Synthesis of Cu-Rich, Al2O3-Stabilized Oxygen Carriers Using a

Feb 22, 2012 - Chemical looping combustion (CLC) is an emerging, new technology for carbon capture and storage (CCS). Copper-based oxygen carriers are...
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Synthesis of Cu-Rich, Al2O3-Stabilized Oxygen Carriers Using a Coprecipitation Technique: Redox and Carbon Formation Characteristics Qasim Imtiaz, Agnieszka M. Kierzkowska, Marcin Broda, and Christoph R. Müller* Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092 Zurich, Switzerland S Supporting Information *

ABSTRACT: Chemical looping combustion (CLC) is an emerging, new technology for carbon capture and storage (CCS). Copper-based oxygen carriers are of particular interest due to their high oxygen carrying capacity and reactivity, low tendency for carbon deposition, and exothermic reduction reactions. In this work, CuO-based and Al2O3-stabilized oxygen carriers with high CuO loadings were developed using a coprecipitation technique. The cyclic redox performance of the synthesized oxygen carriers was evaluated at 800 °C in a laboratory-scale fluidized bed reactor using a reducing atmosphere comprising 10 vol. % CH4 and 90 vol. % N2. The CuO content in the oxygen carrier was found to increase with the pH value at which the coprecipitation was performed. The oxygen carrying capacity of the oxygen carrier containing 87.8 wt % CuO was found to be high (5.5 mmol O2/g oxygen carrier) and stable over 25 redox cycles. Increasing the CuO content further, i.e. > 90 wt %, resulted in materials which showed a decreasing oxygen carrying capacity with cycle number. It was also shown that the incorporation of K+ ions in the oxygen carrier can avoid the formation of the spinel CuAl2O4 and significantly reduce carbon deposition. high oxygen carrying capacity of 20.1 wt %,2 (ii) copper-based oxygen carriers show a high reactivity in both the reduction and the oxidation steps,7 (iii) complete conversion of gaseous, hydro-carbonaceous fuels into CO2 and H2O is thermodynamically favored,8 and (iv) both the reduction and oxidation reactions are exothermic.9 However, due to the low melting point of copper of only 1085 °C, CLC using Cu-based oxygen carriers is limited to comparatively low operating temperatures when compared to alternative oxygen carriers such as Fe2O3.10 Furthermore, the rate of the oxidation reaction of unsupported CuO decreases very quickly with cycle number owing to agglomeration and thermal sintering.11 Hence, to increase the resistance of CuO toward agglomeration and thermal sintering, it has to be stabilized by an inert, high melting point support material such as Al2O3.2 For example, de Diego et al.11 employed three different methods, i.e., mechanical mixing (40, 60, or 80 wt % CuO), coprecipitation (40 wt % CuO), and impregnation (20 or 30 wt % CuO) to synthesize Cu-based oxygen carriers. de Diego et al.11 reported that impregnation of CuO on an inert support was the only successful method to synthesize oxygen carriers that possess a high reactivity and “acceptable” mechanical properties. Mattisson et al.7 prepared Cu-based oxygen carriers (33 wt % CuO) supported by Al2O3 using a dry impregnation

1. INTRODUCTION An emerging and highly promising technology to produce a pure stream of CO2 is chemical looping combustion (CLC).1 In CLC a solid oxygen carrier is used to transfer oxygen from the air to a hydro-carbonaceous fuel, thus, inherently separating CO2 and H2O from the other major components of a flue gas, viz. N2 and unreacted O2. After the condensation of steam, a pure stream of CO2 is obtained which can be readily compressed and stored underground, e.g., in saline aquifers. CLC is a two-stage process comprising a reduction and an oxidation (or regeneration) step. In the reduction step, the fuel is oxidized by the lattice oxygen of the oxygen carrier according to2 CnH2m + (2n + m)MexOy → nCO2 + mH2O + (2n + m)MexOy − 1

(1)

The reduced oxygen carrier is subsequently reoxidized in the regeneration step typically performed using air, viz,2 1 O2 + MexOy − 1 → MexOy (2) 2 An additional advantage of CLC is that the combustion reactions occur at low temperature and in the absence of N2, thus the formation of NOx is greatly reduced.3 Materials considered as oxygen carriers include oxides of Ni, Mn, Fe, and Cu.4−7 Copper-based oxygen carriers have several advantages over other transition metal oxides: (i) CuO has a © 2012 American Chemical Society

Received: Revised: Accepted: Published: 3561

November 29, 2011 February 21, 2012 February 22, 2012 February 22, 2012 dx.doi.org/10.1021/es2042788 | Environ. Sci. Technol. 2012, 46, 3561−3566

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following nomenclature to characterize the synthesis protocol of the oxygen carrier. CuAl is followed by the pH at which the precipitation was performed. For example, CuAl-3.8 is an oxygen carrier which was precipitated at pH 3.8. Characterization of Oxygen Carriers. The surface area and pore volume of the unreacted and recycled oxygen carriers were calculated from N2 isotherms (Quantachrome NOVA 4000e), using the Brunauer−Emmett−Teller (BET)14 and Barrett−Joyner−Halenda (BJH)15 models, respectively. Prior to each N2 adsorption measurement, the unreacted and reacted oxygen carriers were degassed at 300 °C for 2 h. The morphologies of the unreacted and reacted oxygen carriers were imaged by high-resolution scanning electron microscopy (HR-SEM) using a Zeiss Gemini 1530 FEG scanning electron microscope operated at 5 kV. All samples were sputter-coated with platinum prior to imaging. Powder X-ray diffraction (XRD) analysis (Bruker, AXS D8 Advance) was conducted to determine the crystalline phases in the synthesized oxygen carrier. The diffraction patterns were acquired within the range of 2θ = 5°−90° with a step size of 0.0275° per second using Cu Kα radiation and a power of 40 kV × 40 mA. Dried samples were further characterized using Raman spectroscopy (Renishaw RM 1000 Laser Raman Microscope) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy (Thermo Scientific Nicolet iS10). The ATR-FTIR spectra were collected by averaging 32 scans. The spectral resolution was set to 4 cm−1. Extended double scan Raman spectra were obtained by exciting the samples with 1% of the power of a 785-nm diode laser (180 mW total power) for 10 s at 1 cm−1 spectral resolution. Finally, the hardness of the oxygen carrier particles was determined by measuring the force necessary to crush a particle using a Shimpo force gauge equipment. Prior to the measurement, the particles were sieved to a size range of 1.10−2.0 mm. The crushing strength was obtained by averaging 15 measurements obtained from randomly chosen particles. Oxygen Uncoupling Capacity and Carbon Deposition Analysis. To determine the oxygen uncoupling capacity, i.e., the amount of lattice oxygen released in an inert atmosphere, decomposition of the oxygen carriers was carried out in a N2 atmosphere in a thermo-gravimetric analyzer (TGA, Mettler Toledo TGA/DSC 1). In a typical experiment, approximately 30 mg of the oxygen carrier material was heated from room temperature to 500 °C at a rate of 10 °C/min under a flow of N2 of 25 mL/min and held at 500 °C for 60 min to remove moisture. Thermodynamic calculations showed that the partial pressure of O2 is negligible at 500 °C. Subsequently, the temperature was increased to 1100 °C at a rate of 10 °C/min in a flow of N2 (25 mL/min). Assuming that the CuO contained in the sample decomposes to Cu2O, the mass fraction of CuO in the oxygen carrier can be determined according to

technique and reported a high reactivity of the material synthesized in the temperature range investigated, i.e., 750−950 °C, using CH4 as a fuel. The redox properties of CuO/Al2O3 oxygen carriers (14 wt % CuO) prepared using a dry impregnation technique were studied by Adànez et al.12 in the temperature range 700−800 °C using CH4 as a fuel. It was found that the most important parameters affecting the conversion of CH4 were the ratio of oxygen carrier to fuel and the reaction temperature. On the other hand, Chuang et al.,13 contradicting previous results of de Diego et al.,11 reported that Cu-based oxygen carriers manufactured using mechanical mixing or wet impregnation techniques were prone to agglomeration and possessed poor redox characteristics when tested in a fluidized bed reactor. However, oxygen carriers synthesized using a coprecipitation technique maintained a high O2 carrying capacity over many redox cycles. Furthermore, the statistical analysis of Chuang et al.13 showed that the performance of the oxygen carrier synthesized via a coprecipitation technique depends mainly on the pH value at which the precipitation is performed compared to the other synthesis parameters such as calcination time, calcination temperature or calcination atmosphere. Considering the contradicting conclusions of the previous reports that employed the coprecipitation technique to synthesize Cu-based oxygen carriers, the present study aims at obtaining a better understanding of the influence of coprecipitation parameters, such as the pH at which the precipitation is performed or the ratio of Cu2+ to Al3+ in the precursor solution, on the chemical composition and morphological structure of the precipitated and calcined oxygen carriers. The redox characteristics of the oxygen carriers as determined in a fluidized bed are subsequently interpreted in light of the detailed chemical and morphological characterization of the synthesized material. Important facets of this study were (i) to determine the maximum loading of CuO that provides stable redox characteristics and (ii) to assess whether the incorporation of potassium ions into the oxygen carrier originating from the precipitation agent influences carbon formation.

2. EXPERIMENTAL SECTION Preparation of Oxygen Carriers. Cu-based, Al2O3stabilized oxygen carriers were prepared using a coprecipitation technique. Coprecipitation was performed at pH values of 3.8, 5.5, 8.5, and 12.5 using KOH as the precipitating agent. The selection of the pH values was based on preliminary titration experiments (Text S1 and Figure S1 of the Supporting Information, SI). First, a solution containing Cu(NO3)2·2.5H2O and Al(NO3)3·9H2O with a ratio of Cu2+ to Al3+ of 3.0:1.0 to give 82.4 wt % of CuO in the final material was prepared. The total concentration of cations in the solution was 2 M. Subsequently, a 2 M KOH solution was added dropwise to the solution containing the nitrates under magnetic stirring until the desired pH value was reached. The resulting mixture was aged for 150 min and subsequently filtered. During filtration, the precipitate was washed several times with reverse osmosis water (15.0 MΩcm) to remove excess nitrate and alkali ions. Once the electrical conductivity of the filtrate was less than 25 μS/cm, the cake of the washed precipitate was put in an oven at 100 °C for 24 h. The dried oxygen carrier material was subsequently calcined at 800 °C for 2 h in a muffle furnace. Finally, the calcined material was crushed and sieved to a size range of 600−710 μm. Throughout the paper we will use the

wt % CuO =

(m500◦C − m1100◦C) × 9.94 × 100 m500◦C

where m500 °C and m1100 °C are the masses of the oxygen carrier after the removal of moisture and after decomposition, respectively. The factor of 9.94 is based on the stoichiometry of the decomposition reaction 2CuO → Cu2O + 3562

1 O2 2

(3)

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symmetric stretching mode of the NO3− groups, whereas the peaks in the range 1200−1500 cm−1, the main peaks being located at 1270 and 1347 cm−1, are due to the NO3− (v3) antisymmetric stretching vibrations.16 The peaks at 3510, 3552, and 3585 cm−1 can be ascribed to the stretching of the three OH− groups of gerhardtite. Furthermore, the broad band observed in the range 2300−2700 cm−1 for the dried oxygen carriers precipitated at pH values of 3.8, 5.5, and 12.5 can be attributed to the formation of boehmite (AlO(OH)).17 On the other hand, the absence of this band in the Raman spectra of the dried oxygen carrier synthesized at a pH value of 8.5 indicates the precipitation of Al3+ as gibbsite (Al(OH)3).17 The formation of gerhardtite was also confirmed by the ATR-FTIR spectra of the dried oxygen carries (Text S3 and Figure S3 of the SI). XRD Analysis. To identify the chemical composition and crystalline phases of the calcined oxygen carriers, powder XRD analysis was performed. Figure 2 shows the diffractograms of the fresh and cycled (25 redox cycles) oxygen carriers containing 17.6 wt % Al2O3. The XRD measurements of the unreacted oxygen carriers, shown in Figure 2a, revealed the presence of CuO and CuAl2O4 phases. Importantly, Al2O3 was not detected in any of the samples, indicating that Al2O3 was either present in amorphous form or as CuAl2O4. The latter is very likely since, assuming complete precipitation of Al3+, the maximal mass fraction of Al2O3 in the oxygen carrier was 17.6 wt %. The diffractograms of the fully reduced oxygen carriers (Figure S4 of the SI) showed major peaks due to the presence of Cu and very weak peaks for Cu2O and CuO, confirming that CuAl2O4 can be fully reduced to Cu and Al2O3. This observation is in agreement with the Cu−Al−O phase diagram reported by Ingram et al.18 Similarly, Cu is reoxidized to CuO or CuAl2O4 under the oxidizing conditions studied here. Thus, the presence of CuAl2O4 did not influence negatively the oxygen carrying capacity of the oxygen carriers synthesized. However, it is conceivable that the reduction kinetics of CuO and CuAl2O4 differ. The diffractograms of oxygen carriers that have undergone 25 redox cycles in a fluidized bed are shown in Figure 2b. Figure 2b shows that the XRD patterns of the cycled oxygen carriers are similar to that of the unreacted material except for the presence of Cu2O. Cu2O is formed via the decomposition of CuO at 800 °C during the purge with N2 between the reduction and oxidation steps. It is worth mentioning that CuAl2O4 was not detected in the oxygen carrier CuAl-8.5. The absence of CuAl2O4 in CuAl-8.5 could be attributed to the stabilization of CuO by the presence of residual K+ ions. The inhibiting effect of Na+ ions toward spinel formation for the CuO−Al2O3 system has been reported previously by Selim and Youssef.19 The ATR-FTIR spectrum of calcined CuAl-8.5 revealed a very hydrophilic oxygen carrier material (Text S4 and Figure S5 of the SI). CuO Content, Mechanical and Structural Characterization. To unequivocally determine the CuO content of the calcined oxygen carriers, oxygen decoupling experiments were performed. Table 1 summarizes the mass fraction of CuO in the calcined oxygen carriers, the surface area, pore volume, and the crushing strength of the oxygen carriers synthesized, as a function of the pH. The measurements in Table 1 demonstrate that the pH value at which the precipitation was performed strongly influenced the CuO content as well as the structural properties of the oxygen carriers. The content of CuO in oxygen carrier synthesized at pH value of 3.8 was less than the theoretical value indicating that

To study the deposition of carbon during the reduction reaction, approximately 30 mg of oxygen carrier was heated from room temperature to 800 °C at a rate of 10 °C/min in a flow of N2 (25 mL/min) in a TGA. The sample was exposed at 800 °C for 5 min to an atmosphere containing 10 vol. % CH4 and 90 vol. % N2 (25 mL/min) and subsequently cooled under a flow of N2 of 25 mL/min by switching off the furnace resulting in an average cooling rate of approximately 25 °C/ min. The reduced sample was further analyzed using a Raman spectrometer. Fluidized Bed Apparatus. The cyclic redox stability of the oxygen carriers was determined in a laboratory-scale fluidized bed reactor. Detailed information on the experimental setup and protocol can be found in the SI (Text S2 and Figure S2). The molar rate of CO2 production in the reduction step was obtained by multiplying the measured mole fraction of CO2 in the off-gas with the total molar gas flow rate. The total number of moles of CO2 produced was calculated by integrating the molar flow rate of CO2 with respect to time. Assuming full reduction of the theoretical mass of CuO in the oxygen carriers, i.e., assuming 82.4 wt % CuO, the yield of CO2 was defined as yield of CO2 =

measured amount of CO2 theoretical amount of CO2

3. RESULTS AND DISCUSSION Characterization of the Dried Oxygen Carriers. The chemical composition of the dried oxygen carriers was determined using Raman and ATR-FTIR spectroscopy. The Raman spectra of the dried oxygen carriers is shown in Figure 1. Precipitation was performed at the pH values 3.8, 5.5, 8.5,

Figure 1. Raman spectra of the dried oxygen carriers, synthesized using KOH as the precipitating agent. Precipitation was performed at pH values of 3.8, 5.5, 8.5 and 12.5.

and 12.5 using KOH as the precipitation agent. The Raman spectra confirmed that the precipitation of a solution of nitrates of Cu2+ and Al3+ using KOH as the precipitating agent resulted in the formation of gerhardtite, irrespective of the mass fraction of Al2O3 and the pH value at which the precipitation was performed. Gerhardtite, Cu2NO3(OH)3, is a basic copper(II) nitrate. The peaks marked in the Raman spectra shown in Figure 1 agree well with the Raman spectrum of natural gerhardtite.16 The bands in the range 400−550 cm−1 (with peaks located at 532, 477, and 413 cm−1) can be attributed to the OH− deformation modes of the Cu(OH)3− units, whereas the two smaller peaks located at 695 and 722 cm−1 can be assigned to the v4 out of plane bending mode of the NO3− group.16 The peak located at 1048 cm−1 is due to the (v1) 3563

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Figure 2. XRD patterns of the oxygen carriers synthesized using KOH as the precipitating agent. Precipitation was performed at pH values of 3.8, 5.5, 8.5, and 12.5. (a) Unreacted oxygen carriers (calcined at 800 °C for 2 h) and (b) oxygen carriers after being subjected to 25 redox cycles. The following compounds were identified: (▲) CuO, (■) CuAl2O4, and (▼) Cu2O.

Table 1. CuO Content, Mechanical and Structural Parameters of the Synthesized Oxygen Carriers pH

3.8

5.5

8.5

12.5

CuO contents (wt %) surface area (m2/g) pore volume (cm3/g) hardness (N)

70.6 39.6 0.18 39.8

81.3 41.1 0.16 49.1

78.3 2.6 0.03 54.2

87.8 19.5 0.14 41.5

precipitation at pH = 3.8 resulted in an Al-rich oxygen carrier owing to (i) an insufficient amount of KOH to achieve complete precipitation of Cu2+ and Al3+ ions from the solution and (ii) the smaller solubility constant of Al(OH)3 (Ksp = 4.6 × 10−33)20 compared to Cu(OH)2 (Ksp = 2.2 × 10−20).20 Furthermore, the composition of the oxygen carriers was close to the theoretical value of 82.4 wt % CuO at the pH values of 5.5 and 8.5. These results indicate that at pH = 5.5 and 8.5, full precipitation of Al3+ and Cu2+ was achieved. Finally the decomposition experiments established that the precipitation at a high pH value, i.e., 12.5, resulted in oxygen carries with a very high CuO content (87.8 wt %) owing to the formation of [Al(OH)4]−in the strongly alkaline solution. In addition, the BET surface areas were highest and lowest for oxygen carriers synthesized at pH 5.5 and 8.5, respectively. The observation that the surface area is highest at pH = 5.5 can be explained by the formation of high surface area γ-Al2O3 via the calcination of the boehmite, AlO(OH), precursor at 800 °C.21 The lowest surface area was observed for oxygen carriers precipitated at pH = 8.5. This is probably due to the formation of low surface area α-Al2O3 via the calcination of a gibbsite, Al(OH)3.21 Furthermore, the pore volume was found to decrease in the following order of pH values: 3.8 > 5.5 > 12.5 > 8.5. The crushing strength of the unreacted oxygen carrier particles did not seem to vary systematically with the pH value at which precipitation was performed and was within the range of 39.8−71.6 N. The results of the crushing strength measurements indicated that the oxygen carriers synthesized are suitable for use in a fluidized bed reactor. Morphological Characterization. To obtain information about the morphological changes that occur in the oxygen carriers over repeated redox cycles, the unreacted and cycled oxygen carriers were imaged using scanning electron microscopy. The scanning electron micrographs of the oxygen carriers before and after 25 redox cycles are shown in Figure 3. From Figure 3 it can be seen that the unreacted oxygen carriers have a fairly dense structure and the grain size is strongly

Figure 3. SEM micrographs of the unreacted (a−d) and cycled (e−h) oxygen carriers (after 25 redox cycles) prepared at different pH values: (a, e) 3.8, (b, f) 5.5, (c, g) 8.5, and (d, h) 12.5.

influenced by the pH at which precipitation was performed. The oxygen carriers obtained by the calcination of boehmite and gerhardtite were found to have small grains. On the other hand, large grains were observed for the oxygen carrier derived via the calcination of gibbsite and gerhardtite. The smallest average grain size of approximately 10 nm was observed for CuAl-5.5, whereas the largest average grain size of approximately 165 nm was observed for CuAl-8.5. After 25 redox cycles, the oxygen carriers precipitated at pH values of 3.8, 5.5, and 12.5 showed a considerable growth in grain size. The most noticeable change of surface morphology between unreacted and cycled oxygen carrier was observed for CuAl-5.5. Unreacted CuAl-5.5, Figure 3b, showed a structure comprising grains of approximately 10 nm. After 25 redox cycles, Figure 3g, the surface of the oxygen carrier showed cavities and the size of the grains significantly increased by more than a magnitude to an average size of 350 nm. The average grain sizes of CuAl-3.8 3564

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using a gas mixture containing 10 vol. % CH4 and 90 vol. % N2 are shown in Figure 5. The two peaks located at 1315 and 1610

and CuAl-12.5 were found to only increase from 24 to 115 nm and 27 to 94 nm, respectively. Oxygen Carrying Capacity As Determined in a Laboratory-Scale Fluidized Bed Reactor. To access the performance of the oxygen carriers synthesized, experiments were performed at 800 °C in a fluidized bed reactor using a mixture of 10 vol. % CH4 and 90 vol. % N2 as the reducing gas, whereas air was used for reoxidation. The yields of CO2 determined from redox experiments performed over 25 cycles are shown in Figure 4. From Figure 4, it can be seen the oxygen

Figure 5. Raman spectra of the reduced oxygen carriers.

cm−1 in the Raman spectra of oxygen carriers precipitated at pH values of 3.8, 5.5, and 12.5 confirm the presence of graphite on the surface of the oxygen carrier.23 The peak located at 1610 cm−1 can be assigned to the deformation of carbon bonds in the basal planes of the hexagonal structure of graphite, whereas the peak at 1315 cm−1 is due to defects in the graphite structure.24 On the other hand, the Raman spectra of the oxygen carrier precipitated at a pH value of 8.5 did not show any peaks due to carbon formation. This observation may be explained by the hydrophilic nature of CuAl-8.5 owing to the presence of KOH impurities as discussed in the previous section. KOH has a strong affinity to H2O and thus retains H2O formed during the oxidation of methane. It has been reported that the presence of H2O can substantially reduce the formation of carbon, owing to the reaction of adsorbed H2O with carbon precursors during the decomposition of hydrocarbons.25 Implications. In this study Cu-rich, Al2O3 stabilized oxygen carriers were developed for CLC using a coprecipitation technique. The material synthesized at a pH value of 12.5 and containing 87.8 wt % CuO satisfied several important requirements for oxygen carriers in CLC, viz., it possessed (i) an excellent redox-stability over many cycles, (ii) resistance to attrition, i.e., very good mechanical properties, and (iii) the highest oxygen carrying capacity that has been reported so far for Cu-based oxygen carriers. Thus, the oxygen carrier developed here could significantly reduce the solid inventory in a CLC system and thereby decrease appreciably the costs of CO2 capture. Previous reports on CLC highlighted that the deposition of carbon on the oxygen carrier via the decomposition of hydrocarbonaceous fuels results in a decrease in the CO2 capture efficiency of the CLC process. This study demonstrated that the incorporation of KOH into the matrix of the oxygen carrier significantly reduced carbon deposition. Therefore, considering its high reactivity and resistance to carbon formation, the oxygen carriers developed here may also be suitable for the emerging field of CLC for solid fuels. However, further experiments are required to confirm this conclusively. Additionally, the results presented here show that excellent mechanical and structural properties of the oxygen carrier can be obtained by a judicious choice of the pH value at which precipitation is performed due to the formation of favorable Al2O3 precursors. Thus, we believe that this study will aid the

Figure 4. CO2 yield of the oxygen carriers precipitated using KOH as the precipitating agent. Precipitation was performed at the following pH values: (⧫) 3.8, (■) 5.5, (▲) 8.5, and (●) 12.5.

carriers synthesized at pH values of 3.8, 8.5, and 12.5 showed stable yields of CO2 over 25 cycles. On the other hand, the yield of CO2 of the oxygen carrier prepared at a pH value of 5.5 was found to decrease with cycle number probably due to large changes of its surface morphology over the 25 redox cycles tested. In addition, no agglomeration was observed for all the oxygen carriers. Furthermore, the oxygen carrier CuAl-3.8 (70.6 wt % CuO) showed yields of CO2 of less than unity owing to its high Al content as discussed previously. Figure 4 shows that the yields of CO2 of the oxygen carriers CuAl-5.5 and CuAl-8.5 were close to unity indicating that the CuO content of the oxygen carries were close to the theoretical value of 82.4 wt %. The oxygen carrier CuAl-12.5 possessed a yield of CO2 larger than unity which can be attributed to the high CuO content of CuAl-12.5 (87.8 wt %). Increasing the CuO content further, i.e., > 90%, by adjusting the ratio of Cu2+ to Al3+ in the nitrate precursor solution resulted in decreasing yields of CO2 with cycle number (Text S5 and Figure S6 of the SI), probably due to agglomeration and thermal sintering. To summarize, the oxygen carrying capacity was strongly influenced by the pH value at which precipitation was performed and correlated well with the CuO content of the material as determined from oxygen uncoupling experiments. Analysis of Carbon Deposition. One important facet of the work presented here is the characterization of the formation of carbon during the reduction reaction using Raman spectroscopy. Carbon deposition was found to occur for reduction temperatures ≥750 °C via the decomposition of methane according to CH 4(g) → C(s) + 2H2(g)

(4)

Carbon formation would reduce the CO2 capture efficiency of the CLC process, due to the release of CO2 into the atmosphere during the regeneration step.22 Raman spectra of oxygen carriers containing 17.6 wt % Al2O3 reduced for 5 min 3565

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conditions on methane combustion. Ind. Eng. Chem. Res. 2006, 45 (17), 6075−6080. (13) Chuang, S. Y.; Dennis, J. S.; Hayhurst, A. N.; Scott, S. A. Development and performance of Cu-based oxygen carriers for chemical-looping combustion. Combust. Flame 2008, 154 (1−2), 109− 121. (14) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1983, 60 (2), 309−319. (15) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The determination of pore volume and area distributions in porous substances. 1. Computations from nitrogen isotherms. J. Am. Chem. Soc. 1951, 73 (1), 373−380. (16) Frost, R. L.; Martens, W. N.; Rintoul, L.; Mahmutagic, E.; Kloprogge, J. T. Raman spectroscopy of gerhardtite at 298 and 77 K. J. Raman Spectrosc. 2004, 35 (11), 991−996. (17) Frost, R. L.; Ruan, H. D.; Kloprogge, J. T. Comparison of the Raman spectra of Bayerite, Boehmite, Diaspore and Gibbsite. J. Raman Spectrosc. 2001, 32 (9), 745−750. (18) Ingram, B. J.; Mason, T. O.; Asahi, R.; Park, K. T.; Freeman, A. J. Electronic structure and small polaron hole transport of copper aluminate. Phys. Rev. B 2001, 64 (15), 155114−115120. (19) Selim, M. M.; Youssef, N. A. Thermal stability of CuO-Al2O3 system doped with sodium. Thermochim. Acta 1987, 118 (1), 57−63. (20) Clark, R. W.; Bonicamp, J. M. The Ksp − solubility conundrum. J. Chem. Educ. 1998, 75 (9), 1182−1185. (21) Doesburg, E. B. M.; van Hooff, J. H. C. Preparation of catalyst supports and zeolites. In Catalysis, An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis; Moulijn, J. A., van Leeuwen, P., van Santen, R. A., Eds.; Elsevier: Amsterdam, Netherlands, 1993; pp 316. (22) Cho, P.; Mattisson, T.; Lyngfelt, A. Carbon formation on Nickel and Iron oxide-containing oxygen carriers for chemical-looping combustion. Ind. Eng. Chem. Res. 2005, 44 (4), 668−676. (23) Corbella, B. M.; de Diego, L. F.; García-Labiano, F.; Adánez, J.; Palacios, J. M. Characterization study and five-cycle tests in a fixed-bed reactor of titania-supported nickel oxide as oxygen carriers for the chemical-looping combustion of methane. Environ. Sci. Technol. 2005, 39 (15), 5796−5803. (24) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 2000, 61 (20), 14095−14107. (25) Bartholomew, C. H. Carbon deposition in steam reforming and methanation. Catal. Rev.-Sci. Eng. 1982, 24 (1), 67−112.

rational design of highly efficient transition metal-based oxygen carrier material via the coprecipitation technique.

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ASSOCIATED CONTENT

S Supporting Information *

Text S1−S6, Figures S1−S7, Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +41446323440; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Swiss national science foundation (SNF) for financial support (Project 406640_13670011). We also thank Mrs. Lydia Zender for her help with the XRD analysis, Professor H. G. Park for providing access to the Raman Spectrometer, and the Electron Microscopy Centre of the ETH Zurich (EMEZ) for providing access to and training on scanning electron microscopes.

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REFERENCES

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dx.doi.org/10.1021/es2042788 | Environ. Sci. Technol. 2012, 46, 3561−3566