This work investigated the self-activation behavior of large K 2 CO 3 -doped Li 4 SiO 4 sorbent particles. In this selfactivation mechanism, the sorption ability increased as the number of cycles increased. After the sorption−desorption cycles occurred, the sorption ability of the K 2 CO 3 -doped Li 4 SiO 4 sorbent was remarkably enhanced from approximately 2.0 mmol CO 2 /g sorbent to approximately 5.0 mmol CO 2 /g sorbent at 565°C in 10 vol % CO 2 atmosphere. The fresh and used sorbents were then characterized through N 2 adsorption and SEM methods. Results showed that the average pore size increased from 7 to 32 nm and the surface microstructure changed from dense to porous, because the molten eutectic mixture formed by Li 2 CO 3 and Li 2 SiO 3 can facilitate CO 2 diffusion. The formed CO 2 diffusion channel can provide more CO 2 accessibility; this channel can also reduce the CO 2 diffusion resistance through the product layer. Therefore, the sorption ability of the sorbent is enhanced. Meanwhile, the effects of the self-activation temperature were also investigated and the results revealed that the optimal selfactivation temperature is 615°C. Furthermore, under critical conditions, the self-activated sorbent performed more efficiently than the fresh sample. At 450°C under 10 vol % CO 2 atmosphere, the sorption capacity of the self-activated sorbent was approximately 20 times higher than that of the fresh sample. Finally, a pore−core model was also proposed to illustrate the K 2 CO 3 -doped Li 4 SiO 4 self-activation mechanism.
INTRODUCTIONHydrogen is used in various applications, such as ammonia and methanol syntheses, hydrocracking and hydro processing in refineries, and fuel cell production. Currently, hydrogen is mainly generated using the steam methane reforming (SMR) system. However, this traditional method of hydrogen production requires high operation temperatures (800−900°C ), water−gas shift reactors, and purification units for CO 2 removal. 1,2 To overcome the thermodynamic limitations of the SMR system, Carvill 3 proposed a sorption-enhanced SMR (SE-SMR) system, which is a promising technology to replace the conventional SMR system. Compared with the SMR system, the SE-SMR has many advantages, such as relatively low operation temperatures, increased CH 4 conversion and H 2 production, simplified process, and reduced capital cost. The key point of this system is to develop a satisfactory CO 2 solid sorbent. At present, several high-temperature absorbent materials have been proposed as the CO 2 acceptor, such as CaO-based sorbents, 4−7 hydrotalcite-like materials, 8,9 and lithium-based sorbents. 10−12 Among the absorbent materials for CO 2 sorption, lithium orthosilicate (Li 4 SiO 4 ) is one of the most promising because of its large CO 2 capacity, excellent stability, and fast absorption kinetics. The CO 2 sorption process is based on the following reversible reaction:
