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Integrated CO2 capture and conversion as an efficient process for fuels from greenhouse gases Sung Min Kim, Paula M. Abdala, Marcin Broda, Davood Hosseini, Christophe Copéret, and Christoph R. Müller ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03063 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Integrated CO2 capture and conversion as an efficient process for fuels from greenhouse gases Sung Min Kima, Paula M. Abdalaa, Marcin Brodaa, Davood Hosseinia, Christophe Copéretb and Christoph Müllera,*. a

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

ABSTRACT: To mitigate climate change, the reduction of anthropogenic CO2 emissions is of paramount importance. CO2 capture and storage has been identified as a promising short- to mid-term solution, yet the underground storage of CO2 faces often severe public resistance. In this regard, the conversion of the CO2 captured into useful chemicals or fuels is an attractive alternative. Here, we propose and experimentally demonstrate a process that directly integrates CO2 utilization into CO2 capture allowing for the full conversion of the CO2 captured and the selective production of a synthesis gas. The process is attractive both economically and from a process operation point of view, as the coupled reactions are performed in a single reactor. The concentration of (unreacted) CO2 in the off-gas is below 0.08 %, demonstrating the almost full conversion of the CO2 captured in a single, integrated step. Importantly, the process is demonstrated using a nonprecious metal catalyst and an inexpensive naturally occurring CO2 sorbent, viz. limestone. KEYWORDS: CO2 capture and utilization, Calcium looping, Dry reforming of methane, Ni/MgO-Al2O3 DRM catalyst, Limestone-derived CaO-based CO2 sorbent 1. Introduction According to the International Energy Agency (IEA), CO2 capture and storage (CCS) is a technology that is expected to contribute appreciably to the reduction of anthropogenic CO2 emissions.1 Concerning the capture part of CCS, the only technique that has been realized at the industrial scale is amine scrubbing, which is currently used for example for the purification of natural gas.2 However, the implementation of amine scrubbing in the context of CCS is hampered by its high costs (60–107$ per ton of CO2 captured 3-5), reducing the overall efficiency of a power plant by 8.0–12.5 absolute percentage points.6-7 Thus, considerable research efforts are undertaken currently to develop more efficient and, at the same time, less costly CO2 capture technologies. An alternative to amine scrubbing is the capture of CO2 using solid sorbents such as (i) alkaline earth metal oxides, (ii) layered double oxides (LDO)8, (iii) carbon9-10 or (iv) metal organic frameworks (MOF)11-12. Among these solid sorbents, CaO (calcium looping)13-18 has attracted significant attention due to the low price and high abundance of CaO precursors, e.g. limestone, and its high theoretical CO2 uptake capacity of 0.78 gCO2⋅gsorbent-1. In the calcium looping process (Fig. S1), CO2 is captured by CaO through the reversible carbonation reaction (Eq. 1) CaO(s) + CO2 ↔ CaCO3(s) ΔH0298K = ± 178 kJ/mol (1) Techno-economic analyses of the calcium looping process have highlighted its high thermodynamic efficiency, predicting CO2 capture costs in the range of 12–32 $ per ton of CO2, a significant reduction when compared to amine scrubbing.3-5, 19-20 Although CCS has the potential

to contribute to a considerable extent to the reduction of anthropogenic CO2 emissions (according to the IEA by approximately 19% of the total quantity of CO2 to limit the global temperature increase to 2 °C1, 21), there is a growing interest in exploring the possibility to utilize the CO2 captured as a carbon feedstock and convert it to fuels or value-added chemicals.22-29 The capture and subsequent conversion of CO2 is often referred to as CO2 capture and utilization (CCU).30-34 In addition, combining CCS with its conversion to fuels or chemicals may aid the demonstration of the capture technology at the large scale. Nonetheless, a proper assessment of the economic potential of chemical energy storage and CO2 recycling is ultimately required, yet important boundary conditions may depend and vary on currently very dynamic political settings. Generally, to ensure a high efficiency of a CCU process it should (i) be performed in an integrated fashion (ii) yield high CO2 conversions, thus, minimizing energy-intensive separation steps and (iii) avoid the use of expensive chemical resources, such as H2. A rather versatile CO2 and CH4 utilization product is synthesis gas, i.e. a mixture of hydrogen and carbon monoxide, as it can be converted further to hydrocarbon fuels via the established Fischer-Tropsch (FT) process or to methanol (methanol economy)35. In this context, several processes have been proposed. For example, the synthesis gas production via chemical looping reforming that relies on the partial oxidation of methane by a solid oxygen carrier: CH4 + MeOx → CO + 2H2 + MeOx-1, M = Cu, Fe, Mn and Ni.36-43 However, most solid oxygen carriers (Cu-, Mn- and Fe-based materials) yield rather low quantities of H2 and CO, favouring typically the

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The DRM proceeds typically over a transition metal catalyst such as Pt, Ru, Rh, Ni or Co50-53 with Ni-based catalysts being particularly attractive as they combine low costs (when compared to precious metals)54 and a high activity for methane activation55-59. Here, we propose and demonstrate a calcium loopingbased CCU process (Fig. 1) in which CO2 is captured using CaO as the CO2 sorbent in the first step. In the second step, the CO2 sorbent is regenerated, thereby releasing a stream of CO2 in a CH4 atmosphere, which is converted into a synthesis gas via the dry reforming methane, i.e. sorbent regeneration and CO2 conversion are performed simultaneously in a single step via: CaCO3(s) + CH4 → CaO(s) + 2CO + 2H2 ΔH0298K = + 425 kJ/mol (3)

Figure 1. Schematic diagram of the proposed process that integrates the dry reforming of methane directly into the CO2 capture process yielding a synthesis gas. In the first step: CO2 is captured using CaO. In the second step, the CO2 sorbent is regenerated, releasing CO2 that is immediately converted further into a synthesis gas via the DRM.

formation of the over-oxidized products, i.e. H2O and CO2. On the other hand, Ni-based oxygen carriers have been reported to produce with a high selectivity H2 and CO.39-43 In addition, recently the so-called tri-reforming of methane has been proposed, which aims at converting CO2 of a flue gas stream. Here, the endothermic steam and dry reforming reactions are coupled with the exothermic partial oxidation of methane.44-45 Yet, several challenges have to be addressed. This includes: (i) low synthesis gas purity due to high concentration of inert N2 in the flue gas and (ii) unreacted CO2 in the presence of O2 and H2O. A related concept brings a mixture of H2O, CO2 and CH4 in contact with an iron oxide–titania based oxygen carrier. A related concept brings a mixture of H2O, CO2 and CH4 in contact with an iron oxide–titania based oxygen carrier.44 This process was referred to as chemical looping-derived tri-reforming and aims at combining the endothermic steam and dry reforming reactions with the exothermic partial oxidation of methane.44-45 However, obtaining a high yield of synthesis gas via lattice oxygen, instead of molecular oxygen, is still a challenge. A somewhat different concept, is the so-called sorptionenhanced steam methane reforming (CH4 + H2O + CaO → H2 + CaCO3, ΔH0298K = -13 kJ/mol), in which the reforming reaction is performed in the presence of a CO2 sorbent shifting the equilibrium to the product side. This has allowed for the production of high purity hydrogen in a single step (≈ 97%), yet in this process the CO2 captured has to be processed further still or stored underground.18, 46-49 To the best of our knowledge, the simultaneous conversion of the CO2 obtained during regeneration has not been reported yet for a CaO-based CO2 sorbent. The dry reforming of methane (DRM) converts CO2 and CH4, two major greenhouse gases, into an equimolar mixture of hydrogen and carbon monoxide (Eq. 2). CH4 + CO2 → 2H2 + 2CO ΔH0298K = + 247 kJ/mol (2)

In this work, the two-step CCC and CCU process is operated in a cyclic fashion. In the following we demonstrate that using a fluidized bed that contains a mixture of an inexpensive CO2 sorbent and dry reforming catalyst (limestone-derived CaO and Ni supported on MgO-Al2O3, respectively) a synthesis gas with a purity close to 100 % (N2 free basis) and a H2:CO ratio of ~ 1.06 is produced. The CO2 slip of the process is very small, i.e. < 0.08 % of the total off-gas, demonstrating essentially the full conversion of the CO2 captured in a single, integrated step. 2. Experimental section 2.1. Catalyst preparation: A Ni-based catalyst derived from a hydrotalcite precursor, Ni/MgO-Al2O3, was prepared via co-precipitation. First, Ni(NO3)2⋅6H2O, Mg(NO3)2⋅6H2O, and Al(NO3)3⋅9H2O, where the molar ratio Ni : Mg : Al was 8.0 : 58.7 : 33.3, were dissolved in 100 mL of water (reverse osmosis, 15 MΩ·cm) to obtain a 1 M solution that subsequently was added dropwise to 100 mL of an aqueous solution of Na2CO3 (0.2 M). The pH of the mixture was adjusted to 10 ± 0.5 using 2.0 M NaOH. The resulting suspension was stirred at room temperature for 24 h. The precipitate was filtered and washed with DI water until the electrical conductivity of the filtrate was lower than 100 µS/cm. The material was dried overnight at 100 °C and calcined at 800 °C for 5 h using a heating rate of 5 °C/min. The calcined catalyst was ground and sieved to 300–500 µm. 2.2. CO2 sorbent preparation: The CO2 sorbent was derived from limestone (Rheinkalk) via calcination at 800 °C for 2 h in static air. The calcined limestone-derived CO2 sorbent was ground and sieved to 300–400 µm. 2.3. Characterization: A Quantachrome (NOVA 4000e) N2 adsorption analyzer was used to determine the surface area and pore volume of the calcined and reacted limestone and the Ni catalyst. Each sample was degassed at 300 °C for at least 3 h before characterization. The Brunauer et al. (BET)60 and the Barrett et al. (BJH models)61 were used to calculate the surface area and pore size distribution, respectively. Temperature-programmed reduction in hydrogen (H2TPR) was carried out using an Autochem 2920 (equipped with a thermal conductivity detector). In a typical


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experiment 50 mg of the calcined catalyst were loaded in a quartz reactor and heated to 300 °C in an argon atmosphere (50 ml/min) to dehydrate the sample. Subsequently, the materials were cooled down to 50 °C and the gas was switched to 5 vol.% H2/Ar (50 ml/min). Subsequently, the temperature was increased using a heating rate of 5 °C/min. Hydrogen chemisorption was conducted using an Autochem 2920 apparatus. First, the material was reduced in a quartz reactor at 800 °C for 2 h using 5 vol.% H2/Ar. Subsequently the sample was purged with Ar at 400 °C for 30 min and cooled down to 50 °C. The quantity of chemisorbed hydrogen was determined at 30 °C by periodically injecting pulses of 5 vol.% H2/Ar into the bed of the reduced catalyst. The stoichiometry factor of dissociated hydrogen to Ni was taken as 1.0 (H/M).62-64 The crystallinity and chemical composition of the materials were characterized using powder X-ray diffraction (Bruker AXS D8 Advance). The X-ray diffractometer (XRD) was equipped with a Lynxeye superspeed detector and operated at 40 mA and 40 kV using Cu Kα (λ=0.1541 nm) radiation. The diffraction patterns were recorded at ambient conditions in the range 2θ = 590°. The step size was 0.0275° with a time duration per step of 0.8 s. A scanning electron microscope (FEI Magellan 400 FEG), a transmission electron microscope (Philips CM12) and a scanning transmission electron microscope (Hitachi HD2700) were used to characterize the morphology of both the as synthesized and reacted Ni catalyst and limestone. Raman spectroscopy (Thermo Scientific) was used to characterize coke deposition. The Raman spectra were acquired in the range 500–3500 cm-1 using a laser with a wave length of 514.5 nm. The spectral resolution employed was 4 cm−1. 2.4. Cyclic CO2 capture experiments: Cyclic carbonation and calcination reactions were conducted in a fluidized bed reactor (Fig. S2). The fluidized bed reactor was constructed of a quartz tube (length 500 mm and inner diameter 15 mm). A sintered quartz plate, placed 200 mm above the bottom of the reactor was used as the gas distributor. The reactor was placed in an electrically heated, tubular, vertical furnace (Carbolite AAF 1100). The temperature of the bed was controlled via an N-type thermocouple. The flow rates of N2 and CO2 were controlled using mass flow controllers (Bronkhorst ELflow). The concentration of CO2 in the effluent gas was monitored continuously using a non-dispersive infrared analyzer (ABB EL3020). In a typical experiment, 1.0 g of the pre-calcined limestone (300–400 µm) was loaded into the reactor and heated to 800 °C under a flow of N2. Subsequently, the temperature was reduced to 720 °C and carbonation reaction was then conducted in an atmosphere containing 20 vol. % CO2 in N2 (0.2 L/min). After carbonation (15 min), the CO2 sorbent was regenerated (calcined) in N2 (0.5 L/min). In total 10 cycles of the repeated carbonation and calcination reactions were performed.

Additional cyclic CO2 capture and regeneration experiments were performed in a thermogravimetric analyzer (TGA, Mettler Toledo TGA/DSC 1). Here, approximately 10 mg of pre-calcined limestone (300–400 µm) was loaded into an alumina crucible and heated to 800 °C at a rate of 10 °C/min under a flow of N2 (total flow rate of 120 ml/min including a constant purge flow of N2 (25 ml/min) over the microbalance). When a temperature of 800 °C was reached, calcination was performed for 30 min. Subsequently, the temperature was decreased to 720 °C at a rate of 50 °C/min and carbonation (i.e. CO2 capture) was performed for 25 min in 20 vol. % of CO2 in N2 (100 ml/min). After carbonation, the CO2 sorbent was regenerated (calcined) at 720 °C for 15 min in a pure N2 atmosphere (100 ml/min). The cyclic carbonation and calcination reactions were repeated 10 times for each sorbent. 2.5. Cyclic CO2 capture and conversion experiments: Cyclic CO2 capture and conversion tests were conducted in a fluidized bed reactor (Fig. S2). The flow rates of N2, CO2 and CH4 were controlled using mass flow controllers (Bronkhorst EL-flow). The concentrations of H2, CO, CO2 and CH4 in the effluent gas were monitored continuously using non-dispersive infrared analyzers (ABB EL3020 and AO2020). In a typical experiment, 1.0 g of the pre-calcined limestone (300–400 µm) was mixed with 1.0 g of the Ni catalyst (300–400 µm) and heated to 800 °C under a flow of N2 (0.5 L/min). The catalyst was reduced at 800 °C in a flow of 10% H2 in N2 (0.5 L/min) for 2 h. Subsequently, the temperature was reduced to 720 °C and the carbonation reaction was performed for 15 min in an atmosphere containing 20 vol.% CO2 (0.5 L/min, balanced with N2). After carbonation, the CO2 sorbent was regenerated under 2.4 vol.% CH4 in N2 (0.5 L/min). In total 10 cycles of the process were performed. The conversion (CH4 and CO2) and yield (H2 and CO) during the pre-breakthrough stage was calculated as: CH4 or CO conversion ሺ%ሻ = 2

Moles of CH4 or CO2 in the off-gas × 100 Moles of CH4 or CO2 in feed

H2 yield ሺ%ሻ=

CO yield ሺ%ሻ=

Moles of H2 in off-gas × 100 Moles of CH4 in feed × 2

Moles of CO in off × 100 Moles of CH4 + CO2 in feed

Here, we define the CO2 slip as the molar concentration of CO2 in the N2-free off-gas during the CO2 utilization step. The purity of the synthesis gas is defined as the sum of the molar concentrations of H2 and CO in the N2-free off-gas. 3. Results and discussion


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a

b

Chemisorption (g)

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N2 physisorption

c

Grain / particle size [nm]

Material

H2 uptake [μmolNi/g]

SBET 2 [m /g]

Vp 3 [cm /g]

Dp [nm]

CO2 sorbent

-

16

0.13

38.2

140 ± 23

DRM catalyst

262 (13.2%)

163

0.91

2.6

8.4 ± 1.5

a

Value in parenthesis represents the dispersion of Ni using H/Ni = 1.0. bThe specific surface area, pore volume, and pore radius were calculated using BET and BJH models. cThe grains size of the CO2 sorbent and the Ni particle size were measured by HR-SEM and TEM, respectively.

Figure 2. Materials characterization: (a) HAADF-STEM image with (b) elemental mapping and (c) HR-TEM and SAED (inset) of the Ni/MgO-Al2O3 catalyst after reduction (d) HR-SEM of the CO2 sorbent (limestone) calcined at 800 °C. (e) XRD patterns of limestone, calcined limestone and reduced Ni/ MgO-Al2O3 : (△) periclase (MgO-Al2O3 solid solution), (◇) Ni, (○) calcite (CaCO3), (●) lime (CaO) and (▲) portlandite (Ca(OH)2). (f) CO2 release characteristics of limestone as a function of temperature: (–––) 680 °C, (–––) 720 °C and (–––) 760 °C. (g) Table summarizing the physico-chemical properties of the CO2 sorbent and catalyst.

The morphological and structural characteristics of the CO2 sorbent and the dry reforming catalyst employed are shown in Figure 2a-e. CaO is obtained through the calcination of limestone (CaCO3), yielding a CO2 sorbent with an average pore and grain diameter of 47 ± 15 nm and 140 ± 23 nm, respectively, as estimated from HR-SEM (Fig. 2d). The active sites of the DRM catalyst are metallic Ni nanoparticles (8.4 ± 1.5 nm) supported on a periclasetype solid solution (MgO-Al2O3) (Fig. 2a-c). To yield strong metal support interactions, critical to minimize sintering and for maintaining a large number of active sites, the catalyst was derived from a hydrotalcite precursor and pre-reduced at 800°C (H2-TPR shown in Fig. S3). To ensure excellent gas-solid contacting characteristics between the CO2 released and the catalyst, the process was operated in a fluidized bed, which, however, poses additional requirements with regards to the mechanical stability of both the CO2 sorbent and the DRM catalyst. Both limestone and the Ni-catalyst used here are of sufficient mechanical strength to allow operation in a bubbling fluidized bed. An important requirement for the effective coupling of CO2 capture with its subsequent conversion is that the rates of CO2 release (during sorbent regeneration) and CO2 conversion are very similar. Hence, to minimize the slip of unconverted CO2, the CO2 release profile during sorbent regeneration has to match the activity of the Nibased DRM catalyst and for the ease of operation, the CO2

release has to be steady. Figure 2f shows that a regeneration temperature of 760 °C leads to a very rapid decomposition of CaCO3, yielding a highly transient release profile (partially owing to the limited quantity of CaCO3 in the fluidized bed, 1.0 g). On the other hand, reducing the regeneration temperature to 680 °C, a stable, yet slower rate of CO2 release due to thermodynamic limitations is observed, (the equilibrium partial pressure of CO2 is 0.22 atm 65). A good trade-off between the CO2 concentration during regeneration and a stable release profile was obtained for a regeneration temperature of 720 °C, allowing us to study in detail and on a laboratory-scale the coupled process. Under this condition, we obtained a sufficiently stable CO2 volume fraction of 2.4 vol% for 15 min. In the following the coupled CO2 capture and conversion reactions were performed at 720 °C using a mixture of 1.0 g limestone and 1.0 g Ni/MgO-Al2O3. In the first step, the CO2 sorbent was carbonated for 25 min in an atmosphere containing 20 vol.% CO2 in N2 (0.5 L/min). The CO2 concentration in the off-gas during the CO2 capture stage is depicted in Fig. 3 (blue line). The difference between the recorded CO2 concentration in the off-gas during CO2 capture and a blank test (gray dash-dotted line, i.e. a bed without any CO2 sorbent) is equal to the quantity of CO2 captured. The capture capacity of limestone-derived CaO was determined as 0.62 gCO2/gsorbent in the 1st cycle. In the second step a flow of 0.5 L/min of 2.4 vol% CH4 in N2, ensuring a ratio of CH4 to CO2 of approximately 1:1, was used. Figure 3 plots the molar flow rate of CH4, CO, H2


4

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ACS Catalysis

st

Figure 3. Coupled CO2 capture and conversion reactions: molar flow rate of the effluent gas in the 1 cycle of the coupled CO2 capture – conversion process and schematic description of the main processes occurring in the reactor.

and CO2 in the effluent stream of the fluidized bed reactor during the 1st cycle. As the regeneration of the CO2 sorbent is a transient process, due to the “depletion” of the CO2 sorbent with time, three reaction stages can be observed. In the first stage (t = 25–42 min), very little unconverted CO2 (0.08 %) and CH4 (0.06 %) are observed (carbon and hydrogen balance 97 ± 2 and 98 ± 2%, respectively), indicating an almost full conversion of the CO2 released via the dry reforming of methane into a synthesis gas. The ratio of H2 to CO is ~ 1.06-to-1, which is slightly higher than the predicted thermodynamic equilibrium for the dry reforming reaction at 720 °C, viz. 0.94. This indicates that there is probably some methane decomposition (CH4 → C + 2 H2) occurring. In the second, i.e. the breakthrough stage (t = 42–60 min) the molar flow rate of CO decreases gradually as the quantity of CO2 released decreases. This observation is in line with the release profile of CO2 plotted in Figure 2f (decreasing CO2 concentration for t > 42 min). In the breakthrough stage, the molar flow rate of H2 is fairly stable (98 ± 2% hydrogen balance). Owing to the decomposition of CH4 into H2 and carbon, the carbon balance cannot be close in this reaction stage when considering only the gaseous products. The detection of graphite by XRD (Fig. S4a), characteristic D (1250–1350 cm-1) and G (1500–1700 cm-1) carbon bands in the Raman spectra (Fig. S4b) and carbon whiskers by transmission electron microscopy (Fig. S4c) of reacted Ni/MgO-Al2O3 provide evidence for methane decomposition during the breakthrough stage. In the post-breakthrough stage (t > 60 min) the concentration of CH4 increases gradually to ~ 2.4 vol.% indicating that very little methane decomposition and/or DRM proceeds at the end of this stage. Indeed, performing H2 chemisorption of the catalyst after the post-breakthrough

stage, showed a considerable reduction of (accessible) surface Ni. As presented in Table S1 and Figure S4. After the post-breakthrough stage, surface Ni was 10 µmolNi/gcat compared to 262 µmolNi/gcat in the fresh catalyst. In addition, the BET surface area and BJH pore volume of Ni/MgO-Al2O3 was reduced from 163 m2/gcat to 142 m2/gcat and from 0.91 cm3/gcat to 0.26 cm3/gcat, respectively. However, the particle size of Ni, as determined by TEM, increased only marginally from 8.4 ± 1.5 nm to 8.7 ± 2.1 nm after 1st post-breakthrough. These findings indicate that deactivation of the Ni/MgO-Al2O3 catalyst is largely due to coking (and only to a smaller extent due to particle sintering), leading to the poisoning of Ni surface sites and the blockage of pores. It is worth noting that the quantity of surface Ni was recovered (back to 255 µmolNi/gcat) in the subsequent carbonation step. In this stage the carbon deposited is removed by gasification, viz. C + CO2 → 2CO. To evaluate the cyclic stability of the materials and the tolerance of the Ni/MgO-Al2O3 catalyst to the oxidative carbonation conditions, repeated carbonation and regeneration-CO2 conversion cycles were performed. Figure 4a plots the molar flow rate of CO2, CH4, H2 and CO in the effluent gas and the ratio of H2/CO as a function of cycle number. We observe a shift of the time at which pre-breakthrough starts with increasing cycle number (from 42 mins in cycle 1 to 32 mins in cycle 10). Nevertheless, the very high conversion of CH4 and CO2 in the pre-breakthrough stage with a stable H2/CO ratio (≈ 1.06) during all 10 cycles is remarkable (the conversions of CH4 and CO2 decreased only from 97.1% to 96.4% and from 96.3% to 95.3%, respectively, over 10 cycles, Fig. S5a). To elucidate the main deactivation mechanism, the cycled materials (after the regeneration-conversion step)


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st

th

Figure 4. Stability test: (a) molar flow rate and H2/CO ratio of the effluent gas as a function of cycle number: (–––) 1 , (- - -) 5 th and (– · –) 10 cycle. The arrows highlight the trends with cycle number; (b) cyclic CO2 uptake of limestone-derived CaO, determined both in a TGA and a fluidized bed.

were characterized using HR-SEM, TEM, physisorption, XRD, and Raman spectroscopy.

N2

TEM micrographs of cycled Ni/MgO-Al2O3 (Fig. S4d) reveal only a small increase in the Ni particle size from 8.4 ± 1.5 nm to 9.2 ± 2.8 nm after 10th post-breakthrough, in particular considering the harsh operating conditions. As already hypothesized from the slightly higher than expected ratio of H2 to CO, some carbon deposition was observed using XRD (Fig. S4a), Raman (Fig. S4b) and TEM (Fig. S4c and Fig. S4d). This is in agreement with previous studies reporting the formation of filamentous carbon on Ni particles larger than 7 nm under DRM conditions 66. Indeed, we also observe some CO formation (Fig. S6) during the subsequent carbonation step owing to CO2 gasification of the carbon deposited. The XRD data of Ni/MgO-Al2O3 collected after the 2nd carbonation step (Fig. S4a) showed only Bragg reflections due to metallic Ni and MgO (no peaks due to graphite or NiO were detected). It is remarkable that during the carbonation step, the deposited carbon is removed via gasification while the metallic state of Ni is preserved. This characteristic allowed us to quantify the amount of carbon deposited as a function of cycle number by measuring the quantity of CO released during the carbonation steps (Fig. S6). Interestingly, we determined a decreasing quantity of carbon deposited with increasing cycle number (from 19 mmolCO⋅mmolNi-1 in the first to 13 mmolCO⋅mmolNi-1 in the 10th cycle). This observation could be explained by the decreasing duration of the prebreakthrough and breakthrough periods with increasing cycle number. To reduce coke deposition different routes may be successful: (i) high operational temperature (≥ 900 °C) to minimize the exothermic Boudouard reaction (2CO → CO2 + C, ΔH0298K = - 172 kJ/mol); (ii) fabrication of catalysts with high oxygen storage capacity67; (iii) bimetallic catalysts56 and (iv) limitation of the DRM to the pre-breakthrough stage. Comparing the HR-SEM micrographs of fresh and cycled limestone revealed appreciable sintering (Fig. S7a). The

average grain size of limestone increased from approximately 140 ± 23 nm (Fig. 2d) for fresh to 414 ± 51 nm for cycled limestone (Fig. S7a). Sintering was also confirmed by N2 adsorption/desorption measurement, as the BET surface area (16 m2/gsorbent) and BJH pore volume (0.13 cm3/gsorbent) of fresh limestone were reduced to 6 m2/g and 0.05 cm3/g, respectively, after 10 cycles (Fig. S7b). CaO-based CO2 sorbents require a high pore volume in order to avoid the CO2 uptake reaction to become diffusion limited (the molar volume of the product CaCO3, 36.9 cm3⋅mol-1, is approximately twice as large as that of CaO, 16.7 cm3⋅mol-1). Hence, considering the rather stable behavior of the DRM catalyst, the decreasing duration of the pre-breakthrough period can be attributed largely to the deactivation of limestone (the CO2 uptake of limestone decreases from 0.62 gCO2⋅gsorbent-1 to 0.40 gCO2⋅gsorbent-1 during 10 cycles, Fig. 4b). The irreversible “decay” of the textual properties of limestonederived CaO (Fig. S7) leads to a loss of ~ 50% of its initial CO2 uptake (Fig. 4b) within 5 cycles. Future work should, therefore, focus primarily on the development of CaObased CO2 sorbents that possess an improved cyclic stability68-73. To mitigate the sintering-induced decay of the CO2 uptake of CaO-with cycle number, the incorporation of high Tammann temperature stabilizers such as Al2O3, ZrO2 or MgO have been proposed and encouraging results have been obtained.68, 74-76 Besides the addition of stabilizers, the morphological optimization of CaO, e.g. the presence of a large number of meso-pores and grains of size < 100 nm can also improve appreciably the CO2 uptake of CaO.77-79 From a practical point of view, the calcination of CaCO3 would be carried out probably at higher temperatures 80-82 and higher temperature (≥ 900 °C) may be required also to minimize the byproduct formation of coke and H2O in DRM 83-85. To demonstrate the feasibility of the coupled process under a broader range of conditions, we have tested the regeneration of CaCO3 in a stream of pure CH4 at 900 °C (the prior CO2 capture step was performed at


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ACS Catalysis

720 °C). The results of this experiments are shown in Fig. S8. Overall, one can observe that the coupled process performed at 900 °C behaves very similarly to when operated at 720 °C. Although, at 900°C the prebreakthrough period is reduced due to a higher rate of CO2 release (and the limited quantity of CO2 sorbent in the reactor), importantly the almost complete conversion of CH4 and the CO2 captured to H2 and CO was observed during the pre-breakthrough period. The ratio H2 : CO in the synthesis gas was close to unity, i.e. 1.04. Neverthless, the continuous CO2 capture and steady release of CO2 from the sorbent is important for operating the proposed process. That is the amount and cyclic stable CO2 capture capacity of CaO-based CO2 sorbent is crucial. In a practical application, the proposed process can be performed using a continuous feed of CaO-based CO2 sorbent and regeneration as demonstrated in pilot-scale calcium looping plant.86-89 The continuous limestone and fuel feeding enable stable CO2 capture capacity and the saturated sorbent is continuously replaced with a fresh or regenerated sorbent for continuous CO2 capture. Therefore, our proposed process can be implemented to synthesis gas production from saturated CaO sorbent with with zero emission of CO2.

dry reforming of methane directly into CO2 capture using inexpensive CaO as the CO2 sorbent. Importantly the release of CO2 (regeneration of the CO2 sorbent) and the CO2 utilization reaction are performed in a single reactor converting CO2 into a synthesis gas with an approximately equimolar ratio of hydrogen to carbon monoxide and with a minimal CO2 slip. The system developed operates efficiently over more than 10 cycles, and deactivation is mainly due to the sintering of CaO derived from limestone. Our current research effort is thus directed to increase the cyclic stability of CaO-based CO2 sorbents.

The deployment of this process to the large scale will require close attention to the heat demand in the 2nd step of the process due to the endothermicity of both the calcination and the DRM. To tackle this problem, one could envision a retrofitting power plant and other stationary industrial CO2 sources (oxy-combustion process) to provide the heat input without re-introducing N2 90-92 and recover the substantial fraction of heat energy 82, 93-94. For instance, theoretical studies on the thermal integration of high-quality heat from calcium looping systems in power plants have shown the potential to reduce the energy penalty to a range of 6–8%, leading to the reduction of CO2 capture costs by 3.5–5.0% compared to standalone calcium process.95-97 Moreover, an interesting alternative may be the use of solar heat as it has been demonstrated for the production of synthesis gas from CO2 and H2O via redox processes 98-100. In addition one might switch to the use of H2 as the regeneration gas, as this allows the utilization of exothermic CO2 hydrogenation reactions, for example methanation (CO2 + 4H2 → CH4 + 2H2O, ΔH0298K = -253 kJ/mol), methanol synthesis (CO2 + 3H2 → CH3OH + H2O, ΔH0298K = -50 kJ/mol) or Fischer-Tropsch combined reverse water-gas shift reaction (CO2 + 3H2 → - CH2- + 2H2O, ΔH0298K = -128 kJ/mol)101-104, yet in this case the energy requirement to produce H2 has to be taken into consideration. Nonetheless, this work clearly demonstrates that CO2 capture and conversion can be coupled in a single reactor with very high CO2 conversions, thus, minimizing subsequent (energyintensive) purification steps and using CH4 as the reducing gas.

*C. R. M.: Tel., +41 44 632 3440; E-mail, [email protected]

4. Conclusion

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To summarize, we have demonstrated the feasibility of a CO2 capture and conversion process that integrates the

ASSOCIATED CONTENT Supporting Information. Details of methods and materials and characterization of materials by N2 physisorption, SEM, TEM, H2-TPR, XRD and Raman. Description of the regeneration of CaCO3 coupled with DRM at 900 °C.

AUTHOR INFORMATION Corresponding Author

Author Contributions S.M.K, M.B., C.C and C.R.M. conceived the project and designed the experiments. Material synthesis, characterization and performance tests were performed by S.M.K. with the help of P.M.A., M.B. and D.H. All authors contributed to discussions. S.M.K, C.C, P.M.A and C.R.M. wrote the manuscript with contributions from all authors. C.R.M. supervised the work.

Funding Sources This research was funded by ETH Zürich and the Swiss National Funding (SNF)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work has been supported financially by an ETH Research grant (ETH 57 12-2). We thank support of Mrs Lydia Zehnder for XRD measurements, the Scientific Center for Optic and Electron Microscopy (ScopeM) providing access to electron microscopes.

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