본 논문에서는 밀도 함수 이론을 바탕으로 알칼리 금속 산화물 M2O의 리튬과 나트륨을 분석하였다. 알칼리 금속 산화물은 이산화탄소 흡착력이 좋고 이산화탄소가 적습니다. 가압하에서도 효과적으로 이산화탄소를 흡착할 수 있지만, 반응 후에도 효과적으로 흡착할 수 있다는 장점이 있다.
안정성이 높기 때문에 탈착과 그에 따른 이산화탄소 흡착주기를 통한 재활용이 어렵습니다. 비용/효율이 낮아 이산화탄소 흡착제로서 단점이 있다. DFT 계산을 사용하여 산화나트륨에 대한 CO 흡착이 산화리튬에 대한 흡착보다 더 안정적인 것으로 나타났습니다.
산화나트륨의 이산화탄소 흡착에너지 변화. 흡착에너지를 비교한 결과, 표면의 주물질인 나트륨보다 도펀트의 크기가 더 컸다. 전하 변화를 결정하기 위해 Bader 전하 분석이 수행되었습니다.
전압을 인가함에 따라 흡착에너지는 음의 방향으로 증가하는 것으로 나타났다.
Introduction
- Density Functional Theory (DFT)
- Density functional theory and its applications
- Bader charge analysis
- Carbon capture and storage (CCS)
- CO 2 adsorbents
- The characteristics of alkali metal oxides (Li and Na)
Among them, post-combustion technology is mainly used for CO2 capture of coal-fired power generators [40]. Many researchers have attempted to dope or promote various materials in metal-based oxide adsorbents for CO2 capture. The absorption rate of CO2 on Li2ZrO3 was increased tenfold by these dopants.
56] performed sorption experiment using alkali dopants such as Li, Na, K, Rb and Cs on CaO by analysis of Brunauer-Emmett-Teller (BET) surface, and found that the adsorption amount of CO2 was increased after alkali metal doping , except for Li doping. 57] observed that Cu or La doping was effective in increasing the adsorption capacity of CO2 on CeO2. 58] calculated the adsorption energies of CO2 on dopant-free CaO(100) surface and Li-, Na-, K-, Rb-, or Cs-doped CaO(100) surface using theoretical calculations.
They reported that the CO2 adsorption on oxygen surfaces was enhanced by the larger dopant size. In our study, the adsorption of CO2 on both doped and doped Li2O(111) and Na2O(111) surfaces was investigated.
Computational methods
In this study, two different methods are used to enhance CO2 adsorption on M2O(111) surfaces (M = Li or Na). Second, the structural effect using the biaxial deformation of the Rb-doped Na2O(111) surface, which exhibits a negative peak adsorption energy, was considered to enhance CO2.
Results and discussion
Structure optimization
CO 2 adsorption on Alkali metal oxide (Li 2 O and Na 2 O) surface
The adsorption energy of CO2 adsorbate on Li2O(111) and Na2O(111) surface was calculated using the equation (2). 𝐸𝑎𝑑𝑠 = 𝐸𝐶𝑂2/𝑠𝑢𝑟𝑓𝑎𝑐𝑒− (𝐸𝐶𝑂2+ 𝐸𝑠𝑢𝑐𝑟) 2 where 𝐸𝑂2/𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the total energy of CO2 on each surface, and 𝐸𝑢𝑢 is the energy of CO2 on each surface. For CO2 adsorption on dopant-free Li2O(111) and Na2O(111) surfaces, one configuration of CO2 adsorption was considered.
Side and top view of CO2 on the dopant-free (a) Li2O(111) surface and (b) Na2O(111) surface and corresponding adsorption energy of CO2. The adsorption energy of CO2 on a doped-free Na2O(111) surface is affected by the addition of doping substances to the surface. With the exception of Li doping, K or Rb doping improves the CO2 adsorption on the Na2O(111) surface with the negative higher adsorption energy of CO2 (Fig. 4).
For example, doping a Li atom that has a smaller size than a Na atom on the Na2O(111) surface decreases the CO2 adsorption energy. This value was used as an indication for the reasonable desorption limit of CO2 from Na2O surface. All adsorption energies of CO2 on Na2O(111) surface with different dopants were within this thermodynamic limit value.
Specifically, the distance between Li dopant and O1 of CO2 in doped Na2O(111) surface was clearly longer than the distance between K (or Rb) dopant and O1 of CO2 as shown in Table 3 and Fig. 6. The change of the adsorption energy of CO2 as a function of both ionic radius [77] and electronegativity of doping atoms is shown in fig. The adsorption energy of CO2 on the doped Na2O(111) surface becomes negatively higher as the ionic radius (electronegativity) of dopants increases (decreases).
This trend between the ionic dopant radius and the adsorption of CO2 was observed in the other metal oxide system. 56] showed that the adsorption capacity of CO2 on doped CaO is increased with the increased ionic radius of dopants (Cs > Rb > K > Na > . Li) in their experiment. The differences of CO2 adsorption energies on K-doped and the adsorption energy of CO2 on Rb-doped system come from the difference in dopant size rather than the difference in electronegativity.
The similar increasing trend of CO2 adsorption energy with reduced electronegativity of the dopant was also found in the other system. 80] reported that the low electronegativity dopants were useful to enhance the adsorption of CO2 on the SrTiO3 surface in their experiment.
Strain effect on CO 2 adsorption
Bader charge analysis
Here, the charge density difference of CO2 is the sum of the charge density difference of C, O1 and O2. Although the adsorption energy of CO2 on Rb-doped Na2O(111) surface was negatively higher than the CO2 adsorption energy on K-doped system, the charge transferred amount from the doped surface to the adsorbate of CO2 was found to be the same. The difference between CO2 adsorption energy on the K-doped surface and the CO2 adsorption energy on Li-doped system comes from the structural effect associated with the doping size (Figure 9).
The surface environment, which determines the adsorption strength of CO2 on the surface, can be influenced by the size difference between the dopant and the host metal atom. Although there was an obvious enhancement of the CO2 adsorption energy on the Rb-doped Na2O(111) surface compared to the non-strained system by +3%, the ΔQ of adsorbed CO2 was less negative in the strained system than in the non-strained system, as shown. in the Supporting Information. This increase in CO2 adsorption energy due to deformation can be understood by analyzing the distance between C adsorbed CO2 and O surface (𝑑𝐶−𝑂𝑆) as shown in Table 5.
Charge density difference (ΔQ) of adsorbed CO2 and corresponding adsorption energy of CO2 on doped-free and Li-, K- or Rb-doped Na2O(111) surface.
Conclusion
