Titania spheres with nanochannel-restricted NaNO2/MgO for improving cyclic CO2 adsorption stability

ABSTRACT NaNO2/MgO/titania spheres prepared via aerosol-assisted self-assembly (AASA) were used as sorbents for CO2 adsorption at moderate temperature. The titania framework as support would allow MgO to disperse well, thereby increasing the contact between MgO and NaNO2 to enhance carbonation. In this study, the effect of Mg/Ti molar ratio and NaNO2 addition amount on CO2 adsorption was investigated. Results showed that the sorbent prepared by AASA with Mg/Ti molar ratio of 2 following the introduction of 30 wt% NaNO2 presented ∼1 μm particle size with rough sphere surface morphology and mesoporous properties, where the surface area and pore volume were 72 m2/g and 0.18 cm3/g, respectively. With NaNO2 addition, the kinetics and capacity of CO2 adsorption significant increased. In the cyclic adsorption/desorption experiment, the superior stability over the NaNO2/MgO/titania spheres was mainly ascribed to the confined space suppressed the degree of the sintering effect. These results indicated the potential application of the nanochannel-restricted sorbent for rapid, high-capacity, and stable CO2 capture at moderate temperatures. GRAPHICAL ABSTRACT


Introduction
Reducing CO 2 emission from human activity has been a critical issue recently because of global warming [1][2][3].MgO is a potential CO 2 capture sorbent because of its abundance on earth, low costs, low regeneration energy required, and nontoxicity.Its theoretical capacity is 24.8 mmol/g.However, the actual capture capacity is low (<0.5 mmol/g) because of rigid MgCO 3 layer formation on the surface that preventing MgO located in inner layer contact with outer CO 2 gas [4,5].
Due to CO 2 capture phenomenon only occurs on the MgO surface, thus one way to enhance capture capacity is to enlarge MgO surface area.To this purpose, porous MgO sorbents and inert solid-supported nano-sized MgO sorbents have been developed.Several synthesis methods such as sol-gel [6], hydrothermal [7,8], thermal calcination [9][10][11], template [12], solid state reaction [13], evaporation-induced self-assembly (EISA) [14], precipitation [15], and flame spray pyrolysis [16] are reported.Various magnesium precursors including inorganic form and organometallic form are used to prepare MgO-based sorbents.The properties of the MgO-based sorbent, such as surface area, pore volume, pore size distribution, morphology, structure, active sites distribution and surface basicity, are controlled via the synthesis method with different conditions and precursors.
Another way to enhance capture capacity is to introduce inorganic salt on MgO surface to play as promoter role [17][18][19][20].As the inorganic salt is under molten state, it can serve as medium.The MgO can dissolve in medium to form O 2− and Mg 2+ ions.The concentration of these ions and dissolved CO 2 accumulate in medium until critical concentration is reached.
Afterward, nucleation process occurs to generate the MgCO 3 particle.These particles precipitate to form loosely stacked layer instead of rigid layer on MgO surface.Thereby, MgO in the interior of the particle can be utilized [17].Sodium nitrate itself or combined with other alkali metal nitrates commonly use as promotor.Other promotors, such as alkali metal nitrites, are also been reported [21,22].
To gain more MgO surface contact to promotor is also effectively way to enhance CO 2 capture performance.Although the addition of molten salts promoter greatly increasing CO 2 capture capacity, significantly loss occurred after several repeated use.In this study, we fabricated NaNO 2 /MgO within nanochannels in titania spheres that would allow MgO to disperse well, thereby increasing the contact between MgO and NaNO 2 to enhance carbonation.It also expected the degree of the sintering effect would be suppressed within the confined space.We applied a self-built aerosol-assisted self-assembly (AASA) system to prepare NaNO 2 /MgO/titania spheres.The AASA including aerosol spray and evaporation-induced self-assembly concepts can produce high-surface area homogeneous multicomponent metal or transition metal oxide mesoporous materials simply, continuously and easily to scale up [23][24][25].The nanochannels with confined space nature in titania spheres could serve as a nanoreactor for carbonation reaction.The confined space can also be expected to mitigate the degree of MgO sintering during multicycle use [26].With NaNO 2 addition, the kinetics and cyclic capacity of CO 2 adsorption/desorption were discussed.

Sorbent preparation
The preparation of MgO/titania spheres were described as following: 17 ml TBOT, 11.5 ml acetic acid, 5 ml hydrochloric acid, calculated amount of magnesium nitrate hexahydrate with mole ratio of Mg to Ti = 0.3, 0.7, 1.2, 2, 3, and 6 g F127 were dissolved into 60 ml ethanol to form transparent sol solution.The AASA system was presented in Figure S1.The sol solution was transformed to aerosol droplets by lab-made nozzle using compressed air as carrier gas first.The aerosol droplets changed to dry powders when passing through furnace (furnace temperature was set at 400°C) and were collected by a filter in the other side.The collected sample was calcination at 450°C for 5 h to remove template.The microsphere supported Mg-based sorbent was obtained and named as MgTi-x, where x represented mole ratio of Mg/Ti.
The NaNO 2 /MgO/titania spheres were prepared as following description: calculated amount of sodium nitrite relative to MgTi-2 adding weight (10,20,45, and 60 wt%), and the 100 mg MgTi-2 sorbent were mixed in 3 ml of deionized water.After stirring 2 h, the mixture was put into an oven at 105°C overnight to remove solvent.The sorbent obtained was named as MgTi-x-y where y represented NaNO 2 loading weight percentage relative to initial adding spheres.

Sorbent characterization
Scanning electron microscopy (SEM) images were obtained by using Nova Nano SEM 230 instrument.The nitrogen adsorption/desorption isotherms were measured under −196°C by using Micromeritics ASAP 2020 instrument.The samples were degassed at 130°C for 12 h under 1 × 10 −3 Torr environment.The surface area were calculated by Brunauer-Emmett-Teller (BET) equation using adsorption data range of 0.05-0.3 in relative pressure.The total pore volume were calculated at relative pressure of 0.995, while pore size distribution were obtained by Barrett-Joyner-Halenda (BJH) model from adsorption part.Powder X-ray diffraction (XRD) data were measured by using PANalytical X' Pert PRO instrument (Cu Kα λ = 0.154 nm, 45 kV, 40 mA).

CO 2 capture experiment
The CO 2 capture test were performed on TA Instrument SDT-Q600 instrument.The operational temperature was set at 300°C.The pretreatment condition was at 300°C under N 2 gas flow for 60 min.For cyclic test, the capture condition was under CO 2 gas flow for 30 min, and the regeneration condition was under N 2 gas flow for 150 min.The temperature during cyclic process was maintained at 300°C.The in situ infrared (IR) spectrum was recorded by Interspec 200-X equipped with DTGS detector under 300°C.The sample mixed with KBr was filled into stainless steel cup.The resolution was set at 1 cm −1 with 64 scans.The pretreatment stage and regeneration stage were under Ar gas atmosphere for 18 h, while capture stage was under CO 2 gas atmosphere for 60 min.

Properties of MgO/titania spheres
The SEM images of MgO/titania spheres are shown in Figure 1.MgTi-0.3 and MgTi-0.7 present microsphere morphology with a smooth surface.The MgTi-1.2 and MgTi-2 still perform microsphere morphology; however, their surface presents a slight irregularity, and the MgTi-2 can show several fused microspheres.Microsphere morphology is difficult to observe in MgTi-3 because of excessive Mg addition.
The nitrogen adsorption/desorption isotherms of MgO/titania spheres are shown in Figure 2(a)-(e).All adsorption behaviors are similar to the IUPAC type IV (a) isotherm form.The shape of hysteresis loop is similar to the IUPAC type H1 at low Mg loading indicating that these sorbents possess a mesoporous structure with cylindrical pore channels.At high Mg loading as shown in Figure 2(e), the shape of hysteresis loop becomes a little similar to IUPAC type H3, which reveals that some slits porous structure forming.The values of surface area and pore volume are listed in Table 1.The surface area and pore volume of MgTi-0.3 are 156 m 2 /g and 0.19 cm 3 /g, respectively.Increasing the amount of Mg added, the surface area and pore volume increase to 185 m 2 /g and 0.23 cm 3 /g for MgTi-0.7 and 201 m 2 /g and 0.29 cm 3 /g for MgTi-1.2 and then tend to decrease to 168 m 2 /g and 0.20 cm 3 /g for MgTi-2 and 139 m 2 /g and 0.20 cm 3 /g for MgTi-3.The surface area and pore volume increase with low Mg addition, indicating that part of Mg ions incorporate into the skeleton to enlarge the microstructure, but decrease because of the large amount of MgO deposited on the nanochannel surface.Furthermore, small pore size appears with high Mg addition (shown in inset) because of the spheres aggregated or glued together to form pores or slits.
Figure 3 shows the XRD results of MgO/titania spheres.The diffraction pattern of MgTi-0.

CO 2 capture performance of MgO/titania spheres
Figure 4 shows the CO 2 capture performance of MgO/ titania spheres at 300°C and pure CO 2 flowing.The capture capacity of MgTi-0.3,MgTi-0.7,MgTi-1.2,MgTi-2, and MgTi-3 is 0.07, 0.10, 0.22, 0.26, and 0.19 mmole/ g after 60 min, respectively, whereas commercial MgO is 0.13 mmole/g.Low CO 2 capture capacity and poor kinetics performance in the case of MgTi-0.3 and MgTi-0.7 are probably due to insufficient MgO on their nanochannel surface, whereas MgTi-1.2,MgTi-2, and MgTi-3 with adequate MgO on their nanochannel surface show high capture capacity with good capture kinetics.Even only 60 min for adsorption, the capture capacity of MgTi-2 have reached above 95% of the maximum adsorption value at equilibrium.Considering that the CO 2 capture phenomenon only occurs on the surface of MgO, the high capture capacity of MgTi-1.2,MgTi-2, and MgTi-3 in comparison to commercial MgO indicate that MgO is dispersed on their mesoporous sphere nanochannel surface with a small particle size, thus provide lots of active sites to contact CO 2 .MgTi-2 shows the highest capture capacity at 60 min, which is almost two times higher than commercial MgO.The result indicates that Mg/Ti mole ratio of 2 is the optimized Mg loading amount in this study.

Properties of NaNO 2 /MgO/titania spheres
NaNO 2 is loaded onto its surface to further improve the CO 2 capture capacity of MgTi-2.At 300°C, NaNO 2 presents a molten phase (the melting and boiling point of NaNO 2 is 271°C and 320°C, respectively).Thus, NaNO 2 can serve as a promoter.The SEM images of NaNO 2 / MgO/titania spheres are presented in Figure S2.Their surface morphology is rougher compared with that of MgTi-2, and more sheet-like structures are found as NaNO 2 loading increases.
The nitrogen adsorption/desorption isotherm results of NaNO 2 /MgO/titania spheres are shown in Figure 5.The adsorption behaviors are all similar to the IUPAC type IV(a).The shape of hysteresis loops are a little closer to IUPAC H2(a) reveal that most NaNO 2 deposited near the nanochannel entrances.In general, these results indicate that the mesoporosity still maintain as NaNO 2 loading in MgTi-2.However, the nitrogen adsorption amount decreases as NaNO 2 loading increases, which indicates that the surface area and pore volume are reduced.The surface area and pore volume of MgTi-2-10 are 198 m 2 /g and 0.28 cm 3 /g, respectively (Table 1), which are slightly larger than MgTi-2 possibly because of the rough surface morphology.The surface area and pore volume of MgTi-2-20, MgTi-2-45, and MgTi-2-60 are 146 m 2 /g and 0.29 cm 3 /g, 72 m 2 /g and 0.18 cm 3 /g, and 65 m 2 /g and 0.15 cm 3 /g, respectively.The surface area and pore volume decrease with the increase of NaNO 2 loading because of NaNO 2 that is stacked on the pore channel wall or clogged pore entrances.

CO 2 capture performance of NaNO 2 /MgO/ titania spheres
The CO 2 capture performance of NaNO 2 /MgO/titania spheres at 300°C and pure CO 2 flowing are shown in Figure 7.The capture capacity of MgTi-2-10, MgTi-2-20, MgTi-2-45, and MgTi-2-60 after 60 min is 0.67, 1.02, 1.30, and 0.92 mmole/g, respectively.All NaNO 2 /MgO/ titania spheres show significantly higher capture capacity and better CO 2 affinity compared with MgTi-2.According to previous papers in molten nitrate salt system, low salt loading amount may cause incomplete MgO surface coverage, whereas high salt loading amount results in inefficient MgCO 3 formation [17,19].Here, the similar phenomenon occurs in the molten nitrite salt system.Compared with MgTi-2-45, MgTi-2-60 shows low capture capacity because of excessive NaNO 2 loading, whereas MgTi-2-10 and MgTi-2-20 show low capture capacity because of inadequate NaNO 2 loading.The capture amounts of all NaNO 2 / MgO/titania spheres increase rapidly in the first 5 min compared with MgTi-2 because CO 2 need to diffuse in the pore channel to contact MgO, and the molten salt serves as a liquid channel for rapid diffusion [27].The capture amounts of MgTi-2-10, MgTi-2-20, and MgTi-2-45 gradually increase after 5 min, whereas MgTi-2-60 initially becomes nearly plateau and then slowly increases again.This behavior possibly indicates that CO 2 captured by these NaNO 2 /MgO/titania spheres are primarily controlled by Mg 2+ ion generation in molten NaNO 2 medium.In the case of MgTi-2-60, the original Mg 2+ ions in the medium are exhausted in the first 5 min, but the dissolution rate of MgO in the medium is slow.Excessive medium results in longer releasing time for Mg 2+ ions to reach threshold concentration and allow nucleation to occur.Therefore, capturing after 5 min takes time to stop and then starts again to capture.In the other three cases of NaNO 2 /MgO/titania spheres with low NaNO 2 loading, although the original Mg 2+ ions in the medium are almost consumed in <5 min, the nucleation threshold is easy to reach in a short time.The capture data of NaNO 2 /MgO/titania spheres are fitted with the double exponential model [28].The fitting results are shown in Figure S3, and the two exponential constants are listed in Table S1.The results show that the double exponential model fits   well with the capture data.The first-term exponential constant is related to the chemical reaction, whereas the second-term exponential constant is related to diffusion.The first-term exponential constants are higher than the second-term exponential constants in MgTi-2, MgTi-2-10, MgTi-2-20, and MgTi-2-45, indicating that diffusion is dominated by their capture behavior.However, the MgTi-2-60 fitting result shows that the first-term exponential constant is lower than the second-term exponential constant.This result indicates that the chemical reaction is dominated by the MgTi-2-60 capture behavior.The highest capture capacity of MgTi-2-45 indicates that 45 wt% of NaNO 2 loading is the optimized loading amount.
The in situ IR result of the MgTi-2-45 is shown in Figure 8.After 60 min at CO 2 atmosphere, several new peaks appear.Three peaks located at 1345, 1487, and 1549 cm −1 are contributed from unidentate carbonate vibration, whereas the peaks located at 1432 cm −1 are contributed from carbonate ions [29].The appearance of carbonate ions indicate that MgCO 3 particle formation is due to CO 2 reaction with Mg 2+ ions and O 2− ions in molten NaNO 2 medium, whereas unidentate carbonates appear because of MgCO 3 formation on the MgO surface.The appearance of carbonate ions confirms that molten nitrite salt can serve as a medium in CO 2 capture.This additional pathway indicates that the capture capacity enhances remarkably with the introduction of NaNO 2 .The peak at 1255 cm −1 in all stages is contributed from nitrite ions.After regeneration treatment, the spectrum changes to the pattern that is similar to the one recorded before CO 2 join because all carbonate species generated under CO 2 atmosphere nearly disappear.This result implies that the material can be used repeatedly.

Cyclic adsorption/desorption performance
The cyclic adsorption/desorption result of MgTi-2-45 is shown in Figure 9. Relative to the first cycle, the capacity of the second, third, fourth cycle reduces by ∼4%, ∼7%, ∼11%.This consequence could be related to MgCO 3 sintering owing to its poor thermal stability [28] and promoter redistribution [30].The former gives rise to larger particle size formation, while the latter leads to lower promoter promoting efficiency.The capture capacity decreases at the first three cycles and then remains almost the same.That implies both particles sintering and promoter redistribution are suppressed after the first four cycles.We also tested the sorbent without mesoporous microsphere support, Mg-45, and the capacity of the second and sixth cycle reduces by ∼34% and ∼42%.Additionally, the peaks of CO 2 adsorption of Mg-45 shifted later than those of MgTi-2-45.This result indicates that nanochannel space provided by microsphere can effectively mitigate sintering problem, thus improves cyclic performance.As compared to previous reported works [18,21,22], the losses on CO 2 capture capacity were more than 20% during the similar experimental conditions.Therefore, the sorbent prepared in this study exhibited superior cyclic CO 2 adsorption stability.Figure 10 illustrated the potential reaction phenomena during cyclic adsorption/desorption process.The reducing capacity in the beginning indicates that large MgO particle size forms because of the sintering effect to decrease its surface area and Mg 2+ release for molten NaNO 2 efficiency.In the nanochannel space, however, the confined space limits the growth of particles.Therefore, further sintering phenomenon is restrained, and the capacity becomes stable only after several cycles.The morphology of MgTi-2-45 after six cyclic adsorption/desorption process shows a more smooth surface compared with fresh sorbent (Figure S4).This can be explained as surface NaNO 2 reforming under its molten state.

Conclusions
The NaNO 2 /MgO/titania spheres were prepared via continuous AASA, followed by impregnation.The optimized mole ratio of Mg to Ti is 2, and the optimized NaNO 2 loading weight percent based on the mass of adding spheres is 45.The obtained spheres possess a mesoporous texture with nano-sized MgO covered by NaNO 2 located on the nanochannel surface.Part of Mg incorporates into the titania skeleton.The capture capacity reaches 1.30 mmole/g after 60 min with high CO 2 affinity.For cyclic adsorption/desorption process, the capture capacity reduces as low as 11% which only appears in the first few cycles and then becomes stable.The high capture capacity, rapid kinetics performance, and well multicycle stability indicate that good potential application of NaNO 2 /MgO/titania spheres for industrial CO 2 removal at moderate temperature.Future work such as evaluating capture performance under low CO 2 partial pressure should be considered.

Disclosure statement
No potential conflict of interest was reported by the author(s).
3 is similar to MgTi 2 O 5 in JCPDS database (89-6944).This result reveals that Mg can incorporate into the titania framework.The TiO 2 anatase diffraction signals are hard to find, whereas the TiO 2 anatase diffraction signals are found significantly in the absence of Mg addition, indicating that Mg incorporation decreases TiO 2 crystallinity.The intensity of the MgTi 2 O 5 diffraction signals becomes weak in MgTi-0.7 and disappears with the increase of Mg addition, indicating that the crystallinity of MgTi 2 O 5 becomes poor or changes to amorphous state because of MgO formation on their nanochannel surface.The weak MgO diffraction peaks can be found in MgTi-1.2,where peaks are found in MgTi-2 and MgTi-3 with the increase of Mg addition.
The XRD results of NaNO 2 /MgO/titania spheres are shown in Figure6.MgTi-2-10 presents MgO and NaCl diffraction signals.The formation of NaCl is possibly due to the presence of chlorides provided by HCl reagent, which are used during AASA.MgTi-2-20, MgTi-2-45, and MgTi-2-60 all present MgO, Mg(OH) 2 , and NaNO 2 diffraction signals.Mg(OH) 2 appears because water is used as solvent during NaNO 2 loading that results in part of the transformation of MgO to Mg(OH) 2 .More NaNO 2 diffraction peaks appear, and its signal intensity becomes stronger as NaNO 2 loading increases.

Figure 9 .
Figure 9.The six cyclic adsorption/desorption results of MgTi-2-45 (solid line) and Mg-45 (dash line) without microsphere support at 300 o C, where adsorption stage was under pure CO 2 purge and desorption stage was at under pure N 2 purge.

Table 1 .
Surface area and pore volume of microsphere supported MgO-based sorbents with or without NaNO 2 promoter.