In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 1344−1352

In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6‑Stabilized CO2 Sorbents Sung Min Kim,† Wei-Chih Liao,‡ Agnieszka M. Kierzkowska,† Tigran Margossian,‡ Davood Hosseini,† Songhak Yoon,§ Marcin Broda,† Christophe Copéret,*,‡ and Christoph R. Müller*,† †

Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, 8092 Zürich, Switzerland ‡ Department of Chemistry and Applied Sciences, ETH Zürich, Vladimir Prelog Weg 1-5, 8093 Zürich, Switzerland § Institute for Materials Science, University of Stuttgart, Heisenbergstrasse 3, D-70569 Stuttgart, Germany S Supporting Information *

ABSTRACT: CaO is an effective high temperature CO2 sorbent that, however, suffers from a loss of its CO2 absorption capacity upon cycling due to sintering. The cyclic CO2 uptake of CaO-based sorbents is improved by Ca3Al2O6 as a structural stabilizer. Nonetheless, the initially rather stable CO2 uptake of Ca3Al2O6-stabilized CaO yet starts to decay after around 10 cycles of CO2 capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of in situ XRD together with textural and morphological characterization techniques (SEM, STEM, and N2 physisorption) and solid-state 27Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca3Al2O6-stabilized CaO. After a certain number of CO2 capture and regeneration cycles (approximately 10), Ca3Al2O6 transformed into Ca12Al14O33, followed by Al2O3 segregation and enrichment at the surface in the form of small nanoparticles. Al2O3 in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO2 uptake.

C

candidates. Among the solid CO2 sorbents, CaO-based materials show a number of desirable characteristics such as a high theoretical CO2 uptake of 0.78 gCO2/gCaO, earth abundant precursors, e.g., limestone, low price ($9−11 per ton of crushed limestone),25 and fast CO2 uptake and release kinetics, following

O2 capture and storage (CCS) has the potential to contribute to an appreciable extent to the reduction of anthropogenic CO2 emissions allowing to meet the ambitious target to limit the global temperature increase to 2 °C.1−3 According to estimates of the International Energy Agency (IEA), CCS may contribute by 19% to the global reduction target, corresponding to 8.2 Gt CO2/yr.3,4 The highly concentrated stream of CO2 that is obtained after sorbent regeneration may also provide an avenue for the synthesis of value-added chemicals and fuels from CO2 (CCU, carbon dioxide capture and utilization).5−11 From a technological readiness level point of view, amine scrubbing is the “leading” CO2 capture process as it has been implemented on the industrial scale for the removal of CO2 from natural gas,5 yet it is associated with a high energy demand for sorbent regeneration2,12 that translates directly to high CO 2 capture costs ($60−107 per ton of CO 2 captured).13−15 Hence, the search for alternative, less costly, CO2 sorbents is actively pursued. In this context, solid CO2 sorbents, such as layered double oxides (LDO),16 activated carbon,17,18 metal organic frameworks (MOF),19,20 or alkaline earth metal oxides, e.g., calcium oxide,21−24 are interesting © 2018 American Chemical Society

CaO + CO2 ⇄ CaCO3

0 ΔH298K = ±178 kJ/mol

Indeed, techno-economic modeling of CaO-based CO2 capture has estimated the costs of CO2 capture in the range of $12−32 per ton of CO2 captured, i.e., a reduction of 70− 80% when compared to amine scrubbing.13−15,26,27 However, unsupported CaO, as derived through the calcination of limestone, rapidly deactivates with the number of CO2 capture and release cycles. This rapid loss of its CO2 uptake capacity has been attributed largely to a sintering-induced loss of pore volume and surface area owing to the low Tammann Received: December 4, 2017 Revised: January 23, 2018 Published: January 27, 2018 1344

DOI: 10.1021/acs.chemmater.7b05034 Chem. Mater. 2018, 30, 1344−1352

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Chemistry of Materials

to 700−900 °C, CaO is obtained (Figure S1a,b). XRD confirms that both CaO and Ca(OH)2 phases, the latter due to the highly hygroscopic nature of CaO, are present in Ca:Al_90:10 and limestone-derived CaO after calcination at 900 °C (Figure S1c,d). Using XRD, the formation of mixed oxides, i.e., Ca12Al14O33 (≈700−800 °C) and Ca3Al2O6 (≈800−900 °C), in Ca:Al_90:10 is also revealed (Figure S1b,d). STEM with EDX mapping of Ca:Al_90:10 (Figure S2) confirms a homogeneous distribution of Al in the assynthesized material and when calcined at 400 °C. N2 physisorption (Table S1 and Figure S3) of freshly calcined (900 °C) Ca:Al_90:10 demonstrates its comparatively high pore volume and surface area of 0.19 cm3/gsorbent and 29 m2/ gsorbent, respectively. Unlike calcined limestone, Ca:Al_90:10 shows a hierarchical pore size distribution, rich in small mesopores (pore diameter, dp = 4.6 ± 0.4 nm). Upon the removal of the template-derived micelles at 400 °C, followed by CaCO3 formation (400−600 °C), a hysteresis in the N2 isotherm is observed at p/po = 0.85−1.0, indicating the formation of mesopores (Figure S3). Larger mesopores (dp = 18 ± 2.0 nm) are formed during the decomposition of CaCO3 to CaO at 700−900 °C in both calcined limestone and Ca:Al_90:10. Hence, the removal of the template-derived micelles and the decomposition of CaCO3 to CaO results in small (dp < 10 nm) and large mesopores (dp ≥ 10 nm), respectively, leading to a hierarchical porosity in freshly calcined Ca:Al_90:10. Figure 1 plots the 27Al NMR spectra and the corresponding deconvolutions of Al2O3 and calcium-aluminum mixed oxide references that were identified by XRD (Figure S4). 27Al MAS NMR (Figure 1 and Table S2) confirms that the predominant Al species (δiso ppm = 9 and CQ = 2.5 MHz) in the α-Al2O3 reference is in an octahedral configuration and that γ-Al2O3 is composed of tetrahedral (AlIV, δiso ppm = 74 and CQ = 3.2 MHz), pentahedral (AlV, δiso ppm = 40 and CQ = 4.2 MHz), and octahedral (AlVI, δiso ppm = 11 and CQ = 2.1 MHz) Al sites.46−52 In calcium-aluminum mixed oxides (Ca12Al14O33 and Ca3Al2O6), the Al sites are predominantly in tetrahedral coordinations.53−55 While Ca12Al14O33 shows a single dominant AlIV site (δiso ppm = 74 and CQ = 1.1 MHz), Ca3Al2O6 features a broader distribution in the NMR signal, centered at 64 ppm, showing distinct second-order quadrupolar broadening and multiple components (Figure S5). Fitting of the acquired spectrum of Ca3Al2O6 suggests the presence of two types of AlIV sites (δiso = 75 and 79 ppm, CQ = 8.0 and 8.7 MHz, respectively), which can correspond to the two crystallographic sites, i.e., one at the center of a tetrahedral site and another one in a slightly distorted tetrahedral site. Fitting of the 27Al NMR spectra of Ca:Al_90:10 calcined at 800 °C indicates the presence of an AlIV site (δiso = 74 ppm and CQ = 1.0 MHz) that shows similar NMR features as the AlIV site in Ca12Al14O33, albeit with a wider line broadening feature. Increasing the calcination temperature to 900 °C, two AlIV sites were observed for Ca:Al_90:10 (Figure S5): site 1 with 75 ppm (δiso) and 1.2 MHz (CQ), and site 2 with 79 ppm (δiso) and 6.9 MHz (CQ). Site 1 is attributed to an AlIV site with a Ca12Al14O33-like structure,56 and site 2 is in good agreement with Ca3Al2O6 formation, which is corroborated by the diffractogram shown in Figure S1. The appearance of site 2 is presumably indicative of a (partial) phase transformation of Ca12Al14O33 to Ca3Al2O6 consistent with the diffusion and reaction of Ca2+ with Ca12Al14O33 to form Ca3Al2O6 (Ca12Al14O33 + 9CaO → 7Ca3Al2O6).35,57 To summarize, both XRD and 27Al MAS

temperature (TT) of CaCO3 of 533 °C (the operating temperature of the calcium looping process is in the range 650−900 °C28). However, due to the large difference in the molar volume of the product (36.9 cm3/mol for CaCO3) and the reactant (16.7 cm3/mol for CaO), a high pore volume and surface area is critical to avoid diffusion limitation of the CO2 capture reaction. The diffusion coefficient of CO2 in CaO (DCaO = 0.3 cm2/s) is approximately 2 orders of magnitude higher than that in CaCO3 (DCaCO3 = 0.003 cm2/s),28 and it has been estimated that diffusion becomes rate-limiting once the product (CaCO3) layer thickness exceeds 50 nm.28,29 Hence, to improve the structural stability of the material, and in turn the cyclic CO2 uptake, CaO has been stabilized by high Tammann temperature metal oxides, with Ca-Al mixed oxides being arguably the most commonly utilized stabilizer.30−35 However, despite improving the stability of the cyclic CO2 uptake to some extent, also Ca3Al2O6-stabilized CaO shows an appreciable, albeit reduced, decay rate. Intriguingly, it has also been observed commonly that the decay in the CO2 uptake capacity of Ca-Al mixed oxide-stabilized CaO starts after a certain number (≈10) of cycles.36−39 Yet, we currently lack a detailed understanding of this phenomenon and so far the decay in the CO2 uptake of CaO has been attributed rather generally to sintering exhibited by a reduced pore volume and surface area. Indeed, it is unclear whether the changes in pore volume and surface area are the sole reason for deactivation of CaO-based sorbents40−42 and what triggers the loss of the structural stability of Ca3Al2O6-stabilized CaO and in turn its CO2 uptake. However, obtaining a fundamental understanding of the deactivation mechanism is a critical step to improve further the stability of CaO-based CO2 sorbents and to reduce the quantity (of CO2-capture-inactive) stabilizers. Hence, this work aims at identifying the underlying deactivation mechanism of Ca3Al2O6-stabilized CaO by probing in detail the structural and chemical properties of Ca3Al2O6stabilized CaO and changes therein with cycle number. 27Al magic-angle spinning (MAS) solid-state NMR is employed to investigate the structure of the Al sites in the bulk. In addition, dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS)43−45 is applied to probe selectively the structures of the Al sites on the surface of the CO2 sorbent. Combining NMR, in situ XRD and Raman spectroscopy, electron microscopy coupled EDX spectroscopy, and N2 physisorption, we show that the deactivation is triggered by the segregation of Al2O3 from a Ca12Al14O33 structure and its enrichment at the surface in the form of small nanoparticles. Alumina in such a form is not able to stabilize the initially highly porous structure against thermal sintering leading in turn to a reduced CO2 uptake. Ca3Al2O6-stabilized CaO is prepared via an evaporation induced self-assembly (EISA) approach using the triblock copolymer Pluronic P-123 ((PEO)20(PPO)70(PEO)20 polymer) as a structure-directing agent. EISA routes ensure the formation of materials with a high pore volume in the desired meso-porous range (derived from micelles) and the homogeneous distribution between the active phase CaO and the stabilizer Ca3Al2O6. The compositional and morphological changes during the calcination of as-synthesized Ca:Al_90:10 (i.e., a CO2 sorbent with a molar ratio of Ca2+ to Al3+ = 9:1) are summarized in Figures S1 and S2, respectively. After the thermal decomposition of Pluronic P-123 micelles (≈400 °C), CaCO3 is formed and, after increasing further the temperature 1345


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reduced abruptly, marking the transition to the diffuse limited reaction stage. The intersection between the slopes of the two regimes is used to determine the transition time. The CO2 capture rate of both limestone-derived CaO and Ca:Al_90:10 in the kinetically controlled reaction stage is approximately 1.0 × 10−3 g CO2/g CaO/s (possibly limited by external mass transfer characteristics of the TGA) and decreases to 3.0 × 10−5−5.0 × 10−5 g CO2/g CaO/s when entering the diffusionlimited reaction stage. While for both limestone-derived CaO and Ca:Al_90:10, the quantity of CO2 captured in the diffusion-limited reaction stage is stable and independent of the cycle number (0.02−0.05 gCO2/gsorbent), the CO2 uptake of limestone in the kinetically controlled reaction stage is already significantly reduced in the second cycle. For Ca:Al_90:10, the decay in the CO2 uptake in the kinetically controlled reaction stage is more gradual. The reduction of the CO2 uptake in the kinetically controlled reaction stage is in line with the overall cyclic CO2 uptake performance (Figure 2a), indicative that the overall CO2 uptake of a material is largely governed by the quantity of CO2 that is captured in the kinetically controlled reaction stage. Qualitatively, changes in the pore volume with cycle number (Figure 2d,e and Table S3) match trends in the CO2 uptake; i.e., a decreasing pore volume leads to a lower CO2 uptake. Indeed, there is an almost linear relationship between the pore volume of large mesopores and the CO2 uptake in the kinetically controlled carbonation stage (Figure S6c). We observe also that the pore volume in small mesopores decreased rapidly with cycle numbers: from 0.030 cm3/gsorbent (fresh material) to 0.019 cm3/gsorbent (10 cycles) and finally to 0.08 cm3/gsorbent (30 cycles). On the other hand, the pore volume in larger mesopores appears to decrease only after 10 cycles, i.e., at the same time when the reduction of the overall CO2 uptake of the sorbent started. This observation is in agreement with the hypothesis that the CO2 uptake in the kinetically controlled carbonation stage is linked to pore volume available in pores with diameters ≤ 100 nm.28,29 After carbonation, the remaining pore volume in the material is negligible (Figure 2f). Considering the surface area and the CO2 uptake of the material at the end of the kinetically controlled reaction stage, we estimate the critical product layer thickness of CaCO3 as 37 nm for Ca:Al_90:10 and 40 nm for Ca:Al_100:0 and limestone-derived CaO. Hence, the critical product layer seems to be independent of the material composition and in fair agreement with the previously reported value of 50 nm. Hence, these findings confirm that the reduction of pore volume (and surface area) is responsible for the decay of the CO2 uptake of CaO-based CO2 sorbents, independent of their actual composition (e.g., with or without the presence of a stabilizer). The rapid structural change of unsupported CaO (derived from limestone) is also confirmed visually through electron microscopy (Figure 3). Already after 10 cycles, the initially highly porous material has experienced severe sintering, leading to the collapse of its pore structure (in agreement with our previous N2 adsorption measurements, Table S3). On the other hand, the porous morphology of Ca:Al_90:10 is preserved rather well over the first 10 cycles. However, after 30 cycles, also Ca:Al_90:10 shows signs of sintering, but to a lesser extent than unsupported CaO (see also Figure S7). A question that arises from these images is what triggers the rather sudden loss of porosity in Ca3Al2O6-supported CaO, in particular after the initial 10 cycles.

Figure 1. 27Al MAS NMR of Ca:Al_90:10 calcined at 800 and 900 °C, and references: α-Al2O3, γ-Al2O3 (Ca:Al_0:100 calcined at 800 °C), Ca3Al2O6, and Ca12Al14O33, and (b) unit cell structures of Ca12Al14O33, Ca3Al2O6, and α-Al2O3 references. The spectra were fitted with the QUAD Central model in the solid line shape (SOLA) feature of Topspin for the isotropic chemical shift (δiso) and quadrupolar coupling constant (CQ).

NMR confirm the presence of Al in the form of mixed oxides in calcined Ca3Al2O6-stabilized CaO. The cyclic CO2 capture performance of Ca3Al2O6-supported CaO and limestone is assessed in a TGA using realistic operation conditions, i.e., calcination at 900 °C in a pure CO2 atmosphere. Varying the ratio of Ca2+ to Al3+ (Figure 2a and Figure S6), Ca:Al_90:10 is identified as the best material in terms of CO2 uptake after 10 cycles. The reference material, limestone-derived CaO, shows a very high initial CO2 uptake capacity of 0.55 gCO2/gsorbent; however, its CO2 uptake decreases rapidly, yielding only 0.08 gCO2/gsorbent after 30 cycles. A significantly increased cyclic stability is observed for Ca:Al_90:10, reaching 0.34 gCO2/gsorbent after 30 cycles, exceeding the CO2 uptake of limestone-derived CaO by more than 400%. Hence, the Ca3Al2O6 formation increases the cyclic CO 2 uptake of CaO. Yet, we observe an intriguing phenomenon in Figure 1a: The rather stable CO2 uptake of Ca:Al_90:10 during the first 10 cycles is followed by a continuous decay, albeit at a significantly reduced rate when compared to unsupported CaO. Such a behavior has been observed previously in Ca3Al2O6-stabilized CaO,36−39 yet no explanation has been put forward so far. In order to understand this prominent deactivation mechanism of Ca3Al2O6-stabilized CaO, we assess first the temporally resolved carbonation characteristics (Figure 2b,c). The carbonation reaction can be divided into two reaction stages, i.e., a kinetically controlled and a diffusion-limited reaction stage.28,29,58 At the end of the kinetically controlled reaction stage, the rate of carbonation is 1346


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Figure 2. CO2 uptake and textural characterization: (a) Cyclic CO2 capture of limestone-derived CaO and Ca:Al_90:10. The solid line () gives the theoretical CO2 uptake of pure CaO, i.e., 0.78 gCO2/gsorbent. CO2 uptake as a function of carbonation time of (b) limestone-derived CaO and (c) Ca:Al_90:10. The dash line (----) gives the theoretical CO2 uptake of the sorbent assuming the full conversion of the CaO in the material. BJH pore size distribution of (d) limestone (calcined form), (e) Ca:Al_90:10 (calcined form), and (f) Ca:Al_90:10 CO2 (carbonated form) as a function of cycle number. The red (), purple (), green (), and blue () lines refer to sorbents that have undergone 1, 2, 10, and 30 cycles, respectively.

Figure 3. HR-SEM images of the fresh and cycled materials (calcined form).

To answer this question, we first probe changes in the bulk chemical composition of Ca:Al_90:10 as a function of cycle number using in situ XRD. Figure 4a and Figure S8 plot in situ XRD measurements of Ca:Al_90:10. Using Rietveld analysis,

the weight fractions of CaO, Ca12Al14O33, and Ca3Al2O6 are determined as, respectively, 81, 3, and 16 wt % in the freshly calcined material. In the 10th cycle, the quantities of CaO and Ca12Al14O33 increase to 86 and 13 wt %, respectively (Figure 4b 1347


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Figure 4. Structural and chemical characterization: (a) In situ XRD diffractograms for cycles 1−10 (left) and cycles 10−30 (right) in the range 32.5− 33.5°. The red (), green (), and blue () lines refer to the XRD diffractograms at the 1st, 10th, and 30th cycles, respectively. The following compounds were identified: (▲) Ca12Al14O33 and (▼) Ca3Al2O6. (b) The calculated weight fractions of CaO, Ca12Al14O33, and Ca3Al2O6 determined by Rietveld analysis, as a function of cycle number.

75 ppm; 6.9 MHz for δiso = 79 ppm) may suggest the formation of less symmetric and more distorted AlIV sites, possibly due to a phase transformation from Ca3Al2O6 to Ca12Al14O33 in the bulk. In Ca:Al_90:10 that has undergone 10 cycles, only one dominant AlIV site (δiso = 76 ppm and CQ = 1.9 MHz) is observed, indicating a total reconstruction of the AlIV sites,56 which suggests a complete transformation of Ca3Al2O6 to Ca12Al14O33. These characteristic features of the AlIV sites change only negligibly when the cycle number is increased further to 30 cycles (albeit an increase of CQ to 2.0 MHz with δiso at 78 ppm was observed). Combining solid-state 27Al MAS NMR and in situ XRD measurements of Ca:Al_90:10 indicates the successive transformation of calcium-aluminum mixed oxides from Ca3Al2O6 to Ca12Al14O33. One mechanism that could explain the structural collapse of Ca3Al2O6-supported CaO is the demixing of the initially homogeneously distributed (in the CaO matrix) calciumaluminum oxide leading to the segregation of Al2O3 and its migration, for instance, to the surface, where it provides very little structural support. To assess the validity of this hypothesis, analytical methods with preferential surface-sensitivity are essential. Dynamic nuclear polarization (DNP) has been shown to be an effective technique to improve the NMR sensitivity utilizing the large Boltzmann polarization of unpaired electrons (γ = 28024.952 MHz/T).44,45,64−66 In DNP, the solid sample is impregnated typically with a small amount of a nitroxide-based radical solution,67−69 and is cooled down to cryogenic temperatures (ca. 100 K) under MAS conditions. High-energy and high-frequency (gyrotron) microwaves are applied to promote the polarization transfer from the electron to the nuclei, typically 1H. The resulting nuclear hyperpolarization traverses the frozen matrix via 1H spin diffusion of solvent protons, and a final cross-polarization (CP)70 step allows the polarization transfer to the target heteronuclei, e.g., 27 Al. Given the proximity between the hyperpolarized solvents and the surface, the solid surface/surface sites are selectively enhanced compared to the bulk material, hence coining the name “DNP surface enhanced NMR spectroscopy (DNP SENS)”.44,45,71 In this sense, DNP SENS can be used to selectively probe the structural changes of the surface. The DNP surface-enhanced 27Al NMR spectra (Figure 5b) of freshly calcined Ca:Al_90:10 (0 cycles) and Ca:Al_90:10 after 5 cycles show asymmetric signatures, indicating the presence of different Al sites in the sorbents. Fitting of NMR spectra

and Figure S9), whereby the weight fraction of Ca3Al2O6 was reduced to 1 wt % (mass balances for Ca close to 97%). After 30 cycles, the fraction of Ca12Al14O33 decreases further to 11 wt % while the CaO content increased to 89 wt %, indicative of a “demixing” of the calcium and aluminum mixed oxide; Al2O3 is presumably amorphous and not detected by XRD. Our in situ XRD measurements confirm that, over repeated cycles of carbonation and calcination, the fraction of calcium-aluminum mixed oxides decreases. To investigate in more detail the structural changes of the Al species over multiple CO2 uptake and regeneration cycles, solid-state NMR was applied (5, 10, and 30 cycles; Figure 5a

Figure 5. (a) Solid-state 27Al MAS NMR and (b) DNP-SENS 27Al NMR of Ca:Al_90:10 that has undergone 5, 10, and 30 cycles. The isotropic chemical shift (δiso) and quadrupolar coupling constant (CQ) were fitted using solid line shape analysis (SOLA) with the quadrupolar nuclei central transition model.

and Table S2). 2D 27Al multiple-quantum (MQ) MAS NMR experiments59−63 were utilized to enable a clear assignment of the different Al sites (Figure S10). In Ca:Al_90:10 that has undergone 5 cycles, two dominant AlIV sites were observed (δiso = 75 and 79 ppm; CQ = 1.8 and 8.5 MHz, respectively). These NMR signatures resemble the AlIV sites in Ca12Al14O33 and Ca3Al2O6, respectively, yet the increase in the CQ values compared to freshly calcined Ca:Al_90:10 (1.2 MHz for δiso = 1348


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Figure 6. Surface characterization: (a) Raman spectra and (b) HAADF STEM with EDX mapping of freshly calcined Ca:Al_90:10 and Ca:Al_90:10 that has undergone 5, 10, or 30 cycles.

revealed two different Al sites with δiso at 74 ppm (CQ = 0.8− 0.9 MHz) and 77 ppm (CQ = 8.6−9.3 MHz), corresponding to AlIV centers in Ca12Al14O33 and Ca3Al2O6, respectively. After 10 and 30 cycles, the AlIV site at 77 ppm of δiso has almost disappeared. This is attributed to the transformation of Ca3Al2O6 to Ca12Al14O33, which is in agreement with our 27Al solid-state NMR and in situ XRD data. (Figure 5a and 4a). After 10 cycles, a second peak appears with δiso at 8 ppm and CQ of 2.0 MHz, corresponding to an AlVI site that is indicative of the formation of Al2O3, preferentially on the surface.46,47 (see also the spectra of α-Al2O3 and calcined Ca:Al_0:100, Figure S3a) Increasing the number of cycles to 30 led to an increase in the intensity of this AlVI peak. Hence, both conventional and DNPenhanced solid-state 27Al NMR suggest a phase transformation in the mixed oxide from Ca3Al2O6 to Ca12Al14O33 with CO2 uptake and regeneration cycles. Yet, more importantly, the different features observed from these two methods evidence the preferential segregation of Al2O3 to the surface of the sorbent. The formation and surface-segregation of Al2O3 is supported further by Raman spectroscopy (Figure 6a). Freshly calcined Ca:Al_90:10 shows characteristic Ca-O bands (152 and 205 cm−1), O2− bands (279 and 356 cm−1) due to O2− ions in the cage of the Ca-Al mixed oxides, and a broad band (670−920 cm−1) due Al-O in an AlIV framework (these signatures are indicative of the presence of Ca-Al mixed oxides in the fresh, uncycled material72−75). Upon exposure to repeated carbonation and calcination cycles, in particular after 30 cycles, we observe Al-O bands due to aluminum in an AlVI framework (372, 410, 583, 640, and 737 cm−1, corresponding to the Eg, A1g, Eg, A1g, and Eg bands of Al2O3). Hence, Raman spectroscopy further supports the formation, segregation, and surface-enrichment of Al2O3 in reacted Ca:Al_90:10. The last piece of evidence for the proposed mechanism that triggers the structural collapse of the material (and in turn the reduced CO2 uptake) is provided by STEM EDX analysis (Figure 6b). In freshly calcined Ca:Al_90:10, Al is distributed fairly evenly throughout the material due to the formation of a well-mixed, solid solution between the oxides of calcium and aluminum (Figure S2). With increasing number of cycles, the distribution of Al becomes heterogeneous, yet at cycle number 10, the positions of Ca and Al are still overlapping largely. However, after 30 cycles, STEM shows the formation of Al 2 O 3

nanoparticles that have segregated from the bulk and migrated to the surface of rather large, possibly sintered CaO particles. This phenomenon can be linked to the reduction of the material’s pore volume and surface area with cycle number. At the surface, Al2O3 particles are prone to sintering, leading to particle growth. The surface enrichment of Al is in agreement with DNP 27Al NMR and Raman measurements. To summarize, we have synthesized Ca3Al2O6-stabilized, CaO-based CO2 sorbents. The formation of homogeneously distributed, mixed oxides of calcium and aluminum, i.e., Ca3Al2O6 (TT = 771 °C) and Ca12Al14O33 (TT = 725 °C), provides high structural stability and prevents largely the sintering of CaO/CaCO3, translating in turn to a high CO2 uptake, in particular when compared to limestone-derived CaO. In situ XRD measurement during cyclic CO2 capture and regeneration provided evidence for the partial demixing of the calcium-aluminum mixed oxides and the formation of additional CaO. DNP enhanced 27Al NMR and STEM analysis confirmed the formation, segregation, and enrichment of Al2O3 on the surface of Ca:Al_90:10 that has undergone 10 or more cycles. Hence, the formation, segregation, and surface-enrichment of Al2O3 from Ca-Al mixed oxides triggers the sinteringinduced structural collapse of the material and explains the rather sudden decrease in the CO2 uptake of the material. It is hoped that these fundamental insights in the deactivation of calcium oxide based CO2 sorbents will allow us to develop materials with an increased cyclic stability to reduce CO2 capture costs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05034. Experimental details and additional supporting table and figures (N2 physisorption, XRD, TGA, 27Al NMR, SEM, and STEM) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +41 44 632 3440. E-mail: [email protected] (C.R.M.). *Tel.: +41 44 633 9394. E-mail: [email protected] (C.C.). 1349


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Sung Min Kim: 0000-0001-6602-1320 Wei-Chih Liao: 0000-0002-4656-6291 Davood Hosseini: 0000-0001-5261-3149 Christophe Copéret: 0000-0001-9660-3890 Christoph R. Müller: 0000-0003-2234-6902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge ETH (ETH 57 12-2) and the Swiss National Science Foundation (200020_156015) for financial support. We also thank Ms. Lydia Zehnder for her support with the XRD measurements. The Scientific Center for Optic and Electron Microscopy (ScopeM) is acknowledged for providing access to electron microscopes and Dr. René Verel and Dr. Ta-Chung Ong for their help with the Al MAS NMR measurements.

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In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced

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