Thermogravimetry Analysis of CO2 and H2O Reduction from Solar

Jan 3, 2012 - This study addresses the thermochemical production of CO and H2 as high-value solar fuels from CO2 and H2O using reactive Zn ...
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Thermogravimetry Analysis of CO2 and H2O Reduction from Solar Nanosized Zn Powder for Thermochemical Fuel Production Stephane Abanades* Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France ABSTRACT: This study addresses the thermochemical production of CO and H2 as high-value solar fuels from CO2 and H2O using reactive Zn nanoparticles. A two-step thermochemical cycle was considered: Zn-rich nanopowder was first synthesized from solar thermal ZnO dissociation in a high-temperature solar chemical reactor and the reduced material was then used as an oxygen carrier during the CO2 and H2O reduction reactions. The kinetics of CO2 and H2O reduction was investigated by thermogravimetry to demonstrate that the solar-produced nanoparticles react efficiently with CO2 and H2O. Zn started to react from 513 K and almost complete Zn conversion (reaction extent over 95%) was achieved at 633773 K in less than 5 min, thus confirming that the active Zn-rich nanopowder exhibits rapid fuel production kinetics during H2O and CO2 dissociation. The reaction mechanism was best described by a nucleation and growth model with an activation energy of 43 kJ/mol and an oxidant order of 0.8. The high reactivity of zinc was attributed to the specific solar synthesis route involving ZnO thermal dissociation and condensation of Zn vapor as nanoparticles.

1. INTRODUCTION The discovery of feasible means for transforming solar energy into chemical energy in the form of carbon-neutral synthetic fuels is an ambitious goal that provides a sustainable CO2-free and cost-effective pathway to fuel the vehicles and economies of the future. As such, liquid fuels can be directly compatible with existing infrastructures, i.e., distribution network and conversion via combustion engines, and they have a great opportunity to be the replacement for fossil fuels in the 21st century. This study addresses the production of CO and H2 as the building blocks to various solar fuels based on the solar thermochemical dissociation of H2O and CO2 without greenhouse gas emission. The advantage of converting solar radiation to chemical fuels is the production of transportable and long-term storable energy carriers such as syngas that can be further processed to various liquid fuels via catalytic processes. The other major interest of such a process concerns the recycling and up-grading of CO2 emissions into valuable synthetic fuels. The CO2 collected from conventional processes involving fossil fuels combustion is thus used as a raw chemical feedstock that can be recycled,1 rather than being a waste with a cost of disposal. This constitutes an alternative option to underground CO2 sequestration in geologic reservoirs or in the ocean.2 With the right stoichiometric combination of hydrogen and carbon monoxide, synthetic liquid fuels can be produced and then burned to power standard internal combustion engines designed for gasoline or diesel. The targeted solar fuels range from hydrogen and syngas to liquid combustible fuels, such as methanol or even gasoline, diesel, and jet fuel or almost any type of liquid fuel that can be produced from syngas using the FischerTropsch synthesis. The overall process results in the conversion of water and carbon dioxide into fuel from sunlight, which is thus equivalent to a reverse combustion.

These carbon-neutral synthetic fuels can then be used as vehicle fuel or as a feedstock to make plastics and other materials derived from oil. The investigated approach to chemically reenergize carbon dioxide and water into carbon monoxide and hydrogen makes use of activated metal oxide species involved in two distinct steps and concentrated solar energy to drive the endothermic reaction, as represented in Figure 1. Using multistep redox reactions allows reducing the temperature of H2O/CO2 splitting while bypassing the separation issue between H2/CO and O2 since the gas species are produced in separate steps. The H2O-splitting reaction to produce H2 via thermochemical cycles based on ZnO/Zn, SnO2/SnO, Fe3O4/FeO, or mixed metal oxides (ferrites or ceria) was experimentally studied.318 The thermochemical system based on ZnO/Zn redox reactions was recently shown to have the potential to reenergize not only water but also carbon dioxide.1921 ZnO is first reduced at high temperature with concentrated solar radiation as the reaction enthalpy source. Zn then reacts with H2O and/or CO2 to generate H2 and/or CO, and ZnO is recycled back to the first step. The separate reactions of Zn with CO2 and H2O result in the separate production of CO and H2 that can be combined in appropriate amounts for tuning the syngas composition. CO can also be further converted to H2 when combined with water through the water-gas shift reaction (WGSR) for an additional H2 supply. Therefore, solar reactors were designed and developed to produce reactive Zn species and oxygen from ZnO thermal dissociation using solar thermal energy.2225 Other separate studies focused on the reaction of Zn to split H2O and CO2 into H2 and CO. The Zn involved in these studies was either a Received: September 13, 2011 Accepted: December 12, 2011 Revised: December 9, 2011 Published: January 03, 2012

xCO2 þ ðx þ 1ÞH2 O þ solar energy f Cx H2xþ2 ðliquid fuelÞ þ ð1:5x þ 0:5ÞO2 r 2012 American Chemical Society

ð1Þ 741

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Figure 1. H2O and CO2 reduction based on two-step thermochemical ZnO/Zn redox reactions using concentrated solar energy.

When directly feeding an aerosol flow reactor with water vapor and commercial Zn particles (average size of 158 nm), the conversion was only about 24% at 813 K and a gas residence time of 0.6 s.16 Nonisothermal thermogravimetric analysis (TGA) with the same Zn nanopowder dispersed on quartz wool indicated that complete Zn conversion could be achieved during heating up to 800 K at 11.9 K/min.16 The hydrolysis of 70 nm Zn nanocrystal resulted in a complete conversion at 448 K with a residence time of about 10 s, and the activation energy and the reaction order with respect to the water mole fraction were found to be 24 ( 2 kJ/mol and 0.9 ( 0.1, respectively.17 Strong discrepancies can be observed in the estimated activation energy for Zn hydrolysis, ranging from 24 kJ/mol17 to 43 kJ/mol9 to 132 kJ/mol,16 despite the use of submicrometer Zn powders for determining the kinetic parameters. These differences in activation energy can be explained by the preparation methods, chemical composition (fraction of Zn/ZnO, presence of impurities), and morphology of the reacting powders. Moreover, the considered Zn reactant powders are usually almost pure with low content in ZnO, whereas the powders obtained from solar thermal ZnO dissociation contain significant amounts of finely dispersed ZnO impurities because of the simultaneous release of oxygen during synthesis, which improves the reactivity because these ZnO impurities serve as nucleation sites for further oxidation.18,26,27 The H2O- and CO2-splitting reactions using solar Zn produced in a solar reactor prototype suitable for scaling-up were scarcely investigated. For that reason, Zn-rich nanopowder was first synthesized in a high-temperature solar chemical reactor to obtain a representative reduced material. The solar-driven synthesis of Zn material as a reactant was justified because representative samples were produced, while the utilization of commercial standard powders with nonrepresentative characteristics and properties was avoided. The reactivity of the solar Zn nanopowder was then investigated by TGA to compare the chemical conversions and the kinetics obtained during the H2O and CO2 splitting reactions.

commercial grade powder with a large particle size or it was obtained from ZnO carbo-reduction or from pure Zn vaporization and subsequent condensation. Consequently, the observed Zn reactivity differed greatly according to the particle morphology and the Zn preparation method. In particular, surface passivation due to a growing ZnO layer and diffusion mechanisms may hinder the oxidation reaction with H2O or CO2. Previous studies with steam bubbling through molten zinc in the 723773 K range indicated inhibition of the reaction by the formation of a ZnO(s) layer around the steam bubbles.6 When using micrometer-sized commercial pure zinc powders (or Zn obtained from solar ZnO carbo-reduction7), Zn hydrolysis above 673 K showed a fast surface-controlled reaction followed by a slow diffusion-controlled reaction and Zn conversion increased when particle size decreased.8 Likewise, kinetic thermogravimetric studies at 603633 K with commercial submicrometer Zn particles led to a similar reaction mechanism represented by a coreshell model with an initial linear conversion profile attributed to a fast surface reaction (half-order with respect to water vapor mole fraction, y) and followed by a parabolic conversion profile independent of y but dependent on Zn ion diffusion through a ZnO layer.9 The interest of using nanoparticles synthesized through Zn vaporization21 or in a realistic way from concentrated solar power18 was evidenced. Indeed, their large surface-to-volume ratio and their high specific surface area favor their complete oxidation while enhancing the reaction kinetics. The studies involving the simultaneous formation of Zn nanoparticles (by Zn evaporationcondensation) and their in situ hydrolysis by steam-quenching of Zn(g) in an aerosol flow reactor led up to 70% conversion, but the reaction occurred mostly at the walls with ZnO films and filamentary particles deposition, and only pure Zn nanoparticles were collected in the filter.1012 Ernst et al.13 measured up to 90% conversion at temperatures in the 9001273 K range but at the expense of significant wall deposition and low particle yields downstream. Conversions in the range 8796% were obtained in a similar setup at 1023 and 1073 K and with residence times varying from 1.7 to 2.1 s.14,15 However, the energy penalty to evaporate Zn and rapidly cool or quench the flow below the saturation point makes such reactors prohibitive for efficient energy conversion.

2. EXPERIMENTAL SYSTEMS 2.1. Solar Reactor for the Synthesis of Reactive Zn Nanopowders. A solar chemical reactor featuring a cavity-type receiver 742

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Figure 2. Cross-section scheme of the solar reactor based on a refractory ceramic cavity with reactant injection for Zn nanopowder synthesis during high-temperature solar irradiation.

was developed to synthesize the Zn-rich nanopowders at the 1 kWth power scale.25 The considered reaction is the ZnO thermal dissociation (first solar step) for producing the active reduced species used in the next CO2- and H2O-splitting steps. The solar reactor was composed of a glass window and a cylindrical, watercooled reactor shell (Figure 2). The materials for the blackbody cavity-receiver consisted of pure sintered alumina for the cylindrical cavity, yttria-stabilized zirconia felt for the aperture plate, and alumino-silicate for the insulation. The refractory cavity was composed of an alumina tube (30 mm-long and 30 mm i.d.) closed at the bottom by a 7 mm-thick and 37 mm-diameter cylindrical plate and at the front by a 3 mm-thick aperture plate (12 mm-diameter aperture) made of zirconia felt. This cavity was surrounded by the insulation layer. The reactor was installed at the focus of a vertical axis solar furnace composed of a sun-tracking heliostat reflecting vertically the solar irradiation toward a facedown concentrator. The concentrator consisted of a parabolic dish (2 m-diameter, 0.85 m-focal distance) concentrating the incident solar irradiation at the focal point. The power absorbed by the cavity through the aperture was 1050 ( 80 W for a direct normal irradiation (DNI) of 1 kW/m2. The temperature was measured by B-thermocouples in contact with the external cavity walls and by a solar-blind pyrometer pointing the surface of the upcoming oxide inside the cavity through a fluorine window. Once the cavity temperature was reached (1900 K) and steady state operation settled, the reactant was injected continuously as compressed pellets (8 mm-diameter) stacked in a 60 mm-long alumina feeding tube located at the bottom of the cavity. This pile of pellets formed an oxide rod that was pushed upward via a manually rotated screw piston (rod elevation at about 0.9 mm/min) for achieving continuous reactant injection throughout an experimental run (reactant feed rate of about 250 mg/min). Inert gas (4 nL/min of N2) was introduced via the aperture to protect the glass window from the convective flow of product

gases, and a vacuum pump was connected to the outlet for operating at reduced pressure (1520 kPa). The carrier gas and reaction products (O2 and Zn) exited the cavity via a lateral outlet alumina tube. Then, the condensed particles were transported to a specific nanoparticle filter located 0.20 m downstream for their separation from the gas. The collected samples, analyzed via X-ray diffraction (XRD), were composed of a mixture of Zn and ZnO because of the partial recombination reaction with O2, and the mass fraction of Zn species in the solar-produced powder was determined by both a complete oxidation in TGA and by quantifying the hydrogen released from the complete dissolution of the Zn sample in a concentrated aqueous solution (10 wt %) of hydrochloric acid (Zn + 2HCl f ZnCl2 + H2). The typical mean crystallite size of the Zn powder recovered in the filter was estimated with the Scherrer equation and was found to be in the range of 1520 nm. Finally, morphological characterization of the recovered powders was performed via scanning electron microscopy (SEM, Hitachi S-4500), scanning transmission electron microscopy (STEM, Hitachi S-4800), and N2 adsorption/desorption at 77 K for specific surface area characterization (Micromeritics ASAP 2010, preliminary desorption at 573 K). The powder was composed of micronic agglomerates of nanoparticles, with nanoentities typically below 3050 nm (Figure 3). According to the high measured BET specific surface area (18.8 ( 0.1 m2/g) and the mesoporous morphology (0.06 cm3/g for pore sizes between 1.7 and 300 nm), the Zn nanopowders produced with this solar reactor are expected to show favorable kinetics during the H2O and CO2 reduction reactions. This Zn powder was thus used for investigating the reaction kinetics by TGA and for quantifying the Zn conversion as functions of temperature, heating rate, and oxidant mole fraction. 2.2. TGA of the H2O and CO2 Reduction Reactions. The H2O- and CO2-splitting reactions were studied using a Setaram Setsys Evolution thermogravimetric analyzer equipped with a 743

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Figure 3. SEM (left) and STEM (right) imaging of the solar-produced Zn powders.

recovery from the reactor and subsequent handling, thus resulting in about 13 wt % ZnO in the final product. Regarding isothermal TG, the sample was first heated in pure Ar up to the reaction temperature at a heating rate of 20 K/min and the gaseous reactant (CO2 or H2O) was injected once the temperature was reached. During nonisothermal TG, the reactant was injected from the start of sample heating with a linear temperature increase at a controlled heating rate. The CO2 mole fraction ranged between 10% and 100% by adjusting the carrier gas flow-rate (Ar) in the furnace chamber. Steam was generated at a defined relative humidity (RH) by bubbling Ar in a water bath at a controlled temperature, which yields a humid gas with different steam mole fractions: 4.4 ( 0.3% (50% RH, 313 K), 7.0 ( 0.3% (80% RH, 313 K), or 21 ( 1% (90% RH, 333 K). The humid gas was transported to the furnace through a heated tube to avoid condensation. Calibrations of the TG signal with blank runs were carried out to account for the effects of buoyancy and thermal gas expansion on the weight signal and were subtracted from the measurements before analysis. The sample arrangement as a stacked powder in the crucible may cause potential diffusion limitations because access of the reacting gases may be hindered by diffusion. This phenomenon was, however, not observed experimentally because of the very low amount of powder required during TG and was thus negligible. The particle conversion (reaction extent) was calculated dynamically from the global sample mass variation to quantify the reaction progress:

Figure 4. Reaction extents versus time for H2O reduction with Zn during isothermal TG runs at different temperatures and steam mole fractions (H2O mole fraction of 7.0 ( 0.3% (80% RH, 313 K) if not specified, and the time of steam injection corresponds to I at about 14 min).

steam generator. A weighted amount of solar nanopowder (about 35 mg of Zn) was initially loaded inside an alumina crucible (0.17 mL) and then subjected to either isothermal or nonisothermal TG runs. The samples used during each TG experiment came from the same solar-produced powder to facilitate results comparison. The initial mass fractions of reduced Zn species in the samples (wZn) were thus identical in the different TG runs (wZn = 48 ( 1 wt %). An additional sample of Zn nanopowder was prepared by solar vapo-condensation of commercial pure Zn powder (Aldrich, 99.9%) in a specific reactor described by Charvin et al.4 and developed for investigating the Zn/O2 recombination reaction.28 A high initial Zn mass fraction was thus obtained by this technique (wZn = 87 ( 1%), while the produced Zn nanopowder showed identical particle morphology than the one synthesized by solar thermal ZnO dissociation owed to the similar mechanisms occurring during the particle formation process. The nanoparticles synthesized from this technique were not pure Zn because partial oxidation of metallic zinc particles with ambient air occurred during their

α¼

ΔmMZn mwZn ðMZnO  MZn Þ

ð2Þ

where Δm is the sample mass variation (positive because of sample oxidation), m is the initial mass of material in the crucible, wZn is the mass fraction of reduced Zn species in the material, and MZnO and MZn are the molecular weight of ZnO and Zn, respectively. This conversion represents the amount of converted Zn to the initial amount of Zn in the powder. It also corresponds to the amount of H2 or CO produced to the maximum amount that could be produced if the reaction was complete (i.e., H2 or CO yield). The relative uncertainty on the experimental conversion is about 2%, and it is mainly due to the relative uncertainty on the initial Zn mass fraction in the sample. 744

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Figure 6. CO yield at thermodynamic equilibrium as a function of the temperature for the Zn/CO2 system at different CO2/Zn mole ratios (P = 1 atm).

Figure 5. H2 yield at thermodynamic equilibrium as a function of the temperature for the Zn/H2O system at different H2O/Zn mole ratios (P = 1 atm).

3. RESULTS AND DISCUSSION 3.1. TGA of Zn Reactivity during H2O Reduction. H2O reduction via isothermal TG runs was performed for temperatures varying from 633 K to 773 K. Complete conversion was achieved regardless of the reaction temperature (Figure 4). Consequently, the temperature is not a key parameter influencing the reaction rate and the final conversion in the range investigated, although the reaction rate shows a slight temperature dependence. In contrast, the reaction rate strongly depends on the steam content in the flowing gas, with a conversion rate increased by a factor of 2.4 when the H2O mole fraction is increased 3-fold (TG runs at 773 K), resulting in a reaction order with respect to steam of roughly 0.80 ( 0.05. Accordingly, the reaction rate of Zn oxidation is much more sensitive to a steam mole fraction variation than to temperature. Higher steam mole fractions were not investigated because of the strong perturbation of the TG signal. A slight Zn conversion increase was measured before steam injection (limited to 3%), corresponding to the Zn oxidation by residual moisture in the flowing gas adsorbed at the powder surface. Since the conversion varies almost linearly with time during the reaction, the reaction rate is constant and a global zero reaction order can be assumed for Zn oxidation with H2O. Accordingly, the reaction rate can be represented by the following expression:

dα ¼ k½ yH2 O 0:8 dt

Figure 7. Reaction extents versus time for CO2 reduction with Zn during isothermal TG runs (CO2 mole fraction: yCO2 = 50%).

the system. The H2 yield was calculated as the ratio between the amount of targeted product predicted at equilibrium and the input Zn amount in the system (1 mol). According to Figure 5, the H2 yield is maximal at temperatures up to about 9001000 K and it then starts to decrease at higher temperatures because of thermodynamic limitations. The reaction is thus thermodynamically favorable at moderate temperatures (below 900 K), regardless of the H2O/Zn mole ratio. An H2O-deficient composition (H2O/Zn = 0.1:1) does not change the Zn reactivity although the H2 yield is determined by the amount of available H2O reactant. The thermodynamic predictions thus agree with the experimental results. Compared to the existing experimental data on the Zn + H2O reaction,716 the solar-produced Zn used in this study showed the best performances with respect to maximum conversion and kinetic rates. 3.2. TGA of Zn Reactivity during CO2 Reduction. Regarding CO2 reduction from Zn, the thermodynamic study of the system (Figure 6) indicates a wide temperature range in which significant Zn conversion and CO formation occur. The CO yield starts to increase at about 500600 K, and the temperature range of complete Zn conversion is wider when the CO2/Zn mole ratio increases. At low temperatures (below 500900 K depending on the CO2/Zn ratio), CO2 reduction with Zn mainly yields solid

ð3Þ

where k is the kinetic rate constant (2.0 ( 0.1  102 s1 at 773 K) and yH2O is the steam mole fraction. The absence of temperature dependency also suggests a thermodynamic control of the reaction with a final equilibrium state reached rapidly. Hence, thermodynamic equilibrium calculations were performed using HSC Chemistry Outokumpu software29 to predict the H2 yield as a function of the temperature during the H2O reduction reaction. This analysis is based on the Gibbs free enthalpy minimization and assumes ideal mixtures in a closed system without any mass transfer limitation. An excess of inert carrier gas with respect to solid reactant (N2/Zn mole ratio of 100:1) was considered to simulate a continuous gas flow through 745

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C instead of CO according to thermodynamic equilibrium. Thus, an excess of CO2 and temperatures exceeding at least 600 K must be selected to avoid carbon formation during CO2 reduction. At low temperatures, the C yield is enhanced when the CO2/Zn ratio decreases to the detriment of the CO yield. Above 1100 K, the CO yield decreases due to thermodynamic limitations, regardless of the CO2/Zn mole ratio. An optimal temperature range for maximal CO yield thus exists between 900 and 1000 K. The Zn reactivity during CO2 reduction was then investigated experimentally via isothermal TGA. An almost complete Zn conversion was achieved in less than 5 min for temperatures ranging between 633 K and 773 K (CO2 mole fraction of 50%).19 The dependence of the reaction rate on the temperature is weak (especially above 673 K) (Figure 7), similar to the case of H2O reduction. For each temperature, the reaction rate reached a maximum at the inflection point (peak reaction rate for d2α/dt2 = 0), which denotes a progressive increase of the CO2 reduction rate with time before reaching a maximum. The XRD analysis of the solid product after the reaction confirmed the presence of a ZnO phase only, and no evidence of solid C(s) formation was observed (confirmed by TGA in air of the obtained ZnO product). The complete Zn conversion observed during isothermal CO2 and H2O reduction is likely due to the specific synthesis route of particles in the solar reactor. The main mechanisms involve ZnO dissociation followed by condensation of Zn vapors and particle growth combined with partial Zn recombination during growth, because of the presence of O2 that is released simultaneously during the reaction.26 This suggested process for particle formation is a reasonable theory supported by the experimental observations. As a result, Zn is evenly dispersed in the particles and the ZnO sites act as active nuclei to initiate the Zn oxidation reaction in the bulk.27 The use of nanosized particles emphasizes this reaction mechanism since the growth of the nuclei spreads to the whole grains and thus enhances the final reaction extent. The formation of a ZnO diffusion barrier at the particle surface can be thus avoided, which alleviates the diffusion limitation. In a previous study in TGA involving micrometer-sized Zn particles (mean Sauter diameter, 11.4 μm) with low specific surface area (5 m2/g),20 the maximum Zn conversion of approximately 50% only was obtained at 754 K with 20% CO2Ar. High reaction temperatures as well as high CO2 contents led either to strong sample sintering or to the formation of a dense compact ZnO layer, and the reaction was characterized by an initial fast interface-controlled regime followed by a slow diffusioncontrolled regime described by a shellcore kinetic model. At temperatures below the Zn melting point, 693 K, ionic diffusion (Zn2+ and O2) governed the reaction. The use of solarproduced Zn nanoparticles thus totally eliminates the diffusion limitation (ionic diffusion through the ZnO shell inducing slow kinetics), which allows reaching complete conversion. The kinetic rate of CO2 reduction was assumed to obey the following uncoupled form: dα ¼ k½yCO2 m f ðαÞ dt

Figure 8. Master plot analysis comparing the normalized rate data to solid-state kinetic models for CO2 reduction with Zn during isothermal TG run at 633 K (CO2 mole fraction, yCO2 = 50%). The kinetic expressions are detailed by Gotor et al.30.

A master plot analysis was performed for determining the most appropriate kinetic model best describing the reaction mechanism. The normalized differential rate data were compared to various expressions of solid-state kinetic models according to eq 5 in order to identify the appropriate reaction mechanisms.30 This analysis depends only on the kinetic model and allows eliminating the dependence on the other kinetic parameters. ½dα=dt=½dα=dtα ¼ 0:5 ¼ f ðαÞ=f ð0:5Þ

ð5Þ

The most common kinetic models for solidgas reactions pertain to the diffusion models, phase boundary controlled reaction models, nth-order reaction models, and random nucleation and subsequent growth of nuclei models.30 The results (Figure 8) show a good fit over the entire conversion range between the experimental data at 633 K and the two-dimensional growth of nuclei model (A2) represented by the Johnson-Mehl-Avrami equation with f(α) = 2(1  α) [ln(1  α)]1/2. This is consistent with the suggested reaction mechanism involving the Zn oxidation from preexisting ZnO nuclei sites dispersed inside the nanoparticles and growth process to the whole grains. This kinetic analysis also highlights that the diffusion models are not suitable to describe the mechanism, which supports the absence of diffusion limitation that may be induced by the formation of an oxide layer at the particle surface. The identified kinetic mechanism (growth of nuclei model) is consistent with the complete particle conversion (thus meaning a bulk reaction) resulting from the dispersed ZnO impurities that serve as nucleation sites for further Zn oxidation. The presence of ZnO may also induce defects in the Zn lattice structure, which thereby promotes ionic diffusion. The high reactivity of solar Zn nanopowder could thus be the result of combined effects: the bulk dispersion of ZnO that serves as nucleation sites and that improves ionic diffusion through structural defects and the porous nanopowder morphology that enhances gas species diffusion. Nonisothermal TG runs were also performed at different heating rates (10, 15, and 20 K/min) in Ar/CO2 (yCO2 = 50%) with a linear temperature rise to determine precisely the start of Zn oxidation at a temperature of 513 K regardless of the heating rate and to highlight that almost complete conversion (95%) was

ð4Þ

where k is the kinetic rate constant with Arrhenius-type temperature dependency (s1), yCO2 is the CO2 mole fraction, m is the CO2 reaction order, and f(α) is a solid-state kinetic model describing the process mechanism. 746

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Figure 11. Reaction extents versus time for CO2 reduction with Zn during nonisothermal TG runs (constant heating rate of 15 K/min) at different CO2 mole fractions (yCO2 ranging between 10% and 100%).

Figure 9. Reaction extents versus time for CO2 reduction with Zn during nonisothermal TG runs at different heating rates (CO2 mole fraction, yCO2 = 50%). The lines represent the fitting using eq 4 with the A2 model and Ea = 42 ( 6 kJ/mol.

Figure 10. Reaction extents versus temperature for CO2 reduction with Zn during nonisothermal TG runs at different heating rates (CO2 mole fraction yCO2 = 50%).

Figure 12. Logarithm of the Zn conversion rates as a function of ln[yCO2] for CO2 reduction with Zn at a constant heating rate of 15 K/min. The slope equals the kinetic order relative to the CO2 mole fraction: m = 0.82 ( 0.01 (R2 = 0.99) if the data for yCO2 = 1 is not taken into account.

attained at about 673 K (Figure 9). The previously identified A2 model was also used as the fitting kinetic model to check its validity regarding the nonisothermal TG data. Concerning the evolution of the conversion with temperature (Figure 10), it is interesting to point out that the higher the heating rate, the lower the conversion at low temperatures (below about 620 K), this tendency being inversed at higher temperatures. This may be the result of kinetic limitations at low temperatures, whereas the reaction approached thermodynamic equilibrium when temperature increased. This temperature threshold denotes the reaction regime transition from a kinetic to a thermodynamic control. In addition, when the reaction rate increased faster, the heat generated by the exothermal Zn + CO2 reaction may have caused a local increase of temperature in the bulk of the sample and, consequently, further local enhancement of the reaction rate (a high heating rate also promotes the self-heating induced by the reaction enthalpy, which thus favors the reaction completion). The influence of the CO2 mole fraction (10100%) on the Zn reactivity was also examined during dynamic TG runs at a constant heating rate of 15 ( 1 K/min (Figure 11). The start of Zn oxidation was observed at 513 K whatever the CO2 mole fraction

in the feed gas. Although the final conversion does not depend significantly on the CO2 mole fraction, the reaction rate (that determines the time required to reach complete oxidation) tends to increase with the CO2 content in the flowing gas, especially for the lowest CO2 mole fractions below 50%. The variation of the conversion rate at low CO2 mole fractions (1050%) is much higher compared with the variation for high CO2 mole fractions (50% and 100%). The influence of CO2 concentration is thus more pronounced at low CO2 concentrations. This behavior may be explained by the CO2 being absorbed at the surface and by the driving force for the diffusion processes between the core and the particle surface, which increases with the number of adsorbed oxidant molecules according to the mole fraction.31 Indeed, this driving force varies readily from an empty surface to a fully covered one. Then increasing further the CO2 concentration does not modify the surface coverage, and the reaction rate is thus not affected as the whole surface sites should already be occupied by the reactant. Such dynamic TG runs also highlight that the reaction rate (dα/dt) first increased before reaching a maximum (peak reaction rate at the inflection point) when temperature increased 747

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Figure 14. Logarithm of the peak reaction rates as a function of 1/RT for CO2 reduction with Zn (wZn = 87 ( 1%) synthesized by solar vapocondensation. Ea = 42.6 ( 4.6 kJ/mol (R2 = 0.96).

Figure 13. Reaction extents versus time for CO2 reduction with Zn (wZn =87 ( 1%) during isothermal TG runs (CO2 mole fraction, yCO2 = 50%, solar Zn nanopowder synthesized by solar vapo-condensation of commercial Zn). Two identical runs were performed at 623 K with 50 mol % CO2 to check the results reproducibility (superposition).

linearly. The values of the peak reaction rate were used to estimate the CO2 reaction order that was obtained from the slope of the plot of ln[dα/dt] versus ln[yCO2]. A kinetic order relative to the CO2 mole fraction of 0.82 ( 0.01 (R2 = 0.99) was determined (Figure 12). The reaction rate was improved when the CO2 mole fraction increased from 10% to 50%. Increasing further the CO2 mole fraction (up to 100%) did not modify the reaction rate significantly. Consequently, the data for yCO2 = 1 was not taken into account to determine the reaction order. The identified kinetic order is similar to the one obtained with steam instead of CO2 (m = 0.8 at 773 K), and such a value was expected because of the weak influence of the type of oxidizer.9 A previous study with micrometer-sized particles showed that the final reaction extent decreased as the CO2 concentration increased20 because of a more detrimental passivation of the Zn core (formation of a more compact and dense ZnO layer) and a faster transition to the diffusion-controlled regime, leading to reaction inhibition at the end of the interface-controlled regime as a result of a total blockage of the Zn particles. In contrast, high CO2 concentrations were not detrimental to the final reaction extent when using solar Zn nanoparticles (the final conversion in Figure 11 was 93.9% at 50% CO2 and 91.4% at 100% CO2), which is of major importance regarding the recycling of captured CO2 at high concentrations. For a comparison purpose, the Zn nanopowder prepared by solar vapo-condensation was also processed via isothermal TG runs from 603 K to 773 K in order to show the influence of the initial Zn content on CO2 reduction (Figure 13). In this case, the influence of temperature on both the reaction extent and the reaction rate is much more significant than for the Zn nanopowder synthesized by solar thermal ZnO dissociation. Moreover, the final reaction extent does not exceed 80% at the highest temperature investigated (773 K), whereas the reaction was almost complete at this temperature for the powder with a lower initial Zn content (Figure 7). The reaction rate increases sharply during the first minutes of reaction but Zn deactivation occurs suddenly despite the nanosized powder, as previously observed during Zn particles hydrolysis in fixed-bed for hydrogen production.18 This result thus points out that Zn oxidation is favored when the initial powder contains a high fraction of ZnO

Figure 15. Reaction extents versus time for CO2 reduction with Zn (wZn = 87 ( 1%) during nonisothermal TG runs at different heating rates and CO2 mole fractions (solar Zn nanopowder synthesized by solar vapo-condensation of commercial Zn) and associated fittings with A2 model and Ea = 42.6 kJ/mol (lines, cf. eq 4).

(thereby favoring Zn dispersion in the matrix), which leads to the enhanced ionic diffusion in the lattice structure. In contrast, for particles with a high initial Zn mass fraction, the reaction first occurs preferably at the surface with formation of oxide nuclei and subsequent formation of a low-permeable ZnO layer that hinders the reaction progress, due to the limited ionic mobility through the oxide shell.9,18 As a result, the chemical conversion is limited to a threshold value since the core remains unreacted. An activation energy of 42.6 kJ/mol was obtained by linear regression for the CO2 reduction with Zn obtained by solar vapocondensation (Figure 14). Ernst et al.9 obtained a very close value (43 kJ/mol) during the Zn + H2O reaction with submicrometer Zn particles. Nonisothermal TG runs (Figure 15) showed that complete Zn oxidation was possible during a heating ramp, but complete conversion was reached at a minimum temperature of about 830 K (a slightly higher temperature than the one identified in Figure 9), which confirms the hindering effect for high initial Zn content. The reaction rate is thus dependent on the initial Zn content since higher temperatures are required for reaching 748

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Industrial & Engineering Chemistry Research complete particle conversion when the initial Zn content increases. A similar behavior was obtained for micrometer-sized commercial Zn particles during their oxidation with CO2,20 since nearly complete conversion was obtained only during heating up to 1173 K and was attributed to surface cracks in the ZnO layer caused by high Zn(g) partial pressure inside the core above 860 K (after the bursting of the ZnO shell, the Zn(g) exiting through these cracks reacted with CO2). The influence of the CO2 mole fraction (50% and 100%) on the reaction rate is negligible (Figure 15), which agrees with the results obtained with Zn prepared by solar thermal ZnO dissociation (Figure 11). The reaction rate was indeed affected by a change in CO2 content mainly at low CO2 mole fractions below 50%.

ARTICLE

’ ACKNOWLEDGMENT This study was financially supported by ANR (Project Number ANR-09-JCJC-0004-01) and CNRS (Interdisciplinary Energy Program, DISCO2 Project). The author thanks A. Julbe and J. Motuzas (IEM, Montpellier) for the powder characterizations (BET and SEM). The support of R. Garcia from the PROMES technical staff for the design and construction of the solar reactor is acknowledged. ’ REFERENCES (1) Graves, C.; Ebbesen, S. D.; Mogensen, M.; Lackner, K. S. Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable Sustainable Energy Rev. 2011, 15, 1–23. (2) Herzog, H.; Golomb, D. Carbon capture and storage from fossil fuel use. In Encyclopedia of Energy; Elsevier: Amsterdam, The Netherlands, 2004; Vol. 1, pp 277287. (3) Abanades, S. Hydrogen production technologies from solar thermal energy. Green 2011, 1 (2), 209–220. (4) Charvin, P.; Abanades, S.; Lemont, F.; Flamant, G. Experimental study of SnO2/SnO/Sn thermochemical systems for solar production of hydrogen. AIChE J. 2008, 54 (10), 2759–2767. (5) Abanades, S.; Legal, A.; Cordier, A.; Peraudeau, G.; Flamant, G.; Julbe, A. Investigation of reactive cerium-based oxides for H2 production by thermochemical 2-step water-splitting. J. Mater. Sci. 2010, 45, 4163–4173. (6) Berman, A.; Epstein, M. The kinetics of hydrogen production in the oxidation of liquid zinc with water vapor. Int. J. Hydrogen Energy 2000, 25 (10), 957–967. (7) Vishnevetsky, I.; Epstein, M. Production of hydrogen from solar zinc in steam atmosphere. Int. J. Hydrogen Energy 2007, 32 (14), 2791–2802. (8) Lv, M.; Zhou, J.; Yang, W.; Cen, K. Thermogravimetric analysis of the hydrolysis of zinc particles. Int. J. Hydrogen Energy 2010, 35, 2617–2621. (9) Ernst, F. O.; Steinfeld, A.; Pratsinis, S. E. Hydrolysis rate of submicron Zn particles for solar H2 synthesis. Int. J. Hydrogen Energy 2009, 34 (3), 1166–1175. (10) Wegner, K.; Ly, H. C.; Weiss, R. J.; Pratsinis, S. E.; Steinfeld, A. In situ formation and hydrolysis of Zn nanoparticles for H2 production by the 2-step ZnO/Zn water-splitting thermochemical cycle. Int. J. Hydrogen Energy 2006, 31 (1), 55–61. (11) Weiss, R. J.; Ly, H. C.; Wegner, K.; Pratsinis, S. E.; Steinfeld, A. H2 production by Zn hydrolysis in a hot-wall aerosol reactor. AIChE J. 2005, 51 (7), 1966–1970. (12) Melchior, T.; Piatkowski, N.; Steinfeld, A. H2 production by steam-quenching of Zn vapor in a hot-wall aerosol flow reactor. Chem. Eng. Sci. 2009, 64 (5), 1095–1101. (13) Ernst, F. O.; Tricoli, A.; Pratsinis, S. E.; Steinfeld, A. Cosynthesis of H2 and ZnO by in-situ Zn aerosol formation and hydrolysis. AIChE J. 2006, 52 (9), 3297–3303. (14) Hamed, T. A.; Davidson, J. H.; Stolzenburg, M. Hydrolysis of Evaporated Zn in a Hot Wall Flow Reactor. J. Sol. Energy Eng. 2008, 130 (4), 041010. (15) Hamed, T. A.; Venstrom, L.; Alshare, A.; Br€ulhart, M.; Davidson, J. H. Study of a Quench Device for the Synthesis and Hydrolysis of Zn Nanoparticles: Modeling and Experiments. J. Sol. Energy Eng. 2009, 131, 031018. (16) Funke, H. H.; Diaz, H.; Liang, X.; Carney, C. S.; Weimer, A. W.; Li, P. Hydrogen generation by hydrolysis of zinc powder aerosol. Int. J. Hydrogen Energy 2008, 33 (4), 1127–1134. (17) Ma, X.; Zachariah, M. R. Size-resolved kinetics of Zn nanocrystal hydrolysis for hydrogen generation. Int. J. Hydrogen Energy 2010, 35, 2268–2277. (18) Chambon, M.; Abanades, S.; Flamant, G. Kinetic investigation of hydrogen generation from hydrolysis of SnO and Zn solar nanopowders. Int. J. Hydrogen Energy 2009, 34 (13), 5326–5336.

4. CONCLUSION This study focused on the synthesis and the reactivity of nanosized Zn powder for solar fuel production from H2O and CO2. Zn-rich nanopowder was first synthesized in a hightemperature solar chemical reactor. The so-produced solar Zn particles were subsequently used in separate reactions to split H2O and captured CO2 into H2 and CO for the ultimate production of renewable liquid fuels. Complete Zn conversion was achieved at relatively low temperatures (633773 K) with both H2O and CO2 as oxidizers. The influence of the oxidizing gas concentration on the reaction rate was noteworthy only at low mole fractions (typically below 50%), and a reaction order relative to H2O and CO2 of 0.8 was identified. The agreement of experimental results with thermodynamic predictions suggests that the reaction rate is not controlled by diffusion limitation and that the initial presence of ZnO promotes the Zn oxidation reaction. The identified kinetic mechanism of the Zn oxidation reaction was best represented by a growth of nuclei model. The complete Zn conversion during H2O and CO2 reduction comes from the synthesis method during the solar step, which results in the formation of Zn-rich nanoparticles containing ZnO clusters dispersed in the bulk that serve as nucleation sites for further oxidation during the H2O/CO2 splitting step. Consequently, the solar Zn particles containing recombined ZnO react better than purer Zn particles, suggesting that the defects in the Zn lattice structure caused by the presence of ZnO enhance oxygen ion mobility. This effect is combined with the mesoporous morphology that also facilitates access of the gas to the core by diffusion. Thus the high Zn reactivity is likely the result of the improved bulk ionic diffusion combined with the morphology that makes possible gas diffusion in the porous structure. The developed solar reactor produced highly active Zn nanoparticles that can be efficiently used for producing highvalue solar fuels from H2O and CO2, with high particle conversion and rapid fuel production kinetics. The complete cycle for H2 and CO production via two-step ZnO/Zn redox reactions was thus demonstrated in a representative solar process, opening the road toward a complete solar fuel production unit based on solar energy. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +33 4 68 30 77 30. Fax: +33 4 68 30 29 40. E-mail: [email protected]. 749

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(19) Abanades, S.; Chambon, M. CO2 dissociation and upgrading from 2-step solar thermochemical processes based on ZnO/Zn and SnO2/SnO redox pairs. Energy Fuels 2010, 24 (12), 6667–6674. (20) Loutzenhiser, P. G.; Galvez, M. E.; Hischier, I.; Stamatiou, A.; Frei, A.; Steinfeld, A. CO2 Splitting via Two-Step Solar Thermochemical Cycles with Zn/ZnO and FeO/Fe3O4 Redox Reactions II: Kinetic Analysis. Energy Fuels 2009, 23, 2832–2839. (21) Loutzenhiser, P. G.; Galvez, M. E.; Hischier, I.; Graf, A.; Steinfeld, A. CO2 splitting in an aerosol flow reactor via the two-step Zn/ZnO solar thermochemical cycle. Chem. Eng. Sci. 2010, 65, 1855–1864. (22) Muller, R.; Haeberling, P.; Palumbo, R. Further advances toward the development of a direct heating solar thermal chemical reactor for the thermal dissociation of ZnO(s). Solar Energy 2006, 80 (5), 500–511. (23) Schunk, L.; Haeberling, P.; Wepf, S.; Wuillemin, D.; Meier, A.; Steinfeld, A. A Solar Receiver-Reactor for the Thermal Dissociation of Zinc Oxide. J. Sol. Energy Eng. 2008, 130 (2), 021009. (24) Chambon, M.; Abanades, S.; Flamant, G. Design of a lab-scale rotary cavity-type solar reactor for continuous thermal reduction of volatile oxides under reduced pressure. J. Sol. Energy Eng. 2010, 132, 021006. (25) Chambon, M.; Abanades, S.; Flamant, G. Thermal dissociation of compressed ZnO and SnO2 powders in a moving front solar thermochemical reactor. AIChE J. 2011, 57 (8), 2264–2273. (26) Weidenkaff, A.; Steinfeld, A.; Wokaun, A.; Auer, P. O.; Eichler, B.; Reller, A. Direct solar thermal dissociation of zinc oxide: condensation and crystallization of zinc in the presence of oxygen. Solar Energy 1999, 65 (1), 59–69. (27) Weidenkaff, A.; Reller, A. W.; Wokaun, A.; Steinfeld, A. Thermogravimetric analysis of the ZnO/Zn water splitting cycle. Thermochim. Acta 2000, 359, 69–75. (28) Chambon, M.; Abanades, S.; Flamant, G. Solar Thermal Reduction of ZnO and SnO2: Characterization of the Recombination Reaction with O2. Chem. Eng. Sci. 2010, 65 (11), 3671–3680. (29) Roine, A. HSC Chemistry 5.11; Outokumpu Research Oy: Pori, Finland, 2002. (30) Gotor, F. J.; Criado, J. M.; Malek, J.; Koga, N. Kinetic analysis of solid-state reactions: the universality of master plots for analyzing isothermal and nonisothermal experiments. J. Phys. Chem. A 2000, 104, 10777–10782. (31) Moore, W. J.; Lee, J. K. Kinetics of the formation of oxide films on zinc foil. Trans. Faraday Soc. 1951, 47 (5), 501–508.

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