The Experimental Study on CO2 Sorption Properties of Na2ZrO3 at High Temperatures

Introduction

Carbon dioxide (CO₂), the main greenhouse gas, is closely related to global warming and climate change due to combustion of fossil fuels, for example, in power plants. Necessary actions have to be taken to reduce the environmental impact of CO₂. 

CO₂ from fossil fuels is mainly released through flue gas at higher temperatures, typically above 350°C. Due to the high temperature of the gas, most of the conventional physical adsorbents cannot be used because of the decrease in physical adsorption with increasing temperature. By cooling down the temperature of CO₂ in the flue gas, physical adsorbents could be employed but would result in longer desorption cycles. 

To overcome this limitation, the application of chemical sorbents (liquid or solid) at higher temperatures could be the key. Those materials directly absorb CO₂ at high temperatures; no cooling of the gas is required; and an efficient separation of gas mixtures can be realized.

Typical high-temperature CO₂ chemical adsorbents mainly include ammonia adsorbents, calcium-based adsorbents and lithium-based adsorbents [1]. Lithiumbased adsorbents offer the possibility to store and transport CO₂ due to the reaction process which converts CO₂ from the gas state to the solid state [2]. 

Among these alkali metal ceramic adsorbents, Na₂ZrO₃, which is also in the alkali metal group, has lower preparation cost, faster adsorption capacity, and higher adsorption temperature. Therefore, the study of Na₂ZrO₃ has attracted the attention of many investigators.

The adsorption reaction process of Na₂ZrO₃ with CO₂ is shown in the following Eq.(1) [4-7]:

Na₂ZrO₃ + CO₂ ⇆ Na₂CO₃ + ZrO₂ （1）

The adsorption temperature of CO₂ by Na₂ZrO₃ is in the range of 400°C to 800°C [4-6]. When the temperature is lower than 800°C, the reaction proceeds spontaneously and shifts to the side of the products, and Na₂ZrO₃ reacts with CO₂ to form Na₂CO₃. Vice versa, with temperatures higher than 800°C, the reaction proceeds in the reverse direction, and the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition of Na₂CO₃ releases CO₂ and re-forms Na₂ZrO₃. The reversible reaction enables the adsorption and desorption of CO₂ in a cyclic manner. 

In this work, the adsorption-desorption properties of Na₂ZrO₃ for CO₂ were investigated and the effects of the preparation method of Na₂ZrO₃ were compared.

Experimental

The CO₂ cyclic adsorption-desorption performance (measurement program in figure 1) was tested with an STA 2500 Regulus by placing about 10 mg of adsorbent in an alumina crucible and heating it from room temperature to 850°C at a heating rate of 20 K/min under a pure N₂ atmosphere (gas flow 100 ml/min), holding it IsothermalTests at controlled and constant temperature are called isothermal.isothermal for 10 minutes to remove impurities from the sample, and then cooling it down to 650°C at 20 K/min. When the temperature reached 650°C, the atmosphere was switched to a N₂/CO₂ mixture which contained 15% CO₂.

The adsorption reaction was carried out in an IsothermalTests at controlled and constant temperature are called isothermal.isothermal segment for 30 min. Afterwards, the atmosphere was switched back to pure N₂ and the sample was heated to 850°C at 20 K/min. Desorption was characterized in an IsothermalTests at controlled and constant temperature are called isothermal.isothermal segment for 10 minutes at 850°C. The stability of the adsorbent was tested by performing that temperature program 10 times. 

The different sample preparation possibilities for Na₂ZrO₃ are depicted in table 1.

Table 1: Sample preparation of Na₂ZrO₃.

Results and Discussion

Figure 2 shows the TGA curve of the different Na₂ZrO₃ samples synthesized by methods. It can be seen that the mass of each curve significantly increased while CO₂ was present as a reaction partner. After CO₂ was removed from the system, the mass decreased again. When the reaction reached the eighth cycle, the adsorption performance of the four adsorbents stabilized and remained comparable with the ninth and tenth cycle. It can be seen that the Na₂ZrO₃ obtained by the wet mixing method (WM-HD, green; and WM-FD, red) has better adsorption performance than the samples synthesized by the sol-gel method. The adsorption amounts of the four adsorbents were in the following order from the largest to the smallest: WM-HD (18.7%) > WM-FD (17.1%) > SG-FD (16.6%) > SG-HD (15.7%). 

When deriving the TGA curve, shown in figure 2, the mass-loss rate or DTG curve can be obtained, which indicates the change in the rate of weight change with respect to temperature/time. Those curves represent the CO² adsorption rate for the different synthesized conditions of Na₂ZrO₃.

Figure 3 depicts the DTG curve of the CO₂ adsorption of the four adsorbents in the eighth cycle. From the figure, it can be seen that the adsorption rates of adsorbents have, in general, the same trend. However, SG-FD shows the highest adsorption rate compared to the other three samples. Besides that, the rates for SG-HD and WM-HD are similar and sample WM-FD shows the lowest adsorption rate. The Na₂ZrO₃ adsorbent was synthesized by wet-mixing and sol-gel methods, followed by freeze-drying and heated-drying. It can be speculated that the method of sol-gel mixing and freeze-drying is more suitable for the formation of porous structure, and a higher specific surface area could be obtained through this synthetic approach.

Conclusion

The NETZSCH STA 2500 Regulus can be used to investigate the adsorption properties of different materials. In this example, four different synthesized Na₂ZrO₃ samples were investigated, and the CO₂ adsorption properties were characterized. It can be assumed that the synthesis route using the sol-gel method and subsequent freeze-drying leads to a significantly higher surface reactivity. 

By understanding the relationship between synthesis and adsorption properties, the optimum adsorption performance for an individual application can be considered and adjusted accordingly.