Development of an effective bi-functional Ni–CaO catalyst-sorbent for the sorption-enhanced water gas shift reaction through structural optimization and the controlled deposition of a stabilizer by atomic layer deposition

Abstract

The integration of a CaO-based CO₂ sorbent into catalytic schemes to remove CO₂ from the product stream provides an effective means to reduce greenhouse gas emissions of chemical processes and to improve the yield and purity of the desired product. A key requirement for such so-called sorbent-enhanced processes is the availability of cyclically stable CO₂ sorbent. To this end, we have developed CaO-based CO₂ sorbents that combine favourable structural features and a high thermal stability by introducing a thin, conformal layer of Al₂O₃ (forming Ca₃Al₂O₆ with CaO upon calcination) by atomic layer deposition. The structure and pore volume of the sorbent were found to play a key role in its CO₂ capture. Functionalizing such CO₂ sorbents with Ni nanoparticles yielded a highly effective bi-functional material for the sorption-enhanced water-gas shift (SE-WGS) reaction. The material showed a high yield of hydrogen of high purity and minimal CO slip over several cycles of repeated SE-WGS/regeneration operation.

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Introduction

The key contributor to global warming is the increasing emission of anthropogenic carbon dioxide (CO₂) into the atmosphere.^1,2 To limit the global temperature increase to 2 °C compared to pre-industrial levels, the annual global CO₂ emissions have to be reduced to 14 Gt CO₂ by 2050.³ In this context, CO₂ capture and storage (CCS) has the potential to provide a near-to midterm strategy to reduce anthropogenic CO₂ emissions.³ In addition, CO₂ capture and utilization (CCU) may offer a solution to omit CO₂ storage and utilize CO₂ as a carbon source for the production of value-added chemicals and/or fuels.^4–8

Currently, scrubbing relying on aqueous solutions of amines (e.g., monoethanolamine) is widely used to remove CO₂ selectively from a gas stream, e.g., for the purification of natural gas. Yet amine scrubbing has several limitations such as high regeneration costs translating into high costs for CO₂ capture and formation of hazardous side products, e.g., nitrosamines.^9,10 Hence, recent research activities have focused primarily on developing alternative CO₂ capture technologies that rely on solid sorbents, e.g., layered double oxides (LDO),¹¹ zeolites,^12,13 supported amines,¹⁴ activated carbons,^15,16 metal organic frameworks (MOF),^17,18 and alkaline earth metal oxides.^19,20 Among these alternatives, high-temperature calcium looping (CaL) based on the reversible reaction of CaO with CO₂ (i.e., CaO + CO₂ ⇄ CaCO₃, ΔH⁰_298K = ±178 kJ mol⁻¹) has attracted significant attention owing to a number of desirable characteristics of CaO that include its (i) high theoretical CO₂ uptake capacity (0.78 g_CO2 g_CaO⁻¹), (ii) large abundance of natural precursors (e.g., limestone and dolomite), and (iii) the fast kinetics of the CO₂ capture and release reactions. Moreover, the CO₂ capture costs of CaL are estimated to be in the range 12–32 € per t CO₂, which is considerably lower compared to amine scrubbing (34–67 € per t CO₂), pressure swing adsorption (26–33 € per t CO₂) or membrane separation (31–36 € per t CO₂).²¹

The main hurdle concerning the effective utilization of CaO derived from natural precursors (e.g., limestone) is the low Tammann temperature (T_T) of pure CaCO₃ (≈530 °C), which is below the operating temperature of calcium looping (650–900 °C)²² and thus, leads to a low cyclic stability of the material due to sintering. To alleviate the sintering-induced decay of the CO₂ capture capacity of CaO-based CO₂ sorbents, the incorporation of high-T_T stabilizers into the CaO matrix has been proposed. Commonly used stabilizers can be classified into two different groups: (i) those forming a mixed oxide with CaO, such as Al₂O₃, SiO₂, TiO₂ and ZrO₂,^23–26 and (ii) inert stabilizers that do not react chemically with CaO under the relevant operating conditions, such as MgO, Y₂O₃, and ZnO.^27–31 Furthermore, to make calcium looping a viable option at the industrial scale (e.g., operation in circulating fluidized beds) high CO₂ uptakes within relatively short residence times are required. The carbonation of CaO is known to proceed in two reaction regimes,^32,33viz., the kinetically controlled carbonation followed by a sluggish diffusion-limited carbonation regime (the diffusivity of CO₂ in the CaCO₃ product layer, D_CaCO3 = 0.003 cm² s⁻¹, is two orders of magnitude smaller than that in CaO, D_CaO = 0.3 cm² s⁻¹). The transition between these two carbonation regimes is believed to occur when the thickness of the CaCO₃ product reaches ∼50 nm.³² Hence, nano-structuring CaO would minimize the diffusion lengths of CO₂ through the CaCO₃ product layer. In addition to nano-structuring of the sorbent, also the introduction of porosity plays a critical role owing to the large volumetric expansion of the material upon CO₂ capture, as the molar volume of the product (CaCO_3, 36.9 cm³ mol⁻¹) is nearly twice as high as that of the reactant (CaO, 16.7 cm³ mol⁻¹).

Hence, the ideal CaO-based sorbent would exhibit a nano-structured and highly porous morphology that is stabilized by a high-T_T temperature material. Previous studies^{25,28,34–36} have revealed that the scale (micro-, nano- or even atomic level) at which the stabilizer is introduced into the matrix plays an important role for the overall performance of the sorbents. A number of techniques have been adopted to introduce a stabilizer into the CaO including wet-mixing,³⁷ coprecipitation,³⁸ hydration,³⁹ flame spray pyrolysis,⁴⁰ hydrolysis⁴¹ and sol–gel.^42–44 However, these techniques offer only little control over morphological features, such as nanostructure and porosity. More recently, vapor-phase deposition techniques such chemical vapor deposition (CVD)⁴⁵ and atomic layer deposition (ALD)²⁵ have emerged as promising strategies to introduce a stabilizer to a CaO matrix. In particular, ALD offers the possibility of obtaining conformal coatings of a stabilizer, while controlling its thickness on the atomic level. Owing to these unique features of ALD, structurally well-controlled and -characterizable model materials can be realized to investigate the structure–performance relationships of CaO-based sorbents, which has not been conducted in detail yet.

Besides the capture of CO₂, CaO-based sorbents have the potential to be integrated in processes that combine CO₂ capture with a catalytic reaction, e.g., steam reforming^46–50 and/or the water-gas shift reaction^51–54 to yield high-purity H₂. For example, a typical composition of the effluent gas of an industrial water-gas shift unit is approximately 76% H₂, 17% CO₂, 3% CO, and 4% CH₄.⁵⁵ The simultaneous removal of CO₂ from this stream would not only increase substantially the purity of the H₂ produced, but also shift the equilibrium to the product side according to Le Chatelier's principle. These sorption-enhanced processes reduce the operational complexity, since a nearly pure product stream is obtained in a single unit/reactor without further purification steps. Alkali metal-promoted hydrotalcite and Al₂O₃ have been employed for the sorption-enhanced water-gas shift reaction (SE-WGS),^52,56–60 yet with the shortcomings of low CO₂ capture capacities (0.01–0.02 g_CO2 g_sorbent⁻¹) and poor carbonation kinetics. On the other hand, CaO-based CO₂ sorbents are favorable candidates for CO₂ capture purposes owing to their relatively high CO₂ capture capacity (0.3–0.5 g_CO2 g_sorbent⁻¹) and fairly rapid carbonation kinetics at the operating temperatures (i.e., 400–600 °C) of the SE-WGS (H₂O + CO + CaO ⇆ H₂ + CaCO₃, ΔH⁰_298K = −219 kJ mol⁻¹) and SE-SMR (CH₄ + 2H₂O + CaO ⇆ 4H₂ + CaCO₃, ΔH⁰_298K = −13 kJ mol⁻¹) reactions. CaO-based CO₂ sorbents combined with Ni-based catalysts have been employed predominantly for the sorption-enhanced steam methane reforming (SE-SMR). However, high loadings of Ni (i.e., 15–50 wt%) have been required to obtain an acceptable SMR activity, which leads to an appreciable reduction in the CO₂ uptake capacity of the material. For SE-WGS, a relatively low Ni loading (3–10 wt%) has been shown to suffice for high CO conversions.^61–64 Nevertheless, the sintering-induced deactivation of CaO renders them ineffective as a sorbent in a cyclic process.^65,66 Therefore, the development of CaO-based sorbents with a high cyclic stability is vital for the SE-WGS reaction. In addition to a stable CO₂ sorbent, a highly active and stable catalyst at the temperatures of interest is equally important to obtain high yields of high-purity hydrogen. Cu–Zn and Fe–Cr catalysts have been employed in industrial scale water-gas shift reaction units (WGS: H₂O + CO ⇆ H₂ + CO₂, ΔH⁰_298K = −41 kJ mol⁻¹) at low (180–250 °C) and high (300–500 °C) temperatures, respectively.^67–70 Due to the sintering-induced deactivation of Cu-based catalysts, Fe–Cr-based catalysts are often preferred. The strongly carcinogenic, water-soluble hexavalent chromium in Fe–Cr can be leached out under WGS conditions, leading to health and environmental concerns.⁷¹ Therefore, it is highly desirable to develop alternative catalyst formulations containing, e.g., Ni-, Pt-, Pd-, Ru- and Au-based materials for medium-temperature (250–350 °C) WGS.⁶¹ In particular, Ni-based catalysts are attractive as they combine a non-pyrophoric, water-tolerant active phase, relatively low cost and a high activity for CO conversion when compared to its precious metal counterparts, such as Pt, Pd or Ru.^61,69,72

In the context of sorption-enhanced reactions, an important design consideration that needs to be taken into account is the effective interplay between the catalyst and the CO₂ sorbent, which is not easily achieved by the physical mixing of the two components. In this regard, bi-functional catalyst-sorbents are attractive as they provide the means of enhancing both heat and mass transfer between the catalyst and sorbent sites.^73,74 Bi-functional SE-WGS materials using hydrotalcites as the CO₂ sorbent have been manufactured using co-precipitation approaches,^52,75 a method which offers, however, only limited control over the textural and structural characteristics of the material. In addition, hydrotalcites feature a relatively low CO₂ capture capacity when compared to CaO-based sorbents.⁷⁶

Herein, we report the development of a bi-functional catalyst-sorbent containing CaO as the CO₂-capture-active material. The structure of the CaO backbone is stabilized by Al₂O₃ that is introduced by ALD. CaO precursors with different morphologies are investigated, i.e., hollow CaCO₃ realized through a sacrificial templating approach and benchmark materials in the form of commercial CaCO₃, CaCO₃ nanoparticles and limestone. To yield high purity hydrogen, the sorbent is decorated with Ni nanoparticles for SE-WGS operation.

Experimental

Materials

Calcium nitrate tetrahydrate (Acros, 99%), D-(+)-glucose (Sigma-Aldrich, ≥ 99.5%), ethylene glycol (Sigma-Aldrich, 99.8%), trimethyl aluminum (TMA, Pegasus Chemicals, electronic grade), boric acid (Fisher, laboratory reagent grade) CaCO₃ (Fisher) and CaCO₃ nanoparticles (American elements) were used without any further treatment.

Preparation of carbon spheres

Carbon spheres were prepared via the carbonization of D-(+)-glucose under hydrothermal conditions. Specifically, aqueous glucose solutions (2.2 M) were transferred into a teflon-lined stainless steel autoclave. The hydrothermal treatment was carried out at 180 °C for 12 h (autogenous pressure). The resulting material was collected, washed with de-ionized (DI) water and anhydrous ethanol, alternatingly 5 times, and finally dried at 80 °C in an oven overnight.

Preparation of hollow CaCO₃

Dried carbon spheres (1.0 g) were dispersed in an aqueous solution of 10 mmol calcium nitrate and 50 mmol urea (100 ml). The carbon spheres were dispersed by sonication (10 min) followed by vigorous stirring (1 h). Urea hydrolysis to deposit CaCO₃ onto the carbon spheres was performed at 90 °C for 6 h under reflux conditions. The resulting material was collected and washed thoroughly with DI water 5 times. After drying at 120 °C in an oven, hollow CaCO₃ was obtained by the thermal removal of the carbon template in a muffle furnace at 600 °C (5 °C min⁻¹, 2 h).

Atomic layer deposition of Al₂O₃

An Al₂O₃ overcoat was deposited onto non-porous, commercial CaCO₃ and hollow CaCO₃ using a commercial ALD reactor (Picosun R-200) equipped with a system for the coating of powders. For the deposition of Al₂O₃, electronic grade TMA and DI water were used as precursors, and high-purity N₂ served as both carrier and purge gas. The pulse and purge times were kept constant at 0.1 s/15 s/0.1 s/15 s for TMA/N₂/H₂O/N₂. The deposition took place at 300 °C, and the precursor temperatures were fixed at 20 °C. Use of high-resolution TEM allowed us to determine a deposition rate of ∼0.1 nm/ALD cycle, which is in good agreement with the thickness of Al₂O₃ grown simultaneously on a Si wafer and measured by means of ellipsometry (SENTECH SE850).

Bifunctional Ni–CaO catalyst preparation

Bi-functional Ni–CaO materials were prepared by wet impregnation with a solution of nickel acetylacetonate (Ni(acac)₂) dissolved in tetrahydrofuran. Specifically, 0.14 g of Ni(acac)₂ and 1 g of the sorbent, i.e., Al₂O₃-coated or non-coated hollow CaO sorbents were mixed with 50 ml of tetrahydrofuran. After vigorous stirring for 5 h, the materials were dried at 80 °C in an oven overnight, followed by calcination at 900 °C (5 °C min⁻¹) for 2 h in a muffle furnace. The nominal nickel loading was 3.0 wt% as per determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

Material characterization

The elemental composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5100 VDV).

Dried hollow CaCO₃ and as-synthesized Al₂O₃-coated, hollow CaCO₃ (after one half and one full ALD cycle) were collected and characterized by diffuse reflectance FTIR spectroscopy (DRIFT) in the same glove box as the ALD reactor using an Alpha II spectrophotometer (Bruker).

N₂ physisorption was measured using a NOVA 4000e analyzer (Quantachrome Instruments). The adsorption and desorption of N₂ were determined at −196 °C after degassing each sample at 250 °C under vacuum for 2 h. The specific surface area and pore size distribution were calculated using the Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–Halenda (BJH) models, respectively.

The crystallinity and chemical composition of the sorbents were investigated using powder X-ray diffraction (PANalytical Empyrean equipment) with Cu Kα radiation (λ = 1.5418 Å, 45 mA and 40 kV). Each sample was measured in the 2θ range of 10–90° using a step size of 0.016° with a time duration per step of 0.8 s. In the in situ XRD experiments, ∼0.1 g of the CO₂ sorbent was placed in a reaction chamber (XRK 900, Anton Paar). The reaction gases (CO₂ and N₂) were controlled via mass flow controllers (Bronkhorst EL-FLOW MFC). The diffraction patterns during the cyclic carbonation and calcination experiments (following the procedure of the TGA experiments) were recorded in the 2θ range of 10–60° using a step size of 0.02°.

The morphology and textural features of the sorbents were analyzed by high-resolution scanning electron microscopy (HR-SEM, FEI Magellan 400 FEG), and focused ion beam scanning electron microscopy (FIB-SEM, Zeiss, NVision 40). Prior to SEM analysis, the samples were sputter-coated (MED 010) with an ∼5 nm layer of platinum. Transition electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were performed on a FEI Talos F200X operated at 200 kV, which was equipped with a SuperX EDX consisting of four SDD detectors.

CO₂ capture test

The cyclic CO₂ uptake of the samples was measured in a thermogravimetric analyzer (TGA, Mettler Toledo TGA/DSC 3+). In a typical experiment, ∼10 mg of the calcined sorbent was loaded into an alumina crucible (70 μl) and heated to 900 °C (50 °C min⁻¹ heating rate) in N₂, using a flowrate of 150 ml min⁻¹ (measured at ambient temperature and pressure), including a constant N₂ flow (25 ml min⁻¹) as a purge gas over the microbalance. After reaching the reaction temperature, the temperature was maintained for 4 min to completely calcine the material to CaO. Subsequently, the temperature was decreased to 650 °C (50 °C min⁻¹ cooling rate) and the carbonation reaction was performed for 20 min in 20 vol% CO₂ (150 ml min⁻¹) balanced by N₂. Subsequently, the temperature was increased to 900 °C (50 °C min⁻¹ heating rate) in a flow of 30 ml min⁻¹ of CO₂ (25 ml min⁻¹ of N₂ purge gas over the micro-balance) and the sample was calcined for 10 min to regenerate the CO₂ sorbent. The carbonation and calcination reactions were repeated for a given number of cycles for each sorbent.

The cyclic CO₂ capture performance under SE-WGS mimicking conditions (with regards to the temperature and the duration of the carbonation and regeneration reaction), i.e., carbonation at 400 °C using 70 ml min⁻¹ of 20 vol% CO₂ in N₂ for 50 min and regeneration at 800 °C using 50 ml min⁻¹ N₂ for 15 min, was evaluated in the TGA. At the beginning of the experiment the calcined material was heated to 800 °C (10 °C min⁻¹) in N₂ (70 ml min⁻¹) followed by reduction at 500 °C (10 °C min⁻¹) for 2 h in 10 vol% H₂/N₂ (100 ml min⁻¹). Ten repeated cycles of carbonation and regeneration were performed.

Sorption-enhanced water-gas shift reaction (SE-WGS)

The cyclic SE-WGS reaction was carried out in a fixed-bed quartz reactor (∼10.5 mm of internal diameter and 400 mm of length; the bed was placed in the middle of the reactor). In a typical experiment, 0.25 g of the calcined material diluted with SiC (weight ratio of 1 to 10) was used. Prior to the activity test, the material was heated at 800 °C (10 °C min⁻¹) for 1 h in N₂ (100 ml min⁻¹) to ensure the full calcination to CaO. Subsequently, the material was reduced at 500 °C for 2 h in 10 vol% H₂/N₂ (100 ml min⁻¹). The SE-WGS was performed at 400 °C using a total flow rate of 70 ml min⁻¹ (5 ml min⁻¹ CO, 20 ml min⁻¹ H₂O and 45 ml min⁻¹ N₂; GHSV = 16.8 L g⁻¹ h⁻¹ of GHSV). After 50 min of the SE-WGS, the material was heated to 800 °C (20 °C min⁻¹ ramp) and regenerated for 15 min in N₂ (70 ml min⁻¹). In total ten SE-WGS/regeneration cycles were performed. The composition of the off-gas (after the condensation of unreacted steam) was analyzed using a micro-GC (C2V-200, Thermo Scientific) equipped with a thermal conductivity detector (TCD) and using molecular sieve 5A and U-plot cartridges.

Estimation of CO₂ uptake at the kinetically controlled regime

The CO₂ uptake in the kinetically controlled reaction stage was estimated following a model by Pacciani et al. that assumes that the CO₂ capture in the kinetically controlled reaction stage is completed once the volume available in small pores (d_pore < 100 nm) is filled by CaCO₃:^33,77

Results and discussion

Synthesis of the CO₂ sorbents

The synthesis procedure to obtain Al₂O₃-coated, hollow CaO spheres is described schematically in Fig. 1a. Carbon nanospheres (CNS) serving as a sacrificial template were prepared by the hydrothermal treatment of an aqueous solution of glucose (0.22 M) at 180 °C. CaCO₃ was deposited onto carbon spheres by precipitation of an aqueous solution (0.1 M) of Ca(NO₃) with hydrolysed urea at 80 °C, viz., Ca(NO₃)₂ + CO(NH₂)₂ + 2H₂O → CaCO₃ + 2NH₄NO₃, (ΔH⁰_298K = −184 kJ mol⁻¹).

Fig. 1 Al₂O₃-coated, hollow CaO spheres: (a) template-assisted synthesis procedure. (b) DRIFT difference spectra (700–3800 cm⁻¹) after one half and one full ALD cycle. (c) Reaction schematic for the deposition of Al₂O₃ onto CaCO₃via ALD.

The thermal decomposition profile (in air) of as-synthesized CNS and CaCO₃ coated CNS were assessed by TGA (Fig. S1†). In air, the carbon template was removed completely at 500 °C, yielding hollow CaCO₃ structures. Due to the low T_T of CaCO₃, it is important to remove the template at the lowest temperature possible to avoid sintering. After template removal, a thin layer of Al₂O₃ was deposited onto the hollow CaCO₃ spheres via ALD (using alternating pulses of trimethylaluminum (TMA) and steam at 300 °C for a given number of cycles). The final material, i.e., Al₂O₃-coated, hollow spheres of CaO, was obtained by calcination in air at 900 °C.

To probe how the deposition of an Al₂O₃ overcoat onto CaCO₃via ALD proceeds, DRIFT spectra were collected after one half (only TMA pulse injection) and a full (TMA and subsequent steam pulse injection) ALD cycle (Fig. S2†). As the Al₂O₃ precursor (i.e., TMA) is highly reactive with air, the DRIFT analysis was performed in a glovebox. The infrared spectrum of hollow CaCO₃ shows the typically signatures of CO₃²⁻, viz.; (i) out of plane bending (971 cm⁻¹), (ii) symmetric stretching (1550 cm⁻¹), and (iii) combination band of stretch and out of plane vibration (2387–2659 cm⁻¹). The presence of bands (2750–3075 cm⁻¹) due to C–H bonds indicates that there were some organic residues of the carbon sphere. The presence of hydroxyl groups on the surface of the hollow CaCO₃ spheres was confirmed by the presence of O–H bending (1676 cm⁻¹) and O–H stretching (3642 cm⁻¹) vibrations. The O–H stretching band at 3642 cm⁻¹ has been ascribed to hydroxyl ions on interstitial defects of the CaCO₃ lattice framework.⁷⁸Fig. 1b plots the difference DRIFT spectra after one half and one full ALD cycle compared to pristine, hollow CaCO₃. After one half cycle the intensity of the C–H stretching (2750–3075 cm⁻¹) and C–H bending vibrations (1480 cm⁻¹) increased due to the methyl groups of surface-adsorbed (or chemisorbed) TMA. The reduced intensity of the O–H stretching vibrations indicates that, as expected, TMA reacted with surface O–H groups. After the subsequent injection of a pulse of steam, Al–O vibrations (940 cm⁻¹) appeared and the intensity of the C–H vibrations reduced, indicative of the transformation of TMA to hydroxylated aluminium oxide. According to our DRIFT experiments, the following mechanism for the deposition of Al₂O₃ on CaCO₃via ALD can be drawn (Fig. 1c): First TMA reacts with surface hydroxyl groups that are present on CaCO₃. After the exposure to steam, the methyl groups on the surface are hydrolyzed with releasing methane, leading to the formation of a layer of hydroxylated Al₂O₃. Repeating the ALD cycle multiple times leads to the formation of a layer of Al₂O₃ (the surface is hydroxylated) on hollow CaCO₃ spheres.

To label the different sorbents synthesized, the following nomenclature is used throughout the manuscript: “Al (theoretical thickness of the ALD-grown Al₂O₃ layer)–Ca containing phase (either CaCO₃ or CaO)_X′′, where X refers to the CaCO₃ structure that was coated by ALD, viz., hollow CaCO₃ spheres, limestone, commercial, bulk CaCO₃, or nanoparticles (NP) of CaCO₃. For example, the material that contains a 2 nm thick overcoat of Al₂O₃ onto hollow CaCO₃ is referred to as “Al (2 nm)–CaCO₃_hollow”.

Morphological and structural characterization

Fig. 2 visualizes the morphology of the hollow CaCO₃ spheres at different stages of the synthesis process. Scanning electron microscopy (SEM) of the CNS template revealed an average particle diameter of 780 ± 25 nm (Fig. 2a). After the deposition of CaCO₃ by precipitation and subsequent calcination at 600 °C in static air (to remove the CNS template), hollow CaCO₃ spheres were obtained (Fig. 2b). Increasing the calcination temperature to 900 °C in a second step converted CaCO₃ to CaO further, yet lead to the collapse of the hollow sphere structure (Fig. 2c). High-resolution transmission electron microscopy (HR-TEM) of Al₂O₃ coated hollow CaCO₃ spheres, i.e., Al (2 nm)–CaCO₃_hollow (calcined at 600 °C) confirmed that ALD deposited a conformal Al₂O₃ overcoat on the shell-comprising CaCO₃ nanometer-sized grains. The thickness of the Al₂O₃ layer was 2.0 ± 0.3 nm (grown by 20 ALD cycles, Fig. 2e). The high porosity of the hollow CaCO₃ spheres ensured a rapid diffusion of the ALD precursors (TMA and steam) resulting in a homogeneous, conformal Al₂O₃ overcoat on all of the shell-comprising nanometre-sized grains. After calcination at 900 °C in air, the initially clear boundary between CaCO₃ and the Al₂O₃ overcoat disappeared (Fig. 2f). This was due to the formation of a solid solution between Al and Ca, i.e., Ca₃Al₂O₆ (discussed in more detail in the following section). Nevertheless, the hollow structure of Al (2 nm)–CaO_hollow was preserved during calcination at 900 °C (Fig. 2d). Imaging focused ion beam (FIB) cuts of these structures by SEM confirmed that the hollow structure was conserved after high temperature calcination at 900 °C (Fig. 2g and h). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray spectroscopy (EDX) analysis of Al (2 nm)–CaO_hollow (Fig. 2i–l) revealed a homogeneous distribution of Al and Ca in the material, i.e., after calcination the stabilizer Al₂O₃ was not localized as an overcoat anymore.

Fig. 2 Morphological characterization of Al₂O₃-coated, hollow spheres: SEM images of (a) the carbon sphere template, (b) hollow CaCO₃, (c) calcined Al (0 nm)–CaO_hollow and (d) calcined Al (2 nm)–CaO_hollow. HR-TEM images of (e) Al (2 nm)–CaCO₃_hollow and (f) calcined Al (2 nm)–CaO_hollow. (g and h) FIB-SEM and (i–l) HAADF-STEM with EDX analysis of calcined Al (2 nm)–CaO_hollow.

The benchmark materials, Al (2 nm)–CaO_limestone and Al (2 nm)–CaO_bulk, featured rather coarsely-structured morphologies (Fig. 3a–c and d–f, respectively). Similarly, the benchmark CaCO₃ NP, in the absence of an Al₂O₃ coating, i.e., Al (0 nm)–CaCO₃_NP, grew substantially in particle size upon exposure to 900 °C in air (Fig. 3h), whereas the original structure of the spherical nanoparticles was preserved to a large extent when coated by Al₂O₃, viz., Al (1 nm)–CaO_NP (Fig. 3i–l). However, after the calcination of Al (1 nm)–CaO_NP the distribution of Al became more heterogeneous, forming small clusters (Fig. 3j–l). When the thickness of the Al₂O₃ coating was increased further to 12 nm, the core-shell-like morphology of as-synthesized Al (12 nm)–CaCO₃_NP (Fig. 3m and n) transformed into a hollow-sphere-structured morphology after calcination at 900 °C, i.e., Al (12 mn)–CaO_NP (Fig. 3o and p). The formation of these hollow structures was due to the interdiffusion of CaO and Al₂O₃ (Kirkendall effect) upon calcination,⁷⁹ forming the mixed oxide Ca₃Al₂O₆.⁸⁰ The accumulation of voids at the interface through interdiffusion yields small pores.^81,82

Fig. 3 Morphological characterization of the benchmark sorbents: STEM EDX analysis of (a–c) Al (2 nm)–CaO_limestone and (d–f) Al (2 nm)–CaO_bulk. The insets in (a) and (d) represent HAADF-STEM images of Al (2 nm)–CaO_limestone and Al (2 nm)–CaO_bulk, respectively. TEM images of (g) Al (0 nm)–CaCO₃_NP and (h) Al (0 nm)–CaO_NP. (i) HAADF-STEM with (j–l) EDX analysis of Al (2 nm)–CaO_NP, HAADF-STEM with EDX analysis of (m and n) Al (12 nm)–CaCO₃_NP and (o and p) Al (12 nm)–CaO_NP.

To obtain additional information on the structure and composition of the sorbents, in situ XRD experiments were performed on hollow CaCO₃ in the presence or absence of an Al₂O₃ coating. In situ diffraction patterns were acquired during heating in air. For both Al (0 nm)–CaCO₃_hollow and Al (2 nm)–CaCO₃_hollow (Fig. 4a and S3†), the conversion of CaCO₃ to CaO was completed at ∼700 °C; this is in good agreement with the decomposition profile obtained in the TGA (Fig. S1†). Al (2 nm)–CaCO₃_hollow did not show any signatures of crystalline Al-containing phases in the temperature range 600–800 °C (Fig. 4a). The as-synthesized, ALD-grown Al₂O₃ overcoat was initially amorphous. Diffraction peaks due to Ca₃Al₂O₆ appeared at 820 °C and intensified with increasing temperatures (Fig. 4b).

Fig. 4 Physicochemical characterization: (a) in situ XRD diffractograms of Al (2 nm)–CaCO₃ in the temperature range of 600–900 °C. (b) The Ca mol% of CaCO₃, CaO and Ca₃Al₂O₆ in the material calculated by Rietveld refinement as a function of temperature. The symbols represent (■) CaCO₃, (●) CaO and (◆) Ca₃Al₂O₆. (c) Mol% of Al³⁺ in different materials that have been coated by a 2 nm thick layer of Al₂O₃ (determined by ICP-OES) as a function of the surface area of the original sorbent. BJH pore size distribution of (d) CaO_hollow, (e) CaO_NP in the absence, i.e., Al (0 nm) and presence of an ALD-grown Al₂O₃ coating of 2 nm thickness. The arrow indicates the change in the pore volume with increasing thickness of the Al₂O₃ overcoat.

The diffractograms of benchmark CaCO₃ (limestone, bulk and NP) and the calcined derivatives in the absence or presence of an Al₂O₃ coating were acquired by ex situ XRD (Fig. S4†). In the case of the as-synthesized benchmark CaCO₃ (limestone, bulk and NP) with a 2 nm-thick Al₂O₃ overcoat, only CaCO₃ was identified, pointing to an amorphous nature of the as-synthesized Al₂O₃ overcoat. After calcination, Bragg reflections due to CaO and Ca(OH)₂ were typically observed in all of the calcined benchmark sorbents regardless of the presence of an Al₂O₃ coating owing to the decomposition of CaCO₃ to CaO and the formation of Ca(OH)₂ due to the highly hygroscopic nature of CaO. Additionally, Ca₃Al₂O₆ was observed for the calcined, Al₂O₃-coated benchmark sorbents (i.e., limestone, bulk and NP). The observations made by electron microscopy analysis and XRD measurements point to the formation of Ca₃Al₂O₆ through the Kirkendall effect.

For Al (2 nm)–CaO sorbents (hollow, limestone, bulk and NP), the intensity of the diffraction peaks of Ca₃Al₂O₆ increased with the thickness of the Al₂O₃ layer (Fig. S5a†). Rietveld analysis of the calcined, Al-containing samples confirmed a linear relationship between the amount of Ca₃Al₂O₆ formed and the quantity of Al determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Fig. S5b†). This implies that all Al introduced as a stabilizer formed a Ca–Al mixed oxide with CaO, viz., Ca₃Al₂O₆. It is also important to note that the same thickness of the Al₂O₃ overcoat (2 nm) translates to a different quantity of Al in the synthetic and benchmark sorbents owing to different surface areas (Table 1 and S5b†). A higher surface area translates into a higher fraction of Al₂O₃ in the as-synthesized sorbent, as ALD is a surface-dependent, self-limiting deposition process (Fig. 4c).

Table 1 Textural properties of the hollow and benchmark CO₂ sorbents

Sorbent		Ca : Al (molar ratio)	Surface area [m^2 gsorbent^−1]	Pore volume [cm^3 gsorbent^−1]
Hollow CaCO3		100 : 0	9	0.018
Al (2 nm)–CaCO3_hollow		90 : 10	5	0.009
Calcined	Al (0 nm)	100 : 0	18	0.210
	Al (2 nm)	90 : 10	22	0.220
Limestone		100 : 0	1	0.008
Calcined	Al (0 nm)	100 : 0	11	0.123
	Al (2 nm)	99 : 1	12	0.185
	Al (10 nm)	89 : 11	18	0.190
Bulk CaCO3		100 : 0	1	0.002
Calcined	Al (0 nm)	100 : 0	11	0.112
	Al (2 nm)	98 : 2	13	0.187
	Al (10 nm)	88 : 12	17	0.195
CaCO3 nanosphere		100 : 0	19	0.175
Calcined	Al (0 nm)	100 : 0	9	0.021
	Al (1 nm)	90 : 10	22	0.175
	Al (2 nm)	82 : 18	27	0.182

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The textural properties of the sorbents were characterized by N₂ physisorption. For sorbents without an Al₂O₃ overcoat, calcination of CaCO₃ (hollow, limestone and bulk) lead to an increase in both surface area and pore volume owing to the release of CO₂ during the decomposition of CaCO₃ (Table 1). An exception was the calcination of the benchmark CaCO₃ NP (Al (0 nm)–CaCO₃_NP), which lead to a reduction in both the surface area and pore volume from 19 m² g_sorbent⁻¹ and 0.175 cm³ g_sorbent⁻¹ to, respectively, 9 m² g_sorbent⁻¹ and 0.012 cm³ g_sorbent⁻¹. As confirmed by electron microscopy, calcination of Al (0 nm)–CaCO₃_NP resulted in a sintered, non-porous structure.

The deposition of Al₂O₃ onto structures of CaCO₃ affected the textural properties of the materials after calcination (compared to the Al₂O₃-free counterparts). Fig. 4d plots the BJH pore size distribution of hollow CaCO₃ with and without an Al₂O₃ overcoat. After calcination, the surface area and pore volume of Al (2 nm)–CaO_hollow (22 m² g_sorbent⁻¹ and 0.223 cm³ g_sorbent⁻¹) were higher than that of Al (0 nm)–CaO_hollow (18 m² g_sorbent⁻¹ and 0.210 cm³ g_sorbent⁻¹). In particular, the pore volume in pores with d_pore < 10 nm increased significantly when an Al₂O₃ overcoat was deposited. For Al (2 nm)–CaO_NP, the increase in pore volume occured mostly in small pores with d_pore < 10 nm (Fig. 4e). Similarly, the volume in pores with d_pore < 10 nm increased appreciably for Al (2 nm)–CaO_limestone and Al (2 nm)–CaO_bulk when compared to their overcoat-free counterparts (Fig. S6†). The observations made by N₂ physisorption in combination with the results of the XRD measurements and electron microscopy indicate that the interdiffusion of CaO and Al₂O₃ induced by the formation of Ca₃Al₂O₆ upon calcination results in the formation of small pores (d_pore < 10 nm).

CO₂ capture performance of CaO-based CO₂ sorbents

The cyclic CO₂ capture performance of the different CaO-based CO₂ sorbents was assessed in a TGA using industrially relevant operation conditions, i.e., calcination was performed at 900 °C in a CO₂-rich atmosphere. The CO₂ sorbent with a hollow sphere structure without the addition of an Al₂O₃ overcoat (Al (0 nm)–CaO_hollow) showed a significant reduction in its CO₂ uptake with cycle number. The CO₂ uptake decreased from 0.61 g_CO2 g_sorbent⁻¹ in the first cycle to 0.26 g_CO2 g_sorbent⁻¹ in the 10^th cycle (Fig. S7a†). The other extreme, Al (12 nm)–CaO_hollow, which is almost pure Ca₃Al₂O₆, showed no CO₂ uptake as Ca₃Al₂O₆ does not react with CO₂ under the conditions studied here. Nevertheless, the deposition of an Al₂O₃ overcoat (forming Ca₃Al₂O₆ upon calcination) stabilized the cyclic CO₂ uptake of the material, despite the small quantity of Al₂O₃ added (overcoat thicknesses of 0.5 nm and 1.0 nm correspond to an Al³⁺ molar fraction of 2.5% and 5.1% in the sorbent, respectively, as determined by ICP-OES). After ten cycles, Al (0.5 nm)–CaO_hollow and Al (1 nm)–CaO_hollow retained 65% (0.38 g_CO2 g_sorbent⁻¹) and 77% (0.44 g_CO2 g_sorbent⁻¹) of their initial CO₂ uptake capacity, respectively, compared to 42% for the unstabilized sample Al (0 nm)–CaO_hollow. When the thickness of the Al₂O₃ overcoat was increased to 2 nm and 8 nm, the retention value for the CO₂ uptake over ten cycles was further improved to 93% and 97%, respectively, but the overall CO₂ uptake capacity was much lower than with thinner coatings. Fig. 5a plots the CO₂ uptake of hollow-sphere sorbents at the 10^th cycle as a function of the thickness of the ALD-grown Al₂O₃ layer. The parabolic behaviour of the CO₂ uptake, where a sharp increase is followed by a gradual decrease with an increasing thickness of the Al₂O₃ overcoat, suggests that there is a trade-off between the cyclic stability of a sorbent and the availability of CO₂-capture-active CaO. Based on our measurements, the optimal thickness of the Al₂O₃ overcoat was 2 nm (corresponding to 10 mol% of Al³⁺ in the sorbent).

Fig. 5 CO₂ capture performance: (a) CO₂ uptake of Al₂O₃-coated, hollow-spheres after the 10^th cycle as a function of the thickness of the ALD-grown Al₂O₃ layer. The dashed line represents the cyclic CO₂ uptake of limestone, i.e., Al (0 nm)–CaO_limestone, in the 10^th cycle, and the solid line gives the maximum theoretical CO₂ uptake of pure CaO. (b) CO₂ uptake of Al₂O₃-coated, CaO-based CO₂ sorbents as a function of the number of carbonation–regeneration cycles. The dashed line represents the maximum theoretical CO₂ uptake of a sorbent with a molar ratio of Ca²⁺ : Al³⁺ = 9 : 1 (i.e., 25 wt% of Ca₃Al₂O₆) assuming the full conversion of all of the CO₂ capture-active CaO in the material. (c) Temporally resolved CO₂ uptake profiles of the sorbents studied as a function of carbonation time. The solid and dashed lines represent the 1st and 30th carbonation cycle, respectively. (d) CO₂ uptake in the kinetically controlled carbonation stage as a function of pore volume in the 1^st (■) and 30^th (□) cycle. The dashed line represents the CO₂ uptake in the kinetically controlled carbonation stage as predicted by the Pacciani et al. model.^33,77 The fraction of Al³⁺ in Al (2 nm)–CaO_hollow, Al (10 nm)–CaO_limestone, Al (10 nm)–CaO_bulk and Al (1 nm)–CaO_NP was determined as 10 mol% by ICP-OES.

For reference, Fig. S7b† plots the CO₂ uptake of the benchmark sorbents (limestone, bulk and NP) that were also coated with a 2 nm-thick Al₂O₃ layer. The initial CO₂ uptake and stability varied significantly with the type of CaO/CaCO₃ matrix of the backbone. Specifically, the following values were observed for the initial CO₂ uptake and retention of the CO₂ uptake over 30 cycles: (i) Al (2 nm)–CaO_hollow (0.54 g_CO2 g_sorbent⁻¹ and 80%), (ii) Al (2 nm)–CaO_limestone (0.55 g_CO2 g_sorbent⁻¹ and 30%), (iii) Al (2 nm)–CaO_bulk (0.56 g_CO2 g_sorbent⁻¹ and 46%), and (iv) Al (2 nm)–CaO_NP (0.53 g_CO2 g_sorbent⁻¹ and 58%). Owing to differences in the surface areas of the different CaO/CaCO₃ structures that have been coated with a 2 nm-thick layer of Al₂O₃, the molar fraction of Al³⁺ in the material varied between 1–18 mol%. Hence, the number of ALD cycles had to be adjusted to obtain a fixed quantity of 10 mol% of Al³⁺ (as determined by ICP-OES, Table 1) in the benchmark materials, yielding Al (10 nm)–CaO_limestone, Al (10 nm)–CaO_bulk and Al (1 nm)–CaO_NP. The weight fractions of Ca₃Al₂O₆ in the calcined materials, as calculated by the Rietveld analysis, were as follows (Fig. S5b†): Al (2 nm)–CaO_hollow (25%), Al (10 nm)–CaO_limestone (26%), Al (10 nm)–CaO_bulk (24%) and Al (1 nm)–CaO_NP (24%). All of these values were very close to the theoretically expected weight fraction of 25%, which is based on the assumption that all of the Al available is present in the phase Ca₃Al₂O₆. All of these sorbents (both hollow and benchmark morphologies) that contained 10 mol% of Al³⁺ showed nearly identical CO₂ uptakes in the first cycle (Fig. 5b), i.e., in the range 0.49–0.53 g_CO2 g_sorbent⁻¹ (Fig. 5b). However, the decay behavior of the sorbents differed largely. The initial CO₂ uptake and decay rate per cycle were determined as follows: Al (2 nm)–CaO_hollow (0.53 g_CO2 g_sorbent⁻¹ and 0.7%/cycle), Al (10 nm)–CaO_limestone (0.51 g_CO2 g_sorbent⁻¹ and 1.8%/cycle), Al (10 nm)–CaO_bulk (0.50 g_CO2 g_sorbent⁻¹ and 1.8%/cycle) and Al (1 nm)–CaO_NP (0.48 g_CO2 g_sorbent⁻¹ and 2.5%/cycle).

To understand better the effect of the structure of the sorbent on the CO₂ uptake, the temporally resolved carbonation profiles were acquired (Fig. 5c). The two reaction regimes, i.e., kinetically controlled and diffusion-limited, can be approximated by linearly increasing CO₂ uptakes with time, whereby the intersection of the linear approximations can be used to determine the transition point between the two regimes. The CO₂ uptake of Al (2 nm)–CaO_hollow in the kinetically controlled stage of the carbonation reaction reached 0.38 g_CO2 g_sorbent⁻¹ in the 1^st cycle (corresponding to 71% of the overall CO₂ uptake), which reduced to 0.21 g_CO2 g_sorbent⁻¹ in the 30^th cycle (55% retention). For the benchmark sorbents (limestone, bulk and NP), 56–67% of the total CO₂ uptake was captured in the kinetically controlled carbonation regime in the 1^st cycle, i.e., comparable to the sorbent that possessed a hollow-structured morphology. Despite similar quantities of Al³⁺ (10 mol%) in all of the sorbents tested, the benchmark sorbents experienced a significant drop in their CO₂ uptake in the kinetically controlled carbonation regime when compared to Al (2 nm)–CaO_hollow. For all sorbents investigated there appeared to be a linear relationship between the quantity of CO₂ captured in the kinetically controlled carbonation regime and the BJH pore volume for d_pore < 100 nm (Table 1 and Fig. 5d). The measured CO₂ uptake agreed well with the one predicted by the model of Pacciani et al.^33,77 providing further evidence for the assumption that the CO₂ uptake in the kinetically controlled carbonation regime was governed by the filling of the small pores with d_pore < 100 nm.^33,77

Morphological characterization of the cycled CaO-based sorbents

To evaluate changes in the structure of the sorbent over multiple CO₂ capture and regeneration cycles, the cycled CO₂ sorbents were analyzed by electron microscopy. After 30 cycles, the grain size in all of the CO₂ sorbents investigated increased appreciably (Fig. 6a). The growth in grain size (increase compared to the initial grain size) was in the following order: Al (1 nm)–CaO_NP (324%) ≫ Al (10 nm)–CaO_limestone (181%) ≈ Al (10 nm)-CaO_bulk (171%) > Al (2 nm)–CaO_hollow (130%). The trend in the growth of the size of the CaO grains is in line with the rate of decay in the CO₂ uptake, i.e., a higher growth rate of the grain size leads to a higher decay rate in the CO₂ uptake. This is indicative of a sintering-induced decay of the CO₂ uptake in all of the sorbents tested. Furthermore, HAADF-STEM imaging of Al (2 nm)–CaO_hollow (Fig. 6b) reveals that the central void of the hollow spheres was preserved during carbonation and possibly acted as a buffer for the volumetric changes during carbonation and regeneration. Nevertheless, the size of the central void was reduced gradually over repeated cycles of carbonation and regeneration and finally disappeared after 20 cycles. Over 20 cycles, the overall diameter of the hollow spheres reduced from 780 ± 20 nm to 610 ± 15 nm. EDX analysis confirms the aggregation and enrichment of Al-containing phases on the surface of cycled Al (2 nm)–CaO_hollow, in particular from cycle number 10 onwards (Fig. 6b). The segregation and enrichment of Al-containing phases on the surface was also observed in the benchmark materials (Fig. 6c). The surface migration of the stabilizer (Al₂O₃ or Ca–Al mixed oxides) probably triggeried the sintering of the material.⁸³

Fig. 6 Morphological characterization of the cycled CO₂ sorbents: (a) SEM images of freshly calcined and cycled CO₂ sorbents. (b) HAADF-STEM with EDX analysis of Al (2 nm)–CaO_hollow after carbonation as a function of cycle number. (c) HAADF-STEM with EDX analysis of cycled benchmark materials. The molar ratio of Al in the benchmark materials is 10 mol% as determined by ICP-OES. All of the cycled benchmark materials are in the calcined state.

Sorption-enhanced water-gas shift reaction of bi-functional Ni–CaO catalyst

CaO-based CO₂ sorbents have the potential to be incorporated in catalytic processes, yet the possible interaction between the catalyst and the CO₂ sorbent (both stabilizer and CaO) with respect to the formation of a catalytically inactive solid solution needs to be taken into consideration. Here, a bi-functional catalyst-sorbent was prepared by impregnating Ni onto the most promising CO₂ sorbent, i.e., Al (2 nm)–CaO_hollow, and Al (0 nm)–CaO_hollow for reference. The large miscibility gap owing to an appreciable difference in the effective ionic radii of Ca²⁺ (100 pm) and Ni²⁺ (69 pm) in an octahedral coordination⁸⁴ restricts the formation of a CaO–NiO solid solution.⁸⁵ Concerning the interaction between Ni and Al₂O₃ under high-temperature calcination conditions NiAl₂O₄ can form.⁸⁶ The bi-functional material was characterized in detailed after calcination at 900 °C in air. H₂-TPR of both Ni/Al (2 nm)–CaO_hollow and Ni/Al (0 nm)–CaO_hollow showed a single reduction peak at 460–470 °C (Fig. 7a). This low-temperature reduction is ascribed to the reduction of α-type NiO that has only a weak interaction with the support.^87,88

Fig. 7 Characterization of bi-functional Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow: (a) H₂-TPR, (b) XRD diffractograms of the calcined and reduced materials. The symbols represent (●) CaO, (◆) Ca₃Al₂O₆, (▲) Ca(OH)₂, (■) NiO, and (□) Ni. (c) HAADF-STEM images of the reduced materials coupled with EDX mappings of Ca, Ni, and Al.

The chemical composition and structure of the calcined and reduced materials were characterized further by XRD. The following phases were identified in calcined Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow: NiO, CaO and Ca(OH)₂. In Ni/Al (2 nm)–CaO_hollow, also Ca₃Al₂O₆ was detected by XRD. After reduction, Bragg reflections due to Ni were observed in both Ni-containing materials (Fig. 7b). XRD did not provide any evidence that Ni interacted with CaO or Ca₃Al₂O₆, which is in agreement with the findings of the H₂-TPR experiments.

The reduced materials were visualized by HAADF-STEM coupled with EDX analysis (Fig. 7c). After reduction, Ni nanoparticles with a diameter of 11 ± 3.7 nm and 9.7 ± 2.2 nm were observed on Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow, respectively. It is important to note that the hollow-sphere structure of Ni/Al (2 nm)–CaO and the homogeneous distribution of Ca and Al were preserved over wet impregnation, calcination and reduction. However, in the absence of the stabilizer Al₂O₃, i.e., Ni/Al (0 nm)–CaO_hollow, the hollow structure collapsed.

The catalytic activity of the bi-functional materials for the cyclic SE-WGS reaction was assessed in a fixed-bed reactor at 400 °C using a total flow rate of 70 ml min⁻¹ (5 ml min⁻¹ CO, 20 ml min⁻¹ H₂O and 45 ml min⁻¹ N₂; GHSV = 16.8 L g⁻¹ h⁻¹ of GHSV). After 50 min of time on stream (TOS), the material was heated to 800 °C for regeneration (15 min in 50 ml min⁻¹ of N₂). Fig. 8a plots the mole fractions of H₂, CH₄, CO and CO₂ (dry, N₂-free basis) measured in the effluent stream during the first cycle. As the CO₂ capture part is a transient process due to the finite CO₂ capture capacity of the bifunctional material, the overall reaction proceeds in three distinct stages: (i) pre-breakthrough stage, producing high-purity H₂ with full CO conversion; (ii) breakthrough stage, reducing H₂ purity owing to the decreasing CO₂ uptake capacity (and the kinetics) of the sorbent; and (iii) post-breakthrough stage, yielding an off-gas composition close to the conventional WGS process as the CO₂ sorbent has reached its full capacity. High-purity H₂ that requires hardly any additional purification steps is produced only in the pre-breakthrough stage. Hence, a material yielding a long pre-breakthrough period is highly desirable. Using Ni/Al (0 nm)–CaO_hollow, the effluent gas contained 98% H₂ in the pre-breakthrough stage (t = 0–29 min), with negligible quantities of CH₄, CO₂ and CO, yielding 100% CO conversion and 25.3 mmol g⁻¹ of high-purity H₂. This observation confirms that the carbonation reaction reaches equilibrium rapidly, leading to an almost complete uptake of the CO₂ produced in the WGS. In the breakthrough stage (t = 29–40 min), the molar fraction of H₂ decreased gradually, whereas the mole fraction of CO₂ increased. This was due to fact that the carbonation reaction transitioned into the slow, diffusion-limited reaction regime, limiting the uptake of CO₂ in the given residence time of the reactor. At the end of the breakthrough stage, the mole fractions of H₂ and CO₂ were 0.45 and the mole fraction of CO increased from <0.01 to 0.06. The CO conversion was reduced from ≈100% to 94%. For Ni/Al (2 nm)–CaO_hollow, the overall trend was similar to Ni/Al (0 nm)–CaO_hollow; yet, the duration of the pre-breakthrough stage was shorter (t = 0–24 min) when compared to Ni/Al (0 nm)–CaO_hollow (t = 0–29 min). The CO conversion and H₂ yield in the pre-breakthrough of Ni/Al (2 nm)–CaO_hollow were 100% and 21.0 mmol g⁻¹, respectively. This was due to the formation of Ca₃Al₂O₆ in Ni/Al (2 nm)–CaO_hollow, which reduced the quantity of CO₂-capture-active CaO in the material (75%). Concerning the cyclic stability of the bi-functional materials, we observed that the duration of the pre-breakthrough period gradually decreased with number of repeated SE-WGS and regeneration cycles (Fig. 8b). Nevertheless, Ni/Al (2 nm)–CaO_hollow exhibited a good cyclic stability (reduction of the duration of the pre-breakthrough period from 24 to 21 min only over ten cycles), whereas the pre-breakthrough period of Ni/Al (0 nm)–CaO_hollow reduced from 29 min to 10 min. Indeed, the cyclic CO₂ capture capacity of Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow measured during SE-WGS was in very good agreement with the values determined in a TGA under SE-WGS-mimicking conditions (carbonation at 400 °C using 70 ml min⁻¹ of 20 vol% CO₂ in N₂ and regeneration at 800 °C in 70 ml min⁻¹ N₂; Fig. 8c). However, the CO₂ uptake under SE-WGS conditions (carbonation a 400 °C) is lower than the value determined under CO₂ capture conditions (carbonation at 650 °C), most likely due to slower carbonation kinetics. Concerning the window of the reaction temperature of the SE-WGS (∼400 °C), MgO-based CO₂ sorbents might be more favorable.^51,89 The particle size of Ni in Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow increased by a factor of, respectively, 3.9 (43 ± 7 nm) and 2.5 (24 ± 5 nm) over ten cycles of the SE-WGS (Fig. S9†). Nonetheless, both materials maintained a high CO conversion during pre-breakthrough (≈100%) and post-breakthrough (≈94%) over the ten cycles tested. A significant reduction was observed in the pore volume of Ni/Al (0 nm)–CaCO₃_hollow from 0.175 cm³ g_sorbent⁻¹ to 0.021 cm³ g_sorbent⁻¹ after ten cycles of the SE-WGS and regeneration, whereas Ni/Al (0 nm)–CaCO₃_hollow possessed a much higher pore volume of 0.120 cm³ g_sorbent⁻¹ (Table S2†). The reduction of the available pore volume in the material is closely linked to the decay of the CO₂ uptake, leading to a poor SE-WGS stability of Ni/Al (0 nm)–CaO_hollow.

Fig. 8 SE-WGS performance of Ni/Al (0 nm)–CaO_hollow and Ni/Al (2 nm)–CaO_hollow: (a) composition of the effluent during the 1st cycle. Three reaction stages are observed: (i) pre-breakthrough, (ii) breakthrough, and (iii) post-breakthrough. (b) Breakthrough curves of H₂ (red) and CO₂ (green) during the 1^st (solid line), 5^th (dashed line) and 10^th (dotted line) cycle. The arrows indicate the shift of breakthrough with cycle number. (c) CO₂ uptake as determined in a TGA and during SE-WGS conditions. The solid line gives the maximal theoretical CO₂ uptake of pure CaO.

The SE-WGS performance of Ni/Al (2 nm)–CaO_hollow was investigated further in the kinetic regime, i.e., far from the equilibrium CO conversion, using different steam-to-carbon (S/C) ratios and different temperatures (400–600 °C). When increasing the S/C ratio from 1 to 4 an improved WGS performance was observed, i.e., an increase in the CO conversion from 40.5% to 67.4% in the CO₂-free pre-breakthrough stage (Fig. 9a). Furthermore, the CO conversion was increased from 55.8% to 69.5% when the temperature was increased from 400 °C to 500 °C under a total flow rate of 120 ml min⁻¹ (10 ml min⁻¹ CO, 20 ml min⁻¹ H₂O and 90 ml min⁻¹ N₂; GHSV = 28.8 L g_cat⁻¹ h⁻¹). A further increase of the temperature to 600 °C resulted in a decrease of the CO conversion to 61.3% (Fig. 9b). The reduced CO conversion is mostly ascribed to the equilibrium limitation of the SE-WGS above 600 °C (Fig. 9b). The H₂ yield in the CO₂-free pre-breakthrough stage showed an insignificant change with S/C ratio (Fig. 9a), but increased when the temperature was increased from 400 °C to 600 °C (Fig. 9b). This increase is linked to the increase of the CO₂ capture capacity of the sorbent from 0.36 g_CO2 g_sorbent⁻¹ at 400 °C to 0.53 g_CO2 g_sorbent⁻¹ at 600 °C (Fig. 9c). These findings are in line with the linear correlation of the H₂ yield with CO₂ uptake in the pre-breakthrough stage (Fig. 9d). It is noteworthy that the effective CO₂ uptake capacity of Ni/Al (2 nm)–CaO_hollow (0.36–0.53 g_CO2 g_sorbent⁻¹ at 400–600 °C) outperforms alkali metal-promoted hydrotalcite and Al₂O₃ materials that have been studied for the SE-WGS (i.e., 0.01–0.02 g_CO2 g_sorbent⁻¹).^52,56–60

Fig. 9 CO conversion and H₂ yield of Ni/Al (2 nm)–CaO_hollow in the pre-breakthrough stage of SE-WGS as a function of (a) the steam-to-carbon ratio (400 °C using a total flow rate of 110–140 ml min⁻¹ composed of 10 ml min⁻¹ CO, 10–40 ml min⁻¹ H₂O and 90 ml min⁻¹ N₂; GHSV = 26.4–36.8 L g_cat⁻¹ h⁻¹) and (b) the reaction temperature (400–600 °C using a total flow rate of 120 ml min⁻¹ composed of 10 ml min⁻¹ CO, 20 ml min⁻¹ H₂O and 90 ml min⁻¹ N₂; GHSV = 28.8 L g_cat⁻¹ h⁻¹). The dotted and solid lines represent the equilibrium CO conversion of WGS and SE-WGS, respectively. (c) CO₂ uptake of Ni/Al (2 nm)–CaO_hollow at different carbonation temperatures of 400, 500 and 600 °C as a function of the number of carbonation–regeneration cycles. (d) H₂ yield in the pre-breakthrough stage of the SE-WGS (using a total flow rate of 120 ml min⁻¹ composed of 10 ml min⁻¹ CO, 20 ml min⁻¹ H₂O and 90 ml min⁻¹ N₂; GHSV = 28.8 L g_cat⁻¹ h⁻¹) as a function of the CO₂ uptake of Ni/Al (2 nm)–CaO_hollow at 400, 500 and 600 °C.

The SE-WGS performance of bifunctional Ni–CaO is mostly affected by the catalytic activity for achieving a high CO conversion and on the CO₂ capture capacity for obtaining a long pre-breakthrough stage during which high-purity hydrogen is produced. The development of bifunctional catalyst-sorbents thus aims at reducing the quantity of catalyst and stabilizer in the material while providing still a high CO conversion and an effective stabilization of the CaO against sintering. Utilizing inert stabilizers that do not form mixed oxides with CaO under the relevant operation conditions, such as Y₂O₃, MgO and ZnO^27–31 typically allows for the production of materials with higher amounts of CaO, active for CO₂ capture. For higher CO conversions, the development of Ni-based catalysts forming alloys with Cu, Fe, Co and Zn, or with an improved Ni dispersion on the surface have shown promising results and could thus be considered in the further improvement of bifunctional catalyst-sorbent.⁶⁴

Conclusions

To summarize, we have employed atomic layer deposition (ALD) to deposit conformal layers of Al₂O₃ onto different structures of CaCO₃, i.e., hollow CaCO₃ spheres, natural limestone, bulk and nanoparticle-sized CaCO₃. The interaction of the ALD-grown layer of Al₂O₃ with CaO resulted in the formation of CO₂ capture-inactive Ca₃Al₂O₆. Ca₃Al₂O₆ acted as a stabilizer to maintain the original CaCO₃ structure during calcination, in particular for hollow spheres. We found that the CO₂ uptake capacity depends critically on the structure of the sorbent, whereby the volume available in pores with d_pore < 100 nm dictates the CaO conversion in the kinetically controlled carbonation regime. Among the sorbents tested, hollow CaCO₃ spheres coated with a 2 nm thick layer of Al₂O₃ yielded the highest CO₂ uptake over 30 cycles. Functionalizing such hollow CaCO₃ spheres with Ni nanoparticles lead to an effective bi-functional material for the sorption-enhanced water-gas shift reaction (SE-WGS). The newly developed bi-functional material showed a cyclically stable pre-breakthrough period in which high-purity hydrogen was produced with minimal CO slip.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors gratefully acknowledge ETH (ETH 57 12-2) and the Swiss National Science Foundation (200020_156015) for financial support. We would like to thank the Scientific Center for Optical and Electron Microscopy (ScopeM) and the Center for Micro- and Nanoscience (FIRST) at ETH Zurich for providing training and access to electron microscopes.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9se00619b

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