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In-situ XRD and dynamic nuclear polarization surfaceenhanced 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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05034 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Chemistry of Materials

In-situ XRD and dynamic nuclear polarization surface-enhanced NMR spectroscopy unravel the deactivation mechanism of CaO-based, Ca3Al2O6-stabilized CO2 sorbents Sung Min Kima, Wei-Chih Liaob, Agnieszka M. Kierzkowskaa, Tigran Margossianb, Davood Hosseinia, Songhak Yoonc, Marcin Brodaa, Christophe Copéretb,* and Christoph R. Müllera,* a

Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, ETH Zurich, Leonhardstrasse 21, 8092 Zurich, Switzerland b

Department of Chemistry and Applied Sciences, ETH Zürich, Vladimir Prelog Weg 1-5, 8093 Zürich, Switzerland

c

Institute for Materials Science, University of Stuttgart, Heisenbergstrasse 3, D-70569 Stuttgart, Germany

*Corresponding authors Tel.: +41 44 632 3440, [email protected] (Prof. Christoph Müller) Tel. +41 44 633 9394, [email protected] (Prof. Christophe Copéret)

Graphical abstract

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 typically by Ca3Al2O6 as a structural stabilizer. Nonetheless, the initially rather stable CO2 uptake of Ca3Al2O6-stabilized CaO yet starts to decay after around ten 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

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Al NMR (including 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 (approx. 10), Ca3Al2O6 was transformed to Ca12Al14O33 followed by Al2O3 segregation and enrichment at the surface in the form of small nanoparticles. Al2O3 in such a form is not -1ACS Paragon Plus Environment

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able to stabilize effectively the initially highly porous structure against thermal sintering leading in turn to a reduced CO2 uptake.

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CO2 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 valueadded 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 CO2 capture costs (60–107$ per ton of CO2 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 oxide21-24 are interesting 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: CaO + CO2 ⇄ CaCO3

ΔH298K = ± 178 kJ/mol

Indeed, techno-economic modelling of CaO-based CO2 capture have 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 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 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 two 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 oxide 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 Ca3Al2O6stabilized 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 CO2capture-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 Ca3Al2O6-stabilized 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 was 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 could 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 was 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 homogenous 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 Fig. S1 and Fig. S2, respectively. After the thermal decomposition of Pluronic P-123 micelles (≈ 400 °C), CaCO3 is formed and, after increasing further the temperature to 700–900 °C, CaO is obtained (Fig. S1a and S1b). 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 (Fig. S1c and S1d). Using XRD, the formation of mixed oxides, i.e. Ca12Al14O33 (≈ 700–800 °C) and Ca3Al2O6 (≈ 800–900 °C) in Ca:Al_90:10 was also revealed (Fig. S1b and Fig. S1d). STEM with EDX mapping of Ca:Al_90:10 (Fig. S2), confirms a homogeneous distribution of Al in the as-synthesized material and when calcined at 400 °C. N2 physisorption (Table S1 and Fig. 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 showed a hierarchical -4ACS Paragon Plus Environment

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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 was observed at p/po = 0.85–1.0, indicating the formation of mesopores (Fig. 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 resulted in small (dp < 10 nm) and large mesopore (dp ≥ 10 nm), respectively, leading to a hierarchical porosity in freshly calcined Ca:Al_90:10.

Figure 1.

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Al MAS NMR of Ca:Al_90:10 calcined at 800°C and 900 °C, and references: α-Al2O3, γAl2O3 (Ca:Al_0:100 calcined at 800 °C), Ca3Al2O6 and Ca12Al14O33, and (b) unit cell structure of Ca12Al14O33, Ca3Al2O6 and α-Al2O3 references. The spectra were fitted with the QUAD Central model in the Solid lineshape (SOLA) feature of Topspin for the isotropic chemical shift (δiso) and quadrupolar coupling constant (CQ).



Figure 1 plots the

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Al NMR spectra and the corresponding deconvolutions of Al2O3 and calcium-

aluminium mixed oxide references that were identified by XRD (Fig. S4). 27Al MAS NMR (Fig. 1 and Table S2) confirms that the predominant Al species (δiso = 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 = 74 and CQ = 3.2 MHz), pentahedral (AlV, δiso = 40 and CQ = 4.2 MHz), and octahedral (AlVI, δiso = 11 and CQ = 2.1 MHz) Al sites.

46-52

In calcium-aluminium mixed oxides (Ca12Al14O33 and Ca3Al2O6), the Al sites are

predominantly in tetrahedral coordinations.53-55 While Ca12Al14O33 shows a single dominant AlIV site (δiso = 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 (Fig. 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 a 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 (Fig. 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 a AlIV site with a Ca12Al14O33-like structure,56 and site 2 is in a good agreement with Ca3Al2O6 formation, which is corroborated by the diffractogram shown in Fig. 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

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Al MAS NMR confirm the

presence of Al in the form of mixed oxides in calcined Ca3Al2O6-stabilized CaO.


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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.

The cyclic CO2 capture performance of Ca3Al2O6-supported CaO and limestone was 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+ (Fig. 2a and Fig. S6), Ca:Al_90:10 was identified as the best material in terms of CO2 uptake after 10 cycles. The reference material, limestone-derived CaO showed a very high initial CO2 uptake capacity of 0.55 gCO2/gsorbent, however, its CO2 uptake significantly decreases rapidly yielding only 0.08 gCO2/gsorbent after 30 cycles. A significantly increased cyclic stability was 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 increased significantly the cyclic CO2 uptake of CaO. Yet, we -7ACS Paragon Plus Environment


observed 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 assessed first the temporally resolved carbonation characteristics (Fig. 2b and 2c). 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 reduced abruptly, marking the transition to the diffuse limited reaction stage. The intersection between the slopes of the two regimes was 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 was approximately 1.0 × 10-3 g CO2 / g CaO / s (possibly limited by external mass transfer characteristics of the TGA) and decreased to 3.0 × 10-5–5.0 × 10-5 g CO2 / g CaO /s when entering the diffusion-limited 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 2nd 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 (Fig. 2a), indicative that the overall CO2 uptake of a material is largely governed by the quantity of CO2 that is captured in the kinetic controlled reaction stage. Qualitatively, changes in the pore volume with cycle number (Fig. 2d, Fig. 2e 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 kineticallycontrolled carbonation stage (Fig. 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,

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After carbonation, the remaining pore volume in the material is negligible (Fig. 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 -8ACS Paragon Plus Environment

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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).

Figure 3.

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

The rapid structural change of unsupported CaO (derived from limestone) is also confirmed visually through electron microscopy (Fig. 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 showed signs of sintering, but to a lesser extent than unsupported CaO (see also Fig. 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 4.

Structural and chemical characterization: (a) in-situ XRD diffractogram 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 diffractogram at the 1st, 10th and 30th cycle, 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.

To answer this question, we first probed 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 were determined as, respectively, 81 wt. %, 3 wt. % and 16 wt. % in the freshly calcined material. In the 10th cycle, the quantities of CaO and Ca12Al14O33 increased to 86 wt. % and 13 wt. %, respectively (Fig. 4b and Fig. S9), whereby the weight fraction of Ca3Al2O6 was reduced to 1 wt. % (mass balances for Ca closed to 97 %). After 30 cycles, the fraction of Ca12Al14O33 decreased further to 11 wt. % while the CaO content increased to 89 wt. %, indicative of a “de-mixing” of the calcium and aluminium 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-aluminium mixed oxides decreases.


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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) was fitted using Solid lineshape analysis (SOLA) with the quadrupolar nuclei central transition model.

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; Fig. 5a 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 = 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) was 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 was 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 calciumaluminium mixed oxides from Ca3Al2O6 to Ca12Al14O33. -11ACS Paragon Plus Environment


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One mechanism that could explain the structural collapse of Ca3Al2O6-supported CaO is the de-mixing of the initially homogeneously distributed (in the CaO matrix) calcium-aluminium 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).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. 100K) 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 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.

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Al. Given the proximity between the

polarized solution 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 (Fig. 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 revealed two different Al sites with a 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 insitu XRD data. (Fig. 5a and 4a). After 10 cycles, a second peak appears with δiso at 8 ppm and CQ of 2.0 MHz, corresponding to a 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, Fig. S3a) Increasing the number of cycles to 30, led to an increase in the intensity of this AlVI peak. Hence, both conventional and DNP enhanced solid-state

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Al 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.


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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.

The formation and surface-segregation of Al2O3 is supported further by Raman spectroscopy (Fig. 6a). Freshly calcined Ca:Al_90:10 shows characteristic Ca-O bands (152 cm-1 and 205 cm-1), O2- bands (279 cm-1 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 aluminium in an AlVI framework (372 cm-1, 410 cm-1, 583 cm-1, 640 cm-1 and 737 cm-1, corresponding to the Eg, A1g, Eg, A1g, 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 (Fig .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 aluminium (Fig. 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 Al2O3 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.



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To summarize, we have synthesized Ca3Al2O6-stabilized, CaO-based CO2 sorbents. The formation of homogeneously distributed, mixed oxides of calcium and aluminium, 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 de-mixing of the calcium-aluminum mixed oxides and the formation of additional CaO. DNP 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 surfaceenrichment of Al2O3 from Ca-Al mixed oxides triggers the sintering-induced 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.

Associated contents Supporting information Experimental details and additional supporting table and figures (N2 physisorption, XRD, TGA,

27

Al

NMR, SEM and STEM)

Notes The authors declare no conflict of interest.

Acknowledgements 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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