Changyong Qin

DFT Study of Oxygen Dissociation in Molten Carbonate

Using density functional theory method, we have studied the oxygen dissociation in alkali molten carbonate at the B3LYP/6-31G(d) level. The calculated energies were then verified by MP4 and CCSD(T). A four-formula cluster (M2CO3)4, M = Li, Na, and K was used to describe the molten carbonate. It was found that the adsorption of oxygen to molten carbonate is of a chemical type and leads to the formation of CO5(2-) in MC, which was confirmed for the first time by DFT calculations. The energy barrier for its dissociation is calculated to be 197.9, 116.7, and 170.3 kJ/mol in the (M2CO3)4 cluster, M = Li, Na, and K, respectively. If the reaction of O2 + 2CO3(2-) → 2CO4(2-) is approximated as a one-step reaction, the activation energy is estimated to be 96.2, 15.1, and 68.6 kJ/mol, respectively. The reaction rate is first order to the pressure of oxygen. Surprisingly, the reaction of oxygen dissociation has the lowest energy barrier in sodium carbonate, which is consistent with the recent experimental findings. It is very clear that the molten carbonate salt has directly participated in the ORR process and plays an important role as a catalyst in the cathode of SOFCs. The oxygen reduction has been facilitated by MC and enhanced cell performance has been observed.

Energetics of proton transfer in alkali carbonates: a first principles calculation

Recent development of dual-phase ceramic–carbonate composite electrolytes for intermediate-temperature solid oxide fuel cells (SOFCs) has prompted a pressing question as to whether H+ can transfer in molten carbonates and play a role in the enhanced ionic conductivity and improved SOFC performance. In the present study, we use a first principles approach to examine the energetics of H+-transfer in CO32−, Li2CO3 crystals and (Li2CO3)8 clusters. The results indicate that H+-transfer in solid carbonates is difficult, but very facile in a (Li2CO3)8 cluster, a surrogate of molten carbonates.

Can molten carbonate be a non-metal catalyst for CO oxidation?

For the first time, we have examined molten carbonate as a non-metal catalyst for CO oxidation in the temperature range of 300–600 °C. The reaction mechanism was analyzed using a classic Langmuir–Hinshelwood model combined with DFT calculations. It was found that the conversion of CO is greatly enhanced by molten carbonate at about 400 °C and increased to 96% at 500 °C. The reaction process involves four steps, including (1) dissociative adsorption of oxygen, (2) adsorption of CO, (3) surface reaction, and (4) desorption of CO2. DFT modeling reveals the formation of (C2O4)2− and (CO4)2− as the intermediate species, and that the first two steps are exothermic and preferred by chemical equilibrium. The energy barrier of oxygen dissociation to form CO42− is calculated to be 23.0 kcal mol−1, which is in good agreement with the measured overall activation energy of 19.1 kcal mol−1. However, the surface reaction (step 3) has a low energy barrier of 10.8 kcal mol−1 only. This confirms that the oxygen dissociation is the rate determing step in the whole process. Further analysis of the reaction kinetics indicates that the reaction is affected by the CO concentration. With low CO concentration, the reaction is first order with respect to CO and half order to O2. From the current report, it has been proven that molten carbonate can serve as an efficient catalyst for CO oxidation and potentially for other oxidation reactions in the temperature range of 400–600 °C. More studies are demanded to further investigate the reaction mechanism and explore more potential industrial applications.

Proton Transfer in Molten Lithium Carbonate: Mechanism and Kinetics by Density Functional Theory Calculations

Using static and dynamic density functional theory (DFT) methods with a cluster model of [(Li2CO3)8H]+, the mechanism and kinetics of proton transfer in lithium molten carbonate (MC) were investigated. The migration of proton prefers an inter-carbonate pathway with an energy barrier of 8.0 kcal/mol at the B3LYP/6-31 G(d,p) level, which is in good agreement with the value of 7.6 kcal/mol and 7.5 kcal/mol from experiment and FPMD simulation, respectively. At transition state (TS), a linkage of O–H–O involving O 2p and H 1 s orbitals is formed between two carbonate ions. The calculated trajectory of H indicates that proton has a good mobility in MC, oxygen can rotate around carbon to facilitate the proton migration, while the movement of carbon is very limited. Small variations on geometry and atomic charge were detected on the carbonate ions, implying that the proton migration is a synergetic process and the whole carbonate structure is actively involved. Overall, the calculated results indicate that MC exhibits a low energy barrier for proton conduction in IT-SOFCs.