Carbon Dioxide Absorption Behavior and Carbonate Ion Transport of Lithium Orthosilicate / Molten Carbonate Coexistence System
Authors: Mizuhata Minoru , Kanki Kyohei , Maki Hideshi 
Published: February 2020

Recently, efforts to reduce greenhouse gas emissions have been conducted in the wake of the increase related to global warming. Various kinds of silicate and related oxides show large stoichiometry absorption amounts CO2 in a wide temperature range and several of these materials which show reversible reaction are recyclable [1, 2].Among ceramic CO2 absorbents, lithium orthosilicate (Li4SiO4) has a large absorptive capacity (36.7 wt%) and heat resistance. Li4SiO4 absorbs and desorbs CO2with the following equilibrium reaction at the temperature around 993 K.

Li4SiO4 + CO2 ⇄ Li2CO3 + Li2SiO3

Such silicates can be applied to a gas separation material of molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) due to similar temperature between transition temperature (993 K) and operating temperature, which is utilized for carbon dioxide capture storage (CCS) technology of capturing emissions produced from the use of fossil fuels in electricity generation and industrial processes, preventing the carbon dioxide from entering the atmosphere for the above reason [2, 3]. In this study, we focused on the absorption behaviors of CO2 at lower temperature into Li4SiO4 in combination of these two methods and evaluated the physicochemical dependence of surface properties of CO2 absorption reaction.

SiO2 powder and Li2CO3 powder were mixed at the molar ratio Li/Si = 4.0. The mixture was calcined at 973 K for 20 h in air. The resulting powder (Li4SiO4) kept in an Argon atmosphere glove box and mixed with a given amount of K2CO3. Each mixture (2.5 g) was ground for 0-300 minutes using planetary ball-milling equipment.All samples were characterized with XRD, TG-DTA, N2-isotherm, SEM, SEM-EDX, N2-isotherm, ESR and electrochemical measurement.

The relation between the temperature and the absorbed amount to the additive amount was measured. An increase of sample weight by CO2 absorption was confirmed from about 773 K regardless of the increasing additive amount of K2CO3. As a result, only the 5 mol% K2CO3 added system was the least, is evaluated from the viewpoint of not lowering the theoretical capacity of the following contents. Hereafter, the system that hadn’t added K2CO3 will be called Li4SiO4, and the system that had 5 mol% K2CO3 added will be called Li/K.XRD pattern of after the ball-milled of two above-mentioned systems was measured. Specific surface areas of each sample were measured by BET method from the result of N2-isotherm. Their surface areas linearly increased with the milling time increased. In this report, we examined the sample obtained by milled for 120 minutes whose specific surface area was the largest. SEM images shows the sample after the ball milling process had decreased particle sizes and the surface morphologies became complex. The change in the absorption amount according to the grinding processing time of lithium orthosilicate was measured. There are two different absorption processes in ball-milled samples. It was suggested that absorption confirmed around 300-700 K occurred at the surface and above 700 K occurred in the bulk diffusion of CO2. Then, the result of isothermal analysis for above-mentioned two processes at 873 K was calculated. These isotherms were fitted to a double exponential model (Eq. (1)) [4]

V＝A exp(-k1t )＋B exp(-k2t)＋C (1)

The k1
values obtained are at least one order of magnitude higher than those obtained for the k2
constants, over the entire systems. These values are in good agreement with previous reports [4], where the CO2 chemisorption controlled by diffusion process is the limiting step of the whole reaction processes. When seen in detail, the value of A increased with the grinding process and adding the carbonate. These results shows the absorbed amount on the surface increased, and indicates that there were a lot of active spots on the surface from the result of ESR and the external shell, generated from CO2 absorption reaction, became a molten state by adding the carbonate. Also, the value of B was increased with grinding process in Li4SiO4 system. It indicates the external shell was mesoporous. In Li/K system, the reaction place increased at interface because of the external shell became molten state and easily occurred bulk diffusion. Moreover, in Li/K system grinded, both reaction rate and absorption amount become the maximum. Based on the above-mentioned results, we calculated activation parameter at 873 K from Eyring equation. Gibbs free activation energy (ΔG‡
) was decreased with grinding and adding the carbonate showed in Figure 1.

Figure 1