 and H2O), as well as the composition, had little or no impact on the overall system efficiency. Nevertheless, for lower flows of methane, high system efficiency was obtained, however with lower yields of H2 and CO. A system efficiency of around 62%, considering syngas from both the reactors, with a production of syngas composed by 60% H2 and 40% CO was obtained for an isothermal operation at 900 °C or higher, as the optimum for the entire chemical looping cycle. However, the value drops to 16% while considering only the syngas from splitting of CO2 and H2O. The corresponding isothermal system temperature needs to be 900 °C between the reduction and oxidation reactor. In the end, it can be concluded that these results can be taken as a limiting basis for future experimental and theoretical studies in determining the extent of reactions with non-catalytic ceria-based chemical looping CO2 and H2O splitting with methane reduction to evaluate the proposed technology.
- 4.Ambrosini, A., Eric N. Coker, Anthony, M., Mark, A., James A.O., James E.M.: Oxide materials for thermochemical CO2 splitting using concentrated solar energy vision : sunshine to petrol (2012)Google Scholar
- 10.Collins-Martinez, V., Bretado, M.E., Zaragoza, M.M., Gutiérrez, J.S., Ortiz, A.L.: Absorption enhanced reforming of light alcohols (methanol and ethanol) for the production of hydrogen: thermodynamic modeling. Int. J. Hydrogen Energy 38(28), 12539–12553 (2013). https://doi.org/10.1016/j.ijhydene.2012.11.146 CrossRefGoogle Scholar
- 18.Furler, P, Scheffe, J., Marxer, D., Steinfeld, A.: Solar reactors for thermochemical CO2 and H2O splitting via metal oxide redox reactions. In SFERA II SUMMER SCHOOL, Odeillo, France, France (2014). https://sfera2.sollab.eu/uploads/images/networking/SFERA SUMMER SCHOOL 2014 - PRESENTATIONS/Solar Reactor Reduction - Philipp FURLER.pdf
- 22.Hartley, U.W., Ngoenthong, N., Cheenkachorn, K., Sornchamni, T.: CO2 to syngas: metal oxides on stainless steel 316L for micro-channel reactor application. In: International Conference on Chemical and Biochemical Engineering Paris (France), 20–22 July 2015, 8–11 (2015). https://www.researchgate.net/profile/Mahdi_Belguidoum/publication/289540155_AbstractsBook_ICCBE2015/links/5690232b08aec14fa557e115/AbstractsBook-ICCBE2015.pdf
- 32.Liu, F.: Cerium oxide promoted oxygen carrier development and scale modeling study for chemical looping comustion. University of Kentucky (2013). https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1029&context=me_etds
- 39.Rihko-Struckmann, L.K., Datta, P., Wenzel, M., Kai Sundmacher, N.V.R.A., Dharanipragada, H.P., Galvita, V.V., Marin, G.B.: Hydrogen and carbon monoxide production by chemical looping over iron-aluminium oxides. Energy Technology 4(2), 304–313 (2016). https://doi.org/10.1002/ente.201500231 CrossRefGoogle Scholar
- 48.Tescari, S., Agrafiotis, C., Breuer, S., De Oliveira, L., Neises-Von Puttkamer, M., Roeb, M., Sattler, C.: Thermochemical solar energy storage via redox oxides: materials and reactor/heat exchanger concepts. Energy Procedia 49, 1034–1043 (2013). https://doi.org/10.1016/j.egypro.2014.03.111 CrossRefGoogle Scholar
- 53.Warren, K.J., Reim, J., Randhir, K., Greek, B., Carrillo, R., Hahn, D.W., Scheffe, J.R.: Theoretical and experimental investigation of solar methane reforming through the nonstoichiometric ceria redox cycle. Energy Technol. 5(11), 2138–2149 (2017). https://doi.org/10.1002/ente.201700083 CrossRefGoogle Scholar
- 55.Wei, B.: A novel solar-driven system for two-step conversion of CO2 with ceria-based catalysts. KTH Royal Institute of Technology, Stockholm (2014)Google Scholar
- 59.Zhou, Y., Rahman, M.N.: Effect of redox reaction on the sintering behavior of cerium oxide. Acta Materialia 45(9):3635–39 (1997). http://bdm.unb.br/bitstream/10483/4095/2/2011_RicardoOliveiraMonteiroLopes.pdf
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